JPS62130260A - Tool steel for hot working - Google Patents

Abstract

PURPOSE:To obtain a long-lived tool steel for hot working excellent in resistance to heat checks as well as in wear resistance at high temp. without causing too much deterioration in toughness, by specifying a composition consisting of C, Si, Mn, Cr, W, Mo, V, and Fe. CONSTITUTION:The tool steel for hot working has a composition consisting of, by weight, 0.35-0.50% C, <=1.0% Si, <=1.0% Mn, 3.5-5.0% Cr, W and Mo in the amounts satisfying 1/2W+Mo=2.5-4.0%, 1.0-2.5% V, and the balance Fe with inevitable impurities and further containing, if necessary, 1.0-3.0% Co. This steel is excellent in wear resistance at high temp. and resistance to heat checks. On application of above-mentioned steel, the service life of metal dies can be prolonged.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a tool steel for hot working with a long life and excellent high temperature wear resistance and heat check resistance.

(Prior technology) In the field of medium-sized hot forged products such as automobile parts and mechanical parts such as automobile crankshafts, gear materials, and constant velocity joint belts, in recent years, there have been significant technological advances in casting, cold forging, Competition with similar parts made by machining is becoming more intense, and demands for improved dimensional accuracy and yield, as well as more complex shapes, are becoming more severe. In order to meet these demands, in place of the conventional forging method, closed forging, which does not produce flashing, has recently been adopted, which not only improves dimensional accuracy and yield, but also reduces the oxide film that adheres to steel by lowering the forging temperature. It has been proposed to reduce this and also prevent deterioration of the surface condition. However, lowering the forging temperature
Since the deformation resistance of the steel material is increased, the settling and abrasion of the corners of the mold are accelerated.For this reason, the toughness and hardenability are moderately excellent, and the strength and strength in the hot and warm regions are improved. Hot working tool steel with excellent softening resistance, wear resistance, and heat check resistance is now in demand.

Conventionally, the material steel for hot forging molds was 5%C.
R-based 5KD61 and 62@ are commonly used. These have excellent toughness and are less likely to cause large cracks, but their high-temperature strength and softening resistance are still insufficient, so they cannot be used on the working surface of the mold, especially at the corners where high stress acts under high temperatures. Such parts are subject to significant wear and deformation due to plastic flow. Furthermore, in order for the heat check to proceed quickly,
It is necessary to frequently modify the mold by re-polishing, etc.

For molds requiring high temperature strength and softening resistance, medium C-medium Cr 5KDT steel is used. Although this steel has somewhat better high-temperature strength and softening resistance than the 5KD61 and 62 steels mentioned above, it is insufficient to meet the above requirements, and furthermore, its hardenability and toughness as a mold material are not sufficient. Therefore, large cracks may occur from corners or heat cracks, making it unsuitable for use as a long-life mold material.

(Purpose of the Invention) As a result of detailed investigation of the hardness distribution, structural changes, and wear conditions near the working surface due to the use of molds made of conventional steel, the present inventors have found that molds made of conventional steel are required as hot precision mold materials. In order to prevent early wear and tear, the material must be hard at room temperature and have an initial strength (HRC) of 50 or higher, and 6o
It has been found that there is little loss of strength under high temperature conditions of about 0 to 80 degrees Celsius, and it has excellent softening resistance, that is, heat resistance, and also has an appropriate toughness to the extent that it does not cause early heat cracks or large cracks. Along with this, M is added to medium C-medium Cr steel.
By adding ■ together with o and/or W, wear resistance and heat check resistance at high temperatures can be significantly improved without significantly reducing toughness, and in this way, the life of the mold can be significantly extended. The present invention was developed based on the discovery that it can last for a long time.

(Structure of the Invention) The first tool steel for hot working according to the present invention has, in weight percent, C 0.35 to 0.50%, Si 1.0% or less, Mn 1.0% or less, and Cr 3.5 to 5. The second hot working tool steel according to the present invention is characterized by comprising: In addition to the above chemical components, it is characterized by containing CO in a range of 1.0 to 3.0%.

