High strength steel sheet and method for manufacturing the same

Abstract

A method for manufacturing such steel sheet includes continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at 0.01%, the balance being represented by Fe and inevitable impurities, in such a manner that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of soaking when the annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C.

Claims

1. A method for manufacturing high strength steel sheets, comprising continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at 0.01%, the balance being represented by Fe and inevitable impurities, in such a manner that a dew point of an atmosphere in an annealing furnace during the continuous annealing is set to be greater than 45 C., except that the dew point of the atmosphere is controlled to be less than or equal to 45 C. during the course of soaking when the annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to be less than or equal to 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C.

2. The method for manufacturing high strength steel sheets according to claim 1, wherein the chemical composition of the steel sheet further includes one or more elements selected from B at 0.001 to 0.005%, Nb at 0.005 to 0.05%, Ti at 0.005 to 0.05%, Cr at 0.001 to 1.0%, Mo at 0.05 to 1.0%, Cu at 0.05 to 1.0% and Ni at 0.05 to 1.0% in terms of mass %.

3. The method for manufacturing high strength steel sheets according to claim 1, further comprising, after the continuous annealing, electrolytically pickling the steel sheet in an aqueous solution containing sulfuric acid.

4. A high strength steel sheet manufactured by the method described in claim 1 in which a surface portion of the steel sheet extending from the steel sheet surface within a depth of 100 m contains an oxide of one or more selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu and Ni at a rate of not more than 0.060 g/m.sup.2 per single side surface.

5. The method for manufacturing high strength steel sheets according to claim 2, further comprising, after the continuous annealing, electrolytically pickling the steel sheet in an aqueous solution containing sulfuric acid.

6. The method according to claim 1, wherein the steel sheet has a surface portion extending from the steel sheet surface within a depth of 100 m containing an oxide of one or more selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu and Ni at a rate of not more than 0.060 g/m.sup.2 per single side surface is manufactured.

7. The method according to claim 1, wherein the dew point of the atmosphere is allowed to be greater than 45 C. during the course of soaking when the annealing furnace inside temperature is in the range of less than 820 C. or more than 1000 C. as well as during the course of cooling when the annealing furnace inside temperature is less than 750 C.

8. The method according to claim 1, wherein the steel sheet includes Si at 1.3 to 2.0%.

9. The method according to claim 1, wherein the dew point of the atmosphere in the annealing furnace is set in the range of from 40 C. to 10 C., except that the dew point of the atmosphere is controlled to be less than or equal to 45 C. during the course of soaking when an annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to be less than or equal to 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C.

10. The method according to claim 1, wherein the dew point of the atmosphere is controlled to be less than or equal to 45 C. by removing water from said atmosphere in the annealing furnace during the course of soaking when an annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to be less than or equal to 45 C. by removing water from said atmosphere in the annealing furnace during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C.

11. The method according to claim 1, wherein the dew point of the atmosphere in the annealing furnace is set in the range of from 35 C. to 10 C., except that the dew point of the atmosphere is controlled to be less than or equal to 45 C. during the course of soaking when an annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to be less than or equal to 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C.

12. A high strength steel sheet manufactured by the method described in claim 2 in which a surface portion of the steel sheet extending from the steel sheet surface within a depth of 100 m contains an oxide of one or more selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu and Ni at a rate of not more than 0.060 g/m.sup.2 per single side surface.

Description

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(1) The present invention according to exemplary embodiments will be described in detail hereinbelow. In the following description, the unit for the contents of individual elements in the chemical composition of steel is mass % and is indicated simply as % unless otherwise mentioned.

(2) First, there will be described annealing atmosphere conditions that are the most important requirement in the invention and determine the structure of the surface of the steel sheet.

(3) In a high strength steel sheet containing large amounts of Si and Mn, internal oxidation of the surface of the steel sheet can be an origin of corrosion and therefore needs to be prevented as much as possible in order to achieve satisfactory corrosion resistance.

