METHOD FOR MANUFACTURING A HOT-FORMED ARTICLE, AND OBTAINED ARTICLE

Abstract

A method for hot-forming a steel blank into an article including the steps of: a. heating a steel blank to a temperature T1 and holding the heated blank at T1 during a time period t1, wherein T1 is in the range of Ac1 to Ac3+200° C. and wherein t1 is at most 12 minutes, b. transferring the heated blank to a hot-forming tool during a transport time t2 during which the temperature of the heated blank decreases from temperature T1 to a temperature T2, wherein T2 is above Ar1 and wherein the transport time t2 is at most 12 seconds, c. forming the blank in the hot-forming tool into an article and quenching it in the hot-forming tool from a temperature T2 to a temperature T3 at a cooling velocity V2 of 25° C./s or more, d. isothermal holding the article at a temperature T4 for a time period t4, e. wherein temperature T3 and/or temperature T4 is between Ms and Mf and wherein t4 is more than 10 seconds and less than 10 minutes, f. cooling the article from temperature T4 to room temperature at a cooling velocity V4.

The invention also relates to a hot-formed article obtained by the method.

Claims

1. A method for hot-forming a steel blank into an article comprising the steps of: a. heating a steel blank to a temperature T1 and holding the heated blank at T1 during a time period t1, wherein T1 is in the range of Ac1 to Ac3+200° C. and wherein t1 is at most 12 minutes, b. transferring the heated blank to a hot-forming tool during a transport time t2 during which the temperature of the heated blank decreases from temperature T1 to a temperature T2, wherein T2 is above Ar1 and wherein the transport time t2 is at most 12 seconds, c. forming the blank in the hot-forming tool into an article and quenching it in the hot-forming tool from temperature T2 to a temperature T3 at a cooling velocity V2 of 25° C./s or more, d. isothermal holding the article at a temperature T4 for a time period t4, e. wherein temperature T3 and/or temperature T4 is between Ms and Mf and wherein t4 is more than 10 seconds and less than 10 minutes, f. cooling the article from temperature T4 to room temperature at a cooling velocity V4, wherein 1. T3 is between Ms and Mf and wherein T4 is between Bs and Ms, or 2. T3 is between Bs and Ms and wherein T4 is between Ms and Mf, or 3. both T3 and T4 are between Ms and Mf, and wherein the microstructure of the hot-formed article is a complex phase microstructure consisting of, by volume fraction (vol. %) (sum up to 100) at least 80 vol. % bainite and tempered martensite, wherein the tempered martensite is less than 50 vol. %, and at most 20 vol. % ferrite and/or martensite, and/or retained austenite.

2. The method according to claim 1, wherein the article is held at temperature T3 during a holding time 3 of 1 to 180 seconds.

3. The method according to claim 1, wherein the article is heated from temperature T3 to temperature T4 at a velocity V3 of at least 15° C./s.

4. The method according to claim 1, wherein the article is cooled from temperature T3 to temperature T4 at a velocity t3 of at most 15° C./s.

5. The method according to claim 1, wherein the article is cooled from temperature T4 to room temperature at a cooling velocity V4, in the range of 0.1-20° C./s.

6. The method according to claim 1, wherein T1 is in the range of Ac1 to Ac3+150, and wherein the steel blank is heated with a heating velocity V1 in the range of 10-25° C./s.

7. The method according to claim 1, wherein the steel blank comprises in wt %: C: 0.10-0.50, Mn: 0.50-4.00, Si: <2.0, Al: <2.0, Cr: <1.5, Ti: <0.10, B: <0.008, Nb: <0.10, and optionally one or more of the elements selected from V: <0.2, Ca: <0.003, N: <0.005, P: <0.015, S: <0.03, Mo: <0.5, Cu: <1.0, Ni: <1.0, the remainder being Fe and unavoidable impurities.

8. The method according to claim 1, wherein the blank, is provided with a zinc based coating or an aluminium based coating or an organic based coating or any other coating designed to reduce oxidation and/or decarburisation during the hot forming process.

9. The method according to claim 8, wherein the zinc based coating is a coating containing 0.5-3.8 wt % Al, 0.5-3.0 wt % Mg, optionally at most 0.2 wt % of one or more additional elements, unavoidable impurities, the balance being zinc.

