CA2908491C

CA2908491C - High strength steel exhibiting good ductility and method of production via quenching and partitioning treatment by zinc bath

Info

Publication number: CA2908491C
Application number: CA2908491A
Authority: CA
Inventors: Grant Aaron Thomas; Luis G. Garza-Martinez
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: 2019-07-30
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

2 PCTMS2014/038425 High Strength Steel Exhibiting Good Ductility and Method of Production via Quenching and Partitioning Treatment by Zinc Bath [00011 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/galvannealing (HOG) processes such that the resulting steel exhibits high strength and cold formability.
SUMMARY

[0003] The present steel is produced using a composition and a modified HOG
process that together produces a resulting microstructure consisting of generally martensite and austenite (among other constituents). To achieve such a microstructure, the composition includes certain alloying additions and the HOG
process includes certain process modification, all of which are at least partially related to dtiving the transfwmation of austenite to martensite followed by a partial stabilization of atiStenite at room-temperature.
BRIEF DESCRIPTION OF THE FIGURES
100041 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 Nile embodiments given below, serve to explain the principles of the present disclosure.
100051 FIGURE I depicts a sChematic vi6w Of a 11DG temperature profile with a partitioning step performed after galmizingtgalvaonealing.
100061 FIGURE 2 depicts a schematic view of a IIDG temperature profile with a partitioning step pertbrined during galvanizingtgalvannealing.
100071 FIGURE 3 depicts a plot of one embodiment with Rockwell hardness plotted against cooling rate.
100081 FIGURE 4 depicts a plot of another embodiment with Rockwell hardness plotted against cooling raw.
100091 FIGURE 5 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.
100101 FIGURE 6 depicts six photo micrographs of the embodiment of FIG. 3 taken from samples being cooled at various cooling rates.
100111 FIGURE 7 depicts six photo micrographs of the embodiment Of FIG. 4 taken from samples being cooled 41 various cooling rates.
100121 FIGURE 8 depicts six photo micrographs of the embodiment of FIG. 5 taken from samples being cooled at various cooling rates.
100131 FIGURE 9 depicts a plot of tensile data as a function of austenitization temperature for several embodiments, 100141 FIGURF 10 depicts a plot of tensile data as a function of tcustenitization temperature for several embodiments.
NM) FIGURE II. 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.

DETAILED DESCRIPTION
100171 FIG. I 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) hi particular, FIG. I shows a typical hot-dip galvanizing or galvarinealing thermal profile (10) with process modifications shown with dashed lines. In one embodiment the process generally involves austenilization 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 cominuous hot-dip galvaidzing or galvannealing production lines, although such a production line is not required, [00181 As can be seen in FIG. I, 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 AI, 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).
[00191 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 emboclirnents, the galvanizing or galvanncaling process may be emitted entirely and the steel sheet maybe continuously cooled, 10020] 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 M, may be high enough to transform at least some of thn. austenite formed at the peak metal temperature (12) to martensite. In other words the cooling rate may be rapid enough to transform austenite to martansite instead of other non-inartensitic constituents such as ferrite, pcarlite, or bainite which transform at relatively lower cooling rates.
100211 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 ht may be sufficiently great to form an adequate amount of martensite to act as a carbon source to stabilize the.austenne 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 exeessive carbon enrichment of austenite greater than that required to stabilize austenite for the given embodiment).
100221 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) ntay he calculated for a given steel Composition. For Such a calculation, quench temperature (18) corresponds to the retained austenite having an K4 temperature of room temperature after partitioning. Methods for calculating quench temperature (18) ate known in the art and described in.I. G. Speer, A.
M, Streicher, D. K. Matlock, F. Rizzo, and Krauss, "Quenching And Partitioning:
A Fundamentally New Process to Create High Strength Trip Sheet Microstructures," Aystende Formation and .Dger npoyitirm, pp. 505-522, 2003;
and A. M. Streicher, .1. G.. J. Speer, D. K. Matlock, and B. C. De Coornon, "Quenching and Partitioning Response of a Si-Added TR..IP 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 austcnite 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 et 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 (f,,, =1¨ e-I-Ixi 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 Andrew's linear expression):
[0024] Ms( C)= 539¨ 423C ¨ 30.4/167¨ 7.5Si +30Al [0025] 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 austenite. Therefore, for the higher quench temperatures, increasing amounts of fresh martensite form during the final quench. For quench temperatures below the temperature having a maximum amount of retained austenite, an unsatisfactoty 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.
100261 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.
100271 In some embodiments, the steel sheet may be heated above Ms 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. fly 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.
100281 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.

