Austenitic TWIP stainless steel, its production and use

Abstract

The object of the invention is an austenitic stainless steel with high plasticity induced by twinning with innovative chemical composition, and the use thereof in the automobile industry and in all applications wherein both a high resistance to corrosion and a high formability is requested, together with mechanical features of high-resistant steels. The invention also concerns a process for the production of this austenitic stainless steel with high twinning-induced plasticity.

Claims

1. An austenitic stainless steel with high plasticity induced by twinning (TWIP steel) and high mechanical properties and formability defined by: Rp0.2 between 250 and 650 MPa, Rm between 810 and 1200 MPa, A80 between 60 and 100%, comprising the following elements expressed in percentage by weight: C 0.01-0.50; N 0.11-0.50; Mn 6-12; Ni 0.01-6.0; Cu 0.01-6.0; Si 0.01-0.3; Al 0.01-1.5; Cr 11-20; Nb 0.001-0.5; Mo 0.01-2.0; Co 0.2-0.6; optionally, at least one of Ti 0.001-0.5 or V 0.001-0.5; and the remaining portion being Fe and unavoidable impurities.

2. The austenitic stainless steel according to claim 1, further comprising at least one of the following elements with the following percentage by weight: W 0.001-0.5; Hf 0.001-0.5; Re 0.001-0.5; Ta 0.001-0.5.

3. The austenitic stainless steel according to claim 1 comprising the following elements with the following percentage by weight: S+Se+Te<0.5 and/or P+Sn+Sb+As<0.2.

4. The austenitic stainless steel according to claim 1, wherein the following elements have the following percentage by weight: C 0.01-0.15; N 0.11-0.30; Mn 7-10; Cr 16-18; Cu 0.01-3.0; Ni 1.0-5.0; Si 0.01-0.3; Al 0.01-1.5; Nb 0.02-0.3; Co 0.2-0.3; Mo 0.05-1.5.

5. The austenitic stainless steel according to claim 1, wherein the following elements have the following percentage by weight: C+N 0.15-0.5; Cu+Ni 3.0-5.0; Mo+Co 0.21-2.6; Nb+V+Ti 0.05-1.0.

6. The austenitic stainless steel according to claim 1, after a deformation of 30% at room temperature, having a martensite volumetric fraction (+) lower than 5% and which, during a cold deformation, forms twins in quantities, expressed in terms of volumetric fraction, comprised between 2 to 20%.

7. A process for producing the austenitic stainless steel according to claim 1, comprising the following steps: hot deformation of the steel under condition of product obtained by continuous casting or by ingot; or cold deformation with reduction rate higher than 30% of the steel product under condition of annealed hot rolled product or hot rolling raw product, the above-mentioned hot deformation or the above-mentioned cold deformation being followed by a possible recrystallization annealing, at a temperature in the range of 800-1200 C. for a time comprised in the range of 10-600 s, and by cooling at room temperature with a speed in the range of 1 C./s-100 C./s, wherein the austenitic stainless steel has high plasticity induced by twinning (TWIP steel) and high mechanical properties and formability defined by: Rp0.2 between 250 and 650 MPa, Rm between 810 and 1200 MPa, A80 between 60 and 100%, comprising the following elements expressed in percentage by weight: C 0.01-0.50; N 0.11-0.50; Mn 6-12; Ni 0.01-6.0; Cu 0.01-6.0; Si 0.01-0.3; Al 0.01-1.5; Cr 11-20; Nb 0.001-0.5; Mo 0.01-2.0; Co 0.2-0.6; optionally, at least one of Ti 0.001-0.5 or V 0.001-0.5; and the remaining portion being Fe and unavoidable impurities.

8. The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel has an energy absorption of 0.5-0.8 Joules/mm.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A description of embodiments of the invention will be now provided with the help of the figures and of the examples, with the purpose of making to understand objects, features and advantages thereof, not to be meant with limitative purpose.

(2) FIG. 1 shows the comparison, in terms of strain hardening during the cold deformation, of the steel according to the invention (INOX-IP) in the state of cold rolled and annealed strip with two reference steels AISI304 and TWIP steel with high Mn (TWIP-HIGH Mn).

(3) FIG. 2 shows the deformation curve (%) depending upon the tension in MPa at room temperature relevant to a test piece taken from a cold rolled and annealed strip.

(4) FIG. 3 shows the components supporting the automobile body roof (pillars) which can be manufactured with the steel of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) In the examples, PREN is the acronym of Pitting Resistance Equivalent Number and it is an index for the synthetic evaluation of the localized resistance to corrosion.

