JP2008169411A - Steel for die materials - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for die materials which has high softening resistance and excellent wear resistance. <P>SOLUTION: The steel for die materials has a composition consisting of, by mass, 0.15 to 0.55% C, 0.01 to 2.0% Si, 0.01 to 2.5% Mn, 0.01 to 2.0% Cu, 0.01 to 2.0% Ni, 0.01 to 2.5% Cr, 0.01 to 3.0% Mo, 0.01 to 1.0%, in total, of at least either of V and W and the balance Fe with inevitable impurities. In this steel, LMP (LMPmax) giving the maximum value of HRC hardness at room temperature after prescribed heat treatment is ≥17.66, wherein LMP is represented by equation LMP=(TH+273)×(20+log(t))×0.001. In the equation, TH means a holding temperature (°C) at heating at 550 to 700°C and (t) means a holding time (Hr) at heating at 550 to 700°C. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel for mold material excellent in softening resistance suitable for use in molds and tools that require wear resistance. More specifically, the present invention can extend the life of mold products and tool products, thereby reducing the production cost. The present invention relates to a steel for a mold material having excellent softening resistance capable of suppressing the ratio of die cost and tool cost.

  In resin or glass injection molding, die casting, forging, or the like, a molding member such as a die or a tool (hereinafter also simply referred to as “mold material”) is used. Since the life of the mold material greatly affects the production efficiency and the product cost, it is required to extend the life of the mold material in any industrial field. This is because if the life of the mold material is short, the production efficiency due to frequent replacement of the mold material deteriorates, the mold material cost increases, and the proportion of the product cost increases.

Conventionally, when prolonging the life of a mold material, steel containing a large amount of rare metals such as Cr, Mo, V, W, and Co, which is hard at high temperatures and difficult to soften even after long use, that is, “ Steel with high softening resistance has been used. This is because steel with high softening resistance maintains high hardness even at high temperatures and has less defects such as wear.
In particular,
(1) Alloy tool steel (JIS SKD61, JIS SKD62, JIS SKD8), heat resistant steel (JIS SUH1), high speed tool steel (JIS SKH51, JIS SKH55),
(2) Hot tool steel and heat-resistant steel for hot forging in which a large amount of carbides containing rare metals such as Cr, Mo, V, and W exist,
(3) Various developed steels containing a large amount of rare metals such as Cr, Mo, V, W, and Co (for example, the steel described in Patent Document 1 is also a kind),
Has been used as “steel with high softening resistance”.

  However, rare metals such as Cr, Mo, V, W, and Co are expensive and further expensive due to the recent increase in price, which increases the manufacturing cost of products. Therefore, there is a problem that it is difficult to reduce the manufacturing cost even if the life is extended by using steel having high softening resistance.

  Furthermore, since rare metals such as Cr, Mo, V, W, and Co are rare or difficult to extract, there is a problem that the use of a large amount of them reverses resource saving. In addition, the manufacturing process (melting, refining, plastic working, heat treatment) of steels containing these rare metals is complicated, so a large amount of electricity and fossil fuels are used, and greenhouse gases and industrial waste are generated. It has also been pointed out that it causes an increase and goes against environmental burden reduction. Moreover, the complexity of the manufacturing process of the steel containing the rare metal also presents a problem that it is difficult to recycle the steel.

  In order to solve these problems, "steel with a low content of rare elements such as Cr, Mo, V, W, Co" may be used. “Steel with a low content of rare elements” is less expensive because of its low content, and contributes to resource saving and the manufacturing process is not so complicated, which contributes to reducing environmental impact. It is. Furthermore, “steel with a low content of rare elements” has the advantage of being easy to recycle.

Japanese Patent No. 3397250 (paragraph number “0021”)

  However, “steel with a low content of rare elements” has a low content of rare elements such as Cr, Mo, V, W, Co and the like. And solid solution hardening (alloy elements with different sizes from iron atoms dissolve in the ground iron, causing distortion in the crystal structure of the ground iron, which becomes the source of hardness) Therefore, the softening resistance is low, and the life of the mold material using the softening resistance is short. For this reason, when the mold material is “steel with a small content of rare elements”, there is a problem that the mold material is frequently replaced. Even if the mold material itself is inexpensive, if the number of the mold materials manufactured increases, the production efficiency deteriorates and the manufacturing cost increases. Therefore, in the past, “steel with a low content of rare elements” was used as a mold material, and even though the manufacturing cost was reduced, the softening resistance was low, so the mold material was frequently replaced, and the production efficiency could not be increased. Eventually, product manufacturing costs could not be reduced. That is, despite the fact that it is “steel with a low content of rare elements”, there has been no report of a steel with high softening resistance.

The present invention has been made in view of the above circumstances, and a first object of the invention is to provide a steel for a mold material having high softening resistance and excellent thermal fatigue characteristics and wear resistance. An object of the present invention is to provide a steel for a mold material which is excellent in thermal fatigue characteristics and wear resistance despite its low element content. Accordingly, the present invention intends to extend the life of the mold material and reduce the manufacturing cost.
A second object of the present invention is to provide a steel for a mold material that can contribute to resource saving and environmental load reduction.

In order to solve the above problems, the steel for mold material according to the present invention is:
C: 0.15-0.55 mass%, Si: 0.01-2.0 mass%, Mn: 0.01-2.5 mass%, Cu: 0.01-2.0 mass%, Ni: 0.01-2.0% by mass, Cr: 0.01-2.5% by mass, Mo: 0.01-3.0% by mass, and at least one total amount selected from the group consisting of V and W : 0.01 to 1.0% by mass, and the balance is steel for mold material composed of Fe and inevitable impurities,
After soaking at 1010 ° C. to 1050 ° C., it is cooled to 500 ° C. to 550 ° C. at a cooling rate of 200 ° C./min or more, subsequently cooled to 150 ° C. or less at a cooling rate of 100 ° C./min or more, and then 550 The gist is that LMP (LMPmax) that gives the maximum value of HRC hardness at room temperature after heating in a temperature range of from C to 700C is 17.66 or more. However,
LMP = (TH + 273) × (20 + log (t)) × 0.001 (1)
TH is the heating holding temperature [° C.] at 550 ° C. to 700 ° C.,
t is the holding time [Hr] of heating at 550 ° C to 700 ° C.

