JP2015193867A - high toughness hot work tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot work tool steel excellent in toughness by reducing the amount of crystallization carbide during solidification which is harmful to toughness by controlling contents of Ti and N as impurities, and reducing area percentage of carbonitride existing in a steel product with an appropriate homogenization heat treatment.SOLUTION: There is provided a high toughness hot work tool steel containing, by mass%, C of 0.30 to 0.55%, Si of 0.1 to 1.2%, Mn of 0.2 to 1.5%, Ni of 0.1 to 2.0%, Cr of 1.5 to 6.0%, one or two kind of Mo and W with Mo+1/2 W of 0.1 to 2.5%, one or two of V and Nb with V+1/2 Nb of 0.2 to 1.0%, Al of 0.03% or less and the balance Fe with inevitable impurities, N and Ti as inevitable impurities satisfying N≤0.0150%, Ti≤0.005%, and [N]×[Ti]≤3×10(Formula 1).

Description

  The present invention relates to a hot tool steel for a die such as hot forging, casting or die casting.

  Conventionally, as hot tool steel for dies such as hot forging, casting or die casting, JIS G 4404 SKD61, SKD6, SKT, etc., 3Cr-3Mo steel, semi-high steel, etc. are used. . However, in recent years, products that have been divided into multiple parts are often manufactured as a single unit with the aim of improving production efficiency. With conventional steel types, heat cracks and large cracks occur at an early stage. Therefore, there is a demand for hot tool steel having higher strength and higher toughness.

  For example, as a steel material for this hot tool, C: 0.3-0.4%, Mn: 0.2-0.8%, Cr: 4-6%, Mo: 1. 8 to 3%, V: 0.4 to 0.6%, steel materials for hot working tools composed of the remaining iron and inevitable metal impurities and inevitable non-metal impurities. These steel materials are austenitized at 1000 to 1080 ° C. Further, a steel material capable of obtaining a hardness exceeding 45 HRC by tempering at 550 to 650 ° C. has been disclosed as an invention (see, for example, Patent Document 1). By the way, this Patent Document 1 states that the reduction of nitrogen and titanium reduces the toughness of the steel material by reducing the MX-based crystallized product generated during solidification. However, it is not sufficient to make nitrogen and titanium below the specified value, and toughness may not be improved. In the above MX, M represents V and Ti, and X represents C and / or N.

Further, in terms of mass%, C: 0.2 to 0.7%, Cr: 0.5 to 7.0%, one or two of Mo or W at (Mo + 1 / 2W), 0.1 to In the hot working tool of the component composition described in JIS containing 6.0%, V: 3.0% or less, Al is described in 0.04% or less, and the segregation degree of the base composition Cr is Within ± 0.2% by mass, the degree of segregation of (Mo + 1 / 2W) is within ± 0.08% by mass, the degree of segregation of V is within ± 007% by mass, and the maximum diameter observed in the cross-sectional structure is 5 μm or less. An invention of a hot work tool steel excellent in toughness in which aluminum nitride is 2500 pieces / mm ² or less (including 0 pieces) is disclosed (for example, see Patent Document 2). However, in this proposal, it is said that high toughness can be obtained by prescribing the degree of segregation of the base composition C, Mo, V, and suppressing aluminum nitride by reducing Al, but crystallization carbonitriding by segregation during solidification The material is not taken into account, and if the Ti contained in the product contained in the raw materials is not taken into account, the amount of nitride is not sufficiently controlled, and it becomes difficult to obtain the soaking effect, so high toughness is obtained. There may not be.

Japanese Patent No. 4516211 Japanese Patent No. 4441907

  The problem to be solved by the present invention is to control the contents of Ti and N as impurities, thereby reducing the amount of crystallized carbides formed during solidification harmful to toughness, and performing appropriate homogenization heat treatment It is intended to provide a hot tool steel having extremely excellent toughness by reducing the area ratio of carbonitride, which is a crystallized product remaining in a steel product product, to a certain value or less by adding it.

