COLD ROLLED FLAT STEEL PRODUCT FOR PACKAGING AND METHOD FOR PRODUCING A STEEL FLAT PRODUCT

Abstract

A cold rolled steel flat product for packaging made of a low carbon steel having a thickness of less than 0.49 mm and a method of making. The steel flat product has a martensite-free microstructure and represents a standard grade for packaging with tensile strengths from 300 to 550 MPa, which can be produced from a cold-rolled steel sheet with a carbon content from 0.01% to 0.1% by weight by inductive annealing of the steel sheet and subsequent water cooling for quenching the recrystallization-annealed steel sheet. To achieve flatness of 5 I-units or less, the induction annealed steel sheet is first primarily cooled in the manufacturing process to a take-off temperature at a rate of less than 1000 K/s, with the take-off temperature being below the transformation temperature of 723° C., and thereafter a secondary cooling by water cooling with a water temperature of less than 80° C. at a rate of more than 1000 K/s.

Claims

1. A cold rolled flat steel product for packaging with a thickness of less than 0.49 mm and the following composition in terms of weight: C: 0.01-0.1%, Si: <0.03%, Mn: <0.6%, P: <0.1%, S: <0.03%, Al: <0.1%, N: <0.02%, optional Cr: <0.1%, optionally Ni: <0.1%, optional Cu: <0.1%, optional Ti: <0.01%, optional B: <0.005%, optional Nb: <0.01%, optional Mo: <0.02%, optional Sn: <0.03%, residual iron and unavoidable impurities, wherein the flat steel product has a martensite-free microstructure and a flatness of 5 I-units or less.

2. The flat steel product according to claim 1, wherein in an aged state the flat steel product has a yield strength of from 300 MPa to 700 MPa, wherein the aged state of the flat steel product being obtained either naturally by storage at room temperature and/or by painting with subsequent drying or artificially by heating the flat steel product for 20 minutes to temperatures in the range from 200° C. to 210° C.

3. The flat steel product according to claim 1, wherein in an aged state the flat steel product has an elongation at break of 10% or more, wherein the aged state of the flat steel product being obtained either naturally by storage at room temperature and/or by painting with subsequent drying or artificially by heating the flat steel product for 20 minutes to temperatures in the range from 200° C. to 210° C.

4. The flat steel product according to claim 1, wherein the steel microstructure of the flat steel product contains neither martensite nor bainite and no austenite.

5. The flat steel product according to claim 4, wherein the steel microstructure of the flat steel product contains at least substantially only ferrite or ferrite and one or more of the microstructural constituents cementite and pearlite.

6. The flat steel product according to claim 1, wherein the flat steel product has been skin-passed with a skin-passing degree of not more than 5% and has a yield strength of 300 MPa to 500 MPa or wherein the flat steel product has been rerolled with a re-rolling degree of more than 5% and up to 45% and has a yield strength of 430 MPa to 700 MPa.

7. The flat steel product according to claim 1, wherein the flat steel product has an edge corrugation with a maximum corrugation height of 1.5 mm and/or over a length of the flat steel product in the rolling direction of 1 meter has fewer than six corrugations with a corrugation height of more than 1.0 mm.

8. A method of producing the flat steel product according to claim 1, wherein the flat steel product is obtained from a steel having the composition of claim 1 and the method comprises: hot rolling of a slab produced from the steel to obtain a hot-rolled strip, cold rolling of the hot-rolled strip at a reduction ratio of at least 80% to obtain a cold-rolled steel strip with a thickness of less than 0.49 mm, annealing of the cold-rolled steel strip in a continuous annealing furnace, wherein for obtaining a recrystallization-annealed steel strip the cold-rolled steel strip is inductively heated in the continuous annealing furnace to an annealing temperature above the recrystallization temperature and held at the annealing temperature for a predetermined holding time, primary cooling of the recrystallization-annealed steel strip to a take-off temperature which is below the transformation temperature of 723° C. with a maximum primary cooling rate of less than 1000 K/s, secondary cooling of the steel strip to a temperature of less than 80° C. with a secondary cooling rate of more than 1000 K/s.

