JPS61139646A - Nontemper bar steel for hot forging - Google Patents

Abstract

PURPOSE:To obtain the titled bar steel having jumpedly high toughness, by specifying compsn. design of low C steel and hot forging said steel, then cooling said material in hot water. CONSTITUTION:Steel contg. by weight %, 0.06-0.15 C, 0.10-1.00 Si, 0.50-2.00 Mn, 2.00-4.00 Mn+Cr, 0.010-0.030 Ti, 0.0005-0.0030 B, 0.01-0.05 Al, 0.0060 N and the balance Fe is melted in vacuum furnace, said ingot is forged to bar steel having e.g. about 30mm diameter. By heating the bar steel to about 1250 deg.C then cooling it in hot water, material characteristic having about 70-110kg/mm<2> tensile strength and about 10kg-m/cm<2> high toughness is obtd. without quenching and temper treatments. By hot forging then cooling the steel in hot water, component is designed so that matrix structure itself becomes to C bainite structure having high toughness. Thus especially detailed control is unnecessary relating to hot forging condition.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to manufacturing machine parts, etc. after forming by hot forging.
The present invention relates to a non-thermal steel bar for hot forging that can obtain high toughness by cooling in hot water without undergoing the usual quenching and tempering treatments after hot forging.

[Conventional technology]

Many mechanical parts used for automobile parts and the like are manufactured by hot forging from steel bars, forming them, then quenching and tempering (refining) them, and then cutting them. In order to save energy and reduce costs in the production of such parts, heat treatment can be simplified by direct forging quenching using residual heat after hot forging, or non-thermal steel using precipitation hardening of V, Nb, etc. known as technology.

(For example, Automobile Technology Vol. 37 No. 3 p. 242
(1983 or Japanese Patent Application Laid-Open No. 55-82749)
.

However, these so-called micro-alloyed non-thermal steel bars with additions of V, Nb, etc. have a metallographic structure of ferrite.
In pearlitic steel, its toughness is significantly influenced by grain size. Non-tempered steel is used as hot-forged, so its crystal grains are generally extremely coarse and its toughness is often extremely low.Therefore, its application to actual parts is limited, and its application to automobile foot parts, which are important safety parts, is limited. The reality is that it is not used for surrounding parts.

[Problem that the invention seeks to solve]

As a measure to solve the low toughness of hot-forged non-tempered steel, the present inventors previously invented a high-toughness hot-forged non-tempered steel bar that utilizes a low C bainite structure (Patent Application No. 58). -20
9953. (Japanese Patent Application No. 59-138276) The present invention makes it possible to ensure even higher toughness by cooling in hot water after hot forging.

[Means for solving problems]

The gist of the present invention is that, in weight %, C: 0.06-0°15
, Si:0.10-1.00. Mn: 0.50-2.0
09Mn+Cr:'2.00~4.00°Ti:0.0
10-0.030. B: 0.0005-0.003
0. Al: 0.01-0.05. N: 0°0060
This is a non-tempered steel bar for hot forging which can obtain high toughness by discarding it in hot water after hot forging.

[Effect]

The inventors have discovered that the toughness is equivalent to or higher than that of quenched and tempered SCM435 steel, which is a typical steel type of 1-strength steel (10k impact value).
We conducted the following experiments in order to develop a non-heat-treated steel bar for hot forging that has significantly higher toughness than conventional non-heat-treated steel bars for hot forging. went.

C in weight% as sample material: 0.05~O', 20°S
i: 0.10-1. OO, Mn: 0.50~2
．． 00, Mn+Cr:, 2. OO-,4,OO,Ti
: 0°01~0.03. B: 0.0005~0
．． 0030°Afl:O,O1~0,05. N: 0.
0060 or less and the remainder is substantially Fe.
It was melted in a 0 kg vacuum melting furnace and forged into a steel bar with a diameter of 30 mm.

The steel bar thus produced was heated to 1250°C and then cooled in hot water. Thereafter, a test piece was taken from this steel bar and its mechanical properties were investigated. From this test, the results shown in Figure 1 regarding the relationship between C content in steel and tensile strength, and Figure 2 regarding the relationship with impact value were obtained.

The mechanical parts targeted by the present invention have a tensile strength of 70 to 1
10 kg/mrrr, and as for the impact value, the target is a toughness value of 10 kg-m/c n7 or more for the quenched and tempered material of SCM435 steel, which is a representative steel type of strong steel, so the C in the steel is 0. .15% or less, Cr+Mn content is 2
．． It has been found that 0.00 to 4.00% is necessary.