100291 FIci. 2 shows an alternative embodiment of the thermal cycle described above with respect to FIG. I (with a typical galvanizingigalvarmealing 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 austenitc. Of course, like the process of FIG. 1, the present embodiment may also include a peak metal temperature in excess of A.
100301 Neltt,,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 austcnite formed at the peak metal temperature (42) into martcnsite, thus avoiding excessive transformation to non-ntartensitic constituents such as ferrite, pearl ite, banite, andior the like, [0031] The quench (44) may be then ceased at a quench temperature (46).
Like the process of FIG, I, quench temperature (46) is below WL. 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 Ms may be sufficiently great to form an adequate amount of martensite yet be sufficiently low to avoid consuming too much austenite.
100321 The steel sheet is then subsequently reheated (48) to a partitioning temperature (50, 52), Unlike the process of PLO. 1, the partitioning temperature (50, 52) in the present embodiment may be characterized by the galvanizing or galvannealing zinc bath temperature (if galvanizing or galvanncaling 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 i.used. the process may he substantially the same with the exception of a higher bath/partitioning temperature (52).
[00331 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.
100341 In some embodiments the steel sheet may include certain alloying additions to improve the propensity of the steel sheet to form a primarily aiistenitie 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, 0A0,5% molybdenum, 0-0.05%
niobium, other incidental elements, and the balance being iron, 100351 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.
10036) 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-martensitie constituents (e.g., bainite, ferrite, pcarlite), 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-martensitie constituents near the core oldie 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..

109371 in some embodiments manganese may provide additional stabilization of austenite by lowering transformation temperatures of other non-martensitic constituents* as described above. Manganese may further improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure by increasing hardenabitity.
100381 in other embodiments molybdenum may be used to increase hardetuability, [00391 In other embodimentS silicon.and/or aluminum may be provided to reduce the formation of carbides, Ti 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.
'rhos, silicon and/or aluminum additions may be used to further stabilize austenite at room temperature.
[00401 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 shect.
[00411 ln sonic embodiments niobium (or other micm.alloyingelementsõ such as titanium, vanadiurn, 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 earbidefortnation.
100421 In other embodiments, variations in the concentrations of elements and the particular elements selected may be made. Of course, where such variations arc made, it should be understood that such variations may have a.desirable or undesirable effect on the.steefsheet microstructure and/or mechanical properties . in accordance with the properties described above for each given alloying addition.

EXAMPLE I
100431 Embodiments of the steel sheet were made with the compositions set forth in Table 1 below.
[00441 The materials were processed on laboratory equipment according to the following parameters. 'Each sample was subjected to Caceble 1500 treatments using copper cooled wedge grips and the pocket jaw fikture. Samples were austenitized at 1100 C and then cooled to room temperature at various cooling rates between (-100 Us;

Table 1 Cbcotical compe5iiionsin weight %.-1 _______________________________________________________________ i ID - Description I AI I C Co Cr Cu Mn Rio Nb ' Ni I P - Si Sri -1 Ti I V - W
I I
.
V4037i Lab Material 141 I Ø19 1 - -001 <aim 1.54 <0.003 <0003 <0.003 <0.003 1 011 <0_003 0.01 I <0.003 - .1 , _______________________________________________________________ V40313 : Lab Material I 1.29 :0.22 , - 0.20 <0.003 1.08 <0,003 0,02 <0.003 0.02 i 0.01 <0.003 0.01 i <0.003 -...
. V4039 - Lab Material I <0.003 0.20 <0.002 . 0.01 <0.002 2.94 <0.002 - 0.00 <0.002 0.00 i 1.57 <0.002 0.01 ; <0,002 0.00 >

N.) tC) CO
.1=.

I-N) 1-`
in o If (.,..) o 100451 The Rockwell hardness of each of the steel:compositions described in Example and Table I above was taken on the surface of each sample. The results of the tests arc 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.

