PROCESS FOR PRODUCING AN AT LEAST PARTLY QUENCHED AND TEMPERED SHEET STEEL COMPONENT AND AT LEAST PARTLY QUENCHED AND TEMPERED SHEET STEEL COMPONENT

Abstract

The invention relates to a process for producing an at least partly quenched and tempered sheet steel component, where the process comprises the following steps: providing a sheet steel, at least partly austenitizing the sheet steel at a temperature of at least Ac1, at least partly hardening the at least partly austenitized sheet steel to give an at least partly hardened sheet steel component, where the at least partly austenitized sheet steel is cooled to a temperature below Ms, at least partly annealing the at least partly hardened sheet steel component at a temperature of less than Ac1 for producing an at least partly quenched and tempered sheet steel component. A further subject of the invention is an at least partly quenched and tempered sheet steel component.

Claims

1. A process for producing an at least partly quenched and tempered sheet steel component, where the process comprises the following steps: providing a sheet steel; at least partly austenitizing the sheet steel at a temperature of at least Ac1; at least partly hardening the at least partly austenitized sheet steel to give an at least partly hardened sheet steel component, where the at least partly austenitized sheet steel is cooled to a temperature below Ms; and at least partly annealing the at least partly hardened sheet steel component at a temperature of less than Ac1 for producing an at least partly quenched and tempered sheet steel component; wherein the at least partial annealing for producing the at least partly quenched and tempered sheet steel component is carried out at different temperatures in order to establish regions having different properties on the at least partly quenched and tempered sheet steel component, where the at least partial annealing, for generating a region having a first property on the at least partly quenched and tempered sheet steel component, is carried out at a first annealing temperature TP1 between 300° C. and 470° C. and, for generating at least one further region having a further property, is carried out at least one of the following annealing temperatures TP2, TP3, TP4: region having a second property: at a second annealing temperature TP2 between 250° C. and 430° C. with TP2<=TP1−10° C.; and region having a third property: at a third annealing temperature TP3 between 470° C. and less than Ac1; and region having a fourth property: at a fourth annealing temperature TP4 up to 300° C.

2. The process as claimed in claim 1, where the at least partial annealing is carried out temporally immediately after the hardening.

3. The process as claimed in claim 2, where between the regions having different properties on the at least partly quenched and tempered sheet steel component, one or more transition regions are established which have a harmonic transition between the regions having different properties.

4. The process as claimed in claim 3, where a sheet steel having the following chemical composition in wt % is provided: C=0.08 to 0.5; Si+Al>=0.5, with Si+2*Al<5; and Mn=0.5 to 4; balance Fe and unavoidable impurities.

5. The process as claimed in claim 4, wherein the steel sheet has one or more alloy elements from the group (P, S, N, Cr, Mo, Ti, B, Nb, V, Ni, Cu, Sn, Ca, Mg, REM): P up to 0.1; S up to 0.1; N up to 0.1; Cr up to 1.5; Mo up to 1; Ti up to 0.2; B up to 0.01; Nb up to 0.2; V up to 0.5; Ni up to 2; Cu up to 2; Sn up to 0.5; Ca up to 0.1; Mg up to 0.1; and REM up to 0.1; wherein the sheet steel comprises at least one of Cr with at least 0.01 wt % and Mo with at least 0.01 wt %, where Cr and Mo, individually or in combination with Mn, meet the following condition: Mn+Cr+2*Mo>=1 wt %.

6. The process as claimed in claim 5, where the sheet steel is provided as a flat blank or as a preformed part.

7. The process as claimed in claim 6, where the sheet steel is one of hot-rolled and cold-rolled, the steel sheet containing less than 10% of ferrite grains having an equivalent diameter >50 μm.

8. The process as claimed in claim 7, where the at least partial hardening is carried out in at least one press-hardening tool.

9. The process as claimed in claim 8, where, before the at least partial hardening, the at least partly austenitized sheet steel is hot-formed in at least one hot-forming tool.

