JP2016069661A - Steel for mold and mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for mold and a mold securing high hardness and high corrosion resistance, as well as further having good annealing property, difficulties of deposition of grain boundary carbide and difficulties of deposition of pearlite.SOLUTION: A steel for mold is manufactured with a composition containing, 0.200<C<0.380, 0.40<Si<1.50, 0.50<Mn<1.50, 0.40<Ni<1.40, 10.50<Cr<12.50, 0.01<Mo<0.29, 0.002<V<0.32, 0.001<N<0.200, 0.230<C+N<0.460 and the balance Fe with inevitable impurities.SELECTED DRAWING: None

Description

  The present invention relates to a mold steel and a mold, and more particularly to a mold steel and a mold excellent in hardness and corrosion resistance.

In recent years, plastic products in which hard glass fibers are mixed to increase the strength have been increasing. In the injection molding of such plastic products, the wear of the mold becomes obvious. As the mold wears, it is transferred to the product and degrades the surface quality of the product. Products with poor surface quality are not commodities and are discarded. Therefore, it is important that the mold does not wear, and the mold is required to have high hardness in order to ensure wear resistance.
Conventionally, the hardness of a mold used for injection molding of plastic mixed with hard glass fibers is mainly 45 to 55 HRC (from the viewpoint of workability, the hardness is tempered to a lower hardness than the above. Sometimes used).

  In general, a mold for molding a plastic product is provided with a temperature adjusting flow path therein, and cold water, hot water, steam, or the like is supplied to the flow path to adjust the temperature of the mold. However, in a mold having low corrosion resistance, the flow path becomes narrow due to rust, and it becomes impossible to secure a predetermined flow rate (cold water, hot water, steam, etc.), which hinders temperature control. Furthermore, when rust progresses, the flow path is clogged with rust, and the flow path becomes useless. In addition, molds with low corrosion resistance have cracks starting from the rusted part, and the cracks are caused by the progress of the mold, or cold water, hot water, steam, etc. leak from the cracks that penetrate the design surface, which adversely affects resin products. Sometimes. For this reason, the mold is required to have high corrosion resistance.

Molds that require hardness and corrosion resistance for plastic injection molding (including parts that constitute part of the mold) are generally melting → refining → casting → homogenization heat treatment → hot working → intermediate heat treatment → annealing → Manufactured through a process of machining 1 (rough machining) → quenching → tempering → machining 2 (finishing) → mirror polishing or embossing.
Further, surface modification (PVD, CVD, nitriding, shot blasting, shot peening, etc.) may be applied as necessary.

In this manufacturing process, the mold steel is required to be (1) good annealing, (2) no precipitation of grain boundary carbides during intermediate heat treatment, and (3) no pearlite precipitation during quenching.
When the annealing property is poor, complicated and long annealing conditions are required for softening, which leads to an increase in material cost. Accordingly, the mold steel is required to have “good annealing” that is softened to a state where the machining 1 can be performed by a simple heat treatment.

In the above manufacturing process, normalizing and tempering are usually performed as an intermediate heat treatment.
In this normalization and tempering, coarse grain boundary carbides generated by hot working (which precipitate due to a low cooling rate after hot working) can be dissolved, but in some cases coarse grain boundary carbides In addition, the grain boundary carbides are newly precipitated by normalization (because the cooling rate after normalization is small).
The grain boundary carbide generated in this way cannot be eliminated by the subsequent heat treatment (annealing, quenching, tempering).
Similarly, when pearlite is precipitated by quenching, the pearlite cannot be eliminated by subsequent tempering.
In general, the surface of plastic products is required to be smooth, but the grain boundary carbides and pearlite generated as described above become foreign substances dispersed in the matrix, which is an obstacle to obtaining a uniform and smooth surface by mirror polishing. Become.
Grain boundary carbides and pearlite also serve as starting points for fracture due to repeated stresses during use of the mold.
Accordingly, in mold steel for plastic injection molding, “difficult to precipitate grain boundary carbide and pearlite” is required.

Conventionally, JIS SUS420J2 is frequently used for molds and parts that require hardness and corrosion resistance. The component is 0.4C-0.9Si-0.4Mn-0.2Ni-13Cr-0.015N. This SUS420J2 satisfies the condition of (1) good annealing property, and it is cooled to 650 ° C. from 850 to 950 ° C. to 30 ° C./Hr and then left to cool to 87 to 96HRB. Soften.
However, SUS420J2 does not satisfy the above conditions (2) and (3).
That is, in some cases, a remarkable grain boundary carbide is generated in the intermediate heat treatment, and the mirror polishability is hindered.
In addition, SUS420J2 precipitates pearlite even when quenched and cooled at a quenching temperature of 1030 ° C. to 50 ° C./min.
In general, the quenching cooling rate inside the mold is 10 to 40 ° C./min in the temperature range of 550 to 850 ° C. where pearlite is precipitated, and therefore pearlite is inevitably deposited inside the SUS420J2 mold.
The pearlite produced in this way also impairs the mirror polishability. In addition, the precipitated pearlite serves as a starting point, increasing the risk of breakage during use of the mold.