9. The method according to claim 8, wherein the primary cooling is effected by gas cooling and the secondary cooling is effected by water cooling.

10. The method according to claim 8, wherein the steel strip is skin-passed after cooling with a skin-passing degree between 0.2% to 5%, wherein the yield strength of the skin-passed steel strip is between 300 and 500 MPa.

11. The method according to claim 8, wherein the steel strip is skin-passed after cooling with a skin-passing degree of 0.2% to 5% and after skin-passing has a yield strength of 300 MPa to 500 MPa or wherein the steel strip is cold re-rolled after cooling with a re-rolling degree of more than 5% and up to 45% and after re-rolling has a yield strength of between 430 MPa and 700 MPa.

12. The method according to claim 8, wherein the cold-rolled steel strip is inductively heated to the annealing temperature during annealing in the continuous annealing furnace at a heating rate of more than 100 K/s.

13. The method according to claim 8, wherein the cold-rolled steel strip is maintained at the annealing temperature during annealing in the continuous annealing furnace for a holding time of less than 1 second.

14. The method according to claim 8, wherein the annealing temperature is between 725° C. and 769° C.

15. The method according to claim 8, wherein the cold-rolled steel strip is cooled from the annealing temperature to the take-off temperature (Tq) at a primary cooling rate of at least 25 K/s during primary cooling.

16. The method according to claim 8, wherein the surface of the steel flat product immediately after secondary cooling has a surface oxidation with a maximum oxide coverage of 300 C/m.sup.2.

17. The method according to claim 8, wherein the take-off temperature is at a maximum of 600° C.

18. The method according to claim 8, wherein after secondary cooling a tin and/or a chromium/chromium oxide coating and/or an organic coating is applied on at least one surface of the flat steel product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] These and other features as well as advantages of the flat steel products according to the invention and of the method for their production are disclosed in the embodiments and examples described below with reference to the drawings.

[0037] The drawings show:

[0038] FIG. 1: Typical curve of a stress-strain diagram obtained in a tensile test on an aged flat steel product according to the invention, where is the strain and σ is the stress, and ReL represents the lower yield strength, ReH the upper yield strength and Rm the tensile strength;

[0039] FIGS. 2a and 2b: Graphical representation of the strength and elongation at break of flat steel products as a function of the take-off temperature (Tq) of the method according to the invention, where FIG. 2a shows the influence of the take-off temperature (Tq) on the strength (Rp2, ReL, ReH and Rm) and FIG. 2b shows the influence of the take-off temperature (Tq) on the elongation at break A;

[0040] FIGS. 3A and 3B: Graphical representation of the tensile strength and yield strength of flat steel products as a function of the degree of re-rolling during secondary cold rolling and skin pass rolling (skin passing), respectively, where FIG. 3a shows the influence of the degree of re-rolling (NWG) on the tensile strength (Rm) and FIG. 3b shows the influence of the degree of re-rolling (NWG) on the upper yield strength (ReH);

[0041] FIG. 4: Graphical representation of a typical annealing cycle performed in the method according to the invention as a temperature-time diagram.

DETAILED DESCRIPTION

[0042] For the production of flat steel products according to the invention, a slab is cast from a steel melt with a low carbon content (low carbon steel, LC) and is hot-rolled to obtain a hot strip. The components of the steel from which flat steel products according to the invention can be made are explained in detail below:

Composition of the Steel:

[0043] Carbon, C: not less than 0.01% and not more than 0.1%, preferably less than 0.085%;
Carbon increases hardness and strength. Since the aim of the invention is to produce a cold-rolled flat steel product for packaging with a medium strength, e.g. with a yield strength (ReL) in the range from 300 MPa to 700 MPa, the steel contains at least 0.01 wt. % carbon in order to achieve the desired strength even at low re-rolling or skin-passing grades of less than 5%. In order to ensure the rollability of the flat steel product during primary cold rolling and, if necessary, in a second cold rolling step (re-rolling or skin-passing) without reducing the elongation at break, the carbon content should not be too high. Furthermore, as the carbon content increases, a pronounced anisotropy in the form of latency occurs during the production and processing of the flat steel product, since the carbon is present mainly in the form of cementite due to its low solubility in the ferrite lattice of the steel. In addition, as the carbon content increases, the surface quality deteriorates and the risk of slab cracking increases as the peritectic point is approached. This can lead to flatness defects during thermal treatment of the cold-rolled steel flat product. Therefore, limiting the carbon content to a maximum of 0.1% by weight is preferred to avoid the formation of slab cracks and resulting flatness defects. To achieve good flatness and isotropic material properties of the cold-rolled steel sheet, a carbon content of less than 0.085% by weight is particularly preferred. [0044] Manganese, Mn: Not more than 0.6%, preferably more than 0.17%;
Manganese also increases hardness and strength. In addition, manganese improves the weldability and wear resistance of steel. Furthermore, the addition of manganese reduces the tendency to red fracture during hot rolling by binding sulfur to less harmful MnS. Furthermore, manganese leads to grain refinement and manganese can increase the solubility of nitrogen in the iron lattice and prevent diffusion of carbon to the surface of the slab. Therefore, a manganese content of at least 0.17 wt. % is preferable. To achieve the desired strengths, a manganese content of more than 0.2 wt. %, in particular 0.30 wt. % or more, is preferred. However, if the manganese content becomes too high, this is at the expense of the corrosion resistance of the steel and food compatibility is no longer guaranteed. In addition, if the manganese content becomes too high, also the strength of the hot strip becomes too high, which means that the hot strip can no longer be cold rolled. Therefore, the upper limit for the manganese content is 0.6% by weight. [0045] Phosphorus, P: less than 0.1%.
Phosphorus is an undesirable by-product in steels. A high phosphorus content leads in particular to embrittlement of the steel and therefore deteriorates the formability of steel flat products, which is why the upper limit for the phosphorus content is 0.1% by weight. [0046] Sulfur, S: less than 0.03%, preferably more than 0.001%.
Sulfur is an undesirable concomitant element in steels that deteriorates ductility and corrosion resistance. Therefore, no more than 0.03% by weight of sulfur should be present in the steel. On the other hand, complex and cost-intensive measures have to be taken to desulfurize steel, which is why a sulfur content of less than 0.001 wt. % is no longer justifiable from an economic point of view. The sulfur content is therefore in the range from 0.001 wt. % to 0.03 wt. %, particularly preferably between 0.005 wt. % and 0.01 wt. %. [0047] Aluminum, Al: less than 0.1%, preferably more than 0.002%.
Aluminum is required in steel production as a deoxidizing agent for steel quenching. Aluminum also increases scale resistance and formability. For this reason, the aluminum content is preferably more than 0.002% by weight. However, aluminum forms aluminum nitrides with nitrogen, which are disadvantageous because they reduce the amount of free nitrogen and thus reduce the strength achievable by solid solution strengthening with nitrogen. In addition, too high aluminum concentrations can lead to surface defects in the form of aluminum clusters, which also can lead to flatness defects. Therefore, aluminum is used in a concentration of less than 0.1 wt. %. [0048] Silicon, Si: less than 0.03%;
Silicon increases scale resistance in steel and is a solid solution hardener. In steel production, silicon serves as a deoxidizing agent. Another positive effect of silicon on steel is that it increases tensile strength and yield strength. Therefore, a silicon content of 0.003 wt % or more is preferable to achieve the desired strengths. However, if the silicon content becomes too high, and in particular exceeds 0.03 wt. %, the corrosion resistance of the steel may deteriorate and surface treatments, especially by electrolytic coatings, may become more difficult. [0049] Nitrogen, N: less than 0.07% and preferably more than 0.001%.
In steel alloys, nitrogen acts as a solid solution strengthener to increase hardness and strength and has a positive effect on formability, which is reflected, for example, in high values of Erichsen deepening (Erichsen index). Therefore, a nitrogen content of more than 0.001 wt. % is preferable to achieve the desired strength and formability. However, too high nitrogen contents in the molten steel lead to a deterioration of the cold-rollability of the hot strip produced from the molten steel. This is particularly noticeable with a nitrogen content of more than 0.016% by weight. Furthermore, a high nitrogen content in the molten steel increases the risk of defects in the hot-rolled strip, as hot working properties are reduced at very high nitrogen concentrations. Defects in the hot-rolled strip are detrimental to the flatness of the cold-rolled steel sheet. For this reason, the upper limit of the nitrogen content to achieve good flatness is 0.07 wt. % and preferably no more than 0.016 wt. %. [0050] optional: nitride formers, especially niobium, titanium, boron, molybdenum, chromium:
Nitride-forming elements such as aluminum, titanium, niobium, boron, molybdenum and chromium are disadvantageous in the steel of the flat steel products according to the invention because they reduce the proportion of free nitrogen through nitride formation. In addition, these elements are expensive and therefore increase the manufacturing costs. On the other hand, the elements niobium, titanium and boron, for example, have a strength-increasing effect via grain refinement as microalloying constituents without reducing toughness. Therefore, the aforementioned nitride formers can advantageously be added within certain limits as alloying constituents of the steel melt. The steel can therefore (optionally) contain the following alloying constituents by weight: [0051] Titanium, Ti: less than 0.01%; [0052] Boron, B: less than 0.005%, [0053] Niobium, Nb: less than 0.01%, [0054] Chromium, Cr: preferably more than 0.01% to allow the use of scrap in the production of molten steel and to impede the diffusion of carbon on the surface of the slab, but not more than 0.1% to avoid carbides and nitrides, [0055] Molybdenum, Mo: less than 0.02% to avoid excessive increase in recrystallization temperature;

