In recent years, increasingly higher strength has been demanded from steel plate which is used for automobiles, buildings, etc. For example, high strength cold rolled steel plate with an ultimate tensile strength of 900 MPa or more is being rapidly applied as bumpers, impact beams, and other reinforcing members. However, at the time of application of high strength steel plate, it is necessary to solve the problem of prevention of delayed fracture.
“Delayed fracture” is the phenomenon of sudden fracture of a steel member (for example, PC steel wire, bolts) on which a high stress acts under the conditions of use. It is known that this phenomenon is closely related to the hydrogen which penetrates the steel from the environment.
As a factor greatly affecting delayed fracture of steel members, the steel plate strength is known. Steel plate is more resistant to plastic deformation and fracture the higher the strength, so there is a high possibility of use in an environment in which a high stress acts.
Note that, if using a low strength steel member for a member on which a high stress acts, the member plastically deforms and fractures, so delayed fracture does not occur.
In a steel member which is shaped from steel plate such as steel plate for automobile use, the residual stress which occurs after shaping becomes larger the higher the steel plate strength, so there is a high concern over the occurrence of delayed fracture. That is, in a steel member, the higher the strength of the steel, the higher the concern over the occurrence of delayed fracture.
In the past, much effort has been made in the fields of steel bars or thick-gauge steel plate to develop steel materials taking delayed fracture resistance into consideration. For example, in steel bars and steel for bolt use, development has focused on formation of tempered martensite. It has been reported that Cr, Mo, V, and other elements which raise the temper softening resistance are effective for improvement of the delayed fracture resistance (for example, see NPLT 1).
This is art for causing the precipitation of alloy carbides, which act as trap sites of hydrogen, so as to change the mode of delayed fracture from grain boundary fracture to intragranular fracture.
However, the steel which is described in NPLT 1 contains 0.4% or more of C and a large amount of alloy elements, so the workability and weldability which are required from steel sheet deteriorate. Further, to cause the precipitation of alloy carbides, several hours or more of heat treatment is necessary, so the art of NPLT 1 had the problem of manufacturability of steel.
PLT 1 describes using oxides mainly comprised of Ti and Mg to prevent the occurrence of hydrogen defects. However, this art covers thick steel plate and considers delayed fracture after large heat input welding, but both the high workability and delayed fracture resistance which are demanded from steel sheet are not considered.
In steel sheet, since the thickness is small, even if hydrogen penetrates it, it is released in a short time. Further, in terms of workability, steel plate with an ultimate tensile strength of 900 MPa or more had almost never been used before, so the problem of delayed fracture had been treated as small. However, today, use of high strength steel sheet is rising, so development of high strength steel plate with excellent hydrogen embrittlement resistance has become necessary.
Up to now, the art for raising the hydrogen embrittlement resistance almost all relates to steel material which is used at the proof stress or yield stress or less as bolts, steel bars, thick steel plate, and other such products. That is, the prior art is not art covering steel materials (steel plate) such as for members of automobiles where workability (cuttability, press formability, etc.) and, simultaneously, hydrogen embrittlement resistance are sought.
Usually, a member obtained by shaping steel plate has residual stress remaining inside of the member. Residual stress is local, but sometimes exceeds the yield stress of the material steel plate. For this reason, steel plate free of hydrogen embrittlement even if high residual stress remains inside the member has been sought.
Regarding the delayed fracture of steel sheet, for example, NPLT 2 reports about the aggravation of delayed fracture due to work-induced transformation of retained austenite. This considers the shaping of steel sheet. NPLT 2 describes an amount of retained austenite not causing deterioration of the delayed fracture resistance.
That is, the above report relates to high strength steel sheet which has a specific structure. This cannot be said to be a fundamental measure for improvement of the delayed fracture resistance.
PLT 2 describes steel plate for enamelware use which is excellent in fishscale resistance as steel sheet considering hydrogen trapping ability and shapeability. This traps the hydrogen which penetrates steel plate at the time of production as oxides in the steel plate and suppresses the occurrence of “fishscale” (surface defects) which occur after enameling.
However, with the art of PLT 2, the steel plate contains a large amount of oxides inside of it. If oxides disperse in the steel plate at a high density, the shapeability deteriorates, so it is difficult to apply the art of PLT 2 to steel plate for automobile use from which a high shapeability is required. Furthermore, the art of PLT 2 does not achieve both high strength and delayed fracture resistance.
To solve these problems, steel plate in which oxides are precipitated has been proposed (for example, see PLT 3). In such steel plate, the oxides which are dispersed in the steel plate act as trap sites which trap the hydrogen which has penetrated the steel, so dispersion or concentration of hydrogen at locations where stress concentrate and locations where delayed fracture is of a concern is suppressed.
However, to obtain such an effect, steel plate must have oxides dispersed in it at a high density. Strict control of the production conditions is necessary.
Relating to high strength steel plate, for example, there are the arts of PLTs 4 to 9. Further, relating to hot dip galvanized steel plate, for example, there is the art of PLT 10, but as explained above, it is extremely difficult to develop high strength steel plate wherein both delayed fracture resistance and good shapeability are achieved.
PLT 11 discloses ultrahigh strength steel strip which has a tensile strength of 980 N/mm2 or more and is excellent in durability. In this ultrahigh strength steel strip, hydrogen delayed cracking resistance is considered, but basically martensite is used to handle the delayed fracture resistance (conventional method), so the shapeability is insufficient.
PLT 12 discloses high strength steel strip which has a tensile strength of 980 MPa or more and is excellent in hydrogen embrittlement resistance. PLT 13 discloses high strength cold rolled steel plate which is excellent in workability and hydrogen embrittlement resistance.
However, in all of this steel plate, the amount of particles which precipitate inside the grains is large. The hydrogen embrittlement resistance does not reach the level which is currently sought. Therefore, development of high strength steel plate which achieves both delayed fracture resistance and good shapeability has been strongly sought.