Steel, sheet steel product and process for producing a sheet steel product

Patent metadata

Field	Value
Publication number	US-9976205-B2
Application number	US-201314406001-A
Country	US
Kind code	B2
Filing date	Jun 5, 2013
Priority date	Jun 5, 2012
Publication date	May 22, 2018
Grant date	May 22, 2018

Abstract

The invention relates to a steel and to a flat steel product produced therefrom that have optimized mechanical properties and at the same time can be produced at low cost, without having to rely for this on expensive alloying elements that are subject to great fluctuations with regard to their procurement costs. The steel and the flat steel product have for this purpose the following composition according to the invention (in % by weight): C: 0.11-0.16%; Si: 0.1-0.3%; Mn: 1.4-1.9%; Al: 0.02-0.1%; Cr: 0.45-0.85%; Ti: 0.025-0.06%; B: 0.0008-0.002%, the remainder Fe and impurities that are unavoidable for production-related reasons, which include contents of phosphorus, sulfur, nitrogen or molybdenum as long as the following respectively apply for their contents: P: ≤0.02%, S: ≤0.003%, N: ≤0.008%, Mo: ≤0.1%. Similarly, the invention relates to a method for producing a flat steel product that consists of a steel according to the invention.

First claim

The invention claimed is:

1. A steel comprising the following composition (in % by weight)
C: 0.11-0.16%;
Si: 0.1-0.3%;
Mn: 1.4-1.9%;
Al: 0.02-0.1%;
Cr: 0.45-0.85%;
Ti: 0.025-0.06%;
B: 0.0008-0.002%;
the remainder Fe and impurities that are unavoidable for production-related reasons, which include contents of phosphorus, sulfur, nitrogen or molybdenum as long as the following respectively apply for their contents:
P: ≤0.02%
S: ≤0.003%
N: ≤0.008%
Mo: ≤0.05%,
wherein the steel has a yield strength, R p0.2 , of 440-550 MPa a tensile strength of 780-900 MPa, and a microstructure that consists of 60-90% by volume ferrite, including bainitic ferrite, 10-40% by volume martensite, up to 5% by volume residual austenite, and up to 5% by volume other structural constituents that are unavoidable for production-related reasons.

2. The steel as claimed in claim 1 , wherein its Al content is at most 0.05% by weight.

3. The steel as claimed in claim 1 , wherein its Ti content is at most ≤0.055% by weight.

4. The steel as claimed in claim 3 , wherein its Ti content is at most 0.045% by weight.

5. The flat steel product as claimed in claim 1 , wherein the steel has an elongation after fracture A80 of at least 14%, an n 10-20/Ag value of at least 0.11, and a BH2 value of at least 25 MPa.

6. A method for producing a cold-rolled flat steel product constituted as claimed in claim 1 , comprising the following working steps:
a) casting a steel comprising the following composition (in % by weight)
C: 0.11-0.16%;
Si: 0.1-0.3%;
Mn: 1.4-1.9%;
Al: 0.02-0.1%;
Cr: 0.45-0.85%;
Ti: 0.025-0.06%;
B: 0.0008-0.002%;
the remainder Fe and impurities that are unavoidable for production-related reasons, which include contents of phosphorus, sulfur, nitrogen or molybdenum as long as the following respectively apply for their contents:
P: ≤0.02%
S: ≤0.003%
N: ≤0.008%
Mo: ≤0.1% to form a primary product;
b) hot rolling the primary product to form a hot strip with a thickness of 2 to 5.5 mm, the initial hot-rolling temperature being 1000-1300° C. and the final hot-rolling temperature being 840-950° C.;
c) coiling the hot strip to form a coil at a coiling temperature of 480-650° C.;
d) cold rolling the hot strip to form a cold-rolled flat steel product 0.6-2.4 mm thick, the degree of cold rolling achieved by means of the cold rolling being 35-80%;
e) continuous annealing the cold-rolled flat steel product,
e.1) the cold-rolled flat steel product initially being heated in a preheating stage at a heating-up rate of 0.2-45° C./s to a preheating temperature of up to 870° C.,
e.2) the cold-rolled flat steel product subsequently being held at an annealing temperature of 750-870° C. over an annealing period of 8-260 s in a holding stage, the preheated flat steel product optionally being finish-heated to the respective annealing temperature within the holding stage,
e.3) the cold-rolled flat steel product being cooled down after the end of the annealing period at a cooling-down rate of 0.5-110 K/s.

7. The method as claimed in claim 6 , wherein, between working steps a) and b), the primary product is kept at a temperature ≥300° C.

8. The method as claimed in claim 6 , wherein, between working steps a) and b), the primary product is cooled down to room temperature at a cooling-down rate of ≤60° C./h.

9. The method as claimed in claim 7 , wherein, before working step b), the primary product is heated to the respective initial hot-rolling temperature over a heating-up period of up to 500 minutes.

10. The method as claimed in claim 6 , wherein the cold-rolled flat steel product passes through a hot-dip coating, which follows on in the continuous flow from working step e.3), and in that the temperature to which the cold-rolled flat steel product is cooled down in working step e.3) is 455-550° C.

11. The method as claimed in claim 6 , wherein the cold-rolled flat steel product is cooled down to room temperature in working step e.3).

12. The method as claimed in claim 11 , wherein the cold-rolled flat steel product is cooled down to room temperature in at least two cooling steps in working step a3), in that the cold-rolled flat steel product is cooled down to 250-500° C. in the first working step and is held in this temperature range for up to 760 s, and in that the cold-rolled flat steel product is subsequently cooled down to room temperature.

13. The method as claimed in claim 11 , wherein, after the cooling down to room temperature, the cold-rolled flat steel product is electrolytically covered with a metallic protective coating.

Assignees

• Thyssenkrupp Steel Europe Ag

Inventors

• Bocharova Ekatherina
• Ebest Sigrun
• Mattissen Dorothea
• Sebald Roland

Classifications

• C21D8/04

  to produce plates or strips for drawing, e.g. for deep-drawing · CPC title

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D8/0447

  characterised by the heat treatment · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C21D8/0436

  Cold rolling · CPC title