Low-yield-ratio high-strength hot-rolled steel sheet and method of manufacturing the same
AssigneeKawasaki Steel Corporation
InventorsSusumu Masui, Masahiko Morita, Toshiyuki Kato
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Abstract
A low-yield-ratio high-strength hot-rolled steel sheet having the advantageous features of both conventional precipitation-strengthened steel and structure-strengthened steel and free from the drawbacks of these conventional steels, and a manufacturing method characterized in hot-rolling such a steel sheet. The steel sheet has an upper limit of C content of about 0.18 wt. %, and the lower C content limit approximately satisfies C.gtoreq.0.05+Ti/4+Nb/8 in relationship with the content of Ti and/or Nb. The steel sheet also contains about 0.5 to 2.5 wt. % Si and has a structure essentially formed of ferrite in which a carbide of Ti and/or Nb is precipitated, martensite and retained austenite.
Claims3
A low-yield-ratio high-strength hot-rolled steel sheet having a composition essentially consisting of about:
- 0.18 wt. % or less of C;
- more than 0.6 to 2.5 wt. % of Si;
- 0.5 to 2.5 wt. % of Mn;
- 0.05 wt. % or less of P;
- 0.02 wt. % or less of S;
- 0.01 to 0.1 wt. % of Al;
- an element selected from the group consisting of about 0.02 to 0.5 wt. % of Ti and about more than 0.04 to 1.0 wt. % of Nb, and mixtures thereof, said Ti and Nb being present approximately in relation to C according to the following formula:
- C wt. %.gtoreq.0.05+Ti wt. %/4+Nb wt. %/8;
- and
- the balance substantially Fe and incidental impurities;
- wherein the structure of the steel comprises substantially ferrite and martensite containing a precipitated carbide of Ti and/or Nb or said precipitated carbide and residual austenite.
A method of manufacturing a low-yield-ratio high-strength hot-rolled steel sheet, said method comprising the steps of:
- preparing a steel slab having a composition according to the following formula:
- 0.18 wt. % or less of C;
- 0.5 to 2.5 wt. % of Si;
- 0.5 to 2.5 wt. % of Mn;
- 0.05 wt. % or less of P;
- 0.02 wt. % or less of S;
- 0.01 to 0.1 wt. % of Al;
- an element selected from the group of about 0.02 to 0.5 wt. % of Ti and about 0.03 to 1.0 wt. % of Nb, and mixtures thereof, said Ti and Nb being present approximately in relation to C according to the following formula:
- C wt. %.gtoreq.0.05+Ti wt. %/4+Nb wt. %/8;
- and
- the balance substantially Fe and incidental impurities;
- hot-rolling the prepared steel slab;
- finishing the hot-rolling at a temperature of about 820.degree. C. or higher;
- retaining the rolled steel sheet in the range of temperatures of about 820.degree. to 720.degree. C. for at least 10 seconds;
- cooling the steel sheet at a cooling rate of about 10.degree. C./sec or higher;
- and
- coiling temperature of about 500.degree. C. or lower.
Dependencies are inferred from the claim text.