Method for manufacturing a high-strength galvanized steel sheet having excellent formability and crashworthiness

Abstract

A method of manufacturing a galvanized steel sheet having a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.05% or less, a relationship of Ti>4N being satisfied, and the balance comprising Fe and inevitable impurities, and a microstructure containing 60% or more and 95% or less of tempered martensite in terms of area ratio and 5% or more and 20% or less of retained austenite in terms of area ratio.

Claims

1. A method for manufacturing a high-strength galvanized steel sheet having excellent formability and crashworthiness, the method comprising: hot-rolling a steel slab having a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.05% or less, a relationship of Ti>4N being satisfied, and the balance comprising Fe and inevitable impurities at a finish rolling temperature of an A.sub.3 transformation point or higher; after the completion of finish rolling, subsequently cooling the resulting steel sheet to a coiling temperature at an average cooling rate of 30 C./s or more; coiling the steel sheet at the coiling temperature of 300 C. or higher and 550 C. or lower to form a hot-rolled steel sheet; then performing heat treatment on the hot-rolled steel sheet, the heat treatment including heating the hot-rolled steel sheet to an annealing temperature of (A.sub.3 transformation point 20 C.) or higher and (A.sub.3 transformation point +80 C.) or lower at an average heating rate of 5 C./s or more in a temperature range of 500 C. or higher and an A.sub.1 transformation point or lower, holding the steel sheet at the annealing temperature for 10 seconds or more, then cooling the steel sheet from 750 C. to a temperature range of 100 C. or higher and 350 C. or lower at an average cooling rate of 30 C./s or more, subsequently reheating the steel sheet to a temperature of 300 C. or higher and 600 C. or lower, and holding the steel sheet at the temperature for 10 seconds or more and 600 seconds or less; then galvanizing the steel sheet, and optionally performing an alloying treatment, such that the high-strength galvanized steel sheet is formed having a microstructure containing 64% or more and 95% or less of tempered martensite in terms of area ratio and 5% or more and 20% or less of retained austenite in terms of area ratio, the tempered martensite having an average grain diameter of 5 m or less.

2. A method for manufacturing a high-strength galvanized steel sheet having excellent formability and crashworthiness, the method comprising: hot-rolling a steel slab having a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P; 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.05% or less, a relationship of Ti>4N being satisfied, and the balance comprising Fe and inevitable impurities at a finish rolling temperature of an A.sub.3 transformation point or higher; after the completion of finish rolling, subsequently, cooling the resulting steel sheet to a coiling temperature at an average cooling rate of 30 C./s or more; coiling the steel sheet at the coiling temperature of 300 C. or higher and 550 C. or lower to form a hot-rolled steel sheet; then pickling the hot-rolled steel sheet and then cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; performing heat treatment on the cold-rolled steel sheet, the heat treatment including heating the cold-rolled steel sheet to an annealing temperature of (A3 transformation point 20 C.) or higher and (A.sub.3 transformation point +80 C.) or lower at an average heating rate of 5 C./s or more in a temperature range of 500 C. or higher and an A.sub.1 transformation point or lower, holding the steel sheet at the annealing temperature for 10 seconds or more, then cooling the steel sheet from 750 C. to a temperature range of 100 C. or higher and 350 C. or lower at an average cooling rate of 30 C./s or more, reheating the steel sheet to a temperature of 300 C. or higher and 600 C. or lower, and holding the steel sheet at the temperature for 10 seconds or more and 600 seconds or less; then galvanizing the steel sheet, and optionally performing an alloying treatment, such that the high-strength galvanized steel sheet is formed having a microstructure containing 64% or more and 95% or less of tempered martensite in terms of area ratio and 5% or more and 20% or less of retained austenite in terms of area ratio, the tempered martensite having an average grain diameter of 5 m or less.

3. The method for manufacturing a high-strength galvanized steel sheet according to claim 1, wherein the composition further contains at least one selected from the group consisting of, by mass %, Cr: 0.005% or more and 2.00% or less, Mo: 0.005% or more and 2.00% or less, V: 0.005% or more and 2.00% or less, Ni: 0.005% or more and 2.00% or less, and Cu: 0.005% or more and 2.00% or less.

4. The method for manufacturing a high-strength galvanized steel sheet according to claim 1, wherein the composition further contains, by mass %, Nb: 0.01% or more and 0.20% or less.

5. The method for manufacturing a high-strength galvanized steel sheet according to claim 1, wherein the composition further contains, by mass %, at least one selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

6. The method for manufacturing a high-strength galvanized steel sheet according to claim 2, wherein the composition further contains at least one selected from the group consisting of, by mass %, Cr: 0.005% or more and 2.00% or less, Mo: 0.005% or more and 2.00% or less, V: 0.005% or more and 2.00% or less, Ni: 0.005% or more and 2.00% or less, and Cu: 0.005% or more and 2.00% or less.

7. The method for manufacturing a high-strength galvanized steel sheet according to claim 2, wherein the composition further contains, by mass %, Nb: 0.01% or more and 0.20% or less.

8. The method for manufacturing a high-strength galvanized steel sheet according to claim 3, wherein the composition further contains by mass %, Nb: 0.01% or more and 0.20% or less.

9. The method for manufacturing a high-strength galvanized steel sheet according to claim 2, wherein the composition further contains, by mass %, at least one selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

10. The method for manufacturing a high-strength galvanized steel sheet according to claim 3 wherein the composition further contains, by mass %, at least one selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

11. The method for manufacturing a high-strength galvanized steel sheet according to claim 4, wherein the composition further contains, by mass %, at least one selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

Description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(1) The present invention will now be described in detail with reference to exemplary embodiments. A steel sheet of the present invention is a galvanized steel sheet including a substrate and a galvanized layer provided on a surface of the substrate.

(2) First, a description will be made of reasons for selecting the microstructure of a steel sheet serving as the substrate of the steel sheet of the present invention.