  The steel for mold material according to the present invention may further contain Al: 0.002 to 0.5% by mass. The steel for mold material according to the present invention may further contain Co: 0.01 to 2.0% by mass. The steel for mold materials according to the present invention further includes Nb: 0.005 to 0.5 mass%, Ta: 0.005 to 0.5 mass%, Ti: 0.005 to 0.5 mass%, and Zr. : You may contain at least 1 sort (s) chosen from the group which consists of 0.005-0.5 mass%. The steel for mold material according to the present invention may further contain B: 0.0002 to 0.02% by mass. The steel for mold materials according to the present invention further includes S: 0.01 to 2.0 mass%, Ca: 0.0005 to 0.5 mass%, Se: 0.005 to 0.5 mass%, Te: 0. 0.005 to 0.5 mass%, Bi: 0.005 to 0.5 mass%, and Pb: at least one selected from the group consisting of 0.005 to 0.5 mass% may be included.

  In the steel for mold material according to the present invention, the total amount of Cr, Mo, V, W and Co is preferably 6% by mass or less.

  The steel for a mold material according to the present invention is soaked at 1010 ° C. to 1050 ° C., then cooled to 500 ° C. to 550 ° C. at a cooling rate of 200 ° C./min or more, and subsequently 150 at a cooling rate of 100 ° C./min or more. When it is cooled to below ℃ and then heated in the temperature range of 550 ℃ to 700 ℃, the difference between the HRC hardness at room temperature when LMP = 17 and the HRC hardness at room temperature when LMP = 19 is It is desirable that it is 6 or less.

Since the steel for mold material according to the present invention has the above component composition and LMP (LMPmax) giving the maximum value of HRC hardness at room temperature after heat treatment under the above conditions is 17.66 or more, softening resistance Is expensive.
Since the steel for mold materials according to the present invention has a high softening resistance, it is excellent in thermal fatigue characteristics and wear resistance. Therefore, the steel for mold material according to the present invention can extend the life of the mold material.
Moreover, since the steel for mold materials according to the present invention has the above component composition, the effects of dispersion strengthening and solid solution hardening are not weakened. This is also the reason why the softening resistance of the steel for mold material according to the present invention is high.
The steel for mold material according to the present invention has a total amount of Cr, Mo, V, W, and Co of 6% by mass or less, that is, is inexpensive because the content of rare elements is small, and the manufacturing cost can be reduced.
Since the steel for mold materials according to the present invention has a small content of rare elements, it can contribute to resource saving and environmental load reduction.

  Below, the steel for mold materials which concerns on one Embodiment of this invention is demonstrated in detail with reference to drawings. Here, the “mold material” means a member used for molding a mold or a tool.

(Contained components of steel for mold materials)
The steel for mold material according to the present embodiment contains C, Si, Mn, Cu, Ni, Cr, and Mo, and “at least one selected from the group consisting of V and W” as essential elements. The steel for mold material according to the present embodiment is optionally made of Al, Co, “at least one selected from the group consisting of Nb, Ta, Ti and Zr”, B, “S, Ca, Se, Te, Bi, You may contain "at least 1 sort (s) chosen from the group which consists of Pb." The remainder of the steel for mold material according to the present embodiment is made of Fe and inevitable impurities (N, O, P, Mg, etc.).

(Reason for limiting ingredients)
(1) C: 0.15 to 0.55 mass%,
C is an element essential for adjusting the strength of steel. If the C content is too small, the strength required for the mold material used for injection molding, die casting and forging cannot be obtained. If the C content is excessive, coarse carbides are likely to be generated during solidification of the casting, which becomes a starting point of fracture, so that the fatigue strength and impact value when steel becomes a mold material are reduced. Therefore, the content of C is set to 0.15 to 0.55% by mass. The more preferable content of C is 0.2 to 0.5% by mass with which the HRC hardness is 40 to 50 and coarse carbides are not easily formed.

(2) Si: 0.01 to 2.0 mass%
Si dissolves in the ground iron and contributes to solid solution hardening. Si also affects dispersion strengthening through the form of carbide (size, shape, area ratio, etc.). In the case of the steel for a mold material according to the present embodiment, the dispersed particles are mainly Si carbides, and the harder and smaller carbides are dispersed, the harder the steel. As described above, Si affects solid solution hardening and dispersion strengthening.
If the Si content is too small, sufficient softening resistance cannot be obtained. Therefore, in order to ensure softening resistance, a high Si material is preferable. However, if the Si content is excessive, cracking during hot plastic working during material production is likely to occur. Therefore, the Si content is set to 0.01 to 2.0 mass% from the balance between softening resistance and workability. A more preferable Si content is 0.05 to 1.5 mass%.

(3) Mn: 0.01 to 2.5% by mass
Mn is essential as an element for improving hardenability. However, if the content of Mn is excessive, the Ac1 transformation point is lowered, and it becomes difficult to apply the mold made of the steel to a high temperature range (about 650 ° C.). Further, when Mn is contained, spheroidization of carbides during annealing is remarkably inhibited, and it becomes difficult to create a soft state excellent in machinability (annealing deterioration). Then, Mn content was made into 0.01-2.5 mass% from balance, such as the improvement effect of hardenability, Ac1 transformation point, and annealability.

(4) Cu: 0.01 to 2.0% by mass
Cu is useful as an element that improves hardenability. Cu also precipitates alone in the ground iron and contributes to increasing the strength of the steel. However, if the Cu content is excessive, cracks are easily generated during hot plastic working during material production. Therefore, the Cu content is set to 0.01 to 2.0% by mass from the balance between the effect of improving hardenability and hot workability.

