JP2000328179A - Cold tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce cold tool steel having stably high hardness and excellent toughness even in the case of being subjected to quench heating at temp. in a wide temp. range. SOLUTION: This steel contains, by mass, 0.5 to 1.6% C, <=3.0% Si, 0.2 to 2.0% Mn, <=4.0% Ni, 0.2 to 4.0% Cr, 2Mo+W: 0.1 to 8.0%, 0.05 to 3.0% V and 0.02 to 2.0% Nb, and the balance Fe with inevitable impurities, in which non-solid solution MC carbides are present by 0.1 to 5 mass % when being held under heating at 800 to 1100 deg.C. Moreover, the steel may be incorporated with Ti, Ta and Zr and, for increasing its machinability, further with S, Ca, Te, Pb, Se and Bi.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cold tool steel used for a press die, a bending die, a punch die, a drawing die, a die, a punch and the like.

[0002]

2. Description of the Related Art Cold tool steels used for press dies, bending dies, punching dies, drawing dies, dies, punches, etc. are usually used after being subjected to a quenching and tempering treatment to a hardness of about 60 HRC. Often. The heating holding temperature during quenching is
Largely, carbon tool steel (SK), special tool steel (SKS),
Optimal heating conditions are determined for each steel type series, such as cold die steel (SKD), and for each composition, depending on the solid solution state of carbides, crystal grain growth behavior, etc., and appropriate heat treatment is performed. Have been.

For example, in carbon tool steel SK1, 760 to 8
Special tool steel SKS containing 20 ° C, small amount of Cr, W, V, etc.
3, SKD11 containing a large amount of Cr at 800-850 ° C.
The appropriate quenching temperature range varies greatly depending on the type of steel, such as 1000 to 1050 ° C.

[0004] Therefore, in order to perform heat treatment of various steel types in a tool heat treatment line, quenching temperature control for each steel type is important. In addition, fine quenching temperature control is required depending on the composition of the steel. In this way, it is necessary to adjust the quenching temperature and perform the heat treatment.Because of the waiting time for the quenching and the adjustment of the furnace temperature, the conventional heat treatment process requires much time in addition to the actual heat treatment time. It took a long time.

[0005] Further, after quenching, cold tool steel is tempered at 150 to 200 ° C so as not to cause low-temperature temper embrittlement. However, hardening and toughness can hardly be adjusted by this tempering. If the quenching temperature was not adjusted properly due to variations in the furnace temperature, the hardness and toughness after the heat treatment deviated from the target values, and there was a problem that re-heat treatment was required.

[0006]

DISCLOSURE OF THE INVENTION The present invention has a wide range of appropriate quenching temperature, and the quenching temperature does not deteriorate the strength characteristics such as hardness and toughness. It is an object of the present invention to provide a steel having stable high hardness and excellent toughness even when quenched and heated at a temperature in a wide temperature range from a temperature to a quenching temperature for cold die steel.

[0007]

As a result of various studies to solve the above-mentioned problems, it has been found that, in the steel having the chemical composition of the cold work tool steel of the present invention, a suitable amount of V and Nb is added to obtain a stable steel. By forming an appropriate amount and fine MC carbides in the γ phase of steel, the growth of crystal grains is prevented, and even when quenched by heating to a high quenching temperature, it shows high hardness and excellent toughness. Obtained knowledge.

That is, the cold work tool steel of the present invention comprises (1)
In mass%, C: 0.5 to 1.6%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Ni: 4.0% or less, C:
r: 0.2 to 4.0%, 2Mo + W: 0.1 to 8.0
%, V: 0.05-3.0%, Nb: 0.02-2.
0%, the balance being Fe and unavoidable impurities,
When heated and held at a temperature of 800 to 1100 ° C., 0.1 to 5% by mass of undissolved MC carbide is present.

(2) In addition to the above chemical components, T
i: 0.01 to 2.0%, Ta: 0.02 to 2.0%,
Zr: contains any one or more of 0.01 to 2.0%, the balance being Fe and inevitable impurities,
Undissolved M when heated and maintained at a temperature of 00 to 1100 ° C
It is characterized in that C carbides are present in an amount of 0.1 to 5% by mass.

(3) In addition to the chemical component of (1) or (2), S: 0.03 to 0.4
%, Ca: 0.0002 to 0.02%, Te: 0.00
5 to 0.05%, Pb: 0.05 to 0.50%, Se:
0.02 to 0.20%, Bi: 0.015 to 0.15%
Containing at least one of the following, and the balance consisting of Fe and unavoidable impurities. When heated and held at a temperature of 800 to 1100 ° C., undissolved MC carbide is 0.1% by mass.
-5%.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the content of chemical components in the cold tool steel of the present invention will be described below. C: 0.5 to 1.6% C increases the hardness of the steel and forms MC carbides, which are finely dispersed to prevent coarsening of the crystal grains during heating, and furthermore, secondary to tempering. Generates carbides and is added to improve the wear resistance of steel. C content is 0.5
%, The required hardness as a cold tool steel cannot be obtained, so the lower limit of the C content is set to 0.5%. In addition, if C is excessively contained, coarse primary carbides are generated, and toughness is reduced. Therefore, the upper limit of the C content is set to 1.6%.