10. The hot-formed steel article obtained by the method according to claim 1, wherein the steel comprises in wt %: C: 0.10-0.50, Mn: 1.00-3.00 Si: ≤2.0; Al: ≤2.0, Cr: ≤1.5, Ti: ≤0.10, B: ≤0.008, Nb: ≤0.10, and optionally one or more of the elements selected from V: ≤0.2, Ca: ≤0.003, N: ≤0.005, P: ≤0.015, S: ≤0.03, Mo: ≤0.5, Cu: ≤1.0, Ni: ≤1.0, the remainder being Fe and unavoidable impurities, wherein the article has a YS of at least 600 MPa, and a UTS of at least 1000 MPa, and wherein the article has a total elongation (TE) of at least 6% and a bending angle (BA) of at least 45°.

11-15. (canceled)

16. The method according to claim 1, wherein 1. T3 is between Ms and Mf and wherein T4 is between Bs and Ms, or 2. T3 is between Bs and Ms and wherein T4 is between Ms and Mf, or 3. both T3 and T4 are between Ms and Mf, and T3 and T4 are the same, and wherein the microstructure of the hot-formed article is a complex phase microstructure consisting of, by volume fraction (vol. %) (sum up to 100) at least 80 vol. % bainite and tempered martensite, wherein the tempered martensite is equal or less than 40 vol. %, and at most 20 vol. % ferrite and/or martensite, and/or retained austenite.

17. The method according to claim 1, wherein the microstructure of the hot-formed article is a complex phase microstructure consisting of, by volume fraction (vol. %) (sum up to 100) at least 80 vol. % bainite and tempered martensite, wherein the tempered martensite is equal or less than 30 vol. %, and at most 20 vol. % ferrite and/or martensite, and/or retained austenite.

18. The method according to claim 3, wherein the article is heated from temperature T3 to temperature T4 at a velocity V3 of at least 20° C./s.

19. The method according to claim 3, wherein the article is heated from temperature T3 to temperature T4 at a velocity V3 of at least 30° C./s.

20. The method according to claim 4, wherein the article is cooled from temperature T3 to temperature T4 at a velocity t3 of at most 10° C./s.

21. The method according to claim 4, wherein the article is cooled from temperature T3 to temperature T4 at a velocity 3 of at most 8° C./s.

22. The method according to claim 5, wherein the article is cooled from temperature T4 to room temperature at a cooling velocity V4, in the range of 1-15° C./s.

23. The method according to claim 5, wherein the article is cooled from temperature T4 to room temperature at a cooling velocity V4, in the range of 2-10° C./s.

24. The method according to claim 6, wherein T1 is in the range of Ac1 to Ac3+100, and wherein t1 is at most 10 minutes, and wherein the steel blank is heated with a heating velocity V1 in the range of 10-25° C./s.

25. The method according to claim 6, wherein T1 is in the range of Ac3−50 to Ac3+50, and wherein t1 is in the range of 2-8 min, and wherein the steel blank is heated with a heating velocity V1 in the range of 10-25° C./s.

26. The method according to claim 7, wherein the steel blank comprises in wt %: C: 0.15-0.40, Mn: 1.00-3.00, Si: 0.1-2.0, Al: <1.0, Cr: <1.2, Ti: <0.05, B: <0.005, Nb: <0.05, and optionally one or more of the elements selected from V: <0.1 Ca: 0.0003-0.003, N: <0.003, P: <0.015, S: <0.01, Mo: <0.05, Cu: <1.0, Ni: <1.0, the remainder being Fe and unavoidable impurities.

27. The method according to claim 1, wherein the steel blank comprises in wt %: C: 0.15-0.35, Mn: 1.00-2.50, Si: 0.1-1.6, Al: <0.5, Cr: 0.001-1.1, Ti: <0.04, B: <0.005, Nb: 0.001-0.05, and optionally one or more of the elements selected from V: <0.1 Ca: 0.0003-0.003, N: <0.003, P: <0.015, S: <0.01, Mo: <0.05, Cu: <1.0, Ni: <1.0, the remainder being Fe and unavoidable impurities.

Description

[0096] It is a further object of the present invention to provide with an article obtained by any one of the methods described herein.

[0097] FIG. 1 shows a schematic representation of a first embodiment of the method according to the invention.

[0098] FIG. 2 shows a schematic representation of a second embodiment of the method according to the invention.

[0099] FIG. 2 shows a schematic representation of a third embodiment of the method according to the invention.

[0100] In the figures the horizontal axis represents the time t, and the vertical axis represents the temperature T. The hot forming of the blank into the article in the hot forming press is indicated by HF. The time t and temperature T are indicated diagrammatically in the Figures, and so are the cooling and heating velocities V. No values can be derived from the Figures.

