Method for the manufacture of TWIP steel sheet having an austenitic matrix

Abstract

A method for the manufacture of a TWIP steel is provided including: (A) feeding of a slab comprising by weight: 0.5<C<1.2%, 13.0≤Mn<25.0%, S≤0.030%, P≤0.080%, N≤0.1%, Si≤3.0%, 0.051%≤Al≤4.0%, 0.1≤V≤2.5%, and on a purely optional basis, one or more of Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.40%, Ni≤1.0%, Cu≤5.0%, Ti≤0.5%, 0.06≤Sn≤0.2%, the remainder of the composition being made of iron and inevitable impurities resulting from the elaboration, (B) reheating the slab and hot rolling the slab to provide a hot rolled slab, (C) coiling the hot rolled slab to provide a coiled slab, (D) first cold-rolling the coiled slab to provide a first cold rolled slab, (E) recrystallization annealing the first cold rolled slab such that an annealed steel sheet having an UTS.sub.annealed is obtained and (F) second cold-rolling the annealed steel sheet with a reduction rate CR % that satisfies the following equation A: 1216.472−0.98795*UTS.sub.annealed≤(−0.0008*UTS.sub.annealed+1.0124)*CR %.sup.2+(0.0371*UTS.sub.annealed−29.583)*CR %.

Claims

1. A method for producing a TWIP steel sheet comprising: A. feeding a slab comprising by weight: 0.5<C<1.2%, 13.0≤Mn<25.0%, S≤0.030%, P≤0.080%, N≤0.1%, Si≤3.0%, 0.051%≤A|<4.0%, 0.1≤V≤2.5%, the remainder of the composition being made of iron and inevitable impurities resulting from elaboration, B. reheating the slab and hot rolling the slab to provide a hot rolled slab, C. coiling the hot rolled slab to provide a coiled slab, D. first cold-rolling the coiled slab to provide a first cold rolled slab, E. recrystallization annealing the first cold rolled slab such that an annealed steel sheet having an UTS.sub.annealed is obtained, F. second cold-rolling the annealed steel sheet with a reduction rate CR % that satisfies the following equation A:
1216.472−0.98795*UTS.sub.annealed≤(−0.0008*UTS.sub.annealed+1.0124)*CR %.sup.2+(0.0371*UTS.sub.annealed−29.583)*CR %, and G. determining the UTS.sub.annealed before the second cold-rolling.

2. The method according to claim 1, wherein the composition further includes, one or more of Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.40%, Ni≤1.0%, Cu≤5.0%, Ti≤0.5%, and 0.06≤Sn≤0.2%.

3. The method according to claim 1, wherein the amount of Al is above 0.06% in the slab.

4. The method according to claim 1, wherein the reheating is performed at a temperature above 1000° C. and the hot rolling has a final rolling temperature of at least 850° C.

5. The method according to claim 1, wherein the coiling is at a temperature below or equal to 580° C.

6. The method according to claim 1, wherein the first cold-rolling step (D) is realized with a reduction rate between 30 and 70%.

7. The method according to of claim 1, wherein the first cold-rolling step (D) is realized with a reduction rate between 40 and 60%.

8. The method according to claim 1, wherein the recrystallization annealing step (E) is at a temperature between 700 and 900° C.

9. The method according to claim 1, wherein the UTS.sub.annealed obtained after the recrystallization annealing is above 800 MPa.

10. The method according to claim 9, wherein the UTS.sub.annealed is between 800 and 1400 MPa.

11. The method according to claim 10, wherein the UTS.sub.annealed is between 1000 and 1400 MPa.

12. The method according to claim 1, wherein a total elongation obtained after the recrystallization annealing TE % annealed is above 10%.

13. The method according to claim 12, wherein the TE % annealed is above 15%.

14. The method according to claim 13, wherein the TE % annealed is between 30% and 70%.

15. The method according claim 1, wherein the second cold-rolling step (F) is realized with a reduction rate that further satisfies the following equation B: $\frac{\mathrm{CR}\%}{18.2}\le \ln \left(\frac{\mathrm{TEannealed}\%}{10}\right).$

16. The method according to claim 1, wherein the second cold-rolling step (F) is realized with a reduction rate between 1 to 50%.

