TECHNICAL FIELD OF THE INVENTION

The present invention relates to a steel sheet.

This application claims priority based on Japanese Patent Application No. 2017-215829 filed in Japan on Nov. 8, 2017, the content of which is incorporated herein by reference.

RELATED ART

In recent years, for global environmental protection, there has been a demand for an improvement in fuel efficiency of vehicles. With regard to improving the fuel efficiency of vehicles, steel sheets for a vehicle are required to have further high-strengthening in order to reduce the weight of the vehicle body while ensuring safety. As such a high strength steel sheet, a composite structure steel sheet having a composite structure represented by dual-phase steel (DP steel) having a structure in which a hard structure such as martensite or bainite and soft ferrite are combined has been widely used.

Since such DP steel is generally a high alloy, alloying elements such as Mn are segregated in a direction parallel to a sheet thickness direction in a melting step. Since this segregation portion is stretched by hot rolling or cold rolling, the segregation portion is continuous in a band shape in a layered manner (hereinafter, this referred to as microsegregation). In case of DP steel, a hard phase is generated in this microsegregation portion. As a result, the hard phase becomes a structure that is continuous in a band shape. It is known that such a structure in which a hard phase generated due to microsegregation is continuous in a band shape significantly deteriorates hole expansibility and bendability.

As a technique for solving the problems caused by the above-mentioned microsegregation in DP steel, for example, Patent Document 1 describes a steel sheet in which Mn is diffused by being held in a temperature range of 1200° C. or higher and 1300° C. or lower for 0.5 h or longer and 5 h or shorter before a hot rolling step, whereby a ratio C1/C2 between an upper limit C1 and a lower limit C2 of the Mn concentration in a cross section of the steel sheet in a sheet thickness direction is 2.0 or less. In this steel sheet, it is disclosed that variation in stretch flangeability is significantly reduced by setting C1/C2 to 2.0 or less.

On the other hand, high-strengthening of the steel sheet causes a reduction in ductility, and a difficulty in cold press forming. Therefore, there is a demand for a material that is relatively soft and easy to form during forming, and has a large bake-hardening amount during coating baking after the forming. That is, for further high-strengthening of vehicle components, a steel sheet having high bake-hardenability is required. Bake-hardening is a strain aging phenomenon that occurs when interstitial elements (carbon and nitrogen) are locked to dislocations that are introduced by press forming (hereinafter, also referred to as “pre-strain”), during coating baking at a high temperature (150° C. to 200° C.).

However, the DP steel sheet containing a large amount of ferrite as described in Patent Document 1 has a problem that the bake-hardenability is generally low.

As a technique for improving the bake-hardenability, for example, Patent Document 2 describes a cold-rolled steel sheet in which a high bake-hardening amount is secured by including a hard structure consisting of bainite and martensite as the primary structure, and limiting the ferrite fraction to 5% or less.

However, as a result of examinations by the present inventors, it was found that in the cold-rolled steel sheet described in Patent Document 2, while a certain bake-hardenability can be obtained if the pre-strain is 1%, sufficient bake-hardenability cannot be obtained in a case where the pre-strain is small (for example, 0.5%). That is, in the cold-rolled steel sheet of Patent Document 2, it is necessary to increase the pre-strain in order to obtain high bake-hardenability.

PRIOR ART DOCUMENT

Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2010-065307[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2008-144233

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

As described above, in steel sheets for a vehicle, excellent bake-hardenability has to be secured in order to meet the demand for further high-strengthening in the future. On the other hand, in a case where it is necessary to increase the pre-strain in order to obtain high bake-hardenability, this cannot be applied to a member having low workability at the time of press forming or the like. Moreover, an increase in pre-strain causes a decrease in ductility, so that application to a member that require excellent ductility is also difficult.

The present invention has been made in view of the above problems. An object of the present invention is to provide a steel sheet excellent in bake-hardenability capable of obtaining a sufficient bake-hardening amount even with a pre-strain of 0.5%.

Means for Solving the Problem

The present inventors have intensively studied to solve the above problems. As a result, it was clarified that among two types of segregation, center-line segregation and microsegregation, it is important to reduce microsegregation of alloying elements, and to form a structure containing 95% or more of a hard structure in which a dislocation density is increased in order to improve bake-hardenability.

In the related art, in a structure containing 95% or more of a hard structure, no hard phase is generated in a band shape, and therefore microsegregation is hardly considered. However, the present inventors found that dislocations introduced by pre-strain are uniformized by reducing microsegregation in a structure containing 95% or more of a hard structure and furthermore, bake-hardenability is improved by increasing a dislocation density during manufacturing.

A steel sheet of the present invention that can achieve the above object is as follows.

(1) A steel sheet according to an aspect of the present invention includes, as a chemical composition, by mass %: C: 0.05% to 0.30%; Si: 0.2% to 2.0%; Mn: 2.0% to 4.0%; Al: 0.001% to 2.000%; P: 0.100% or less; S: 0.010% or less; N: 0.010% or less; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.100%; Cu: 0% to 1.00%; Ni: 0% to 1.00%; Mo: 0% to 1.00%; Cr: 0% to 1.00%; W: 0% to 0.005%; Ca: 0% to 0.005%; Mg: 0% to 0.005%; a rare earth element (REM): 0% to 0.010%; B: 0% to 0.0030%; and a consisting of Fe and impurities, in which a metallographic structure contains, by area ratio, 95% or more of a hard structure and 0% to 5% of residual austenite, by mass % in a cross section in a thickness direction, C1/C2 which is a ratio of an upper limit C1 of a Mn content to a lower limit C2 of the Mn content is 1.5 or less, and a bake-hardening amount BH is 150 MPa or more.

(2) In the steel sheet according to (1), the chemical composition may include, by mass %, one or two or more of: Ti: 0.003% to 0.100%; Nb: 0.003% to 0.100%; and V: 0.003% to 0.100%, and a total amount of Ti, Nb, and V is 0.100% or less.

(3) In the steel sheet according to (1) or (2), the chemical composition may include, by mass %, one or two or more of: Cu: 0.005% to 1.00%; Ni: 0.005% to 1.00%; Mo: 0.005% to 1.00%; and Cr: 0.005% to 1.00%, and a total amount of Cu, Ni, Mo, and Cr is 1.00% or less.

(4) In the steel sheet according to any one of (1) to (3), the chemical composition may include, by mass %, one or two or more of: W: 0.0003% to 0.005%; Ca: 0.0003% to 0.005%; Mg: 0.0003% to 0.005%; and a rare earth element (REM): 0.0003% to 0.010%, and a total amount of W, Ca, Mg, and the rare earth element (REM) is 0.010% or less.

(5) In the steel sheet according to any one of (1) to (4), the chemical composition may include, by mass %: B: 0.0001% to 0.0030%.

Effects of the Invention

According to the above aspect of the present invention, a steel sheet excellent in bake-hardenability can be provided by controlling the microsegregation of alloying elements in the steel sheet and increasing the dislocation density in the hard structure. This steel sheet is excellent in press formability, and further high-strengthened by being baked during coating after press forming, so that the steel sheet is suitable as a structural member of a vehicle and the like. In the present invention, excellent bake-hardenability means that the bake-hardening amount (BH amount) when a heat treatment is performed at 170° C. for 20 minutes after adding 0.5% pre-strain is 150 MPa or more.

