CA2910012C

CA2910012C - High strength steel exhibiting good ductility and method of production via in-line heat treatment downstream of molten zinc bath

Info

Publication number: CA2910012C
Application number: CA2910012A
Authority: CA
Inventors: Grant Aaron Thomas; Jose Mauro B. Losz
Original assignee: AK Steel Properties Inc
Current assignee: Cleveland Cliffs Steel Properties Inc
Priority date: 2013-05-17
Filing date: 2014-05-16
Publication date: 2020-02-18
Anticipated expiration: 2034-05-16
Also published as: AU2014265214B2; WO2014186689A3; WO2014186722A2; WO2014186722A3; EP2997172B1; CN105247090A; RU2632042C2; KR101776242B1; KR20190101504A; KR20160007647A; TW201502284A; KR20160007646A; CA2908491A1; TWI616538B; AU2014265262B2; BR112015027447A2; CA2910012A1; JP2018178262A; TW201446974A; KR20170104158A

Abstract

Steel with high strength and good formability is produced with compositions and methods for forming austenitic and martensitic microstructure in the steel. Carbon, manganese, molybdenum, nickel copper and chromium may promote the formation of room temperature stable (or meta- stable) austenite by mechanisms such as lowering transformation temperatures for non-martensitic constituents, and/or increasing the hardenability of steel. Thermal cycles utilizing a rapid cooling below a martensite start temperature followed by reheating may promote formation of room temperature stable austenite by permitting diffusion of carbon into austenite from martensite.

Description

High Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Heat Treatment Downstream of Molten Zinc Bath Grant A. Thomas Jose Mauro B. Losz [0001]
BACKGROUND
[00021 It is desirable to produce steels with high strength and good formability characteristics. However, commercial production of steels exhibiting such characteristics has been difficult due to factors such as the desirability of relatively low alloying additions and limitations on thermal processing capabilities of industrial production lines. The present invention relates to steel compositions and processing methods for production of steel using hot-dip galvanizing/galvanncaling (HDG) processes such that the resulting steel exhibits high strength and cold formability.
SUMMARY
100031 The present stool is produced using a composition and a modified IIDG process that together produces a resulting microstructure consisting of generally martcnsite and austenite (among other constituents). To achieve such a microstructure, the composition includes certain alloying additions and the HDG
process includes certain process modification, all of which are at least partially related to driving the transformation of austenite to martensite followed by a partial stabilization of austenite at room-temperature.
BRIEF DESCRIPTION OF THE FIGURES
[0004] The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the general description given above, and the detailed description of the embodiments given below, serve to explain the principles of the present disclosure.
[0005] FIGURE 1 depicts a schematic view of a HDG temperature profile with a partitioning step performed after galvanizing/galvannealing.
[0006] FIGURE 2 depicts a schematic view of a HDG temperature profile with a partitioning step performed during galvanizing/galvannealing.
[0007] FIGURE 3 depicts a plot of one embodiment with Rockwell hardness plotted against cooling rate.
[0008] FIGURE 4 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.
[0009] FIGURE 5 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.
[0010] FIGURE 6 depicts six photo micrographs of the embodiment of FIG. 3 taken from samples being cooled at various cooling rates.
[0011] FIGURE 7 depicts six photo micrographs of the embodiment of FIG. 4 taken from samples being cooled at various cooling rates.
[0012] FIGURE 8 depicts six photo micrographs of the embodiment of FIG. 5 taken from samples being cooled at various cooling rates.
[0013] FIGURE 9 depicts a plot of tensile data as a function of austenitization temperature for several embodiments.

2 [0014] FIGURE 10 depicts a plot of tensile data as a function of austenitization temperature for several embodiments.
[0015] FIGURE 11 depicts a plot of tensile data as a function of quench temperature for several embodiments.
[0016] FIGURE 12 depicts a plot of tensile data as a function of quench temperature for several embodiments.