(5) Ni: 0.01 to 2.0 mass%
Ni is useful as an element that improves hardenability. However, if the Ni content is excessive, the same harmful effects as in the case where the Mn content is excessive are caused. Therefore, for the same reason as Mn, the content of Ni is set to 0.01 to 2.0% by mass.

(6) Cr: 0.01 to 2.5% by mass
Cr not only improves hardenability, but is also useful as an element that forms carbides and increases the strength of steel. However, if the Cr content is excessive, the softening resistance is greatly reduced. Therefore, the content of Cr is set to 0.01 to 2.5% by mass from the balance between the hardenability improvement effect and the softening resistance. The upper limit of the Cr content in the present embodiment is lower than that of conventional steel for mold materials or heat resistant steel (for example, JIS SKD61, SKD62, SKH51, SKH55, SUH1, etc.). A more preferable content of Cr is 0.1 to 2.0% by mass.

(7) Mo: 0.01-3.0 mass%
Mo is useful as an element that not only improves hardenability but also forms carbides to increase the strength of steel. In particular, Mo has a large effect of increasing the softening resistance of steel. However, if the content of Mo is excessive, saturation of characteristics (softening resistance and the like) and an increase in manufacturing cost are caused. The content of Mo was set to 0.01 to 3.0% by mass from the balance of the effect of improving hardenability, softening resistance, and manufacturing cost. The upper limit of the Mo content is lower than that of conventional steel for mold materials or heat-resistant steel (for example, JIS SKH51, SKH55, etc.). A more preferable Mo content is 0.5 to 2.5% by mass.

(8) Total amount of at least one selected from the group consisting of V and W: 0.01 to 1.0% by mass
V and W not only improve hardenability, but are useful as elements for forming carbides and increasing the strength of steel. V and W are particularly effective in increasing the softening resistance. However, if the content of at least one of V and W is excessive, not only saturation of characteristics (hardenability, high strength, softening resistance, etc.) and an increase in production cost will be caused, but also coarseness during solidification of casting. Since it becomes easy to produce a carbide | carbonized_material and it becomes a starting point of destruction, the fatigue strength and impact value at the time of steel becoming a mold material are reduced. The effects of V and W on steel are almost the same. Therefore, the content of at least one selected from the group consisting of V and W is 0.01 to 1.0% by mass in total from the balance of the effect of improving hardenability, softening resistance, manufacturing cost, and the like. The total amount of these contents is lower than the upper limit of conventional steel for mold materials and heat-resistant steel (for example, JIS SKD8, SKH51, SKH55, etc.). A more preferable total amount of V and W content is 0.2 to 0.8% by mass.

(9) Al: 0.002 to 0.5 mass%
Al is a selective additive element (hereinafter simply referred to as “selective additive element”) that can be optionally added to steel. Al is an element that forms AlN and prevents coarsening of crystal grains during quenching. Al precipitates by forming an intermetallic compound and contributes to dispersion strengthening. However, if the Al content is excessive, not only saturation of characteristics (crystal grain coarsening prevention, dispersion strengthening, etc.) and an increase in production cost will be caused, but the inclusion alumina will be increased. Since this is the starting point of fracture, it reduces the fatigue strength and impact value when steel becomes a mold material. Therefore, the content of Al is set to 0.002 to 0.5 mass% from the balance between the effect of preventing the coarsening of crystal grains and the characteristics.

(10) Co: 0.01 to 2.0 mass%
Co is an optional element and contributes to solid solution hardening and dispersion strengthening by the same action as in the case of Si. Furthermore, Co has the effect of increasing the oxidation resistance of steel. If the Co content is too small, sufficient softening resistance cannot be obtained. Therefore, in order to ensure softening resistance, a high Co material is preferable. However, if the Co content is excessive, not only the manufacturing cost is increased, but also characteristics (oxidation resistance, softening resistance, etc.) are saturated, Lack of profit.
Therefore, the content of Co is set to 0.01 to 2.0% by mass from the balance between characteristics and manufacturing cost. The upper limit of the Co content is lower than that of conventional steel for mold materials and heat-resistant steel (for example, JIS SKD8, SKH55, etc.). A more preferable Co content is 0.1 to 1.8% by mass.

(11) Nb: 0.005 to 0.5 mass%
Nb is a selective additive element, and is an element that forms NbC, NbN, or the like and prevents coarsening of crystal grains during quenching. However, if the Nb content is excessive, saturation of characteristics (such as prevention of coarsening of crystal grains) and an increase in production cost are caused. Moreover, NbC and NbN crystallized relatively coarsely reduce the fatigue strength and impact value when steel becomes a mold material. Therefore, the content of Nb is set to 0.005 to 0.5 mass% from the balance of the effect of preventing the coarsening of crystal grains, the manufacturing cost, the material, and the like.

(12) Ta: 0.005 to 0.5 mass%
Ta is a selective additive element and is an element having the same effect as Nb. The content of Ta was set to 0.005 to 0.5% by mass as in the case of Nb.

(13) Ti: 0.005 to 0.5 mass%
Ti is a selective additive element and is an element having the same effect as Nb. The Ti content was set to 0.005 to 0.5% by mass, as in the case of Nb.

(14) Zr: 0.005 to 0.5 mass%
Zr is a selective additive element and is an element having the same effect as Nb. The content of Zr was set to 0.005 to 0.5% by mass as in the case of Nb.

(15) B: 0.0002 to 0.02 mass%
B is a selective additive element, an element that segregates at the austenite grain boundaries and suppresses the precipitation of the ferrite phase, and remarkably increases the hardenability of the steel. However, if the content of B is excessive, saturation of characteristics (ferrite phase precipitation suppression, hardenability, etc.) and an increase in production cost are caused, resulting in poor profit. The content of B is set to 0.0002 to 0.02 mass% from the balance between the effect of improving the hardenability and the manufacturing cost.