Other Optional Components:

[0056] In addition to the residual iron (Fe) and unavoidable impurities, the molten steel may also contain other optional constituents, such as optionally copper, Cu: expediently more than 0.002% to allow the use of scrap in the production of the molten steel, but less than 0.1% to ensure food compatibility; [0057] optionally nickel, Ni: expediently more than 0.01% to allow the use of scrap in the production of molten steel and to improve toughness, but less than 0.1% to ensure food compatibility; [0058] optional Tin, Sn: preferably less than 0.03%;

Manufacturing Method of the Flat Steel Product:

[0059] With the steel composition described above, a steel melt is produced for the production of flat steel products according to the invention, which is first continuously cast and, after cooling, cut into slabs. The slabs are then reheated to preheating temperatures of more than 1100° C., in particular 1200° C., and hot-rolled to produce a hot-rolled strip with a thickness in the range from 1 to 4 mm.

[0060] The final rolling temperature during hot-rolling is preferably above the Ar3 temperature in order to remain austenitic, and is in particular between 800° C. and 920° C.

[0061] The hot-rolled strip is coiled into a coil at a predetermined and suitably constant coiling temperature (reel temperature, HT). The coiling temperature is preferably below Ar1 in order to remain in the ferritic range, preferably in the range from 500° C. to 750° C., and particularly preferably is less than 640° C. in order to achieve a homogeneous distribution of carbides and a cementite precipitation as fine as possible.

[0062] For the production of a packaging steel in the form of a thin flat steel product in the thickness range of 0.49 mm or less (ultrafine sheets), the hot-rolled strip is cold-rolled, expediently with a thickness reduction (degree of reduction or cold rolling degree) of at least 80% and preferably in the range from 85% to 98%. The width of the steel strip is preferably 1200 mm or less and is particularly preferably in the range from 700 mm to 1100 mm. To restore the crystal structure of the steel destroyed during cold rolling, the cold-rolled steel strip is then recrystallizing annealed in an annealing furnace at an annealing temperature above the recrystallization temperature. This is done, for example, by passing the flat steel product in the form of a cold-rolled steel strip through a continuous annealing furnace in which the steel strip is inductively heated to the annealing temperature and held at the annealing temperature for a predetermined holding time. The heating rate at which the cold-rolled steel strip is inductively heated to the annealing temperature during annealing in the continuous annealing furnace is more than 100 K/s and preferably more than 300 K/s, in particular between 400 K/s to 600 K/s.

