JPH03260010A - Production of non-heattreated steel bar for hot forging and production of hot forged non-heattreated parts - Google Patents

Abstract

PURPOSE:To obtain a non-heattreated steel bar for hot forging having high strength and high toughness by subjecting a cast billet in which respective contents of C, Si, Mn, Cr, V, Al, and N are specified and which is cast under the prescribed temp. conditions to rolling without delay. CONSTITUTION:A steel having a composition consisting of, by weight, 0.15-0.35% C, 0.1-0.5% Si, 1-3% Mn, <=1% Cr, 0.05-0.3% V, 0.005-0.02% Sol.Al, <0.01% N, and the balance Fe is refined. Then, this steel is cast into a billet through the temp. range from 1000 deg.C to the solidification point at >=20 deg.C/min cooling rate by a continuous casting method. Subsequently, this cast billet is subjected, without delay, to rolling into a steel bar without cogging. Further, this steel bar is hot-forged at >=1000 deg.C and cooled at 0.2-1.2 deg.C/sec cooling rate, by which hot forged non-heattreated parts having a structure containing >=30% bainite can be obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing a steel bar for mechanical parts used in the fields of automobiles and industrial machinery, and a method for manufacturing non-heat-treated mechanical parts using the same steel bar. It is something.

[Conventional technology]

Many of the mechanical parts used for automobiles and industrial machinery are manufactured by processing carbon steel for machine structures or alloy steel bars into rough shapes through hot forging, and then undergoing quenching and tempering (quenching and tempering) and cutting. It is manufactured after.

However, in recent years, the use of so-called non-tempered steel, which can be used as hot-forged by omitting the above-mentioned tempering treatment, has become widespread. The toughness of the resulting parts is low, limiting the range of parts to which it can be applied.

Based on the actual situation as described in 2, Japanese Patent Application Laid-Open No. 62
-253725, patent application filed by the applicant in 1983
There are proposals described in Japanese Patent Application Laid-Open No. 3-318279, as well as Japanese Patent Application Laid-open Nos. 60-103161, 61-139646, and 64-220.

[Problem to be solved by the invention]

The above-mentioned proposals described in Japanese Unexamined Patent Publication No. 62-253725 and Japanese Patent Application No. 63-318279 aim to prevent coarsening of crystal grains by adding Ti etc. and using its carbonitride. In the hot forging temperature range, crystal grains inevitably become coarse and the effect is not sufficient.

Also, JP-A-60-103161, JP-A-61-13
9646 and JP-A-64-220 propose increasing the amount of alloying elements such as Cr+B+Ti, Mo, etc., and changing the as-hot-forged structure to a bainitic structure to obtain high toughness. However, there is still no one that is fully satisfactory in terms of both cost and effectiveness.

The present invention aims to solve the above-mentioned problems and to provide a technology for producing a high-strength, high-toughness non-tempered steel bar for hot forging, and for producing hot-forged non-tempered parts using the bar. be.

[Means to solve the problem]

In order to achieve high strength and high toughness, the present invention adjusts the chemical composition so that the as-hot-forged structure becomes a &iI weave containing bainite, and also solidifies during the continuous casting stage of the steel bar manufacturing process. to control the subsequent cooling rate;
This was achieved by controlling hot forging and subsequent cooling conditions.

That is, the gist of the present invention is as follows.

(1) C in weight%: 0.15% or more and 0.35% or less,
Si: 0.1% or more and 0.5% or less, Mn: l, QQ
% or more and 3.00% or less, Cr: 1.0% or less, V
: 0.05% or more and 0.30% or less, sol, A/
: 0.005% or more and less than 0.020%, N: 0.
A steel containing less than 100% 0.0100%, with the remainder essentially consisting of unavoidable impurities and Fe, is melted by a normal method, and then cast in a temperature range of 20°C/l1l from the solidification point to 1000°C using a continuous casting method.
Cast into a slab under conditions such that the cooling rate is higher than that of In,
A method for producing a non-thermal steel bar for hot forging having high strength and high toughness, the method comprising immediately rolling the steel bar into a steel bar without performing blooming rolling.