(4) Promoting the internal oxidation of Si and Mn can improve phosphatability. However, it also leads to a decrease in corrosion resistance. Thus, it is necessary that corrosion resistance be improved by suppressing internal oxidation while good phosphatability be ensured by an approach other than promoting the internal oxidation of Si and Mn. As a result of studies, the present invention provides first that in order to ensure phosphatability, oxides of such elements as Si and Mn that have been formed by surface segregation during the course of heating in annealing are reduced during the course of soaking at a relatively high temperature, and the oxygen potential at an early stage of cooling is lowered to prevent the occurrence of oxidation, thereby decreasing the amounts of oxides on the steel sheet surface and improving phosphatability. Further, internal oxidation is substantially suppressed from occurring in the surface portion of the steel sheet with the result that corrosion resistance is improved.

(5) These effects are obtained by performing annealing in a continuous annealing facility in such a manner that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of soaking when the annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C. By controlling the dew point in this manner, surface oxides formed during the course of heating are reduced and the amounts of oxides on the steel sheet surface and decreased. Because the oxygen potential in the annealing atmosphere is low, internal oxides are not substantially formed. As a result, excellent phosphatability to prevent the occurrence of non-covered regions or uneven results of conversion treatment, as well as higher corrosion resistance are obtained.

(6) The annealing furnace inside temperature of interest during the course of soaking is limited to be in the range of not less than 820 C. and not more than 1000 C. for the following reasons. If the temperature is less than 820 C., surface oxides of elements such as Si and Mn cannot be reduced sufficiently even if the reducing ability is increased by lowering the dew point to not more than 45 C. Further, the temperature is limited to be not more than 1000 C. because any temperature higher than 1000 C. is disadvantageous because the equipment (such as rolls) in the annealing furnace is degraded and costs are increased.

(7) The range of the annealing furnace inside temperature in which the dew point is controlled during the course of cooling is limited to be not less than 750 C. for the following reasons. When the temperature is in the range of not less than 750 C., components in the steel start to undergo surface segregation. If the dew point of the atmosphere is not controlled to become not more than 45 C. when the temperature is in this range, steel components are allowed to undergo surface segregation. Such surface segregation can be suppressed by controlling the dew point of the atmosphere to become not more than 45 C. If the temperature is less than 750 C., surface oxides cannot be reduced at such a low temperature even if the dew point of the atmosphere is lowered. Thus, the range of the annealing furnace inside temperature (in which the dew point is to be controlled) during the course of cooling is limited to be not less than 750 C.

(8) Next, the chemical composition of the high strength steel sheets of interest according to embodiments of the present invention will be described.

(9) C: 0.01 to 0.18%

(10) Carbon increases workability by forming phases such as martensite in the steel microstructure. In order to obtain this effect, carbon needs to be added at not less than 0.01%. On the other hand, adding carbon in excess of 0.18% causes a decrease in ductility as well as deteriorations in quality and weldability. Thus, the C content is limited to be not less than 0.01% and not more than 0.18%.

(11) Si: 0.4 to 2.0%

(12) Silicon increases the strength and the ductility of steel and is therefore an effective element for achieving a good quality. In order to obtain the objective strength in the present invention, silicon is advantageously added at not less than 0.4%. Steel sheets having a Si content of less than 0.4% cannot achieve a preferred strength of interest in the invention and are substantially free of problems in terms of phosphatability. On the other hand, adding silicon in excess of 2.0% results in the saturation of steel strengthening effects as well as the saturation of ductility enhancement. Further, achieving an improvement of phosphatability becomes difficult. Thus, the Si content is limited to be not less than 0.4% and not more than 2.0%.

(13) Mn: 1.0 to 3.0%

(14) Manganese is an effective element for increasing the strength of steel. In order to ensure mechanical characteristics and strength, the Mn content needs to be not less than 1.0%. On the other hand, adding manganese in excess of 3.0% causes difficulties in ensuring weldability as well as in ensuring the balance between strength and elongation. Thus, the Mn content is limited to be not less than 1.0% and not more than 3.0%.

(15) Al: 0.001 to 1.0%

(16) Aluminum is added for the purpose of deoxidizing molten steel. This purpose is not fulfilled if the Al content is less than 0.001%. The deoxidizing effect for molten steel is obtained by adding aluminum at not less than 0.001%. On the other hand, adding aluminum in excess of 1.0% increases costs and results in an increase in the amount of surface segregation of aluminum, thereby making it difficult to improve phosphatability. Thus, the Al content is limited to be not less than 0.001% and not more than 1.0%.