[0063] The annealing temperature is preferably between 725° C. and 769° C. and the holding time is preferably less than 1 second, in particular between 0.3 and 0.9 seconds, more preferably less than 0.7 seconds and in particular between 0.50 and 0.60 seconds.

[0064] After the holding time has elapsed, the recrystallization-annealed steel strip is cooled in a primary cooling step at a primary cooling rate of less than 1000 K/s to a take-off temperature (Tq) which is below the recrystallization temperature and is preferably less than 715° C., in particular less than 600° C. Preferably, the primary cooling rate is less than 400 K/s and, for example, in the range from 100 K/s to 400 K/s. Immediately after primary cooling, a secondary cooling of the steel strip to a temperature of less than 80° C. takes place at a secondary cooling rate of more than 1000 K/s. Preferably the secondary cooling rate is more than 1500 K/s and, for example, is in the range from 1600 K/s to 3000 K/s. The primary cooling preferably is carried out by gas cooling, in particular a gas jet, and the secondary cooling is preferably performed by water cooling, in particular by immersing the steel strip in a water bath with a water temperature of 80° C. or less.

[0065] A typical annealing cycle for annealing and cooling the steel strip is shown in FIG. 4. Therein, Tm is designating the (maximum) annealing temperature to which the steel strip is heated in the annealing furnace, T.sub.q is the take-off temperature at which secondary cooling is initiated, and T.sub.f is designating the water temperature of the water cooling system at which secondary cooling takes place. The steel strip is inductively heated to the annealing temperature within a short heating time t.sub.1, which is preferably less than 2 seconds and lies, for example, between 1.0 and 2.0 seconds. The steel strip is held at the annealing temperature for a holding time t.sub.H=t.sub.2−t.sub.1 until t.sub.2 and then cooled. Cooling is carried out in two steps, namely a primary cooling step with gas cooling (up to time t.sub.3), in which the steel strip is cooled at a low cooling rate to the take-off temperature T.sub.q, and a secondary cooling step beginning at time t.sub.3, in which the steel strip is cooled with water at a very high cooling rate to the water temperature T.sub.f.

[0066] Examples of embodiments of the invention and comparative examples are explained below.

Examples of Embodiments of the Invention

[0067] Flat steel products (steel sheets) were produced from steel melts with the alloy compositions listed in Table 1 by hot rolling followed by (primary) cold rolling.

[0068] The cold-rolled flat steel products were then subjected to recrystallization annealing in a continuous annealing furnace by inductively heating the flat steel products at a high heating rate to an annealing temperature above the recrystallization temperature and holding them at the annealing temperature for a holding time. At the end of the holding time, the steel strips were cooled in two stages, with a primary cooling by means of gas cooling at a low cooling rate to a take-off temperature T.sub.q followed by a secondary cooling with rapid quenching in a water bath at a predetermined water temperature T.sub.f. After cooling, the steel strips were cold re-rolled with a re-rolling degree of 5% to 45% or skin passed with a skin pass degree of 0.2% to a maximum of 5%. The process parameters during annealing and cooling and the subsequent secondary cold rolling or skin-passing are listed in Table 2, and the material parameters obtained are shown in Table 3. Therein [0069] Tm is the annealing temperature (in ° C.), [0070] t.sub.H is the dwell time (in s), [0071] dT/dt_K1 is the cooling rate during primary cooling (in K/s), [0072] Tq is the take-off temperature (in ° C.) at the end of primary cooling, [0073] Tf is the water temperature (in ° C.) during secondary cooling, [0074] D is the thickness of the steel strip after re-rolling or skin passing (in mm), [0075] NWG is the degree of re-rolling during secondary cold rolling (in %) or the degree of skin-passing (in %) during skin-passing, [0076] Rp0.2 or Rp2 is the 0.2% Yield strength (in MPa) (in rolling direction), [0077] Rp0.5 or Rp5 is the 0.5% Yield strength (in MPa) (in rolling direction), [0078] ReL is the lower yield strength (in MPa) (in rolling direction), [0079] ReH is the upper yield strength (in MPa) (in rolling direction), [0080] A is the elongation at break (in %) (in the rolling direction), [0081] Rm is the tensile strength (in MPa) (in rolling direction) and [0082] P is the flatness (in U-units) measured according to the standard defined in “ASTM A 1030/A1030M-11”.