(3) A steel sheet serving as a substrate of a high-strength galvanized steel sheet of the present invention preferably has a microstructure containing 60% or more and 95% or less of tempered martensite in terms of area ratio and 5% or more and 20% or less of retained austenite in terms of area ratio, or further containing 10% or less (including 0%) of ferrite in terms of area ratio and/or 10% or less (including 0%) of martensite in terms of area ratio, the tempered martensite having an average grain diameter of 5 m or less.

(4) Tempered Martensite: 60% or More and 95% or Less in Terms of Area Ratio

(5) In the present invention, the formation of tempered martensite is preferred in order to ensure the strength and formability, in particular, stretch flangeability of the steel sheet. When the area ratio of tempered martensite is less than 60%, it is difficult to achieve both a tensile strength (TS) of 1,200 MPa or more and a hole expansion ratio () of 50% or more. On the other hand, when the area ratio of tempered martensite exceeds 95%, the total elongation (EL) significantly decreases and satisfactory formability cannot be achieved. Accordingly, the area ratio of tempered martensite is set to 60% or more and 95% or less. Note that the area ratio of tempered martensite is preferably 60% or more and 90% or less, and more preferably 70% or more and 90% or less.

(6) Retained Austenite: 5% or More and 20% or Less in Terms of Area Ratio

(7) In the present invention, the formation of retained austenite is preferred in order to ensure formability of the steel sheet. Retained austenite is effective in improving the total elongation (EL). In order to sufficiently exhibit this effect, it is necessary to set the area ratio of retained austenite to 5% or more. On the other hand, when the area ratio of retained austenite exceeds 20%, the hole expansion ratio () significantly decreases and stretch flangeability deteriorates. Accordingly, the area ratio of retained austenite is set to 5% or more and 20% or less. Note that the area ratio of retained austenite is preferably 10% or more and 18% or less.

(8) Furthermore, in the present invention, the microstructure of the steel sheet (substrate) is preferably a dual-phase microstructure including tempered martensite and retained austenite. In the case where the steel sheet (substrate) contains ferrite and/or martensite, it is necessary to limit the amounts thereof to the ranges described below.

(9) Ferrite: 10% or Less (Including 0%) in Terms of Area Ratio

(10) When the area ratio of ferrite exceeds 10%, it is difficult to achieve both a tensile strength (TS) of 1,200 MPa or more and a hole expansion ratio () of 50% or more. Accordingly, the area ratio of ferrite is set to 10% or less (including 0%).

(11) Martensite: 10% or Less (Including 0%) in Terms of Area Ratio

(12) When the area ratio of martensite exceeds 10%, the hole expansion ratio () significantly decreases and stretch flangeability deteriorates. Accordingly, the area ratio of martensite is set to 10% or less (including 0%).

(13) Furthermore, in the present invention, other phases (e.g., bainite and pearlite) may also be contained as long as tempered martensite, retained austenite, ferrite, and martensite satisfy the respective area ratios described above. However, from the standpoint of the strength, the total area ratio of the other phases is preferably 15% or less.

(14) Average Grain Diameter of Tempered Martensite: 5 m or Less

(15) In the present invention, it is advantageous to refine the tempered martensite in order to ensure crashworthiness. As described above, it is believed that when the grain diameter of tempered martensite is made small, in dynamic deformation of a steel sheet caused at the time of the crash of an automobile, the number of propagation paths of cracks increases, crash energy is dispersed, and it becomes possible to absorb lager crash energy. When the average grain diameter of tempered martensite exceeds 5 m, the above-described effect of improving crashworthiness cannot be sufficiently obtained. Accordingly, in embodiments of the present invention, the average grain diameter of tempered martensite is set to 5 m or less. Note that the average grain diameter of tempered martensite is preferably 3 m or less.

(16) Herein, in the present invention, the terms area ratio of tempered martensite, area ratio of ferrite, area ratio of martensite, and area ratios of the other phases refer to the area proportion of respective phases to an observation area in the case where the microstructure of a steel sheet serving as a substrate is observed. Each of the area ratios is determined as follows. A cross section in the thickness direction of the steel sheet is polished, and then corroded with 3% nital. Subsequently, a position located at from an edge of the steel sheet in the thickness direction is observed with a scanning electron microscope (SEM) at a magnification of 1,500. Each of the area ratios is determined by image processing using Image-Pro manufactured by Media Cybernetics, Inc.

(17) In the present invention, the area ratio of retained austenite is determined as follows. A steel sheet is polished to a position located at from an edge of the steel sheet in the thickness direction, and further polished by 0.1 mm by chemical polishing. With regard to this polished surface, the integrated reflection intensities of a (200) plane, a (220) plane, and a (311) plane of fcc iron (austenite) and a (200) plane, a (211) plane, and a (220) plane of bcc iron (ferrite) are measured with an X-ray diffractometer using the K line of Mo. A proportion of austenite is determined from an intensity ratio of the integrated reflection intensity obtained from each the planes of fcc iron (austenite) to the integrated reflection intensity obtained from each of the planes of bcc iron (ferrite). This proportion of austenite is defined as the area ratio of retained austenite.

(18) In the present invention, the average grain diameter of tempered martensite is determined as follows. A cross section parallel to the rolling direction of the steel sheet is observed with a scanning electron microscope (SEM) at a magnification of 1,500. The total of the area of the tempered martensite present in a field of view is divided by the number of tempered martensite crystal grains to determine the average area of the tempered martensite crystal grains. The power of the average area of the tempered martensite crystal grains is defined as the average grain diameter (corresponding to one side of a square (square approximation)).

(19) Next, the reasons for selecting the composition of the steel sheet (substrate) of the present invention will be described. Note that the notation of % representing a composition below means mass % unless otherwise stated.

(20) C: 0.05% or More and 0.5% or Less

(21) Carbon (C) is an essential element in order to form a low-temperature transformed phase such as tempered martensite and to increase the tensile strength (TS). When the C content is less than 0.05%, it is difficult to ensure 60% or more tempered martensite in terms of area ratio. On the other hand, when the C content exceeds 0.5%, the total elongation (EL) and spot weldability are degraded. Accordingly, the C content is set to 0.05% or more and 0.5% or less. Preferably, the C content is 0.1% or more and 0.3% or less.