17. The method according claim 16, wherein the second cold-rolling step (F) is realized with a reduction rate between 1 and 25%.

18. The method according claim 16, wherein the second cold-rolling step (F) is realized with a reduction rate between 26 and 50%.

19. The method according to claim 1, wherein after the second cold-rolling step (F), a hot-dip coating step (F′) is performed.

20. The method according to claim 19, wherein the the hot-dip coating step (F′) is performed with an aluminum-based bath or a zinc-based bath.

21. The method according to claim 20, wherein the aluminum-based bath comprises less than 15% Si, less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, the remainder being Al.

22. The method according to claim 20, wherein the zinc-based bath comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

23. The method as recited in claim 1 wherein after the second cold-rolling the steel sheet has a UTS above 1200 MPa.

24. The method as recited in claim 1 wherein after the second cold-rolling the steel sheet has a TE above 10%.

25. The method as recited in claim 1 wherein the reheating is performed at a temperature above 1000° C., the hot rolling has a final rolling temperature of at least 850° C., the coiling temperature is at a temperature below or equal to 580° C. and the first cold-rolling step (D) is realized with a reduction rate between 30 and 70%.

26. The method as recited in claim 3 wherein the first cold-rolling step (D) is realized with a reduction rate between 30 and 70%.

27. The method as recited in claim 8, wherein the recrystallization annealing step (E) is at a temperature between 750 and 850° C. and for a duration of 10 to 500 seconds.

28. The method as recited in claim 1, wherein the determining UTS.sub.annealed is between the recrystallization annealing and the second cold-rolling.

29. The method of claim 28, further comprising determining a value of 1216.472−0.98795*UTS.sub.annealed before the second cold-rolling.

30. The method as recited in claim 17, wherein the second cold-rolling step (F) is realized with a reduction rate between 1 and 15%.

31. A method for producing a TWIP steel sheet comprising: A. feeding a slab comprising by weight: 0.5<C<1.2%, 13.0<Mn<25.0%, S≤0.030%, P≤0.080%, N≤0.1%, Si≤3.0%, 0.051%≤Al≤4.0%, 0.1≤V≤2.5%, the remainder of the composition being made of iron and inevitable impurities resulting from elaboration, B. reheating the slab and hot rolling the slab at a temperature above 1000° C. and hot rolling it to a final rolling temperature of at least 850° C. to provide a hot rolled slab, C. coiling the hot rolled slab at a temperature below or equal to 580° C. to provide a coiled slab, D. first cold-rolling the coiled slab with a reduction rate between 30 and 70% to provide a first cold rolled slab, E. recrystallization annealing the first cold rolled slab between 700 and 900° C. such that an annealed steel sheet having an UTS.sub.annealed is obtained, F. second cold-rolling the annealed steel sheet with a reduction rate CR % that satisfies the following equation A:
1216.472−0.98795*UTS.sub.annealed≤(−0.0008*UTS.sub.annealed+1.0124)*CR %.sup.2+(0.0371*UTS.sub.annealed−29.583)*CR %.

32. The method according to claim 31, wherein after the second cold-rolling step (F), a hot-dip coating step (G) is performed.

33. The method as recited in claim 31, wherein the recrystallization annealing step (E) is at a temperature between 750 and 850° C. and for a duration of 10 to 500 seconds.

34. The method as recited in claim 31, wherein the second cold-rolling step (F) is realized with a reduction rate between 1 and 15%.

35. The method as recited in claim 31, wherein the recrystallization annealing step (E) is at a temperature higher than 750° C.

36. The method of claim 31, further comprising determining a value of 1216.472−0.98795*UTS.sub.annealed before the second cold-rolling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figures:

(2) FIG. 1 illustrates one embodiment according to the present invention.