(16) S: 0.01 to 2.0% by mass
S is a selective additive element, and is an element that forms MnS and improves the machinability of steel. However, if the S content is excessive, saturation of characteristics (such as improvement of machinability) and an increase in manufacturing cost are caused. Further, MnS crystallized relatively coarsely reduces the fatigue strength and impact value when steel becomes a mold material. Therefore, the content of S is set to 0.01 to 0.2% by mass from the balance of the effect of improving machinability, the manufacturing cost, the material, and the like. In addition, when using steel for the shape | mold material (tool or metal mold | die) of a shape where machinability does not become a problem, the one where S content is low is desirable.

(17) Ca: 0.0005 to 0.5 mass%
Ca is a selective additive element and is an element that improves the machinability of steel. Ca is also an element that improves the hot workability of steel by changing the form of inclusions in the steel. However, when the content of Ca is excessive, saturation of characteristics (improvement of machinability, improvement of hot workability, etc.) and an increase in manufacturing cost are caused. Therefore, the content of Ca is set to 0.0005 to 0.5 mass% from the balance of the effect of improving the machinability and the manufacturing cost.

(18) Se: 0.005 to 0.5 mass%
Se is a selective additive element and is an element that improves the machinability of steel. However, if the Se content is excessive, a large amount of Se compound is produced, and the fatigue strength and impact value when steel becomes a mold material are reduced. Therefore, the content of Se is set to 0.005 to 0.5 mass% from the balance of the effect of improving machinability and characteristics (fatigue strength, impact value, etc.).

(19) Te: 0.005 to 0.5 mass%
Te is a selective additive element and is an element that improves the machinability of steel. However, if the Te content is excessive, a large amount of Te compound is produced, and the fatigue strength and impact value when steel becomes a mold material are reduced. Therefore, the content of Te is set to 0.005 to 0.5 mass% from the balance of the effect of improving machinability and properties (fatigue strength, impact value, etc.).

(20) Bi: 0.005 to 0.5 mass%
Bi is a selective additive element and is an element that improves the machinability of steel. However, if the Bi content is excessive, Bi particles dispersed in the steel are increased, and hot workability is greatly reduced. Therefore, the Bi content is set to 0.005 to 0.5 mass% from the balance between the effect of improving machinability and hot workability.

(21) Pb: 0.005 to 0.5 mass%
Pb is a selective additive element that improves the machinability of steel. However, if the Pb content is excessive, the amount of Pb particles dispersed in the steel is increased, and hot workability is greatly reduced. Therefore, the content of Pb is set to 0.005 to 0.5 mass% from the balance between the effect of improving machinability and hot workability.

(22) Total amount of at least one selected from the group consisting of Cr, Mo, V, W, and Co: 6% by mass or less The content of rare elements (Cr, Mo, V, W, Co) is thus limited. This is because the manufacturing cost can be reduced, and it can contribute to resource saving and environmental load reduction. Certainly, if the content of Cr, Mo, V, W, and Co is 6% by mass or less, the effects of solid solution hardening and dispersion strengthening are less likely to appear, and this works in a disadvantageous manner in terms of wear resistance and the like. However, since the steel for mold material according to the present embodiment has the above component composition and LMP (LMPmax) giving the maximum value of HRC hardness at room temperature is 17.66 or more, high softening resistance is obtained, and the conventional steel and Equivalent thermal fatigue characteristics and wear resistance can be obtained. Therefore, the content of the rare element is set to 6% by mass or less.

(LMP, LMPmax and reasons for limitation)
Next, LMP, LMPmax of the steel for mold materials according to the present embodiment having the above component composition and the reason for limitation thereof will be described.

(23) LMP (LMPmax) ≧ 17.66 giving maximum value of HRC hardness at room temperature
LMP is a parameter that is widely used when evaluating a change in hardness due to tempering of a steel material, and is also referred to as a “tempering parameter”. LMP is defined by the following equation (1), and is used to define tempering conditions (temperature TH and time t) for giving a desired HRC hardness to the steel material. Among LMPs, LMPmax is a value that defines tempering conditions (temperature TH and time t) that give the maximum value of HRC hardness (HRCmax) at room temperature.
LMP = (TH + 273) × (20 + log (t)) × 0.001 (1)
Here, TH is a tempering holding temperature [° C.], and t is a tempering holding time [Hr].
LMPmax was set to 17.66 or more if the steel satisfies this condition, it can be said that the steel has a high softening resistance, and the mold material manufactured using this steel is used for the production of injection molded products and die cast products. This is because even if it is applied, it is difficult to soften, so that it is difficult to cause problems such as wear (see FIG. 1). In addition, the steel for mold materials according to the present embodiment is soaked at 1010 ° C. to 1050 ° C., then cooled to 500 ° C. to 550 ° C. at a cooling rate of 200 ° C./min or more, and subsequently cooled at 100 ° C./min or more. HRC hardness at room temperature when LMP = 17 and HRC hardness at room temperature when LMP = 19 when cooled at a rate of 150 ° C. or lower and then heated in a temperature range of 550 ° C. to 700 ° C. The difference is 6 or less. This is because sufficient softening resistance is considered to be obtained. Details will be described later.

(Procedure for obtaining LMPmax)
Here, a procedure for obtaining LMPmax will be described.
(A) After soaking a plurality of square bars (11 mm × 11 mm × 60 mm) made of steel of an arbitrary component within the above component composition in a temperature range of 1010 ° C. to 1050 ° C. for 30 minutes, 200 ° C./min Cool to 500 ° C. to 550 ° C. at the above cooling rate, then cool to 100 ° C./min to 150 ° C. or less (quenching), and then 550 ° C. to 700 ° C. (appropriate temperature for each square bar) Perform reheating (tempering) for an arbitrary time (distributed at an appropriate temperature interval for each square bar) at an interval) (LMP for each square bar is the tempering temperature TH [° C] at this time in equation (1)) And tempering time t [Hr]), and thereafter, the HRC hardness is measured for each square bar at room temperature. The reason for setting the quenching speed to 100 ° C./min or more is to obtain “a structure mainly composed of martensite” when the steel is used as a mold material. Further, the temperature range of 550 ° C. to 700 ° C. is a temperature range that can be reached in actual production when the steel becomes a mold material, for example, hot forging, and softening in actual production of the steel. It is because it is suitable for evaluation of resistance.
(B) Next, for each square bar, a graph is created in which the HRC hardness measured at room temperature is plotted against the LMP calculated by equation (1) (see FIG. 2). The graph shows either a pattern of (i) monotonically decreasing or (ii) having a local maximum. At this time, the maximum value of the HRC hardness at room temperature is defined as HRCmax. And LMP which gives the maximum value (HRCmax) of HRC hardness in room temperature is set to LMPmax.
By the above procedure, LMP (LMPmax) giving the maximum value of HRC hardness at room temperature is obtained.