  As described above, in addition to high hardness and high corrosion resistance, the mold for plastic injection molding has (1) excellent annealing, (2) no precipitation of grain boundary carbides during intermediate heat treatment, and (3) quenching. The property that pearlite does not precipitate is sometimes required, but conventionally, steel for molds and molds satisfying these properties have not been provided.

  In addition, the steel containing 10.5-12.5Cr which is the range of this invention is disclosed by the following patent document 1-patent document 7. FIG. However, as shown below, these steels are not plastic injection mold steels, and have different uses from the present invention. In addition, essential elements and characteristics of interest are also different.

Patent Document 1 discloses 40 to 47 HRC free cutting tool steel. However, the material described in Patent Document 1 is not mentioned as an injection mold for plastics with high hardness and high corrosion resistance, and S is an essential addition for free cutting, and the hardness level is lower than that of the present invention. This is different from the present invention. If this steel is applied to a plastic injection mold, it is easily estimated that a predetermined mirror property cannot be secured due to the influence of free cutting components and that the wear resistance is inferior.
Moreover, the Example which contained Cr in the range of 7.05 to 15.0% is not disclosed, and the effect of containing Cr within the same range is not proved. There is no focus on annealing and precipitation of grain boundary carbides and pearlite.

  Patent Document 2 discloses a free-cutting tool steel of 45 to 63 HRC. However, the one described in Patent Document 2 is also different from the present invention in that there is no mention as a high hardness, high corrosion resistance plastic injection mold, and S is an essential addition for free cutting. If this steel is applied to a plastic injection mold, it is easily estimated that the predetermined specularity cannot be secured due to the influence of the free cutting component. There is no focus on annealing and precipitation of grain boundary carbides and pearlite.

  Patent Document 3 discloses an alloy steel for hot working. However, the one described in Patent Document 3 is not mentioned as an injection mold for plastics with high hardness and high corrosion resistance, and there are cases where the basic components are C, Si, REM, and N. It is easily estimated that corrosion resistance is not obtained. Moreover, about Cr as a selection element, there is no indication of the Example in the range of 2.5-13.0Cr, and the effect of containing Cr within the same range is not proved by the Example. There is no focus on annealing and precipitation of grain boundary carbides and pearlite.

  Patent Document 4 discloses a die casting steel having a carbide area ratio of 5.5 to 30% and excellent melt resistance. However, in the thing of patent document 4, Ni is not essential, and even if it adds, it is as low as 0.2% (Example), and the effect of high Ni is not proven, Mo + 0.5W is essential, but at least It is as high as 1.95% (Example) and is different from the present invention in that the effect of low Mo has not been proved. Further, C is very much because a large amount of carbide is formed. When applied to plastic injection molds, it is easily presumed that the specularity and corrosion resistance decrease due to the influence of carbides, and that the destruction starts from carbides. There is no focus on annealing and precipitation of grain boundary carbides and pearlite.

Patent Document 5 discloses a steel wire having a diameter of 4.5 to 20 mm for a spring. However, the one described in Patent Document 5 is different from the present invention in that there is no mention as a plastic injection mold and V is not essential.
Even when V is selectively added, it is as high as 0.5% (Example), and the effect of low V has not been proved. Of course, a steel wire having a diameter of 4.5 to 20 mm cannot be applied to a mold. There is no focus on annealing and precipitation of grain boundary carbides and pearlite.

  Patent Documents 6 and 7 disclose stainless steel pipes for oil wells. Those described in these patent documents are different from the present invention in that there is no mention as a plastic injection mold and Ni, Mo, and V are not essential. Moreover, Si is as low as 0.31% (Example) or less, and the effect of high Si has not been proved. The selectively added Ni is as high as 1.63% (Example) at the minimum, and the effect of low Ni has not been proved. The selectively added Mo is as high as at least 0.75% (Example), and the effect of low Mo has not been proved. Of course, steel pipes cannot be applied to molds. There is no focus on annealing and precipitation of grain boundary carbides and pearlite.

  On the other hand, high Cr steel for plastic injection molds is disclosed in Patent Document 8 and Patent Document 9 below. However, those described in these patent documents have a Cr addition amount as high as 12.5% or more, which is different from the present invention.

  Patent Document 10 discloses a plastic injection mold steel in which the addition amount of Cr overlaps with that of the present invention. However, the present invention is directed to a component range of Si, Mn, Ni, and Mo that is not disclosed as an example of this Patent Document 10, and has found an effect that is not found in the technology disclosed in this Patent Document.

