Steel for Warm Working, Warm Working Method Using the Steel, and Steel Material and Steel Component Obtainable Therefrom

Abstract

There are provided a steel for warm working, to be subjected to warm working as various structures, components of cars, and the like, a warm working method thereof, and a steel material and a steel component obtainable from the warm working method.

[Solving Means] A steel is to have a particle dispersion type fiber structure formed in the matrix by warm working. The steel is characterized in that the total amount of the dispersed second-phase particles at room temperature is 7×10⁻³ or more in terms of volume fraction, and the Vickers hardness (HV) is equal to or larger than the hardness H of the following equation (2):

H=(5.2−1.2×10⁻⁴λ)×10²  (2)

when the steel is subjected to any of annealing, tempering, and aging treatments in the as-unworked state under conditions such that a parameter λ expressed by the following equation (1):

λ=T(log t+20)(T; temperature(K), t; time(hr))  (1)

is 1.4×10⁴ or more in a prescribed temperature range of 350° C. or more and Ac1 point or less. This steel is taken as the steel for warm working.

Description

TECHNICAL FIELD[0001]The present invention relates to a steel to be worked into various structures, components of cars, and the like for use. More particularly, it relates to a steel for warm working to be subjected to warm working, and a warm working method thereof, and a steel material and a steel component obtainable from the warm working method.BACKGROUND ART[0002]In recent years, there has been a demand for a tougher and higher performance high strength steels than ever with an increase in size of structures, and a reduction in weight of car components or the like. As the means for improving the toughness of the steel, conventionally, there are generally known: (1) reduction of impurity elements such as P and S causing embrittlement, (2) refinement and reduction of inclusions, (3) addition of alloy elements, (4) reduction of carbon, (5) crystal grain refinement, (6) refinement of dispersed second-phase particles such as carbide particles, and the like.[0003]Out of these, crysta...

AI Technical Summary

Benefits of technology

[0062]As for the steel for warm working of the first invention, the softening resistance when the steel is heated, namely, the thermal stability and the total amount of the matrix structure and the dispersed second-phase particles are controlled. As a result, a particle dispersion type fibrous structure can be formed when the steel is subjected to warm working, and the Vickers hardness after warm working can be set at 3.7×102 or more. As a result, there is provided a steel for warm working capable of being tremendously improved in toughness while keeping the tensile strength of 1.2 GPa or more at ordinary temperatures.

[0063]In accordance with the second invention, the structure of the steel for warm working as the prior working structure is transformed into an ultrafine duplex structure including dispersed second-phase particles such as carbide particles finely dispersed therein by using martensite transformation or bainite transformation. As a result, it becomes possible to effect the formation of a fiber structure even in the inside with efficiency when the steel is subjected to warm working. In addition to this, it becomes possible to largely improve the delayed fracture resistance characteristics.

[0064]In accordance with the third invention, the alloy composition excellent in cost efficiency and recyclability can achieve strengthening of the steel obtained when the steel is subjected to warm working.

[0065]In accordance with the fourth invention, it is possible to disperse dispersed second-phase particles which are finer and excellent in hydrogen trapping property. Further, it is possible to strengthen the steel material obtained when the steel is subjected to warm working, and to largely enhance the toughness in a low temperature range, and the delayed fracture resistance characteristics.

[0066]In accordance with the fifth invention, it is possible to improve the toughness further to a lower temperature range.

[0067]In accordance with the sixth invention, while working the steel for warm working into a desired shape, a fiber structure can be formed to obtain a high toughness. Incidentally, as the equipment, warm working equipment which has been conventionally put into practical use can be utilized. Therefore, the invention has a very high practical utility.

Problems solved by technology

Thus, the reduction in grain size to as ultrafine as 0.5 μm or less is very difficult with a thermo-mechanical treatment of a steel intended for mass production.

However, such an ultrafine grain steel shows almost no uniform elongation, and the elongation is mostly caused by ununiform deformation due to necking, resulting in a large reduction in ductility.

In general, strengthening of a steel unfavorably largely reduces various characteristics such as ductility, toughness, delayed fracture resistance characteristics, fatigue characteristics, formability.

Particularly, when a very versatile low alloy martensitic steel is strengthened to 1.2 GPa or more, it is remarkably reduced in toughness, delayed fracture resistance characteristics, and the like.

(a) High Temperature Tempering

For this reason, for a steel for a mechanical structure particularly requiring toughness, tempering is generally carried out in the vicinity of 650° C. However, within such a temperature range, the dispersed second-phase particles also grow with ease during tempering, and hence the reduction in strength of the steel is inevitable.

Therefore, strengthening only by high temperature tempering has its limitation.

However, it is very difficult to suppress the growth of ultrafine austenite grains.

Whereas, excessive refinement of crystal grains promotes the diffusion type phase transformation at the grain boundary, which unfavorably makes hardening difficult, and also causes other problems.

Further, there is also another problem that quenching crack is caused during cooling after working.

However, for such steel materials as those having a tensile strength of more than 1.2 GPa, cold forging thereof is very difficult because of the strength.

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