SE547321C2 - A high strength steel strip or sheet, and a method for producing the same - Google Patents

Abstract

The invention relates to a high strength steel strip or sheet comprising in wt. %: 0.15 - 0.25 C, 0.5 -1.8 Si, 2.0 - 2.7 Mn, 0.03 -1.2 Al, 0.6 - 2.0 Si Al. The strip or sheet having a tensile strength (Rm) of 980 - 1110 MPa, a microstructure including 10-25 vol% retained austenite. The retained austenite fulfil: a mechanical stability (kp) of 5 - 30, a carbon content xcy of the retained austenite of 0.8 -1.2, and RA * kp / xcy ≥ 200. The invention also relates to a method including double annealing cycle for producing the strip or sheet. (Fig. 1)

Claims (8)

1. A high strength steel strip or sheet having: a) b) d) a composition consisting of the following elements in Wt. %: C 0.15 - 0.25 Si 0.5 - 1.8 Mn 2.0 - 2.7 Al 0.03 -1.2 Si+ Al 0.6 - 2.0 Optionally Ti í 0.Nb í 0.V í 0.Cr í 0.B í 0.005 Mo í 0.balance Fe apart from impurities; a tensile strength (Rm) 980 - 1100 MPa Wherein Rm is derived according to the European norm EN 10002 Part 1, Wherein the samples are taken in the longitudinal direction of the strip; a microstructure comprising of in vol. %: retained austenite 10 - 25 Wherein the amount of retained austenite is measured by means of the saturation magnetization method described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61-64, and the other phases are determined by use of scanning electron microscope (SEM) at 2000 times magnification; Wherein the retained austenite fulfil: 5 - 30, kp as defined by LudWigson and Berger in J. Iron Steel Inst. 1969, vol. 207, 10 pp. 63, a mechanical stability (kp) Where p=1: VVO _ Vy Vy _ ß _ kpxe V70 ...initial retained austenite content Vy ...retained austenite content after deformation s true strain; a carbon content of the retained austenite (in wt.%) xcy 0.8 -1.2, wherein xcy is determined by the formula: ay= 3.556+0.0453 xcy+ 0.00095 xMn+ 0.0056 xAi Where xMn, and xA1 are the content of manganese, and aluminium in austenite (in wt.%), and wherein the contents for xMn and xA1 are assumed to be equal the nominal content, and the lattice parameter av is obtained from the y reflections in X-ray diffraction (XRD) measurements using the following equation described in N.H. van Dijk, A.M. Butt, L. Zhao, J. Sietsma, S.E. Offerman, J .P. Wright, and S. van der Zwaag, Thermal stability of retained austenite in TRIP steels studied by synchrotron X-ray diffraction during cooling, Acta Materialia 5 3 (2005) 5439-5447; and RA * kp / xcy

2. The steel strip or sheet according to claim 1, wherein the microstructure fulfils at least one of the following requirements in vol. %, preferably all the requirements: double tempered martensite 15 - 60 tempered martensite 0 - 70 bainite 0 - 70 fresh martensite + carbides 0 - 10 bainite + tempered martensite 15 - 70 other phases 0 -

3. The steel strip or sheet according to claim 1 or 2, wherein the microstructure fulfils at least one of the following requirements in vol. %, preferably all the requirements: double tempered martensite 30 - 55 tempered martensite 10 - 40 bainite 5 - 30 fresh martensite + carbides 0 - 5 bainite + tempered martensite 25 - 50 other phases 0 -

4. The steel strip or sheet according to any one of the preceding claims, wherein one or more of the following mechanical properties are fulfilled: YS yield strength (RPM) HER Hole Expansion Ratio (Ä) TE Total Elongation (A30) TS*TE*HER/500 - 900 MPa E30% 215% 27000MPa%

5. The steel strip or sheet according to any one of the preceding claims, wherein one or more of the following mechanical properties are fulfilled: TS, Tensile Strength (Rm) YS yield strength (RPM) HER Hole Expansion Ratio (Ä) TE Total Elongation (A30) TS*TE*HER/980-1050 MPa 600-800 MPa 35 - 60 % 20 - 30 % 8000 - 12000 MPa

6. The steel strip or sheet according to any one of the preceding claims, wherein the retained austenite fulfils one or more of the following: AR ECD mechanical stability (kp) carbon content in retained austenite (xcy) RA* kp/.xcy 2 2.0 5 0.55 10-0.9 -1.1 250 -

7. The high strength steel strip or sheet according to claim 1 having a composition consisting of the following alloying elements (in wt. %): C Si Mn Al Si+ Al Optionally Ti Nb V Cr 0.17 -0.23 0.5 - 1.6 2.2 -2.6 0.03 -1.0 0.7 -1.50.1 50.1 50.1 50.B Mo S 0.005 S 0.balance Fe apart from impurities;

8. A method for producing a high strength steel strip or sheet according to any one of claims 1 - 7, comprising: g) Providing a cold ro11ed steel having a composition as defined in item a) of the claims: h) heating the strip at a rate (I-ÉRÉ) of to 1 - 20 °C/s to a first annealing temperature (Tan1) between 800 “C and 1000 °C, and soaiïirig for 10 - 300 s (ftani). i) cooling the strip at a rate (CR1) of 10 - 100 °C/s to a first quenching temperature (TQ1) between MF and (MS-20) °C, j) heating the cooled strip at a rate (HR2) of 1-100 °C/s to a first partitioning temperature (Toa1) in the range of the first quenching temperature (TQ1) + 10 °C to 500 °C, and partitioning the strip for 204000 s (toal), k) cooiiiig the strip to a teihperature below 50 °C at a rate (CRÉZ) of 1 - 50 °C/s, 1) heating the strip at a rate (HRÉ) of to 1 - 20 °C/s to a second annealing temperature (Tan2) between 700 °C, and 900 °C, and soaking for 10 - 300 s (tanfi), m) cooling the strip at a rate (CR3) of 10 - 100 °C/s to a second quenching temperature (TQ2) in the range of 150 - 500 °C, n) partitioning the strip for 20 -(toa2) at a second partitioning temperature (Toa2) in the range of the second quenching temperature (TQ2) to 500 °C. o) coohng the strip to a teniperature below 50 °C, p) optionahy' niaking sheets fittan the strips.