JPS61238941A - Untempered steel for hot forging - Google Patents

Abstract

PURPOSE:To obtain untempered steel for hot forging requiring no heat treatment after forging by incorporating specific percentage of C, Si, Mn, Cr, acid-soluble Al, Ti and B to Fe. CONSTITUTION:The untempered steel for hot forging consisting of, by weight, 0.05-0.15% C, 0.05-1.00% Si, 1.30-2.50% Mn, 0.30-2.00% Cr, 0.015-0.060% acid-soluble Al, 0.010-0.100% Ti, 0.0003-0.0005% B, and the balance Fe with inevitable impurities is prepared, which requires no normalizing heat treatment after forging and has superior toughness and ductility as well as high strength.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a non-tempered steel for hot forging that does not require heat treatment such as quenching and tempering after forging.

<Conventional technology> Conventionally, mechanical structural parts used in automobiles and construction machinery have been made of mechanical structural carbon steel or alloy steel, which is hot forged, reheated, quenched and tempered, etc. It is manufactured by applying heat treatment to give it strength characteristics depending on the purpose and use. However, due to the cost of a large amount of thermal energy in the 1-fertilization treatment, an increase in the number of processing steps, an increase in the number of products in progress, etc., the manufacturing cost of the above-mentioned mechanical structural parts inevitably increases.

Therefore, in recent years, in the manufacture of forged steel products, JI
Carbon steel for mechanical structures specified in SG4051 and JISG
The material is so-called non-thermal steel for hot forging, which is made by adding trace amounts of precipitation hardening alloy elements such as V, Nb, and Ti to manganese steel and manganese chromium steel for machine structures specified in 4106. Non-thermal forged steel products, which can obtain the desired properties without the need for heat treatment through heating, forging, and subsequent cooling, are attracting attention, and some have already been put into practical use.

Normally, such non-thermal forged steel products have a temperature of 1100 to 1350
Forging is started at a temperature of 1,000 to 1,300 degrees Celsius, and after the forging is finished at a temperature of 1,000 to 1,300 degrees Celsius, it is manufactured by cooling in the atmosphere, blast cooling, or fountain cooling.

<Problems to be solved by the invention> However, non-tempered forged steel products manufactured using non-tempered steel for continuous hot forging, which has already been put into practical use, are inferior to heat-treated forged steel products. The same strength is ensured, but the toughness and ductility are inferior. This is because the strength of non-thermal forged steel products is mainly ensured by the strength of pearlite and the strength due to the precipitation of carbonitrides such as V and Nb, and the structure is ferrite and pearlite transformed from coarse austenite grains. This is due to this. For this reason, attempts have been made to improve the toughness and ductility by optimizing the ratio of chemical components such as C, Mn, and V in the raw steel and by lowering the forging temperature. We have not yet reached the point where we can secure this. especially,
When using conventional non-temperature steel for hot forging as a material, if the tensile strength is 90 kgf/mm2 or more, the
The absorbed energy in the mU Notch Charpy impact test is 3.
The toughness decreases extremely to 2 kgf-m or less, and cannot exhibit sufficient toughness as a structural material.

Therefore, the object of the present invention is to provide a hot forged material that effectively contains additive elements, does not require tempering heat treatment after forging, has high strength, excellent toughness and ductility, and has good hardenability and machinability. The objective is to provide non-thermal treated steel for use in the manufacturing industry.

Means for Solving Problems〉 The inventors added elements that impart high strength and excellent toughness and ductility to conventional non-tempered steel in various component ratios to make test steels, and As a result of conducting tensile tests, Charpy impact tests, etc. on the test steel, and conducting repeated research, it was possible to find the optimum chemical composition ratio so that the structure of the test steel exhibits bainite or ferrite-bainite when cooled after hot forging. This heading constitutes the present invention. The non-tempered steel for hot forging of the first invention contains C: 0.05 to 0.15% by weight (hereinafter referred to as weight%), Si: 0.05 to 1.00%,
Mn: 1.30-2.50%, Cr: 0.30-2
．． 00%, acid soluble A, 9: 0.015-0.060%
, Ti: O, 010-0.100%, B: 0.
0003 to 0.0050%, with the remainder consisting of Fe and unavoidable impurities.

Further, the non-temperature steel for hot forging with improved hardenability of the second invention contains each element from C to B of the first invention in the same ratio range, and furthermore, Nb=≦0 .20%, Mo
:≦0.50%, V:50.30%, and the balance is Fe and inevitable impurities.