Comparative Examples

[0083] For comparison purposes, the steel sheets with the alloy composition according to Table 1 were subjected to recrystallization annealing in a continuous annealing furnace by inductively heating the flat steel products at a heating rate of more than 100 K/s to an annealing temperature above the recrystallization temperature and holding them at the annealing temperature for a holding time and then cooling them in a single step by means of water cooling at a cooling rate of more than 1000 K/s. The flat steel products of the comparative examples were then subjected to a single step cooling process at a cooling rate of more than 1000 K/s. After cooling, the steel strips were cold re-rolled with a re-rolling degree of 5% to 45% or skin-passed with a skin-passing degree of 0.2% to a maximum of 5%. The process parameters for annealing and cooling and the subsequent secondary cold rolling or skin-passing of these comparative examples are given in Table 2, and the material parameters obtained are shown in Table 3.

[0084] The specimens of the examples according to the invention and the comparative examples were subjected to artificial aging after skin-passing or secondary re-rolling by keeping the specimens at an aging temperature of approx. 200° C. to 210° C. for more than 20 minutes. Tensile tests were carried out on the aged specimens to determine the mechanical properties (according to DIN EN 102020 and EN ISO 6892). A typical stress-strain diagram of an aged specimen is shown in FIG. 1.

[0085] For skin-passed specimens with a degree of re-rolling <5%, R.sub.eL is specified as the yield strength in the aged condition. For re-rolled specimens with a degree of re-rolling ≥5%, Rex or the 0.5% proof stress (R.sub.p0.5) is specified as the yield strength in the aged condition, since R.sub.eL cannot always be determined for these specimens. In general, the following applies to the determination of the yield strength: if ReL can be determined, this parameter should generally be used; if this is not the case, the use of the Rp0.2% yield strength is recommended as a suitable parameter as long as a plateau in the Lüders strain range is evident; in all other cases, the maximum, ReH and/or Rm should be used.

[0086] As can be seen in Table 3, a multiphase microstructure with a martensite phase has formed in the comparative examples (Examples 1, 6 to 10, 15 to 17 and 20 to 22), which is why these examples cannot be qualified as being in the scope of the invention. This can be explained by the high take-off temperature and the rapid cooling during secondary cooling. For the process control during cooling of the recrystallization-annealed steel strip, therefore, take-off temperatures T.sub.q below 690° C. are preferred to avoid a martensite phase. Due to the multiphase microstructure, these examples have a higher yield strength and tensile strength compared with the examples according to the invention, which lie outside the preferred ranges of maximum 500 MPa for the yield strength (for single cold-rolled steel strips with a skin pass degree of less than 5%). Furthermore, examples 6 to 10, 12, 15 to 17 as well as examples 20 to 22 exhibit flatness defects, which is represented by high flatness values of more than 5 I-units, which is why these examples do not qualify as being in the scope of the invention. In example 12, no martensite phase has formed. However, due to a higher water temperature during secondary cooling, as compared to Examples 2 to 5 and Example 11 according to the invention (which are of the same steel No. 6), this sample exhibits flatness defects reflected by flatness values of 17.3 I-units, which are outside the range of the invention.

[0087] In order to determine the influence of the take-off temperature T.sub.q on the strength and elongation at break, tensile tests with different take-off temperatures T.sub.q were carried out on aged specimens according to Table 1. The results of these tests are shown graphically in FIGS. 2a and 2b, wherein FIG. 2a shows the influence of the take-off temperature T.sub.q on the Yield Strength (Rp2, ReL, ReH and Rm) and FIG. 2b shows the influence of the take-off temperature T.sub.q on the elongation at fracture A. From FIG. 2a, an increase in strength can be observed at take-off temperatures Tq above approx. 650° C., whereas the take-off temperature T.sub.q has only little influence on the elongation at break, as can be seen from FIG. 2b.