(17) P: 0.005 to not more than 0.060%

(18) Phosphorus is one of elements that are inevitably present in steel. An increase in cost is expected if the P content is reduced to below 0.005%. Thus, the P content is specified to be not less than 0.005%. On the other hand, any P content exceeding 0.060% leads to a decrease in weldability and causes a marked deterioration in phosphatability to such an extent that it becomes difficult to improve phosphatability even by the present invention. Thus, the P content is limited to be not less than 0.005% and not more than 0.060%.

(19) S: 0.01%

(20) Sulfur is one of inevitable elements. The lower limit is not particularly limited. However, the presence of this element in a large amount causes decreases in weldability and corrosion resistance. Thus, the S content is limited to be not more than 0.01%.

(21) In order to control the balance between strength and elongation, one or more elements selected from 0.001 to 0.005% of B, 0.005 to 0.05% of Nb, 0.005 to 0.05% of Ti, 0.001 to 1.0% of Cr, 0.05 to 1.0% of Mo, 0.05 to 1.0% of Cu and 0.05 to 1.0% of Ni may be added as required.

(22) The appropriate amounts of these optional elements are limited for the following reasons.

(23) B: 0.001 to 0.005%

(24) The effect in promoting hardening is hardly obtained if the B content is less than 0.001%. On the other hand, adding boron in excess of 0.005% results in a decrease in phosphatability. Thus, when boron is added, the B content is limited to be not less than 0.001% and not more than 0.005%. However, it is needless to mention that boron may not be added when the addition of this element is considered to be unnecessary in view of an improvement in mechanical characteristics.

(25) Nb: 0.005 to 0.05%

(26) The effect in adjusting strength is hardly obtained if the Nb content is less than 0.005%. On the other hand, adding niobium in excess of 0.05% results in an increase in cost. Thus, when niobium is added, the Nb content is limited to be not less than 0.005% and not more than 0.05%.

(27) Ti: 0.005 to 0.05%

(28) The effect in adjusting strength is hardly obtained if the Ti content is less than 0.005%. On the other hand, adding titanium in excess of 0.05% results in a decrease in phosphatability. Thus, when titanium is added, the Ti content is limited to be not less than 0.005% and not more than 0.05%.

(29) Cr: 0.001 to 1.0%

(30) The effect in promoting hardening is hardly obtained if the Cr content is less than 0.001%. On the other hand, adding chromium in excess of 1.0% results in the surface segregation of chromium and a consequent decrease in weldability. Thus, when chromium is added, the Cr content is limited to be not less than 0.001% and not more than 1.0%.

(31) Mo: 0.05 to 1.0%

(32) The effect in adjusting strength is hardly obtained if the Mo content is less than 0.05%. On the other hand, adding molybdenum in excess of 1.0% results in an increase in cost. Thus, when molybdenum is added, the Mo content is limited to be not less than 0.05% and not more than 1.0%.

(33) Cu: 0.05 to 1.0%

(34) The effect in promoting the formation of a retained -phase is hardly obtained if the Cu content is less than 0.05%. On the other hand, adding copper in excess of 1.0% results in an increase in cost. Thus, when copper is added, the Cu content is limited to be not less than 0.05% and not more than 1.0%.

(35) Ni: 0.05 to 1.0%

(36) The effect in promoting the formation of a retained -phase is hardly obtained if the Ni content is less than 0.05%. On the other hand, adding nickel in excess of 1.0% results in an increase in cost. Thus, when nickel is added, the Ni content is limited to be not less than 0.05% and not more than 1.0%.

(37) The balance after the deduction of the aforementioned elements is represented by Fe and inevitable impurities.

(38) Next, there will be described a method for manufacturing the high strength steel sheets according to embodiments of the invention as well as the reasons why the conditions in the method are limited.

(39) In an embodiment, a steel having the above-described chemical composition is hot rolled and is thereafter cold rolled to give a steel sheet, and subsequently the steel sheet is annealed in a continuous annealing facility. In the present invention, the annealing is carried out in such a manner that the dew point of the atmosphere is preferably controlled to become not more than 45 C. during the course of soaking when the annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C. In the above processing of steel, it is possible to anneal the hot rolled steel sheet without subjecting it to cold rolling.

