JP2013213255A - Hot working die steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide hot working die steel in which hot strength and hardenability are ensured by optimizing the amounts of alloy elements added, and lengthening of die life span is achieved by controlling the amount of undissolved carbide at quenching, thereby imparting high levels of strength and toughness to the die steel for hot working.SOLUTION: A hot working die steel comprises, by mass, 0.30-0.50% C, 0.10-0.30% Si, 0.40-0.80% Mn, ≤0.005% S, 1.00-2.00% Ni, ≥1.00% but <2.80% Cr, ≥1.00% but <1.80% Mo, 0.10-0.40% V, ≤0.10% Nb, >0.0060% but ≤0.0100% N, ≤0.005% Ti and the balance being Fe and unavoidable impurities. In the die steel, the crystal grain size number after quenching is ≥7.0 and the structure upon heating for quenching comprises substantially austenite and undissolved MC type carbide, nitride and carbonitride.

Description

  The present invention relates to a hot die steel excellent in hardness and toughness used for hot forging, hot extrusion, casting, die casting and the like, and also excellent in machinability.

  In order to extend the life of the mold, it is necessary to satisfy both hardness and crack resistance (toughness) of the mold steel. The reason for this is that in a hot forming mold, the mold surface softens and wears due to contact and sliding with a high-temperature workpiece. In this case, if significant wear occurs on the mold surface, the intended product shape cannot be molded, and the mold reaches the end of its life. Further, the mold surface is exposed to a heating-cooling cycle by repetition of heat input from the workpiece to the mold and heat removal from the mold during application of the lubricant or mold release agent. The mold surface expands when heated and is given compressive stress. This softening during heating reduces the mold surface strength, and microscopic buckling occurs on the mold surface where the strength is reduced. Conversely, during cooling, a tensile stress due to shrinkage is applied to the mold. If the tensile stress at this time exceeds the strength of the softened mold surface, thermal fatigue cracks (heat check, heat crack) starting from buckling occur on the mold surface. Thermal fatigue cracks are transferred to a molded product, which impairs the design of the molded product, degrades the product quality, deteriorates the releasability between the molded product and the mold, and decreases the productivity. Cause. In addition, the extension of thermal fatigue cracks can lead to accelerated wear due to fine chipping or dropping off of the surface, and further to cracking of the mold.

  That is, in order to prolong the life of the mold steel for hot forming, it is required to maintain both strength at high temperatures and toughness that suppresses the progress of cracks. JIS-SKD61 is generally used for hot press forging, hot extrusion and die casting dies, and JIS-SKT4 is generally used for hot hammer forging dies (all are JIS G 4404). reference.). JIS-SKD61 is a mold steel that combines both strength and toughness at a relatively high level. However, premature breakage often occurs due to cracking during use, and the toughness is not always sufficient. Moreover, the toughness of JIS-SKD61 is insufficient to suppress the extension of thermal fatigue cracks. JIS-SKT4 emphasizes toughness so that it can withstand a large impact caused by hammer forging, but lacks wear resistance due to low softening resistance. Further, when the die-carved surface is repeatedly pulled down for the purpose of reclaiming, the hardenability is low, resulting in a decrease in hardness at the center, and cracking or settling occurs due to insufficient strength. Furthermore, since the applicable hardness is low, the wear resistance and strength are insufficient, and it is not suitable for use in hot press forging or hot extrusion.

  As a conventional prior art, high strength and toughness heat that can significantly improve wear and sag due to heat tracks, large cracks, or softening in molds used for forging large products. Interstitial tool steel has been proposed (see, for example, Patent Document 1). However, since the present invention does not limit the amount of N in the steel, a coarse crystallized product of nitride or carbonitride may be formed, and the toughness may be insufficient. Furthermore, since there is no restriction on the amount of Ti in the steel, fine carbonitrides of 5 μm or less effective for suppressing the coarsening of crystal grains may be reduced, resulting in insufficient toughness. .

