JP6714334B2 - Hot work tool steel with excellent thermal conductivity and toughness - Google Patents

Description

TECHNICAL FIELD The present invention relates to a die steel, and more particularly to a die steel used in a thermal environment of 500° C. or higher, such as die casting and hot stamping.

In recent years, in the die-casting field, mechanical strength and thermal load on the die-casting die have been increased due to the higher strength of aluminum parts for the purpose of weight reduction of automobiles and the shortening of the part molding processing pitch for the purpose of improving productivity. It is increasing. As a result, the mold is prone to problems such as large cracks and heat checks. In order to cope with these problems, a die material that is excellent in hardenability and toughness is required. Further, in order to meet the demand for improving the speed of the production cycle for the purpose of improving the productivity described above, high thermal conductivity is required as a material property. As a conventional technique or a similar technique focusing on these points, there are techniques disclosed in the following patent documents, but each has its problems.

That is, as a conventional technique, there has been proposed a method for reducing the adverse effect of grain boundaries due to segregation of steel having high mechanical strength and wear resistance (see, for example, Patent Document 1). However, since the proposed method of reducing the adverse effect of segregation of steel has no limitation on N that affects carbide formation, when N is added in excess, a large amount of N remains in the matrix, resulting in thermal conductivity. There is a problem that

Further, steel capable of ensuring hardenability, thermal conductivity, and softening resistance, steel for molds, and inventions relating to molds using the same, particularly steels applicable to large-section molds, steels for molds And a mold using the same has been proposed (see, for example, Patent Document 2). However, this proposed steel has a problem that the ΔC value represented by C-Ceq deviates from the invention of the present application, so that the amount of alloying elements remaining in the matrix increases and the thermal conductivity decreases.

Furthermore, a hot work tool steel having excellent thermal diffusivity, toughness and complete hardenability, which has high thermal conductivity and mechanical durability, and high yield strength at room temperature and high temperatures of over 600° C. has been proposed. (See, for example, Patent Document 3). However, the Mn amounts shown in the examples of the proposed steels are as low as 0.23% or less, so that the hardenability is insufficient and the toughness is reduced.

Similarly, a hot work tool steel having excellent thermal diffusivity, hardness and wear resistance has been proposed (see, for example, Patent Document 4). However, since the Mn amounts shown in the examples of the proposed steels are as low as 0.6% or less, the hardenability is insufficient and the toughness is reduced.

Japanese Patent Publication No. 2007-538154 JP, 2009-242820, A Special table 2012-522886 gazette Japanese Patent Publication No. 2014-508218

The problem to be solved by the invention of the present application is that it has a higher level of toughness and thermal conductivity than the steel proposed in the patent document that is the above-mentioned prior art, and is applicable to die casting and hot stamping. It is to provide mold steel.

Means for solving the above problems of the present application, the first means, in mass%, C: 0.20 to 0.60%, Si: 0.03 to less than 0.50%, Mn: 0. More than 60 to less than 1.50%, Cr: 0.21 to less than 4.00%, Mo: 0.5 to 5.0%, 2Mo+W: 3.0 to less than 13.0%, N: 0.010 to Containing less than 0.030% , optionally containing W: less than 4.0%, V: less than 1.00%, Nb: less than 1.0% as optional components, balance Fe and unavoidable When it is made of impurities and Ceq: 0.20% or more and ΔC=C−Ceq, ΔC: −0.300 to 0.300% and B value: 0.74 to 0.94. Is a hot work tool steel having excellent thermal conductivity and toughness.
However, Ceq=0.06×%Cr+0.063×%Mo+0.033×%W+0.2×%V+0.1×%Nb, where the% element represents the mass% of each element, and ΔC=C -Ceq in Ceq is mass %, and further, Ceq is a carbon equivalent mainly used as a standard for the amount of C necessary when all the added elements become carbides, where ΔC is in the steel. From the relationship between C and the amount of each carbide forming element, it is an index considering the amount of solid solution element which is important in improving the thermal conductivity. Further, B value=(0.063×%Mo+0.033×%W+0.2×%V+0.1×%Nb)/Ceq, and the% element in this case indicates the mass% of each element.

