JP2701438B2 - Manufacturing method of non-tempered free-cutting forged steel with high hot ductility - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-refined free-cutting forged steel product having a high hot ductility using a slab produced by a continuous casting method as a starting material.

2. Description of the Related Art Free-cutting steel is a steel in which the machinability is particularly improved by adding an appropriate element, and the free-cutting imparting elements include S, Pb, and S.
e, Te, Bi and Ca are known. These elements are added singly or in combination, but various free-cutting steels have been devised in consideration of performance such as machinability and strength and plastic workability.

Among them, as free-cutting steels of carbon steel and alloy steel for machine structure, which emphasizes strength, V, Nb
A non-heat treated steel to which S and Te are added in combination is used to improve the machinability.

However, since this steel has low hot ductility, the slab produced by the continuous casting method has a drawback that cracks easily occur when it is formed into a billet by rolling.

Problems to be Solved by the Invention As described above, the high-temperature ductility is low, and when a continuous cast slab is rolled into a billet, the hot-ductility of a free-cutting steel that cannot be produced by the continuous casting method due to cracking is improved. The inventor has conducted various studies for the purpose of providing a free-cutting forged steel product that can be manufactured by a casting method.

As a result, the cause of high-temperature cracking that occurs during billet rolling of continuous cast slabs is that AlN precipitates at the austenite grain boundaries and that strong precipitation strengthening elements such as V precipitate at the austenite grain boundaries. This is because the relative ductility of the grain boundaries with respect to the inside is reduced, and microcracks are generated on the surface of the cast slab or directly below the surface when straightening and bending the continuous cast slab, and the crack is opened during slab rolling. But I understand.

Based on this conclusion, the solution to the above problem was to refine the crystal grains and add it as a deoxidizer.
Al, N entering from the material and the outside are limited to a range where AlN is not generated, and ZrN is precipitated by adding Zr, and N is fixed by ZrN.

The addition amount of V, which is a precipitation strengthening element, is suppressed to a range where the high-temperature ductility does not decrease.

I found that.

The present invention provides a method for producing a non-heat treated free-cutting forged steel product having high hot ductility using a slab produced by a continuous casting method as a starting material based on the above findings.

Means for Solving the Problems In order to achieve the above object, a method for producing a non-heat treated free-cutting forged steel product having a high hot ductility according to the present invention is as follows: C 0.70% or less; %, P 0.035% or less, S 0.01 to 0.50%, Te 0.10% or less, B 0.0002 to 0.0020%, V 0.15% or less, and Zr and N under the conditions satisfying the relationship of the following formula. , The balance is made of Fe and steel of unavoidable impurities, and a billet obtained by rolling a slab manufactured by a continuous casting method, 1000 to
After heating to 1300 ° C and performing forging with a reduction of 50% or more in a temperature range of 850 ° C or more, immediately accelerated cooling at a cooling rate of 0.3 to 1.0 ° C / sec to obtain a non-refined free-cutting forged steel product.

In the present invention, the components of the free-cutting steel are limited for the following reasons.

C is necessary to ensure the strength of the steel, but if the content exceeds 0.70%, the base material and the welded portion are deteriorated, so the content of C is set to 0.70% or less.

Si is an effective component as a deoxidizing agent for steel, but if its content exceeds 1.5%, inclusions increase and ductility decreases, so it was set to 1.5% or less.

Mn is necessary to improve the strength and toughness of steel, but if its content exceeds 2.0%, toughness deteriorates,
Since segregation increases, the content is set to 2.0% or less.

Although P is effective for toughening steel, the content of the steel of the present invention must be limited to 0.035% or less in order to exhibit its performance stably, and the content is desirably as small as possible.

S is indispensable for improving machinability, but 0.010
%, The effect of addition is not recognized, and if it exceeds 0.50%, the hot workability deteriorates. Therefore, the content was limited to 0.010 to 0.50%.

Te is effective in improving machinability, but if its content exceeds 0.10%, hot workability deteriorates, so 0.10%
It was as follows.