[0101] A steel blank is heated up to the austenitizing temperature T1 above Ac.sub.1 at a heating rate of 15° C./s and held at aT1 for a time period t. Then the heated blank is transferred from the furnace to the hot forming press, during which cooling of the blank to temperature T2 by air occurs to some extend. Care is taken that the temperature T2 does not decrease below the Ar1 temperature before the blank is placed in the hot-forming press. The blank is then hot-formed into an article and cooled down to temperature T3 at a cooling rate of >25° C./s and the cooling is interrupted and the article is held at T3 for a time period t3. Thereafter, the three embodiments follow different routes. Finally, the formed article is cooled down to room temperature at a cooling rate V4.

[0102] The values for the temperatures T3, T4 and cooling or heating velocities V3 and V4, and the holding time t3 and t4 depend on the different embodiments as shown in the FIGS. 1-3.

[0103] The different temperatures are explained below.

[0104] Ac1: Temperature at which, during heating, austenite starts to form.

[0105] Ac3: Temperature at which, during heating, transformation of the ferrite into austenite ends.

[0106] Ar1: The temperature at which transformation of austenite to ferrite is completed during cooling.

[0107] Ar3: The temperature at which austenite begins to transform to ferrite during cooling.

[0108] Bs: Temperature at which, during cooling, transformation of the austenite into Bainite starts.

[0109] Ms: Temperature at which, during cooling, transformation of the austenite into martensite starts.

[0110] Mf: Temperature at which, during cooling, transformation of the austenite into martensite ends.

[0111] Critical phase transformation temperatures are determined by dilatometer experiments.

[0112] The invention will be elucidated by means of the following, non-limiting examples making reference to the accompanying figures. Table 1 shows the steel composition used in a method according to the present invention. The tables 2, 3 and 4 give the process parameters and mechanical properties for the different steel types of Table 1, and for the process types of the three embodiments. The results are discussed below.

TABLE-US-00001 TABLE 1 Steel compositions (wt. %). A.sub.c3 M.sub.s Steel C Si Mn Cr Al P S Ti Nb B (° C.) (° C.) B 0.31 1.47 1.66 0.05 0.02 0.006 0.004 0.003 <0.005 <0.0001 865 330 C 0.29 1.55 2.0 0.005 0.008 0.005 0.005 0.004 <0.005 <0.0001 830 310 D 0.26 1.54 1.5 1.05 0.009 0.005 0.005 0.004 <0.005 <0.0001 850 340 E 0.29 0.29 1.23 0.48 0.03 0.012 0.005 0.025 0.031 0.0031 831 360 F 0.26 1.48 1.77 0.003 0.015 0.010 0.008 0.007 <0.005 0.0003 855 340 G 0.25 0.21 2.4 0.003 0.018 0.012 0.009 0.005 <0.005 0.0019 800 330 H 0.22 1.0 2.1 0.004 0.03 0.005 0.004 0.004 <0.005 <0.0001 845 375 J 0.31 1.53 1.98 1.04 0.011 0.01 0.01 0.027 <0.005 <0.0001 805 285

TABLE-US-00002 TABLE 2 Process parameters and mechanical properties of process type 1 T1 t1 T2 t2 T3 t3 T4 t4 YS UTS TE BA Steel ° C. min ° C. s ° C. s ° C. s MPa MPa % ° Note* B1 900 6 25 0 25 0 1147 1881 4.5 35 Ref. B2 900 6 780 6 260 5 350 60 907 1371 7.2 63 Inv. B3 900 6 780 6 200 5 350 60 1046 1337 7.7 68 Inv. B4 900 6 780 6 220 5 350 60 736 1314 9.2 70 Inv. B5 900 6 780 6 220 5 450 60 574 1058 22 79 Inv. B6 820 6 780 5 260 5 380 600 856 1319 15 75 Inv. C1 880 6 25 0 25 0 1283 1941 6.2 41 Ref. C2 880 6 760 6 300 5 400 30 1252 1782 7.8 55 Inv. C3 880 6 760 6 300 5 400 60 1319 1718 9.1 55 Inv. C4 880 6 760 6 300 5 400 120 1184 1511 9.8 52 Inv. C5 880 6 760 6 300 5 400 180 1412 1606 6.6 62 Inv. D1 880 6 25 0 25 0 1269 1865 5.7 47 Ref. D2 880 6 760 6 300 5 400 30 1184 1714 8.7 53 Inv. D3 880 6 760 6 300 5 400 60 1116 1564 7.6 50 Inv. D4 880 6 760 6 300 5 400 120 1341 1596 6.2 57 Inv. D5 880 6 760 6 300 5 400 180 1362 1590 6.9 55 Inv. E1 900 2 25 0 25 0 1101 1671 5.8 60 Ref. E2 810 2 750 5 340 0 400 180 509 1166 11.9 65 Inv. *V2 = −40° C./s; V3 = 20° C./s; V4 = −5° C./s.