(40) Hot Rolling

(41) Hot rolling may be performed under usual conditions.

(42) Pickling

(43) It is preferable to perform a pickling treatment after hot rolling. In the pickling step, black scales formed on the surface are removed and the steel sheet is subjected to cold rolling. Pickling conditions are not particularly limited.

(44) Cold Rolling

(45) Cold rolling is preferably carried out with a draft of not less than 40% and not more than 80%. If the draft is less than 40%, the recrystallization temperature becomes lower and the steel sheet tends to be deteriorated in mechanical characteristics. On the other hand, because the steel sheet of the invention is a high strength steel sheet, cold rolling the steel sheet with a draft exceeding 80% increases not only the rolling costs but also the amount of surface segregation during annealing, possibly resulting in a decrease in phosphatability.

(46) The steel sheet that has been cold rolled or hot rolled is annealed and then subjected to a conversion treatment.

(47) In an annealing furnace, the steel sheet undergoes a heating step in which the steel sheet is heated to a predetermined temperature in an upstream heating zone, a soaking step in which the steel sheet is held in a downstream soaking zone at a predetermined temperature for a prescribed time, and a cooling step.

(48) As described above, the annealing is performed in such a manner that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of soaking when the annealing furnace inside temperature is in the range of not less than 820 C. and not more than 1000 C. as well as that the dew point of the atmosphere is controlled to become not more than 45 C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750 C. The thus-annealed steel sheet is thereafter subjected to a conversion treatment. Because the dew point of the atmosphere is usually higher than 40 C., the dew point is controlled to become not more than 45 C. by absorbing and removing water in the furnace with a water absorber.

(49) If the volume fraction of hydrogen gas in the atmosphere is less than 1 vol %, the activation effect by reduction cannot be obtained and phosphatability is deteriorated. Although the upper limit is not particularly limited, costs are increased and the effect is saturated if the fraction exceeds 50 vol %. Thus, the volume fraction of hydrogen gas is preferably not less than 1 vol % and not more than 50 vol %. The gas components in the annealing furnace except hydrogen gas are nitrogen gas and inevitable impurity gases. Other gas components may be present as long as they are not detrimental in achieving the advantageous effects of the invention.

(50) After the steel sheet is cooled from the temperature range of not less than 820 C. and not more than 1000 C., hardening and tempering may be performed as required. Although the conditions for these treatments are not particularly limited, it is desirable that tempering be performed at a temperature of 150 to 400 C. The reasons are because ductility tends to be lowered if the temperature is less than 150 C. as well as because hardness tends to be decreased if the temperature is in excess of 400 C.

(51) According to the present invention, good phosphatability can be ensured even without performing electrolytic pickling. However, it is preferable that electrolytic pickling be performed in order to remove trace amounts of oxides that have been inevitably generated by surface segregation during annealing and thereby to ensure better phosphatability.

(52) The electrolytic pickling conditions are not particularly limited. However, in order to efficiently remove the inevitably formed surface oxides of silicon and manganese after the annealing, alternating electrolysis at a current density of not less than 1 A/dm.sup.2 is desirable. The reasons why alternating electrolysis is selected are because the pickling effects are low if the steel sheet is fixed to a cathode as well as because if the steel sheet is fixed to an anode, iron that is dissolved during electrolysis is accumulated in the pickling solution and the Fe concentration in the pickling solution is increased with the result that the attachment of iron to the surface of the steel sheet causes problems such as dry contamination.

(53) The pickling solution used in the electrolytic pickling is not particularly limited. However, nitric acid or hydrofluoric acid is not preferable because they are highly corrosive to a facility and require careful handling. Hydrochloric acid is not preferable because chlorine gas can be generated from the cathode. In view of corrosiveness and environment, the use of sulfuric acid is preferable. The sulfuric acid concentration is preferably not less than 5 mass % and not more than 20 mass %. If the sulfuric acid concentration is less than 5 mass %, the conductivity is so lowered that the bath voltage is raised during electrolysis possibly to increase the power load. On the other hand, any sulfuric acid concentration exceeding 20 mass % leads to a cost problem because a large loss is caused due to drag-out.

