JP2006002237A - Steel for plastic mold excellent in texturability and machinability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for plastic molds which has both high machinability and high texturability. <P>SOLUTION: The steel comprises, by mass%, over 0.25 to 0.40% C, 0.05 to less than 0.50% Si, over 0.9 to 1.5% Mn, up to 0.01% P, 0.02 to less than 0.08% S, 0.05 to 0.30% Ni, 0.30 to less than 1.35% Cr, 0.02 to less than 0.50% Mo, 0.02 to 0.30% V, 0.02 to 0.10% Zr, and less than 0.035% Al, with the balance being Fe and impurities, and is characterized in that K is up to 0.63, Ceq is 0.72 to 0.85, (wherein K and Ceq are represented by the following formulae), and concerning sulfide inclusions larger than 50 μm<SP>2</SP>in size, the inclusions with an aspect ratio of at most 3.8 exist 2.5 pieces per mm<SP>2</SP>, and the inclusions with an aspect ratio of at least 7.0 exist 4.0 pieces per mm<SP>2</SP>. K=0.01(C%)+0.60(Si%)+0.12(Mn%)+3.5(P%)+4.10(S%)-0.19(Mo%)+0.10(Cr%)-0.10(Ni%). Ceq=(C%)+(Si%)/10+((Mn%)-32/55×(S%))/5+(Ni%)/23+(Cr%)/9+(Mo%)/2+(V%)/2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steel for plastic molds that has good machinability and is excellent in texture processing.

  Since the manufacturing cost of the plastic mold accounts for a large proportion of the machining cost of the mold, workability, that is, machinability is required. Conventionally, carbon steel for machine structure such as S55C and alloy steel for machine structure such as SCM440 have been used as mold steel, but in recent years, further reduction in mold production cost has been demanded. Various steel types with improved machinability have been proposed.

For example, in JP-A-8-13088 (Patent Document 1), an appropriate amount (0.01 to 0.10%) of S is added under C: 0.05 to 0.20% to reduce Si. Thus, a steel which has improved the machinability and prevented the occurrence of irregularities by reducing the difference in hardness between the weld heat affected zone and its surroundings is disclosed in Japanese Patent No. 3141735 (Japanese Patent Laid-Open No. 9-49067). No.) gazette (Patent Document 2) discloses a steel whose machinability is improved by adding Si, which has been considered harmful to the machinability, as high as 0.5 to 2.5%. No. -12196 (Patent Document 3) discloses a steel in which S is added in a large amount of 0.06 to 0.20% and the Mn / S ratio is specified to 10 to 14 to prevent deterioration of toughness. In JP 2000-303140 A (Patent Document 4), the steel of Patent Document 1 is S While decreasing to 0.02 to 0.04%, increase C to more than 0.20 to 0.30% to increase the amount of pearlite and add 1.30 to 1.50% Cr to ensure hardenability Steel has been disclosed.
JP-A-8-13088 Japanese Patent No. 3141735 JP-A-10-121196 JP 2000-303140 A

However, in recent years, not only is machinability improved, but the surface of the mold is often subjected to hard embossing in order to improve the surface quality of the molded product and promote mold release. There has been a demand for excellent grain-workability for steel.
Even in the conventional technology, the reduction in the unevenness of the embossing process has been achieved to some extent by the reduction in Si, but the excellent machinability and the embossing processability have not yet been combined. That is, in the steel of Patent Document 1, since the amount of C is small, it is indispensable to add various alloy elements to secure the strength, and sufficient embossing workability cannot be obtained for segregation of these alloy elements. Moreover, in steel of patent document 2, in order to improve machinability by high Si formation, there exists a problem in embossing workability, and it cannot be said that sufficient embossing workability is obtained. Further, the steel of Patent Document 3 has a large amount of S, and large MnS is easily stretched in the direction of pressure forging, so that a good texture is not obtained. In addition, although the steel of Patent Document 4 is intended to improve the texture processing by reducing Si, the amount of alloying elements such as Cr is large in order to ensure hardenability, and sufficient texture processing is obtained. It is not done.
This invention is made | formed in view of this problem, and it aims at providing the steel for plastic molds which has the outstanding machinability and the embossing workability.