  Furthermore, hot tool steel excellent in machinability and tool life used for hot forging dies, extrusion dies, die casting dies, etc. has been proposed (for example, see Patent Document 2). However, the Mo content in the example of the proposed invention is 0.63% or less, significantly less than 0.30 to 2.00% in the claims, exceeding 0.63% and exceeding 2.00. % Is not shown at all in the examples that are excellent as the invention of Patent Document 2. Therefore, the amount of Mo of the invention of Patent Document 2 is substantially smaller than the amount of Mo in the present invention described later, and is different from the present invention.

  Furthermore, as hot working tools, tools for forging presses and forging dies as well as die casting tools, extrusion dies and especially mandrels for light metals and steel, or plastic injection molds and profile manufacturing as plastic molding tools A die for use has been proposed (see, for example, Patent Document 3). However, in this document, paragraph 0012 mentions that the appropriate amount of S is 0.025 to 0.030%, and the inventive steel shown in the examples is positively added with 0.027% or more of S. . On the other hand, as described later, since the S of the present application is 0.005% or less, the invention of Patent Document 3 is different from the present invention in this respect. Furthermore, similarly, the steel No. shown in Table 3 which is an example of the cited document 3 is shown. Q9277, Q9278, Q9279, Q9280, Q9286, and Q9287 are all C ≦ 0.38% and V ≧ 0.71%, which are also different from the scope of the present invention.

  Further, there has been proposed a mold steel that increases heat fatigue characteristics and softening resistance, thereby suppressing heat check and water-cooled hole cracking and extending the life (see, for example, Patent Document 4). However, in all of the inventive steels of Examples shown in Table 3, the Mn content is 0.86% or more, which is substantially higher than the Mn content of the present invention. Furthermore, in the thing of patent document 4, quenching temperature is limited to 1010-1050 degreeC, and in this, MC which is an insoluble carbide required at the time of quenching heating is insufficient to apply to this invention. The toughness may be reduced due to the coarsening of crystal grains. As described above, in any of the prior arts of Patent Documents 1 to 4 which are the prior art, the definition of the structure at the time of quenching heating is different from that of the present invention.

JP-A-6-88163 Japanese Patent No. 4186340 Special table 2011-517729 gazette JP 2008-56982 A

  The problem to be solved by the present invention is to secure hardness, hot strength and hardenability by optimizing the amount of alloying elements added, and also to control the amount of undissolved carbide during quenching, The aim is to propose a hot mold steel that achieves a long life of the mold by combining the strength and toughness of the mold steel for processing at a high level.

  The means of the present invention for solving the above-mentioned problems is a means relating to hot die steel. That is, in the means of claim 1, C: 0.30 to 0.50%, Si: 0.10 to 0.30%, Mn: 0.40 to 0.80%, S: 0.0. 005% or less, Ni: 1.00 to 2.00%, Cr: 1.00% or more and less than 2.80%, Mo: 1.00% or more and less than 1.80%, V: 0.10 to 0 .40%, Nb: 0.10% or less, N: more than 0.0060% and 0.0100% or less, Ti: 0.005% or less, preferably Ti: 0.003% or less, and the balance Fe and A steel composed of inevitable impurities. The grain size number after quenching of this steel is 7.0 or more, and the structure during quenching heating is substantially austenite and insoluble in MC type carbides, nitrides, and carbonitriding. It is a hot mold steel characterized by being a product.

  In the means of claim 2, part or all of Mo of the steel component of claim 1 is replaced with twice the amount of W, that is, 1.00% ≦ Mo + 0.5W <1.80%, The steel component content is the same as in claim 1 and the steel is composed of the balance Fe and unavoidable impurities, the grain size number after quenching is 7.0 or more, and the structure during quenching heating is substantially austenite and non-austenite. It is a hot mold steel characterized by solid solution MC type carbides, nitrides, and carbonitrides.

  According to the third aspect of the present invention, in addition to the steel component of the first aspect, the steel further comprises, by mass%, Co: 2.0% or less, the balance being Fe and inevitable impurities, and the structure during quenching heating. Is a hot mold steel characterized by being MC type carbides, nitrides, and carbonitrides which are substantially insoluble in austenite.

  In the means of claim 4, in addition to the steel component of claim 2, the steel further contains, by mass%, Co: 2.0% or less, the balance being Fe and inevitable impurities, and crystals after quenching. Hot mold steel having a grain size number of 7.0 or more, and the structure of this steel during quenching heating is substantially austenite and insoluble MC type carbides, nitrides, and carbonitrides It is.