(54) The temperature of the electrolytic solution is preferably not less than 40 C. and not more than 70 C. Because the bath temperature is raised by the generation of heat by continuous electrolysis, the pickling effect may be low if the temperature is less than 40 C. Further, maintaining the temperature below 40 C. is sometimes difficult. Furthermore, a temperature exceeding 70 C. is not preferable in view of the durability of the lining of the electrolytic cell.

(55) The high strength steel sheets of embodiments of the present invention are obtained in the above manner.

(56) As a result, the inventive steel sheet according to exemplary embodiments of the invention has a characteristic structure of the surface described below.

(57) An oxide of one or more selected from Fe, Si, Mn, Al and P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni has been suppressed from being formed in a surface portion of the steel sheet extending from the steel sheet surface within a depth of 100 m, and the total amount of such oxides formed is limited to not more than 0.060 g/m.sup.2 per single side surface.

(58) In a high strength steel sheet containing Si and a large amount of Mn, internal oxidation of the surface of the steel sheet can be an origin of corrosion and therefore needs to be prevented as much as possible in order to achieve satisfactory corrosion resistance.

(59) Thus, the present invention first provides that in order to ensure phosphatability, the oxygen potential in the annealing step is lowered and thereby the activities of easily oxidized elements such as Si and Mn in the surface portion of base iron are lowered. In this manner, the external oxidation of these elements is suppressed and consequently phosphatability is improved. Further, internal oxidation is also suppressed from occurring in the surface portion of the steel sheet with the result that corrosion resistance is improved. These effects become apparent by preventing the surface portion of the steel sheet which extends from the surface of the base steel sheet within a depth of 100 m from the formation of an oxide of one or more selected from Fe, Si, Mn, Al and P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni such that the total amount of such oxides formed is not more than 0.060 g/m.sup.2. If the total amount of formed oxides (hereinafter, referred to as internal oxidation amount) is in excess of 0.060 g/m.sup.2, corrosion resistance is deteriorated. The effect in the improvement of corrosion resistance is saturated even when the internal oxidation amount is reduced to less than 0.0001 g/m.sup.2. Thus, the lower limit of the internal oxidation amount is preferably 0.0001 g/m.sup.2.

EXAMPLE 1

(60) Hereinbelow, embodiments of the present invention will be described in detail based on EXAMPLES.

(61) Hot rolled steel sheets with a steel composition described in Table 1 were pickled to remove black scales and were thereafter cold rolled to give cold rolled steel sheets with a thickness of 1.0 mm. Cold rolling was omitted for some of the steel sheets. That is, as-descaled hot rolled steel sheets (thickness: 2.0 mm) were also provided.

(62) TABLE-US-00001 TABLE 1 (mass %) Steel code C Si Mn Al P S Cr Mo B Nb Cu Ni Ti A 0.04 0.1 1.9 0.04 0.01 0.003 B 0.03 0.4 2.0 0.04 0.01 0.003 C 0.09 0.9 2.1 0.03 0.01 0.004 D 0.13 1.3 2.0 0.03 0.01 0.003 E 0.09 1.7 1.9 0.03 0.01 0.003 F 0.08 2.0 2.1 0.03 0.01 0.003 G 0.11 1.3 2.8 0.04 0.01 0.003 H 0.12 1.3 2.0 0.95 0.01 0.003 I 0.12 1.3 2.0 0.04 0.06 0.004 J 0.12 1.3 2.1 0.03 0.01 0.008 K 0.12 1.3 1.9 0.02 0.01 0.003 0.7 L 0.12 1.3 2.0 0.04 0.01 0.003 0.12 M 0.12 1.3 2.1 0.03 0.01 0.003 0.005 N 0.12 1.3 2.0 0.05 0.01 0.003 0.001 0.04 O 0.12 1.3 1.9 0.03 0.01 0.004 0.11 0.2 0.3 P 0.12 1.3 1.9 0.04 0.01 0.003 0.003 0.03 Q 0.12 1.3 2.0 0.03 0.01 0.004 0.05 R 0.20 1.3 2.1 0.04 0.01 0.003 S 0.12 2.1 1.9 0.04 0.01 0.003 T 0.12 1.3 3.1 0.04 0.01 0.004 U 0.12 1.3 2.0 1.10 0.01 0.004 V 0.12 1.3 1.9 0.03 0.07 0.003 W 0.12 1.3 2.1 0.04 0.01 0.015 Underlines indicate outside the inventive range.