100461 Light optical micrographs were taken in the longitudinal through thickness direction near the center of each sample for each of the compositions of Example I. 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 Ibr each composition with each micrograph representing a sample subjected to a different cooling rate.

[00471 A critical cooling rate for each of the compositions of Ekample 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 avoid the formation of non-martensitic transformation products.

The results of these tests are as follows:
100481 V4037: 70 CJs 10049] V4038: 75 'Cis [00501 V4039: 7 C/s = EXAMPLE 5 100511. 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 or 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 Fla 1. As a result V4039 was subjected to annealing at 600 C. for 2 hours in 100%1.12 atmosphere=after hot rolling, but prior to cold rolling. All materials were reduced during cold rolling. about 75% to imm. Results for some of the material compositions SOL forth in Table 2 after hot rolling and cold rolling are shown in Tables 3 and 4, respectively.

. , 1 1 , 1-1 t.
Ig N ':"':, , Cil 03 61 a N. N.
i 0 0 0 0g a I 0,0 . . 011 %8 8 8 ia 0 ___________________________________________ ¨ ¨ ¨
-2 a a a '..-j 8 a a a 8 0 a 6 0 9 o F., 6 _ ---------------- ¨
i an el 0) d 6 6 0 0 0 0 t:' 5,,..) , m aa ifi g 0 40 N. 0 r..a o0c.a c4 ca a 0000:,i;1(c`gf f i t.., :...4 v V V
al g 'E . , -i q 0 a 0 d 0 F. ________________________________________________________ tea t=-1 ,S
<5 6 a-: 6 'fi a-7 ,-: . ,.: ,-: Q g 6: a g g ,., 71 --+.--------e A c I 2 ,ct. 2 X 3 2 8 4.1 01 2 ..; ,-, 1.1 N "
0 0 a 0 6 6 _ 0 0 0 0 0 6 6 0 d 0 (a '. .0 (a 0 ,0 -- ,ro ,0 .5. al .0 Ir 9aa 'ai'gf,47,SEtE2.

a 3 3 3 3 3 3 3 5 3 3 3 3 2 f ..-- ry N. I-- 1.1 co al En t, to C).-- ssi 4, 2 2 8 1 i 2 8 F F! 8 f 8N-I
F

-fable 3- Teinsile Dols, Post Hot Roiling , Yield Strength . Total Uniform Upper YS 1 Lower YS offset UTS Hardness Heat YPE (%) Elongation Elongation HRA
MPa ksi 1 MPa I ksi MPa ksi IlaPn ksi (2.1 %

, 0 NIA ____________ N/A I N/A -1 N/A 375 54 652 95 26 15 V4063 0 N/A 1 N/A ' N/A WA 380 55 648 94 26 .

V4039. 0 NIA NIA NIA N/A 640 93 1085 157 .. 14 t--- 9 1 67 V4339' 0 NIA N/A N/A
NIA 603 88 748 109 20 I 10 i 51 (annealed) -4 ______________________________________________________________ 63 0.6 645 94 637 92 633 92 883 128 20 11 --1 0.5 610 89 1 605 88 611 89 876 127 22 12 1 ...
V4062 I 1.1 507 74 _ 501 73 5C6, 73 712 103 I 0.7 505 73 502 -73 502 73 713 103 24 ¨12 .. 57 V4078-1 1 0.8 427 62 416 60 425 __ 62 594 86 32 18 [ 51 0 V4078-2 I 0.6 525 76 I 519 T7r- 525 76 685 99 I 21 I I 56 >
V4049-1 I 1,8 364 53 361 52 I 361 52 ________________________ 544 79 30 17 48 o V4079-2 1 1,2 497 72 481 70 I 489 71 I 639 93 , 24 I 13 52 N.) VD
=
[ensile lest performrd in uansversc direction for V4039 o co .h.
tg I-N) Table 4 -Tensile Data, Post Cold Rolling 1-`
Ul i Yield o Total to Strength ISIS Uniform Hardness I
Heat 0.2% Onset ____ Elic"gali" Elongation Vs HRA (.,..) _______________ MPa Icsi MPa i ltsi o V4037 927 i 134 971 I 141 4.9 1.4 64 V4063 1046 1 152 1101 1 160 2.4 1.3 65 V4038 1001 146 1054 1 153 5.5 1,6 .. 65 V4039 1149 167 12161 176 4.4 1.5 68 1.14060 1266 184 1393 102 5.4 119 69 V4061 1187 1 172 1-7-127-9 186 4.3 1.7 68 V4062 1111 161 1185 172 4.3 1,7 66 V4078-1 1047 j 152 1105 160 3.5 1.4 .. 65 V4075-2 1154 167 1209 1 175 4,2 1.4 .. 66 V4079-1 1_ 932 i 135 1 975 141 4.5 1.4 64 V4079-2 1034 ' 150 I 1078 , 156 3.9 1.3 66 [00531 The compositions cif Example 5 were subjected to Gleehle dilatomety.
feeble dilatomety was performed in vacuum using a l01.6x25.4x1 nun 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 it to. the di latometrie data and the point at which the dilatometric data deviated from linear behavior was taken as the transformation temperature of interest (e.g., Ai, A3, Ms). The resulting transformation temperatures are tabulated in Table 5.
(11054I Gleeble methods were also used to measure a critical cooling rate for each of the compositions of Example 5. The first method utilized Glecble dilato.mety, as described above. The second method utilized measurements Of Rockwell hardness.