  The reasons for limiting the steel components of the hot mold steel will be described below. In addition,% shows the mass%.

C: 0.30 to 0.50%
C is an element necessary for ensuring sufficient hardenability and forming carbides to obtain wear resistance and high temperature strength. If C is lower than 0.30%, sufficient high-temperature strength and wear resistance cannot be obtained. On the other hand, when C is more than 0.50%, solidification segregation is promoted and toughness is inhibited. Therefore, C is set to 0.30 to 0.50%.

Mn: 0.40 to 0.80%
Mn is an element necessary for ensuring hardenability. However, it is insufficient if Mn is less than 0.40%. On the other hand, when Mn exceeds 0.80%, the workability of the steel material is lowered. Therefore, Mn is set to 0.40 to 0.80%.

S: 0.005% or less When S is contained in excess of 0.005%, it combines with Mn to form MnS and inhibit toughness. Therefore, S is set to 0.005% or less.

Ni: 1.00 to 2.00%
Ni is an element that improves hardenability and toughness. However, if Ni is less than 1.00%, there is no effect of improving hardenability and toughness. On the other hand, if Ni is contained in an amount of more than 2.00%, the high temperature strength and machinability of the steel material are hindered. Therefore, Ni is set to 1.00 to 2.00%.

Cr: 1.00% or more and less than 2.80% Cr is an element that improves hardenability. However, if Cr is less than 1.00%, the improvement of hardenability is insufficient. On the other hand, when Cr is 2.80% or more, excessive Cr-based carbides are formed during quenching and tempering, and the high temperature strength and softening resistance are lowered. Therefore, Cr is made 1.00% or more and less than 2.80%.

Mo: 1.00% or more and less than 1.80% Mo is an element necessary for obtaining precipitated carbides that contribute to hardenability, secondary hardening, wear resistance, and high-temperature strength. Further, fine carbides that have become insoluble during quenching are elements that have an effect of suppressing the coarsening of crystal grains. However, if Mo is less than 1.00%, the above effect cannot be obtained. On the other hand, even if Mo is added in an excess of 1.80% or more, the effect is not only saturated, but also segregation is promoted and carbides are coarsely aggregated to reduce toughness. Further, excessive addition of Mo increases the cost. Therefore, Mo is made 1.00% or more and less than 1.80%.

V: 0.10 to 0.40%
When V is contained in an amount of 0.10% or more, fine and hard MC type carbides, nitrides, and carbonitrides are precipitated during tempering, contributing to high temperature strength and wear resistance, and during quenching. Fine carbides and carbonitrides suppress the coarsening of crystal grains and suppress the decrease in toughness. However, if V is less than 0.10%, the above effect cannot be obtained. On the other hand, when V is more than 0.40%, coarse carbides, nitrides, and carbonitrides are crystallized during solidification, thereby inhibiting toughness. Moreover, cost increases. Therefore, V is set to 0.10 to 0.40%.

Nb: 0.10% or less Nb precipitates fine and hard MC-type carbides, nitrides, and carbonitrides during tempering and contributes to high-temperature strength and wear resistance. Moreover, at the time of quenching, fine carbides and carbonitrides suppress the coarsening of crystal grains and suppress the decrease in toughness. However, if Nb is more than 0.10%, coarse carbides, nitrides, and carbonitrides are crystallized during solidification, thereby inhibiting toughness. Moreover, cost increases. Therefore, Nb is made 0.10% or less.

N: more than 0.0060% and 0.0100% or less N combines with V and Nb to form MC type nitrides and / or carbonitrides, and contributes to hardness and wear resistance. Those MC-type nitrides and / or carbonitrides suppress grain coarsening during quenching and improve toughness. However, when N is 0.0060% or less, the above effect is not obtained. On the other hand, if N exceeds 0.0100% and too much, bonding with V and Nb at higher temperatures is promoted in the segregation part (alloy element enrichment part) that cannot be avoided in the solidification process. The MC type nitrides and / or carbonitrides thus produced become coarse, and conversely, toughness is inhibited. Therefore, N is set to more than 0.0060% and 0.0100% or less.