(3) FIG. 2 illustrates another embodiment according to the present invention.

DETAILED DESCRIPTION

(4) The following terms will be defined: UTS: ultimate tensile strength, UTS.sub.annealed: ultimate tensile strength obtained after the recrystallization annealing, TE: total elongation, TE.sub.annealed: total elongation obtained after the recrystallization annealing and CR %: reduction rate of the second cold-rolling.

(5) In accordance with an embodiment of the present invention, a method for producing a TWIP steel sheet comprising the following steps: A. feeding of a slab having steel sheet comprising by weight: 0.5<C<1.2%, 13.0≤Mn<25.0%, S≤0.030%, P≤0.080%, N≤0.1%, Si≤3.0%, 0.051%≤Al≤4.0%, and on a purely optional basis, one or more elements such as Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.40%, Ni≤1.0%, Cu≤5.0%, Ti≤0.5%, V≤2.5%, 0.06≤Sn≤0.2%, the remainder of the composition being made of iron and inevitable impurities resulting from the elaboration, B. Reheating such slab and hot rolling it, C. A coiling step, D. A first cold-rolling, E. A recrystallization annealing such that an annealed steel sheet having an UTS.sub.annealed is obtained and F. A second cold-rolling with a reduction rate CR % that satisfies the following equation A:
1216.472−0.98795*UTS.sub.annealed≤(−0.0008*UTS.sub.annealed+1.0124)*CR %.sup.2+(0.0371*UTS.sub.annealed−29.583)*CR %

(6) Without willing to be bound by any theory it seems that when the method according to the present invention is applied, in particular when the reduction rate of the second cold-rolling satisfies the equation A, it makes it possible to obtain a TWIP steel sheet having improved mechanical properties, specially a higher strength.

(7) Regarding the chemical composition of the steel, C plays an important role in the formation of the microstructure and the mechanical properties. It increases the stacking fault energy and promotes stability of the austenitic phase. When combined with a Mn content ranging from 13.0 to 25.0% by weight, this stability is achieved for a carbon content of 0.5% or higher. In case there are vanadium carbides, a high Mn content may increase the solubility of vanadium carbide (VC) in austenite. However, for a C content above 1.2%, there is a risk that the ductility decreases due to for example an excessive precipitation of vanadium carbides or carbonitrides. Preferably, the carbon content is between 0.4 and 1.2%, more preferably between 0.5 and 1.0% by weight so as to obtain sufficient strength.

(8) Mn is also an essential element for increasing the strength, for increasing the stacking fault energy and for stabilizing the austenitic phase. If its content is less than 13.0%, there is a risk of martensitic phases forming, which very appreciably reduce the deformability. Moreover, when the manganese content is greater than 25.0%, formation of twins is suppressed, and accordingly, although the strength increases, the ductility at room temperature is degraded. Preferably, the manganese content is between 15.0 and 24.0% and more preferably 17.0 and 24.0% so as to optimize the stacking fault energy and to prevent the formation of martensite under the effect of a deformation. Moreover, when the Mn content is greater than 24.0%, the mode of deformation by twinning is less favored than the mode of deformation by perfect dislocation glide.

(9) Al is a particularly effective element for the deoxidation of steel. Like C, it increases the stacking fault energy which reduces the risk of forming deformation martensite, thereby improving ductility and delayed fracture resistance. However, Al is a drawback if it is present in excess in steels having a high Mn content, because Mn increases the solubility of nitrogen in liquid iron. If an excessively large amount of Al is present in the steel, the N, which combines with Al, precipitates in the form of aluminum nitrides (AlN) that impede the migration of grain boundaries during hot conversion and very appreciably increases the risk of cracks appearing in continuous casting. In addition, as will be explained later, a sufficient amount of N must be available in order to form fine precipitates, essentially of carbonitrides. Preferably, the Al content is below or equal to 2.0%. When the Al content is greater than 4.0%, there is a risk that the formation of twins is suppressed decreasing the ductility. Preferably, the amount of Al is above 0.06%, advantageously above 0.1% and more preferably above 1.0%.