TABLE-US-00003 TABLE 3 Process parameters and mechanical properties of process type 2 T1 t1 T2 t2 T3 t3 T4 t4 YS UTS TE BA Steel ° C. min ° C. s ° C. s ° C. s MPa MPa % ° Note* D6 880 6 760 5 350  30 320 120 1210 1590 7.1 50 Inv. D7 880 6 760 5 350  60 320 120 1132 1523 8.5 63 Inv. D8 880 6 760 5 350 120 320 120 1076 1456 9.2 66 Inv. D9 880 6 760 5 350 180 320 120 1053 1415 10.5 70 Inv. *V2 = −40° C./s; V3 = −3° C./s; V4 = −5° C./s.

TABLE-US-00004 TABLE 4 Process parameters and mechanical properties of process type 3 T1 t1 T2 t2 T3 = T4 t3 + t4 YS UTS TE BA Steel ° C. min ° C. s ° C. s MPa MPa % ° Note* F1 900 2 25 0 1115 1661 6.5 43 Ref. F2 900 2 860 5 310 20 1074 1619 9.0 52 Inv. F3 900 2 860 5 310 30 950 1515 9.3 52 Inv. F4 900 2 860 5 310 60 968 1533 11.2 66 Inv. F5 900 2 860 5 310 120 984 1482 8.0 79 Inv. F6 900 2 860 5 270 20 1112 1651 8.9 45 Inv. F7 900 2 860 5 270 30 863 1455 8.9 54 Inv. F8 900 2 860 5 270 60 983 1547 9.1 56 Inv. F9 900 2 860 5 270 120 896 1438 8.0 62 Inv. G1 850 2 25 0 1129 1639 6.2 62 Ref. G2 850 2 730 5 300 20 1076 1577 7.6 64 Inv. G3 850 2 730 5 300 30 1063 1527 6.3 72 Inv. G4 850 2 730 5 300 60 1032 1493 7.1 80 Inv. G5 850 2 730 5 300 120 964 1433 7.9 88 Inv. G6 850 2 730 5 260 20 1072 1589 7.7 61 Inv. G7 850 2 730 5 260 30 1088 1579 7.2 68 Inv. G8 850 2 730 5 260 60 1051 1543 7.2 76 Inv. G9 850 2 730 5 260 120 992 1527 7.8 79 Inv. H1 900 2 25 0 1050 1450 6.0 41 Ref. H2 900 2 860 5 300 40 923 1489 8.2 52 Inv. H3 900 2 860 5 300 60 962 1499 8.2 50 Inv. H4 900 2 860 5 340 20 1028 1502 8.8 56 Inv. H5 900 2 860 5 340 40 951 1437 8.7 52 Inv. H6 900 2 860 5 340 60 897 1434 8.8 56 Inv. I1 900 2 25 0 980 1402 5.9 43 Ref. I2 900 2 860 5 300 20 865 1376 6.5 88 Inv. I3 900 2 860 5 300 40 755 1371 8.4 92 Inv. I4 900 2 860 5 300 60 1033 1360 7.0 79 Inv. *V2 = −50° C./s; V4 = −5° C./s

[0113] Examples with Steel Compositions B, C and D

[0114] Steel blanks with dimensions of 200 mm×110 mm×1.5 mm have been prepared from a cold-rolled steel sheet having the composition B or C as given in Table 1. The Ac3 and Ms temperatures were determined using dilatation tests.

[0115] The blanks were first heated at 880° C. (T1) in a box furnace for 6 min and then transported to a hot-forming apparatus. The hot-forming was performed in laboratory scale by Schuler SMG company, Germany (hereafter SMG press). The SMG press tools were preheated to a temperature of 450° C. or 300° C. (T3). The blanks were transferred to the SMG press in 10 s and press-quenched to T3 temperature into an article. The article was transferred to a muffle furnace preheated at a temperature of 350° C. or 400° C. (T4) and austempering or partitioning and/or tempering heat treatment was applied (Q-P-T treatment).

