JPH03211227A - Production of hot forged non-heat-treated steel having high strength and high toughness - Google Patents

Abstract

PURPOSE:To produce a hot forged non-heat-treated steel having high strength and high toughness and increased in tensile strength and yield strength by subjecting a steel having a specific composition containing C, Si, Mn, S, V, N, Cr, etc., and Ti, etc., to hot forging and to ageing at specific temp. and then allowing this steel to stand to be cooled. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.10-0.60% C, 0.80-3.0% Si, <=3.0% Mn, 0.050-0.30% S, 0.030-0.30% V, 0.005-0.060% N, further one or more kinds among <=3.0% Cr, <=3.0% Ni, <=1.0% Mo, and <=2.0% Cu, further one or more kinds among 0.001-0.050% Ti, 0.005-0.10% Nb, and 0.005-0.10% Al, and the balance Fe with inevitable impurities and further containing, if necessary, one or more kinds among 0.005-0.50% Pb, 0.001-0.050% Ca, 0.001-0.20% Te, 0.010-0.50% Se, and 0.010-0.50% Bi is hot-forged. Subsequently, this steel is aged at 200-600 deg.C and then allowed to stand to be cooled. By this method, the hot forged non-heat-treated steel having high strength and high toughness which has >=85kgf/mm<2> tensile strength and >=70kgf/mm<2> yield strength and in which machinability is improved, if necessary, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing hot forged non-thermal treated steel for mechanical structures having excellent strength, toughness and stiffness.

[Prior Art] Conventionally, in the manufacture of mechanical parts that require high strength and high toughness, a thermal treatment such as quenching and tempering is performed after hot forging into a predetermined shape. However, thermal refining treatment requires many steps and requires a large amount of thermal energy, leading to an increase in manufacturing costs. For this reason, in recent years, from the viewpoint of reducing the number of steps and saving energy, the development of hanging steel, that is, non-tempered steel, has been carried out by omitting the tempering treatment. If the aim is to improve the strength, non-tempered steel containing V is sufficient. However, this steel type has poor toughness, especially low-temperature toughness, and is inadequate as a material for use in automobile suspension parts that require high strength and high toughness.It also has extremely poor machinability and lacks part productivity. Furthermore, in recent years, there has been a trend in which yield strength has become more important in component design, and there is a desire to develop materials with so-called high yield ratios, which have as high a yield strength as possible for the same tensile strength.

On the other hand, Japanese Patent Application Laid-open No. 56-38448 discloses that Si
, Strengthening of base steel by increasing Mn, etc., and Ti,
By increasing the strength of the steel material by precipitation strengthening of V and Nb, and increasing the N content in the steel to 0.29% T1 or more, the precipitates of Ti, V and Nb, which are mainly nitrides, are generated, and the old steel is strengthened. The austenite grain size has been refined to improve the toughness of the steel material, and a material has been shown that can maintain excellent tensile strength and toughness without any subsequent heat treatment as it is hot forged.

[Lesson 1 that the invention attempts to solve! ! [I] However, the current situation is that even with the use of such materials, sufficient yield strength has not yet been secured.

The purpose of the present invention is to have a tensile strength of 85 kgf/mm2 or more,
It has sufficient toughness and excellent rigidity, and also has a capacity of 70 kgf/a.
It is an object of the present invention to provide a method for producing hot forged non-tempered steel having a high yield strength of m' or more.

[11! Means for Solving the Problem] The present inventors have conducted intensive studies to provide a method for improving the yield strength of hot-forged non-tempered steel that has high strength, high toughness, and excellent stiffness. The present invention was made based on the new knowledge that yield strength can be increased without deteriorating tensile strength, toughness, and machinability by performing aging treatment after forging, which has not been done in the past. This is what I did.

That is, the gist of the manufacturing method according to the first invention is that C: 0.10 to 0.60% Si: 0.80 to 3.0% Mn: 3.0% or less S: Contains 0.050 to 0.30% V: 0.030 to 0.05 0: 0.005 to 0.050%, and further includes Cr: 3.0% or less Ni: 3.0% or less MO: 1. Contains one or more of 0% or less Cu: 2.0% or less, further Ti: 0.001 to 0.050% Nb: 0.005 to 0.10% A-text 70.00
After hot forging steel containing one or more of 5 to 0.10% and the remainder consisting of Fe and unavoidable impurities, it is heated at 200℃ to 60℃.
Tensile strength of 85kgf/ by aging and cooling to 0℃
mm' or more, and a yield strength of 70 kgf/mm2 or more.

Next, the gist of the manufacturing method according to the second invention is that in addition to the composition of the first invention steel, Pb: 0.005 to 0.50% by weight, Ca: O, OO1 to 0,050%Te: 0
, 001-0, 20% Se: 0.010-0.05 0- After hot forging steel containing one or more of 0.
Bow tensile strength is 85kgf/ by cooling to 0℃ after aging.
The present invention is characterized in that it enables the production of hot-forged non-tempered steel having a yield strength of 70 kgf/mm'' or more, high strength, high toughness, and excellent rigidity.

