JPH0250910A - Manufacturing method of mold steel plate with good thermal fatigue properties - Google Patents

Abstract

PURPOSE:To improve an extremely thick steel plate having excellent heat fatigue characteristic by executing heat-treatment under specific temp. condition after hot-rolling low-medium carbon low alloy steel slab containing the specific quantities of Ni, Cr, Mo, etc., to the thick plate. CONSTITUTION:The steel slab of composition containing 0.15-0. 50 wt.% C, 0.05-0.75% Si, 0.25-2.00% Mn, 0.25-4.00% Ni, 1.00-4.00% Cr, 0.15-3.00% Mo, 0.005-1.50% Sol. Al and 0.0020-0.0500% N or further, one or more kinds among 0.002-0.002%, Ti, 0.0003-0.0100% B, 0.005-0.200% Zr and 0.003-0.50% V, is hot- rolled to the extremely thick plate material having >=250mm thickness. After reheating this to the temp. of Ac3 point+30 deg.-Ac3 point+200 deg.C of this steel by using water soluble quenching liquid near the water or oil, rapidly cooled quenching is executed at <=300 deg.C and successively, tempering is executed at the temp. of <=Ac1 point. The extremely thick steel plate having excellent heat fatigue characteristic for using to hot forging die, plastic die, etc., is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing an extra-thick steel plate for molds having a thickness of 250+ nm or more used for hot forging molds, plastic molds, etc.

[Prior art] JIS 5KDIlil (
0,4C-1,005j-0,4Mn-5Cr-1,3
Mo-1, OV series) and S K T 4 (0,55C-
0,25S j-0,75Mn], 7 Nj-(1,9
Cr-0,4Mo system) is used. Since these steel materials are originally expensive, there is a need to reduce the cost of molds.

In order to reduce costs, there are steel materials that use age hardening and reduce the amount of alloy added, as described in Japanese Patent Application Laid-Open No. 63-38557, but even though the field of application is steel for plastic molds, the hardness is relatively low. However, since the thermal fatigue properties are not good, it cannot be applied to hot forging dies, and it does not necessarily lead to cost reduction in terms of the total cost of the dies.

On the other hand, there is a method of reducing the amount of alloy added by water quenching, but as stated in Japanese Patent Publication No. 131-34182, when thin high C-Cr-Mo steel is subjected to normal water quenching, the surface layer of the steel material hardening is significant and quench cracking occurs.

Therefore, it is necessary to suppress the amount of alloy added and to extend the life of the thermal fatigue layer, and there is a desire to develop steel materials that can achieve this.

[Problems to be Solved by the Invention] Therefore, an object of the present invention is to provide a method for inexpensively manufacturing a steel plate for molds having good thermal fatigue properties.

[Means for Solving the Problems] The present invention has been made in order to advantageously solve the problem of developing steel materials for extending the life of molds.
The gist is that C: 0.15 to 0 in weight ratio.
．． 50%, Si: 0.05-0.75%, Mn.

0.25-2.00%, Ni: more than 0.25-4.00%
, Cr: more than 1.00 to 4.00%, Mo: 0.15 to 3
．． 00%, soΩ, toΩ20, 005~1.50%, N
・Basic component: 0.0020-0.0200%, additionally 0.002-0.200% Ti, B:
0.0003-0.0100%, Zr: 0.110
5-0.200%, V: 0.003-0500%, a steel containing one or more of 0.003-0500%, the balance consisting of Fe and unavoidable impurities is hot worked, A c a +30 ° C
-A c a + After reheating to 200°C, quenching and cooling from this temperature to 3 [10°C or less using water or a water-soluble quenching liquid close to Ura, and then tempering at a temperature of A c i or less. This is a method for manufacturing mold steel sheets with good thermal fatigue properties.

[Function] The inventors have made various detailed improvements based on the recognition that in order to extend the life of molds using JIS 5KD61 and SKT 4, it is necessary to improve the thermal fatigue properties of the material. conducted research.

As a result, it has been found that thermal fatigue properties can be improved by combining the ingredients and heat treatment method of the present invention.

That is, in the present invention, it is possible to suppress the formation of upper bainite by adding Ni and to improve high-temperature hardness by forming an intermetallic compound of NiAβ, and at the same time, the intermetallic compound is a stress concentration source compared to a carbide. They found that the method of the present invention improves these properties and successfully develops steel materials for molds with good thermal fatigue properties. be.