  As the amount of alloying elements increases, microsegregation at the time of solidification increases, and as a result, it has been reported that microsegregation does not disappear completely even when forging / rolling is performed, and that uneven embossing tends to occur. It was. However, as elements that promote segregation, S, P, Si for generating MnS, and Cr, Mo when added more than necessary are known. However, when the present inventor conducted a detailed investigation on the segregation of the alloy elements, the other elements other than the above have an effect on the segregation, and Mo, which has been said to have an adverse effect on the segregation, is rather segregated. It was found that wrinkle workability was improved by reducing Furthermore, in order to improve the machinability, it is important that large and spherical sulfide inclusions are present in a predetermined amount or more. On the other hand, even if the inclusions are large, As with segregation, it was found that the textured workability was lowered. The present invention has been completed based on such knowledge.

That is, the plastic mold steel of the present invention is mass%,
C: more than 0.25%, 0.40% or less,
Si: 0.05% or more, less than 0.50%,
Mn: more than 0.9%, 1.5% or less,
P: 0.01% or less,
S: 0.02% or more, less than 0.08%,
Ni: 0.05% or more, 0.30% or less,
Cr: 0.30% or more, less than 1.35%,
Mo: 002% or more, less than 0.50%,
V: 0.02% or more, 0.30% or less,
Zr: 0.02% or more, 0.10% or less,
Al: less than 0.035%, the balance is Fe and inevitable impurities, and when K and Ceq are expressed by the following formula, K: 0.63 or less, Ceq: 0.72 or more, 0.85 or less Satisfied,
Sulfide type having a size of 50 μm ² or more and an aspect ratio of 3.8 or less of sulfide inclusions A of 2.5 pieces / mm ² or more, a size of 50 μm ² or more and an aspect ratio of 7.0 or more Inclusion B is 4.0 pieces / mm ² or less.
K = 0.01 (C%) + 0.60 (Si%) + 0.12 (Mn%) + 3.5 (P%)
+ 4.10 (S%) -0.19 (Mo%) +0.10 (Cr%) -0.10 (Ni%)
Ceq = (C%) + (Si%) / 10 + ((Mn%) − 32/55 × (S%)) / 5
+ (Ni%) / 23+ (Cr%) / 9+ (Mo%) / 2+ (V%) / 2

In the plastic mold steel,
C: more than 0.25%, 0.35% or less,
S: 0.03% or more, less than 0.05%,
Ni: 0.10% or more, 0.20% or less,
Cr: 0.50% or more, less than 1.0%,
Mo: 005% or more, 0.25% or less,
V: 0.05% or more, 0.30% or less,
Al: less than 0.005% K: 0.57 or less,
Ceq: preferably 0.74 or more and 0.81 or less, or more, sulfide inclusions A at 3.5 pieces / mm ² or more and sulfide inclusions B at 3.0 pieces / mm ² The following is preferable.

The plastic mold steel according to the present invention has a K value obtained by quantifying the segregation tendency of the constituent elements under a predetermined component of 0.63 or less, and a Ceq value obtained by quantifying the increase tendency of the hardness (strength) of the component. The number of spherical inclusions having an aspect ratio of 3.8 or less is specified for sulfide type inclusions having a size of 50 μm ² or more, which are specified in .72 to 0.85 and further affect the machinability and texture processing. Since the number of stretched inclusions having an aspect ratio of 7.0 or more is 4.0 pieces / mm ² or less at 5 pieces / mm ² or more, both the machinability and the texture processing are excellent.

First, the reasons for limiting the components (mass%) of the plastic mold steel of the present invention will be described.
C: more than 0.25%, 0.40% or less C is an element necessary for ensuring strength. In order to improve machinability, it is effective to reduce the amount of C. However, if the amount of C is reduced, the strength decreases, and it is necessary to increase the amount of alloy element added to ensure the required strength. As a result, the embossing processability decreases. That is, if the C content is 0.25% or less, it is difficult to ensure the strength even if alloying elements are added within a range that does not impair the machinability, while if it exceeds 0.40%, the machinability decreases. For this reason, the lower limit of the C amount is 0.25% (excluding 0.25%), and the upper limit is 0.40%, preferably 0.35%.

Si: 0.05% or more, less than 0.50% Si needs to be added as a strength improving element and 0.05% or more as a deoxidizing element. As a result, the embossing processability decreases. For this reason, the lower limit of the Si content is 0.05%, preferably 0.10%, and the upper limit is 0.50% (not including 0.50%), preferably 0.40% or less.