Example 1

(6) Three different 1.0-thick cold strip samples were obtained from cold rolling of slabs produced by a continuous casting plant. The hot strips were cold rolled (50% reduction) and subjected to final recrystallization annealing according to the modes shown in Table 1.

(7) TABLE-US-00001 TABLE 1 Furnace Soaking Heating rate temperature time Cooling rate ( C./s) ( C.) (s) ( C./s) 20 1000 90 50

(8) The chemical compositions of the considered steels are reported in the following table.

(9) TABLE-US-00002 TABLE 2 Exam- ple C N Mn Ni Cu Si Al Cr Nb Mo Co 1.1 0.05 0.2 9.5 2 2 0.2 1.5 18 0.09 0.2 0.6 (inv.) 1.2 0.1 0.2 9 1 4 0.25 0.001 18 0.1 1.5 0.5 (inv.) 1.3 (com- 0.04 0.10 9 2 4 0.25 0.001 18 para- tive)

(10) Table 3 shows the mechanical properties relevant to the steel of table 2.

(11) TABLE-US-00003 TABLE 3 Yield Tensile Rp0.2 strength Example (Mpa) UTS (MPa) A80 (%) 1.1 (inv.) 360 850 90 1.2 (inv.) 370 810 84 1.3 345 710 45 (comparative)

(12) The steels of the examples 1.1 and 1.2 show mechanical properties according to those of the present invention. The samples 1.1 and 1.2, deformed by 30% at room temperature, have both a percentage of twins higher than 8% and almost total lack of martensite (+). FIG. 1 shows the comparison, in terms of hardening during cold deformation, of the steel related to the example 1.1 with the two reference steels AISI304 and TWIP steel with high Mn (TWIP-HIGH Mn).

(13) The microstructure of the steel of example 1.1. after a deformation by 30% at room temperature has a martensite (+) percentage lower than 1%. The percentage of twins, assessed by means of optical microscope, resulted to be 10%. The steel of the example 1.3, instead, has a poor TWIP effect during deformation (the fraction of twins present after the deformation by 30% is lower than 1%).

(14) The corrosionistic properties of the subject examples are shown in the following table 4.

(15) TABLE-US-00004 TABLE 4 Critical Crevice Temperature Example PREN EP (mV) ( C.) Co 1.1 (inv.) 22 300-500 10-15 1.2 (inv.) 26 300-500 15-20 1.3 20 400-500 5-15 (comparative)

(16) The products related to the examples 1.1 and 1.2 can be used for manufacturing automobile components requiring a good resistance to corrosion and a high mechanical resistance together with an excellent capability of energy absorption, such as the structural elements of automobiles. FIG. 3 shows the pillars of an automobile which can be obtained with the steels according to the examples 1.1 and 1.2. The pillars are the body portions whereupon the roof is supported and which have great importance for the structural strength of the body high portion.

Example 2

(17) Two 10.0 mm-thick wire rods were obtained from hot rolling of billets produced by a continuous casting plant. The conditions of final recrystallization annealing of the wire rods are shown in the following table.

(18) TABLE-US-00005 TABLE 5 Furnace Soaking temperature time Cooling rate ( C.) (s) ( C./s) 1000 120 50

(19) The chemical composition of the subject wire rods is shown in the following table.

(20) TABLE-US-00006 TABLE 6 Ex- am- ple C N Mn Ni Cu Si Al Cr Nb Mo Co Ti 2.1 0.12 0.13 7 3 2 0.25 1.5 18 0.3 0.2 0.5 0.1 (inv.) 2.2 0.25 0.35 9.5 2 0 0.2 1.5 10.5 (com- para- tive)

(21) Table 7 shows the mechanical features related to the steel of table 6.

(22) TABLE-US-00007 TABLE 7 Yield Tensile Rp0.2 strength Example (Mpa) UTS (MPa) A80 (%) 2.1 (inv.) 320 780 88 2.2 410 860 52 (comparative)

(23) The mechanical properties of the steel 2.1 are excellent. In fact, the sample 2.1, deformed by 30% at room temperature, has a percentage of twins higher than 8% and total lack of martensite (+). On the contrary the chemical composition 2.2 shows a poor ductility.

(24) The microstructure of the steel 2.2, deformed by 30% at room temperature, in fact, has a percentage of twins lower than 1%. The low fraction of twins produced during the deformation explains the low work hardening of the material and then the poor obtained ductility. FIG. 2 shows the diagram tension-deformation at room temperature of the steel related to the example 2.1.