10. The process as claimed in claim 9, where the at least partial hardening is carried out in at least one press-hardening tool or additionally in the at least one hot-forming tool, which is actively cooled.

11. The process as claimed in claim 8, where the at least partial annealing is carried out in at least one annealing tool which has at least two differently temperature-conditioned regions.

12. An at least partly quenched and tempered sheet steel component, produced more particularly as claimed in claim 1 wherein the at least partly quenched and tempered sheet steel component has regions having different properties: a first region having a first property, comprising a microstructure with residual austenite between 3% and <35%, 35% to 97% martensite, up to 30% bainite and unavoidable structure constituents, and at least one further region having a further property, comprising at least one of the following properties: a second region having a second property, comprising a structure having a residual austenite fraction which is lower by comparison with the first region, balance martensite and optionally bainite and unavoidable structure constituents; and a third region having a third property, comprising a structure having a residual austenite fraction which is lower by comparison with the first region and, if present, by comparison with the second region, balance martensite and optionally bainite and unavoidable structure constituents; and a fourth region having a fourth property, comprising a structure having <3% residual austenite, balance martensite and optionally bainite and unavoidable structure constituents.

13. The sheet steel component as claimed in claim 12, where the at least partly quenched and tempered sheet steel component comprises the following chemical composition in wt %: C=0.08 to 0.5; Si+Al>=0.5, with Si+2*Al to 5; and Mn=0.5 to 4, balance Fe and unavoidable impurities.

14. The sheet steel component as claimed in claim 16, where the sheet steel component has a first region having a residual austenite stability value S_RA>=0.3590 nm and/or a structure hardness value Hv_rC and at least one further region having the following residual austenite stability values S_RA and/or structure hardness values Hv_rC: the second region with an S_RA which is less than the S_RA of the first region, and with an Hv_rC which is greater by at least 10 Hv than the Hv_rC of the first region, and the third region with an S_RA which is less than the S_RA of the first region and, if present, of the second region, and/or with an Hv_rC which is less by at least 10 Hv than the Hv_rC of the first region, and the fourth region, if residual austenite >0 and <3% is present, with S_RA<0.3950 nm and/or with an Hv_rC which is greater by at least 40 Hv than the Hv_rC of the first region and, if present, greater by at least 10 Hv than the Hv_rC of the second region, where the residual austenite stability value S_RA is determined with the following formula: S_RA=G_RA−0.0002 nm*% Si−0.0006 nm*% Al+0.0004 nm*% Mn, where G_RA defines the lattice constant of the residual austenite, where the structure hardness value Hv_rC of the first region meets the following condition: Hv_rC<320+800*(% C+% N)+75*(% Nb){circumflex over ( )}0.5.

15. The sheet steel component as claimed in claim 14, where the sheet steel component, between the regions having different properties, has one or more transition regions, where the transition region or regions space the various regions from one another with a transverse extent (Q) of at least 5 mm.

16. The sheet steel component of claim 13, further comprising one or more alloy elements from the group (P, S, N, Cr, Mo, Ti, B, Nb, V, Ni, Cu, Sn, Ca, Mg, REM): P up to 0.1, S up to 0.1, N up to 0.1, C up to 1.5, M up to 1, Ti up to 0.2, B up to 0.01, Nb up to 0.2, V up to 0.5, Ni up to 2, Cu up to 2, Sn up to 0.5, Ca up to 0.1, Mg up to 0.1, REM up to 0.1,

Description

[0105] Specific embodiments of the invention are elucidated in more detail below with reference to the drawing. The drawing and accompanying description of the resultant features should not be read as limiting on the respective embodiments, instead serving to illustrate the exemplary embodiment. Furthermore, the respective features can be utilized with one another and also with features of the above description for possible further developments and improvements of the invention, specifically in the context of additional embodiments which are not illustrated.