3 DETAILED DESCRIPTION
[0017] FIG. 1 shows a schematic representation of the thermal cycle used to achieve high strength and cold formability in a steel sheet having a certain chemical composition (described in greater detail below). In particular, FIG. 1 shows a typical hot-dip galvanizing or galvannealing thermal profile (10) with process modifications shown with dashed lines. In one embodiment the process generally involves austenitization followed by a rapid cooling to a specified quench temperature to partially transform austenite to martensite, and the holding at an elevated temperature, a partitioning temperature, to allow carbon to diffuse out of martensite and into the remaining austenite, thus, stabilizing the austenite at room temperature. In some embodiments, the thermal profile shown in FIG. 1 may be used with conventional continuous hot-dip galvanizing or galvannealing production lines, although such a production line is not required.
[0018] As can be seen in FIG. 1, the steel sheet is first heated to a peak metal temperature (12). The peak metal temperature (12) in the illustrated example is shown as being at least above the austenite transformation temperature (A1) (e.g., the dual phase, austenite + ferrite region). Thus, at the peak metal temperature (12), at least a portion of the steel will be transformed to austenite. Although FIG. 1 shows the peak metal temperature (12) as being solely above A1, it should be understood that in some embodiments the peak metal temperature may also include temperatures above the temperature at which ferrite completely transforms to austenite (A3) (e.g., the single phase, austenite region).
[0019] Next the steel sheet undergoes rapid cooling. As the steel sheet is cooling, some embodiments may include a brief interruption in cooling for galvanizing or galvannealing. In embodiments where galvanizing is used, the steel sheet may briefly maintain a constant temperature (14) due to the heat from the molten zinc galvanizing bath. Yet in other embodiments, a galvannealing process may be used and the temperature of the steel sheet may be slightly raised to a galvannealing temperature (16) where the galvannealing process may be performed. Although,