(10) Correspondingly, the nitrogen content must be 0.1% or less so as to prevent the precipitation of AlN and the formation of volume defects (blisters) during solidification. In addition, when elements capable of precipitating in the form of nitrides, such as vanadium, niobium, titanium, chromium, molybdenum and boron, the nitrogen content must not exceed 0.1%.

(11) Optionally, the amount of V is below or equal to 2.5%, preferably between 0.1 and 1.0%. Preferably, V forms precipitates. Preferably, the volumic fraction of such elements in steel is between 0.0001 and 0.025%. Preferably, vanadium elements are mostly localized in intragranular position. Advantageously, vanadium elements have a mean size below 7 nm, preferably between 1 and 5 nm and more preferably between 0.2 and 4.0 nm.

(12) Silicon is also an effective element for deoxidizing steel and for solid-phase hardening. However, above a content of 3%, it reduces the elongation and tends to form undesirable oxides during certain assembly processes, and it must therefore be kept below this limit. Preferably, the content of silicon is below or equal to 0.6%.

(13) Sulfur and phosphorus are impurities that embrittle the grain boundaries. Their respective contents must not exceed 0.030 and 0.080% so as to maintain sufficient hot ductility.

(14) Some Boron may be added up to 0.005%, preferably up to 0.001%. This element segregates at the grain boundaries and increases their cohesion. Without intending to be bound to a theory, it is believed that this leads to a reduction in the residual stresses after shaping by pressing, and to better resistance to corrosion under stress of the thereby shaped parts. This element segregates at the austenitic grain boundaries and increases their cohesion. Boron precipitates for example in the form of borocarbides and boronitrides.

(15) Nickel may be used optionally for increasing the strength of the steel by solution hardening. However, it is desirable, among others for cost reasons, to limit the nickel content to a maximum content of 1.0% or less and preferably between below 0.3%.

(16) Likewise, optionally, an addition of copper with a content not exceeding 5% is one means of hardening the steel by precipitation of copper metal. However, above this content, copper is responsible for the appearance of surface defects in hot-rolled sheet. Preferably, the amount of copper is below 2.0%. Preferably, the amount of Cu is above 0.1%.

(17) Titanium and Niobium are also elements that may optionally be used to achieve hardening and strengthening by forming precipitates. However, when the Nb or Ti content is greater than 0.50%, there is a risk that an excessive precipitation may cause a reduction in toughness, which has to be avoided. Preferably, the amount of Ti is between 0.040 and 0.50% by weight or between 0.030% and 0.130% by weight. Preferably, the titanium content is between 0.060% and 0.40 and for example between 0.060% and 0.110% by weight. Preferably, the amount of Nb is above 0.01% and more preferably between 0.070 and 0.50% by weight or 0.040 and 0.220%. Preferably, the niobium content is between 0.090% and 0.40% and advantageously between 0.090% and 0.200% by weight.

(18) Chromium and Molybdenum may be used as optional element for increasing the strength of the steel by solution hardening. However, since chromium reduces the stacking fault energy, its content must not exceed 1.0% and preferably between 0.070% and 0.6%. Preferably, the chromium content is between 0.20 and 0.5%. Molybdenum may be added in an amount of 0.40% or less, preferably in an amount between 0.14 and 0.40%.

(19) Furthermore, without willing to be bound by any theory, it seems that precipitates of vanadium, titanium, niobium, chromium and molybdenum can reduce the sensitivity to delayed cracking, and do so without degrading the ductility and toughness properties. Thus, preferably, at least one element chosen from titanium, niobium, chromium and molybdenum under the form of carbides, nitrides and carbonitrides are present in the steel.