Based on the above knowledge, Si:'0.10~1.00%,
Ti: 0.010-0.030%, B: 0.00
05-0.0030%, Afl: 0.01-0.05%
, N: 0.0060% or less in steel containing C: 0.
06-0.15%, Cr+Mn: 2.00-4.0
If the 0% balance is essentially Fe, the tensile strength can be increased to 70 to 110 kg/mr by cooling in hot water after hot forging.
It was discovered that an impact value of 10 kg-m/c' rr or more could be obtained, and the present invention was completed.

The reasons for the limitations of the present invention will be explained below.

C is one of the important elements that determines the strength and toughness of products, and if it is less than 0.06%, the amount of alloying elements added to obtain the necessary strength will be too large, which is uneconomical, so it should be 0.06% or more. And so. On the other hand, if it exceeds 0.15%, the toughness decreases, so the upper limit was set at 0.15%.

Si is an element necessary for deoxidation, and needs to be 0.10% or more. On the other hand, if more than 1.00% is added, the strength becomes too high than necessary, so 1.00% is set as the upper limit.

Mn is an element that controls the strength of products along with deoxidation and C and Cr, and is necessary to combine with S in steel to prevent embrittlement during hot working of steel, and therefore 0.50
% or more is required. Moreover, if it exceeds 2.00%, machinability decreases and manufacturing difficulties increase, so the upper limit was set at 2.00%.

Cr, like the above-mentioned C and Mn, is necessary to adjust the strength of the product, and in balance with the amount of Mn, the tensile strength is 70 to 11.
2 in the amount of Cr + Mn to obtain 0 kg/mrrr.
．． 00-4.00% is required.

Ti is an element necessary to fix N so that B (described below) can work effectively, and if it is less than 0.010%, the effect of fixing N is not sufficient, and if it is added in excess of 0.030%, it will not be effective. Since the effect is saturated, the upper limit was set at 0.030%.

B is an element necessary to improve the hardenability of steel and has a value of 0.00
If it is less than 0.0030%, the effect is small, and even if it is added in excess of 0.0030%, no commensurate effect can be obtained.

If N exceeds 0.0060%, the amount of Ti required to fix N becomes too large and toughness decreases, so the upper limit is set to 0.
．． 0060%.

In addition to the above components, in order to improve machinability, 0.07
It is effective to add up to 0% S or up to 0.30% Pb.

Next, the method of using the steel of the present invention will be described.

The composition of the steel of the present invention is designed so that the matrix structure itself becomes a highly tough C bainite structure by cooling in hot water after hot forging, so there is no need for particularly detailed management of hot forging conditions. do not.

In order to cool the product in hot water after hot forging, a simple quenching tank is prepared after the hot forging machine, and the water temperature is first raised to 95°C or higher, after which the forged product is continuously charged and cooled. Stable cooling of hot water is possible. It is particularly suitable for production lines that currently use forging and direct quenching, and can be implemented by filling water instead of quenching oil.

〔Example〕

EXAMPLES The effects of the present invention will be explained in more detail with reference to Examples below. Steel having the chemical composition shown in Table 1 was melted and made into a steel bar with a diameter of 70φ using a conventional method.

This steel bar was heated to 1250℃ and then hot forged to a diameter of 35φ.
It was molded into a shaft and immediately cooled in hot water after forging. A medium-length test piece and an impact test piece were taken from this shaft, and the materials were investigated, and the results shown in Table 2 were obtained.

As is clear from Table 2, the material of the present invention has a tensile strength of 70 to 1
10kg/mrr? It can be seen that it has a high impact value of 10 kg-m/cn (below).The example numbered 6 in the table has a high impact value, but the strength is too high. , No. 7 is an example in which the strength was too low.

〔Effect of the invention〕

As mentioned above, the non-tempered steel bar of the present invention, after hot forging,
By cooling in hot water, the tensile strength of 70 to 110 kg/m2 can be achieved by 10 k without quenching or tempering.
The material has a high toughness of g-m/c ni' or higher, and can be applied to important safety parts such as automobile suspension parts.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between C content and tensile strength of the steel of the present invention. FIG. 2 is a diagram showing the relationship between C content and impact value of the steel of the present invention. Patent Applicant Nippon Steel Corporation Agent Patent Attorney Yasushi Furushima 1st Procedural Amendment Date of July 1985

Claims (1)

[Claims] In weight %, C: 0.06-0.15 Si: 0.10-1.00 Mn: 0.50-2.00 Mn+Cr: 2.00-4.00 Ti: 0. 010 to 0.030 B: 0.0005 to 0.0030 Al: 0.01 to 0.05, the rest is substantially Fe, and high toughness can be obtained by cooling in hot water after hot forging. Non-temperature steel bar for forging.