JPS61264129A - Manufacture of unrefined hot forged article having high strength and toughness - Google Patents

Abstract

PURPOSE:To provide superior strength and toughness to a hot forged article by heating a steel obtd. by adding prescribed amounts of Mn and Cr to a machine structural carbon steel to a prescribed low temp., hot forging the heated steel and cooling the resulting hot forged article so as to reduce the amount of proeutectoid ferrite in the article. CONSTITUTION:A steel contg., by weight, 0.25-0.6% C, 0.1-1% Si, 1-2% Mn and 0.3-1% Cr is heated to the Ac3 transformation point - 1,050 deg.C and hot forged. The resulting hot forged article is cooled so as to form a steel structure contg. proeutectoid ferrite by an amount F(%) represented by the formula [where C% is the C content (wt%) in the steel]. The structure is a ferrite-pearlite structure having <=0.2mum space between pearlite lamellae.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for manufacturing a high-strength, high-toughness non-temperature hot forged product. This invention relates to a method for manufacturing non-thermal forged products for machine structures having excellent strength and toughness.

(Prior art) Conventionally, hot forged products for machine structures are generally produced by hot forging medium carbon steel or low alloy steel material, then reheating, quenching, and tempering, that is, refining treatment to achieve the desired purpose. , and are provided with strength and toughness depending on the purpose. However, the thermal refining treatment requires a large amount of thermal energy, and the manufacturing cost inevitably increases due to an increase in the number of processing steps, an increase in the number of products in progress, and the like.

Therefore, in recent years, in the production of hot forged products for machine structures,
In order to simplify the manufacturing process, and in particular to omit tempering heat treatment after hot forging, various non-thermal hot forging steels and
A method for manufacturing non-thermal hot forged products has been proposed. Most of these conventional non-temperature hot forging steels are precipitation-hardening non-temperature steels made by adding small amounts of so-called precipitation-hardening alloying elements such as V, Nbs Ti, and Zr to medium carbon steel. The aim is to precipitate these elements in the cooling process after hot forging and obtain high strength through precipitation hardening.

For example, in Japanese Patent Publication No. 58-2243, a small amount of V is added to medium carbon steel, which is then heated to a temperature of 1,100°C or higher and die-forged, and then heated to 500°C for 10~b. , describes a method for producing a non-thermal forged product having a ferrite-pearlite structure in which fine V carbonitrides are precipitated in ferrite. However, when using such precipitation hardening non-tempered steel, the steel is heated to 1000 to 1100°C as described above in order to sufficiently dissolve the precipitation hardening alloying elements in the steel before hot forging. It is necessary to heat the forged product to a higher temperature, and as a result, the grain structure in the forged product becomes significantly coarsened, making it impossible to obtain sufficient toughness.

In order to solve these problems, the amount of precipitation hardening elements added to the steel material and the forging method should be reduced as much as possible (for example, Japanese Patent Application Laid-Open No. 55-82750), low C, high Mn, etc. For example, Japanese Patent Application Laid-Open No. 54-121225), controlling the type of precipitates (for example, Japanese Patent Application Laid-Open No. 56-3
8448 (Japanese Patent Publication No. 8448), and micronization of crystal grains by controlled cooling (for example, Japanese Unexamined Patent Application Publication No. 169723/1984), none of these methods has been able to produce a non-tunable material with excellent strength and toughness. Obtaining quality hot forged products is
However, it is not easy.

(Purpose of the Invention) In order to solve the above-mentioned problems, the inventors of the present invention have developed a non-temperature steel alloy with high strength and toughness, without using precipitation hardening elements and without heat treatment after hot forging. As a result of intensive research in order to obtain high-quality hot forged products, we found that carbon steel for mechanical structures with a predetermined amount of Mn and Cr added was heated to a predetermined low temperature range before hot forging to coarsen the austenite crystal grains. In addition, by cooling after hot forging, the amount of pro-eutectoid ferrite in the forged product is reduced, and by refining the pearlite structure, strength and toughness are immediately improved without heat refining after hot forging. The present invention was achieved by discovering that a forged product can be obtained.