Means for solving the above-mentioned problems are, in the first means, mass%, C: 0.30 to 0.55%, Si: 0.1 to 1.2%, Mn: 0.2 to 1 0.5%, Ni: 0.1-2.0%, Cr: 1.5-6.0%, and further comprising any one or two of Mo and W, Mo + 1 / 2W: 0.0. 1 to 2.5%, further comprising any one or two of V and Nb, V + 1 / 2Nb: 0.2 to 1.0%, and Al: 0.03% or less And N and Ti as the inevitable impurities satisfy N ≦ 0.0150%, Ti ≦ 0.005%, and [N] × [Ti ] ≦ 3 × 10 ^-5 ... satisfy the equation (1), wrought molding ratio specified by JIS: If you and forging to annealed state above 4s In cross-section in the tissue of the steel material is a high tenacity hot work tool steel, wherein the area ratio of carbo-nitride having a length more than 5μm is 0.4% or less.

In the second means, in mass%, C: 0.30 to 0.55%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Ni: 0.1 to 2 0.0%, Cr: 1.5 to 6.0%, further comprising any one or two of Mo and W, Mo + 1 / 2W: 0.1 to 2.5%, It consists of any one or two of V and Nb, contains V + 1 / 2Nb: 0.2-1.0%, further contains Al: 0.03% or less, and consists of the balance Fe and inevitable impurities. N and Ti as inevitable impurities satisfy N ≦ 0.0150%, Ti ≦ 0.005%, and [N] × [Ti] ≦ 3 × 10 ⁻⁵ (Formula 1 ) Forging and forming ratio specified by JIS: 5 μm in the cross-sectional structure of steel when forged to 4 s or more and annealed. The area ratio of the carbonitride having the above length is 0.4% or less, and the as-cast steel ingot is within a temperature range of 1200 to 1300 ° C. and 1334-73 × [C] -12 [ Si] —a high toughness hot tool steel characterized by performing a homogenization heat treatment at a temperature satisfying a homogenization heat treatment temperature (° C.)> 0.

The invention according to the claims of the present application is the above means, so that the area ratio of the carbonitride which is a crystallized substance is 0.4% or less, and when the homogenization heat treatment temperature is performed, A hot work tool steel having excellent toughness having no void due to the occurrence of a molten layer, no coarse primary carbide, and an impact value of 40 J / cm ² or more is obtained.

  The chemical component content and components in the present invention will be described below. In addition,% is the mass%.

C: 0.30 to 0.55%
C is an element necessary for ensuring sufficient hardenability and forming carbides to obtain the hardness, wear resistance and strength of steel, and is an element which suppresses the austenite grain coarsening during quenching. . If C is lower than 0.30%, sufficient strength and wear resistance cannot be obtained. On the other hand, when C is more than 0.55%, solidification segregation is promoted, and crystallization of coarse carbides or generation of a molten layer is likely to occur during soaking, thereby inhibiting toughness. Therefore, C is 0.30 to 0.55%, preferably 0.30 to 0.50%.

Si: 0.1-1.2%
Si is an element necessary for ensuring the deoxidation effect and hardenability in steelmaking. If Si is less than 0.1%, these effects are not exhibited. On the other hand, if the Si content is more than 1.2%, the toughness is reduced, and the thermal conductivity, which is an important physical property value for hot tool steel, is reduced. Therefore, Si is set to 0.1 to 1.2%, preferably 0.2 to 1.1%.

Mn: 0.2 to 1.5%
Mn is an element necessary for ensuring the deoxidation effect and hardenability in steelmaking. If Mn is less than 0.2%, these effects cannot be obtained sufficiently. On the other hand, when Mn exceeds 1.5%, toughness and machinability are lowered. Therefore, Mn is set to 0.2 to 1.5%, preferably 0.3 to 1.2%.

Ni: 0.1 to 2.0%
Ni is an element that improves hardenability and toughness. If Ni is less than 0.1%, these effects cannot be obtained. On the other hand, even if Ni is more than 2.0%, there is almost no improvement in characteristics, and the A ₁ transformation point is lowered to deteriorate the heat resistance, and the machinability is also deteriorated. Therefore, Ni is set to 0.1 to 2.0%, preferably 0.1 to 1.5%.