Ti: 0.005% or less, desirably 0.003% or less Ti produces MC-type carbides, nitrides, carbonitrides at a temperature higher than V and Nb during solidification, and uses them as nuclei for V and / or Nb MC-type carbides, nitrides, and carbonitrides grow coarsely, thus impairing toughness. When Ti is more than 0.005%, V and Nb are consumed as coarse MC-type carbides, nitrides, and carbonitrides, and fine MC-type carbides, nitrides, and carbonitrides of 5 μm or less are reduced. Therefore, the effect of suppressing the coarsening of crystal grains during quenching is reduced. Therefore, Ti is set to 0.005% or less, preferably 0.003% or less.

The size of the crystal grain after quenching is a crystal grain size number of 7.0 or more. When the crystal grain size number is less than 7.0, the crystal grain is too large and the toughness is lowered. Therefore, the size of the crystal grains after quenching is set to a crystal grain size number: 7.0 or more.

MC type carbides, nitrides, carbonitrides that are substantially austenite and insoluble in quenching heating If carbides other than MC type remain in an insoluble solution, they precipitate during tempering and contribute to secondary hardening Carbide is reduced, and sufficient tempering hardness and high temperature strength cannot be obtained. Therefore, MC type carbides, nitrides, and carbonitrides in which the structure during quenching heating is substantially insoluble with austenite are used.

Invention according to claim 2: A part or all of Mo of the invention according to claim 1 is replaced with W in double amount. That is, in mass%, 1.00% ≦ Mo + 0.5W <1.80%
W is an element that can obtain the same effect as Mo. However, in order to obtain the same effect as Mo, W needs to be twice the amount of Mo by mass%, and 1.00% ≦ Mo + 0.5W. On the other hand, W, like Mo, not only saturates the effect, but also promotes segregation and reduces toughness by coarse aggregation of carbides. In addition, the cost increases. Therefore, Mo + 0.5W <1.80%.

Invention according to claim 3: Invention in which steel component of the invention according to claim 1 is added by mass% and Co: 2.0% or less Co is made 2.0% or less because the base is strengthened and the high temperature Strength can be improved. On the other hand, if the Co content is more than 2.0%, the toughness is hindered and the cost increases. Therefore, Co added to the steel component of the invention according to claim 1 is made 2.0% or less.

Invention according to claim 4: Invention in which the steel component of the invention according to claim 2 is added by mass% and Co: 2.0% or less Co is made 2.0% or less because of strengthening the base and high temperature Strength can be improved. On the other hand, if the Co content is more than 2.0%, the toughness is hindered and the cost increases. Therefore, Co added to the steel component of the invention according to claim 2 is made 2.0% or less.

  By adopting the above means, the present invention provides a hot mold steel in which the strength and toughness are combined at a high level and the life of the mold is increased.

It is a figure which shows the relationship between the quenching temperature of the steel of the symbol A of the example of this invention, and a crystal grain size number. Is a diagram showing the relationship between quenching temperature and M ₂ C-type volume percentage of carbide of steel symbols A and D of the present invention embodiment.

  A steel having the composition of the present invention and the comparative example shown in Table 1 in mass% and the balance consisting of Fe and inevitable impurities is melted using a 1 ton vacuum melting furnace, and then formed into an ingot. The ingots were agglomerated and subjected to a homogenization heat treatment at 1240 ° C. for 10 hours, followed by hot forging with a forging ratio of about 6S to produce a steel material having a diameter of 160 mm.

  The steel material manufactured above was heated to 920-980 degreeC, and it quenched. In this case, using a block of 25 mm × 25 mm × 25 mm indexed from the middle part of each steel material, the soaking temperature was maintained at 30 ° C. for 30 minutes at the quenching temperature shown in Table 2, and then stirred at 50 ° C. It was carried out by oil cooling to the oil.

  The crystal grain size (old austenite crystal grain size) number after quenching of * 2 was determined in accordance with “steel—microscopic test method of crystal grain size” defined in JIS G 0551.