JP-A-57-73171 JP-A-57-73172 JP 58-113352 A JP 2007-197784 A JP 2007-314815 A JP 2008-297602 A JP 2009-167476 A Japanese Patent Laid-Open No. 8-253846 JP-T-2004-503676 Special table 2010-539325 gazette

  The present invention is based on the circumstances as described above, and after securing high hardness and high corrosion resistance, a mold having better annealing, difficulty in precipitation of grain boundary carbides, and difficulty in precipitation of pearlite. It was made for the purpose of providing steel and molds.

  Accordingly, the first aspect relates to a mold steel, and is 0.200 <C <0.380, 0.40 <Si <1.50, 0.50 <Mn <1.50 in mass%. , 0.40 <Ni <1.40, 10.50 <Cr <12.50, 0.01 <Mo <0.29, 0.002 <V <0.32, 0.001 <N <0.200 0.230 <C + N <0.460, with the balance having the composition of Fe and inevitable impurities.

Normally, in the steel for molds, the following components can be included as inevitable impurities within the following ranges.
P ≦ 0.05, S ≦ 0.003, Cu ≦ 0.30, Al ≦ 0.10, W ≦ 0.10, O ≦ 0.01, Co ≦ 0.10, Nb ≦ 0.004, Ta ≦ 0.004, Ti ≦ 0.004, Zr ≦ 0.004, B ≦ 0.0001, Ca ≦ 0.0005, Se ≦ 0.03, Te ≦ 0.005, Bi ≦ 0.01, Pb ≦ 0. 03, Mg ≦ 0.02, REM ≦ 0.10, and the like.

  According to a second aspect of the present invention, in the first aspect of the present invention, at least one of 0.10 <W ≦ 5.00 and 0.10 <Co ≦ 4.00 is further contained by mass%.

  According to a third aspect of the present invention, in any one of the first and second aspects, 0.004 <Nb ≦ 0.100, 0.004 <Ta ≦ 0.100, 0.004 <Ti ≦ 0.100 in mass%. It further contains at least one of 0.004 <Zr ≦ 0.100.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the composition further contains 0.10 <Al <1.20 in mass%.

  A fifth aspect of the present invention is characterized in that in any one of the first to fourth aspects, 0.30 <Cu ≦ 3.0 is further contained in mass%.

  A sixth aspect of the present invention is characterized in that in any one of the first to fifth aspects, 0.0001 <B ≦ 0.0050 is further contained in mass%.

  In a seventh aspect of the present invention, in any one of the first to sixth aspects, 0.003 <S ≦ 0.050, 0.0005 <Ca ≦ 0.2000, 0.03 <Se ≦ 0.50 in mass%. It further contains at least one of 0.005 <Te ≦ 0.100, 0.01 <Bi ≦ 0.50, 0.03 <Pb ≦ 0.50.

An eighth aspect of the present invention relates to a mold, and is made of the steel according to any one of the first to seventh aspects.
In the present invention, the “mold” includes not only the mold body but also mold parts such as pins that are assembled and used. Furthermore, the metal mold | die which consists of steel of this invention and the surface-treated thing is contained.

The present invention as described above is characterized in that SUS420J2 is reduced in C, reduced in Cr, increased in Mn, increased in Ni and added with Mo to suppress precipitation of grain boundary carbides and pearlite. is there.
According to the present invention, it is possible to suppress precipitation of grain boundary carbides and pearlite while ensuring hardness, corrosion resistance, and annealing properties as in SUS420J2.
In SUS420J2, since the precipitated carbide is Cr-based carbide, it is effective to reduce Cr in order to suppress the precipitation of carbide, but on the other hand, if Cr is excessively reduced, the corrosion resistance and annealing properties deteriorate. End up.
Therefore, in the present invention, 10.50 <Cr <12.50 is set without excessively reducing Cr, and by adding an appropriate amount of Mn, Ni, and Mo under this Cr amount, the grains can be obtained while ensuring good annealing. Sedimentation of boundary carbide and pearlite was suppressed.

In the present invention, the N content is increased to compensate for the decrease in hardness due to the lower C content. Furthermore, the effect of supplementing the hardness was obtained by the secondary hardening of Mo by adding Mo.
Moreover, the annealing property comparable to SUS420J2 was ensured by not excessively increasing Mn, Ni, and Mo, and the corrosion resistance comparable to SUS420J2 was ensured by low C and not excessively reducing Cr.
In the present invention, V is further added to pin the austenite grain boundaries with carbides during quenching and maintain fine crystal grains. This is to compensate for the decrease in Cr-based carbides during quenching due to the reduction of C and Cr with V-based carbides. In addition, a part of V that dissolves during quenching exhibits the effect of supplementing the hardness by secondary curing.