Mn: more than 0.9%, 1.5% or less Mn is an important element from the viewpoint of deoxidation and securing strength, and its action is insufficient at 0.9% or less, while segregation is performed at more than 1.5%. The tendency becomes remarkable, and the embossing process is deteriorated by promoting the formation of the band structure. For this reason, the lower limit of the amount of Mn is 0.9% (not including 0.9%), preferably 1.0%. On the other hand, the upper limit is 1.5%, preferably 1.4%.

P: 0.01% or less P is an impurity element and has a tendency to segregate, so that it is preferable that the amount is as small as possible because it reduces the embossing workability.

S: 0.02% or more and less than 0.08% S is an element effective for improving machinability. For this reason, the lower limit of the S amount is 0.02%, preferably 0.03%, and the upper limit is 0.08% (not including 0.08%), preferably 0.05. % Is good.

Ni: 0.05% or more, 0.30% or less Mo: 0.02% or more, 0.50% or less Ni and Mo are effective elements that improve the hardenability without deteriorating the texture. When Ni is less than 0.05% and Mo is less than 0.20%, the effect is too small. On the other hand, when Ni exceeds 0.30%, the effect of addition is saturated, resulting in high costs, and when Mo is 0.50% or more, machinability is reduced. To come. Therefore, the lower limit of Ni is 0.05%, preferably 0.10%, and the upper limit is 0.30%, preferably 0.20%. The lower limit of Mo is 0.02%, preferably 0.05%, and the upper limit is 0.50% (excluding 0.50%), preferably 0.25%. It is good.

Cr: 0.30% or more and less than 1.35% Cr is an important element from the viewpoint of securing strength. However, if it is too much, segregation tendency becomes remarkable and the embossing workability deteriorates. For this reason, the lower limit of the Cr content is 0.30%, preferably 0.50%, while the upper limit is 1.35% (excluding 1.35%), preferably 1.0. % (Excluding 1.0%).

V: 0.02% or more, 0.30% or less V is not only an element indispensable for improving the strength, but also an element having a small decrease in machinability for the improvement in strength. If it is less than 0.02%, such an action is too small. On the other hand, even if it exceeds 0.30%, the action becomes saturated, resulting in high raw material costs. For this reason, the lower limit of the V amount is 0.02%, preferably 0.05%, while the upper limit is 0.30%, preferably 0.20%.

Zr: 0.02% or more, 0.10% or less Zr has the effect of spheroidizing MnS inclusions, thereby improving the machinability and simultaneously reducing the expanded MnS. Also improves. If it is less than 0.02%, such an action is too small. On the other hand, if it exceeds 0.10%, the action is saturated, resulting in high raw material costs. For this reason, the lower limit of the amount of Zr is 0.02%, preferably 0.04%, while the upper limit is 0.10%, preferably 0.08%.

Al: less than 0.035% Al may be added as a deoxidizing element. However, the addition of Al causes the oxide inclusions to become hard alumina oxides, resulting in a decrease in machinability. Furthermore, the extension which affects the production | generation of MnS is accelerated | stimulated and the machinability and the embossing workability fall. For this reason, the smaller the amount, the better, but in the present invention, addition of less than 0.035% is allowed. Preferably it is good to stop at less than 0.005%.

K: 0.63 or less K = 0.01 (C%) + 0.60 (Si%) + 0.12 (Mn%) + 3.5 (P%)
+ 4.10 (S%) -0.19 (Mo%) +0.10 (Cr%) -0.10 (Ni%)
The K value is a scale representing the influence of alloy elements on microsegregation during solidification. When this value exceeds 0.630, the segregation tendency becomes prominent as will be apparent from the examples described later, and the embossing workability deteriorates. It becomes like this. For this reason, the upper limit of the K value is 0.63, preferably 0.57. The equation for defining K is an equation obtained from the knowledge that the reverse V segregation density of the killed steel ingot has a correlation with the average buoyancy acting on the residual molten steel in the coexisting solid solution phase (Hida et al. Based on "Steel", Vol. 67, 1981, p954-958), the coefficient was corrected by experiments by the inventors.