(22) Si: 0.01% or More and 2.5% or Less

(23) Silicon (Si) is an element that is effective in improving the balance between the tensile strength (TS) and the total elongation (EL) by contributing to solid solution hardening of a steel. In addition, Si is an element that is effective in forming retained austenite. In order to achieve these effects, it is necessary to set the Si content to 0.01% or more. On the other hand, a Si content exceeding 2.5% causes a decrease in the total elongation (EL) and deterioration of the surface quality and weldability. Accordingly, the Si content is set to 0.01% or more and 2.5% or less. Preferably, the Si content is 0.7% or more and 2.0% or less.

(24) Mn: 0.5% or More and 3.5% or Less

(25) Manganese (Mn) is an element that is effective in increasing the strength of a steel, and is an element that promotes the formation of a low-temperature transformed phase such as martensite in a cooling step after hot rolling and a cooling step from an annealing temperature described below. In order to achieve these effects, it is necessary to set the Mn content to 0.5% or more.

(26) On the other hand, when the Mn content exceeds 3.5%, the total elongation (EL) significantly decreases, thereby deteriorating formability. Accordingly, the Mn content is set to 0.5% or more and 3.5% less. Preferably, the Mn content is 1.5% or more and 3.0% less.

(27) P: 0.003% or More and 0.100% or Less

(28) Phosphorus (P) is an element that is effective in increasing the strength of a steel. In order to achieve this effect, it is necessary to set the P content to 0.003% or more. On the other hand, when the P content exceeds 0.100%, crashworthiness of the steel is decreased by grain boundary segregation of P. Accordingly, the P content is set to 0.003% or more and 0.100% or less.

(29) S: 0.02% or Less

(30) Sulfur (S) is a harmful element that is present as an inclusion such as MnS and deteriorates crashworthiness and weldability. Therefore, it is preferable to reduce the S content as much as possible in the present invention. However, in consideration of the manufacturing cost, the S content is set to 0.02% or less.

(31) Al: 0.010% or More and 0.5% or Less

(32) Aluminum (Al) is an element that acts as a deoxidizer, and is preferably added in a deoxidizing step in the steelmaking. In order to achieve this effect, it is necessary to set the Al content to 0.010% or more. On the other hand, when the Al content exceeds 0.5% and a continuous casting process is employed, a risk of slab cracking during continuous casting increases. Accordingly, the Al content is set to 0.010% or more and 0.5% or less. Preferably, the Al content is 0.02% or more and 0.05% or less.

(33) B: 0.0002% or More and 0.005% or Less

(34) Boron (B) is an element that is effective in suppressing the formation of ferrite from austenite grain boundaries and forming a low-temperature transformed phase in a cooling step after hot rolling and a cooling step from an annealing temperature described below. In order to achieve this effect, it is necessary to set the B content to 0.0002% or more. On the other hand, when the B content exceeds 0.005%, the effect is saturated, and thus the effect that is worth the cost is not obtained.

(35) Accordingly, the B content is set to 0.0002% or more and 0.005% or less. Preferably, the B content is 0.0005% or more and 0.003% or less.

(36) Ti: 0.05% or Less, and Ti>4N

(37) Titanium (Ti) is an element that is necessary to effectively utilize B, which has the above-described effect, by forming a Ti nitride to fix N in a steel. Boron (B) exhibits the above effect in a solid-solution state, but is easily bonded to N in the steel and precipitates in the form of BN. Boron in the form of the precipitated state loses the above effect. Consequently, in the present invention, by incorporating Ti, which has an affinity with N stronger than the affinity of B with N, nitrogen is fixed in a high-temperature range so as to suppress the precipitation of BN. In order to achieve this effect, it is necessary to set the Ti content to be larger than 4N content (mass %). On the other hand, even if Ti is excessively incorporated, the effect of suppressing the precipitation of BN is saturated, and the total elongation (EL) decreases. Accordingly, the Ti content is set to 0.05% or less and so as to satisfy the relationship Ti>4N.

(38) The basic composition in embodiments of the present invention has been described above. In addition to the above basic composition, at least one selected from the group consisting of Cr: 0.005% or more and 2.00% or less, Mo: 0.005% or more and 2.00% or less, V: 0.005% or more and 2.00% or less, Ni: 0.005% or more and 2.00% or less, and Cu: 0.005% or more and 2.00% or less may be contained.

(39) Each of Cr, Mo, V, Ni, and Cu is an element that is effective in forming a low-temperature transformed phase such as martensite in a cooling step after hot rolling and a cooling step from an annealing temperature described below. In order to achieve this effect, it is preferable to incorporate at least one element selected from Cr, Mo, V, Ni, and Cu in an amount of each element of 0.005% or more. On the other hand, when the content of each of these elements exceeds 2.00%, the above effect is saturated, and thus the effect that is worth the cost is not obtained. Accordingly, the content of each of Cr, Mo, V, Ni, and Cu is preferably set to 0.005% or more and 2.00% or less.

(40) Furthermore, in the present invention, 0.01% or more and 0.20% or less of Nb may be further contained in addition to the above basic composition.

(41) Niobium (Nb) is an element that forms a carbonitride and thus that is effective in increasing the strength of a steel by precipitation hardening. In order to achieve this effect, it is preferable to set the Nb content to 0.01% or more. On the other hand, when the Nb content exceeds 0.20%, the effect of increasing the strength is saturated and the total elongation (EL) may decrease. Accordingly, the Nb content is preferably set to 0.01% or more and 0.20% or less.

(42) In the present invention, in addition to the above basic composition, at least one selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less may be contained.

(43) Each of Ca and a rare-earth metal (REM) is an element that is effective in controlling the form of a sulfide, and is an element that is effective in improving formability of a steel sheet. In order to achieve this effect, it is preferable to incorporate at least one element selected from of Ca and REM in an amount of each element of 0.001% or more. On the other hand, when the content of each of these elements exceeds 0.005%, the cleanliness of the steel may be adversely affected. Accordingly, the content of each of Ca and REM is preferably set to 0.001% or more and 0.005% or less.

(44) In the steel sheet of the present invention, components other than the above are Fe and inevitable impurities.