  The present invention as described above is particularly suitable as a steel for plastic injection molds and steels for rubber molds including injection molding, and is a cold press mold for steel sheets, a hot stamp mold for steel sheets, It is also suitable as a steel for molds such as a tableting mold for solidifying drug powder into tablets.

Next, the reasons for limiting each chemical component in the present invention will be described below.
“Chemical composition of claim 1”
0.200 <C <0.380
When C ≦ 0.200, it is difficult to stably obtain high hardness (45 HRC or more) necessary for ensuring high wear resistance. In 0.380 <= C, corrosion resistance and weldability will fall. Further, when 0.380 ≦ C, grain boundary carbides and pearlite are likely to precipitate.
A preferable range of C is 0.205 <C <0.370, which is excellent in the balance of characteristics, and more preferably 0.210 <C <0.360.

0.40 <Si <1.50
When Si ≦ 0.40, the machinability during machining deteriorates. Further, when Si ≦ 0.40, it is difficult to obtain a high hardness (45 HRC or more) necessary for ensuring high wear resistance. There is also an inconvenience that unevenness of carbide distribution in the annealed metal structure becomes large due to low Si.
On the other hand, when 1.50 ≦ Si, the thermal conductivity is greatly reduced. In order to increase the productivity of injection molding, it is necessary to shorten the solidification time of the plastic injected into the mold. However, if the thermal conductivity of the mold decreases, the solidification time becomes longer and the productivity decreases.
In addition, when 1.50 ≦ Si, grain boundary carbides and pearlite are likely to precipitate. Delta ferrite is also likely to occur. If the delta ferrite remains, it will adversely affect the mirror polishability and may be a starting point for mold destruction. Delta ferrite tends to precipitate at higher temperatures, so high Cr high Si steels must be homogenized and hot worked at low temperatures to avoid delta ferrite, and it is difficult to reduce segregation by lowering the temperature. This adversely affects mirror polishability and texture processing.
A preferable Si range is 0.55 <Si <1.35, which is excellent in the balance of these characteristics, and more preferably 0.83 <Si <1.25.

FIG. 1 shows the influence of Si content on machinability.
0.33C-0.93Mn-0.93Ni-12.3Cr-0.12Mo-0.20V-0.020N as a basic component, material with varying Si content is machined, and the processing tool is worn The cutting distance until the end of life was evaluated. The larger the cutting distance, the better the material machinability.
As shown in FIG. 1, the machinability is improved with an increase in Si, and the improvement is particularly remarkable when 0.40 <Si. Therefore, in the present invention, 0.4 <Si is specified.

0.50 <Mn <1.50
When Mn ≦ 0.50, the effect of stabilizing austenite and suppressing precipitation of grain boundary carbides and pearlite is small.
On the other hand, if 1.50 ≦ Mn, the annealability deteriorates. Further, when 1.50 ≦ Mn, the thermal conductivity is greatly reduced.
A preferable range of Mn is 0.55 <Mn <1.30, which is excellent in the balance of various properties, and more preferably 0.83 <Mn <1.20.
For the stabilization of austenite (suppression of precipitation of grain boundary carbides and pearlite), Ni addition is very effective in the case of high Cr steel. However, a large amount of Ni causes a significant cost increase. Therefore, an increase in material cost is suppressed by using Mn, which is an element that stabilizes austenite as in Ni and is inexpensive.

FIG. 2 shows the influence of the amount of Mn on the critical cooling rate of pearlite precipitation.
0.33C-0.93Si-0.71Ni-12.3Cr-0.23Mo-0.22V-0.021N as a basic component, the cooling rate from 1030 ° C. is applied to the material whose amount of Mn is changed. When the temperature was changed, the lowest cooling rate at which perlite did not precipitate was evaluated. The smaller the critical cooling rate, the more preferable it is because pearlite is less likely to precipitate.
As shown in FIG. 2, the critical cooling rate decreases as Mn increases and reaches 10 ° C./min at 0.50 Mn. In general, the quenching speed inside the mold is 10 to 40 ° C./min in the temperature range of 550 to 850 ° C. where pearlite precipitates. Therefore, if the critical cooling rate of pearlite deposition is 10 ° C./min, the actual mold There is very little risk of pearlite formation even when the mold is quenched. Therefore, in the present invention, 0.50 <Mn is specified.

0.40 <Ni <1.40
When Ni ≦ 0.40, the effect of stabilizing austenite and suppressing precipitation of grain boundary carbides and pearlite is small. Also, the effect of suppressing delta ferrite is small.
On the other hand, if 1.40 ≦ Ni, the annealability deteriorates. In addition, the thermal conductivity is greatly reduced.
A preferable Ni range is 0.60 <Ni <1.20, which is excellent in the balance of various characteristics, and more preferably 0.85 <Ni <1.08.

