Hot work tool steel with excellent thermal conductivity

Abstract

To provide a hot work tool steel provided with high hot workability, coefficient of thermal conductivity, hardness, and toughness, and capable of suitably applicable in die cast and hot stamping.SOLUTION: A hot work tool steel is made of steel comprising, in mass%, C: 0.35 to 0.70%, Si: 0.50% or less, Mn: 0.50% or less, Ni: 0.10 to 2.00%, Cr: 0.30 to 2.00%, Mo: 0.50 to 3.00%, Cu: 0.10 to 1.80%, Ti: 0.001 to 0.150%, V: 0.10 to 0.55%, B: 0.0001 to 0.0100%, and the balance made of Fe and unavoidable impurities, wherein Ni / Cu≥0.45, K value shown in formula (1) is 72 or less, in quenched and tempered state, in martensite single phase or a mixed structure of martensite and bainite, an area ratio of martensite is 80% or more. In addition, formula (1): K={(25+10 VM) / VM}×(1.33Sisol+0.50Mnsol+0.24 Nisol+0.52Crsol+0.33Mosol+0.39Cusol+1.0).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot work tool steel such as a die steel, and more particularly to a hot work tool steel used in high temperature environments such as die casting and hot stamping. [Background technology]

[0002] In recent years, in the die casting industry, the mechanical and thermal loads on die casting dies have increased due to the need to increase the strength of aluminum parts to reduce the weight of automobiles and shorten part forming processing intervals to improve productivity. As a result, dies are more susceptible to problems such as wear and large cracks. To address these issues, die materials with excellent hardness and toughness are required.

[0003] Furthermore, in hot stamping, problems arise with the die due to contact with the high-strength workpiece and wear caused by scale that forms on the surface of the workpiece, so high hardness is required of the die material.

[0004] Furthermore, die casting and hot stamping dies have cooling circuits built in, and the cooling efficiency of the cooling water flowing through these circuits has a significant impact on production cycle speed. One way to improve cooling efficiency is to increase the thermal conductivity of the die. Therefore, in order to meet the demand for faster production cycle speeds, which is intended to improve productivity as mentioned above, high thermal conductivity is a necessary material characteristic.

[0005] Furthermore, when considering the actual production of the above-mentioned die material, manufacturability, that is, high hot workability, is also required.

[0006] The applicant has previously developed a hot work tool steel containing, in mass%, C: more than 0.35% to 0.70%, Si: 0.01% to 1.20%, Mn: 0.01% to 1.50%, Cr: 0.35% to 4.00%, Cu: 0.10% to 2.50%, Ni: 0.10% to 2.99%, V: more than 0.10% to 0.55%, B: 0.0001% to 0.0100%, and O: 0.0050% or less, and containing either one or both of Mo and W, and The present invention proposes a hot work tool steel characterized by the following: Mo: 3.00% or less, W: 6.00% or less, Mo+1 / 2W: 0.50% to 3.00%, with the balance being Fe and unavoidable impurities; B+O+N: 0.0420% or less; and the value of K shown in the formula (K=36.04-12.65C-1.58Mo-0.79W-9.34Mn-19.01Al+0.69Ni-0.34Cu) being 15.6 or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2021-127846 Summary of the Invention [Problem to be solved by the invention]

[0008] However, unlike the present invention, the above proposed inventions do not focus on the amount and structure of the solid solution elements in the matrix, and in some cases the thermal conductivity remains below 30.0 W / m K. Therefore, it cannot be said that a stable and sufficient thermal conductivity can be obtained. Therefore, a more stable thermal conductivity is desired.

[0009] Therefore, the problem to be solved by the present invention is to provide a hot work tool steel that combines high hot workability, thermal conductivity, hardness, and toughness, and is suitable for use in die casting, hot stamping, and the like. [Means for solving the problem]