Further, the non-tempered steel for hot forging with improved machinability of the third invention contains each element from C to B of the first invention in the same ratio range, and further has S: ≦012%. , Pb
≦0.30%, Ca: ≦0.01%, Te: 5
It is characterized by containing 0.10% of at least one kind, with the remainder consisting of Fe and unavoidable impurities.

In addition, the non-temperature steel for hot forging with improved hardenability and modified machinability of the fourth invention contains each element from C to V of the second invention in the same ratio range, and , containing each element from S to Te of the third invention in the same ratio range,
The balance is characterized by consisting of Fe and unavoidable impurities.

The reasons for limiting the chemical components of the present invention will be described below.

C is 0.25 to 0.60% in conventional non-annealed steel for hot forging that utilizes precipitation strengthening of carbonitrides such as V and Nb to ensure the required strength as forged. It was necessary to contain some amount. This is because securing strength mainly depends on the strength of pearlite and the strength due to carbonitride precipitation. However, in the steel of the present invention, the strength after forging is ensured mainly by strengthening by bainite precipitation, so it is possible to reduce carbon. C for improved toughness
The lower the content, the more effective it is, but if it is less than 0.05%, it is necessary to add a large amount of hardenability-improving elements such as Mn and Or to precipitate bainite, which is uneconomical, so the lower limit is set to 0. .05%. On the other hand, if the content exceeds 0°15%, it is easy to obtain the required strength, but the deterioration of toughness increases, so the upper limit is set at 0°15%.

It is necessary to contain 0.05% or more of Si to deoxidize and strengthen the ferrite base, but if it is avoided in a large amount exceeding 1.00%, toughness and machinability will decrease, so the upper limit should be set to 1.
00%.

Mn is an element that greatly contributes to improving hardenability and is an essential element for the steel of the present invention. In order to make the structure mainly composed of low carbon bainite in a cooled state after forging, a content of at least 1.30% is required, although it depends on the amount of other elements added. However, if the content exceeds 2.50%, martensite may be formed in areas where the cooling rate is high, such as at the ends of forged steel products, so the upper limit was set at 2.50%.

Cr is also an element that improves hardenability like Mn, and in order to obtain a bainite structure appropriately, the content should be adjusted to 0.30 to 2.
．． 00% range.

Acid-soluble AjJ is effective in deoxidizing effect and refining crystal grain size, but in order to effectively exhibit this effect, it is necessary to contain at least 0.015%.

However, even if the content exceeds 0.060%, the effect will be saturated and it will also have a detrimental effect on machinability, so the content was limited to a range of 0.015 to 0.060%. .

Ti is a strong carbonitride-forming element and is effective in fixing free Ti. The steel of the present invention must contain B, and in order to fully exhibit the hardenability-improving effect of B, it is necessary to fix N in the steel and prevent free N from existing as much as possible. Further, since Ti carbonitride suppresses coarsening of austenite crystal grains during forging, the bainite structure precipitated as a result of forging is also refined, improving toughness.

In order to exhibit such an effect, it is necessary to contain 0.01% or more, but since containing more than 0.100% reduces machinability, the upper limit is set to 0.100%.

B is an element that is particularly effective in obtaining a low carbon bainite structure because, as is well known, B can greatly improve the hardenability of hypo-eutectoid steel when added in a small amount. Hardenability is improved by Mo, Mn
It is also possible to add large amounts of hardenability-improving elements such as Cr, Ni, Cu, etc.; however, in that case, the addition of these alloying elements reduces the cost of the steel material compared to that of conventional hardening and tempering treatments. The price exceeds that of mechanical structural steel and low-alloy steel, and the benefits of non-thermal conditioning are lost. Therefore, the addition of B is essential, and in order to obtain a stable hardenability improvement effect, the content should be adjusted to 0.0003 to 0.000.
It was limited to a range of 50%.

Nb, Mo, and V all form a solid solution in the austenite structure when heated before forging, and are effective in improving hardenability even in small amounts. Therefore, they should be added as necessary, and the upper limit of each content should be set. 0.20%, 0.50%.

It was set to 0.30%.

S, Pb, Ca, and Te all improve the machinability of steel, so when machining products after forging, they shall be added as necessary, and the upper limit of each content shall be , 0.12%, 0.30%, 001%, 0°10%
And so.