FIG. 3 shows the effect of Ni content on the critical cooling rate of pearlite precipitation.
0.33C-0.94Si-1.01Mn-12.3Cr-0.23Mo-0.22V-0.021N as a basic component, the cooling rate from 1030 ° C. is applied to the material in which the amount of Ni is changed. When the temperature was changed, the lowest cooling rate at which perlite did not precipitate was evaluated.
As shown in FIG. 3, the critical cooling rate decreases with an increase in Ni and becomes 10 ° C./min at 0.40 Ni. Therefore, in the present invention, 0.40 <Ni is specified.

10.50 <Cr <12.50
When Cr ≦ 10.50, the corrosion resistance deteriorates. On the other hand, when Cr ≦ 10.50, the annealability is also deteriorated.
On the other hand, when 12.50 ≦ Cr, grain boundary carbides and pearlite are likely to precipitate, and the mirror polishability deteriorates. Also, delta ferrite is likely to precipitate.
A preferable Cr range is 10.75 <Cr <12.48, which is excellent in the balance of various properties, and more preferably 11.00 <Cr <12.46.

0.01 <Mo <0.29
When Mo ≦ 0.01, the effect of suppressing the precipitation of pearlite is poor. In addition, when Mo ≦ 0.01, there is almost no contribution of secondary curing, so that it is difficult to stably obtain a hardness of 45 HRC or higher when tempering at a high temperature.
On the other hand, if 0.29 ≦ Mo, the annealability deteriorates. Also, delta ferrite is likely to precipitate.
A preferable range of Mo is 0.02 <Mo <0.28, which is excellent in the balance of various characteristics, and more preferably 0.03 <Mo <0.27.

FIG. 4 shows the influence of the amount of Mo on the delta ferrite area ratio.
0.33C-1.04Si-0.56Mn-0.42Ni-12.4Cr-0.22V-0.011N The area ratio of delta ferrite of the structure tempered at 550 ° C. was evaluated.
As shown in FIG. 4, delta ferrite hardly precipitates when Mo is reduced, and the area ratio becomes zero at 0.29 Mo. Therefore, in the present invention, Mo <0.29 is specified.

0.002 <V <0.32
When V ≦ 0.002, the effect of maintaining fine austenite crystal grains during quenching is poor, and the risk of the mold breaking during use due to a decrease in toughness increases. Further, when V ≦ 0.002, there is almost no contribution of secondary curing, so that it is difficult to stably obtain 45 HRC or more when tempering at a high temperature.
On the other hand, when 0.32 ≦ V, the effect of maintaining the fine crystal grains is saturated and the cost is increased. Further, the nitride of V is liable to precipitate, and on the contrary, the toughness is lowered. Further, in this case, since N does not contribute to the increase in strength, the effect of adding N becomes very small.
A preferable V range is 0.005 <V <0.31, which is excellent in the balance of various characteristics, and more preferably 0.008 <V <0.30.

0.001 <N <0.200
When N ≦ 0.001, the effect of increasing the hardness is poor, and it is difficult to stably obtain 45 HRC or more. Further, N greatly affects the solid solution temperature of the V-based carbide, and the lower the N, the more the V-based carbide dissolves at a lower temperature. Therefore, when N ≦ 0.001, the effect of maintaining fine austenite crystal grains during quenching is achieved. Is also scarce.
When 0.200 ≦ N, the effects of increasing the strength and maintaining fine crystal grains are saturated. Further, when 0.200 ≦ N, the refining time and cost required for N addition increase, leading to an increase in material cost. Further, when 0.200 ≦ N, V nitride is liable to precipitate and the toughness is lowered.
A preferable range of N is 0.004 <N <0.195, which is excellent in the balance of various characteristics, and more preferably 0.007 <N <0.190.

0.230 <C + N <0.460
When C + N ≦ 0.230, the effect of increasing the hardness is poor, and it is difficult to stably obtain 45 HRC or more. Further, since V-based carbides during quenching are reduced, the effect of maintaining fine austenite crystal grains is poor.
On the other hand, when 0.460 ≦ C + N, the effects of increasing strength and maintaining fine crystal grains are saturated. If 0.460 ≦ C + N, the refining time and cost required for N addition increase, leading to an increase in material cost. In particular, when 0.200 ≦ N, coarse V-based carbides increase and V nitrides are also likely to precipitate, resulting in a decrease in toughness.
A preferable C + N range is 0.235 <C + N <0.450 which is excellent in the balance of various characteristics, and more preferably 0.240 <C + N <0.445.

“Chemical composition of claim 2”
Since the steel of the present invention contains a large amount of Cr, the softening resistance is low, and it is difficult to ensure 45 HRC when the tempering temperature is high. In such a case, W or Co may be selectively added to ensure strength. W increases strength by precipitation of carbides. Co increases the strength by solid solution in the base material, and at the same time contributes to precipitation hardening through a change in carbide form. In particular,
0.10 <W ≦ 5.00
0.10 <Co ≦ 4.00
It is sufficient to contain at least one kind (one element).
If any element exceeds a predetermined amount, saturation of characteristics and significant cost increase are caused.