(45) Next, a method for manufacturing the steel sheet of the present invention will be described.

(46) The high-strength galvanized steel sheet of the present invention is manufactured by, for example, hot-rolling a steel slab having the above-described composition at a finish rolling temperature of an A.sub.3 transformation point or higher; after the completion of finish rolling, subsequently cooling the resulting steel sheet to a coiling temperature at an average cooling rate of 30 C./s or more; coiling the steel sheet at a coiling temperature of 300 C. or higher and 550 C. or lower to form a hot-rolled steel sheet; then performing heat treatment on the hot-rolled steel sheet, the heat treatment including heating the hot-rolled steel sheet to an annealing temperature of (A.sub.3 transformation point20 C.) or higher and (A.sub.3 transformation point+80 C.) or lower at an average heating rate of 5 C./s or more in a temperature range of 500 C. or higher and an A.sub.1 transformation point or lower, holding the steel sheet at the annealing temperature for 10 seconds or more, then cooling the steel sheet from 750 C. to a temperature range of 100 C. or higher and 350 C. or lower at an average cooling rate of 30 C./s or more, subsequently reheating the steel sheet to a temperature of 300 C. or higher and 600 C. or lower, and holding the steel sheet at the temperature for 10 seconds or more and 600 seconds or less; then galvanizing the steel sheet, and optionally performing an alloying treatment.

(47) Alternatively, in the above method, the hot-rolled steel sheet after coiling may be pickled and then cold-rolled to form a cold-rolled steel sheet, the above-described heat treatment may be performed on the cold-rolled steel sheet, a galvanizing treatment may then be performed, and the alloying treatment may be optionally performed.

(48) In the present invention, a method for producing a steel is not particularly limited, and a known producing method using a converter, an electric furnace, or the like, can be employed. From the standpoint of suppressing macro segregation, a steel slab is preferably manufactured by a continuous casting process. Alternatively, a slab may be manufactured by another known casting process such as an ingot making-slabbing process or a thin-slab continuous casting process. In hot-rolling a steel slab after casting, the steel slab may be once cooled to room temperature, and may then be reheated in a heating furnace and rolled. Alternatively, a steel slab after casting may be charged in a heating furnace without being cooled to room temperature, and may be heated and then rolled. Alternatively, in the case where a steel slab after casting maintains a temperature equal to or higher than a predetermined temperature, an energy-saving process, in which direct rolling is performed after slight retention of heat, may also be employed. Note that in the case where a steel slab is heated (or reheated) in a heating furnace, the heating temperature of the steel slab is preferably set to 1,100 C. or higher in order to dissolve a carbide and to suppress an increase in a rolling load during hot rolling. On the other hand, in order to suppress an increase in scale loss, the heating temperature of the steel slab is preferably set to 1,300 C. or lower.

(49) Rough rolling and finish rolling are performed on the steel slab obtained as described above. In the present invention, conditions for the rough rolling need not be particularly limited. In performing the finish rolling, from the standpoint of suppressing a trouble during rolling, the trouble being concerned about when the heating temperature of the steel slab is low, a rough bar may be heated after the rough rolling. Furthermore, a so-called continuous rolling process may be employed in which rough bars are joined together and then subjected to continuous finish rolling.

(50) In the present invention, it is preferred that a hot-rolled steel sheet (or a cold-rolled steel sheet) before an annealing treatment described below have a microstructure of bainite or martensite having a high dislocation density. For this purpose, the finish rolling temperature, cooling conditions subsequent to the finish rolling, and the coiling temperature are specified as follows.

(51) Finish Rolling Temperature: A.sub.3 Transformation Point or Higher

(52) When the finish rolling temperature is lower than an A.sub.3 transformation point, ferrite is formed during rolling, and austenite formed during an annealing treatment and during a step of heating a hot-rolled steel sheet (or a cold-rolled steel sheet) to an annealing temperature, the annealing treatment and the step of heating being described below, is coarsened. As a result, a fine tempered martensite microstructure cannot be obtained as the microstructure of the substrate of the finally obtained galvanized steel sheet, and crashworthiness of the steel sheet decreases. In finish rolling, anisotropy of a hot-rolled steel sheet is increased, which may result in a decrease in formability after cold rolling and annealing. Setting the finish rolling temperature to the A.sub.3 transformation point or higher is effective in solving this problem. Accordingly, the finish rolling temperature is set to the A.sub.3 transformation point or higher.

(53) In order to reduce the rolling load and to make the shape and the material quality of the hot-rolled steel sheet uniform, it is preferable to perform lubricated rolling, through which a coefficient of friction is adjusted to be 0.10 to 0.25, in all passes or some of the passes of the finish rolling.

(54) Average Cooling Rate to Coiling Temperature: 30 C./s or More

(55) After the finish rolling, when the average cooling rate to a coiling temperature is less than 30 C./s, ferrite is formed during cooling, and austenite formed during an annealing treatment and during a step of heating a hot-rolled steel sheet (or a cold-rolled steel sheet) to an annealing temperature, the annealing treatment and the step of heating being described below, is coarsened. As a result, a fine tempered martensite microstructure cannot be obtained as the microstructure of the substrate of the finally obtained galvanized steel sheet, and crashworthiness of the steel sheet decreases. Accordingly, the average cooling rate to the coiling temperature is set to 30 C./s or more.

(56) Coiling Temperature: 300 C. or Higher and 550 C. or Lower

(57) When the coiling temperature exceeds 550 C., coarse ferrite and pearlite are formed, and austenite formed during an annealing treatment and during a step of heating a hot-rolled steel sheet (or a cold-rolled steel sheet) to an annealing temperature, the annealing treatment and the step of heating being described below, is coarsened. As a result, a fine tempered martensite microstructure cannot be obtained as the microstructure of the substrate of the finally obtained galvanized steel sheet, and crashworthiness of the steel sheet decreases. On the other hand, when the coiling temperature is lower than 300 C., the shape of the hot-rolled steel sheet is deteriorated. Accordingly, the coiling temperature is set to 300 C. or higher and 550 C. or lower. Preferably, the coiling temperature is 400 C. or higher and 530 C. or lower.