Cr: 1.5-6.0%
Cr is an element that improves hardenability and is effective in improving wear resistance. However, when Cr is less than 1.5%, these effects cannot be obtained sufficiently. On the other hand, if the Cr content is more than 6.0%, a large amount of Cr-based carbides are formed during quenching and tempering, and the toughness, high-temperature strength and softening resistance are lowered. Therefore, Cr is 1.5 to 6.0%, preferably 1.5 to 5.5%.

Mo + 1 / 2W: 0.1-2.5%
Mo or W is an element necessary for obtaining precipitated carbides that contribute to hardenability, secondary hardening, and wear resistance. Furthermore, Mo or W has the effect that the fine carbide which became insoluble at the time of quenching suppresses the coarsening of crystal grains. However, if Mo + 1 / 2W composed of one or two of Mo or W is less than 0.1%, the above effect cannot be obtained. On the other hand, even if Mo + 1 / 2W composed of one or two of Mo or W is added in excess, the effect is saturated, and the toughness is reduced due to coarse aggregation of carbides, and the cost is increased. Therefore, Mo + 1 / 2W is set to 0.1 to 2.5%, preferably 0.3 to 2.2%.

V + 1 / 2Nb: 0.2-1.0%
V or Nb is an element that precipitates fine and hard carbides and carbonitrides during tempering and contributes to strength and wear resistance. In addition, V or Nb suppresses coarsening of crystal grains and suppresses a decrease in toughness during quenching because of fine carbides and fine carbonitrides. However, if V + 1 / 2Nb composed of one or two of V and Nb is less than 0.2%, the above effect cannot be obtained. On the other hand, when there is too much V + 1 / 2Nb which consists of 1 type or 2 types of V or Nb, the coarse crystallized substance will be produced | generated at the time of solidification and toughness will be inhibited. Therefore, V + 1 / 2Nb is set to 0.2 to 1.0%, preferably 0.2 to 0.8%.

Al: 0.03% or less When Al is more than 0.03%, oxides and nitrides are increased, which tends to be the starting point of cracking, and the fatigue strength and toughness of the steel are reduced. Therefore, Al is set to 0.03% or less, preferably 0.02% or less.

N and Ti as inevitable impurities: N ≦ 0.0150%, Ti ≦ 0.005%, and [N] × [Ti] ≦ 3 × 10 ⁻⁵ (Formula 1) are satisfied.
N and Ti as inevitable impurities satisfy N ≦ 0.0150%, Ti ≦ 0.005%, and satisfy [N] × [Ti] ≦ 3 × 10 ⁻⁵ (Formula 1). In some cases, the generation of crystallized products during solidification is suppressed. However, when [N] × [Ti] exceeds 3 × 10 ⁻⁵ , the crystallized product is difficult to dissolve, and the carbonitride in the steel material tends to be coarse. Therefore, N and Ti as unavoidable impurities satisfy N ≦ 0.0150%, Ti ≦ 0.005%, and [N] × [Ti] ≦ 3 × 10 ⁻⁵ (Formula 1). To do.

JIS standard forging ratio: Area ratio of carbonitride having a length of 5 μm or more in the cross-sectional structure of an annealed steel material forged to 4 s or more: 0.4% or less JIS standard forging ratio Cracks generated by stress concentration on carbonitride when the area ratio of carbonitride having a length of 5 μm or more in the cross-sectional structure of the annealed steel material forged to 4 s or more exceeds 0.4% Advances along the carbonitride and becomes easy to connect to a nearby carbonitride, resulting in a decrease in toughness. Therefore, the area ratio of carbonitride having a length of 5 μm or more in the cross-sectional structure of the annealed steel material forged to a forging forming ratio of 4 s or more is set to 0.4% or less.