In particular, after samples were subjected to Oleeble testing at range of coo Ihig 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 'IRA Were considered significant. The critical cooling rate to avoid non-martensitie transformation product Was taken as the highest cooling rate for which the hardness was lower than 2 point URA than the makirninn hardness. The resulting critical cooling rates are also tabulated in Table 5 for some of the compositions listed in Example 5.
rod,: 5- Transinromion Trinpersturc$ mul (Altai{ Cooling Role from Gleale Dilanrnely Critical Conlin Mlle !fent As en A, CO KM ('CA) Olaibie cchIci Diiatnindry Hardon,Ts_ ' r-)/4037 737 970 409 loconcluskie 05 , 9403/1 791 9110 441 (;5 V1039 750 874 39.1 <10 V.101,1 , 075 900 325 40 i 33 V1074- I 750 92.5 450 40 V4078-2 799. I um, 4,15. 40 V*79-1 801 1 1000 430 .10 V4079-2 .r 759 990 425 r 40 =

100551 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 etaL described above. The results of the calculations are tabulated below in Table 6 for some of the compositions listed in Example 5, Table 6 ¨ Qfituclacmperalm'e Rod Thearelleal Maxim um of Hard ited Austenite Thecrelical 1101 QT (C) m --shunt' . 0,15 V4063 27S cus V4039 270 0.15 V4060 191 035 .
V4061 196 I0.35 V4062 237 = 0;31 V40'7S- 276 276 0.16 V4179-I 273 0.16 V4979-2 272 0.16 1006] The samples of the compositions of Example 5 were subjected to the thermal profiles shown in FIGS. I 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. 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 teinperatinVand FIC3S, 11-12 show tensile strength data plotted against quench temperature. Additionally, where the thermal cycles were performed using cilcchle 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 tbr 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.

