JPH08209235A - Production of cast steel product having low yield ratio - Google Patents

Abstract

PURPOSE: To provide a method for producing a low alloy cast steel product in which low yield ratio for high tensile strength and anti-earthquake design is realized. CONSTITUTION: A low alloy cast steel casting material contg. <=0.2% C, <=1% Si, <=2% Mn, <=2% Ni, <=1% Cr, <=1% Mo, <=0.5% V, <=1% Cu, and the balance substantial Fe is hardened from the temp. range (a) of the Ac3 point or above, is thereafter hardened from the two phase temp. range of the Ac1 to the Ac3 point (preferably, Ac1 +30 deg.C to the Ac1 +80 deg.C) and is then subjected to tempering treatment. Otherwise, it is subjected to normalizing treatment of executing hardening from the temp. range (b) of the Ac3 point or above and thereafter executing air cooling from the two phase temp. range of the Ac1 to the Ac3 point and is then subjected to tempering treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low yield ratio cast steel product useful as a civil engineering / construction steel aggregate or a joint material.

[0002]

2. Description of the Related Art Conventionally, JIS G5102 cast steel products for welding structures, JI, have been used as steel aggregates and joint materials in the field of civil engineering and construction.
Centrifugal casting pipes for welded structures of SG5201 are used. Recently, due to the demand for weight reduction of structural members due to the increase in the number of buildings, high tensile strength is required, and from the viewpoint of seismic design, high deformability is emphasized and the yield ratio (yield strength / tensile strength) There has been a demand for a low yield ratio steel material having a low value (about 0.8 or less).

[0003]

To increase the tensile strength of steel,
For example, addition of alloying elements such as Ni, Cr, Mo, quenching,
Although it is effective to carry out tempering, etc., these methods increase the yield strength and lower the yield ratio (about 0.8 or less).
Is difficult to meet. The present invention is to provide a low-alloy cast steel product for a welded structure, while maintaining tensile strength, to reduce the yield strength and to provide a method for producing a low-yield ratio cast steel product capable of satisfying a request for a low yield ratio. is there.

[0004]

A first method for producing a low yield ratio cast steel product according to the present invention is C: 0.2% or less, Si:
1.0% or less, Mn: 2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, V:
0.5% or less, Cu: 1.0% or less, balance substantially Fe
The low alloy cast steel cast material consisting of is quenched from a temperature range of A _c3 or higher, is quenched from a two-phase temperature range of A _{c1 to} A _c3 , and is then tempered. . The second manufacturing method of the present invention is a normalizing treatment in which the above-mentioned low alloy steel casting material is quenched from a temperature range of A _c3 points or higher, and then air-cooled from a two-phase temperature range of A _{c1 to} A _c3 points. And then tempering.

[0005]

[Operation] A cast steel product obtained by subjecting the above-mentioned low alloy steel cast material to ordinary quenching and tempering treatment has a mixed phase structure in which tempered martensite phase, bainite phase and ferrite phase are mixed, and this product has high tensile strength. However, due to the increase in yield strength, it exhibits a high yield ratio. On the other hand, during quenching and tempering, the temperature range of A _{c1 to} A _c3 points (austenite-
This is a book that decides to carry out quenching (two-phase quenching treatment) by rapid cooling (water-cooling) from the ferrite two-phase region or normalizing (two-phase normalizing treatment) by gradually cooling (air cooling) from the two-phase temperature region The cast steel product obtained by the method of the invention is mainly composed of a ferrite phase and exhibits a mixed phase structure in which a fine bainite phase and a tempered martensite phase are mixed therein, and this one does not impair the tensile strength and is about 0.8 or less. It has a low yield ratio of.

The reasons for limiting the chemical composition of the cast steel material according to the present invention are as follows. C: 0.2% or less C is a solid solution strengthening element, but if it exceeds 0.2%, the ductility decreases, weld cracking easily occurs, and the suitability as a weld structural material is impaired. Is the upper limit. It is preferably 0.03 to 0.2%.

Si: 1.0% or less Si is added as a deoxidizing element in the steel melting process. The amount is sufficient up to 1.0%, and if it exceeds that, the toughness of the steel is impaired and the weldability deteriorates.