“Chemical composition of claim 3”
If the quenching heating temperature becomes high or the quenching heating time becomes long due to unexpected equipment troubles, etc., there is a concern about deterioration of various characteristics due to coarsening of crystal grains. In such a case, Nb—Ta—Ti—Zr can be selectively added, and coarsening of austenite crystal grains can be suppressed by fine precipitates formed by these elements. In particular,
0.004 <Nb ≦ 0.100
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
What is necessary is just to contain at least 1 sort (s) of these.
When any element exceeds a predetermined amount, carbides, nitrides, and oxides are excessively generated, and the impact value and the mirror polishability are reduced.

“Chemical component of claim 4”
Similarly, in order to suppress the coarsening of the austenite crystal grains, 0.10 <Al <1.20
Can be contained. Al combines with N to form AlN, has the effect of suppressing the movement of austenite grain boundaries (ie, grain growth), and is effective in maintaining fine grains.
Moreover, since Al forms nitrides in steel and contributes to precipitation strengthening, it also has the effect of increasing the surface hardness of the nitriding steel. It is effective to use a steel material containing Al for a mold for nitriding for higher wear resistance.
However, when Al exceeds a predetermined amount, the thermal conductivity and toughness are reduced.

“Chemical component of claim 5”
In recent years, the size of molds tends to increase as parts become larger and integrated. Large molds are difficult to cool. For this reason, when quenching a large metal mold of steel with low hardenability, ferrite, pearlite, and coarse bainite are precipitated during quenching, and various properties deteriorate. The steel of the present invention has a considerably high hardenability, and there are few such concerns. However, even when a very large mold is processed by a quenching method with a low cooling strength, Cu can be added to further improve the hardenability. In particular,
0.30 <Cu ≦ 3.0
May be contained. Cu also has the effect of increasing hardness by aging precipitation. When Cu exceeds a predetermined amount, segregation becomes prominent, leading to a decrease in specular polishing properties and embossing properties.

“Chemical component of claim 6”
Addition of B is also effective as a measure for improving hardenability. In particular,
0.0001 <B ≦ 0.0050
Containing.
Note that when B forms BN, the effect of improving the hardenability is lost, and therefore B needs to be present alone in the steel. Specifically, it is sufficient that nitride is formed with an element having an affinity for N stronger than B, and B and N are not bonded. Examples of such elements include the elements listed in claim 3. Even if the element of claim 3 is present at the impurity level, it has an effect of fixing N, but depending on the amount of N, it may be added within the range specified in claim 3. Even if B is combined with N in steel to form BN, if surplus B is present alone in the steel, it enhances hardenability.
B is also effective in improving machinability. In order to improve machinability, BN may be formed. BN is similar in nature to graphite and lowers cutting resistance while improving chip friability. In addition, when B and BN exist in steel, hardenability and machinability are improved at the same time.

“Chemical component of claim 7”
For improving the machinability, it is also effective to selectively add S-Ca-Se-Te-Bi-Pb. In particular,
0.003 <S ≦ 0.050
0.0005 <Ca ≦ 0.2000
0.03 <Se ≦ 0.50
0.005 <Te ≦ 0.100
0.01 <Bi ≦ 0.50
0.03 <Pb ≦ 0.50
What is necessary is just to contain at least 1 sort (s) of these.
If any element exceeds a predetermined amount, machinability is saturated, hot workability is deteriorated, impact value and specular polishing are deteriorated.

  According to the present invention as described above, there are provided a mold steel and a mold that can satisfy all the characteristics of hardness, corrosion resistance, annealing, difficulty of precipitation of grain boundary carbides, and difficulty of precipitation of pearlite. be able to.

It is the figure which showed the influence of Si amount which acts on machinability. It is the figure which showed the influence of the amount of Mn which has on pearlite precipitation. It is the figure which showed the influence of the amount of Ni which has on pearlite precipitation. It is the figure which showed the influence of Mo amount which acts on delta ferrite precipitation.

The 20 steel types shown in Table 1 were subjected to tests to evaluate the quenching and tempering hardness, corrosion resistance, annealing, difficulty of precipitation of grain boundary carbides, and difficulty of precipitation of pearlite.
The five types of comparative steels are used for applications that require hardness and corrosion resistance. Comparative steel 1 is JIS SUS420J2, comparative steel 2 is JIS SUS403, comparative steel 3 is JIS SUH1, and comparative steel 4 is JIS. SUH600 and comparative steel 5 are steels sold in the city.
The materials of 20 steel types shown in Table 1 were manufactured by the following procedure. First, molten steel was cast into a 50 kg ingot, and then homogenized at 1240 ° C. And it was finished in the shape of a bar with a rectangular cross section of 60 mm × 45 mm by hot forging. Subsequently, normalizing by heating to 1020 ° C. and quenching and tempering by heating at 670 ° C. were performed. Furthermore, after heating to 860 degreeC or 930 degreeC, it annealed by annealing slowly at 30 degreeC / Hr. Test pieces were cut out from the steel bars and used for various tests.