[0106] In the drawing

[0107] FIG. 1) shows a schematic flow chart of one embodiment of the process of the invention according to a first working example,

[0108] FIG. 2) shows a schematic flow chart of one embodiment of the process of the invention according to a second working example,

[0109] FIG. 3) shows a schematic flow chart of one embodiment of the process of the invention according to a third working example,

[0110] FIG. 4) shows a schematic flow chart of one embodiment of the process of the invention according to a fourth working example,

[0111] FIG. 5) shows a schematic flow chart of one embodiment of the process of the invention according to a fifth working example,

[0112] FIG. 6) shows a schematic flow chart of one embodiment of the process of the invention according to a sixth working example,

[0113] FIG. 7) shows a schematic perspective view of a quenched and tempered sheet steel component according to a first working example,

[0114] FIG. 8) shows a schematic perspective view of a quenched and tempered sheet steel component according to a second working example, and

[0115] FIG. 9) shows a schematic perspective view of a quenched and tempered sheet steel component according to a third working example.

[0116] FIGS. 1 to 6 represent schematic flow charts of various embodiments of the process of the invention.

[0117] (0) Identifies a device or apparatus for the shaping of a sheet steel, in which the sheet steel is shaped or formed, more particularly given a near-net shape, preferably by cold shaping or forming, in order to provide a preformed sheet steel for the ongoing operation. The device (I) comprises means for shaping the sheet steels. The device (0) may be configured in the form of one or more tools.

[0118] (I) Identifies a device or apparatus for at least partly austenitizing a provided sheet steel, in which the sheet steel is austenitized at a temperature of at least Ac1, more particularly at least Ac3 or above Ac3. The device (I) comprises means for at least partly heating the sheet steels provided. The sheet steel provided may in particular also be completely heated or austenitized.

[0119] The device (I) may be configured in the form of a furnace, as in the form of a continuous furnace, for example.

[0120] (II) Identifies a device or apparatus for at least partly hardening an at least partly austenitized sheet steel, in which the at least partly austenitized sheet steel is hardened to give an at least partly hardened sheet steel component, where the at least partly austenitized sheet steel is cooled to a temperature below Ms. The device (II) comprises means for actively cooling the at least partly austenitized sheet steels, said means comprising, for example, at least one tool and/or a medium for the hardening. The at least one tool may be configured as a press-hardening tool (II.1), as a hot-forming and press-hardening tool (II.2), as a hot-forming, press-hardening and annealing tool (II.2, III) or a press-hardening and annealing tool (II.1, III). The at least one tool may additionally have further functions, and may, for example, comprise means for trimming and/or making holes (IV).

[0121] (III) Identifies a device or apparatus for at least partly annealing an at least partly hardened sheet steel component, in which the at least partly hardened sheet steel component is quenched and tempered to give an at least partly quenched and tempered sheet steel component, where the at least partly hardened sheet steel component is annealed at a temperature of less than Ac1. The device (III) comprises means for actively temperature-conditioning the at least partly hardened sheet steel components, said means comprising, for example, at least one tool and/or one medium for annealing, where different temperature zones are provided in order to allow different regions (2, 3, 4, 5) having different properties to be established on the sheet steel component (1) for at least partial quenching and tempering. The at least one tool may be configured as an annealing tool (III.1) separately or integrated in a tool, more particularly for hot-forming and/or press hardening (II.1, II.2). The at least one tool may additionally have further functions, and may, for example, comprise means for trimming and/or making holes (IV).

[0122] (IV) Identifies a device or apparatus for afterworking an at least partly quenched and tempered sheet steel component, in which the at least partly quenched and tempered sheet steel component is afterworked, more particularly cut and/or given holes. The device (IV) comprises means for machining the at least partly quenched and tempered sheet steel components. Where the device (IV) comprises means for trimming and/or making holes, the means in question may be thermal means, in the form of a laser, for example, or mechanical means, such as one or more cutting and/or punching tools, for example. The device (IV) may be configured separately or integrated in a tool, more particularly for hot-forming and/or press hardening (II.1, II.2) or annealing (III).