4 in other embodiments, the galvanizing or galvannealing process may be omitted entirely and the steel sheet may be continuously cooled.
[0020] The rapid cooling of the steel sheet is shown to continue below the martensite start temperature (Ms) for the steel sheet to a predetermined quench temperature (18). It should be understood that the cooling rate to Ms may be high enough to transform at least some of the austenite formed at the peak metal temperature (12) to martensite. In other words the cooling rate may be rapid enough to transform austenite to martensite instead of other non-martensitic constituents such as ferrite, pearlite, or bainite which transform at relatively lower cooling rates.
[0021] As is shown in FIG. 1, the quench temperature (18) is below M,. The difference between the quench temperature (18) and M, may vary depending on the individual composition of the steel sheet being used. However, in many embodiments the difference between quench temperature (18) and M, may be sufficiently great to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive "fresh"
martensite upon final cooling. Additionally, quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench (e.g., to avoid excessive carbon enrichment of austenite greater than that required to stabilize austenite for the given embodiment).
[0022] In many embodiments, quench temperature (18) may vary from about 191 C to about 281 C, although no such limitation is required. Additionally, quench temperature (18) may be calculated for a given steel composition. For such a calculation, quench temperature (18) corresponds to the retained austenite having an WL temperature of room temperature after partitioning. Methods for calculating quench temperature (18) are known in the art and described in J. G. Speer, A.
M.
Streicher, D. K. Matlock, F. Rizzo, and G. Krauss, "Quenching And Partitioning:
A Fundamentally New Process to Create High Strength Trip Sheet Microstructures," Austenite Formation and Decomposition, pp. 505-522, 2003;
and A. M. Streicher, J. G. J. Speer, D. K. Matlock, and B. C. De Cooman, "Quenching and Partitioning Response of a Si-Added TRIP Sheet Steel," in Proceedings of the International Conference on Advanced High Strength Sheet Steels for Automotive Applications, 2004.
[0023] The quench temperature (18) may be sufficiently low (with respect to MO to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive "fresh" martensite upon the final quench.
Alternatively, the quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench and creating a situation where the potential carbon enrichment of the retained austenite is greater than that required for austenite stabilization at room temperature. In some embodiments, a suitable quench temperature (18) may correspond to the retained austenite having an M, temperature of room temperature after partitioning. Speer and Streicher c-al. (above) have provided calculations that provide guidelines to explore processing options that may result in desirable microstructures. Such calculations assume idealized full partitioning, and may be performed by applying the Koistinen-Marburger (KM) relationship twice (jin =1¨ e-1.100-2 (AT)) first to the initial quench to quench temperature (18) and then to the final quench at room temperature (as further described below). The Ms temperature in the KM
expression can be estimated using empirical formulae based on austenite chemistry (such as that of the well known in the art Andrew's linear expression):
100241 Ms( C) = 539¨ 423C ¨ 30.4Mn ¨ 7.5S1 +30 Al 10025] The result of the calculations described by Speer et al. may indicate a quench temperature (18) which may lead to a maximum amount of retained austenite. For quench temperatures (18) above the temperature having a maximum amount of retained austenite, significant fractions of austenite are present after the initial, quench; however, there is not enough martensite to act as a carbon source to stabilize this austenitc. Therefore, for the higher quench temperatures, increasing amounts of fresh mattensite form during the final quench. For quench temperatures below the temperature having a maximum amount of retained austenite, an unsatisfactory amount of austenite may be consumed during the initial quench and there may be an excess amount of carbon that may partition from the martensite.
[0026] Once the quench temperature (18) is reached, the temperature of the steel sheet is either increased relative to the quench temperature or maintained at the quench temperature for a given period of time. In particular, this stage may be referred to as the partitioning stage. In such a stage, the temperature of the steel sheet is at least maintained at the quench temperature to permit carbon diffusion from martensite formed during the rapid cooling and into any remaining austenite.
Such diffusion may permit the remaining austenite to be stable (or meta-stable) at room temperature, thus improving the mechanical properties of the steel sheet.
[0027] In some embodiments, the steel sheet may be heated above Mc to a relatively high partitioning temperature (20) and thereafter held at the high partitioning temperature (20). A variety of methods may be utilized to heat the steel sheet during this stage. By way of example only, the steel sheet may be heated using induction heating, torch heating, and/or the like. Alternatively, in other embodiments, the steel sheet may be heated but to a different, lower partitioning temperature (22) which is slightly below M,. The steel sheet may then be likewise held at the lower partitioning temperate (22) for a certain period of time. In still a third alternative embodiment, another alternative partitioning temperature (24) may be used where the steel sheet is merely maintained at the quench temperature. Of course, any other suitable partitioning temperature may be used as will be apparent to those of ordinary skill in the art in view of the teachings herein.
[0028] After the steel sheet has reached the desired partitioning temperature (20, 22, 24), the steel sheet is maintained at the desired partitioning temperature (20, 22, 24) for a sufficient time to permit partitioning of carbon from martensite to austenite.
The steel sheet may then be cooled to room temperature.