(Structure of the Invention) The method for producing a high-strength, high-toughness, non-temperature forging steel according to the present invention includes, in weight percent, C0.25-0.60%, Si0.10-1.00%, Mn 1.00-1.00%. Ac, the steel containing 2.00% and 0.30 to 1.00% Cr, whose temperature is above the transformation point and whose temperature is 10
After hot forging by heating to a temperature of 50°C or less, cooling the steel structure, the amount of pro-eutectoid ferrite is F (%), F≦85-1000% (%) (However, 0% indicates the content in weight percent in the steel.

), and the ferrite-pearlite structure is such that D≦0.20 (μm), where D (μm) is the pearlite-lamella spacing.

First, conventional carbon steel for machine structures (S28C-S58C)
and 80% steel to which predetermined amounts of Mn and Cr are added.
After heating to 0 to 1000°C, rolling 0 to 50% and letting it cool, a hot forging simulation was performed to produce a plate material with a thickness of 15 cranes, and about the test pieces obtained from these hot forging simulation treated materials. The relationship between tensile strength and Charpy impact value is shown in Figure 1. From this, it has been found that the toughness of the above-mentioned carbon steel for mechanical structures is improved by adding Mn and Cr at the same strength. Incidentally, when the tensile strength of the steel according to the present invention is T S (kgf/mm2) and the Charpy impact value is CIV (kgf/mm"), (TS)・(C1y) l/2 is 230 or more. Figure 1 shows that it has excellent strength and toughness.

Next, the amount of pro-eutectoid ferrite and pearlite lamella spacing in the hot forging simulation treated material, and the strength and
The relationship with toughness balance is shown in Figure 2. In other words, the microstructure of the treated material whose toughness has been improved by the addition of Mn and Cr is F≦85-1000% (%), where the amount of pro-eutectoid ferrite is F (%) (however, 0% is steel It shows the content in weight%.

) It is understood that F is small.

It was also found that the pearlite lamella spacing was smaller than 0.20 μm.

That is, according to the present invention, a steel made by adding a predetermined amount of Mn and Cr to a conventional carbon steel for machine structural use is used as a raw material steel, and by cooling this steel after hot forging, the forged product can be made under a microscope. The amount F of ferrite in the structure is suppressed to a predetermined value or less, and the pearlite lamella spacing is 0.20. ct
By making it smaller than m, it is possible to obtain a hot forged product with an excellent balance of strength and toughness without heat refining.

Next, the reasons for limiting the chemical components of the species used in the method of the present invention will be explained.

C needs to be added in an amount of 0.25% or more in order to give the hot forged product according to the present invention the strength necessary as a mechanical structural component. However, when added in excess, toughness and machinability are impaired, so the upper limit of the amount added is set at 0.60%.

Si is an element that is necessary as a deoxidizing agent in steel manufacturing and is also necessary to strengthen ferrite. In order to effectively obtain such effects, in the present invention, 0.1
It is necessary to add 0% or more, but when adding too much, inclusions such as SiO□ increase and reduce toughness and machinability, so the upper limit of the addition amount is set at 1.00%. do.

As mentioned above, Mn and Cr are essential elements in the steel used in the method of the present invention in order to keep the amount of pro-eutectoid ferrite and the pearlite-lamella spacing within a predetermined range after hot forging. At least 1.00% Cr
It is necessary to add at least 0.30%. However, adding too many of these elements
Not only is it economically disadvantageous, but it also increases the susceptibility to quench cracking in parts that undergo induction hardening after forging, so the upper limit for the amount of Mn added is 2.00.
%, Cr is 1.00%.

Aβ is 0,01 for deoxidizing steel and refining grains.
It is necessary to add 0% or more, but 0.060%
When the value exceeds this value, the machinability of the steel deteriorates.

Accordingly, the amount of AID added is in the range of 0.010-0.060%.

In addition, in the present invention, in addition to the above-mentioned elements, at least one selected from the group consisting of 0.1% or less Ti and 0.1% or less Nb may be added to the steel material used, in order to refine the grains, if necessary. One type of element can be added. In addition, in order to improve machinability, S 0.15% or less, Pb 0.30% or less, Ca 0.020% or less, Se 0.30% or less, Te 0.30% or less, Bio, 10% or less, and Ce0. It is also possible to add at least one element selected from the group consisting of 20% or less.

Next, the forging conditions in the method of the present invention will be explained.