  The high-temperature strength, which is the high-temperature characteristic of * 3, was determined by indexing a block-shaped specimen having a side of 25 mm from the middle circumference of each steel material and tempering to 44 to 46 HRC by quenching and tempering. These 44 to 46 HRC values are the initial hardness measured with a Rockwell hardness meter after removing the scale layer on the surface of the test material. These test materials were held at 650 ° C. for 50 hours, and after these steel materials were air-cooled, the scale layer on the surface of the steel materials was removed again, and then measured with a Rockwell hardness meter. That is, it was evaluated by ΔHRC, which is a degree of hardness reduction.

  * 4 Toughness was evaluated by the energy required for fracture by Charpy impact test. The test pieces used for these were taken from the direction perpendicular to the rolling direction at the center of the forged material having a diameter of 140 mm. Further, these test pieces were tempered to 44 to 46 HRC by quenching and tempering, 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 machinability of * 5 was evaluated based on the tool life until each steel material in an annealed state was drilled with a SKH51 φ5 mm drill with a 10 mm deep hole and broken.

  * 6 AL, AH, DL and DH of comparative steels are obtained by quenching the same material as steel A or D of the present invention at different temperatures. Note that AL indicates that the material is A but the quenching temperature is lower than A, and AH indicates that the material is A but the quenching temperature is higher than A.

  A comparative example will be described below. First, the symbol “a1” of the comparative example will be described as an example, and “a” of “a1” of the comparative example corresponds to “A” of the present invention. However, “1” of “a1” in the comparative example indicates that the content of any of the steel components of A is either too much or too little. In the case of “a1”, any component indicates Ti, and in this case, it indicates that there are too many components. Whether it is too much or too little is different in each case, one is indicated by 1 and the opposite is indicated by 2. Therefore, the following explanation is referred to here.

  Since a1 of the comparative example has a Ti content that is higher than that of A of the present invention, coarse MC-type carbides, nitrides, carbonitrides are formed, and 5 μm or less of undissolved at the time of quenching Since the amount of MC decreases and the crystal grains become coarse, the toughness is inferior.

  The f1 of the comparative example is inferior in toughness because the amount of N added is larger than that of F of the present invention, and coarse MC-type carbides, nitrides, and carbonitrides are formed.

  The f2 of the comparative example is inferior in toughness because the amount of N added is too small compared with F of the present invention, and the amount of MC of 5 μm or less of undissolved solution is reduced during quenching and the crystal grains become coarse. Yes.

  Since c1 of a comparative example has more C addition amount than C of this invention, it promotes segregation and reduces toughness.

  Since c2 of a comparative example has too little C addition amount than C of this invention, a required quantity of carbide precipitation cannot be obtained and high temperature strength is inferior.

  The comparative example h1 is inferior in toughness because the amount of Si added is less than H of the present invention and the hardenability is lowered. Moreover, the machinability is getting worse.

  Since h2 of a comparative example has too much Si addition amount than H of the present invention, the ductility of the base structure is lowered and the toughness is inferior.

  The e1 of the comparative example has a lower amount of Mn than E of the present invention, and the toughness is reduced due to insufficient hardenability.

  In the comparative example e2, since the amount of Mn added is larger than that of E of the present invention, the annealing hardness becomes harder and the machinability becomes worse.

  Since d1 of a comparative example has too much S content rather than D of this invention, toughness has fallen.

  Since b1 of a comparative example has less Ni addition amount than B of this invention, hardenability is inadequate and toughness is falling.

  Since b2 of a comparative example has too much Ni addition amount than B of this invention, high temperature strength deteriorates and machinability is also inferior.

  Since g1 of the comparative example has a smaller Cr addition amount than G of the present invention and sufficient hardenability cannot be obtained, toughness is lowered.

Since g2 of the comparative example has too much Cr addition amount than G of the present invention, the amount of MC and M2C effective for high-temperature strength is reduced and softening resistance is lowered.
Comparative example a2 has less Mo addition than A in the present invention, and sufficient high-temperature strength is not obtained.

  Since a3 of a comparative example has too much Mo addition amount than A of this invention, the coarse aggregation of the carbide | carbonized_material arises and the toughness is reduced. Even if it is added in an amount exceeding the proper amount, the high-temperature strength is saturated.

  E3 of the comparative example has a smaller V addition amount than E of the present invention, and a sufficient high-temperature strength is not obtained. Moreover, since the amount of undissolved MC of 5 μm or less during quenching decreases and the crystal grains become coarse, the toughness is inferior.