[0029] FIG, 2 shows an alternative embodiment of the thermal cycle described above with respect to FIG. 1 (with a typical galvanizing/galvannealing thermal cycle shown with a solid line (40) and departures from typical shown with a dashed line). In particular, like with the process of FIG. 1, the steel sheet is first heated to a peak metal temperature (42). The peak metal temperature (42) in the illustrated embodiment is shown as being at least above Al. Thus, at the peak metal temperature (42), at least a portion of the steel sheet will be transformed to austenite. Of course, like the process of FIG. 1, the present embodiment may also include a peak metal temperature in excess of A3.
[0030] Next, the steel sheet may be rapidly quenched (44). It should be understood that the quench (44) may be rapid enough to initiate transformation of some of the austenite formed at the peak metal temperature (42) into martensite, thus avoiding excessive transformation to non-martensitic constituents such as ferrite, pearlite, banite, and/or the like.
[0031] The quench (44) may be then ceased at a quench temperature (46).
Like the process of FIG. 1, quench temperature (46) is below M,. Of course, the amount below Ms may vary depending upon the material used. However, as described above, in many embodiments the difference between quench temperature (46) and M, may be sufficiently great to form an adequate amount of martensite yet be sufficiently low to avoid consuming too much austenite.
[0032] The steel sheet is then subsequently reheated (48) to a partitioning temperature (50, 52). Unlike the process of FIG. 1, the partitioning temperature (50, 52) in the present embodiment may be characterized by the galvanizing or galvannealing zinc bath temperature (if galvanizing or galvannealing is so used). For instance, in embodiments where galvanizing is used, the steel sheet may be re-heated to the galvanizing bath temperature (50) and subsequently held there for the duration of the galvanizing process. During the galvanizing process, partitioning may occur similar to the partitioning described above. Thus, the galvanizing bath temperature (50) may also function as the partitioning temperature (50).
Likewise, in embodiments where galvannealing is used, the process may be substantially the same with the exception of a higher bath/partitioning temperature (52).
[0033] Finally, the steel sheet is permitted to cool (54) to room temperature where at least some austenite may be stable (or meta-stable) from the partitioning step described above.
[0034] In some embodiments the steel sheet may include certain alloying additions to improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure and/or to improve the mechanical properties of the steel sheet. Suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.15-0.4% carbon, 1.5-4% manganese, 0-2%
silicon or aluminum or some combination thereof, 0-0.5% molybdenum, 0-0.05%
niobium, other incidental elements, and the balance being iron.
[0035] In addition, in other embodiments suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.15-0.5% carbon, 1-3%
manganese, 0-2% silicon or aluminum or some combination thereof, 0-0.5%
molybdenum, 0-0.05% niobium, other incidental elements, and the balance being iron. Additionally, other embodiments may include additions of vanadium and/or titanium in addition to, or in lieu of niobium, although such additions are entirely optional.
[0036] In some embodiments carbon may be used to stabilize austenite. For instance, increasing carbon may lower the Ms temperature, lower transformation temperatures for other non-martensitic constituents (e.g., bainitc, ferrite, pearlite), and increase the time required for non-martensitic products to form.
Additionally, carbon additions may improve the hardenability of the material thus retaining formation of non-martensitic constituents near the core of the material where cooling rates may be locally depressed. However, it should be understood that carbon additions may be limited as significant carbon additions may lead to detrimental effects on weldability.

[0037] In some embodiments manganese may provide additional stabilization of austenite by lowering transformation temperatures of other non-martensitie constituents, as described above. Manganese may further improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure by increasing hardenability.
[0038] In other embodiments molybdenum may be used to increase hardenability.
[0039] In other embodiments silicon and/or aluminum may be provided to reduce the formation of carbides. It should be understood that a reduction in carbide formation may be desirable in some embodiments because the presence of carbides may decrease the levels of carbon available for diffusion into austenite.
Thus, silicon and/or aluminum additions may be used to further stabilize austenite at room temperature.
[0040] In some embodiments, nickel, copper, and chromium may be used to stabilize austenite. For instance, such elements may lead to a reduction in the M, temperature. Additionally, nickel, copper, and chromium may further increase the hardenability of the steel sheet.
[0041] In some embodiments niobium (or other micro-alloying elements, such as titanium, vanadium, and/or the like) may be used to increase the mechanical properties of the steel sheet. For instance, niobium may increase the strength of the steel sheet through grain boundary pinning resulting from carbide formation.
[0042] In other embodiments, variations in the concentrations of elements and the particular elements selected may be made. Of course, where such variations are made, it should be understood that such variations may have a desirable or undesirable effect on the steel sheet microstructure and/or mechanical properties in accordance with the properties described above for each given alloying addition.

[0043] Embodiments of the steel sheet were made with the compositions set forth in Table 1 below.
[0044] The materials were processed on laboratory equipment according to the following parameters. Each sample was subjected to Gleeble 1500 treatments using copper cooled wedge grips and the pocket jaw fixture. Samples were austenitized at 1100 C and then cooled to room temperature at various cooling rates between 1-100 C/s.