Mn: 2.0% or less Mn is a deoxidizing / desulfurizing element and an element having an effect of improving hardenability. However, since a large amount of Mn deteriorates weldability, 2.0% is an upper limit. And

Ni: 2.0% or less Ni enhances the hardenability, reduces the mass effect in the quenching treatment of thick wall materials (wall thickness of about 50 mm or more) used particularly for building columns, etc. Effective in improving toughness. For this purpose, the addition amount up to 2.0% is sufficient, and addition of a large amount exceeding that not only impairs the economical efficiency but also deteriorates the weldability. Preferably, it is 0.4 to 2.0%.

Cr: 1.0% or less Cr enhances the hardenability and partially forms carbides to enhance the strength of steel by the precipitation hardening action. However, if it exceeds 1.0%, the toughness of the steel decreases and the weldability is impaired.
This is the upper limit.

Mo: 1.0% or less Mo is an element that enhances hardenability and tempering resistance. Further, a part of Mo forms carbides and contributes to the strength improvement of steel. However, if added in a large amount, the toughness of steel is impaired and the weldability deteriorates, so it is made 1.0% or less.

V: 0.5% or less V improves the hardenability and forms carbides to increase the strength of the steel, but if it exceeds 0.5%, the weldability is impaired. The upper limit.

Cu: 1.0% or less Cu is an element effective for improving strength and ductility.
However, if it exceeds 1.0%, the weldability of steel is impaired, so this is made the upper limit. It is preferably 0.2% or less.

Impurity components such as P and S are allowed to be mixed within the range inevitably accompanied by the usual alloy melting technology,
Mixing P of 0.04% or less and S of 0.04% or less does not impair the gist of the present invention.

The low yield ratio cast steel product of the present invention is a low alloy steel cast product having the above chemical composition (a) after quenching (rapid cooling from the austenite single phase temperature range of A _c3 points or more), A _{c1 to} A _{c.
After the step of performing} "two-phase quenching" that quenches from _c3 point (austenite-ferrite two-phase temperature range) and then tempering, or (b) after quenching (quenching from austenite single-phase temperature range of A _c3 point or more) , A _{c1 to} A _c3 points (austenite-ferrite two-phase temperature range) are subjected to "two-phase normalizing" in which they are air-cooled, and then tempered. Quenching from a temperature of A _c3 or higher may be water cooling. The heating temperature in the two-phase quenching treatment performed after the quenching is preferably in the range of A _c1 point + 30 ° C. to A _c1 point + 80 ° C. from the viewpoint of the low yield ratio. Water cooling is suitable for cooling from the same temperature range. On the other hand, the heating temperature for the two-phase normalizing is preferably in the temperature range of A _c1 point to A _c1 point + 30 ° C. The tempering treatment performed after the two-phase quenching or the two-phase normalizing is performed by heating and holding in an appropriate temperature range of A _c1 point or less as in the tempering in the normal quenching / tempering treatment, and then air cooling.

[0016]

【Example】

[Example 1] [1] Sample material Centrifugal casting pipe made of the following low alloy steel (pipe diameter 550, wall thickness 6
(0, mm) is heat treated to obtain the test material. Chemical composition (Wt%): C 0.10, Si 0.23, Mn 0.84,
P 0.013, S 0.011, Ni 0.06, Cr 0.10, Mo 0.
50, V 0.09, Cu 0.14. A _c1 point: 730 ° C, A _c3 point: 890 ° C.

[2] Mechanical Properties Table 1 shows the heat treatment conditions and the measurement results of the mechanical properties of each test material. Inventive Example No. 11 is an example of the first manufacturing method (quenching → two-phase quenching → tempering) of the present invention.
o.12 to 14 are examples of the second manufacturing method (quenching → two-phase normalizing → tempering) of the present invention. Comparative Example No. 15 is an example that has been subjected to normal quenching and tempering treatment, and Comparative Example No. 16
In this example, quenching and two-phase normalizing are performed, but the subsequent tempering process is omitted.

In Table 1, Comparative Example No. 15 (performed by normal quenching and tempering treatment) has a high tensile strength, but has no effect of improving the yield ratio, and the yield ratio greatly exceeds 0.8. . Comparative Example No. 16 (which is subjected to quenching and two-phase normalizing but omits tempering treatment) has low yield strength and low toughness, and is not suitable as a structural material. On the other hand, Invention Examples Nos. 11 to 14 have high tensile strength and also have a low yield ratio of 0.8 or less as an effect of lowering the yield point. In addition, ductility and toughness are also good.