Furthermore, since the plate is thick, the surface cools slowly and the hardness of the surface hardened portion during quenching is low, so quench cracking does not occur.

Next, the reasons for limiting the component system in the present invention are as follows.

C dissolves in martensite or precipitates as a carbide and has the effect of increasing the hardness of steel. In order to exhibit this effect, a content of 0.15% or more is required, but 0.5% or more is required.
If it exceeds 0%, repair weldability of the mold will be impaired, so the content was limited to 0.15 to 0.50%.

Si has the effect of improving the hardness of steel at low cost, but 0.
This effect will not be exhibited unless it is 0.05% or more, and 0.75%
If the content exceeds 0.05 to 0.0, the toughness will decrease.
Limited to 75%.

Mn has the effect of improving the hardness and toughness of steel at low cost.
It is essential to ensure the hardness and toughness of the base steel plate,
For this purpose, the content must be 0.25% or more. On the other hand 2
If the content exceeds .00%, specularity and fatigue properties will deteriorate due to the formation of MnS.

For this reason, the content was limited to 0.25 to 2.00%. N
Although i has the effect of improving hardenability and increasing toughness by introducing cross-slip, this effect is not exhibited unless it exceeds 0.25%. If there are too many, it becomes too expensive, so set the upper limit to 4.
．． 00%. Therefore, the content should be increased from more than 0.25 to 4.0
It was limited to 0%.

Cr has the effect of improving hardenability and improving the hardness of steel as a precipitate, but this effect is not exhibited unless it exceeds 1.00%, and it is not economical when it exceeds 4,00%. . For this reason, the content was limited to more than 1.00% to 4.00%.

MO has the effect of improving hardenability and improving the hardness of steel as a precipitate, but this effect is not exhibited unless it is 0.15% or more, and it is not economical if it exceeds 3.00%.

For this reason, the content was limited to 0.15-300%.

soβ and Aρ are necessary as deoxidizing elements during the production of base steel sheets, and in order to make their effect stable, so and Q
, 0.005% or more in AΩ is required. On the other hand, 1.50%
If the content exceeds 100%, hot workability during steel plate processing will be significantly reduced. Therefore, the content of SOΩ and Aβ is 0.005 to 1
Limited to 50%.

N precipitates as AgN during the production of the base steel sheet, prevents coarsening of γ grains, and is required in an amount of 0.0020% or more in order to obtain this effect. Moreover, if the content exceeds 0.0500%, giant AΩN will precipitate, resulting in a decrease in toughness.

For this reason, the N content was limited to 0.0020 to 0.0500%.

Next, Ti, B, and zr are added in the second invention.

I will talk about ■.

These components are added because they have the uniform effect of improving the toughness of steel, but in order to ensure the desired effect on the above action, the lower limit of each content is set such that Ti:0.
002%, B: 0.0003%, ZrO, 005%
, ■・0.003% is required.

However, Ti: 0.200% and B: 0, respectively.
．． 0200%, Zr: 0.200%, V: 0.5
Even if the content exceeds the upper limit of 00%, the action and effect will be saturated or the effect will be reduced.
The content of each was determined as above.

Next, hot working is performed by rolling or forging to a predetermined monthly rate, and the higher the degree of working per cycle, the better.

Next, the heat treatment method will be described.

The heat treatment method of the present invention ranges from A C3+ at a high temperature of 30°C or higher to 300°C using water or a water-soluble quenching liquid close to Uura.
The basic method is a quenching process in which the material is cooled to a temperature of Ac1 or less, and a quenching and tempering process in which the material is tempered at a temperature of Ac1 or less after quenching, and if residual austenite is present after the quenching process, sub-zero treatment is added after the tempering process.

During the quenching process, A c a +30°C-Ac3
After austenizing at +200°C, the material is rapidly cooled to 300°C or less for quenching.

The reason why the austenitizing temperature is set to A c 3+ 30°C or higher is to completely austenitize in consideration of the temperature distribution in the industrial heat treatment furnace, and the upper limit is set to A c a
The reason for setting the temperature to +200°C is to prevent the γ grains from becoming significantly coarser.

The reason why the quenching end temperature was set to 300°C or lower is that the M8 point is 300°C or higher and martensite is sufficiently transformed.