<Hardening and tempering hardness>
A 15 mm × 15 mm × 25 mm block was used as a test piece. The block was heated to 1030 ° C. and held for 60 minutes, then rapidly cooled at 100 ° C./min and baked, and then tempered at 470 to 520 ° C. for 2 hours. The maximum hardness obtained in this tempering temperature range was evaluated. The quenching and tempering hardness is preferably 45 HRC or more in order to ensure wear resistance.
The results are as shown in Table 2. Since the comparative steel 2 and the comparative steel 4 are low C, a hardness of 45 HRC or higher cannot be obtained, but all other steel types are 45 HRC or higher. In other words, for the inventive steel, 45 HRC or more necessary for ensuring wear resistance is obtained. Of course, the hardness can be lowered by adjusting the tempering conditions.

<Corrosion resistance>
About the test piece, what evaluated the said quenching tempering hardness was diverted. The specimen after the hardness measurement was mirror-polished, the rusting state after being exposed to an environment of 98% humidity and 50 ° C. for 24 hours was photographed, and the area ratio of rust was evaluated by image processing. If the area ratio of rust was 1% or less, the test was “O”, and if the area ratio of rust was more than 1%, the test was rejected and “X”.
The results are as shown in Table 2. Since the comparative steel 3 is high C and low Cr, its corrosion resistance is poor. Other comparative steels and inventive steels have high corrosion resistance due to high Cr.
From the results of quenching and tempering hardness and corrosion resistance shown in Table 2, it was confirmed that the invention steel 15 type, comparative steel 1, and comparative steel 5 have both high hardness and high corrosion resistance. Even if the mold is actually manufactured, these steel types are considered to exhibit high hardness and high corrosion resistance if the heat treatment conditions are appropriate.

<Annealing>
A 15 mm × 15 mm × 25 mm block was used as a test piece. The block was heated to 860 ° C. (Comparative Steel 2 / Comparative Steel 3. Comparative Steel 4) or 930 ° C. (other steel types) and held for 240 minutes, then cooled to 650 ° C. at 30 ° C./Hr, and then released. It was cold. Then, the HRB hardness of the test piece was measured, and it was confirmed whether it was softened to the hardness which can be machined easily. If it was 100HRB or less, it was determined to be “◯” if it passed, and if it exceeded 100HRB, it was rejected, indicating “x”.
The results are as shown in Table 3. The comparative steel 3 and the comparative steel 5 have a hardness after annealing exceeding 100 HRB, and are not sufficiently softened and are “x”. Since the comparative steel 3 has high Si, the solid solution hardening contributes greatly, and the hardness is high even after annealing. Since the comparative example 5 was high Ni and good hardenability, it did not become a structure of spheroidized carbide and ferrite, but became bainite, and thus became high hardness.
With regard to the comparative steel 3 and the comparative steel 5, there is a high possibility that the tool life will be shortened or the processing efficiency will be reduced by roughing the mold even during the actual mold manufacture.
On the other hand, for other steel types including the inventive steel, the hardness after annealing is 100 HRB or less, and it is considered that such a problem does not occur.

<Difficulty of precipitation of grain boundary carbide>
This was evaluated by an experiment simulating the factory manufacturing process. A block of 15 mm × 15 mm × 25 mm was used as a test piece, and conditions according to hot working and intermediate heat treatment were given to this block.
As condition A where grain boundary carbides are most likely to precipitate, heating at 1240 ° C. and cooling at 2 ° C./min are given, and heating at 1020 ° C. and cooling at 2 ° C./min are given as simulation of normalization, and tempering from 100 ° C. To 670 ° C. and cooled to room temperature.
Further, as Condition B in which the grain boundary carbide hardly precipitates, heating at 1240 ° C. and cooling at 50 ° C./min are given, and heating at 1100 ° C. and cooling at 50 ° C./min are given as simulation of normalization. Heated to 670 ° C. by tempering and cooled to room temperature.
The metal structures of these two-level test pieces were photographed and the degree of precipitation of grain boundary carbides was evaluated. Specifically, it is calculated what percentage of the grain boundary length is covered by the carbide, and if it is 30% or less, the precipitation of the grain boundary carbide is considered to be slight, and “O” exceeds 30%. If this is the case, it will be rejected as “x” because the carbides will be conspicuous.