(2) Heating the steel bar described in 1 above to 1150°C or higher
Hot forging is performed at a temperature of 00°C or higher, and then 0.2~
A method for producing a hot-forged non-annealed part having high strength and toughness, characterized by cooling at a cooling rate in the range of 1.2°/sec to obtain a structure containing 30% or more of bainite.

[For production]

The effects and reasons for limitations of each requirement of the present invention will be explained below.

CTC is an important component for adjusting the strength and toughness of non-MT steel parts; if it is less than 0.15%, the strength will be too low, while if it exceeds 0.35%, the toughness of the resulting parts will decrease. , all of which must be avoided.

Si: Si is necessary as a deoxidizing agent, and if it is less than 0.1%, the effect is insufficient, and if it is added in excess of 0.5%, the effect is saturated, so the upper limit was set at 0.5%.

Mn: Mn is required in an amount of 1.00% or more to improve hardenability and adjust the strength and toughness of parts, but if added in a large amount exceeding 3.00%, toughness will be impaired and manufacturing problems may occur. This should be avoided as it increases the difficulty.

Cr: Similar to Mn, Cr is an element that has the function of increasing hardenability and adjusting strength and toughness, but since it is less effective and more expensive than Mn, the amount added is limited to 1.0%. .

V: V has an extremely large effect of improving hardenability, and
When steel is cooled after hot forging, it precipitates as carbonitrides ((2)) and is an effective element for increasing the hardness of parts. In order to have that effect, 0.05% or more is required, but 0.05% or more is required.
If added in excess of 30%, the strength will increase too much and the toughness will decrease, so it must be avoided.

sol, N: sol, jV is O, O for deoxidation
O 5% or more is required, but if it is 0.020% or more, the amount of high melting point alumina, which is a deoxidation product, increases, which may cause nozzle clogging during continuous casting, and must be avoided. In addition, in combination with the amount of N described later, A/
Since the amount of N precipitated increases and may cause cracking during straightening of the slab, the upper limits of the amounts of sol and AI must be strictly controlled from this point of view as well.

It is known that NUN combines with M in steel to generate A/N and has the effect of refining grains, and when aiming for this effect, it is common sense to increase the amount of N and N added. Ta. In the case of non-temperature steel for hot forging, which has the same purpose as the present invention, it has usually been added with the aim of achieving this effect.

However, according to the research conducted by the present inventors, A7N becomes a solid solution in the steel at temperatures above 1150°C, which is the heating temperature range for hot forging, and not only can no grain refinement effects be expected. , sot, and AI mentioned above, so it is necessary to control the amount added to less than 0.0100%.

In claim (1), 1 from the freezing point by continuous casting method.
If slabs are manufactured at a cooling rate of less than 20'C/win in the temperature range of 000℃, the structure of the slabs will become coarse, the material quality of the parts will deteriorate, and the A/N will become coarse during the cooling process of manufacturing slabs. Continuous cutting may cause various problems, such as cracking during straightening of precipitated cast slabs, or coarsening of MnS, which improves machinability during parts machining, and increased anisotropy in the material properties of parts. It is necessary to perform casting under conditions such that the casting method provides a temperature range of 1000° C. from the freezing point and a cooling rate of 20° C./min or more.

After forming a slab under the above conditions, it is immediately rolled into a steel bar without blooming. Needless to say, by blooming rolling,
This should be avoided as it requires extra costs.

In claim (2), the heating temperature of hot forging is 1150
If the temperature is less than °C or the forging temperature is less than 1000''C, the load during the forging process will be excessive and the life of the expensive forging die will be significantly shortened, so it must be avoided.