As the water-soluble quenching fluid similar to oil used in the present invention, for example, Kocon Quenchant E, a trade name manufactured by Union Carbide Company, USA, is preferably used.

Next, tempering has the effect of improving toughness and releasing quenching strain by spheroidizing carbides and tempering the quenched structure. For this purpose, it is preferable to perform tempering two or more times at a temperature of A C1 or lower. When tempering is performed in one time, A c
It is sufficient to set the tempering temperature at two or more levels at a temperature of 1 or less and maintain the tempering temperature in sequence for an arbitrary period of time during the heating process.

Sub-zero treatment works to prevent mold deformation during mold processing and mold use by completely converting the retained austenite into martensite when residual austenite exists after quenching.

In order to fully exhibit this effect, after quenching and before tempering, dry ice or liquid nitrogen may be used to cool the material to below the martensitic transformation end point (Mf point).

[Example] The components of the Example and Comparative Example are shown in Table 1, and the product board thickness heat treatment conditions are also shown in Table 1.

Incidentally, the steel was melted in a converter, and after melting, it was made into slabs by a conventional method, and each slab was heated to 1250° C. and rolled into a thick plate. Further, each steel plate was tempered at 550 to 650°C.

The mechanical test values of the obtained extra-thick steel plates are shown in Table 2, and FIGS. 1 and 2 show the hardness distribution and thermal fatigue properties of Example Steels 4 and 16 of the present invention.

Figure 3 shows a thermal fatigue test piece.

Note that the numbers in the figure indicate dimensions (unit: m/m).

However, from Table 2 and the drawings, it is clear that the products according to the present invention have a long thermal fatigue life, small variations in hardness, and good toughness with a high level of toughness. On the other hand, the comparative examples showed short thermal fatigue life, large hardness variations, and low toughness levels.

[Effects of the invention] As described in detail above, heat treatment is performed by cooling and quenching from Ac +30°C to Ac3 + 200°C to 300°C or less using water or a water-soluble quenching fluid similar to oil, and tempering at AC1 or less. Therefore, it is possible to obtain a steel plate for molds that is hard, has a uniform hardness distribution in the thickness direction, has a high toughness value, and has good thermal fatigue properties.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the hardness distribution of Example 4 of the present invention and Comparative Example 16, FIG. 2 is a graph showing the thermal fatigue life in the thickness direction of Example 4 of the present invention and Comparative Example 16, and FIG. 3a. b is an explanatory diagram showing the shape of a test piece used in a thermal fatigue test. Agent Patent Attorney Tatsuo Chanoki Fig. surface distance @ (A77L) Fig. 10θ 2θθ 300 Base force/distance (ynyyt) 4θ0 First cause (a) Part 13t, ys--- (b) ←/7-Hand, selling correction 1 ■ di (spontaneous) Specification 1, page 3, table 2, r 394/461 in the HB (surface/center) column of steel N013. ” Amended on September 2, 1986.

Claims (1)

[Claims] 1. C: 0.15-0.50%, Si: 0.05-0.75 in weight ratio
%, Mn: 0.25 to 2.00%, Ni: more than 0.25
4.00%, Cr: more than 1.00 to 4.00%, Mo: 0
．． 15-3.00%, sol. Al: 0.005-1.
50%, N: 0.0020 to 0.0500%, and the balance is Fe and unavoidable impurities.
After reheating to Ac_3+30℃~Ac_3+200℃, 3
A method for producing a steel plate for molds having good thermal fatigue properties, characterized by quenching and cooling to a temperature of 00°C or lower, and then tempering at a temperature of Ac_1 or lower. 2. C: 0.15-0.50%, Si: 0.05-0.75 in weight ratio
%, Mn: 0.25 to 2.00%, Ni: more than 0.25
4.00%, Cr: more than 1.00 to 4.00%, Mo: 0
．． 15-3.00%, sol. Al: 0.01-1.0
0%, N: 0.0020-0.0500%, and furthermore, Ti: 0.002-0.200%, B: 0.0003-0.0100%, Zr: 0.005-0. 200%, V: 0.003 to 0.500%, for a mold with good thermal fatigue properties according to claim 1, using steel containing one or more of the following, with the balance consisting of Fe and inevitable impurities. Method of manufacturing steel plates.