Example 2 [1] Specimen Material Centrifugal casting pipe made of the following low alloy steel (pipe diameter 800, wall thickness 8
(0, mm) is heat treated to obtain the test material. Chemical composition (Wt%): C 0.10, Si 0.29, Mn 1.05,
P 0.013, S 0.011, Ni 0.68, Cr 0.08, Mo 0.
30, V 0.09, Cu 0.05. A _c1 point: 735 ° C, A _c3 point: 875 ° C.

[2] Mechanical Properties Table 2 shows the heat treatment conditions and the measurement results of mechanical properties of each test material. Inventive Example No. 21 is an example of the first manufacturing method (quenching → two-phase quenching → tempering) of the present invention.
o.22 is an example of the second manufacturing method of the present invention (quenching → two-phase normalizing → tempering). Comparative Example No. 23 is an ordinary quenching / tempering treated material, and Comparative Example No. 24 is an example in which only normal normalizing (after heating to the _{Ac 3} point or more and then air cooling) is performed.

In Table 2, invention examples No. 21 and No. 22
Has a high strength (590 N / mm ² class or higher) and a low yield ratio of 0.8 or lower, has excellent ductility and toughness, and has significantly improved material properties as a structural material. In particular, No. 22 has material properties that combine high strength of 590 N / mm ² or higher and low yield ratio of 0.7 or lower. On the other hand,
Comparative Example No. 23 (normal quenching / tempering material) has high strength, but it cannot satisfy the low yield ratio (0.8 or less).
Although Comparative Example No. 24 (normalized material) satisfied the low yield ratio, its strength did not reach that of the invention example (590 N / mm ² class or higher) and its impact characteristics were also inferior.

FIG. 1 shows the metallographic structure of the sample material No. 21 of the invention example, and FIGS. 2 and 3 show the metallographic structures of the sample materials No. 23 and No. 24 of the comparative examples, respectively. There is. Test material N
o.22 (Fig. 2) is a mixed structure mainly composed of bainite phase and a small amount of ferrite phase, and sample No. 24 (Fig. 3) is a mixed structure of coarse ferrite phase and pearlite phase (partly bainite phase). In contrast to this, the test material No. 21 of the invention example
It is observed that (FIG. 1) has a mixed structure in which fine ferrite phase is the main component and fine bainite phase is dispersed.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

The low alloy cast steel products produced by the method of the present invention have high tensile strength and high toughness as a heat treatment effect, and at the same time, have a low yield ratio (about 0.8 or less) and also have a good elongation and toughness. It is excellent, suitable as a seismic design material such as a pillar material for high-rise buildings, and enables its weight reduction.

[Brief description of drawings]

FIG. 1 is a drawing-substituting micrograph (magnification 400) showing an example of a metal structure of a cast steel product.

FIG. 2 is a drawing-substituting micrograph (magnification: 400) showing an example of a metal structure of a cast steel product.

FIG. 3 is a drawing-substitute micrograph (magnification: 400) showing an example of a metal structure of a cast steel product.

Claims (2)

[Claims] 1. C: 0.2% or less, Si: 1.0% or less, Mn: 2.0% or less, Ni: 2.0% or less, Cr:
1.0% or less, Mo: 1.0% or less, V: 0.5% or less, Cu: 1.0% or less, the balance is a low alloy cast steel casting material consisting essentially of Fe, at a temperature of A _c3 point or more. A method for producing a low yield ratio cast steel product, which comprises quenching quenching from a region, quenching quenching from a two-phase temperature region of A _{c1 to} A _c3 points, and then tempering. 2. C: 0.2% or less, Si: 1.0% or less, Mn: 2.0% or less, Ni: 2.0% or less, Cr:
1.0% or less, Mo: 1.0% or less, V: 0.5% or less, Cu: 1.0% or less, the balance is a low alloy cast steel casting material consisting essentially of Fe, at a temperature of A _c3 point or more. A method for producing a low yield ratio cast steel product, which comprises quenching from a zone, quenching, air-cooling from a two-phase temperature region of A _{c1 to} A _c3 , and then tempering.