(20) Optionally, tin (Sn) is added in an amount between 0.06 and 0.2% by weight. without willing to be bound by any theory, it is believed that since tin is a noble element and does not form a thin oxide film at high temperatures by itself, Sn is precipitated on a surface of a matrix in an annealing prior to a hot dip galvanizing to suppress a pro-oxidant element such as Al, Si, Mn, or the like from being diffused into the surface and forming an oxide, thereby improving galvanizability. However, when the added amount of Sn is less than 0.06%, the effect is not distinct and an increase in the added amount of Sn suppresses the formation of selective oxide, whereas when the added amount of Sn exceeds 0.2%, the added Sn causes hot shortness to deteriorate the hot workability. Therefore, the upper limit of Sn is limited to 0.2% or less.

(21) The steel can also comprise inevitable impurities resulting from the development. For example, inevitable impurities can include without any limitation: O, H, Pb, Co, As, Ge, Ga, Zn and W. For example, the content by weight of each impurity is inferior to 0.1% by weight.

(22) According to an embodiment of the the present invention, the method comprises the feeding step A) of a semi product, such as slabs, thin slabs, or strip made of steel having the composition described above, such slab is cast. Preferably, the cast input stock is heated to a temperature above 1000° C., more preferably above 1050° C. and advantageously between 1100 and 1300° C. or used directly at such a temperature after casting, without intermediate cooling.

(23) The hot-rolling is then performed at a temperature preferably above 890° C., or more preferably above 1000° C. to obtain for example a hot-rolled strip usually having a thickness of 2 to 5 mm, or even 1 to 5 mm. To avoid any cracking problem through lack of ductility, the end-of-rolling temperature is preferably above or equal to 850° C.

(24) After the hot-rolling, the strip has to be coiled at a temperature such that no significant precipitation of carbides (essentially cementite (Fe,Mn).sub.3C)) occurs, something which would result in a reduction in certain mechanical properties. The coiling step C) is realized at a temperature below or equal to 580° C., preferably below or equal to 400° C.

(25) A subsequent cold-rolling operation followed by a recrystallization annealing is carried out. These additional steps result in a grain size smaller than that obtained on a hot-rolled strip and therefore results in higher strength properties. Of course, it must be carried out if it is desired to obtain products of smaller thickness, ranging for example from 0.2 mm to a few mm in thickness and preferably from 0.4 to 4 mm.

(26) A hot-rolled product obtained by the process described above is cold-rolled after a possible prior pickling operation has been performed in the usual manner.

(27) The first cold-rolling step D) is performed with a reduction rate between 30 and 70%, preferably between 40 and 60%.

(28) After this rolling step, the grains are highly work-hardened and it is necessary to carry out a recrystallization annealing operation. This treatment has the effect of restoring the ductility and simultaneously reducing the strength. Preferably, this annealing is carried out continuously. Advantageously, the recrystallization annealing E) is realized between 700 and 900° C., preferably between 750 and 850° C., for example during 10 to 500 seconds, preferably between 60 and 180 seconds.

(29) According to embodiments of the present invention, the UTS value of a steel sheet obtained after the recrystallization annealing is called UTS.sub.annealed. Preferably, after the recrystallization annealing step E), the annealed steel sheet has an UTS.sub.annealed above 800 MPa, preferably between 800 and 1400 MPa and more preferably between 1000 and 1400 MPa.

(30) Preferably, the TE value of a steel sheet obtained after the recrystallization annealing is called TE annealed. In this preferred embodiment, the steel sheet has a TE.sub.annealed above 10%, preferably above 15% and more preferably between 30 and 70%.

(31) Then, the second cold-rolling is realized with a reduction rate that satisfies the equation A.

(32) In a preferred embodiment, the second cold-rolling step F) is realized with a reduction rate CR % that further satisfies the following equation B:

(33) $\frac{\mathrm{CR}\%}{18.2}\le \ln \left(\frac{\mathrm{TEannealed}\%}{10}\right)$

(34) Without willing to be bound by any theory, it seems that when the method according to the present invention is applied, in particular when the reduction rate of the second cold-rolling further satisfies the above equation, it makes it possible to obtain a TWIP steel sheet having further improved mechanical properties, specially a higher elongation.