According to the method of the present invention, the above-described element-containing solution 14
woA When hot forging is performed by heating to a temperature above the C3 transformation point and below 1050 ° C., followed by natural cooling or accelerated cooling, and the amount of pro-eutectoid ferrite is F (%), F≦85 140C%C%) (1) (However, 0
% means the amount added in weight % in steel. ) When the lamella spacing is D (μm), it is necessary to have a ferrite-pearlite structure that satisfies the following (D≦0.20 (μm)) (2).

The heating temperature before forging needs to be at least the Ac3 transformation point in order to obtain a uniform austenite structure. On the other hand, the heating temperature before forging needs to be 1050° C. or lower to avoid coarsening of crystal grains. That is, 1
This is because when heating to a temperature higher than 050° C., the crystal grains become coarser and even though the strength is improved, the toughness is further reduced.

As mentioned above, in the method of the present invention, it is necessary to satisfy the conditions (1) and (2) above. High quality hot forged products can be obtained.

Regarding cooling after hot forging, in the case of large parts, it may be difficult to obtain a required microscopic structure when naturally cooling after hot forging. In such cases, after hot forging, it is possible to control the microstructure to satisfy the above conditions by performing accelerated cooling instead of natural cooling to obtain a high-strength, high-toughness non-thermal forged product. I can do it.

(Effects of the Invention) As described above, according to the present invention, a predetermined amount of Mn and Cr
By heating machine structural steel to which a predetermined amount of is added to a predetermined low temperature range before forging, coarsening of crystal grains is prevented.
Furthermore, during cooling after hot forging, the amount of pro-eutectoid ferrite is reduced and the pearlite structure is refined, so
High strength, high toughness, non-thermal forging steel can be obtained.

(Examples) Examples of the present invention are listed below, but the present invention is not limited to these Examples in any way.

Example Steel having the chemical composition shown in Table 1 was heated to 800 to 1200°C.
A plate material with a thickness of 15 fi was manufactured by performing a hot forging simulation process in which the material was heated to 0 to 50%, and then allowed to cool.

The filA of the present invention is obtained by adding predetermined amounts of Mn and Cr to comparative mD, which is a conventional carbon steel for mechanical structures, to give it the chemical composition according to the present invention. Comparative steel B is one in which the amount of C added exceeds the range specified in the present invention;
By increasing the amount, pro-eutectoid ferrite is reduced and high strength is attempted. Furthermore, steel C is S45C
It is a precipitation hardening type non-temperature steel in which V is added to m.

The mechanical properties of the above-mentioned hot forging simulation-treated plate materials made of the above-mentioned inventive steel and comparative steel were determined by taking JIS No. d tensile test pieces and JIS No. a impact test pieces from the above-mentioned sheets parallel to the rolling direction, and measuring Tensile and impact tests were conducted and evaluated. The results are shown in FIG. 1 and Table 2. In addition, regarding the comparative steel, 85
Table 2 also shows the mechanical properties of the samples heated at 0°C, water-cooled, and tempered at 600°C.

As is clear from the above results, according to the present invention, conventional carbon steel for mechanical structures, or comparative steel made by adding a large amount of C to it, can be made into precipitation-hardened non-thermal steel by addition of It is possible to obtain a non-thermal hot forged product that has both superior strength and toughness compared to high-strength steel.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amounts of Mn and Cr added in mechanical structural steel and the tensile strength and impact value of non-tempered hot forged products, and Figure 2 is a graph showing the strength of hot forged products. - It is a graph showing the relationship between toughness balance, pro-eutectoid ferrite area ratio, minimum pearlite lamella spacing, and C content.

Claims (1)

[Claims] (1) Steel containing 0.25 to 0.60% C, 0.10 to 1.00% Si, 1.00 to 2.00% Mn, and 0.30 to 1.00% Cr by weight at Ac_3 transformation point or higher and 1
After hot forging by heating to a temperature of 050°C or less,
When the steel structure is cooled and the amount of pro-eutectoid ferrite is F (%), F≦85-140C% (%) (However, C% indicates the content by weight% in the steel.) A method for producing a high-strength, high-toughness non-thermal hot forged product, characterized in that the ferrite-pearlite structure is such that D≦0.20 (μm), where the pearlite-lamella spacing is D (μm).