Ceq: 0.72 or more, 0.85 or less Ceq = (C%) + (Si%) / 10 + ((Mn%) − 32/55 × (S%)) / 5
+ (Ni%) / 23+ (Cr%) / 9+ (Mo%) / 2+ (V%) / 2
The Ceq value shows the influence on the hardness of the alloying element in the steel in terms of the amount of C. As is clear from the examples described later, when the Ceq is too low, less than 0.720, it is stable. If the required hardness (Hv 190 to 210) cannot be satisfied, and if it is too high (over 0.850), even if the tempering temperature is increased, the hardness will be over Hv 210, resulting in a portion that is too high. And wrinkle processability are reduced. For this reason, the lower limit of Ceq should be 0.72, preferably 0.74, while the upper limit should be 0.85, preferably 0.81. Hardness and strength have a correlation, and in the steel of the present invention, the required strength is satisfied by setting the hardness to Hv 190 to 210. The equation of Ceq is an empirical formula well known to those skilled in the art.

  The constituent components of the steel of the present invention are as described above, and consist of the balance Fe and inevitable impurities. In addition to the above-described components, Cu, Pb, Bi, REM, Nb, and Ti may be added in appropriate amounts (about 4% or less in total) as elements that improve machinability and grain workability. In the above components, Zr is cited as a form control element for sulfide inclusions, but Ca, Mg, Te, Se, etc. are also conceivable. However, it is necessary to be cautious about Ca because it tends to form large oxides and may reduce the texture and fatigue strength.

The structure of the steel of the present invention is basically a two-phase structure of ferrite and pearlite. According to experiments by the present inventor, large inclusions having a size of 50 μm ² or more among sulfide inclusions present in the structure It was found that the effect on the machinability and the embossing workability was particularly great.

In order to improve machinability, it is effective that a large number of large and spherical sulfide inclusions exist. In the present invention, as will be apparent from the examples described later, spherical sulfide having an aspect ratio of 3.8 or less is effective. The number of physical inclusions A is 2.5 pieces / mm ² or more, preferably 3.5 pieces / mm ² or more. On the other hand, if there are a large number of large and expanded sulfide inclusions, the machinability of the stretched portion and the surrounding portion will be different, so that the embossing processability is deteriorated. For this reason, as will be apparent from the examples described later, the number of spherical sulfide inclusions B having an aspect ratio of 7.0 or more is 4.0 pieces / mm ² or less, preferably 3.0 pieces / mm ² or less. To do.

  The sulfide inclusions are mainly Mn sulfides represented by MnS, but Zr, Ca, Mg, Te, Se sulfides are also targeted. These composite sulfides may also be used. Or, further, other elements including Fe, Cr, Ti, and O, which are known to be contained in sulfides, may be used.

In producing the steel for plastic molds of the present invention, a steel having a predetermined component is melted, the cast piece is subjected to hot forging, then subjected to normalization as necessary, and the hardness is set to Hv 190 to 210. Tempering is performed to adjust to. In particular, in order to control the size of sulfide inclusions, the action of Zr is large in terms of components, but in production, the cooling rate during casting, the forging temperature of hot forging, and the forging pressure ratio are controlled. It is effective. In addition, the size can be controlled by controlling the deoxidizing element.
A typical manufacturing method is as follows. In an electric furnace, raw materials are charged and the components are adjusted, and then ladle refining and vacuum processing are performed. The main purpose of ladle refining and vacuum treatment is to reduce oxide inclusions harmful to machinability. Thereafter, it is cast into a steel ingot. The casting speed is effective for controlling the size of MnS. For increasing the size, the casting speed is slow, for example, 15 ° C./min or less, preferably about 10 ° C./min or less. After casting, after heating and holding at 1100 ° C. or higher, forging is performed. Increasing the heating temperature is effective for increasing the size of MnS. Moreover, in heating, it is necessary to make the temperature sufficiently rise to the inside of the steel ingot. Furthermore, reheating during the forging process is also effective for making MnS into a large spheroid. The forging ratio at the time of forging varies depending on the required specifications of the product, and in order to ensure the internal quality, a forging pressure ratio of a certain level or more is necessary, but for increasing the size of MnS, it is not increased more than necessary. It is desirable to do so. For example, a forging pressure ratio of about 2 to 4, preferably about 2 to 3 is recommended. In addition, the addition of forging from the opposite direction is effective in ensuring large spheroidization by suppressing the expansion of MnS while ensuring the internal quality.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limitedly interpreted by this Example.