A second means is, in addition to the element of the first means, in mass%, containing Ni: 2.0% or less, consisting of balance Fe and unavoidable impurities, and Ceq: 0.20% or more, When ΔC=C−Ceq, ΔC: −0.300 to 0.300% and B value: 0.74 to 0.94, heat having excellent thermal conductivity and toughness. Tool steel.
However, Ceq=0.06×%Cr+0.063×%Mo+0.033×%W+0.2×%V+0.1×%Nb, where the% element represents the mass% of each element, and ΔC=C C in -Ceq is% by mass, and Ceq is mainly used as a standard for the amount of C necessary when all the added elements become carbides, where ΔC is the same as C in steel. This is an index considering the amount of solid solution element which is important in improving the thermal conductivity in relation to the amount of carbide forming elements. Further, B value=(0.063×%Mo+0.033×%W+0.2×%V+0.1×%Nb)/Ceq, and the% element in this case indicates the mass% of each element.

In the first means and the second means, high temper softening resistance is required to obtain high thermal conductivity. That is, by applying a relatively high tempering temperature, by precipitating the carbide forming elements in the steel as carbides, the elements that form a solid solution in the matrix structure and reduce the thermal conductivity are reduced, and the thermal conductivity is reduced. To raise. In order to obtain high temper softening resistance, it is effective to add elements such as Mo and W, and subsequently add elements such as V and Nb. On the other hand, when only these elements are added, not only the hardenability becomes insufficient and it becomes difficult to obtain sufficient hardness, but also a structure such as bainite is generated during the quenching, which may result in a low toughness. On the other hand, in order to improve the toughness, it is effective to add an element such as Cr having a relatively large effect of improving the hardenability. Under such a circumstance, the B value is a value of an equation considering the balance of the carbide forming elements to be added in order to have both thermal conductivity and toughness.

By adopting the invention of the first means and the second means of the present application, since the impact value showing the toughness is high and the thermal conductivity is high as compared with the conventional hot work tool steel, it is suitable for die casting and hot stamping. Applicable mold steel can be obtained.

Before describing the embodiments of the present invention, the reasons for limiting each constituent element of the constituent elements of the hot work tool steel of the invention according to the claims of the present application and the conditions thereof will be described. In addition,% in each component element is mass %.

C: 0.20 to 0.60%
C is an element for ensuring sufficient hardenability and quenching and tempering hardness and forming carbide to obtain wear resistance and high temperature strength. When C is less than 0.20%, the hardness, high temperature strength and wear resistance of the hot work tool steel cannot be sufficiently obtained. On the other hand, when C is more than 0.60%, segregation of components and carbides in the hot work tool steel is promoted and toughness is reduced. In addition, the amount of solute C in the hot work tool steel increases, which lowers the thermal conductivity. Therefore, C is set to 0.20 to 0.60%.

Si: less than 0.50% Si is an element that contributes to the improvement of hardenability and hardness. However, if the Si content is more than 0.50%, the thermal conductivity is greatly reduced. Therefore, Si is set to less than 0.50%.

Mn: more than 0.60 and less than 1.50% Mn has a hardenability of less than 0.60%, and an abnormal structure such as bainite is formed during quenching, which lowers toughness. On the other hand, when Mn is contained at 1.50% or more, the matrix of the hot work tool steel is embrittled and toughness and hot workability are deteriorated. Therefore, Mn is set to more than 0.60 and less than 1.50%.

Cr: less than 4.00% Cr is an element that improves hardenability. However, if Cr is contained in a large amount of 4.00% or more, it promotes the agglomeration and coarsening of Cr-based carbides and reduces the toughness, high temperature strength and softening resistance. When Cr is contained in 4.00% or more, the peak of secondary hardening due to tempering shifts to a lower temperature side, more alloy elements remain in the matrix after tempering, and the thermal conductivity is lowered. ..