[For production] The present invention will be explained in detail below.

First, is C an effective element for increasing the strength of forged products? If it is less than 0.10%, the strength will be insufficient, and if it is less than 0.10%, the strength will be insufficient.
If it exceeds 0%, the toughness will deteriorate, so the content should be reduced to 0.1%.
The content was set at 0 to 0.60%.

Next, Si is added for the purpose of increasing the strength by solid solution hardening, but if it is less than 0.80%, the effect is insufficient, while if it exceeds 3.0%, the effect is saturated, or rather Since it causes deterioration of toughness, the content should be reduced to 0.8
The content was set at 0 to 3.0%.

Further, Mn and S exist as MnS in steel and contribute to the refinement of the structure, but the effect is insufficient when S is less than 0.050%. Also, Mn: more than 3.0%, S: 0.30
If the Mn content exceeds 3.0%, the effect becomes saturated and the toughness deteriorates.

Furthermore, V and N contribute to the refinement of the structure through the precipitation behavior of VN, or when V: less than 0.030% and N: 0.
0.005%, the effect is insufficient; on the other hand, V
If the content exceeds V : 0.30% or N : 0.060%, the effect will be saturated and the toughness will deteriorate.
:0.030-0.30%, N:0. O05～o,o
It was set as ao%.

In addition, C, Mo, Ni, and Cu are effective elements for increasing the strength of forged products, but from an economical point of view, the content should be reduced to 3.0% or less for Cr and 1.0% for Mo. Hereinafter, Ni: 3.0% or less, Cu: 2.0% or less.

In addition, in the steel of the present invention, for the purpose of grain size adjustment, ^2, T
i. One or more types of Nb are added. However, Affi: less than 0.005%, Ti: 0.0
01%, Nb: less than 0.005%, the effect is insufficient, while shoulder: more than 0.10%, Ti: 0.05
Above 0%, Nb: above 0.10%, the effect is saturated,
Rather, it deteriorates the toughness, so A, Q: 0.005 to 0
．． 10%, Ti: 0.001-0.050%, Nb: 0
．． 005 to 0.010%.

Furthermore, in the non-tempered steel according to the present invention, Pb, Ca
One or more of Te, Se, and Bi are added for the purpose of improving machinability. However, Pb: less than 0.005%, Ca: 0.001%
The effect is insufficient if it is less than 0.001%, Te: less than 0.001%, Sa: O, less than 010%, Bi: less than 0.010%, Pb: more than 0.50%, Ca: more than 0.050%, T
e: more than 0.20%, Se: more than 0.50%, Bi: 0.5
If it exceeds 0%, the effect will be saturated and the toughness will deteriorate.
a: 0.001-0.050%, Te: 0.001-0
．． 20%, Se: 0.010-0.50%, Bi: 0
．． 0.010% to 0.50%.

The above is the basic composition of the steel of the present invention. When steel bars with these compositions are hot forged and cooled to room temperature, the structure is mainly bainite, with a small amount of martensite and retained austenite mixed therein. As it is, there is no problem in terms of tensile strength and toughness, but the yield strength is insufficient compared to heat-treated steel. Therefore, we focused on improving yield strength by applying aging treatment to reduce the mobility of dislocations in the martensitic phase. However, the aging temperature is 200
Below 600°C, the effect is insufficient, and above 600°C, both tensile strength and yield strength decrease significantly. Therefore, the aging temperature was set to 200°C to 600°C. Tensile strength of 85 kgf/a can be achieved by cooling after aging according to these conditions.
It is possible to produce hot-forged, non-thermal treated steel that has a yield strength of 70 kgf/+m" or more and has excellent strength, high toughness, and excellent rigidity.

[Example] Examples of the method for manufacturing non-thermal steel having high strength, high toughness, and excellent rigidity according to the present invention will be shown below.

Example-1 Steels having the compositions (1), (2), and 6.7 shown in Table 1 were melted in a high-frequency furnace, cast, and then rolled into round bars with a diameter of 50 mm. These were heated to 1250° C. and then hot forged into one beam. The forging finishing temperature is 1050°C, and after that the cooling rate is 1.
Cooled to room temperature at 0°C/sec. Some of these are 30
Aging treatment was performed at 0°C for 60 minutes. JIS No. J tensile test piece and J
IS No. 3 impact test pieces were taken and tensile strength and -50℃,
Charpy impact value at 20°C was determined. On the other hand, some products have a thickness of 30IIIl after being cast.
rolled into a steel plate. The rolling finish temperature was 1050°C, and thereafter, the material was cooled to room temperature at a cooling rate of 1.0°C/sec. This steel plate was used before aging and after aging (300℃ x 60℃).
m1n) was evaluated. As a guideline for rigidity, when drilling a blind hole with a depth of 20mm using a 5KH9 (φ5) drill without cutting oil and with an initial speed of 0.1ma+/rev, the drill life is 5000a+ for the total depth of the hole.
The cutting speed (m/ff1in) was used.