(Reason for limiting LMPmax)
The basis for setting the LMP (LMPmax) that gives the maximum value of the HRC hardness at room temperature to 17.66 or more will be described with reference to FIG. The target is six types of steel (in the figure, steel 1 to steel 6) in which LMPmax is changed by component adjustment. From these steels, hot forging dies for bearing races (bearing races) were prepared and tempered to HRC53. The material to be forged was JIS SUJ2, and was forged by heating to 1240 ° C.
Forging was performed at a rate of 100 shots per minute, and the amount of die wear after 50000 shots was compared.

  FIG. 1 shows the result of the forging. In the figure, LMPmax on the horizontal axis was determined for each of Steel 1 to Steel 6 by the above method. According to the figure, it can be seen that the amount of wear of steel decreases as LMPmax increases. Further, according to the figure, there is an inflection point in the vicinity of LMPmax = 17.7, and when the LMPmax exceeds this value, the amount of wear of the steel rapidly decreases. This means that even if the initial hardness (hardness before use for forging) is common, if the property against softening resistance is different, the wear resistance is also different. Even when the initial hardness was changed, the inflection point was in the vicinity of LMPmax = 17.7, and when the LMPmax exceeded this value, the amount of wear of the steel rapidly decreased.

In general, when the tempering time is about 1 Hr, it is known that when the tempering temperature is 610 ° C. or higher, the hardness of the steel rapidly decreases. When this condition is applied to the formula (1), the LMP becomes 17.66 or more. Therefore, even if the LMP exceeds 17.66, if the steel has a small decrease in hardness, it can be said that the softening resistance is sufficiently high, and the amount of wear can be reduced.
From the above, LMP (LMPmax) ≧ 17.66 giving the maximum value of HRC hardness at room temperature was set.

(Function)
Since the steel for mold materials according to the present embodiment has the above component composition, it has the necessary hardness as steel for mold materials by solid solution hardening and dispersion strengthening. In addition, since the steel for mold material according to the present embodiment has the above-described component composition, the influence of dispersion strengthening is weak because carbides are not coarsened (cause of poor softening resistance) even when heated. I wo n’t.
The steel for mold material according to the present embodiment has the above component composition and has a high softening resistance because LMP (LMPmax) giving the maximum value of HRC hardness at room temperature is 17.66 or more. Therefore, although the steel for mold material according to the present embodiment has a low content of rare elements, it exhibits softening resistance higher than that of conventional steel and is excellent in thermal fatigue characteristics and wear resistance.

(Example A)
(Preparation of test piece)
Test pieces (invention steels 1 to 32, comparative steels 1 to 5) having the components shown in Table 1 were prepared. A blank in the table means that the element is not actively added, and the content is an impurity level. Moreover, although not described in the table, the inventive steel and the comparative steel also contain N, O, P, Mg and the like at the impurity level.

  Inventive steels 1-32 are inexpensive steels that fall within the category of low alloy steels, with Cr + Mo + V + W + Co being as low as 6% by mass or less. Comparative steels 1 to 4 are expensive steels that have Cr + Mo + V + W + Co exceeding 6 mass% and are classified as medium alloys or high alloys. The comparative steels 1 to 5 are commercially available for mold materials, and are used, for example, for dies for forging heated materials.

Next, the inventive steels 1 to 32 and the comparative steels 1 to 5 were processed into 11 mm × 11 mm × 60 mm square bars. Each square bar is heated to 1030 ° C., held for 30 minutes, then cooled to 500 ° C. to 550 ° C. at a cooling rate of 200 ° C./min or higher, and then cooled to 150 ° C. or lower at a rate of 120 ° C./min. did. By this quenching, a structure mainly composed of martensite when used as a mold material was obtained. In addition, only high-speed tool steel (Comparative Steel 4) was heated to 1200 ° C. in order to sufficiently dissolve the alloy elements. Subsequently, tempering was performed. This tempering was performed by holding each steel in a temperature range of 550 ° C. to 700 ° C. for 1 Hr to 100 Hr.
Thereafter, the HRC hardness of each steel at room temperature was measured. When LMP was calculated by applying the tempering conditions at this time to the formula (1), the tempering was LMP = 16.46 to 21.41. Further, the tempering time at 575 ° C. was adjusted to obtain LMPmax and HRCmax of each steel. Table 2 summarizes the Cr + Mo + V + W + Co content, HRCmax, LMPmax, and tempering time [575] at 575 ° C. of each steel.

(Evaluation of specimen)
FIGS. 2A to 2D show the correlation between LMP and HRC hardness of Invention Steel 1 and Comparative Steels 2 and 4. FIG.
The hardened invention steel 1 becomes about HRC43 when it is tempered under the condition of LMP≈17.6 (for example, 600 ° C. × 1.5 Hr) (dotted circle in FIG. 2B). When applied, there is an increase in temperature during use, but a decrease in hardness is very unlikely (broken arrow in FIG. 2 (b)), and the wear of the mold is considered to be less than that of the comparative steel 2. This is because the inventive steel 1 is considered to be less likely to be worn because the LMPmax is larger (higher softening resistance) than the comparative steel 2.