Table 1 Chemical compositions in weight %.
ID Description Al C Co Cr Cu Mn Mo Nb Ni P
Si Sn Ti V
V4037 Lab Material 1.41 0.19 - 0.01 <0.003 1.54 <0.003 <0.003 <0.003 <0.003 0.11 <0.003 0.01 <0.003 -V4038 Lab Material 1_29 0.22 - 0.20 <0.003 1.68 <0.003 0.02 <0.003 0.02 0.01 <0.003 0.01 <0.003 -oc crN
ch V4039 Lab Material <0.003 0.20 <0.002 0.01 <0.002 2.94 <0.002 0.00 <0.002 0.00 1.57 <0.002 0.01 <0.002 0.00 oc IRNO
=
oc [0045] The Rockwell hardness of each of the steel compositions described in Example 1 and Table 1 above was taken on the surface of each sample. The results of the tests are plotted in FIGS. 3-5 with Rockwell hardness plotted as a function of cooling rate. The average of at least seven measurements is shown for each data point. The compositions V4037, V4038 and V4039 correspond to FIGS. 3, 4, and

5, respectively.

[0046] Light optical micrographs were taken in the longitudinal through thickness direction near the center of each sample for each of the compositions of Example 1. The results of these tests are shown in FIGS. 6-8. The compositions V4037, V4038, and V4039 correspond to FIGS. 6, 7, and 8, respectively.
Additionally, FIGS. 6-8 each contain six micrographs for each composition with each micrograph representing a sample subjected to a different cooling rate.

[0047] A critical cooling rate for each of the compositions of Example 1 was estimated using the data of Examples 2 and 3 in accordance with the procedure described herein. The critical cooling rate herein refers to the cooling rate required to form martensite and minimize the formation of non-marten sitic transformation products. The results of these tests are as follows:
[0048] V4037: 70 C/s [0049] V4038: 75 C/s [0050] V4039: 7 C/s [0051] Embodiments of the steel sheet were made with the compositions set forth in Table 2 below.
[0052] The materials were processed by melting, hot rolling, and cold rolling. The materials were then subjected to testing described in greater detail below in Examples 6-7. All of the compositions listed in Table 2 were intended for use with the process described above with respect to FIG. 2 with the exception of V4039 which was intended for use with the process described above with respect to FIG. 1. Heat V4039 had a composition intended to provide higher hardenability as required by the thermal profile described above with respect to FIG. 1. As a result V4039 was subjected to annealing at 600 C for 2 hours in 100% H2 atmosphere after hot rolling, but prior to cold rolling. All materials were reduced during cold rolling about 75% to lmm. Results for some of the material compositions set forth in Table 2 after hot rolling and cold rolling are shown in Tables 3 and 4, respectively.

C) n.) l.0 r-, Table 2 Chemical compositions in weight %.
o o 0 1-.
Iv Heat Description c Mn Si Al Mo Cr Nb B "