Ingot as cast: Perform homogenization heat treatment at a temperature satisfying the homogenization heat treatment temperature (° C.)> 0 with 1334-73 × [C] -12 [Si] − in the range of 1200 to 1300 ° C. Casting If the steel ingot as it is is subjected to a homogenization heat treatment at a temperature lower than 1200 ° C., the solid solution of the crystallized product and the diffusion of the solid solution element become insufficient. On the other hand, as-cast steel ingot is 1334-73 × [C] -12 [Si] -homogenization heat treatment temperature (° C.) ≦ 0, or when the homogenization heat treatment is performed above 1300 ° C. A molten layer is generated, resulting in coarsening of primary carbides, deterioration of toughness and deterioration of hot workability. Therefore, the as-cast steel ingot is subjected to homogenization heat treatment at a temperature satisfying the range of 1200 to 1300 ° C and satisfying 1334-73 × [C] -12 [Si] -homogenization heat treatment temperature (° C)> 0.

Here, an embodiment of the present invention will be described.
First, after melting the steel of the composition containing the chemical components listed in Table 1 of the present invention and the comparative example shown in Table 1 with the balance consisting of Fe and inevitable impurities using a 1 ton vacuum melting furnace, The ingot was agglomerated. These ingots were held at the homogenization heat treatment temperature set in Table 2 for 12 hours and subjected to the homogenization heat treatment. Steel material was manufactured by forging. In the present invention, in addition to the above-described vacuum melting furnace, secondary melting may be performed by ESR (electroslag remelting) method or VAR (vacuum arc remelting) method.

  Using the 60 mm square cube obtained by indexing from the middle part of each steel material produced above, that is, the part having a radius of 40 mm, stirring is carried out for 30 minutes at a quenching temperature of 880 to 1030 ° C., followed by stirring 50 It was put into oil at 0 ° C. and quenched by oil cooling.

Further, these test pieces were tempered at a tempering temperature of 500 ° C. or higher, and tempered to 44 to 46 HRC mainly used in hot tool steel. The toughness of the obtained steel material was evaluated by the energy required for destruction by the Charpy impact test. The test pieces used for these were taken from the vertical direction of the central part of the forged material having a diameter of 160 mm, and a U-notch having a depth of 2 mm defined in JIS Z 2242 was processed into a surface perpendicular to the rolling direction. The toughness of the good or bad determination, typically because it is a minimum 20 J / cm ² required for hot tool steel, judged by whether 40 J / cm ² or more impact value twice to obtain did.

  The amount of crystallized material, which is carbonitride, was measured by observing a 10 mm square test piece having a length of 16 mm on a surface parallel to the rolling direction from the center of a forged material having a diameter of 160 mm. In other words, after the specimen was mirror-polished by final buffing, the specimen was observed at 400 times with an optical microscope, and 30 fields of view where there were many carbonitride crystallized substances were selected. The area ratio of the deposit was calculated and the amount of crystallized substance was shown as the carbonitride area ratio (%).

As already described, No. of the example of the present invention consisting of the chemical components indicated by mass% in Table 1 in the embodiment of the present invention. 1-16 and Comparative Example No. 17 to 31 will be further described. As inevitable impurities of the invention according to claim 1 and claim 2 of the present invention, mass%, N ≦ 0.0150% and Ti ≦ 0.005%, and [N] × [Ti] ≦ 3 × 10 −5 (1) The length of 5 μm or more in the cross-sectional structure of the annealed steel material that has been forged to 4 s or more in Table 2 is satisfied. The carbon nitride having an area ratio of 0.4% or less is further included, and in addition to this requirement, the invention according to claim 2 includes a homogenization heat treatment temperature in the range of 1200 to 1300 ° C. and 1334- The homogenization heat treatment is performed at a temperature satisfying the homogenization heat treatment temperature (° C.)> 0 with 73 × [C] -12 [Si] −. These are shown in Table 2 for the presence or absence of a molten layer and the impact value (in the T direction) of 40 J / cm ² or more. In addition, said [N] and [Ti] have shown the mass% of N and Ti, respectively.