  The comparative steel 2 is a typical steel material of a forging die that is used after being tempered to about HRC43 due to the problem of toughness. When the tempered comparative steel 2 is tempered under the condition of LMP≈18.45 (for example, 615 ° C. × 6 Hr), it becomes about HRC43 (dotted circle in FIG. 2C), and is applied to forging in this state. Then, it is considered that the hardness decreases due to the temperature rise during use (broken line arrow in FIG. 2C), and the wear of the mold becomes remarkable. This is because the comparative steel 2 is considered to be easily worn because the LMPmax is smaller than the inventive steel 1 (low softening resistance).

The comparative steel 4 is a typical steel material of a forging die that is used after being tempered to about HRC60 when it is desired to increase wear resistance at the expense of toughness. When the tempered comparative steel 4 is tempered under the condition of LMP≈17.9 (for example, 615 ° C. × 1.5 Hr), it becomes about HRC 60 (dotted circle in FIG. 2 (d)). As the temperature rises, the hardness is greatly reduced (broken line arrow in FIG. 2D), and becomes about HRC45.
This hardness is almost the same as that of Invention Steel 1 (HRC43), it is about 2 harder than that of Invention Steel 1 after use, and the initial hardness (hardness before forging) is significantly harder than that of Invention Steel 1. The wear of the mold is also smaller than that of Invention Steel 1. However, the difference in wear is relatively small, and the effect of improving wear resistance is small even though expensive steel is used. The reason is considered that the comparative steel 4 has a small LMPmax (low softening resistance). Moreover, as a result of sacrificing toughness, the mold of the comparative steel 4 has a risk of breaking early.

  From the above, in the forging die, it is considered that the softening resistance can be increased by selecting a steel having a large LMPmax as the steel for the mold material. In this case, it is considered that the maximum value of HRC hardness may be reduced when importance is attached to toughness, and the maximum value of HRC hardness may be increased when importance is attached to wear resistance.

Based on this finding, when the results in Table 2 are evaluated, the inventive steels 1 to 32 are considered to have high softening resistance because all of LMPmax is 17.66 or more. Among them, the inventive steel 2 is considered to be particularly excellent in wear resistance since the HRCmax is high, and the inventive steel 3 is considered to be particularly excellent in toughness since the HRCmax is low. Any one of the inventive steels 1 to 32 may be used as the steel for the mold material depending on the application.
On the other hand, Comparative Steels 1 to 5 are considered to have low softening resistance because LMPmax was less than 17.66. Among them, comparative steels 1 to 3 and 5 have high initial hardness, but are considered to have a large decrease in hardness in a high temperature state (see FIG. 2). Incidentally, as described above, the comparative steel 4 has a small effect of improving the wear resistance although it is expensive.
Moreover, although the comparative steel 5 is an example with little content of a rare element, LMPmax is small. Therefore, it can be seen that LMPmax cannot be increased only by reducing the content of rare elements. In order to raise LMPmax, it has confirmed that it was necessary to provide a specific composition component like invention steel.

Next, a method for increasing LMPmax will be described with reference to FIG.
FIG. 3 is a graph showing the correlation between LMP and HRC hardness for explaining the effects of Cr and Mo on the softening resistance. FIG. 3 (a) shows the same when the Cr content is increased or decreased. The figure (b) shows the correlation when the content of Mo is increased or decreased. According to the figure, it is understood that in order to increase LMPmax, it is preferable to (1) reduce the Cr content and / or (2) increase the Mo content. Although illustration is omitted, the contents of V and W can be said to be the same as the contents of Mo. Si and Co do not form carbides, but may affect the softening resistance through changes in the form of carbides or solid solution strengthening, so that appropriate amounts may be included (see Table 1).

(Example B)
(Evaluation test of test punch-Part 1)
A test piece (invention steel and comparative steel) was used as a steel material to prepare a punch tempered to HRC50 to HRC60, and hot forging was performed using this to measure the amount of wear. Note that the punch is a mold used to make a hole by pushing into a workpiece.
Punch steels shown in Table 3 were prepared, and hot forging punches were produced. The steel for punching after roughing is heated to 1030 ° C. (only high-speed tool steel of comparative steel 4 is 1200 ° C. in order to sufficiently dissolve the alloy elements), and after holding for 30 minutes, the cooling rate is 200 ° C./min or more. Then, it was cooled to 500 ° C. to 550 ° C. and then rapidly cooled to 150 ° C. or less at a cooling rate of 100 ° C./min or more. Subsequently, the steel for punching was tempered under appropriate conditions (heated in a temperature range of 550 ° C. to 700 ° C.), and the hardness H was adjusted to HRC 50 to HRC 60.
Further, the punching steel was finished to obtain a test punch (invention steel punch, comparative steel punch) having a normal dimension and having a schematic shape shown in FIG. The inventive steel punch is a punch produced using the inventive steel, and the comparative steel punch is a punch produced using the comparative steel.

  The state of hot forging is shown in FIG. 4 (b). Carbon steel heated to 1000 ° C. is used as a work material, a test punch is incorporated in a die, and a lubricant (mineral oil is mixed with graphite powder). Liquid) was applied to the punch for each shot and performed at a rate of 60 shots / minute. And the amount of wear near the front-end | tip outer peripheral part of the punch for a test was measured.

The amount of wear is the difference between the initial diameter before hot forging in the vicinity of the outer periphery of the tip of the test punch (dotted circle in FIG. 4C) and the diameter after forging a specified number of shots (the following equation) (2)), the state where the amount of wear was 1 mm or more was determined as the life.
Amount of wear [mm] = 20.5 [mm] -d [mm] (2)
The reason why the vicinity of the outer periphery of the test punch tip was used for the wear amount measurement corresponds to an area where the deformation speed of the part of the workpiece hit by the outer periphery is very high, so that the test punch was damaged by wear. It is easy to receive. Further, the excessive wear near the outer peripheral portion greatly affects the product dimensions and product surface properties of the workpiece, and when the wear amount exceeds a certain value, the punch reaches the end of its life.