The reason for limiting the chemical components in the steel of the present invention will be explained.

C. In addition to ensuring the hardenability of steel, tempering hardness of HRC50 or higher, and high-temperature strength, it also forms carbides with carbide-forming elements such as V, Mo, and W, suppresses coarsening of crystal grains, and improves crystal grains. It is added to improve temper softening resistance and abrasion resistance by refining and forming carbides during tempering. In order to effectively exhibit such effects, it is necessary to add at least 0.35% of C to the steel of the present invention. However, when adding too much,
C reduces toughness and also reduces softening resistance.
The upper limit of is 0.50%. In particular, 0.38 to 0.43
A range of % is preferred.

Si is added to improve the high temperature strength and oxidation resistance of the steel of the present invention. However, when adding too much, the toughness deteriorates, so the upper limit of the amount added is set to 1.0%.

Mn is added to improve the hardenability and oxidation resistance of the steel of the present invention. However, if added in excess, machinability deteriorates, so the amount added should be 1.0% or less.

In the present invention, Cr is an essential element to ensure hardenability and wear resistance as a mold material. also reduces temper softening resistance and wear resistance. However, adding too much leads to a decrease in toughness, so the upper limit of the amount added is set at 5.0%. Particularly preferred is a range of 3.8 to 4.5%.

Mo precipitates fine carbides during tempering, increases temper softening resistance and high temperature strength, and improves wear resistance and heat check resistance. It also solidly dissolves in dough to improve hardening.

Like Mo, W also precipitates fine carbides during tempering, increases temper softening resistance and high temperature strength, and improves wear resistance and heat check resistance.

Furthermore, when used as a mold, the oxide film formed on the mold surface becomes denser when exposed to high temperatures of 600°C or higher, reducing the frictional resistance between the mold and the steel material during forging. have

Mo and W are essential elements in order to provide the above-mentioned effects to the steel of the present invention, but in order to effectively obtain such effects, the total amount of A amount and W amount of Mo, ie, ! It is necessary that 4Mo+w be in the range of 2.5 to 4.0%. I/
When 2MO+W is added in excess of 4.0%, there is a strong tendency to form coarse carbides, which deteriorates toughness. It should be noted that Mo has a large effect of improving toughness, while W
Since M o has a large effect of improving wear resistance, the addition ratio of these elements is selected appropriately depending on the purpose and application. In particular, in order to simultaneously improve toughness and wear resistance, Mo
It is preferable to set it as the range of 2.8-3.2% and Wl, 2-1.6%.

In the steel of the present invention, carbides that are difficult to dissolve in solid solution are formed in the dough, which not only improves wear resistance, but also dissolve in solid solution in the dough during quenching and heating, and precipitate moldy carbides during tempering. It is the most important element for providing excellent softening resistance and strength at high temperatures. Furthermore, (2) improves toughness by making crystal grains finer, and has the effect of improving heat check resistance by combining with low softening resistance and high-temperature strength. In order to effectively obtain this effect, it is necessary to add at least 1.0%, but when adding too much, coarse carbides are formed and the toughness is deteriorated.
It also reduces the hardenability, so the upper limit of the amount added is 2.
．． 5%.

Co is medium C-medium W and M as in the steel of the present invention.
o In the -V series hot working tool steel, it is added to suppress the tendency of carbides to coarsen due to tempering and to improve high temperature strength and softening resistance. Furthermore, like W, when used as a mold, the oxide film that forms on the mold surface when exposed to high temperatures of 600°C or higher is made denser, reducing the frictional resistance between the mold and the steel material during forging. It also has the effect of providing excellent abrasion resistance in combination with the heat insulation and protection effect on the mold surface. In order to effectively obtain such effects,
It is necessary to add at least 1.0%, but when adding too much, the toughness of the steel deteriorates, so the upper limit of the amount added is 3.0%.

(Effects of the Invention) As described above, the hot working tool steel of the present invention has medium C
- Molds made by adding V together with Mo and/or W to medium Cr steel have excellent wear resistance and heat check resistance at high temperatures, and have a long life. Further, by subjecting the mold to nitriding treatment, it is possible to further extend the life of the mold.