--, N.) 1 4=
0 V4037 Lab Material 0.19 1.54 0.11 1.41 0 0.009 0 ' 0.0007 r-r r-.
r cre cs=
I V1307 1 Lab Material 0.19 1.53 1_48 0.041 0 0 0 0.0005 ere I-I V4063 Lab Material 0.19 1.6 0.11 1.34 0 0.003 0 0.0007 r-.
to V4038 Lab Material 022 1.68 0.007 1.29 0 0.2 0.021 0.0008 V4039 Lab Material 0.2 2.94 1.57 <0.030 <0.002 0.005 0.002 NM
V1305 Lab Material 0_2 294 1_57 0 0 0 0 0.0006 V4107 Lab Material 0.18 4.03 1.63 0.005 0 0 0 I 0.0008 V4108 Lab Material 0.18 5.06 1_56 0.004 0 0 0 0.0009 ;IN V4060 Lab Material 0.4 1.2 1.97 0.003 0 0.19 0,007 0.0005 _ V4061 Lab Material 0.41 1.2 0_98 0.003 0 0.003 o 0.0004 V4062 Lab Material o.n 1.18 0.012 1.16 0 0.003 o 0.0007 V4078-1 Lab Material 0.2 1.67 0.1 1.41 0.28 0.003 <0.003 0.0007 V4078-2 Lab Material 0.2 1.67 0.1 1.41 0.27 <0.003 0.051 0.0007 V4079-1 Lab Material 0.19 1.94 0.098 1.43 <0.003 <0.003 <0.003 0.0007 , V4079-2 : Lab Material 0.19 1.96 0.099 1.41 <0.003 <0.003 0.051 0.0007 *0 n 1-i 4:--a Ca t.4 C) Table 3 - Tensile Data, Post Hot Rolling Kt to i-. Yield Strength Total Uniform ...
-0 UpperYS Lower YS UT
Hardness o Heat YPE (%) 0.2% Offset Elongation Elongation 0 H RA
1-.
MPa ksi MPa Etsi MPa ksi MPa ksi (2") % 14 d N.) V4063 0 N/A N/A N/A N/A 375 54 652 95 26 4.
n.) 0 N/A N/A N/A N/A 380 55 648 94 26 , 15 ' 53 -...
i--, 0 V4039* 0 _ N/A N/A N/A N/A 640 _ 9 67 no i-. - -c., c, -.1 V4039* - 0 10 61 coo i (annealed) to i 4 k , I-. i 0 V4060 0.6 645 94 I 0.5 610 89 605 88 611 89 i-. r--0 N/A N/A N/A 0 496 72 790 115 22 , 11 to V4061 0 N/A N/A N/A 0 , 507 74 1.1 507 _ 74 501 73 , 506 _ 73 _ 712 103 26 ' 12 50 0.7 505 73 502 73 502 73 _. _ 713 103 V4078-1 0.8 427 , 62 416 80 425 V4078-2 - 0-6 525 I 76 519 - 75 , 525 .._ V4079-1 1.8 364 L_ 53 361 , 52 , 361 r 52 544 79 30 17 45 V4079-2 1.2 497 72 481 70 489 *Tensile test performed in trans-versc direction for V4039 Table 4 -Tensile Data, Post Cold Rolling -Yield Strength WM 1 Total Unifr.rm Hardness ' Heat 0.2% Offset Elongation e1on0ation % H RA
MPa ksi 1 MPa ksi (r) %
-V4037 927 134 _ 971 141 4.8 1.4 64 V4063 1046 152 1101 160 2.4 1.3 65 -V4038 1001 145 1054 153 5.5 1.6 65 V4039 1149 167 ., 1216 176 4.4 1.5 , 68 -V4060 1266 , 184 _1393 202 5.4 1.9 , 69 _ V4061 1187 172 1279 186 4.3 1.7 68 IV
_ _ n V4062 1111 161 _1185 172 4.3 1.7 66 ,-3 V4078-1 1047 152 1105 160 3.6 1_4 - 65 i V4078-2 1154 167 _ 1209 175 4.2 1.4 66 V4079-1 932 135 975 141 _ 4.6 14 64 -, o ' i--, V4079-2 1034 150 - 1078 156 3.9 1.3 66 4--O"
ta OD
C.44 dl "

[0053] The compositions of Example 5 were subjected to Gleeble dilatomety.
Gleeble dilatomety was performed in vacuum using a 101.6x25.4x1 mm samples with a c-strain gauge measuring dilation in the 25.4 mm direction. Plots were generated of the resulting dilation vs. temperature. Line segments were fit to the dilatometric data and the point at which the dilatometric data deviated from linear behavior was taken as the transformation temperature of interest (e.g., A1, A3, MO. The resulting transformation temperatures are tabulated in Table 5.
[0054] Gleeble methods were also used to measure a critical cooling rate for each of the compositions of Example 5. The first method utilized Gleeble dilatomety, as described above. The second method utilized measurements of Rockwell hardness.