[0116] The experimental results with respect to the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UL) total elongation (TL) and banding angle (BA), are given in Table 2 and Table 3. The tensile properties were measured in the samples with the stress direction being parallel to the rolling direction. The 3-point “guided bending tests” were conducted on samples with dimensions 40 mm×3025 mm. The length direction of the samples was parallel to the rolling direction of steel sheets. Parallel bending tests where the bending axis is perpendicular to the rolling direction of the sheets were carried out. For this method, a former and two supporting cylinders were used in order to bend the steel sheets. The cylinders and the punch were mounted in a tensile testing machine. The load cell is used to measure the punch force and the displacement of the crosshead gives the punch displacement. The experiments were stopped at different bending angles and the bent surface of the specimen was inspected for identification of failure in order to determine the bending angle.

[0117] Examples with Steel Composition E

[0118] The present inventive method has been applied to a steel composition E, as shown in Table 2, similar to the known 22MnB5, but with a C content higher than from this of 22MnB5. The Ac1, Ac3, and Ms temperatures were determined using dilatation tests. A cold-rolled steel sheet having the composition E and a thickness of 1.5 mm has been prepared and blanks with dimensions of 600 mm×110 mm were cut. A thermal cycle according to the invention was applied to simulate the hot press forming process using a continuous annealing simulator (CASIM). The blanks were first heated to a variable soak temperature of 900° C. or 810° C. (T1) and held at said temperature for a soak time of 2 min (t1). The transfer of the heated blanks from the furnace to the press forming apparatus was simulated by cooling down slowly the blanks to 750° C. (T2) and at a cooling rate of 3° C./s. The blanks are cooled down to a temperature T3 at a cooling rate of 40° C./s (V2) and isothermal held for t3 and subsequently cooled down to room temperature at a cooling rate of 3.5° C./s (V4). In said experiments T3=T4. The process parameters, the tensile properties and the bending angle are given in Table 2.

[0119] Examples with Steel Compositions F, G, H, I and J

[0120] Steel compositions F, G, H, I and J according to table 1 have be used. Steel blanks with dimensions of 600 mm×110 mm×1.5 mm or 230 mm×110 mm×1.5 mm have been prepared from a cold-rolled steel sheets having the composition F, G and H, I, J respectively as given in Table 1.

[0121] The blanks of steel F and G were heat treated in a Continuous Annealing Simulator (CASIM), those of steels H, I and J in a Hot Dip Annealing Simulator (HDAS). Whatever be the apparatus used for heat treatments, it was ensured that the thermal cycles were simulated accurately.

[0122] The blanks of steels F and G were first heated to 900° C. and 850° C. (T1) respectively in CASIM and soaked for 2 min (t1). Then the blanks were cooled to 860° C. (for T1=900° C.) and 730° C. (for T1=850° C.) in 10 s to simulate the transfer of the blanks from reheating furnace to the hot press (T2). Then, either from 860 or from 730° C., the blanks were cooled at a rate of 40° C./s to an isothermal holding temperature below the Ms of these steels (here T3=T4 and the two isothermal steps are combined into one step). Then, the blanks of steel F were isothermally held at 310 and 270° C. for time spans of 0, 20, 30, 60 and 120 s (t3=t4) and then cooled to room temperature at a rate of 5° C./s. On the other hand, the blanks of steels H and I were isothermally held at 300 and 260° C. for time durations of 0, 20, 30, 60 and 120 s (t3=t4) before cooling to room temperature at a rate of 5° C./s.

[0123] The blanks of steels H and I were heated to 900° C. (T1) in HDAS apparatus and soaked for 2 min (t1). Then the blanks were cooled to 860° C. (T2) in 10 s to simulate the transfer of the blanks from reheating furnace to the hot press, before cooling them at a rate of 50° C./s to 300 or 340° C. (here also T3=T4 and the two isothermal steps are combined into one step). These two temperatures are below Ms of steels H and I as evident from Table 1. Then, the blanks of steels H and I were isothermally held at either 300 or 340° C. for time spans of 0, 20, 40 and 60 s (t3=t4) following cooling to room temperature at a rate of 5° C./s.