(Example) The steel of the present invention will be explained in detail below using examples.

Example 1 Table 1 shows the chemical compositions of the inventive steel and comparative steel, and shows the heat treatment conditions for quenching and tempering these steels and the degree of decrease in hardness, i.e., softening, when held at a temperature of 600°C for a predetermined time. The resistance is shown in Table 2. According to the steel of the present invention, it maintains a tempering hardness of HRC50 or more, and compared to comparative steels,
It is clear that both have excellent softening resistance.

Table 3 also shows the 0,
The steel of the present invention has excellent high-temperature strength, as shown by the 2% yield strength.

Next, FIG. 1 shows the results of evaluating the heat check resistance of the inventive steel and comparative steel in a fluidized bed thermal fatigue test. In this thermal fatigue test, the test piece was alternately immersed in a test machine consisting of two fluidized bed constant temperature baths using alumina particles as a heating medium, and heating and cooling were repeated. Thermal cycle conditions were a high temperature bath at 700° C. and a low temperature bath at 150° C., and the samples were immersed for 15 seconds and 45 seconds, respectively, and the growth rate of cracks was measured using a microscope.

It is clear that the steel of the present invention has much better heat check resistance than the comparative steel.

Example 2 A behergya for an automobile was manufactured at a temperature of 1000 to 1050°C by closed forging using a 1000 ton mechanical press using molds made of the above-mentioned steels B and C of the present invention and comparative mF. We investigated the lifespan of molds until they were discarded and the causes of discarding. The results are shown in Table 4. According to the mesh mold of the present invention, the heat check is shallow, the mold repair time is significantly shortened, and the service life is improved.

Example 3 The invention steel C and comparative steel F were used for two-wheeled vehicles at a temperature of 1000 to 1050°C in closed forging with a 1600-ton mechanical press using nitrided roughing dies. We manufactured a crank and investigated the lifespan of the mold until it was discarded and the causes of its discard. As the results are shown in Table 4, the life of the steel mold of the present invention is improved.

Example 4 Using molds made of the invention steel A and comparison steel F, 10
Bolts were manufactured at a temperature of 0.000C, and the lifespan of the mold until it was discarded and the causes of its discard were investigated. As shown in Table 4, the steel molds of the present invention have a long life, but the comparative steel molds suffer from significant corner wear and have a short life.

Example 5 The present invention! Shafts were manufactured at a temperature of 1000°C by closed forging using a 2000-ton hydraulic press using molds made of FaA and comparative mF, and the lifespan of the molds until they were discarded and the causes of their disposal were investigated. Ta. As shown in Table 4, the comparison steel mold was discarded due to sagging and a relatively deep heat check, but according to the steel mold of the present invention, the shallow heat check was the cause of the discard, and the mold The time required for repairs is shortened, and the service life is also improved.

[Brief explanation of drawings]

FIG. 1 is a graph showing the crack growth of the present invention steel and comparative steel in a thermal fatigue test, and FIG. 2 is a plan view and a cross-sectional view showing a test piece used in the thermal fatigue test. Patent applicant: Kobe Steel, Ltd. Japan Koshuha Steel Co., Ltd. Agent: Patent attorney Ittsu Makino Department Figure 1 Zaike IL/rod! ((xlO) Procedural amendment (voluntary) January 4, 1986

Claims (2)

[Claims] (1) In weight% C 0.35-0.50%, Si 1.0% or less, Mn 1.0% or less, Cr 3.5-5.0%, 1/2W+Mo 2.5-4.0%, V 1.0-2.5%, Tool steel for hot working, consisting of balance iron and unavoidable impurities. (2) in weight% C 0.35-0.50%, Si 1.0% or less, Mn 1.0% or less, Cr 3.5-5.0%, 1/2W+Mo 2.5-4.0%, V 1.0-2.5%, Co　1.0~3.0% Tool steel for hot working, consisting of balance iron and unavoidable impurities.