(25) The corrosionistic properties of the steels at issue are shown in the following table.

(26) TABLE-US-00008 TABLE 8 Critical Crevice Temperature Example PREN EP (mV) ( C.) Co 2.1 (inv.) 22 400-600 10-15 2.2 16 100-200 <5 (comparative)

Example 3

(27) Three samples of the same hot rolled strip with thickness of 2.0 mm were subjected to three different recrystallization annealing cycles shown in the following table with the purpose of verifying the effect of the annealing cycle on the final microstructure and on the mechanical properties.

(28) TABLE-US-00009 TABLE 9 Heating Furnace Keeping Cooling speed temperature time speed Example ( C.) ( C.) (s) ( C./s) 3.1 (inv.) 30 800 90 50 3.2 (inv.) 20 1100 60 50 3.3 0.01 700 36000 0.1 (comparative)

(29) The chemical composition of the exemplified samples is shown in the following table 10.

(30) TABLE-US-00010 TABLE 10 C N Mn Ni Cu Si Al Cr Nb Mo Co Ti V Ta 0.1 0.25 8.5 2 1 0.2 0.1 17 0.05 1.0 0.05 0.08 0.1 0.1

(31) The following table shows the mechanical properties related to the 3 examined samples.

(32) TABLE-US-00011 TABLE 11 Yield Tensile Rp0.2 strength Example (Mpa) UTS (MPa) A80 (%) 3.1 (inv.) 580 910 50 3.2 (inv.) 320 780 92 3.3 380 680 39 (comparative)

(33) In case of the example 3.1 the annealing at low temperature determined a partial recrystallization and a very fine grain size (about 1 m). This allows obtaining a higher yielding stress value even if a high residual ductility is still kept.

(34) The product related to the example 3.2 has mechanical features significantly higher than those of any stainless steel of the previous state of art. The properties of the steel of the example 3.3, instead, are significantly lower due to the precipitation of carbides during the annealing cycle. The microstructure of the example 3.3, after deformation by 30% at room temperature, is characterized by a percentage of martensite (+) of 8%. The fraction of twins, assessed by optical microscope, resulted to be lower than 1%. The low fraction of twins produced during the deformation explains the low work hardening of the material and then the poor obtained ductility.

(35) The corrosionistic properties of the herein exemplified steels are shown in the following table.

(36) TABLE-US-00012 TABLE 12 EP Critical Crevice Example PREN (mV) Temperature ( C.) 3.1, 3.2 (inv.) 21 200-400 5-10 1.3 (comparative) 21 100 <5

(37) In the steel of the comparative example 3.3 the not suitable process conditions determined mechanical and corrosionistic properties not appropriate for the application in the automotive field.

Example 4

(38) Two 1.5 mm-thick strip samples of a steel according to the invention were obtained from hot rolling and subsequent cold rolling (50% reduction rate) and final annealing. The annealing conditions are shown in table 13.

(39) TABLE-US-00013 TABLE 13 Heating Furnace Soaking Cooling rate temperature time rate ( C.) ( C./s) (s) ( C./s) 150 35 90 50

(40) The chemical composition of the subject samples are shown in the following table.

(41) TABLE-US-00014 TABLE 14 Ex- am- ple C N Mn Ni Cu Si Al Cr Mo Co Nb Ta W 4.1 0.1 0.15 6.5 2 3 0.2 1.0 18 2 0.2 0.1 0.07 0.1 (inv.) 4.2 0.1 0.09 8 4 2 1.0 1.5 18 (com- para- tive)

(42) Table 15 shows the mechanical properties related to the examples of table 14.

(43) TABLE-US-00015 TABLE 15 Yield Tensile Rp0.2 strength Example (Mpa) UTS (MPa) A80 (%) 4.1 (inv.) 420 910 70 4.2 360 820 45 (comparative)

(44) The microstructure of the example 4.1 is characterized by a volumetric fraction of twins higher than 8% at a 30% deformation. Upon observing with the optical microscope the microstructure of the steel related to the example 4.2, deformed by 30%, the presence of twins was not revealed.

(45) The corrosionistic properties of the steel considered in the present example are shown in table 16.

(46) TABLE-US-00016 TABLE 16 Crevice Critical Temperature Example PREN Ep (mV) ( C.) 4.1 (inv.) 27 400-600 20-30 4.2 (comp.) 19 300-400 10-15

(47) The product obtained in the example 4.1 according to the invention underlined a high mechanical resistance together with a good resistance to corrosion and ductility. Such functional property makes this product more suitable than the comparative steel 4.2 for implementing automobile components.