[0010] The present invention provides a hot work tool steel that contains, in mass%, 0.35 to 0.70% C, 0.50% or less Si, 0.50% or less Mn, 0.10 to 2.00% Ni, 0.30 to 2.00% Cr, 0.50 to 3.00% Mo, 0.10 to 1.80% Cu, 0.001 to 0.150% Ti, 0.10 to 0.55% V, 0.0001 to 0.0100% B, and the balance being Fe and unavoidable impurities, wherein the mass ratio of Ni to Cu is Ni / Cu≧0.45, the value of K shown in formula (1) is 72 or less, and the steel is in a quenched and tempered state, and the structure is a martensite single phase or a mixed structure of martensite and bainite, with the martensite ratio being 80% or more in terms of area fraction. Note that equation (1) is: K = {(25 + 10V M ) / V M}×(1.33Si sol +0.50Mn sol +0.24Ni sol +0.52Cr sol +0.33Mo sol +0.39Cu sol +1.0), where V M : Volume fraction of matrix phase, M sol : The amount of solid solution of component element M in the matrix during tempering (mass%). The right side of formula (1) contains the sol Substitute the percentage value of . [Effects of the Invention]

[0011] The hot work tool steel of the present invention has a reduction of 70% or more when the Gleeble test is carried out at 1100°C in the steel ingot state, and has high hot workability. Furthermore, the hot work tool steel of the present invention, when quenched and tempered, has a high thermal conductivity of 30.0 W / m K or more at room temperature, a high hardness of 48 HRC or more at room temperature, and a Charpy impact value of 20 J / cm at room temperature. 2 Therefore, the hot work tool steel of the present invention has high hot workability, thermal conductivity, hardness, and toughness, and is a hot work tool steel that can be suitably applied to die casting, hot stamping, etc. DETAILED DESCRIPTION OF THE INVENTION

[0012] Prior to describing the embodiments of the present invention, the reasons for specifying the composition of each component of the hot work tool steel of the present invention, the reasons for specifying the value of K in formula (1), and the reasons for specifying the structure in a quenched and tempered state will be explained. Note that % in the component composition means % by mass.

[0013] C: 0.35 to 0.70% C is a component that strengthens the matrix by dissolving in solid solution and promotes precipitation strengthening by forming carbides and carbonitrides during tempering. If the C content is too low, sufficient quench-temper hardness cannot be obtained. From this perspective, C content is set to 0.35% or more. Preferably, C content is set to 0.55% or more. On the other hand, if C content is too high, segregation is promoted, coarse carbides and carbonitrides are formed, and toughness is reduced. From this perspective, C content is set to 0.70% or less.

[0014] Si:0.50% or less Si has a deoxidizing effect in steelmaking and is also an essential component for ensuring hardenability. If the Si content is low, the deoxidizing effect and hardenability are not fully exhibited. From this viewpoint, the Si content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the Si content is too high, it dissolves in the matrix, reducing thermal conductivity. Furthermore, it promotes segregation, reducing toughness. Therefore, from these viewpoints, the Si content is set to 0.50% or less. Furthermore, the Si content is preferably 0.30% or less.

[0015] Mn: 0.50% or less, Mn has a deoxidizing effect in steelmaking and is an essential component for ensuring hardenability. If the Mn content is low, the deoxidizing effect and hardenability are not fully exhibited. From this viewpoint, Mn is preferably 0.01% or more, and more preferably 0.1% or more. If there is too much Mn, it dissolves in the matrix and reduces thermal conductivity. It also inhibits the spheroidization of carbides during annealing, reducing machinability. Therefore, from these viewpoints, Mn is set to 0.50% or less. Preferably, Mn is 0.40% or less.

[0016] Ni: 0.10~2.00%, Ni is a component that improves hardenability and suppresses a decrease in toughness due to bainite formation. It also prevents red embrittlement caused by Cu. If the Ni content is too low, it is not possible to improve hardenability, reduce toughness, or prevent red embrittlement, so the Ni content is set to 0.10% or more. Preferably, the Ni content is 0.40% or more. On the other hand, if the Ni content is too high, it dissolves in the matrix and reduces thermal conductivity. It also inhibits the spheroidization of carbides during annealing, reducing machinability. Furthermore, it increases costs. Therefore, from these perspectives, the Ni content is set to 2.00% or less. Preferably, the Ni content is 1.50% or less.

[0017] Cr: 0.30~2.00%, Cr is an element that improves hardenability and prevents a decrease in toughness due to bainite formation. If the Cr content is low, sufficient hardenability cannot be obtained. From this viewpoint, the Cr content is set to 0.30% or more. Preferably, the Cr content is set to 0.50% or more. On the other hand, if the Cr content is too high, it dissolves in the matrix, reducing thermal conductivity. Furthermore, excessive carbides are formed, reducing toughness. From these viewpoints, the Cr content is set to 2.00% or less. Preferably, the Cr content is set to 1.5% or less.