[0123] FIG. 1 shows four separate devices (I, II, III, IV) in which the process of the invention for producing a quenched and tempered sheet steel component (1) of the invention can be implemented. A flat sheet steel is provided and is austenitized completely at a temperature above Ac3 in a furnace (I). The austenitized sheet steel is subsequently removed from the furnace (I) and transferred using suitable transfer means into a hot-forming and press-hardening tool (II.2), in which the austenitized sheet steel is hot-formed and cooled to a temperature below Ms and therefore hardened to give a sheet steel component. The hardened sheet steel component is subsequently transferred using suitable transfer means into an annealing tool (III), in which the hardened sheet steel component is annealed at different temperatures to give a quenched and tempered sheet steel component (1) having regions (2, 3, 4, 5) with different properties. The quenched and tempered sheet steel component (1) may lastly be transferred using suitable transfer means into a tool (IV) for trimming and/or making holes, by means of laser, for example. After the trimming and/or hole-making in the tool (IV), the fully fabricated, quenched and tempered sheet steel component (1) can be removed.

[0124] In the second embodiment in FIG. 2, the devices (III, IV) are combined in one device or in one tool in comparison to the first embodiment in FIG. 1. The annealing tool (III) possesses the additional function of additionally machining or remachining, more particularly cutting and/or making holes in, the sheet steel component for quenching and tempering, this being accomplished, for example, via cutting and/or punching tools (IV) additionally integrated or disposed in or on the annealing tool (III).

[0125] In the third embodiment in FIG. 3, the devices (II, III, IV) are combined in one device, such as a transfer press, for example, or in one tool. The hot-forming and press-hardening tool (II.2) is an annealing tool (III) at the same time and additionally possesses cutting and/or punching tools (IV). The embodiment may also be such that the devices (II, III, IV) are installed separately, or at least partly separately, from one another in one device.

[0126] In the fourth embodiment in FIG. 4 the devices (II, III) are combined in one device or in one tool. The tool comprises a hot-forming, press-hardening and annealing tool (II.2, III). The remachining device (IV) is configured separately.

[0127] In accordance with the first embodiment of FIG. 1, the fifth embodiment in FIG. 5 also shows four separate devices (0, I, II, III); in contradiction to the first four embodiments, in which a substantially flat sheet steel is provided in the form of a blank and subsequently austenitized, here and in the sixth embodiment as well, a preformed steel sheet is provided for the austenitizing. Since the preformed steel sheet preferably already has a near-net-shaped geometry, there is also no need for hot-forming, and so the hardening is carried out in a press-hardening tool (II.1) in the device (II). Remachining in a further device, not shown, is conceivable as and when required.

[0128] In the sixth embodiment in FIG. 6 the devices (II, III) are combined in one device or in one tool. The tool comprises a press-hardening and annealing tool (II.1, III). The remachining device (IV) is configured separately.

[0129] In an investigation, a strand was cast in a continuous casting unit from three melts A, B and C having the chemical composition indicated in table 1, and each strand was divided into slabs. The slabs were subsequently heated through in a walking beam furnace at temperatures above 1100° C. and hot-rolled on a hot strip line to give a hot strip of 3.2 mm. The hot strips were conditioned and subsequently cold-rolled to give cold strips of 1.5 mm. The cold strips produced from melts A and C were coated conventionally with an aluminum and silicon coating, whereas the cold strip produced from melt B remained uncoated. The cold strips produced from melts A and C and the cold strip from melt B were each separated to form seven steel sheets, which were subjected to cold forming in a device (0), the sheets being provided in each case in the form of a preformed steel sheet.