[0088] In order to determine the change in strength during re-rolling (secondary cold rolling or skin-passing), tests were carried out on specimens according to the invention and on specimens of the comparative examples with different re-rolling degrees (NWG). The results of these tests are shown graphically in FIGS. 3a and 3b, where FIG. 3a shows the influence of the degree of re-rolling (NWG) on the tensile strength (Rm) and FIG. 3b shows the influence of the degree of re-rolling (NWG) on the upper yield strength (ReH). It can be seen from FIGS. 3a and 3b that the degree of rerolling causes a smaller increase in strength in the samples according to the invention than in the comparative examples. The flat steel products according to the invention can therefore be kept within the moderate strength range with yield strengths between 430 MPa and 700 MPa, even in the case of double reduced (DR) specimens with re-rolling degrees of 5% to 45%. This allows the formation of flat steel products with moderate strength and lower thicknesses of 0.21 mm or less, by double cold rolling. At skin pass degrees of 5% or less, the flat steel products of the invention have yield strengths in the range of 300 MPa to 500 MPa.

[0089] In the tables, the European convention of using a comma to separate the integral part of a number from the decimal part is used.

TABLE-US-00001 TABLE 1 Mass % Steel C Mn P Si Al Cu Cr Nb Ti S Sn B Mo Ni N 1 0.021 0.21 0.010 0.005 0.022 0.014 0.021 <0.0010 <0.0010 0.010 0.002 0.002 0.004 0.020 0.0025 2 0.025 0.25 0.012 0.020 0.031 0.017 0.030 <0.0010 <0.0010 0.004 0.003 — 0.003 0.015 0.0054 3 0.037 0.23 0.011 0.020 0.032 0.008 0.022 <0.0010 <0.0010 0.005 0.002 — 0.001 0.016 0.0043 4 0.058 0.29 0.009 0.010 0.031 0.011 0.046 <0.0010 <0.0010 0.003 0.004 — 0.004 0.020 0.0059 5 0.069 0.26 0.011 0.016 0.061 0.010 0.054 <0.0010 <0.0010 0.009 0.006 — 0.002 0.020 0.0040 6 0.067 0.29 0.010 0.005 0.016 0.009 0.020 <0.0010 <0.0010 0.010 0.002 — 0.001 0.018 0.0111

TABLE-US-00002 TABLE 2 Steel D NWG Tm tH dT/dt_K1 Tq Tf Example [Tab. 1] [mm] [%] [° C.] [s] [K/s] [° C.] [° C.] 1 Comparative Example 6 0.205 1.0 769 1.4 50 695 25 2 Invention 6 0.205 8.0 769 1.4 50 685 25 3 Invention 6 0.205 1.0 769 1.4 50 650 25 4 Invention 6 0.205 1.0 769 1.4 50 600 25 5 Invention 6 0.205 1.0 769 1.4 50 550 25 6 Comparative Example 6 0.205 1.0 769 1.4 50 735 25 7 Comparative Example 6 0.205 1.0 769 1.4 50 745 25 8 Comparative Example 6 0.205 1.0 769 1.4 50 735 17 9 Comparative Example 6 0.205 1.0 769 1.4 50 735 35 10 Comparative Example 6 0.205 1.0 769 1.4 50 735 55 11 Invention 6 0.205 8.0 769 1.4 50 685 17 12 Comparative Example 6 0.205 8.0 769 1.4 50 685 35 13 Invention 3 0.185 1.0 769 1.4 50 650 25 14 Invention 3 0.185 1.0 769 1.4 50 650 25 15 Comparative Example 3 0.185 1.0 769 1.4 50 735 25 16 Comparative Example 3 0.185 1.0 769 1.4 50 735 17 17 Comparative Example 3 0.185 1.0 769 1.4 50 735 55 18 Invention 1 0.185 1.0 769 1.4 50 650 25 19 Invention 1 0.210 1.0 769 1.4 50 600 25 20 Comparative Example 1 0.210 1.0 769 1.4 50 735 25 21 Comparative Example 1 0.210 1.0 769 1.4 50 735 17 22 Comparative Example 1 0.210 1.0 769 1.4 50 735 55 23 Invention 2 0.190 1.0 769 1.4 50 650 25 24 Invention 2 0.190 1.0 769 1.4 50 600 25 25 Comparative Example 2 0.190 1.0 769 1.4 50 735 25 26 Comparative Example 2 0.190 1.0 769 1.4 50 735 17 27 Comparative Example 2 0.190 1.0 769 1.4 50 735 55 28 Invention 4 0.185 1.0 769 1.4 50 650 25 20 Invention 4 0.185 1.0 769 1.4 50 600 25 30 Comparative Example 4 0.185 1.0 769 1.4 50 735 25 31 Comparative Example 4 0.185 1.0 769 1.4 50 735 17 32 Comparative Example 4 0.185 1.0 769 1.4 50 735 55 33 Invention 5 0.200 1.0 769 1.4 50 650 25 34 Invention 5 0.200 1.0 769 1.4 50 600 25 35 Comparative Example 5 0.200 1.0 769 1.4 50 735 25 38 Comparative Example 5 0.200 1.0 769 1.4 50 735 17 39 Comparative Example 5 0.200 1.0 769 1.4 50 735 55