The components shown in Table 1 were melted in an experimental vacuum induction heating furnace, solidified by controlling the average cooling rate at the time of casting, and the obtained cast piece was hot forged into a 100 mm × 100 mm square. Table 2 also shows the average cooling rate during casting, the heating temperature in hot forging, and the average forging pressure ratio.
The obtained forged piece was subjected to normalization at 880 ° C. × 2 h, and then tempered. At this time, the tempering temperature T and the cooling rate CR after tempering were adjusted within a standard range (T: 580 to 640 ° C., CR: 1 to 5 ° C./min).

Using the steel material produced as described above, a structure observation test piece, an embossing test piece, and a cutting test piece were collected from a portion 25 mm deep from the surface.
Tissue observation was performed using EPMA having an image analysis function for the purpose of observing the size and form of sulfide inclusions. The exposed area per sample was about 120 mm ² , the area per inclusion, the major axis and the minor axis were measured, and the aspect ratio (major axis / minor axis) was determined. Table 2 shows the number of large-diameter sulfides having a size of 50 μm ² or more and having an aspect ratio of 3.8 or less and A having an aspect ratio of 7.0 or more.
In the cutting test, a milling test using a carbide tool was performed, and the cutting length at which the tool wear amount was 2 mm was evaluated as the tool life. The cutting conditions were a down cut with a rotation speed = 1050 rpm, a feed rate = 250 mm / min, a cutting depth = 5 mm, and a cutting width = 15 mm.
Wrinkle processability is evaluated by polishing the test piece to paper # 2000, etching with ferric chloride aqueous solution, and measuring the surface roughness using a stylus type roughness measuring machine. did. The measurement results Ra (nm) are also shown in Table 2.

From Tables 1 and 2, the invention examples satisfying the conditions for the number of components and sulfides according to the present invention are excellent in both machinability and texture. However, even if the component conditions are satisfied, Samples Nos. 32 and 34 whose production conditions are inappropriate are not satisfied with the number of sulfides, the cutting lengths are 13.8 m and 13.6 m, and the machinability is reasonable. However, the embossing processability was remarkably deteriorated.

Claims (3)

mass%
C: more than 0.25%, 0.40% or less,
Si: 0.05% or more, less than 0.50%,
Mn: more than 0.9%, 1.5% or less,
P: 0.01% or less,
S: 0.02% or more, less than 0.08%,
Ni: 0.05% or more, 0.30% or less,
Cr: 0.30% or more, less than 1.35%,
Mo: 002% or more, less than 0.50%,
V: 0.02% or more, 0.30% or less,
Zr: 0.02% or more, 0.10% or less,
Al: less than 0.035%, the balance is Fe and inevitable impurities, and when K and Ceq are expressed by the following formula, K: 0.63 or less, Ceq: 0.72 or more, 0.85 or less Satisfied,
Sulfide type having a size of 50 μm ² or more and an aspect ratio of 3.8 or less of sulfide inclusions A of 2.5 pieces / mm ² or more, a size of 50 μm ² or more and an aspect ratio of 7.0 or more Steel for plastic molds with inclusions B of 4.0 pieces / mm ² or less and excellent in texture and machinability.
K = 0.01 (C%) + 0.60 (Si%) + 0.12 (Mn%) + 3.5 (P%)
+ 4.10 (S%) -0.19 (Mo%) +0.10 (Cr%) -0.10 (Ni%)
Ceq = (C%) + (Si%) / 10 + ((Mn%) − 32/55 × (S%)) / 5
+ (Ni%) / 23+ (Cr%) / 9+ (Mo%) / 2+ (V%) / 2 A component according to claim 1, wherein
C: more than 0.25%, 0.35% or less,
S: 0.03% or more, less than 0.05%,
Ni: 0.10% or more, 0.20% or less,
Cr: 0.50% or more, less than 1.0%,
Mo: 005% or more, 0.25% or less,
V: 0.05% or more, 0.30% or less,
Al: less than 0.005%,
K: 0.57 or less,
The steel for plastic molds according to claim 1, which satisfies Ceq: 0.74 or more and 0.81 or less. The plastic mold steel according to claim 1 or 2, wherein the sulfide inclusion A is 3.5 pieces / mm ² or more and the sulfide inclusion B is 3.0 pieces / mm ² or less.