JPH07102316A - Heat treatment for steel casting - Google Patents

Abstract

PURPOSE:To produce a stable and superior steel casting with low yield ratio by subjecting a steel casting for structural use to hardening or to normalizing treatment under specific temp. conditions and then applying heating and air cooling. CONSTITUTION:A steel casting for welded structure is first heated up to an annealing temp., e.g. 930 deg.C, higher than the Ac3 transformation point of the steel casting, held at the temp. for proper time, and subjected to hardening by water cooling or air cooling or to normalizing treatment. Subsequently, this heat treated steel casting is heated up to a temp., e.g. 810 deg.C, not lower than its Ac1 transformation point and lower than the Ac3 transformation point, held at the temp. for proper time, and then air-cooled. By this method, the yield ratio of this steel casting can be reduced to a value as low as <=0.75, and the steel casting excellent in deformability and toughness can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for a cast steel product, which comprises casting a structural cast steel product and then performing a heat treatment for lowering the yield ratio of the cast steel product.

[0002]

2. Description of the Related Art When a yield ratio (yield point / tensile strength) is lowered by appropriately treating a structural cast steel product, the cast steel product becomes excellent in deformability and becomes tenacious. A structure made of the cast steel product is less likely to be destroyed in an abnormal condition such as an earthquake than a structure made of a cast steel product having a high yield ratio. Therefore, a heat treatment for reducing the yield ratio is generally performed on the cast structural steel product. By the way, as a conventional heat treatment for lowering the yield ratio, a heat treatment for performing normalizing by heating to an appropriate normalizing temperature higher than Ac ₃ point (A ₃ point at the time of heating), or Ac ₃ point A heat treatment has been adopted in which after heating to a higher quenching temperature as appropriate, quenching is performed, and then to a suitable tempering temperature lower than the Ac ₁ point (A ₁ point during heating) and tempering. For example, when the cast steel product is a welded structure cast steel product (JIS standard: SCW) or a welded structure centrifugal force cast steel product (JIS standard: SCW-CF), after heating to 930 ° C. higher than the Ac ₃ point. ,
Normalizing treatment by air cooling (see FIG. 3), or quenching and tempering treatment by heating to 930 ° C. higher than Ac ₃ point and water cooling, and then heating to 600 ° C. lower than Ac ₁ point and air cooling (FIG. 4) (See) was adopted.

[0003]

However, when the above-mentioned normalizing treatment or quenching and tempering treatment is applied to the cast steel product, the yield ratio of the cast steel product does not decrease sufficiently as expected (specifically, Specifically, it will be 0.80 or more),
For example, in order to obtain a cast steel product having an excellent low yield ratio of 0.75 or less, there is a problem that the above-mentioned conventional method is unsuitable. The present invention has been made by paying attention to such an actual situation, and in the case of obtaining a structural cast steel product which can be dealt with even under extremely severe conditions, the yield is performed at a low cost which is not so different from the cost of the conventional method. It is an object of the present invention to provide a heat treatment method by which a cast steel product having an excellent low yield ratio of ratio: 0.75 or less can be obtained.

[0004]

A method for heat treatment of a cast steel product according to the present invention (hereinafter referred to as a method of the present invention) is a cast steel for casting a structural cast steel product and then performing a heat treatment for lowering the yield ratio of the cast steel product. A method for heat treating a cast steel product, the method comprising:
After quenching or normalizing by heating to an appropriate temperature higher than ₃ points, the cast steel product subjected to the primary heat treatment is subjected to an appropriate temperature higher than Ac ₁ point and lower than Ac ₃ point (that is, Ac ₁
It is characterized in that it is air-cooled after being heated to an appropriate temperature of ₃ points to Ac ₃ points).