(58) Through the above steps, a hot-rolled steel sheet having a microstructure of bainite or martensite having a high dislocation density, that is, a microstructure containing a large number of nucleation sites of austenite is obtained. In the present invention, this hot-rolled steel sheet is preferably heated to an annealing temperature and soaked at the annealing temperature under the conditions described below, thereby forming fine austenite.

(59) Average Heating Rate in Temperature Range of 500 C. or Higher and A.sub.1 Transformation Point or Lower: 5 C./s or More

(60) By heating the hot-rolled steel sheet in a temperature range of 500 C. or higher and an A.sub.1 transformation point or lower, which is a recrystallization temperature range of the steel of the present invention, at an average heating rate of 5 C./s or more, recrystallization during the temperature increase by heating is suppressed so as to refine austenite formed at the A.sub.1 transformation point or higher. When the average heating rate is less than 5 C./s, recrystallization of ferrite occurs during the temperature increase by heating, and strain (dislocation) that has been introduced to the steel sheet (hot-rolled steel sheet) is released. Therefore, grain refining of austenite becomes insufficient. Accordingly, the average heating rate in the temperature range of 500 C. or higher and the A.sub.1 transformation point or lower is set to 5 C./s or more.

(61) Annealing Temperature: (A.sub.3 Transformation Point20 C.) or Higher and (A.sub.3 Transformation Point+80 C.) or Lower

(62) When the annealing temperature is lower than (A.sub.3 transformation point20 C.), austenite is not sufficiently formed, and the microstructure of the steel sheet desired in the present invention cannot be obtained. On the other hand, when the annealing temperature exceeds (A.sub.3 transformation point+80 C.), austenite is coarsened, and the microstructure of the steel sheet desired in the present invention cannot be obtained. Accordingly, the annealing temperature is set to (A.sub.3 transformation point20 C.) or higher and (A.sub.3 transformation point+80 C.) or lower.

(63) Holding Time at Annealing Temperature (Soaking Time): 10 Seconds or More

(64) When the holding time (soaking time) at the annealing temperature is less than 10 seconds, austenite is not sufficiently formed, and the microstructure of the steel sheet desired in the present invention cannot be obtained. Accordingly, the holding time (soaking time) at the annealing temperature is set to 10 seconds or more.

(65) In the present invention, after the soaking at the annealing temperature, cooling is conducted under the following conditions, whereby part of fine austenite is subjected to martensite transformation to obtain a microstructure containing fine untransformed austenite and fine martensite.

(66) Average Cooling Rate from 750 C.: 30 C./s or More

(67) When the average cooling rate from 750 C. is less than 30 C./s, a large amount of ferrite is formed during cooling, and the microstructure of the steel sheet desired in the present invention cannot be obtained. Accordingly, the average cooling rate from 750 C. is set to 30 C./s or more. Preferably, the average cooling rate from 750 C. is 50 C./s or more.

(68) Cooling Stop Temperature: 100 C. or Higher and 350 C. or Lower

(69) By cooling the steel sheet to a temperature range of 100 C. or higher and 350 C. or lower at the above average cooling rate, the microstructure containing fine untransformed austenite and fine martensite is obtained. When the cooling stop temperature at the above average cooling rate exceeds 350 C., martensite transformation does not sufficiently occur. On the other hand, when the cooling stop temperature at the above average cooling rate is lower than 100 C., the amount of untransformed austenite significantly decreases. Accordingly, the cooling stop temperature at the above average cooling rate is set to 100 C. or higher and 350 C. or lower. Preferably, the cooling stop temperature is 200 C. or higher and 300 C. or lower.

(70) In the present invention, the resulting steel sheet is subsequently reheated to the following temperature and held at the temperature, then galvanized, and optionally subjected to an alloying treatment. During the soaking, during the galvanizing treatment, and the optional alloying treatment, fine martensite is transformed to tempered martensite, and part of fine untransformed austenite is transformed to bainite or pearlite. Subsequently, when the steel sheet is cooled to room temperature, untransformed austenite remains as austenite or transforms to martensite. In embodiments of the present invention, since the martensite before reheating has a fine microstructure, the tempered martensite obtained by tempering also has a fine microstructure. Furthermore, tempered martensite having an average grain diameter of 5 m or less is obtained.

(71) Reheating Temperature: 300 C. or Higher and 600 C. or Lower

(72) By setting the reheating temperature to 300 C. or higher and 600 C. or lower, and holding the steel sheet at this temperature for 10 seconds or more, fine martensite is tempered to form tempered martensite. Here, since the martensite has a fine microstructure, the tempered martensite obtained by the tempering also has a fine microstructure, and thus tempered martensite having an average grain diameter of 5 m or less is obtained. In untransformed austenite, the concentration of carbon (C) proceeds and the untransformed austenite is stabilized as retained austenite. However, part of the untransformed austenite may transform to martensite. When the reheating temperature is lower than 300 C., tempering of martensite is insufficient, and stability of retained austenite also becomes insufficient. Consequently, a steel sheet (substrate) microstructure having 60% or more of tempered martensite in terms of area ratio and 5% or more of retained austenite in terms of area ratio cannot be obtained. On the other hand, when the reheating temperature exceeds 600 C., untransformed austenite is easily subjected to pearlite transformation, and the microstructure desired in the present invention cannot be obtained. Accordingly, the reheating temperature is set to 300 C. or higher and 600 C. or lower. Preferably, the reheating temperature is 350 C. or higher and 500 C. or lower.

(73) Holding Time at Reheating Temperature: 10 Seconds or More and 600 Seconds or Less

(74) When the holding time at the reheating temperature is less than 10 seconds, tempering of martensite is insufficient, and stability of retained austenite also becomes insufficient. Consequently, a steel sheet (substrate) microstructure having 60% or more of tempered martensite in terms of area ratio and 5% or more of retained austenite in terms of area ratio cannot be obtained. On the other hand, when the holding time at the reheating temperature exceeds 600 seconds, untransformed austenite easily transforms to bainite or pearlite, and the microstructure desired in the present invention cannot be obtained. Accordingly, the holding time at the reheating temperature is set to 10 seconds or more and 600 seconds or less. Preferably, the holding time at the reheating temperature is 20 seconds or more and 300 seconds or less.