[0130] As outlined in the fifth embodiment in accordance with FIG. 5, the total of 21 steel sheets provided were fully austenitized above Ac3 in a furnace (I) at a furnace temperature of 920° C. for a duration of 300 s; see table 2. The austenitized steel sheets were transferred into a press-hardening tool (II.1) in a transfer time of 7 s, and in this tool the austenitized steel sheets were cooled or quenched and thus hardened to give sheet steel components. The temperature of the press-hardening tool (II.1) was a uniform 224° C. for the AS-coated steel sheets and a uniform 240° C. for the uncoated steel sheets, with the closed duration of the press-hardening tool (II.1) being 6 s in each case. The withdrawal temperatures measured for each of the hardened sheet steel components are apparent from table 2. Temporally immediately after the hardening, the hardened sheet steel components were transferred into an annealing tool (III), where cooling below Mf was prevented. The annealing tool (III) possessed four differently temperature-conditionable zones, allowing the establishment of regions (2, 3, 4, 5) having up to four different properties on the sheet steel component (1) for quenching and tempering, as well. The temperatures established in the respective zones of the annealing tool (III), and also the annealing temperatures TP1 to TP4 measured in the corresponding regions (2, 3, 4, 5) having different properties, on the quenched and tempered sheet steel components (1) on withdrawal from the annealing tool (III), and also the corresponding closed times of the annealing tool (III) for the purpose of establishing the different properties, are apparent from table 2. The quenched and tempered sheet steel components (1) of embodiments 1, 6 and 7 are shown illustratively in FIGS. 7, 8 and 9, schematically in a perspective view.

[0131] Though not depicted here, it is possible to produce sheet steel components which have only partial austenitization, only partial hardening and only partial quenching and tempering.

[0132] FIG. 7 shows a quenched and tempered sheet steel component (1) having a first region (2) having a first property and a fourth region (5) having a fourth property, where a transition region (1.1) separates the two regions (2, 5) from one another with a defined distance in the transverse extent (Q), the transverse extent (Q) being at least 10 mm.

[0133] FIG. 8 shows a quenched and tempered sheet steel component (1) having three first regions (2) having a first property, two third regions (4) having a third property, and one fourth region (5) having a fourth property, where transition regions (1.1) separate the different regions (2, 4, 5) from one another, in each case with a defined distance in the transverse extent (Q). The three first regions (2) are present in sections on the quenched and tempered sheet steel component (1), with two third regions (4) being present between the three first regions (1). The fourth region (5) defines an end section on the quenched and tempered sheet steel component (1).

[0134] FIG. 9 shows by comparison with FIG. 8 a quenched and tempered sheet steel component (1) having two first regions (2) having a first property, one second region (3) having a second property, two third regions (4) having a third property, and one fourth region (5) having a fourth property, where transition regions (1.1) separate the different regions (2, 3, 4, 5) from one another, in each case with a defined distance in the transverse extent (Q). The transition region (1.1) between the first third region (4) and the second first region (2) has wider dimensioning in its transverse extent (Q) by comparison with the other transition regions (1.1).

[0135] Table 3 provides a detailed overview of the different properties established in the respective regions (2, 3, 4, 5) on the quenched and tempered sheet steel components (1) by the process of the invention, as indicated in table 2.

[0136] The annealing temperatures (TP1, TP2, TP3, TP4) refer to the temperature in the corresponding regions (2, 3, 4, 5) on the quenched and tempered sheet steel component (1) on or shortly after withdrawal from the annealing tool (III). They may not and do not have to correspond to the tool temperatures in the zones which are in contact with the regions (2, 3, 4, 5).

[0137] Measurement Methods

[0138] Hv JC: Vickers hardness (Hv1)

[0139] A_RA, GRA: both parameters were ascertained from the x-ray diffractogram, in accordance with DIN 13925 “X-ray diffractometry of polycrystalline and amorphous materials” using the Rietveld method.