  In the comparative example e4, the amount of addition of V is excessive as compared with E of the present invention, and coarse carbonitrides are crystallized during solidification to inhibit toughness.

  In the comparative example f3, the amount of Nb added is larger than that of F of the present invention, and coarse carbonitrides are crystallized during solidification to inhibit toughness.

  In d2 of the comparative example, the amount of Co added is excessive as compared with D of the present invention, and the toughness is inhibited.

  Since the quenching temperatures of the comparative examples AL and DL are too lower than those of A and D of the present invention, the solid solution of the alloy elements at the time of quenching is insufficient, and the carbides precipitated at the time of tempering are reduced. For this reason, sufficient high-temperature strength is not obtained.

  Since AH and DH of comparative examples have a quenching temperature that is higher than A and D of the present invention, the amount of unhardened MC of 5 μm or less at the time of quenching decreases, leading to grain coarsening and toughness. It is getting worse.

  FIG. 1 shows a steel material made of steel A shown in Table 1 of the present invention, heated to 850 to 1000 ° C. and kept soaked for 30 minutes, and then poured into 50 ° C. stirring oil and quenched by oil cooling. The quenching temperature and grain size No. of the steel material. It is a graph which shows the relationship. From this graph, it can be seen that when the quenching temperature exceeds 980 ° C., undissolved MC type carbides, nitrides and carbonitrides are insufficient, and the crystal grains are coarsened.

FIG. 2 shows a steel material composed of A steel and D steel shown in Table 1 of the present invention, heated to 850 to 1000 ° C. and kept soaked for 30 minutes, and then poured into oil at 50 ° C. with stirring. steel was hardened by a graph showing the relationship between the quenching temperature and M ₂ C volume ratio (vol%). From this graph, it can be seen that when the quenching temperature is less than 930 ° C., carbides other than the MC type do not completely dissolve.

  As can be seen from the results of Tables 1 and 2 above and the results of FIGS. 1 and 2, the present invention can optimize the hardness of the mold steel by optimizing the addition amount of the alloy element as the hot mold steel. In addition, ensuring hot strength and hardenability, and controlling the amount of carbide eliminates the decrease in the amount of MC less than 5 μm in solid solution at the time of quenching and prevents coarsening of crystal grains and has excellent toughness And, it is a hot die steel that has a controlled life.

Claims (4)

  In mass%, C: 0.30 to 0.50%, Si: 0.10 to 0.30%, Mn: 0.40 to 0.80%, S: 0.005% or less, Ni: 1.00 ~ 2.00%, Cr: 1.00% or more and less than 2.80%, Mo: 1.00% or more and less than 1.80%, V: 0.10 to 0.40%, Nb: 0.10 %, N: more than 0.0060% and 0.0100% or less, Ti: 0.005% or less, and the balance Fe and inevitable impurities, the grain size number after quenching of this steel is 7 A hot die steel characterized by having a structure of 0.0 or more and a structure of MC type carbides, nitrides, and carbonitrides which are substantially insoluble in austenite and quenching and heating in the quenching heating.   A part or all of Mo in the steel component according to claim 1 is replaced with twice the amount of W, that is, 1.00% ≦ Mo + 0.5W <1.80%, and the content of other steel components is The steel of the balance Fe and inevitable impurities as in claim 1 and having a grain size number of not less than 7.0 after quenching and an MC type carbide whose structure during quenching heating is substantially austenite and insoluble. , Nitride, and carbonitride, hot mold steel.   In addition to the steel components according to claim 1, a steel comprising Co: 2.0% or less in mass%, the balance being Fe and inevitable impurities, and the grain size number after quenching is 7.0 or more A hot die steel characterized in that the microstructure during quenching heating is substantially austenite and insoluble MC type carbides, nitrides, and carbonitrides.   In addition to the steel components according to claim 2, the steel further comprises mass%, Co: 2.0% or less, the balance being Fe and inevitable impurities, and the grain size number after quenching is 7.0. As described above, a hot die steel characterized in that the structure during quenching heating is substantially MC-type carbide, nitride, and carbonitride which are not dissolved in austenite.

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