B is effective in improving the hardenability of steel and is effective in increasing the strength. However, if its content is less than 0.0002%, the effect of its addition does not increase, and if it exceeds 0.0020%, the toughness deteriorates.
0.0002 to 0.0020%.

V is effective for increasing strength, but its content is 0.15%
If it exceeds, the hot workability is deteriorated, so the content is made 0.15% or less.

Zr fixes solid solution N as Zr N, suppresses precipitation of Al N which lowers high-temperature ductility, and promotes the effect of improving the hardenability of B, but completely fixes solid solution N by its content. Otherwise, these effects decrease, so that the contents of Zr and N are set to satisfy the following formula. Note that N is set to 0.0075% or less.

The forging conditions were determined for the following reasons.

The heating temperature for forging requires high-temperature heating for low load and uniform plastic deformation.
If the temperature is less than 1300 ° C., the workability decreases.If the temperature exceeds 1300 ° C., the toughness is deteriorated due to the coarsening of crystal grains.
00-1300 ° C.

A lower forging temperature is more effective for refining the crystal grains, but if the forging temperature is lower than 850 ° C., the workability is reduced.

The larger the rolling reduction, the more effective it is to refine the crystal grains.
If it is less than 50%, the grain size is insufficient and the toughness deteriorates.
% Or less.

Cooling after forging is effective for toughening, but 0.3 ° C / sec
If it is less than 10%, since the toughening is insufficient, accelerated cooling is performed at 0.3 ° C./sec or more. However, rapid cooling over 1.0 ° C / sec deteriorates toughness, so the upper limit was set at 1.0 ° C / sec.

Action 700-1000 by limiting the chemical composition of free-cutting steel
As a result of the improved high-temperature ductility at ° C, a slab produced by the continuous casting method can be rolled into a billet without cracking.

Example No. 1 steel according to the practice of the present invention, whose chemical components are shown in Table 1.
No. 6 to No. 6 and Comparative steel Nos. 7 to N deviating from the components of the present invention
A slab smelted in a 150 kg vacuum melting furnace for o.9 was heated to 1200 ° C, and then drawn by a high-temperature tensile test at 700 to 1000 ° C. Table 1 shows the results. FIG. 1 shows the relationship between the drawing and the test temperature of the inventive steel No. 1 and the comparative steel No. 7 in a high-temperature tensile test.

From the above results, it can be seen that all of the steels according to the present invention have a large drawing at high temperatures, and are excellent in high-temperature ductility as compared with comparative steels.

Effect of the Invention According to the present invention, Al and N are limited to a range that does not produce AlN, and Zr is added to fix N as ZrN.
By using a steel in which the addition of steel is limited to the range where the hot ductility does not decrease, a non-refined free-cutting forged steel product with high hot ductility can be obtained without cracking during billet rolling even when manufactured by a continuous casting method. be able to.

[Brief description of the drawings]

FIG. 1 shows the steel type No. 1 according to the embodiment of the present invention and the comparative steel type N
9 is a graph showing the relationship between drawing and test temperature in the high temperature tensile test of o.7.

Continuation of the front page (56) References JP-A-1-188622 (JP, A) JP-A-61-291955 (JP, A) JP-A-60-17061 (JP, A) JP-A-63-241113 (JP) , A) JP-A-53-108825 (JP, A)

Claims (1)

(57) [Claims] 1. Weight ratio: C 0.70% or less, Si 1.50% or less, Mn 2.0% or less, P 0.035% or less, S 0.01 to 0.50%, Te 0.10% or less, B 0.0002 to 0.0020%, V 0.15% The following is contained, and Zr and N are contained under the condition satisfying the relationship of the following formula, The balance is made of Fe and steel of unavoidable impurities, and a billet obtained by rolling a slab manufactured by a continuous casting method, 1000 to
Heat-treated to 1300 ° C, forged at a reduction rate of 50% or more in a temperature range of 850 ° C or higher, and immediately accelerated cooling at a cooling rate of 0.3 to 1.0 ° C / sec. Manufacturing method.