(63) Next, the cold rolled steel sheets and the hot rolled steel sheets were introduced into a continuous annealing facility. The steel sheet was passed through the annealing facility while the dew point was controlled as described in Table 2 when the annealing furnace inside temperature was in the range of not less than 820 C. and not more than 1000 C. during the course of soaking as well as when the annealing furnace inside temperature was in the range of not less than 750 C. during the course of cooling. The annealed steel sheet was thereafter subjected to water hardening and then to tempering at 300 C. for 140 seconds. Subsequently, electrolytic pickling was performed by alternating electrolysis in a 5 mass aqueous sulfuric acid solution at 40 C. under current density conditions described in Table 2 while switching the polarity of the sample sheet between anodic and cathodic alternately each after 3 seconds. Thus, sample sheets were prepared. The dew point in the annealing furnace was basically set at 35 C. except when the dew point was controlled in accordance with the furnace temperature. The gas components in the atmosphere included nitrogen gas, hydrogen gas and inevitable impurity gases. The dew point was controlled by removing water in the atmosphere by absorption. The hydrogen concentration in the atmosphere was basically set at 10 vol %.

(64) With respect to the obtained sample sheets, TS and El were measured in accordance with a tensile testing method for metallic materials described in JIS Z 2241. Further, the sample sheets were tested to examine phosphatability and corrosion resistance, as well as the amount of oxides present in a surface portion of the steel sheet extending immediately from the surface of the steel sheet to a depth of 100 m (the internal oxidation amount). The measurement methods and the evaluation criteria are described below.

(65) Phosphatability

(66) Phosphatability was evaluated by the following method.

(67) A conversion treatment liquid (PALBOND L3080 (registered trademark)) manufactured by Nihon Parkerizing Co., Ltd. was used. A conversion treatment was carried out in the following manner.

(68) The sample sheet was degreased with degreasing liquid FINE CLEANER (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., and was thereafter washed with water. Subsequently, the surface of the sample sheet was conditioned for 30 seconds with surface conditioning liquid PREPAREN Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. The sample sheet was then soaked in the conversion treatment liquid (PALBOND L3080) at 43 C. for 120 seconds, washed with water and dried with hot air.

(69) The sample sheet after the conversion treatment was observed with a scanning electron microscope (SEM) at 500 magnification with respect to randomly selected five fields of view. The area ratio of the regions that had not been covered with the chemical conversion coating was measured by image processing. Phosphatability was evaluated based on the area ratio of the non-covered regions according to the following criteria. The symbol indicates an acceptable level.

(70) : not more than 10%

(71) x: more than 10%

(72) Corrosion Resistance after Electro-Coating

(73) A 70 mm150 mm test piece was cut out from the sample sheet that had been subjected to the above conversion treatment. The test piece was cationically electro-coated with PN-150G (registered trademark) manufactured by NIPPON PAINT Co., Ltd. (baking conditions: 170 C.20 min, film thickness: 25 m). Thereafter, the edges and the non-test surface were sealed with an Al tape, and the test surface was cut deep into the base steel with a cutter knife to create a cross cut pattern (cross angle: 60), thereby preparing a sample.

(74) Next, the sample was soaked in a 5% aqueous NaCl solution (55 C.) for 240 hours, removed from the solution, washed with water and dried. Thereafter, an adhesive tape was applied to the cross cut pattern and was peeled therefrom. The exfoliation width was measured and was evaluated based on the following criteria. The symbol indicates an acceptable level.

(75) : The exfoliation width on one surface was less than 2.5 mm.

(76) x: The exfoliation width on one surface was 2.5 mm or more.

