High-strength cold-rolled steel sheet and method for manufacturing the same (as amended)

Abstract

A high-strength cold-rolled steel sheet having a tensile strength (TS) of 1150 MPa or more and excellent elongation and delayed fracturing resistance, as well as a method for manufacturing the steel sheet. The steel sheet has a chemical composition containing, by mass %, C: 0.16% to 0.30%, Si: 1.20% to 2.20%, Mn: 1.50% to 2.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.007% or less, at least one of Ti and Nb, and the balance being Fe and inevitable impurities, wherein the combined mass % of Ti and Nb is 0.04% to 0.15%, and a multi-phase microstructure including ferrite having an average crystal grain diameter of 3 μm or less in an amount of 45% to 65% in terms of volume fraction, tempered martensite having an average crystal grain diameter of 2 μm or less in an amount of 35% to 55% in terms of volume fraction, and optionally non-recrystallized ferrite having an average crystal grain diameter of 5 μm or less in an amount of 5% or less.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is the U.S. National Phase application of PCT International Application No. PCT / JP2015 / 006346, filed Dec. 21, 2015, and claims priority to Japanese Patent Application No. 2015-038507, filed Feb. 27, 2015, the disclosures of both applications being incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to a high-strength cold-rolled steel sheet and a method for manufacturing the steel sheet, and in particular, to a high-strength, cold-rolled steel sheet which can be used for structural parts of, for example, an automobile.BACKGROUND OF THE INVENTION[0003]Nowadays, since CO2 emission regulations are being tightened due to increasing concern about environmental problems, the weight reduction of an automobile body for increasing fuel efficiency is an issue to be addressed in the automobile field. Therefore, since there has been progress in reducing the t...

AI Technical Summary

Benefits of technology

[0009]Therefore, an object of certain embodiments of the present invention is, by solving the problems with the conventional techniques described above, to provide a high-strength cold-rolled steel sheet excellent in terms of elongation and delayed fracturing resistance and a method for manufacturing the steel sheet.

[0010]In order to increase elongation and delayed fracturing resistance, it is possible to achieve excellent elongation and delayed fracturing resistance by adding specified amounts of Ti and / or Nb, by controlling the volume fractions of ferrite, tempered martensite, and non-recrystallized ferrite in a steel sheet microstructure in specified proportions, and by finely making crystal grains of the steel sheet microstructures.

[0012]In order to achieve a tensile strength (TS) of 1150 MPa or more, the hardness of martensite is sufficiently increased by adding a specified amount of C, that satisfactory elongation is achieved by preventing martensite grains from combining with each other through the control of the volume fraction of tempered martensite as a result of adding Ti and / or Nb in order to finely make crystal grains of ferrite and martensite, and that fine carbides of Ti and / or Nb contribute to an improvement in delayed fracturing resistance as a result of such carbides functioning as the trap sites of hydrogen. Since the fine carbides of Ti and / or Nb not only have an influence on the trap sites of hydrogen but also increase the hardness of ferrite, such carbides also have a function of improving tensile strength (TS). In addition, performing heating to an annealing temperature at an optimized heating rate makes crystal grains fine, which results in an improvement in elongation and delayed fracturing resistance. Moreover, in the case where the volume fraction of non-recrystallized ferrite is equal to or less than a specified value, it is possible to improve strength without causing any negative influence on elongation or delayed fracturing resistance.

[0038]According to embodiments of the present invention, it is possible to obtain a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1150 MPa or more and a high elongation which is accompanied by excellent formability. In addition, this high-strength cold-rolled steel sheet has excellent delayed fracturing resistance. According to embodiments of the present invention, it is possible to stably obtain a high-strength cold-rolled steel sheet excellent in terms of elongation and delayed fracturing resistance in which fracturing does not occur for 100 hours, for example, when it is immersed in hydrochloric acid having a temperature of 25° C. and a pH of 1 under a stress corresponding to an elongation of 15.0% or more.

Problems solved by technology

Moreover, in the case of the technique according to Patent Literature 2, although there is an example having a tensile strength (TS) of 1150 MPa or more, it is not possible to achieve sufficient elongation with which it is possible to perform press forming in order to obtain a complex shape.

In addition, since there is no mention of delayed fracturing resistance in Patent Literature 1 or Patent Literature 2, the technique according to Patent Literature 1 or Patent Literature 2 does not satisfy necessary and sufficient condition which is required for an automotive steel sheet, because an increase in strength is accompanied by a risk of delayed fracturing occurring due to hydrogen entering from a natural environment, for example, when the steel sheet is used for an automobile.

As described above, in the case of a conventional high-strength steel sheet having a tensile strength (TS) of 1150 MPa or more, since it is difficult to achieve excellent delayed fracturing resistance while achieving a sufficient elongation for excellent formability, it is a fact that a steel sheet which is fully satisfactory in terms of these properties (tensile strength, elongation, and delayed fracturing resistance) has not yet been developed.