<Effects of the Invention> The non-tempered steel for hot forging of the present invention optimally contains additive elements so that its structure exhibits bainite or ferrite-bainite when cooled after hot forging, and
Since hardenability-improving elements and machinability-improving elements are added as necessary, there is no need for tempering heat treatment after forging, making it cheaper to manufacture than conventional tempered steel, and it can be left cooled after forging. It has high strength, excellent toughness and ductility comparable to conventional tempered steel, and also has good hardenability and machinability.

<Example> Hereinafter, the present invention will be explained in detail with reference to Examples.

Table 1 listed at the end shows the chemical composition, heat treatment conditions, and results of the above tests of the test steels subjected to the tensile test and Charpy impact test. These test steels were melted with adjusted components and rolled to a diameter of 72 mm. The above test steel was first cut to a length of 250 mm, and then cut to a length of 1250 mm.
It was heated to °C and forged into a round bar with a diameter of 50 mm using a hammer. The finishing temperature during final molding was 1150°C, and then
It was left to cool in the air. Sample w4 No. 13 is 553C steel as an example of tempered steel, and after hot forging, it is reheated as usual and subjected to quenching and tempering treatment. JIS No. 4 (parallel part diameter 14 m) in one direction from the center of the forged round bar.
mφ) tensile test piece TS and Tsuki S3 (2 mm U notch) impact test piece CH were taken to investigate the mechanical properties of the forged and drawn material. In Table 1, test jl! Nos. I to 7 are inventive steels within the scope of the claims of the present invention, and Samples 11iNo. 8 to 13 are comparative steels for the above-mentioned inventive steels. As is clear from the table, the steels of the present invention generally have a lower C content and a higher Mn content than the comparative steels, and contain Nb, which is not found in the comparison steels.

Regarding the tensile test and impact test results in the table, the present invention steels (Nos. 1 to 7) are different from conventional non-tempered steels (No. 13).
) has strength and toughness that are comparable to those of On the other hand, among the comparative steels that are outside the scope of the claims,
Although the required strength can be ensured by increasing the carbon content of the test steels N018 to IO, their toughness values are low. In addition, test steel No. 11
Although the required toughness value can be obtained by reducing the carbon content, the strength is insufficient. Among the steels of the present invention, test steel No. 5
, 7 exhibits a bainite structure and has a tensile strength of 97.
4kgf/mm'.

1.02. It not only has a high Okgf/mm" but also excellent toughness. For this reason, by using the non-tempered steel of the present invention as a raw material steel, it can be reheated after conventional hot forging.

For example, parts such as steering knuckles and front axles, which were previously manufactured through expensive heat treatment such as quenching and tempering, can be manufactured at low cost by simply cooling after hot forging.

Claims (4)

[Claims] (1) C: 0.05 to 0.15% by weight (hereinafter referred to as weight%),
Si: 0.05-1.00%, Mn: 1.30-2.5
0%, Cr: 0.30-2.00%, acid-soluble Al: 0.
015-0.060%, Ti: 0.010-0.100
%, B: 0.0003 to 0.0005%, the remainder F
Non-thermal steel for hot forging consisting of e and inevitable impurities. (2) C: 0.05-0.15%, Si: 0.05-1
．． 00%, Mn: 1.30-2.50%, Cr: 0.3
0-2.00%, acid-soluble Al: 0.015-0.060
%, Ti: 0.010-0.100%, B: 0.000
Contains 3 to 0.0005%, and further includes Nb: ≦0.20%
, Mo:≦0.50%, V:≦0.30%, and the remainder consists of Fe and unavoidable impurities. (3) C: 0.05-0.15%, Si: 0.05-1
．． 00%, Mn: 1.30-2.50%, Cr: 0.3
0-2.00%, acid-soluble Al: 0.015-0.060
%, Ti: 0.010-0.100%, B: 0.000
Contains 3 to 0.0005%, further S: ≦0.12%,
Pb:≦0.30%, Ca:≦0.01%, Te:≦0
．． A non-tempered steel for hot forging with improved machinability, containing at least 10% of Fe and unavoidable impurities. (4) C: 0.05-0.15%, Si: 0.05-1
．． 00%, Mn: 1.30-2.50%, Cr: 0.3
0-2.00%, acid-soluble Al: 0.015-0.060
%, Ti: 0.010-0.100%, B: 0.000
Contains 3 to 0.0005%, and further includes Nb: ≦0.20%
, Mo:≦0.50%, V:≦0.30%, S:≦0.12%, Pb:≦0.
Non-thermal treatment for hot forging with improved hardenability and machinability, containing at least one of the following: 30%, Ca: ≦0.01%, Te: 0.10%, with the remainder being Fe and unavoidable impurities. steel.