(77) Workability

(78) To evaluate workability, a JIS No. 5 tensile test piece was sampled from the sample sheet in a direction that was 90 relative to the rolling direction. The test piece was subjected to a tensile test at a constant cross head speed of 10 mm/min in accordance with JIS Z 2241, thereby determining the tensile strength (TS/MPa) and the ductility (El %). For steel sheets with TS of less than 650 MPa, workability was evaluated to be good when TSEl22,000 and to be bad when TSEl<22,000. For steel sheets with TS of 650 MPa to 900 MPa, workability was evaluated to be good when TSEl20,000 and to be bad when TSEl<20,000. For steel sheets with TS of not less than 900 MPa, workability was evaluated to be good when TSEl18,000 and to be bad when TSEl<18,000.

(79) The internal oxidation amount, namely, the amount of internal oxidation from the steel sheet surface to a depth of 100 m was measured by an impulse furnace fusion-infrared absorption method. It should be noted that the amount of oxygen present in the starting material (namely, the high strength steel sheet before annealing) should be subtracted. Thus, in one embodiment of the invention, surface portions on both sides of the continuously annealed high strength steel sheet were polished by at least 100 m and thereafter the oxygen concentration in the steel was measured. The measured value was obtained as the oxygen amount OH of the starting material. Further, the oxygen concentration was measured across the entirety of the continuously annealed high strength steel sheet in the sheet thickness direction. The measured value was obtained as the oxygen amount OI after internal oxidation. The difference between OI and OH (=OIOH) was calculated wherein OI was the oxygen amount in the high strength steel sheet after internal oxidation and OH was the oxygen amount in the starting material. The difference was then converted to an amount per unit area (namely, 1 m.sup.2) on one surface, thereby determining the internal oxidation amount (g/m.sup.2).

(80) The results and the manufacturing conditions are described in Table 2.