Mo: 0.5-5.0%
Mo is an element that contributes to hardenability, secondary hardening during tempering, wear resistance, high temperature strength, and softening resistance. In addition, Mo is an element that easily forms carbides at a higher tempering temperature, which influences and shifts the peak of secondary hardening to the high temperature side. Therefore, in order to obtain high thermal conductivity, it is desirable to apply a high tempering temperature, but if Mo is less than 0.5%, high thermal conductivity cannot be obtained. Therefore, Mo needs to be 0.5% or more. However, if the Mo content is more than 5.0%, the amount of Mo remaining in the matrix increases and the thermal conductivity decreases. Further, Mo promotes the agglomeration/coarsening of carbides and reduces the toughness. Moreover, since Mo is an expensive element, the cost becomes high. Therefore, Mo is set to 0.5 to 5.0%.

W: 4.0% or less W, like Mo, is an element that contributes to hardenability and secondary hardening during tempering, wear resistance, high temperature strength, and softening resistance, and at a higher tempering temperature. Since it is an element that easily forms a carbide, which influences the peak of the secondary hardening to the high temperature side, it can be added if necessary. Therefore, in order to obtain a high thermal conductivity, it is desirable to apply a high tempering temperature, but if W is more than 4.0%, W remaining in the matrix increases and the thermal conductivity decreases. W promotes agglomeration/coarsening of carbides and reduces toughness. In addition, W is an expensive element, so the cost becomes high. Therefore, W is set to 4.0% or less.

2Mo+W: 3.0 to less than 13.0% (however, the above Mo: 0.5 to 5.0% and W: less than 4.0%).
As described above, Mo and W are elements that contribute to hardenability, secondary hardening during tempering, wear resistance, high temperature strength, and softening resistance. Further, Mo and W are elements that easily form carbides at a higher tempering temperature, which influences and shifts the peak of secondary hardening to the high temperature side. By the way, it is desirable that Mo and W can be applied with a high tempering temperature in order to obtain high thermal conductivity. However, if 2Mo+W is less than 3.0% within the range of Mo: 0.5 to 5.0% and W: less than 4.0%, the above-described secondary curing cannot be obtained. On the other hand, when 2Mo+W is 13.0% or more, Mo and W remaining in the matrix of the hot work tool steel increase, and the thermal conductivity decreases. Further, when 2Mo+W is 13.0% or more, it promotes agglomeration and coarsening of Mo and W-based carbides, lowers toughness, and is expensive because it is an expensive element. In particular, in order to obtain the same effect as W with Mo, it is necessary to add Mo in an amount twice as much as W as described above. Therefore, 2Mo+W is 3.0 to less than 13.0% (however, Mo is in the range of 0.5 to 5.0% and W is less than 4.0%).

V: less than 1.00% V is an element that precipitates fine and hard MC-type carbonitrides during tempering and contributes to high temperature strength and wear resistance, so V can be added as necessary. Further, the carbonitrides of V do not completely dissolve at the time of quenching, but a part thereof does not dissolve. Therefore, these carbonitrides suppress coarsening of crystal grains and suppress deterioration of toughness. On the other hand, when V is contained in an amount of 1.00% or more, the V remaining in the matrix of the hot work tool steel increases and the thermal conductivity decreases. In addition, it promotes the agglomeration and coarsening of the V-based carbides and reduces the toughness. Further, if V is contained in an amount of 1.00% or more, the cost becomes high.

Nb: less than 1.0% Nb is an element having the same effect as V. That is, Nb is an element that precipitates fine and hard MC-type carbonitrides during tempering and contributes to high-temperature strength and wear resistance, so Nb can be added as necessary. Further, the carbonitride of Nb is not completely dissolved at the time of quenching, and a part thereof is not solid-solved. Therefore, these carbonitrides suppress coarsening of crystal grains and suppress deterioration of toughness. On the other hand, when Nb is contained in an amount of 1.0% or more, the amount of Nb remaining in the matrix of the hot work tool steel increases and the thermal conductivity decreases. In addition, it promotes aggregation and coarsening of Nb-based carbides and reduces toughness. Further, if Nb is contained in an amount of 1.0% or more, the cost becomes high.

N: less than 0.030% N is an element that achieves the same effect as C. However, if N is added in excess of 0.030% or more, the amount of N in the matrix of the hot work tool steel increases, and the thermal conductivity of the steel decreases. Therefore, N is set to less than 0.030%.