These results are shown in Table 2.

In Table 2, ■ and ■ are non-tempered steels having high strength and high toughness according to the first invention, and 6.7 is a comparative steel.

As is clear from Table 2, the steels of the present invention (2) and (2) both before and after aging have a tensile strength of 85 kgf/ll1m" or more, a low-temperature toughness of 3.0 kgf-m7cm" or more, and a It can be seen that it has room temperature toughness of .5 kgf-m/cm' or more. However, the yield strength is insufficient before aging. However, by adding aging, the yield strength of all of them increases to 70 kgf/a+m2 or more. On the other hand, Comparative Steel 6 is an example in which the C content is below the range of the present invention, and the toughness is equivalent to that of the developed steel, but the strength is insufficient both before and after aging. On the other hand, comparison wI7
This is a case where the S content is below the range of the present invention, and although there is no problem in strength, low temperature toughness and room temperature toughness are insufficient both before and after aging.

Example-2 In the same manner as in Example 1, steels with compositions 8 to 10 shown in the 'tS1 table were melted in a high frequency furnace, and after casting, a diameter of 50 m+ was produced.
It was rolled into a round bar. After heating these to 1250°C, l
Hot forged into a beam. The forging finishing temperature is 1050°C, and after that the cooling rate is 1. Cooled to room temperature at Otl:/sec. Some of them were aged at 300°C for 60 minutes. J in the longitudinal direction from the center of one of these beams
An IS J tensile test piece and a JIS No. 3 impact test piece were taken, and the tensile strength and Charpy impact value at -50°C and 20°C were determined. On the other hand, some of the products were melted in a high frequency furnace, cast, and then rolled into steel plates with a thickness of 30 mm. The rolling finishing temperature was 1050° C., and the material was then allowed to cool in the atmosphere. This steel plate was used before and after aging (3
The stiffness was evaluated at 00°C x 60a+in. As a guideline for rigidity, it is exactly the same as in Example 1, and the cutting speed (
a+/win) was used. These results are shown in Table 3.

In Table 3, ■ to ■ are non-thermal treated steels according to the second invention, and 8 to 10 are comparative steels. As is clear from Table 3, all of ■~■ are 85kgf/mn+ after the statute of limitations.
” or more, has a yield strength of 70 kgf/am2 or more, and has a low temperature toughness of 3.0 kgf-m/c or more, 6.
．． It can be seen that it has room temperature toughness of 5 kgf-m7 cm" or more. Comparative steel 8 is an example in which the Si content is lower than the range of the present invention, and the toughness is the same as the developed steel, but the strength is different before and after aging. On the other hand, comparative steel 9.10
These are cases in which the contents of S and N are respectively lower than the range of the present invention, and although the strength is not a problem, the low-temperature toughness and room-temperature toughness are insufficient both before and after aging.

Regarding rigidity, as can be seen by comparing Tables 2 and 3, machinability elements (Pb, Ca%Te.

It can be seen that the rigidity of specimens 1, 2, and 2 containing Se and Bi) is better than that of specimens 2 and 2 containing no Se and Bi.

[Effects of the invention] As stated above, by using the steel of the present invention,
It has a high tensile strength of kgf/mm2 or more, and it is possible to obtain high toughness and excellent stiffness, and furthermore, by aging after hot forging, it is possible to obtain a high yield strength of 70 kgf/mm2 or more.

This makes it possible to omit the thermal refining treatment that was conventionally required and to reduce the manufacturing cost associated with it, and the industrial effects are extremely significant.

4 others

Claims (1)

[Claims] 1% by weight: C: 0.10-0.60% Si: 0.80-3.0% Mn: 3.0% or less S: 0.050p-0.30% V: 0 Contains .030 to 0.30% N: 0.005 to 0.060%, and further contains Cr: 3.0% or less Ni: 3.0% or less Mo: 1.0% or less Cu: 2.0% or less It further contains one or more of the following: Ti: 0.001-0.050% Nb: 0.005-0.10% Al: 0.005-0.10% , after hot forging steel with the balance consisting of Fe and unavoidable impurities, 200°C to 60°C
Tensile strength of 85kgf/ by aging and cooling to 0℃
A method for producing a high-strength, high-toughness hot-forged non-thermal steel having a yield strength of 70 kgf/mm^2 or more. 2% by weight, and further steel components: Pb: 0.005-0.50% Ca: 0.001-0.050% Te: 0.001-0.20% Se: 0.010-0.50% Bi 2. The method for producing a high-strength, high-toughness hot-forged non-thermal steel according to claim 1, characterized in that the steel contains one or more of the following: 0.010 to 0.50% and has excellent rigidity.