Si: 3.0% or less Si is an element added to improve the pearlite hardenability and the bainite hardenability and to increase the tempering hardness. However, excessive content lowers the toughness. Is set to 3.0%.

Mn: 0.2-2.0% Mn is added to improve pearlite hardenability and bainite hardenability. If the Mn content is less than 0.2%, hardenability cannot be improved, so the lower limit of the Mn content is set to 0.2%. If the content is excessive, the generation of retained austenite causes the toughness to be reduced. Therefore, the upper limit of the Mn content is set to 2.0%.

Ni: 4.0% or less NI is added to improve hardenability. If the Ni content exceeds 4.0%, the retained austenite increases and it becomes difficult to secure the required hardness, and the machinability decreases. Therefore, the upper limit of the Ni content is set to 4.0%.

Cr: 0.2-4.0% Cr is added to improve hardenability and temper hardness. If the Cr content is less than 0.2%, the effect is small, so the lower limit of the Cr content is set to 0.2%. But,
If the content exceeds 4.0%, the machinability of the steel decreases due to the increase in carbide having high hardness, so the upper limit of the Cr content is set to 4.0%.

2Mo + W: 0.1-8.0% Mo and W are both added to improve the bainite hardenability and increase the tempering hardness. One or two types of Mo and W are obtained by adding W equivalent = 2 × (Mo content) +
0.1 to 8.0% (W content) is contained. If the W equivalent is less than 0.1%, the effect is not sufficient.
The lower limit of the equivalent is 0.1%. If the W equivalent exceeds 8.0%, the primary carbides that are hardly dissolved increase and the toughness of the steel decreases, so the upper limit of the W equivalent is set to 8.0%.

V: 0.05-3.0% V is an essential element added to prevent crystal grains from growing and coarsening during quenching and heating. In order to exhibit the above effects, it is necessary to contain 0.05% or more. However, if the content exceeds 3.0%, the amount of hardly soluble primary carbides increases and the toughness and machinability of the steel decrease, so the upper limit of the V content is set to 3.0%.

Nb: 0.02 to 2.0% Nb is an essential element added to prevent crystal grains from growing and coarsening during quenching and heating, like V.
If the Nb content is less than 0.02%, there is no effect of suppressing crystal grain growth, so the lower limit of the Nb content is set to 0.02%. However, when the content exceeds 2.0%, the amount of hardly soluble primary carbides increases and the toughness and machinability of the steel decrease, so the upper limit of the Nb content is set to 2.0%.

Ti: 0.01-2.0%, Ta: 0.0
2 to 2.0%, Zr: 0.01 to 2.0% Ti, Ta, and Zr each form a primary carbide to suppress the growth of crystal grains during quenching and heating. Two or more can be contained. If the content of each element is small, the effect is not obtained. Therefore, the lower limit of the content is set to 0.01% for Ti and Zr and T for Zr, respectively.
a is set to 0.02%. If each of these elements is excessively contained, the amount of hardly soluble primary carbides increases and the toughness and machinability of the steel decrease. Therefore, the upper limit of each content is set to 2.0%.

S: 0.03-0.4%, Ca: 0.0
002-0.02%, Te: 0.005-0.05%,
Pb: 0.05-0.50%, Se: 0.02-0.2
0%, Bi: 0.015 to 0.15% In order to improve the machinability of steel, one or more of S, Ca, Te, Pb, Se, and Bi may be added. it can. If the content of each element is small, the effect is not obtained. Therefore, the lower limit of the content is set to S: 0.03, respectively.
%, Ca: 0.0002% Te: 0.005%, Pb:
0.05%, Se: 0.02%, Bi: 0.015%.

However, if S is excessively contained, the hot workability, toughness and hardness of steel are reduced, so the upper limit of the content is set to 0.4%. Further, if the Ca content is excessive, the toughness of the steel decreases, so the upper limit of the content is set to 0.02%. If the Te content is excessive, the hot workability and toughness of the steel decrease, so the upper limit of the content is set to 0.05%. If the Pb content is excessive, the hot impact resistance of steel decreases, so the upper limit of the content is set to 0.50%. If the Se content is excessive, the toughness of the steel decreases, so the upper limit of the content is set to 0.10%. If Bi is excessively contained, the toughness of the steel is impaired. Therefore, the upper limit of the content is set to 0.15%.

