A HIGH STRENGTH STEEL PRODUCT AND A PROCESS TO PRODUCE A HIGH STRENGTH STEEL PRODUCT

Abstract

A high strength steel product and a process for producing a high strength steel product, the high strength steel product being useful for producing frame components for vehicles and automobiles.

Claims

1. A high strength steel product with a chemical composition comprising: 0.20-0.55 wt. % C; 1.00-3.50 wt. % Mn; 0.05-2.50 wt. % Cr; 0.50-3.00 wt. % Si; 0.01-1.00 wt. % Al; 1.00-4.00 wt. % of Σ (Si+Al); 1.50-6.00 wt. % of Σ (Cr+Mn); at most 0.050 wt. % P; at most 0.020 wt. % S; at most 0.010 wt. % N; and optionally one or more of: 0.05-0.50 wt. % Cu; 0.05-1.00 wt. % Ni; 0.05-0.50 wt. % Mo; 0.01-0.10 wt. % Nb; 0.01-0.10 wt. % Ti; 0.01-0.10 wt. % V; 0.0003-0.0050 wt. % B; 0.01-0.15 wt. % of Σ (Nb+Ti+V); 0.0003-0.0100 wt. % of Σ (Ca+REM); remainder iron and inevitable impurities; and wherein the microstructure comprises at least 40% partitioned martensite and 60-90% of Σ (partitioned martensite+bainitic ferrite) and 5-35% of retained austenite; and wherein the retained austenite comprises an average C content of 0.90% or more.

2. The high strength steel product of claim 1, having a tensile strength of at least 1300 MPa and/or a total elongation of at least 13%.

3. The high strength steel product of claim 1, wherein the chemical composition comprises 0.25-0.50 wt. % C and/or 1.50-3.00 wt. % Mn and/or 0.20-2.00 wt. % Cr and/or 0.80-2.50 wt. % Si and/or 0.05-0.70 wt. % Al and/or 1.30-3.50 wt. % of Σ (Si+Al) and/or 2.00-4.00 wt. % of Σ (Cr+Mn).

4. The high strength steel product of claim 1, wherein Cu and/or Ni and/or Mo and/or Nb and/or Ti and/or V and/or B belongs to the inevitable impurities.

5. The high strength steel product of claim 1, wherein the microstructure comprises 0-15% of proeutectoid ferrite.

6. The high strength steel product of claim 1, wherein the microstructure comprises 0-5% of fresh martensite.

7. The high strength steel product of claim 1, wherein the microstructure comprises 0-1.5% of carbides.

8. The high strength steel product of claim 1, wherein the microstructure comprises at least 50% of partitioned martensite.

9. A process for producing a high strength steel product according to claim 1 comprising the steps of: A. providing a cold rolled steel having the following composition: 0.20-0.55 wt. % C; 1.00-3.50 wt. % Mn; 0.05-2.50 wt. % Cr; 0.50-3.00 wt. % Si; 0.01-1.00 wt. % Al; 1.00-4.00 wt. % of Σ (Si+Al); 1.50-6.00 wt. % of Σ (Cr+Mn); at most 0.050 wt. % P; at most 0.020 wt. % S; at most 0.010 wt. % N; and optionally one or more of: 0.05-0.50 wt. % Cu; 0.05-1.00 wt. % Ni; 0.05-0.50 wt. % Mo; 0.01-0.10 wt. % Nb; 0.01-0.10 wt. % Ti; 0.01-0.10 wt. % V; 0.0003-0.0050 wt. % B; 0.01-0.15 wt. % of Σ (Nb+Ti+V); 0.0003-0.0100 wt. % of Σ (Ca+REM); remainder iron and inevitable impurities; B. heat treating the cold rolled steel at a temperature T2 above Ac3−20° C. for a duration t2 of between 1 and 300 s; C. cooling the annealed steel, at a cooling rate V4 of at least 25° C./s to a quenching temperature T4 between Ms and Mf; D. heat treating the annealed steel at a partitioning temperature T5 between Bs and Ms for a duration t5 of between 15 and 150 s; and E. cooling to ambient temperature, to obtain a steel product wherein the microstructure comprises at least 40% partitioned martensite and 60-90% of Σ (partitioned martensite+bainitic ferrite) and 5-35% of retained austenite; and wherein the retained austenite comprises an average C content of 0.90% or more.

10. The process according to claim 9, wherein at least part of the process is performed in a hot forming press, and wherein the steel is hot-press-formed during step C.

11. The process according to claim 9, wherein between step B and C, the steel is cooled to a temperature T3 above Ar3, preferably between 680-800° C.

12. The process according to claim 9, wherein the process further comprises a coating step, wherein the steel product is provided with a metallic coating by means of plating or hot-dipping.

13. The process according to claim 12, wherein the coating step is performed during step D.

14. The process according to claim 12, wherein the coating step is performed after step D, at a temperature T6 above Bn and below Bs, preferably in the range of 450° C. to 500° C. for a duration t5+t6 of between 15 and 150 s.

15. A component selected from a car or truck component, a component of a body in white, a component of a frame or a subframe, or a component of a structure or engineering project, said component having been produced from the steel product according to claim 1.

16. A component selected from a car or truck component, a component of a body in white, a component of a frame or the subframe, or a component of a structure or engineering project, said component having been produced from the steel product produced according to the process of claim 9.

17. The process according to claim 9, wherein the process further comprises a coating step, wherein the metallic coating is an aluminium based alloy.

18. The process according to claim 9, wherein the process further comprises a coating step, wherein the metallic coating is a zinc based alloy.

Description

[0125] FIG. 1—This figure shows a representative process according to the invention and the various process parameters used during the process.