(35) Preferably, the second cold-rolling step F) is realized with a reduction rate between 1 to 50%, preferably between 1 and 25% or between 26 and 50%. It allows the reduction of the steel thickness. Moreover, the steel sheet manufactured according to the aforesaid method, may have increased strength through strain hardening by undergoing a re-rolling step. Additionally, this step induces a high density of twins improving thus the mechanical properties of the steel sheet.

(36) After the second cold-rolling, a hot-dip coating step G) can be performed. Preferably, step G) is realized with an aluminum-based bath or a zinc-based bath.

(37) In a preferred embodiment, the hot-dip coating step is performed with an aluminum-based bath comprises less than 15% Si, less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, the remainder being Al.

(38) In another preferred embodiment, the hot-dip coating step is performed with a zinc-based bath comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

(39) The molten bath can also comprise unavoidable impurities and residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath.

(40) For example, the optionally impurities are chosen from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additional element being inferior to 0.3% by weight. The residual elements from feeding ingots or from the passage of the steel sheet in the molten bath can be iron with a content up to 5.0%, preferably 3.0%, by weight.

(41) For example, an annealing step can be performed after the coating deposition in order to obtain a galvannealed steel sheet.

(42) A TWIP steel sheet having an ultimate tensile strength (UTS) above 1200 MPa, preferably between 1200 and 1600 MPa is thus obtained. Preferably, the total elongation (TE) is above 10%, more preferably above 15% and more preferably between 15 and 50%.

Example

(43) In this example, TWIP steel sheets having the following weight composition were used:

(44) TABLE-US-00001 Grade C % Si % Mn % P % Cr % Al % % Cu % V % N 1 0.595 0.205 18.3 0.035 — 0.782 1.7 0.18 0.01 2 0.88 0.508 17.96 0.03 0.109 2.11 0.15 0.093 0.0044 3 0.876 0.502 17.63 0.032 0.108 2.78 0.149 0.384 0.0061 4 1.04 0.505 17.69 0.034 0.108 2.8 0.147 0.447 0.0069

(45) Firstly, the samples were heated and hot-rolled at a temperature of 1200° C. The finishing temperature of hot-rolling was set to 890° C. and the coiling was performed at 400° C. after the hot-rolling. Then, a 1.sup.st cold-rolling was realized with a cold-rolling reduction ratio of 50%. Thereafter, a recrystallization annealing was performed at 750° C. during 180 seconds. The UTS annealed and TE.sub.annealed obtained after the recrystallization annealing step were determined.

(46) Afterwards, the 2.sup.nd cold-rolling was realized with different cold-rolling reduction ratios. Results are shown in the following Table:

(47) TABLE-US-00002 2.sup.nd cold- UTS.sub.annealed TE rolling Equation A UTS Equation B Trials Grade (MPa) (%) (%) satisfied (MPa) satisfied TE (%) 1  1 980 ND 11 No 1095 ND ND 2* 1 980 ND 30 Yes 1425 ND ND 3* 2 1053 67 15 Yes 1292 Yes 37 4* 2 1053 67 30 Yes 1476 Yes 16 5* 3 1100 36 15 Yes 1352 Yes 21 6* 3 1100 36 30 Yes 1659 No 7 7* 4 1140 37 15 Yes 1420 Yes 19 8* 4 1140 37 30 Yes 1741 No 8 *examples according to the present invention; ND = not done

(48) Results show that when the method according to the present invention is applied, in particular when the equation A is satisfied, the mechanical properties of the TWIP steel sheet are highly improved.

(49) FIG. 1 shows the value of UTS obtained after the second cold-rolling for Trials 1 to 8. For Trials 2 to 8, Equation A is satisfied meaning that UTS is highly improved.

(50) FIG. 2 shows the value of TE obtained after the second cold-rolling for Trials 3 to 8. For Trials 3, 4, 5 and 7, Equation B is further satisfied which means that both UTS and TE are highly improved.