(81) TABLE-US-00002 TABLE 2 Annealing furnace Dew- point Dew- ( C.) at Dew- point 750 C. Steel point ( C.) at and Internal Si Mn ( C.) at 820 C. above in Soaking oxidation Current Steel (mass (mass below and cooling temp. amount Electrolytic density No. code %) %) 820 C. above zone ( C.) (g/m.sup.2) pickling A/dm.sup.2 1 D 1.3 2.0 35 25 50 850 0.235 Not performed 2 D 1.3 2.0 35 35 50 850 0.136 Not performed 3 D 1.3 2.0 35 40 50 850 0.076 Not performed 4 D 1.3 2.0 35 45 50 850 0.055 Not performed 5 D 1.3 2.0 35 50 50 850 0.014 Not performed 6 D 1.3 2.0 35 60 50 850 0.009 Not performed 7 D 1.3 2.0 35 50 25 850 0.193 Not performed 8 D 1.3 2.0 35 50 35 850 0.129 Not performed 9 D 1.3 2.0 35 50 40 850 0.069 Not performed 10 D 1.3 2.0 35 50 45 850 0.054 Not performed 11 D 1.3 2.0 35 50 60 850 0.011 Not performed 12 D 1.3 2.0 35 50 50 750 0.005 Not performed 13 D 1.3 2.0 35 50 50 800 0.008 Not performed 14 D 1.3 2.0 35 50 50 820 0.012 Not performed 15 D 1.3 2.0 35 50 50 900 0.026 Not performed 16 D 1.3 2.0 35 50 50 950 0.033 Not performed 17 D 1.3 2.0 35 50 50 850 0.013 Performed 1 18 D 1.3 2.0 35 50 50 850 0.012 Performed 5 19 D 1.3 2.0 35 50 50 850 0.012 Performed 10 20 A 0.1 1.9 35 50 50 850 0.003 Not performed 21 B 0.4 2.0 35 50 50 850 0.006 Not performed 22 C 0.9 2.1 35 50 50 850 0.008 Not performed 23 E 1.7 1.9 35 50 50 850 0.025 Not performed 24 F 2.0 2.1 35 50 50 850 0.037 Not performed 25 G 1.3 2.8 35 50 50 850 0.016 Not performed 26 H 1.3 2.0 35 50 50 850 0.044 Not performed 27 I 1.3 2.0 35 50 50 850 0.018 Not performed 28 J 1.3 2.1 35 50 50 850 0.012 Not performed 29 K 1.3 1.9 35 50 50 850 0.011 Not performed 30 L 1.3 2.0 35 50 50 850 0.012 Not performed 31 M 1.3 2.1 35 50 50 850 0.013 Not performed 32 N 1.3 2.0 35 50 50 850 0.012 Not performed 33 O 1.3 1.9 35 50 50 850 0.012 Not performed 34 P 1.3 1.9 35 50 50 850 0.011 Not performed 35 Q 1.3 2.0 35 50 50 850 0.013 Not performed 36 R 1.3 2.1 35 50 50 850 0.012 Not performed 37 S 2.1 1.9 35 50 50 850 0.046 Not performed 38 T 1.3 3.1 35 50 50 850 0.013 Not performed 39 U 1.3 2.0 35 50 50 850 0.041 Not performed 40 V 1.3 1.9 35 50 50 850 0.022 Not performed 41 W 1.3 2.1 35 50 50 850 0.011 Not performed Corrosion resistance after electro- TS El No. Phosphatability coating mpa % TS El Workability Remarks 1 x x 1077 19.9 21432 Good COMP. EX. 2 x x 1033 18.2 18801 Good COMP. EX. 3 x 1041 19.1 19883 Good COMP. EX. 4 1025 19.3 19783 Good INV. EX. 5 1038 19.6 20345 Good INV. EX. 6 1031 19.2 19795 Good INV. EX. 7 x x 1011 19.6 19816 Good COMP. EX. 8 x x 1013 19.5 19754 Good COMP. EX. 9 x 1028 18.2 18710 Good COMP. EX. 10 1064 19.5 20748 Good INV. EX. 11 1066 19.6 20894 Good INV. EX. 12 x x 854 24.1 20581 Good COMP. EX. 13 x 974 22.5 21915 Good COMP. EX. 14 999 21.4 21379 Good INV. EX. 15 1166 19.4 22620 Good INV. EX. 16 1195 19.5 23303 Good INV. EX. 17 1040 20.1 20904 Good INV. EX. 18 1035 20.6 21321 Good INV. EX. 19 1041 20.5 21341 Good INV. EX. 20 680 28.6 19448 Bad COMP. EX. 21 1001 20.4 20420 Good INV. EX. 22 1022 21.4 21871 Good INV. EX. 23 1033 21.9 22623 Good NV. EX. 24 1126 18.4 20718 Good INV. EX. 25 1064 19.6 20854 Good INV. EX. 26 1055 19.5 20573 Good INV. EX. 27 1136 18.4 20902 Good INV. EX. 28 1032 19.5 20124 Good INV. EX. 29 1051 19.3 20284 Good INV. EX. 30 1055 19.1 20151 Good INV. EX. 31 1025 20.1 20603 Good INV. EX. 32 1065 19.4 20661 Good INV. EX. 33 1074 19.8 21265 Good INV. EX. 34 812 25.6 20787 Good INV. EX. 35 1045 19.5 20378 Good INV. EX. 36 1256 14.2 17835 Bad COMP. EX. 37 x 1195 16.3 19479 Good COMP. EX. 38 1114 15.2 16933 Bad COMP. EX. 39 x x 1065 19.3 20555 Good COMP. EX. 40 x 1122 17.5 19635 Good COMP. EX. 41 x 1074 19.2 20621 Good COMP. EX.

(82) From Table 2, the high strength steel sheets manufactured by the inventive method were shown to be excellent in phosphatability, corrosion resistance after electro-coating and workability in spite of the fact that these high strength steel sheets contained large amounts of easily oxidized elements such as Si and Mn. On the other hand, the steel sheets obtained in COMPARATIVE EXAMPLES were poor in at least one of phosphatability, corrosion resistance after electro-coating and workability.

INDUSTRIAL APPLICABILITY

(83) The high strength steel sheets according to the present invention are excellent in phosphatability, corrosion resistance and workability, and can be used as surface-treated steel sheets for reducing the weight and increasing the strength of bodies of automobiles. Besides automobiles, the inventive high strength steel sheets can be used as surface-treated steel sheets having a corrosion resistant film on the base steel sheet in a wide range of applications including home appliances and building materials.