JPH01176055A - Non-heat treated steel for hot forging having excellent machinability - Google Patents

Abstract

PURPOSE:To obtain the title steel having excellent cutting workability before forging and requiring no thermal refining treatment after hot forging by adding specific elements for alloy to a steel for hot forging. CONSTITUTION:The steel contg., by weight, 0.30-0.60% C, 0.10-0.60% Si, 0.50-2.00% Mn, <0.030% P, 0.010-0.120% S, 0.020-0.060% Al, 0.002-0.015% N and 0.05-0.40% V, contg. at least one kind among 0.005-0.050% Nb, 0.005-0.050% Ti, 0.15-0.60% Cr and 0.04-0.30% Pb and the balance consisting Fe is heated to 900-1,300 deg.C, is subjected to hot rolling in the range of austenitic single phase having an Ar3 transformation point or above and is thereafter cooled at the cooling speed of <=5 deg.C/min. The non-heat treated steel for hot forging having <=200 HB hardness, having excellent cutting workability before forging, requiring no thermal refining treatment such as quenching and tempering after hot forging and furthermore having excellent machinability can be obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention is a hot forging tool that not only can omit the quenching and tempering treatment after hot forging, but also has excellent cutting workability before forging. It concerns non-tempered steel.

(Conventional technology) Hot forged parts for machine structures such as automobiles and industrial machinery have traditionally been made of carbon steel for machine structures or alloy steel for machine structures, and after forging, they are subjected to quenching and tempering treatment, so-called refining treatment. After being adjusted to the required strength and toughness, it is finished by machining.

Such thermal refining treatment is necessary to ensure mechanical properties, but it requires a large amount of thermal energy and increases the number of processing steps, leading to an increase in component manufacturing costs. Therefore, in recent years, in order to save energy and reduce costs by omitting processes, so-called non-thermal steel has been developed, which can be used as hot forged without heat treatment.
It is being put into practical use in some mechanical structural parts. These are ■
By adding precipitation-strengthening elements such as , Nb, and Ti, the required strength can be obtained in the cooling process after hot forging. (For example, Japanese Patent Publication No. 58-53709, Japanese Patent Application Laid-Open No. 56-38448).

(Problem to be solved by the invention) However, the above-mentioned non-tempered steel has a higher hardness than conventional steel due to precipitation strengthening even before forging, so there is a problem in the pre-forging process. there were.

In other words, in the manufacturing process of hot forged parts, it is necessary to cut the material into predetermined dimensions before forging, and billet shear cutting, saw cutting, gas cutting, etc. are used, but billet shear cutting, saw cutting, etc. In cutting, an increase in hardness is undesirable because it shortens the life of the cutting tool.

In this way, conventional non-temperature steel for hot forging has the effect of omitting heat treatment after forging, but on the other hand, the material hardness before forging is higher than conventional steel for heat treatment, so billet shearing during the cutting process is required. There remained a problem in that the cutting processability with a saw machine was poor.

This invention advantageously solves the above-mentioned problems, and it goes without saying that it has excellent non-thermal refining properties, as well as excellent cutting workability in the cutting process before forging. The purpose is to propose.

(Means for Solving the Problems) As a result of intensive research to solve the above problems, the inventors found that in order to obtain good cutting workability in the cutting process before forging, the hardness of the steel material must be 20 in Prinell hardness (HB)
We have learned that it is best to set it to 0 or less.

The inventors then conducted extensive studies on the composition of steel materials in order to achieve the above-mentioned hardness characteristics, and finally found a composition that would achieve the desired objective, and completed this invention.

This invention is based on the above knowledge. That is, this invention has the following characteristics: C: 0.30 to 0.60wtχ (hereinafter simply expressed in %), Si: 0.10 to 0.60%, Mn: 0.50 to 2.00% or less, P: 0.0
30% or less, S: 0.010 to 0.120%, Al:
0.020 to 0.060%, N: 0.002
-0.015% and V: 0.05-0.40%, and Nb: 0.005-0.050%, Ti: 0.
005-0.050%, Cr: 0.15-0.60
% and Pb: 0.04 to 0.30%, the remainder is substantially composed of Fe, and has a hardness of 200 or less in IB. It is a non-thermal steel for hot forging with excellent properties.

This invention will be explained in detail below.

First, the reason why the composition of this invention steel was limited to the above range will be explained.

C: 0.30-0.60% C is an element necessary to obtain strength, and at least 0.3% is required to ensure the desired strength as hot forged.
0% is required, but if it exceeds 0.60%, the hardness will become too high and cause deterioration of toughness and machinability, so C should be 0%.
．． It needs to be in the range of 30 to 0.60%.

Si: 0.10 to 0.60% Si is an element necessary to act as a deoxidizer in steelmaking and to ensure strength as hot forged.
．． 10% is required, but if it exceeds 0.60%, toughness and machinability will be impaired, so Si should be 0.10 to 0.6%.
It needs to be within the range of 0%.

Gate: O, SO~2.00% Mn is an element useful for improving strength and toughness, and 9
By heating to 00 to 1300°C, it becomes a sufficient solid solution and imparts the strength and toughness required as a non-tempered steel. Mn is required to be at least 0.50% to ensure strength as hot forged, but if it exceeds 2.00%, hardening will be significant and toughness will deteriorate, so Mn should be 0.50 to 2. It needs to be within the range of .00%.

P: 0.030% or less P is a harmful element that embrittles the steel material, and it is desirable to reduce it as much as possible, but it is acceptable at 0.030% or less.

S: 0.010 to 0.120% S causes deterioration of toughness, but on the other hand contributes to machinability, so 0.010% or more is required.