[0140] S_RA: calculated from G_RA according to specified formula

[0141] Abbreviations:

[0142] Table 1:

[0143] Surf: coating surface, U: uncoated, AS: aluminum-silicon-coated

[0144] A_F40: fraction (number %) of ferrite grains having an equivalent diameter >40 μm

[0145] Table 2:

[0146] T_ToolA: tool temperature, press-hardening tool

[0147] T_Abs: temperature of component on withdrawal from press-hardening tool

[0148] Z_Abs: press-hardening tool closed time

[0149] T_ToolX: temperature of annealing tool in tool region X (X:1-4)

[0150] TPX: component temperature in the region in contact with tool region X of the annealing tool on withdrawal from annealing tool

[0151] Z_Temp: annealing tool closed time

[0152] Table 3:

[0153] Hv_rC: Vickers hardness (Hv1)

[0154] A_RA: fraction of residual austenite in the structure (vol. %)

[0155] G_RA: lattice constant of residual austenite

[0156] S_RA: calculated from G_RA by the formula indicated in the text; describes the residual austenite stability

[0157] The process of the invention enables the production of cost-favorable sheet steel components having goal-directed properties, more particular vehicle body parts such as, for example, A pillars, B pillars or both side and crossmembers, and also combinations thereof, such as a door ring, for example. The process of the invention is applicable not only to monolithic sheet steels of constant thickness, but also to monolithic sheet steels of varying thickness (tailored rolled blanks). The process of the invention, furthermore, can also be applied generally to tailored products, examples being at least two steel sheets joined to one another, in the form of patchwork blanks or tailored welded blanks, with differing thickness and/or grade.

TABLE-US-00001 TABLE 1 Melt wt. % wt. % A C indicates data missing or illegible when filed

TABLE-US-00002 TABLE 2 T_furn Z-furn Z_trans T_toolA T_Abs Z_Abs T_tool1 Component Melt ° C. ° C. ° C. ° C. 1 A 920 300 7 224 265 6 410 2 A 920 300 7 224 264 6 3 A 920 300 7 224 4 A 920 300 7 224 5 A 920 300 7 224 262 420 6 A 920 300 7 224 360 7 A 920 300 7 224 258 6 390 8 B 920 300 7 240 283 6 410 9 B 300 7 240 284 420 10 B 300 7 240 282 420 11 B 300 7 240 275 12 B 920 300 7 240 279 6 420 13 B 920 300 7 240 284 6 14 B 920 300 7 240 390 15 B 920 300 7 410 16 B 300 7 420 17 B 920 300 7 218 261 6 420 18 B 920 300 7 218 251 6 19 B 920 300 7 218 257 6 420 20 B 300 7 21 B 300 7 TP1 T_tool2 TP2 T_tool3 TP3 T_tool4 TP4 Z_Temp Component ° C. ° C. ° C. ° C. ° C. ° C. ° C. 1 299 40 2 414 20 50 30 3 413 520 501 30 4 405 40 10 5 414 385 381 550 533 20 6 348 570 549 30 59 100 7 345 341 530 510 70 102 50 8 402 300 40 9 20 49 30 10 411 520 30 11 450 405 402 40 10 12 385 382 550 13 570 551 30 100 14 381 345 508 70 99 50 15 398 305 40 16 418 20 30 17 520 30 18 442 405 40 71 10 19 417 385 381 550 533 20 348 553 30 61 100 21 380 340 530 505 70 50 indicates data missing or illegible when filed

TABLE-US-00003 TABLE 3 First region Second region Third region Fourth region Component % m m % m m % m m % nm nm 1 3 12 0.3610 10 2 3 399 11 0.3623 4 12 10 5 405 11 11 6 11 7 11 10 3 8 11 0.3612 415 9 9 10 0.3612 10 391 10 0.3612 0.3627 11 405 10 12 11 10 13 14 11 10 472 2 15 12 16 17 408 14 3 0.3621 18 432 19 20 13 21 418 12 3 indicates data missing or illegible when filed