Figures 1 and 2 show the results of examining changes in material properties by cooling at various cooling rates after hot forging and changing the microstructure fraction of bainite and ferrite/pearlite. It can be seen that when the content is 30% or more, stable high strength and toughness can be obtained. Since the cooling rate limit at this time was 0.2°C/sec, the lower limit of the cooling rate after forging was set to 0.2°C/sec or more. On the other hand, if the cooling rate increases, the structure becomes mainly lower bainite, which is close to martensite, and the strength of the part becomes extremely hard, which must also be avoided. This limit is 1.2”C
/sec.

〔Example〕

The present invention will be further explained below with reference to Examples.

Example 1 50 kg of steel having the chemical components shown in Table 1 was melted in a vacuum melting furnace, cast into an ingot, and then made into a steel bar with a diameter of 30 mm. At this time, the cooling rate in the temperature range from the freezing point to 1000°C was 25°C/va in.

This steel bar was used as a raw material, and was heated to 1200°C and then cooled at a cooling rate of 0.6°C/sec for the purpose of simulating a non-tempered part. A test piece was cut out from this steel bar and a material test was conducted. The results are shown in Table 1.

It can be seen from Table 1 that the parts according to the examples of the present invention have both high strength and high toughness.

Example 2 Steel having the chemical composition shown in Table 2 was melted and cast into a continuous slab having a cross-sectional size of 162 x 162 mm.

At this time, the cooling rate in the temperature range from the freezing point to 1000°C was 45° (:/min).

Table 2 shows the casting conditions and the quality of the slabs, and it can be seen that according to the present invention, slabs with good quality can be manufactured.

Example 3 Steel having the chemical composition shown in Table 3 was melted and cast into a continuous slab of 350x560 ceramic with a cross-sectional size of 162x162mm. At this time, the cooling rate in the temperature range from the freezing point to 1000°C is 45° (: /ll1in, 9'(
/sin.

A slab of 350 x 560 mm is made into 162 x 1 by blooming rolling.
After rolling into a 62mm steel billet, the other 162 x 162
The slab of the cabinet was directly rolled into round steel with a diameter of 50mm+.

This round steel was hot forged into steel bars with a diameter of 25 mm under various conditions shown in Table 4, and after cooling, the material properties were investigated and are shown in Table 4.

From the results in this table, it can be seen that the non-tempered parts of the present invention have both high strength and toughness.

〔Effect of the invention〕

As shown above, the present invention is an industrially useful invention that provides a manufacturing technology for non-tempered parts having both high strength and high toughness.

[Brief explanation of drawings]

Figures 1 and 2 show changes in material properties by cooling at various cooling rates after hot forging to change the microstructure fraction of bainite and ferrite/pearlite. The relationship between the fraction and tensile properties, and FIG. 2 is a diagram showing the relationship between the bainite fraction and impact value. 67- Figure 1 Beday 1 minute: #- (su)

Claims (2)

[Claims] (1) C: 0.15% or more and 0.35% or less, S by weight%
i: 0.1% or more and 0.5% or less, Mn: 1.00% or more and 3.00% or less, Cr: 1.0% or less, V: 0.05%
0.30% or more, sol. Steel containing Al: 0.005% or more and less than 0.020%, N: less than 0.0100%, and the remainder essentially consisting of unavoidable impurities and Fe is melted by a normal method, and the solidification point is lowered by a continuous casting method. to 1000℃
A hot rolling method having high strength and high toughness, characterized in that it is cast into a slab at a cooling rate of 20°C/min or more in a temperature range of A method for manufacturing non-tempered steel bars for forging. (2) Heating the steel bar according to claim 1 to 1150°C or higher;
Hot forging is performed at a temperature of 000℃ or higher, and then 0.2~
A method for producing a hot-forged non-thermal refined part having high strength and high toughness, characterized by cooling at a cooling rate in the range of 1.2° C./sec to obtain a structure containing 30% or more of bainite.