Ceq: 0.20% or more Ceq is a carbon equivalent that requires 0.20 or more in order to secure a sufficient amount of elements to obtain temper softening resistance, where Ceq is 0.20% or more. To do. In the present invention, Ceq=0.06*%Cr+0.063*%Mo+0.033*%W+0.2*%V+0.1*%Nb. That is, Ceq indicates the standard of carbon equivalent required when all the elements indicated by% element in this formula become carbides.

ΔC=(C−Ceq): −0.300 to 0.300%, preferably −0.100 to 0.200%
If ΔC is less than -0.300%, not only it becomes difficult to obtain sufficient quenching and tempering hardness, but also the amount of alloying elements remaining in the matrix of the hot work tool steel increases and the thermal conductivity decreases. Further, it promotes segregation of alloy components and reduces toughness. On the other hand, when ΔC exceeds 0.300%, excessive C and N remain in the matrix of the hot work tool steel, which lowers the thermal conductivity. Further, it promotes segregation of C and N and segregation of carbides, and reduces toughness. Therefore, ΔC=(C−Ceq): −0.300 to 0.300%, preferably −0.100 to 0.200%. In addition, C of (DELTA)C=(C-Ceq) is mass %.

B value: 0.74 to 0.94
If the B value is lower than 0.74 (that is, the proportion of Cr added is large from the Ceq equation), high temper softening resistance cannot be obtained, that is, high tempering temperature cannot be applied, and heat The amount of alloying elements remaining in the matrix of the intermediate tool steel increases and the thermal conductivity decreases. On the other hand, when the B value is higher than 0.94, the hardenability is insufficient and the toughness is reduced. Note that B value=(0.063×% Mo+0.033×% W+0.2×% V+0.1×% Nb)/Ceq. Here,% element indicates mass% of each element. By the way, in order to obtain high thermal conductivity, high temper softening resistance is required. That is, by applying a relatively high tempering temperature, the elements that form a solid solution in the matrix structure and reduce the thermal conductivity by precipitating the carbide forming elements in the steel as carbides, and increase the thermal conductivity .. In order to obtain high softening resistance, it is effective to add elements such as Mo and W, and subsequently elements such as V and Nb. On the other hand, when only these elements are added, not only the hardenability becomes insufficient and it becomes difficult to obtain sufficient hardness, but also a structure such as bainite is generated during the quenching, which may result in a low toughness. To improve the toughness, it is effective to add an element such as Cr having a relatively large effect of improving the hardenability. As can be seen from the above, the formula for the B value is a formula that considers the balance of the carbide-forming elements to be added in order to have both thermal conductivity and toughness.

Ni: 2.0% or less Ni is an element necessary for improving hardenability in the invention according to claim 2 of the present application. However, when Ni is more than 2.0%, it is an expensive element and the cost becomes high. Therefore, Ni is set to 2.0% or less.

Next, modes for carrying out the invention of the present application will be described. Table 1 shows No. 1 of the present application. With respect to the invention steels of 18 kinds of Examples A to R and the comparative steels of 13 kinds of Comparative Examples, the chemical composition of each element contained in each is shown in mass %. The balance of each chemical component of the examples and comparative examples shown in Table 1 of the present application is Fe, and although not shown in Table 1, these steels have the chemical components shown in Table 1 and the balance of Fe. In addition, it contains inevitable impurities. Further, in Table 1, the mass% of 2Mo+W, Ceq, ΔC, and B value are the No. of the invention steels of each example. The respective values are shown for 18 types of A to R and 13 types of comparative steels 1 to 13 which are comparative examples.

The invention steels, which are the examples and the comparative examples, made of the chemical components and the balance Fe and inevitable impurities shown in Table 1, were melted, cast into 2 ton ingots, and then hot forged. Each block having a cross-sectional area of 100 mm×220 mm having a wrought forming ratio of about 4 S was finished. Note that the invention steel Nos. Sixteen kinds of A to P are example steels of the invention according to claim 1 (first means), and the invention steel Nos. Since two kinds of Q to R contain Ni as a chemical component, they are steels of the embodiment of the invention according to claim 2 (second means). Similarly, the comparative steel Nos. 12 is steel of a comparative example of the invention according to claim 2 (second means) of the present application.