  Table 3 summarizes the steel materials for punching used in the hot forging test, their basic characteristics, tempering conditions, and their wear resistance evaluation. Among the tempering conditions of the punch, the hardness H represents the hardness of each punch tempered before hot forging, and HRC50 to HRC53, which are representative values of hot forging, are set as the initial values of the comparative steel punches 4 and 5. Since the hardness is an extremely hard example, it was set to HRC60 or higher). The heating temperature and the holding time indicate the temperature and time for tempering to obtain this hardness H. In the wear resistance evaluation of each punch, the number shown in each shot column is the wear amount obtained by the equation (2), and “x” means that the wear amount exceeds 1 mm and the life is reached. .

Inventive steel punch 1 and comparative steel punches 1 to 3 have H≈HRC53 and the same hardness at the start of forging. Inventive steel punch 1 has less wear than comparative steel punches 1 to 3. Further, the inventive steel punches 2 to 14 with H≈HRC50 have an initial hardness of about 3 lower than that of the comparative steel punches 1 to 3 with H≈HRC53, but the wear amount is almost equal to that of the comparative steel punches 1 to 3. Therefore, it was found that the inventive steel punches 1 to 14 were not as hard as the comparative steel punch 4 (H = 60.1) having a very high initial hardness, but had excellent wear resistance. In addition, the comparative steel punch 5 given extremely high initial hardness (H = 63.2) in order to maximize the wear resistance is broken to less than 1000 shots because of its low toughness, and the wear resistance is evaluated. could not. Thus, it was found that if too much wear resistance is emphasized, premature fracture is likely to occur, and even if expensive steel is used, there is no meaning at all.
Further, the comparative steel punch 6 was in the same condition as the inventive steel in that there were few rare elements, but the LMPmax was low and the wear was large. Therefore, it was found that LMPmax cannot be increased and the wear resistance cannot be increased only by reducing rare elements.

  The amount of wear when the initial hardness is common is determined by the softening resistance and the amount of carbide. Since the invention steel punch has Cr + Mo + V + W + Co of 6 mass% or less, the amount of carbide is smaller than that of the comparative steel punch in which Cr + Mo + V + W + Co exceeds 6 mass%, but the wear amount is almost the same (except for comparative steel punches 4 to 6). Therefore, it was found that the softening resistance was high. From this, even with inexpensive steel with few rare elements, if the softening resistance is increased as in the invention steel, it is tempered to about HRC50 to HRC53, which is a typical hardness of a hot forging die, It was found that the wear resistance equal to or higher than that of “conventional steel containing many carbides with Cr + Mo + V + W + Co exceeding 6 mass%” can be realized. Further, the above results agreed well with the results shown in FIG. The invention steel punch has an LMPmax exceeding 17.66, but the comparative steel punch has an LMPmax lower than this value.

(Example C)
(Evaluation test of test punch-Part 2)
A test piece (invention steel and comparative steel) was used as a steel material to prepare a punch that had been tempered to H≈HRC44, and this was used for hot forging to measure the amount of wear. The reason why H≈HRC44 is used is that a mold that places importance on toughness is used at about HRC44. Each punch is manufactured in the same procedure as the punch shown in Table 3, using the inventive steel 1, comparative steel 1 (JIS SKD61), and comparative steel 2 (hot tool steel A), which are frequently used at about HRC44, as steel materials. did. The procedure of the hot forging test was also the same as that shown in Table 3.

Table 4 summarizes the steel materials for punching used in the hot forging test, the basic characteristics thereof, the tempering conditions, and the wear resistance evaluation.
The amount of wear of the inventive steel punch 15 was smaller than that of the comparative steel punch 7 and was equivalent to that of the comparative steel punch 8 in which Cr + Mo + V + W + Co exceeded 9 mass%. The inventive steel punch 15 has an HRCmax smaller than the comparative steel punches 7 and 8, but an LMPmax larger than the comparative steel punches 7 and 8. Therefore, when compared with the same initial hardness, it was found that the inventive steel punch 15 which is harder to soften has less wear. The comparative steel punch 8 was less worn than the comparative steel punch 7 because the comparative steel punch 8 has a large content of Cr, Mo, V, etc. and a large amount of hard carbide particles. It is done.
The amount of wear is determined by the softening resistance and the amount of carbide. From the results in Table 4, it was found that if the softening resistance is increased even if the amount of carbide is small, the wear resistance equivalent to or higher than that of steel with a large amount of carbide can be realized. . Therefore, even with an inexpensive steel with a small amount of rare elements, if the softening resistance is increased as in the case of the invention steel, in the state of being tempered to about HRC44 with an emphasis on toughness, “Cr + Mo + V + W + Co contains more than 6% by mass of carbide. It was found that the wear resistance equivalent to or better than the conventional steel to be achieved can be realized.

(Example D)
(Test Punch Evaluation Test-3)
A test piece (invention steel and comparative steel) was used as a steel material, and a punch tempered to H≈HRC40 was prepared. Using this, hot forging was performed, and the amount of wear was measured. The reason why H≈HRC40 is used is that a mold that places importance on machinability is used at about HRC40. Each punch was manufactured in the same procedure as the punches shown in Tables 3 and 4 using the inventive steel 3 and comparative steel 1 (JIS SKD61) frequently used at about HRC 40 as steel materials. The procedure of the hot forging test was also the same as that shown in Tables 3 and 4.

Table 5 shows the steel materials for punching used in the hot forging test, their basic characteristics, tempering conditions, and their wear resistance evaluation.
The invention steel punch 16 has Cr + Mo + V + W + Co of about 2% by mass, and the amount of wear of the invention steel punch 16 is the same as that of the comparative steel punch 9 in which Cr + Mo + V + W + Co exceeds 7% by mass. That was all. This is presumably because the inventive steel punch 16 has an LMPmax larger than 17.66 and larger than the comparative steel punch 9, so that the softening resistance is increased and the wear resistance is improved.
Therefore, even with an inexpensive steel with a small amount of rare elements, if the softening resistance is increased as in the case of the invented steel, many carbides with a Cr + Mo + V + W + Co content exceeding 7% by mass after being tempered to about HRC40 with emphasis on machinability. It was found that the wear resistance equivalent to or better than that of “conventional steel containing” can be realized.