[0124] For the blanks of steel J the following heat treatment procedures were followed in HDAS. The blanks were heated to 900° C. (T1) in HDAS apparatus and soaked for 5 min (t1). Then the blanks were cooled to 860° C. (T2) in 10 s to simulate the transfer of the blanks from reheating furnace to the hot press, before cooling them at a rate of 50° C./s to 300, 325, 350, 375 and 400° C. (here also T3=T4 and the two isothermal steps are combined into one step). These temperatures lie above the Ms of steel J as can be seen from Table 1. Then, the blanks were isothermally held at those temperatures for time spans of 0, 600, 1800 and 3600 s (t3=t4) following cooling to room temperature at a rate of 5° C./s.

[0125] It is to be mentioned that for the blanks of steels F, G, H and I the processes described above are basically one-step quenching and partitioning (Q&P) process since T3=T4<Ms. For steel J, it is austempering process (T3=T4>Ms) which was used for bainitic transformation to take place. When t3 (=t4 as well) was 0 s, this specific blank basically represent the reference sample which experienced the thermal cycles of a standard hot forming, i.e. without application of the Q&P or austempering step. The process parameters and mechanical properties of steels are given in Table 4.

[0126] The tensile properties and bendability of the steels are presented in Table 2 through 4 along with their heat treatment process parameters. According to thermal cycles applied, steels C, D, E have gone through two-step low temperature process combining bainitic and martensitic transformation, tempering and partitioning (Table 2). Steels F, G, H and I have undergone one-step heat treatment below Ms causing martensitic transformation and partitioning to take place (Table 3). In steel J, only bainitic transformation took place during the one-step austempering step (Table 4).

In the reference samples in Tables 2-4, predominantly martensitic microstructures formed as usual for standard hot forming process. The microstructure developments in the steels due to the modified process of this invention are described below.

[0127] In steel C, in case of conditions C1 to C5, during first isothermal holding at T3, bainitic transformation took place and during the second holding at T4, again bainitic transformation took place enriching the remaining austenite in carbon and increasing the effective bainite content in the final microstructure. For conditions C6 to C9, during first holding (T3) a small amount of martensite formed as T3 was below the Ms of steel C. Then during second holding at T4, bainitic transformation took place in an accelerated manner as presence of small amount of martensite is known to accelerate bainitic transformation kinetics. During final cooling to room temperature, in all the conditions high amounts of retained austenite were obtained with some amounts of fresh martensite. For steel D, in conditions D6 to D9 similar phase transformations as in C6 to C9 took place except that more amount of martensite was formed at T3. Additionally, in conditions C6 to C9 and D6 to D9 tempering of the initially formed martensite and carbon portioning from this martensite to austenite would take place.

For steel D, in conditions D1 to D5 during first isothermal holding bainitic transformation took place at T3, and then at T4 some amount of martensite formed. At T4, tempering of this martensite and carbon portioning from martensite to austenite occurred. During final cooling to room temperature, some fresh martensite formed and some austenite remained untransformed. For steel E, in condition E2, the austenitizing temperature T1 is in the intercritical range and one step austempering took place during isothermal holding (T3=T4), with carbon enrichment in austenite. The final microstructure includes ferrite (15 vol. %), bainite/tempered martensite and retained austenite. The presence of ferrite increases the elongation but at expense of strength. Certain amount of austenite transformed to martensite in steels F, G, H and I after quenching to the isothermal holding temperature (T3=T4) since this temperature is below the Ms of the respective steels. The amount of this initial martensite varied with the temperature of quenching—a lower temperature in a particular steel would form a higher amount of initial martensite. The well-known Koistinen-Marburger formula can be used to estimate this initial martensite fraction. Then during the isothermal holding the martensite would temper, and at the same time carbon will partition from martensite to austenite. Carbon partitioning will be more for higher Si content in steel and vice versa. Then during final cooling to room temperature some fresh martensite will form depending on the carbon enrichment in austenite, and some amount of austenite will remain as retained austenite.
For steel J, the isothermal holding temperatures (300-400° C.) were above the Ms of the steel. Therefore, during this austempering bainitic transformation took place —primarily carbide free bainite formed causing carbon enrichment in austenite. After isothermal holding, on cooling the blanks to room temperature, some martensite formed and certain amount of austenite remained untransformed due to carbon enrichment during austempering.

[0128] From Tables 2-4, it is evident that due to the described different multiphase microstructures both the total elongation and bending angle of the steels in the innovative processes are improved in comparison with the reference conditions, and most importantly these are higher than reference values for steel I (Table 3) which is standard 22MnB5 steel subjected to standard hot forming thermal cycle in its reference condition. This is beneficial for enhanced energy absorption capacity of the hot stamped steels in use.