[0005]

According to the method of the present invention in which, after the quenching or normalizing is performed, heating is performed at an appropriate temperature of Ac _{1 to} Ac ₃ and air cooling is performed, an excellent low yield ratio of 0.75 or less is obtained. It was empirically confirmed by a test to be described later that a ratio of cast steel products was obtained. Moreover, after the quenching or normalizing, the cost of heating to an appropriate temperature of Ac ₁ point to Ac ₃ point and air cooling is the conventional quenching and tempering treatment in view of the heat cycle. Therefore, the method of the present invention can be carried out at a low cost which is not so different from the cost of the conventional method.

[0006]

As a result, according to the above-mentioned method of the present invention, when it is desired to obtain a structural cast steel product that can be dealt with under extremely severe conditions, the yield ratio can be reduced at a low cost which is not so different from that of the conventional method. A cast steel product having an excellent low yield ratio of 0.75 or less can be obtained, thereby achieving the object of the present invention.

[0007]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the method of the present invention in which a cast steel product for welding structure (JIS standard: SCW) or a centrifugal cast steel product for welding structure (JIS standard: SCW-CF) is used as a processed product. The heat treatment conditions of the examples are shown. The heat treatment conditions will be specifically described. First, the cast steel product is first heated to an appropriate quenching temperature (specifically, 930 ° C.) higher than the Ac ₃ point, and an appropriate time (specifically, After holding for 2 hours), quenching treatment is performed by water cooling or air cooling. Then, the cast steel product after the quenching treatment is heated to an appropriate temperature (specifically, 810 ° C.) higher than the Ac ₁ point and lower than the Ac ₃ point for an appropriate time (specifically, After holding for 1 hour), air cooling is performed. A tensile test piece was sampled from the cast steel product subjected to such heat treatment, and a tensile test using the tensile test piece was conducted to examine the yield point and the tensile strength.
It was confirmed that it was 4.5 kg / mm ² (338 N / mm ² ), and the tensile strength was 54.6 kg / mm ² (535 N / mm ² ). Therefore, the yield ratio was 0.63, and it was confirmed that a cast steel product having an excellent low yield ratio of yield ratio: 0.75 or less was obtained by the method of the present invention.
The elongation was 26.0%.

Next, another embodiment will be described. FIG. 2 shows a second embodiment in which the method of the present invention was carried out using a welded structure cast steel product (JIS standard: SCW) or a welded structure centrifugal force cast steel product (JIS standard: SCW-CF) as an object to be treated. The heat treatment conditions are shown. The heat treatment conditions will be specifically described. First, the cast steel product is first appropriately tempered at a temperature higher than the Ac ₃ point (specifically, 920 ° C.).
After being heated to, and held for an appropriate time (specifically, 2 hours), normalizing treatment is performed by air cooling. Thereafter, the cast steel product after the normalizing treatment is subjected to an appropriate temperature higher than the Ac ₁ point and lower than the Ac ₃ point (specifically, 8
After heating to 10 ° C. and holding for an appropriate time (specifically, 1 hour), air cooling is performed. A tensile test piece was taken from the cast steel product subjected to such heat treatment, a tensile test was performed using the tensile test piece, and the yield point and the tensile strength were investigated to obtain the yield ratio. In the case of yield as well: 0.7
It was confirmed that a cast steel product having an excellent low yield ratio of 5 or less was obtained.

[Brief description of drawings]

FIG. 1 is an explanatory view showing heat treatment conditions (first embodiment) of the present invention.

FIG. 2 is an explanatory view showing heat treatment conditions (second embodiment) of the present invention.

FIG. 3 is an explanatory diagram showing conventional heat treatment conditions.

FIG. 4 is an explanatory diagram showing conventional heat treatment conditions.

Claims (1)

[Claims] 1. A method for heat treatment of a cast steel product, comprising the step of casting a structural cast steel product and then performing a heat treatment for lowering the yield ratio of the cast steel product, wherein the cast steel product is heated to an appropriate temperature higher than the Ac ₃ point. Then, after quenching or normalizing, the primary heat treated cast steel product is heated to an appropriate temperature higher than the Ac ₁ point and lower than the Ac ₃ point, and then air-cooled.