In particular, after samples were subjected to Gleeble testing at range of cooling rates, Rockwell hardness measurements were taken. Thus, Rockwell hardness measurements were taken for each material composition with a measurement of hardness for a range of cooling rates. A comparison was then made between the Rockwell hardness measurements of a given composition at each cooling rate.
Rockwell hardness deviations of 2 points HRA were considered significant. The critical cooling rate to avoid non-martensitic transformation product was taken as the highest cooling rate for which the hardness was lower than 2 point HRA than the maximum hardness. The resulting critical cooling rates are also tabulated in Table 5 for some of the compositions listed in Example 5.
Table 5 ¨ Transformation Temperatures and Critical Cooling Rate from Gleeble Dilatomety Critical Cooling Rate Heat A1 ( C) A3 ( C) M, ( C) (ous) Gleeble Cileeble/
Dilatometry Hardness V4037 737 970 469 Inconclusive 65 V4039 750 874 394 <10 6 [0055] The compositions of Example 5 were used to calculate quench temperature and a theoretical maximum of retained austenite. The calculations were performed using the methods of Speer et al., described above. The results of the calculations are tabulated below in Table 6 for some of the compositions listed in Example 5.
Table 6¨ Quench Temperature and Theoretical Maximum of Retained Austenite õ
f(y) Theoretical Heat QT ( C) Maximum V4037 281 0.15 V4063 278 0.15 V4038 270 0.18 V4039 203 0.2 V4060 191 0.35 V4061 196 0.36 V4062 237 0.31 V4078-1 276 0.16 V4078-2 276 0.16 V4079-1 273 0.16 V4079-2 272 0.16 [00561 The samples of the compositions of Example 5 were subjected to the thermal profiles shown in FIGS. 1 and 2 with peak metal temperature and quench temperature varied between samples of a given composition. As described above, only composition V4039 was subjected to the thermal profile shown in FIG. 1, while all other compositions were subjected to the thermal cycle shown in FIG. 2. For each sample, tensile strength measurements were taken. The resulting tensile measurements are plotted in FIGS. 9-12. In particular, FIGS. 9-10 show tensile strength data plotted against austenitization temperature and FIGS. 11-12 show tensile strength data plotted against quench temperature. Additionally, where the thermal cycles were performed using Gleeble methods, such data points are denoted with "Gleeble."
Similarly, where thermal cycles were performed using a salt bath, such data points are denoted with "salt."
[0057] Additionally, similar tensile measurements for each composition listed in Example 5 (where available) are tabulated in Table 7, shown below. Partitioning times and temperatures are shown for example only, in other embodiments the mechanisms (such as carbon partitioning and/or phase transformations) occur during non-isothermal heating and cooling to or from the stated partitioning temperature which may also contribute to final material properties.

Table 7 - Tensile Data, Post Partitioning Peak Quench Heat Te 0.2% Ultimate Total Metal Partitioning Partitioning Elongation x UTS (Mpa Temp Yield Tensile Elongation mp Temp ( C) ( C) Strength Strength (%) ( C) 800 250 466 30 419 818 27 22,424 800 250 466 30 416 807 28 22,345 850 250 466 30 553 862 25 21,805 V1307 850 250 466 30 535 847 25 21,336 900 250 466 30 548 854 24 20,144 800 250 400 30 445 898 22 19,675 900 250 466 30 566 856 23 19,594 800 250 400 30 432 889 22 19,478 800 160 466 15 746 1317 23 29,630 800 200 466 15 716 1332 19 25,309 800 250 466 15 718 1403 18 25,115 V4060 800 200 466 15 632 1309 19 24,746 800 250 466 15 701 1379 18 24,407 800 160 466 15 845 1311 18 23,986 850 250 466 15 891 1291 18 23,749 850 250 466 15 735 1223 19 23,729 850 300 466 15 443 657 32 20,763 921 200 466 30 325 612 34 20,633 850 250 466 15 405 696 30 20,543 921 300 466 30 380 591 34 20,090 921 356 466 30 386 592 34 20,078 921 400 466 30 388 588 34 19,937 940 200 466 30 362 598 33 19,906 V4037 850 200 466 15 427 687 28 19,022 940 200 466 30 353 592 32 18,989 980 200 466 30 341 612 31 18,897 900 300 466 15 493 727 26 18,767 850 200 466 15 447 702 27 18,600 850 300 466 15 404 678 27 18,435 980 200 466 30 347 611 30 18,387 940 200 466 30 330 548 33 18,253 980 200 466 30 345 612 29 17,939 850 300 466 15 481 754 26 19,536 918 400 466 30 377 681 27 18,461 918 286 466 30 357 695 26 18,348 918 200 466 30 363 697 26 18,193 918 300 466 30 354 696 26 17,949 850 300 466 15 457 773 23 17,777 800 250 400 60 821 1299 15 19,225 800 250 400 60 821 1298 15 18,945 V4039 900 250 400 60 923 1273 15 18,593 850 250 400 60 874 1278 14 18,142 900 250 400 60 913 1258 14 17,984 800 160 466 15 746 1317 23 29,630 800 200 466 15 716 1332 19 25,309 800 250 466 15 718 1403 18 25,115 800 200 466 15 632 1309 19 24,746 V406 800 250 466 15 701 1379 18 24,407 800 160 466 15 845 1311 18 23,986 850 250 466 15 891 1291 _ 18 23,749 850 250 466 15 735 1223 19 23,729 800 200 466 30 942 1319 17 22,422 850 200 466 15 695 1222 16 19,070 750 250 466 15 553 985 20 19,902 750 250 466 15 581 918 21 18,996 750 200 466 15 478 813 23 18,778 V4062 750 250 466 15 480 816 22 17,944 750 200 466 15 536 790 23 17,936 V4107 850 250 400 60 776 1382 13 17,824 900 250 400 60 923 1642 11 17,401 850 250 400 60 952 1620 11 17,337 V 850 300 466 15 448 783 24 19,016 850 300 466 15 492 761 24 17,888 900 250 466 30 713 843 21 17,946 V4078-2 850 300 466 15 689 859 20 17,525 850 300 466 15 671 871 20 17,503 , , , 100581 It will be understood various modifications may be made to this invention without departing from the scope of it. Therefore, the limits of this invention should be determined from the appended claims.