[0018] Mo: 0.50-3.00%, Mo is a component that promotes secondary hardening during tempering and increases quench-and-temper hardness. If the Mo content is low, sufficient quench-and-temper hardness cannot be obtained. From this viewpoint, the Mo content is set to 0.50% or more. Preferably, the Mo content is set to 1.00% or more. On the other hand, if the Mo content is too high, it dissolves in the matrix, reducing thermal conductivity. In addition, excessive carbides are formed, reducing toughness. This also leads to increased costs. Therefore, from these viewpoints, the Mo content is set to 3.00% or less. Preferably, the Mo content is set to 2.50% or less.

[0019] Cu: 0.10-1.80% Cu is an ingredient that improves hardenability and suppresses a decrease in toughness due to bainite formation. If the Cu content is low, sufficient hardenability cannot be obtained. From this viewpoint, Cu content is set to 0.10% or more. Preferably, Cu content is set to 0.50% or more. On the other hand, if there is too much Cu, it dissolves in the matrix and the thermal conductivity decreases. Furthermore, if there is too much Cu, red embrittlement occurs and hot workability deteriorates. Therefore, from these viewpoints, Cu content is set to 1.80% or less.

[0020] Ti: 0.001 to 0.150%, Ti is an element that reduces N in steel by forming nitrides, promotes the solid solution of B, and improves hardenability. If Ti is too little, these effects are not achieved. From these viewpoints, Ti content is set to 0.001% or more. On the other hand, if Ti is too much, coarse nitrides are formed, reducing toughness. From these viewpoints, Ti content is set to 0.15% or less. Preferably, Ti content is 0.10% or less.

[0021] V: 0.10~0.55%, V is a component that promotes secondary hardening during tempering and increases quench-and-temper hardness. If the V content is low, sufficient quench-and-temper hardness cannot be obtained. From this perspective, the V content is set to 0.10% or more, and preferably 0.25% or more. On the other hand, if the V content is too high, excessive carbides and carbonitrides are formed, reducing toughness and increasing costs. From these perspectives, the V content is set to 0.55% or less, and preferably 0.45% or less.

[0022] B: 0.0001~0.0100%, B is a component that improves hardenability and suppresses a decrease in toughness due to bainite formation. If the content is too low, sufficient hardenability cannot be obtained. From this viewpoint, the B content is set to 1 ppm or more, and preferably 5 ppm or more. On the other hand, if the B content is too high, excessive nitrides are formed at the prior γ grain boundaries, resulting in a decrease in hot workability. From this viewpoint, the B content is set to 100 ppm or less, and preferably 75 ppm or less. That is, the B content is set to 0.0001 to 0.0100% (i.e., 1 to 100 ppm). Preferably, the B content is set to 0.0005 to 0.0075% (i.e., 5 to 75 ppm).

[0023] Ni / Cu≧0.45 This is an index focusing on the mass ratio of Ni to Cu, Ni / Cu. When there is more Cu than Ni and Ni / Cu is less than 0.45, red embrittlement occurs and hot workability deteriorates. Therefore, the ratio of Ni to Cu in the present invention is set to Ni / Cu≧0.45.

[0024] The value of K in formula (1) is 72 or less. Equation (1): K={(25+10V M ) / V M}×(1.33Si sol +0.50Mn sol +0.24Ni sol +0.52Cr sol +0.33Mo sol +0.39Cu sol +1.0) In addition, V M M is the volume fraction of the matrix phase excluding the crystallized carbides remaining in an undissolved state during quenching and the precipitated carbides during tempering. sol is the amount (mass%) of alloying element M dissolved in the matrix during tempering, and the right side of formula (1) contains the value of each M sol Substitute the percentage value of . This K value is an index of thermal conductivity, and if the hot work tool steel of the present invention has a K value of 72 or less, the thermal conductivity at room temperature after quenching and tempering will be high, at 30.0 W / m·K or more. If the K value is greater than 72, the thermal conductivity will not be sufficient. Since the thermal conductivity of Si, Mn, Ni, Cr, Mo, and Cu decreases when they are dissolved in the matrix, the amount of each of these alloy elements M dissolved in the matrix should be adjusted to M. sol This is the basis for the calculation.