Test pieces were taken from each of the block materials obtained above, quenched at 980 to 1080° C., and after tempering, tempered twice at 520 to 650° C. That is, the quenching temperature and tempering temperature shown in Table 2 were applied to the heat treatment of the test pieces of each steel type shown in Table 1.

The toughness in Table 2 was evaluated by the Charpy impact test. The test pieces used were taken from the rolling direction of the central portion of the block material, which was the above-mentioned forged material. These test pieces were hardened at 980 to 1080° C., tempered twice at 520 to 650° C. after hardening, tempered to 45 HRC, and finished into 2 mm U-notch Charpy test pieces. If the impact value in the Charpy impact test is 30 J/cm ² or more, the toughness is ◎, and if the impact value in the test is 20 J/cm ² or more and less than 30 J/cm ² , the toughness is O, and the impact value in the same test is If it was lower than 20 J/cm ^{2, the} toughness was evaluated as x.

The laser flash method was used to measure the thermal conductivity. A test piece for measuring thermal conductivity having a diameter of 10 mm×1 mm was sampled from the above block material and used as the test piece. These test pieces were quenched at the quenching temperature shown in Table 2 and then subjected to heat treatment at the tempering temperature to measure the thermal conductivity. If the measured thermal conductivity is 40 W/m·K or more, it is marked as ◎, if it is 30 W/m·K or more and less than 40 W/m·K, it is marked as ○, and if it is lower than 30 W/m·K, it is marked as ×. The results are shown in Table 2.

In the invention steels according to claim 1 and claim 2 of the present application, as shown in Table 2, the toughness evaluated by the Charpy impact test is No. In each of the invention steels shown in A to R, the impact value is 20 J/cm ² or more and less than 30 J/cm ² , or the impact value is 30 J/cm ² or more, and the laser flash method is used. The measured thermal conductivity is 30 W/m·K or more and is less than 40 W/m·K, or is 40 W/m·K or more, and both are hot tools having excellent thermal conductivity and toughness. It is steel. On the contrary, in the comparative steel of the invention of the present application, as shown in Table 2, the toughness evaluated by the Charpy impact test is No. In the comparative steels shown in 1 to 13, No. 1, No. 2, No. 4-6, No. The impact values of Nos. 8 to 11 were lower than 20 J/cm ² and were ×, and No. 3, No. 7, No. Only 4 cases of 12 to 13 are ◯. The thermal conductivity measured using the laser flash method is No. 1-3, No. 6-8, No. 12, No. 13 is lower than 30 W/mK, and further No. 4, No. 5, No. 9 to 11 are ◯ of 30 W/m·K or more and less than 40 W/m·K, and about 69% is steel having low toughness and 62% is steel having low thermal conductivity.

Claims (1)

% By mass, C: 0.20 to 0.60%, Si: 0.03 to less than 0.50%, Mn: more than 0.60 to less than 1.50%, Cr: 0.21 to 4.00% Less, Mo: 0.5 to 5.0%, 2Mo+W: 3.0 to less than 13.0%, N: 0.010 to less than 0.030%,
Further, as an optional component, W: less than 4.0%, V: less than 1.00%, Nb: less than 1.0% is optionally contained,
When the balance is Fe and unavoidable impurities and Ceq: 0.20% or more and ΔC=C−Ceq, ΔC: −0.300 to 0.300%, B value: 0.74 to 0.94. A hot work tool steel having excellent thermal conductivity and toughness characterized by:
However, Ceq=0.06×%Cr+0.063×%Mo+0.033×%W+0.2×%V+0.1×%Nb, in which the% element indicates the mass% of each element, and ΔC=C−Ceq C in% by mass, further Ceq is a standard for the amount of C mainly necessary when all the added elements become carbides, where ΔC is the heat amount from the relationship between C in steel and the amount of each carbide forming element. It is an index considering the amount of solid solution element which is important in improving conductivity, and B value=(0.063×%Mo+0.033×%W+0.2×%V+0.1×%Nb)/Ceq. The% element is the mass% of each element.