(75) The galvanizing treatment is preferably conducted by dipping the steel sheet obtained above in a galvanizing bath at 440 C. or higher and 500 C. or lower, and then galvanizing the steel sheet while controlling the amount of coating by gas wiping or the like. In the case where the galvanized layer is alloyed, an alloying treatment is then preferably performed by holding the steel sheet in a temperature range of 450 C. or higher and 600 C. or lower for 1 second or more and 30 seconds or less. As for the galvanizing bath, in the case where the alloying treatment is not performed, it is preferable to use a galvanizing bath containing Al in an amount of 0.12% or more and 0.22% or less. In contrast, in the case where the alloying treatment is performed, it is preferable to use a galvanizing bath containing Al in an amount of 0.08% or more and 0.18% or less.

(76) A description has been made of a case where the above-described heat treatment is performed on a hot-rolled steel sheet and a galvanizing treatment is then performed. Alternatively, in the present invention, a hot-rolled steel sheet may be pickled and then cold-rolled to form a cold-rolled steel sheet, the above-described heat treatment may be performed on the cold-rolled steel sheet, the galvanizing treatment may then be performed, and the alloying treatment may be optionally performed. In the case where cold rolling is performed, the conditions for the cold rolling are not particularly limited. However, the cold rolling reduction is preferably set to 40% or more. Furthermore, in order to reduce the rolling load during the cold rolling, hot-rolled steel sheet annealing may be performed on the hot-rolled steel sheet after coiling.

(77) In addition, temper rolling may be performed on a steel sheet obtained after the galvanizing treatment and the optional alloying treatment in order to, for example, correct the shape or to adjust the surface roughness of the steel sheet. Furthermore, a paint treatment such as a resin coating or an oil-and-fat coating may also be performed.

EXAMPLES

(78) Steels having the compositions shown in Table 1 were produced in a converter, and continuously cast to obtain steel slabs. These steel slabs were heated to 1,200 C. Subsequently, rough rolling was performed, and finish rolling was performed at the finish rolling temperatures shown in Tables 2 and 3. Subsequently, the resulting steel sheets were cooled to a coiling temperature at an average cooling rate of 30 C./s, coiled at the coiling temperatures shown in Table 2 and 3 to form hot-rolled steel sheets having a thickness of 2.3 mm. Heat treatment was performed on the hot-rolled steel sheets. As for some of the steel slabs, after the steel slabs were formed into hot-rolled steel sheets having a thickness of 3.0 mm, the hot-rolled steel sheets were pickled and then cold-rolled to form cold-rolled steel sheets having a thickness of 1.4 mm. Heat treatment was performed on the cold-rolled steel sheets. The heat treatment conditions are shown in Tables 2 and 3. The heat treatment of all the steel sheets was conducted in a continuous galvanizing line. The steel sheets (substrates) after the heat treatment were dipped in a galvanizing bath at 460 C. containing Al in an amount of 0.15% by mass to form a galvanized layer with a coating weight (per one side) of 35 to 45 g/m.sup.2. Thus, galvanized steel sheets were obtained. Furthermore, for some of the steel sheets, after the galvanized layer was formed, an alloy treatment was conducted at 520 C., and the resulting steel sheets were cooled at a cooling rate of 10 C./s. Thus, galvannealed steel sheets were obtained.

(79) TABLE-US-00001 TABLE 1 A.sub.1 A.sub.3 transformation transformation Chemical composition (mass %) point point Steel C Si Mn P S Al N Ti B Others Ti/N ( C.) ( C.) Remark A 0.13 1.5 2.5 0.022 0.003 0.029 0.003 0.02 0.001 6.67 723 878 Invention Example B 0.41 1.4 1.8 0.019 0.001 0.034 0.002 0.03 0.003 15 724 820 Invention Example C 0.20 1.0 2.0 0.020 0.003 0.400 0.002 0.02 0.002 10 719 851 Invention Example D 0.08 0.5 3.2 0.008 0.005 0.037 0.003 0.03 0.0005 Cr: 0.41 10 703 798 Invention Example E 0.25 1.8 2.1 0.025 0.002 0.026 0.004 0.04 0.001 Mo: 0.20 10 734 874 Invention Example F 0.12 0.5 1.3 0.013 0.002 0.028 0.002 0.03 0.004 V: 0.10 15 720 866 Invention Example G 0.19 1.5 2.2 0.016 0.004 0.032 0.003 0.03 0.003 Ni: 0.51 10 715 856 Invention Example H 0.11 0.7 2.7 0.009 0.002 0.029 0.003 0.02 0.002 Cu: 0.19 6.67 705 829 Invention Example I 0.22 1.0 1.9 0.015 0.005 0.031 0.004 0.02 0.002 Nb: 0.04 5 720 849 Invention Example J 0.10 1.7 2.4 0.011 0.002 0.021 0.002 0.02 0.003 Ca: 0.004 10 729 900 Invention Example K 0.35 1.1 0.9 0.007 0.004 0.030 0.004 0.03 0.002 REM: 0.002 7.5 733 849 Invention Example L 0.02 1.5 2.1 0.020 0.001 0.021 0.004 0.03 0.001 7.5 731 920 Comparative Example M 0.15 1.3 4.2 0.016 0.003 0.044 0.003 0.02 0.003 6.67 697 807 Comparative Example N 0.15 0.9 0.4 0.009 0.002 0.032 0.002 0.01 0.002 5 739 910 Comparative Example O 0.15 1.5 1.4 0.009 0.002 0.032 0.002 0.001 737 908 Comparative Example P 0.12 1.2 1.5 0.010 0.003 0.035 0.003 0.03 10 731 897 Comparative Example Q 0.16 1.0 2.0 0.015 0.003 0.031 0.006 0.01 0.002 1.67 720 860 Comparative Example