  No. of the example of the present invention shown in Table 2. 1-No. No. 16 has a carbonitride area ratio of 0.4% or less, a homogenization heat treatment temperature of 1200 to 1300 ° C., and 1334-73 × [C] -12 [Si] —. Temperature (° C.)> 0 is satisfied, no molten layer is generated, the impact value (T direction) is 40 J / cm 2 or more, and the toughness is high.

On the other hand, in the comparative example shown in Table 2, No. No. 17 does not undergo homogenization heat treatment. No. 18 has N higher than 0.015%. No. 28 has a Ti content of more than 0.005% as shown in Table 1, and many carbonitrides with a length of 5 μm or more are present in the steel, and the impact value (T direction) is less than 40 J / cm ^2. The toughness is low and x.

  Comparative Example No. 19, and no. No. 20 has poor hardenability and low toughness.

  Comparative Example No. 21, and no. No. 24 has a small amount of precipitated carbides, cannot suppress the coarsening of crystal grains during quenching heating, and has low toughness.

  Comparative Example No. 22, no. 23 and no. No. 26 is low in toughness because of a large amount of coarse crystallized carbide even after the homogenization heat treatment, and segregation is remarkable, and X.

  Comparative Example No. No. 25 is x because oxides and nitrides are generated too much and toughness is insufficient.

  Comparative Example No. In No. 27, Mn is 2.2%, which is too much than 1.5%, and brittleness of the base occurs, and sufficient toughness cannot be obtained.

  Comparative Example No. 29, C is 0.65%, more than 0.55%, and 1334-73 × [C] -12 [Si] -homogenization heat treatment temperature (° C.) ≦ 0. The melted layer is generated, the primary carbide is coarsened, and segregation is also remarkable, so that the toughness is insufficient, and x.

  Comparative Example No. No. 30 is x because the homogenization heat treatment temperature is lower than 1150 ° C. and 1200 ° C., so that the solid solution of the crystallized product in the segregation part does not sufficiently progress and the toughness becomes insufficient.

  Comparative Example No. No. 31 has a homogenization heat treatment temperature of 1320 ° C. and higher than 1300 ° C., and 1334-73 × [C] -12 [Si] −homogenization heat treatment temperature (° C.) ≦ 0. A layer is generated, the primary carbide becomes coarse, and the toughness is insufficient.

Claims (2)

In mass%, C: 0.30 to 0.55%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Ni: 0.1 to 2.0%, Cr: 1.5 to 6.0%, further comprising any one or two of Mo and W, Mo + 1 / 2W: 0.1 to 2.5%, and either V or Nb It consists of one or two kinds, V + 1 / 2Nb: 0.2 to 1.0%, further containing Al: 0.03% or less, the balance Fe and unavoidable impurities steel, N and Ti as unavoidable impurities satisfy N ≦ 0.0150%, Ti ≦ 0.005%, satisfy [N] × [Ti] ≦ 3 × 10 ⁻⁵ , and have a JIS-specified forging ratio. In the state where the steel material forged for 4 s or more was annealed, the carbon nitride having a length of 5 μm or more in the cross section of the steel material was an area ratio of 0. % High tenacity hot work tool steel which is characterized in that less. In mass%, C: 0.30 to 0.55%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Ni: 0.1 to 2.0%, Cr: 1.5 to 6.0%, further comprising any one or two of Mo and W, Mo + 1 / 2W: 0.1 to 2.5%, and either V or Nb It consists of one or two kinds, V + 1 / 2Nb: 0.2 to 1.0%, further containing Al: 0.03% or less, the balance Fe and unavoidable impurities steel, N and Ti as unavoidable impurities satisfy N ≦ 0.0150%, Ti ≦ 0.005%, satisfy [N] × [Ti] ≦ 3 × 10 ⁻⁵ , and have a JIS-specified forging ratio. In the state where the steel material forged for 4 s or more was annealed, the carbon nitride having a length of 5 μm or more in the cross section of the steel material was an area ratio of 0. %, And the ingot as cast is in a temperature range of 1200 to 1300 ° C. and satisfies 1334-73 × [C] -12 [Si] -homogenization heat treatment temperature (° C.)> 0. High toughness hot work tool steel characterized by homogenization heat treatment at temperature.