TABLE-US-00003 TABLE 3 in rolling direction structure Steel Rp0.2 ReL ReH A Rm martensite Flatness Example [Tab. 1] [MPa] [MPa] [MPa] [%] [MPa] [Yes/No] [I-Unit] 1 Comparative Example 6 588 571 621 14 587 Yes 1.5 2 Invention 6 564 545 592 16 552 No 1.3 3 Invention 6 498 486 516 15 497 No 1.2 4 Invention 6 479 473 487 17 489 No 1.4 5 Invention 6 473 445 480 18 476 No 0.9 6 Comparative Example 6 599 581 630 13 597 Yes 20.2 7 Comparative Example 6 605 586 637 14 598 Yes 36.1 8 Comparative Example 6 598 578 631 13 598 Yes 12.0 9 Comparative Example 6 591 572 622 13 597 Yes 36.5 10 Comparative Example 6 490 475 502 16 473 Yes 43.5 11 Invention 6 561 549 590 15 554 No 1.2 12 Comparative Example 6 558 542 583 16 550 No 17.3 13 Invention 3 430 436 421 16 424 No 1.7 14 Invention 3 402 397 406 16 405 No 1.8 15 Comparative Example 3 513 493 526 14 498 Yes 19.6 16 Comparative Example 3 521 501 533 16 506 Yes 10.3 17 Comparative Example 3 408 393 419 18 389 Yes 40.1 18 Invention 1 363 366 366 16 359 No 1.2 19 Invention 1 319 325 336 16 326 No 1.4 20 Comparative Example 1 428 408 438 15 411 Yes 21.7 21 Comparative Example 1 432 412 444 16 417 yes 11.4 22 Comparative Example 1 334 319 348 28 317 yes 42.9 23 Invention 2 384 385 388 15 382 No 1.3 24 Invention 2 336 346 358 16 346 No 1.5 25 Comparative Example 2 457 423 458 14 460 Yes 20.0 26 Comparative Example 2 462 442 472 13 446 Yes 12.1 27 Comparative Example 2 347 332 358 24 329 Yes 42.8 28 Invention 4 450 458 443 15 444 No 2.3 29 Invention 4 419 418 429 14 424 No 2.2 30 Comparative Example 4 559 539 568 13 542 Yes 18.5 31 Comparative Example 4 568 548 581 13 553 Yes 9.7 32 Comparative Example 4 429 414 437 20 410 Yes 39.6 33 Invention 5 500 487 517 15 498 No 1.5 34 Invention 5 480 474 488 17 490 No 1.8 35 Comparative Example 5 599 581 633 13 597 Yes 17.2 36 Comparative Example 5 600 557 613 16 582 Yes 12.5 37 Comparative Example 5 486 466 498 18 481 Yes 45.1