'forge '7 -ikoisito 011A, Post Por66oning Peak Heat Quench 0,2% Ultimate Total Metal Partitioning Partitioning TE x UTSIMpa Temp Yield Tensile Elongation Temp . Temp irC) Time {0 x %1 Strength Strength (%) _____ 800 250 466- . 30 419 818- 27 22 424 800 250 466 30. ' 416 807 28 -- 22,345 _____ 8'5-0 250 466 30 553 862 25 21,805 - --350 250 466 30 __ 535 847 25 __ 21,338 V1307 __ 74.5-0- - j'5'0-- - 465 r 30 548 854 -- 24 -- 29 144 ' --no - --- 256 4-6-0 30 445 898 22 19675 '- _ t 900 250 466 30 566 856 23 ' 19,594 .....
800 - 250 400 30 432 889 22 19,478 800 160 466- 15 746 1317 , 0 29630 SOO 200 466 15 716 1332 19 25,309 _ 800 250 466 15 718 1403 18 " 25,115 V40 800 200 466 15 632 1300 19 - 24,746 800 250 466 15 __ 701 1379 18 .L 24,407 800 160 466 15 845 1311 18 25,9158 , 850 , 250 466 15 891 1291 18 '-----i3,749 i 850 250 466 15 735 1223 __ 19 23,729 - _ 850 300 466 i 15 . 443 =657 32 29,763 921 1 200, 1 466 ________ 30 325 _ 612 34 20633 350 250 466 15 403 698 50 20,543 -_____ 921 300 1 30 380 591 34 -- 20,090 ,...._ 921 356 466 30 , 385 592 34 20,078 , 940 ________ 200 J 466 36 362 598 33 19,906 V4037 850 _________________ 200 I 466 - 15 427 587 28 19,022 940 200 ' 466 30 353 592 32 18,989 980 200 466 30 341 612 31 13,897 900 I- 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 30 347 011 30 18,387 -940 200 466 30 330 548 33 18,253 -980 200 I 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 , V4038 1-- g 18 200 468 30 363 697 26 18,193 918 300 468 30 _ 354 696 26 17,949 850 300 468 15 457 773 23 _17 777 1 800 250 400 60 821 1299 15 19,225 , 800 250 400 GO 821 1298 15 18,045 V4039 900 250 400 60 923 1273 15 18,593 2 850 ' ' 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 1 18 25,115 800 200 466 f5 632 1309 19 24,746 800 250 466 15 1---'731 1379 18 24 407 V4060 ' -KO 160 466 15 845 1311 18 23,986 850 250 __ 466 15 891 1291 18 23,749 850 ___ 250 466 15 735 1223 19 23,729 890 200 468 30 942 1319 17 22,422 850 260 - 7- 4613 15 __ '---95 1222 16 19,070 750 250 468 15 553 985 20 19,962 V4061 -- :
_________ -750- - 250 - - 4613 ' - 15 581 910 21 16,9ps_ 750 200 466 15 478 613 23 , , V4062 750 250 465 15 480 -8'16 22 17.944 ,..._ 750 - 200 466 15,_, 55êT 790 23 17,936.-, V410710 __ 1 250 400 60 776 -T 1382 13 -r 17,824 ... ....._ 900 250 400 60 923 '__I 1642 11 17A01 V4108 850 250 400 60 . __ 952 1620 11 17,337 V4078-1 1850 300 466 15 448 783 24 19,016 i 850 ' 300 466 15 492 761 24 17,888 , 900 250 466 30 713 843 - if 17,946 V4078-2 850 300 466 15 689 859 20 17,525 ______ 850 300 - 466 -, 15 671 871 20 17,503 f00581 It will he undo stood various modifications may be made to this invention without departing from the spirit and scope of it. Therefore, the limits of this invention should be &termined from the appended

Claims

What is claimed is:

1. A method for processing a steel sheet, the steel sheet comprising the following elements by weight percent:
0.15-0.5% carbon;
1-3% manganese;
2% or less silicon, aluminum, or some combination thereof;
0.5% or less molybdenum;
0.05% or less niobium; and thc balance being iron and other incidental impurities;
the method comprising the steps of:
(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 (12) by cooling at a cooling rate, wherein T2 is between 160 and 400° C and 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 efitical 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;
(c) re-heating the steel sheet to a partitioning temperature, wherein the partitioning temperature is sufficient to penii it diffusion of carbon within the structure of the steel sheet;
(d) 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;
(e) subjecting the steel sheet to a hot dip galvanizing or galvannealing process during the step of stabilizing 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
s.

4. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.15-0.4% carbon;
1.5-4% 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.

5. A method for processing a steel sheet, the steel sheet comprising the following elements by weight percent:
0.15-0.5% carbon;
1-4% 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 the steps of:
(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 (12) by cooling at a cooling rate, wherein T2 is between 160 and 400° C and is below the martensite start temperature (Ms), wherein the cooling rate is sufficiently rapid to transform austenite to martensite, wherein the cooling rate is sufficiently rapid such that the formation of bainite and other non-martensitic transformation products are substantially suppressed;

(c) 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;
(d) stabilizing austenite by holding the steel sheet at the partitioning temperature for a holding time sufficient to permit diffusion of carbon from martensite to austenite;
(e) subjecting the steel sheet to a hot clip galvanizing or galvannealing process during the step of stabilizing austenite; and (f) cooling the steel sheet to room temperature.

6. The method of claim 5, wherein the steel sheet comprises 1-3% manganese.

7. The method of claim 5 or 6, wherein the partitioning temperature is above Ms.