(80) TABLE-US-00002 TABLE 2 Finish Hot-rolling Heat treatment conditions rolling coiling Annealing Cooling stop Reheating Coated steel temperature temperature Heating rate temperature Annealing time Cooling rate temperature temperature Reheating time sheet No. Steel ( C.) ( C.) Cold rolling ( C./s) *1 ( C.) (s) *2 ( C./s) *3 ( C.) ( C.) (s) *4 Alloying treatment Remark 1 A 900 500 Performed 5.2 890 60 30 250 450 40 Performed Invention Example 2 900 500 Performed 5.8 890 60 30 200 450 40 Not performed Invention Example 3 900 600 Performed 5.8 890 60 30 250 450 40 Performed Comparative Example 4 900 500 Performed 6.0 750 60 60 250 450 50 Performed Comparative Example 5 900 500 Performed 5.6 980 60 60 250 450 50 Performed Comparative Example 6 900 500 Performed 5.8 900 60 60 80 450 50 Performed Comparative Example 7 B 850 550 Not performed 5.1 870 90 100 220 500 50 Performed Invention Example 8 850 550 Not performed 3.5 870 80 100 200 500 50 Performed Comparative Example 9 850 550 Not performed 6.2 840 5 100 200 500 50 Performed Comparative Example 10 850 650 Not performed 5.5 840 60 120 250 420 50 Performed Comparative Example 11 850 500 Not performed 6.5 860 40 100 50 400 50 Performed Comparative Example 12 C 900 550 Not performed 6.2 860 120 30 270 450 60 Not performed Invention Example 13 900 550 Not performed 6.1 860 60 15 200 450 60 Not performed Comparative Example 14 900 550 Not performed 6.0 860 60 30 80 450 120 Not performed Comparative Example 15 D 900 500 Not performed 5.5 800 150 70 230 350 70 Performed Invention Example 16 900 500 Not performed 5.8 800 60 150 30 350 70 Performed Comparative Example 17 900 500 Not performed 6.0 800 90 100 370 450 70 Performed Comparative Example 18 E 900 500 Performed 10 900 75 80 240 400 30 Performed Invention Example 19 900 600 Performed 10 900 70 80 250 500 10 Performed Comparative Example 20 900 500 Performed 9.5 900 60 80 240 650 50 Performed Comparative Example 21 900 500 Performed 9.8 900 75 80 200 250 50 Performed Comparative Example *1 Average heating rate from 500 C. to A.sub.1 transformation point ( C./s) *2 Holding time at annealing temperature (s) *3 Average cooling rate from 750 C. to cooling stop temperature ( C./s) *4 Holding time at reheating temperature (s)

(81) TABLE-US-00003 TABLE 3 Coated Finish Hot-rolling Heat treatment conditions steel rolling coiling Heating Annealing Annealing sheet temperature temperature rate temperature time No. Steel ( C.) ( C.) Cold rolling ( C./s) *1 ( C.) (s) *2 22 F 900 450 Performed 5.2 890 300 23 900 450 Performed 5.8 890 300 24 900 450 Performed 6.0 890 300 25 G 900 500 Performed 5.8 890 60 26 900 500 Performed 7.2 870 90 27 H 900 500 Performed 6.5 860 40 28 I 900 500 Performed 6.0 890 120 29 J 950 400 Performed 5.5 900 150 30 K 900 450 Performed 5.2 900 50 31 L 940 500 Performed 5.8 940 60 32 M 850 500 Performed 5.2 850 60 33 N 950 500 Performed 5.2 950 75 34 O 920 500 Performed 5.2 920 75 35 P 910 500 Performed 6.5 910 75 36 Q 900 500 Performed 5.2 900 75 Coated Heat treatment conditions steel Cooling Cooling stop Reheating Reheating sheet rate temperature temperature time Alloying No. ( C./s) *3 ( C.) ( C.) (s) *4 treatment Remark 22 50 300 500 40 Not performed Invention Example 23 50 300 500 700 Not performed Comparative Example 24 50 300 500 0 Not performed Comparative Example 25 130 200 450 120 Performed Invention Example 26 80 220 430 10 Performed Invention Example 27 100 200 400 50 Performed Invention Example 28 30 270 440 150 Not performed Invention Example 29 150 200 350 70 Performed Invention Example 30 70 240 400 30 Performed Invention Example 31 30 270 400 40 Performed Comparative Example 32 80 200 400 50 Performed Comparative Example 33 80 300 400 50 Performed Comparative Example 34 40 200 400 50 Performed Comparative Example 35 30 200 400 50 Performed Comparative Example 36 30 200 400 80 Performed Comparative Example *1 Average heating rate from 500 C. to A.sub.1 transformation point ( C./s) *2 Holding time at annealing temperature (s) *3 Average cooling rate from 750 C. to cooling stop temperature ( C./s) *4 Holding time at reheating temperature (s)

(82) Test specimens were prepared from the coated steel sheets (Nos. 1 to 36) obtained above, the area ratios of tempered martensite, retained austenite, ferrite, and martensite, and the average grain diameter of the tempered martensite were determined in accordance with the methods described above.

(83) Note that, in determining the area ratios, image processing was conducted using commercially available image processing software (Image-Pro manufactured by Media Cybernetics, Inc.).

(84) Furthermore, the tensile strength, the total elongation, the hole expansion ratio (stretch flangeability), and crash energy absorption (crashworthiness) were determined in accordance with the test methods described below.

(85) <Tensile Test>

(86) JIS No. 5 tensile test specimens (JIS 22201) were prepared from the coated steel sheets (Nos. 1 to 36) in a direction perpendicular to the rolling direction. A tensile test in accordance with JIS Z 2241 was conducted at a strain rate of 10.sup.3/s to measure the tensile strength (TS) and the total elongation (EL).

(87) <Hole Expansion Test>

(88) Test specimens each having a size of 150 mm150 mm were prepared from the coated steel sheets (Nos. 1 to 36). A hole expansion test was conducted three times for each coated steel sheet in accordance with a hole expansion test method (JFST1001-1996) specified in the standard of the Japan Iron and Steel Federation. An average hole expansion ratio (%) was determined from the results of the test performed three times to evaluate the stretch flangeability.