Claims

1. A method for processing a steel sheet, the steel sheet comprising the following elements by weight percent:
0.15-0.4% carbon;
manganese;
2% or less silicon, aluminum, or some combination thereof;
0.5% or less molybdenum;
0.05% or less niobium; and the balance being iron and other incidental impurities;
the method comprising:
(a) heating the steel sheet to a first temperature (T1), wherein T1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite;
(b) cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the martensite start temperature (M s), wherein the cooling rate is sufficiently rapid to transform austenite to martensite;
(c) subjecting the steel sheet to hot dip galvanizing or galvannealing while cooling the steel sheet to T2;
(d) re-heating the steel sheet to a partitioning temperature, wherein the partitioning temperature is sufficient to permit diffusion of carbon within the structure of the steel sheet;
(e) stabilizing austenite by holding the steel sheet at the partitioning temperature for a holding time, wherein the holding time is of a period of time sufficient to permit diffusion of carbon from martensite to austenite; and (f) cooling the steel sheet to room temperature.

2. The method of claim 1, wherein the hot dip galvanizing or galvannealing occurs above Ms.

3. The method of claim 1, wherein the partitioning temperature is above M.

4. The method of claim 1, wherein the partitioning temperature is below M
s.

5. A method for processing a steel sheet of a selected composition, the steel sheet comprising the following elements by weight percent:
0.15-0.4% carbon;
1.5-4% manganese;
2% or 1 ess silicon, aluminum, or some combination thereof;
0.5% or less molybdenum;
0.05% or less niobium; and the balance being iron and other incidental impurities;
the method comprising:
(a) heating the steel sheet to a first temperature (T1), wherein T1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite;
(b) cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the martensite start temperature (M s), wherein the cooling rate is sufficiently rapid to transform austenite to martensite, wherein the cooling rate is defined for the selected composition of the steel sheet by a critical cooling rate that results in a room temperature hardness of the steel sheet that is no lower than 2 HRA below a maximum room temperature hardness of the steel sheet, to avoid the formation of non-martensitic transformation products;
(c) subjecting the steel sheet to hot dip galvanizing or galvannealing while cooling the steel sheet to T2;
(d) re-heating the steel sheet to a partitioning temperature, wherein the partitioning temperature is sufficient to permit diffusion of carbon within the structure of the steel sheet;
(e) stabilizing austenite by holding the steel sheet at the partitioning temperature for a holding time, wherein the holding time is of a period of time sufficient to permit diffusion of carbon from martensite to austenite; and (f) cooling the steel sheet to room temperature.

6. The method of claim 5, wherein the hot dip galvanizing or galvannealing occurs above M.

7. The method of claim 5, wherein the partitioning temperature is above M
s.

8. The method of claim 5, wherein the partitioning temperature is below M.