[0025] Structure after quenching and tempering: Single martensite phase, or a mixed structure of martensite and bainite, with the area ratio of martensite being 80% or more If the alloy of the present invention contains a large amount of bainite, an incompletely hardened phase, after quenching and tempering, its toughness will be significantly reduced. This will result in insufficient die life when used in hot stamping or die casting dies. Therefore, to ensure toughness, the area ratio of martensite must be 80% or more. The bainite structure also increases when the cooling rate during quenching is reduced.

[0026] <Example> 100 kg of steels having chemical compositions shown in Inventive Steels No. 1 to 21 in Table 1 and Comparative Steels No. 22 to 42 in Table 2, with the balance being Fe and unavoidable impurities, were melted in a vacuum induction melting furnace, and the resulting steel ingots were hot forged into blocks 65 mm wide and 30 mm high. Test pieces 8 mm in diameter and 100 mm long were prepared using parts of the forged and wrought materials, and their hot workability was evaluated.

[0027] The remaining forged and stretched material was annealed at 870°C, and then round bars measuring 20 mm in diameter and 160 mm in length were taken from the midpoint between the surface and the center. These round bars were held at 1030°C, then quenched by air cooling, and tempered twice at 570-670°C. After this, the structure was observed, and the thermal conductivity, quenched and tempered hardness, and toughness were investigated. However, for No. 42, the cooling rate during quenching was slowed to increase the bainite structure, and the proportion of martensite after quenching and tempering was adjusted to less than 80%. The results are shown in Tables 1 and 2.

[0028] (hot workability) The hot workability was evaluated by a Gleeble test, which was carried out at 700°C to 1300°C using the above-mentioned test pieces having a diameter of 8 mm and a length of 100 mm.

[0029] (structural observation) The surface of the quenched and tempered specimen parallel to the forging direction was polished to a mirror finish, and then etched in a nitric acid alcohol solution. The microstructure was observed using a scanning electron microscope (SEM). 2In the region, bainite and martensite structures were identified, and when bainite was confirmed, the area ratio of martensite was calculated as a percentage using image analysis software. All of the invention steels Nos. 1 to 21 in the examples had a mixed structure of martensite and bainite.

[0030] (thermal conductivity) The laser flash method was used to measure thermal conductivity. After quenching and tempering, the samples were finished into disks with a diameter of 10 mm and a diameter of 1 mm, and then subjected to testing. Thermal conductivity was measured at room temperature.

[0031] (Hardness) The hardness after quenching and tempering was measured at room temperature using a Rockwell hardness tester on the surface of the quenched and tempered sample perpendicular to the forging direction.

[0032] (toughness) Toughness was evaluated by Charpy impact testing at room temperature. Test specimens were prepared from quenched and tempered samples. The test specimens were 2 mm U-notch Charpy test specimens, with the notch processed on the surface parallel to the forging direction.

[0033] [Table 1]

[0034] [Table 2]

[0035] Note 1) The remainder of the chemical composition in Tables 1 and 2 is Fe and unavoidable impurities. Note 2) The "-" notation for the components in Table 2 indicates the amount of unavoidable impurities, i.e., Ti: less than 0.001%, B: less than 1 ppm. Note 3) The M ratio in Tables 1 and 2 refers to the proportion (area fraction) of martensite in the mixed structure of martensite and bainite. An M ratio of 100% means a single martensite phase structure. Note 4) The underlined components in Table 2 indicate that the components are outside the scope of the present invention or that the effects are inferior.

[0036] [Table 3]

[0037] [Table 4]

[0038] Note 1) The underlined items in Table 4 indicate that they are outside the scope of this invention. Note 2)Si sol ,Mn sol ,Ni sol ,Cr sol ,Mo sol ,Cu sol indicates the amount (mass %) of Si, Mn, Ni, Cr, Mo, and Cu dissolved in the matrix, respectively. Note 3)V M indicates the volume fraction (vol %) of the matrix.

[0039] Amount of solid solution during tempering M sol and matrix volume fraction V M was calculated using thermodynamic equilibrium calculation software (Thermo-Calc). Based on the obtained values, the K value of each steel was calculated. The results are shown in Tables 3 and 4.