The cold tool steel of the present invention can be heat-treated using the same heat treatment equipment as for conventional cold tool steel. The quenching heating temperature is 800 to 1100 ° C.
By heating in this temperature range, MC carbide of 0.1% to 5% by mass% can be finely dispersed in the γ phase. If the quenching heating temperature is less than 800 ° C., the carbides are insufficiently penetrated and the hardness cannot be increased, and if the quenching heating temperature exceeds 1100 ° C., MC carbides that inhibit the growth of crystal grains form a solid solution. As a result, crystal grains grow and the steel becomes brittle.

[0023]

EXAMPLES The steel shown in Table 1 was melted by a high-frequency induction furnace to produce a 1-ton steel ingot. After heating the steel ingot at 700 ° C. for 3 hours, the steel ingot was subjected to a low-temperature annealing of standing to cool, and the skin was shaved.
It was forged at 100 ° C. to obtain a steel bar having a diameter of 100 mm. After heating the steel bar at 850 ° C. for 4 hours, the steel bar was cooled at a cooling rate of 10 ° C./h for 6 hours.
The material was gradually cooled to 50 ° C., and thereafter subjected to air-cooled spheroidizing annealing to obtain a spheroidizing annealing material.

[0024]

[Table 1]

From the D / 4 position of the spheroidized annealing material,
Specimen for measuring hardness and grain size, Charpy specimen,
Cut out each test piece coarse material of the test piece for carbide extraction, 800 ° C
After heating at ~ 1100 ° C for 0.5 hour and oil cooling,
After tempering at 0 ° C. × 2 hours, it was finely processed and subjected to each test.

Hardness test: 10 mm × 10 mm × 10 mm
Rockwell hardness tester C
The hardness was measured on a scale.

Measurement of crystal grain size: The same test piece as the hardness test piece was measured for crystal grain size according to the crystal grain size measurement method specified in JIS G 0551. The average value of the crystal grain size numbers in 10 visual fields was used as a representative value.

Charpy impact test: A Charpy test was performed using a 10R notch Charpy test piece of 10 mm × 10 mm × 55 mm cut out from the forging direction to obtain a Charpy value.

Carbide extraction test: 10 mm × 10 mm × 1
Using a test piece of 0 mm, electrolytic extraction was performed using 2% hydrochloric acid + 2% citric acid aqueous solution as an electrolytic solution, and MC carbide was separated as an extraction residue, the mass was measured, and the mass% of MC carbide was calculated. The collected carbide is identified by X-ray diffraction,
It was confirmed to be MC carbide.

Table 1 shows the results of each test. As is clear from Table 1, in Comparative Examples 1-4 containing no V or Nb or containing only V, those quenched at a quenching temperature of 1100 ° C (Comparative Examples 1 and 3) had a quenching temperature of 800 ° C.
The amount of MC carbide is sharply reduced to 0.1% or less as compared with those quenched by the method (Comparative Examples 2 and 4), the crystal grains grow, and the Charpy value is significantly reduced. In Comparative Example 5 in which the Nb content was excessive, the quenching temperature was
In each case at 0 ° C., a large amount of 5% or more MC carbide was contained, and although the crystal grain size was fine, the Charpy value was low.

On the other hand, in the embodiment of the present invention, the amount of MC carbide is in the range of 0.5 to 5% and the crystal grain size is fine even when quenched at either 800 ° C. or 1100 ° C. The hardness shows a high value of around 60 HRC, and the Charpy value also shows a high value.

[0032]

As described above, according to the present invention, even if the treatment is performed at a quenching temperature in a wide range of 800 to 1100 ° C., crystal grains do not grow and the hardening is stable. And a cold tool steel having excellent toughness can be provided. This eliminates the need for complicated quenching temperature management for each type of steel and each steel composition when performing heat treatment in the tool heat treatment line, eliminating the need to wait for the heat treatment and adjusting the furnace temperature. This greatly contributes to improving the efficiency of the heat treatment process.

Claims (3)

[Claims] 1. In mass%, C: 0.5 to 1.6%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Ni: 4.0% or less, Cr: 0 0.2 to 4.0%, 2Mo + W: 0.1 to 8.0%, V: 0.05 to 3.0%, Nb: 0.02 to 2.0%, and the balance from Fe and unavoidable impurities Becomes 80
Undissolved MC when heated and maintained at a temperature of 0 to 1100 ° C
Cold tool steel characterized in that 0.1 to 5% by mass of carbide is present. 2. In addition to the above chemical components, any one of the following: Ti: 0.01 to 2.0%, Ta: 0.02 to 2.0%, Zr: 0.01 to 2.0% The cold tool steel according to claim 1, wherein the cold tool steel contains one or more kinds. 3. In addition to the above chemical components, S: 0.03 to 0.4%, Ca: 0.0002 to 0.02%, Te: 0.005 to 0.05%, Pb: 0 0.05 to 0.50%; Se: 0.02 to 0.20%; Bi: 0.015 to 0.15%. The cold tool steel according to claim 2.