The results are as shown in Table 4. In the high C comparative steel 1 and the comparative steel 3, film-like or point-sequence-like carbides are remarkably observed at the grain boundaries, and the result is “x”. This tendency is remarkable in the high Cr comparative steel 1.
In the steel of the judgment “x”, a considerable amount of carbide exists in the grain boundary even under the condition B in which the grain boundary carbide hardly precipitates. In comparative steel 1 and comparative steel 3, grain boundary carbides become conspicuous even in the actual mold manufacturing process, and there is concern about deterioration of mirror surface polishing and cracking during use of the mold.
On the other hand, for other steel types including the invention steel, the precipitation is slight even under the condition A where the grain boundary carbide is likely to precipitate, and the degree of precipitation of the grain boundary carbide is judged to be slight even in the actual mold. . That is, it is considered that the mirror polishability is lowered and the risk of cracking is low.

<Difficult to deposit pearlite>
A specimen cut into φ4 mm × 10 mm was used as a test piece, heated to 1030 ° C. and held for 60 minutes, and then cooled to room temperature at various cooling rates (9 to 80 ° C./min). After cooling, the test piece was cut and the cross-sectional metal structure was observed at a magnification of 400 times to confirm the presence or absence of pearlite precipitation. If pearlite did not precipitate, it was accepted as “◯”, and even if slight precipitation was observed, it was rejected as “x”.
The results are as shown in Table 5. The comparative steel 1 and the comparative steel 3 are “x” when the quenching cooling rate is low. The quenching and cooling rate inside the mold is 10 to 40 ° C./min in the temperature range of 550 to 850 ° C. where pearlite is precipitated, so that pearlite is precipitated inside the mold using comparative steel 1 or comparative steel 3. Becomes inevitable, and the mirror polishability deteriorates. There is also an increased risk of destruction while the mold is in use.
On the other hand, with regard to other steel types including the inventive steel, it can be determined that pearlite does not precipitate at any cooling rate, and pearlite does not precipitate even when the mold is actually quenched.

Table 6 summarizes the above evaluation results.
The comparative steel 2, the comparative steel 3, and the comparative steel 4 have difficulty in any of the basic performances of high hardness and high corrosion resistance.
The comparative steel 1 can be judged to be easy to precipitate grain boundary carbides and pearlite in a particularly large mold, and there is a problem that the mirror polishability deteriorates and the risk of cracking increases.
The comparative steel 5 has difficulty in annealing and has a problem that the tool life and productivity in machining are reduced. As described above, each comparative steel has a problem in at least one item.
On the other hand, the invention steel type 15 has no problem in any item. Invented steel has the basic properties of high hardness and high corrosion resistance, and has annealing properties, difficulty in precipitation of grain boundary carbides, and difficulty in precipitation of pearlite. Therefore, even in an actual mold, in addition to high hardness and high corrosion resistance, it can be expected to exhibit high specularity and difficulty in cracking.

Although the embodiment of the present invention has been described in detail above, this is merely an example.
For example, it is also effective to use the steel of the present invention after being subjected to surface shot blasting, nitriding treatment, PVD treatment, CVD treatment, plating treatment or other surface modification treatment.
It can also be applied to powders and plates used to make molds by additive molding of powders and plates, and can be used for repairing welding of die bodies and parts as rods, etc. The present invention can be implemented with various modifications.

Claims (8)

0.200 <C <0.380 by mass%
0.40 <Si <1.50
0.50 <Mn <1.50
0.40 <Ni <1.40
10.50 <Cr <12.50
0.01 <Mo <0.29
0.002 <V <0.32
0.001 <N <0.200
0.230 <C + N <0.460
And the balance has a composition of Fe and unavoidable impurities. In Claim 1, In mass%, 0.10 <W <5.00
0.10 <Co ≦ 4.00
A mold steel characterized by further containing at least one of the following. In any one of Claim 1, 2, 0.004 <Nb <= 0.100 in the mass%.
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
A mold steel characterized by further containing at least one of the following. In any one of Claims 1-3, 0.10 <Al <1.20 in the mass%
Furthermore, it contains steel for metal mold | die characterized by the above-mentioned. In any one of Claims 1-4, 0.30 <Cu <= 3.0 by mass%
Furthermore, it contains steel for metal mold | die characterized by the above-mentioned. In any one of Claims 1-5, 0.0001 <B <= 0.0050 in the mass%
Furthermore, it contains steel for metal mold | die characterized by the above-mentioned. In any one of Claims 1-6, 0.003 <S <= 0.050 in the mass%.
0.0005 <Ca ≦ 0.2000
0.03 <Se ≦ 0.50
0.005 <Te ≦ 0.100
0.01 <Bi ≦ 0.50
0.03 <Pb ≦ 0.50
A mold steel characterized by further containing at least one of the following.   A mold made of the steel according to any one of claims 1 to 7.

Images