[0040] Inventive Steels Nos. 1 to 21 satisfy the chemical composition, Ni / Cu ratio, K value, and M ratio specified in the present application. When the Gleeble test was carried out in the steel ingot state at 1100°C, all of them had a reduction of area of ​​72.2% or more, and had high hot workability. Furthermore, in the quenched and tempered state, they had a thermal conductivity of 30.4 W / m K or more at room temperature, a hardness of 48 HRC or more at room temperature, and a Charpy impact value of 28.8 J / cm2 From the above, it was confirmed that a hot work tool steel was obtained that combines high hot workability, thermal conductivity, hardness, and toughness, and is suitable for use in die casting, hot stamping, and the like.

[0041] Comparative steel No. 22 has an insufficient amount of C and is therefore insufficient in hardness. Comparative Example No. 23 contains too much C and has poor toughness. Comparative steel No. 24 has an excessive amount of Si, the value of formula K is exceeded, and the thermal conductivity and toughness are poor. Comparative steel No. 25 has an excessive amount of Mn, the value of formula K is exceeded, and the thermal conductivity is poor. Comparative steel No. 26 has an insufficient amount of Ni, poor hot workability, a low M ratio, an excessive amount of bainite, and poor toughness. Comparative steel No. 27 contains too much Ni, so the value of the formula K is exceeded and the thermal conductivity is poor. Comparative steel No. 28 has an insufficient Cr content, a low M ratio, and an excessive bainite structure, and is therefore inferior in toughness. Comparative steel No. 29 has an excessive amount of Cr, so the value of formula K is exceeded, and the thermal conductivity and toughness are poor. Comparative steel No. 30 has an insufficient Mo content and therefore lacks hardness. Comparative steel No. 31 contains too much Mo, the value of formula K is exceeded, and the thermal conductivity and toughness are poor. Comparative steel No. 32 has an insufficient amount of Cu, a low M ratio, and an excessive amount of bainite, resulting in poor toughness. Comparative steel No. 33 has an excessive amount of Cu, which results in poor hot workability, an excess value of the formula K, and poor thermal conductivity. Comparative steel No. 34 had an insufficient amount of Ti, which did not reduce the N content in the steel and did not promote the solid solution of B, resulting in a structure with a low M ratio and excessive bainite, and therefore poor toughness. Comparative steel No. 35 contains too much Ti and is inferior in toughness. Comparative steel No. 36 has an insufficient amount of V and therefore lacks hardness. Comparative steel No. 37 contains too much V and is inferior in toughness. Comparative steel No. 38 has an insufficient amount of B, a low M ratio, and an excessive amount of bainite, resulting in poor toughness. Comparative steel No. 39 contains an excessive amount of B, and has poor hot workability. Comparative steel No. 40 has a high Cu content relative to Ni, resulting in a low Ni / Cu value, and therefore has poor hot workability. For comparative steel No. 41, the value of formula K is exceeded, and the thermal conductivity is poor. Comparative steel No. 42 has a low M ratio and a structure with an excessive amount of bainite, and therefore has poor toughness.

Claims

[Claim 1] In mass%, C: 0.35-0.70%, Si: 0.50% or less, Mn: 0.50% or less, Ni: 0.10-2.00%, Cr: 0.30-2.00%, Mo: 0.50-3.00%, Cu: 0.10 to 1.80%, Ti: 0.001 to 0.150%, V: 0.10-0.55%, B: 0.0001 to 0.0100%, The balance is Fe and unavoidable impurities, Ni / Cu≧0.45, The value of K shown in formula (1) is 72 or less, In a quenched and tempered state, The structure is a martensite single phase or a mixed structure of martensite and bainite, with the area ratio of martensite being 80% or more. Hot work tool steel characterized by: Equation (1): K = {(25 + 10V)} M ) / V M }×(1.33Si sol +0.50Mn sol +0.24Ni sol +0.52Cr sol +0.33Mo sol +0.39Cu sol +1.0), However, in formula (1), V M : volume fraction of matrix phase, M sol : The amount of solid solution of component element M in the matrix during tempering (mass%), and the right side of the formula is sol Substitute the percentage value of .

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