  According to Examples A to D described above, if the content of rare metals such as Cr, Mo, V, W, and Co is at least higher than that of the conventional steel, the inventive steel has higher softening resistance than that of the conventional steel, It was found that the same or better wear resistance can be achieved. It was also found that the superiority or inferiority of wear resistance in actual production can be judged based on LMPmax obtained by a simple experiment of a specific method.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. In the above embodiment, the present invention is applied to hot forging. However, the present invention can be widely applied to mold materials such as injection molding and die casting of resin and glass.
Further, the inventive steels in the above examples are high in softening resistance, alloy-saving and environmentally friendly steels. Therefore, if this is applied to injection molding, die casting, forging, etc., the production efficiency is improved and the product manufacturing cost is increased. Reduction, further resource saving and environmental load reduction are expected.

  In addition, in the quenching of the mold material made of the inventive steel, the heating temperature is 1010 ° C. to 1050 ° C., but the conditions relating to the heating time are not particularly limited, and the heating time in which the required amount of alloy element is dissolved depends on the steel material components. To be selected. In quenching the mold material, it is desirable to avoid precipitation of ferrite phase and pearlite phase. If the mold is large, the quenching speed will be low even if it is oil-cooled.If the mold is left to cool, the quenching speed will be large, but the structure after quenching is mainly martensite and bainite regardless of the size of the mold. What is necessary is just to obtain the organization. Further, in the quenching of the mold material, the cooling method of the mold material is not limited at all, and a cooling method that satisfies the above-described conditions may be selected according to the size of the mold material. A plurality of cooling methods may be combined.

  The tempering conditions are not particularly limited, and can be arbitrarily selected in consideration of LMPmax / HRCmax / required hardness / manufacturability. For example, after the first tempering of 605 ° C. × 2 Hr, the second time of 595 ° C. × 3 Hr and the third time of 575 ° C. × 2 Hr may be performed to set the hardness to HRC43. This is because there is no significant difference in characteristics from the case where H = HRC 43 is adjusted by one tempering.

  Since the steel for mold material according to the present invention is excellent in softening resistance, it is suitable for use in mold materials such as injection molding, die casting and forging of resin and glass. Further, since the steel for mold material according to the present invention is inexpensive, it is extremely useful industrially for manufacturers of steel for mold materials, and manufacturers of resin / glass products and die cast products.

It is the graph which showed the relationship between the amount of wear and LMPmax. (A)-(d) is the graph which showed the correlation with LMP and HRC hardness about invention steel 1, comparative steel 2, and comparative steel 4. FIG. (A), (b) is the graph which showed the correlation with LMP and HRC hardness, and is the graph which showed the influence of Cr and Mo which respectively have on softening resistance. (A) is a figure which shows the schematic shape of a test metal mold | die (punch), (b) is a figure which shows the condition of hot forging, (c) demonstrates the definition of the amount of wear near punch tip. FIG.

Claims (8)

C: 0.15-0.55 mass%,
Si: 0.01 to 2.0 mass%,
Mn: 0.01 to 2.5% by mass,
Cu: 0.01-2.0 mass%,
Ni: 0.01 to 2.0% by mass,
Cr: 0.01 to 2.5% by mass,
Mo: 0.01-3.0 mass%, and
A total amount of at least one selected from the group consisting of V and W: 0.01 to 1.0% by mass, the balance being steel for mold material composed of Fe and inevitable impurities,
After soaking at 1010 ° C. to 1050 ° C., it is cooled to 500 ° C. to 550 ° C. at a cooling rate of 200 ° C./min or more, subsequently cooled to 150 ° C. or less at a cooling rate of 100 ° C./min or more, and then 550 A steel for a mold material, characterized in that LMP (LMPmax) giving a maximum value of HRC hardness at room temperature after heating in a temperature range of from C to 700C is 17.66 or more.
However,
LMP = (TH + 273) × (20 + log (t)) × 0.001 (1)
TH is the heating holding temperature [° C.] at 550 ° C. to 700 ° C.,
t is the holding time [Hr] of heating at 550 ° C to 700 ° C. Furthermore,
The steel for mold materials according to claim 1, comprising Al: 0.002 to 0.5 mass%. Furthermore,
Co: 0.01-2.0 mass% is contained, The steel for mold materials of Claim 1 or 2 characterized by the above-mentioned. Furthermore,
Nb: 0.005 to 0.5 mass%,
Ta: 0.005 to 0.5 mass%,
Ti: 0.005 to 0.5 mass%, and
Zr: At least 1 sort (s) chosen from the group which consists of 0.005-0.5 mass% is contained, The steel for mold materials in any one of Claim 1 to 3 characterized by the above-mentioned. Furthermore,
B: 0.0002-0.02 mass% is contained, The steel for mold materials in any one of Claim 1 to 4 characterized by the above-mentioned. Furthermore,
S: 0.01-2.0 mass%,
Ca: 0.0005 to 0.5 mass%,
Se: 0.005 to 0.5 mass%,
Te: 0.005 to 0.5 mass%,
Bi: 0.005 to 0.5 mass%, and
The steel for mold materials according to any one of claims 1 to 5, comprising at least one selected from the group consisting of Pb: 0.005 to 0.5 mass%.   The steel for mold materials according to any one of claims 1 to 6, wherein the total amount of Cr, Mo, V, W and Co is 6 mass% or less.   After soaking at 1010 ° C. to 1050 ° C., it is cooled to 500 ° C. to 550 ° C. at a cooling rate of 200 ° C./min or more, subsequently cooled to 150 ° C. or less at a cooling rate of 100 ° C./min or more, and then 550 The difference between the HRC hardness at room temperature when LMP = 17 and the HRC hardness at room temperature when LMP = 19 is 6 or less when heated in a temperature range of 750C to 700C. The steel for mold materials according to any one of claims 1 to 7.

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