However, if it exceeds 0.120%, it will adversely affect the toughness after hot forging, so S should be 0.010 to 0.120%.
% range.

Al: 0.020-0.060% Al is added as a deoxidizing agent, but it is also an element useful for refining crystal grains. % or more is required. However, 0.0
If it exceeds 60%, it will promote coarsening of crystal grains and deteriorate machinability, so Al should be 0.020 to 0.
．． It is necessary to set the range to 0.060%.

N: 0.002~0.015χ N is a useful element that coexists with A1, V, Nb, Ti, etc. to refine crystal grains and improve strength and toughness. I need.

However, if it exceeds o or oiso%, the toughness will deteriorate, so N needs to be in the range of 0.0020 to 0.0150%.

V: 0.05 to 0.40% ■ is a main element to ensure strength and toughness as hot forged, and precipitates as carbonitrides during cooling after hot forging and is removed during tempering treatment. Allows for omission, at least 0
．． 05% is required. However, if it exceeds 0.40%, it will not only harden significantly and deteriorate its toughness, but also be disadvantageous from an economical point of view.
limited to the range of

Nb, Ti: 0.005-0.050% Both Nb and Ti exist mainly as carbonitrided precipitates, similar to ■, and effectively contribute to grain refinement and precipitation hardening of hot forged products. However, if the content is less than 0.005%, the effect of addition is poor, and if it exceeds 0.05%, the effect is saturated, so Nb and Ti are 0.005 to 0.0.
It was limited to a range of 5%.

Cr: 0.15 to 0.60% Cr effectively contributes to improving the strength and toughness of hot forged products by strengthening by solid solution and refining the structure, but when the content is 0.15%, If it is less than 0.60%, the effect of adding Cr will be poor, while if it exceeds 0.60%, the toughness will decrease.
．． It was limited to a range of 15 to 0.60%.

Pb: 0.04 to 0.30% Pb, like S, is an element that improves machinability, and is added when even higher machinability is required. However, if the content is less than 0.04%, the addition effect is poor, and if it exceeds 0.30%, hot workability deteriorates, so Pb was limited to a range of 0.04 to 0.30%. .

Next, a method for manufacturing this invention steel will be explained.

Now, the steel adjusted to a predetermined composition is then heated to a temperature of 900"C or higher in order to efficiently roll it. However, if the temperature exceeds 1300"C, the yield will decrease due to scale loss and the thermal energy will decrease. Since this is economically disadvantageous, the heating temperature is preferably about 900 to 1300°C. The subsequent rolling must be carried out in Ar, at a temperature above the transformation point, that is, in the austenite single phase region.

This is because in order to obtain a ferrite/pearlite structure suitable for cutting, it is preferable to perform the process at an Ar point of 3 or higher, and in this point rolling in the T+α two-phase region results in a finer structure than necessary and a high hardness. I don't like being too much. Subsequent cooling needs to be performed at a relatively slow cooling rate at least in the Ar3 to Ar+ transformation point temperature range.

This is because carbonitrides such as V, Nb, and Ti, which cause precipitation hardening, partially precipitate in austenite, but most of them precipitate during the γ→α transformation. This is because speed has a large effect on carbonitride size and texture. As a result of many experiments, we found that if the cooling rate exceeds 5° (/1 Iin), the hardness becomes too high due to the finer carbonitride size and finer grain structure. It is preferable to do this at a value of /win or less.

In this way, in the pre-forging cutting process, a non-tempered IT steel with a hardness of HB of 200 or less, which is necessary to obtain good cutting performance, can be obtained.

(Function) The feature of this invented steel is that, unlike conventional non-tempered steel, the hardness in the material state before forging is HB 200 or less, and after hot forging, it has a hardness of HB, Nb, Ti, etc. Needless to say, precipitation strengthening eliminates the need for quenching and tempering.

(Example) Steels (A to ■) having the structures shown in Table 1 were melted and made into billets by the usual method, and then hot rolled under the following conditions. φ, while steel type E-1 was rolled into a 32 mmφ steel bar.

In Table 1, steel types A-H are the target steels of this invention,
On the other hand, the steel type (■) is 545C, which is a comparison steel.

Table 2 shows the results of the hardness measurement of the thus obtained steel bar and the billet shear cutting test.

Table 2
1] As is clear from Table 2, steel according to the present invention (Nal
~8) are all hardness levels that provide good cutting workability of HB200 or less, and the conventional example (Nn9, steel type I
) was about the same hardness.

On the other hand, a comparative example (k[0
- 17), the tool life was longer than that of the conforming example.

The tool life is compared and evaluated based on the amount of wear on the tool after cutting N119 (345C) 500 times, and the number of cuts it takes to reach the same amount of wear.According to this invention, the tool life is at the same level as conventional steel. It was confirmed that cutting workability was obtained.

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain a non-heat-treated steel for hot forging, which not only has excellent non-heat-treated properties but also has greatly improved cutting workability before hot forging. This not only expands the scope of application of such steel types, but also helps reduce costs.

Claims (1)

[Claims] 1. C: 0.30 to 0.60 wt%, Si: 0.10 to 0.60 wt%, Mn: 0.50 to 2.00 wt%, P: 0.030 wt% or less, S : 0.010 to 0.120 wt%, Al: 0.020 to 0.060 wt%, N: 0.002 to 0.015 wt%, and V: 0.05 to 0.40 wt%, and Nb: 0 Contains at least one selected from .005 to 0.050 wt%, Ti: 0.005 to 0.050 wt%, Cr: 0.15 to 0.60 wt%, and Pb: 0.04 to 0.30 wt%. A non-temperature steel for hot forging with excellent machinability, characterized in that the remainder is essentially Fe and has a hardness of HB 200 or less.