(89) [Impact Tensile Test]

(90) Test specimens each having a width of a parallel portion of 5 mm and a length of 7 mm were prepared from the coated steel sheets (Nos. 1 to 36) so that a tensile test direction is a direction perpendicular to the rolling direction. A tensile test was conducted at a strain rate of 2,000/s using an impact tensile tester to which a Hopkinson bar method is applied. Absorbed energy (AE) up to an amount of strain of 5% was determined to evaluate crash energy absorption (crashworthiness) (refer to Tetsu to Hagane (Journal of the Iron and Steel Institute of Japan), The Iron and Steel Institute of Japan, vol. 83 (1997), No. 11, p. 748-753). The absorbed energy (AE) was determined by integrating a stress-true strain curve in a range of the amount of strain of 0% to 5%. The evaluation results are shown in Tables 4 and 5.

(91) TABLE-US-00004 TABLE 4 Microstructure *5 Tensile property values Absorbed Coated F M TM TM Tensile Total energy steel area area area Retained average grain strength elongation up to sheet ratio ratio ratio area ratio diameter TS EL TS EL 5% No. (%) (%) (%) (%) (m) Others (MPa) (%) (MPa-%) AE(MJ/m.sup.3) (%) AE/TS Remark 1 0 0 78 12 3.4 B 1333 16 21328 79 56 0.059 Invention Example 2 0 0 85 15 3.5 1378 17 23426 80 63 0.058 Invention Example 3 0 0 80 11 7.5 B 1340 15 20100 56 52 0.042 Comparative Example 4 28 0 55 12 3.6 B 1098 20 21960 50 35 0.046 Comparative Example 5 0 0 80 10 8.5 B 1314 14 18396 51 57 0.039 Comparative Example 6 0 0 97 3 3.1 1312 7 9184 74 72 0.056 Comparative Example 7 0 8 64 14 4.3 B + P 1306 18 23508 68 50 0.052 Invention Example 8 0 9 70 15 7 8 B + P 1319 17 22423 61 50 0.046 Comparative Example 9 15 0 53 3 3.5 B + P 1135 18 20430 58 38 0.051 Comparative Example 10 0 10 50 7 8.8 B 1583 14 22162 65 35 0.041 Comparative Example 11 0 0 95 4 2.2 1426 9 12834 97 70 0.068 Comparative Example 12 0 7 66 13 3.3 B 1391 17 23647 77 55 0.055 Invention Example 13 25 0 53 12 3.1 B 1091 22 24002 52 28 0.048 Comparative Example 14 0 0 96 4 3.3 1322 8 10576 68 68 0.051 Comparative Example 15 0 5 81 7 2.8 B 1202 12 14424 74 56 0.062 Invention Example 16 0 0 98 2 2.8 1169 7 8183 81 65 0.069 Comparative Example 17 0 54 23 5 2.6 B 1407 6 8442 80 34 0.057 Comparative Example 18 0 0 74 13 2.1 B 1460 17 24820 98 58 0.067 Invention Example 19 0 0 75 13 5.6 B 1492 16 23872 69 62 0.046 Comparative Example 20 0 0 75 2 2.3 P 1123 14 15722 65 32 0.058 Comparative Example 21 0 19 80 1 2.1 1520 6 9120 85 40 0.056 Comparative Example *5 F: Ferrite M: Martensite TM: Tempered martensite : Austenite P: Pearlite B: Bainite

(92) TABLE-US-00005 TABLE 5 Microstructure *5 Tensile property values Absorbed F M TM TM Tensile Total energy Coated area area area Retained average grain strength elongation up to steel ratio ratio ratio area ratio diameter TS EL TS + EL 5% sheet No (%) (%) (%) (%) (m) Others (MPa) (%) (MPa-%) AE(MJ/m.sup.3) (%) AE/TS Remark 22 0 4 80 8 1.9 B 1216 14 17024 76 62 0.063 Invention Example 23 0 0 80 1 1.9 B + P 957 13 12441 68 76 0.071 Comparative Example 24 0 10 80 0 1.8 B 1277 6 7662 94 53 0.074 Comparative Example 25 0 0 84 16 2.7 1467 17 24939 93 59 0.063 Invention Example 26 0 9 81 10 2.5 1501 14 21014 98 55 0.065 Invention Example 27 0 0 85 7 2.9 B 1218 13 15834 70 60 0.057 Invention Example 28 4 8 68 14 3.0 B 1387 17 23579 75 50 0.054 Invention Example 29 3 4 86 6 1.8 B 1204 12 14448 84 66 0.070 Invention Example 30 0 5 78 10 2.4 B 1435 14 20090 81 54 0.056 Invention Example 31 64 5 29 2 2.8 644 25 16100 29 62 0.045 Comparative Example 32 0 26 72 2 2.9 1475 8 11800 80 31 0.054 Comparative Example 33 23 0 64 0 3.0 B + P 929 14 13006 45 29 0.048 Comparative Example 34 25 0 66 2 3.0 B 914 15 13710 45 30 0.049 Comparative Example 35 28 0 66 0 2.9 B 908 15 13620 41 31 0.045 Comparative Example 36 26 0 62 1 3.2 B 915 15 13725 44 29 0.048 Comparative Example *5 F: Ferrite M: Martensite TM: Tempered martensite : Austenite P: Pearlite B: Bainite

(93) In Comparative Examples, in any of the tensile strength (TS), the total elongation (EL), the hole expansion ratio (), and the absorbed energy (AE) up to an amount of strain of 5% in the case where the tensile test was conducted at a strain rate of 2,000/s, a satisfactory property cannot be achieved. In contrast, in Examples of the present invention, a high strength, e.g., tensile strength TS: 1,200 MPa or more, and excellent formability including a total elongation EL of 12% or more and a hole expansion ratio of 50% or more are achieved. Furthermore, in addition to a desired strength and formability, all Examples of the present invention preferably have a ratio (AE/TS) of the absorbed energy (AE) up to an amount of strain of 5% in the case where the tensile test was conducted at a strain rate of 2,000/s to the static tensile strength (TS) of 0.050 or more, and thus exhibit excellent crashworthiness.