JP2009235562A - Steel for cold press die excellent in machinability, heat treatment dimensional change characteristic and impact characteristic, and press die - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a press die for various kinds of cold working as represented by press dies for high tension working of automobile parts for which machinability, heat treatment dimensional change characteristics, impact characteristics, and peeling resistance of surface treatment films are required, and a press die. <P>SOLUTION: There is provided the steel for the cold press die containing, by mass%, 0.80 to 0.89% C, 1.0 to <1.4% Si, 0.1 to 1.0% Mn, 0.030 to 0.070% S, 7.5 to 8.5% Cr, 0.05 to 0.2% Ni, 0.9 to 1.6% one or two kinds of Mo and W in terms of Mo+1/2W, 0.03 to 0.3% one or two kinds of V and Nb in terms of V+1/2 Nb, the balance Fe and inevitable impurities, and is excellent in the machinability, the heat treatment dimensional change characteristics, the impact characteristics. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to various cold working press dies, such as high-tensile press dies for automobile parts that require machinability, heat treatment sizing characteristics, impact characteristics, and surface treatment film peeling resistance. The present invention relates to steel and press dies.

  Conventionally, in press dies for automobiles, especially bending dies, punching dies, drawing dies, etc., with the expansion of high-tensile application to automobile parts, high C-high Cr-based materials such as JIS SKD11, which have high hardness and excellent wear resistance. Although cold die steel is used, SKD11 has poor machinability, and in the case of a deep drawing die that requires heavy cutting, reduction of the machining cost in die production has become an issue, and recently, SKD11 has improved the machinability of SKD11. The improved steel has been widely used.

  However, these dies are roughly processed and finished from the raw material to the mold shape, and then heat treatment (quenching and tempering) is performed, and assembly, trial driving, rework, and surface treatment are completed. Therefore, the efficiency of the reworking process by changing the size during assembly and the reworking process at the time of trial placement greatly contributes to cost reduction, so that zero heat treatment size change and machinability in a quenching and tempering state are required.

  Therefore, for example, the tool steel disclosed in Japanese Patent Application Laid-Open No. 2007-197746 (Patent Document 1) is Ni containing Si: 1.40 to 2.6% and Cu: 0.01 to 0.50%. It is Cr, Mo, V steel, and cold die steel disclosed in JP 2006-152356 A (Patent Document 2) is Ni: 0.3 to 1.5%, Cu: 0.1 The cold die steel disclosed in Japanese Patent Laid-Open No. 2006-169624 (Patent Document 3) is Cr, Mo steel containing ~ 1.0% and Al: 0.1-0.7%. , Ni: 0.3-1.5%, Cu: 0.1-1.0%, Al: Cr, Mo steel containing 0.1-0.5%, observed in its cross-sectional structure This regulates the distribution of carbides.

  Further, the cold working mold disclosed in Japanese Patent Application Laid-Open No. 2006-28584 (Patent Document 4) has Ni: 0.3 to 1.5%, Cu: 0.1 to 1.0%, Al : Cr, Mo steel containing 0.1 to 0.7%, and cold tool steel disclosed in Japanese Patent Application Laid-Open No. 2007-77442 (Patent Document 5) is Cr: 9 to 12% Cr, Mo, Ni steel containing carbon, and carbides observed in the cross-sectional structure thereof are regulated.

Furthermore, according to the example, the tool steel disclosed in Japanese Patent Laid-Open No. 2001-107181 (Patent Document 6) is Cr, Mo steel containing C: 0.43 to 0.81%, and C And the amount of Cr are regulated. Moreover, the cold tool steel currently disclosed by Unexamined-Japanese-Patent No. 11-92971 (patent document 7) is C: 1.10-1.35%, Cr: Cr containing 9.00-12.00% , Mo, and V steels, each of which restricts the amount of C and Cr, has been proposed.
JP 2007-197746 A JP 2006-152356 A JP 2006-169624 A JP 2006-28584 A JP 2007-77442 A JP 2001-107181 A Japanese Patent Laid-Open No. 11-92871

  Patent Document 1 described above is a Ni, Cr, Mo, V steel containing Si: 1.40 to 2.6% and Cu: 0.01 to 0.50%, where Si is too high and the toughness is high. There is a problem that it decreases, and Cu is an essential element. Patent Documents 2 and 3 are Cr and Mo steels containing Ni: 0.3 to 1.5%, Cu: 0.1 to 1.5%, Al: 0.1 to 0.7%, Ni And is an invention using Cu and Al as essential elements. Similarly, Patent Document 4 is a Cr and Mo steel containing Ni: 0.3 to 1.5%, Cu: 0.1 to 1.0%, Al: 0.1 to 0.7%. However, the addition of Ni, Cu, and Al causes problems such as a decrease in the amount of solid solution C, a sufficient quenching and tempering hardness cannot be obtained, and hot workability during production deteriorates. Further, the addition of Ni also deteriorates the machinability at the time of die machining.

  Patent Document 5 is Cr, Mo, Ni steel containing Cr: 9 to 12%, and there is a problem that Cr is too high and machinability and toughness are deteriorated. Moreover, patent document 6 is Cr, Mo steel containing C: 0.43-0.81%, Comprising: C is too low and abrasion resistance and hardness are inadequate. Furthermore, Patent Document 7 is a Cr, Mo, V steel containing Cr: 9.00 to 12.00%, and there is a problem that the machinability and toughness are deteriorated because Cr is too high.

  In order to solve the problems as described above, the inventors have made extensive developments, and as a result, the alloy element addition amount causing the heat treatment size change is optimized and the homogenization treatment is performed, so that By suppressing the heat treatment size change and improving the machinability after the heat treatment, it contributes to cost reduction in mold production. We also offer steel for cold press dies and press dies that have excellent impact characteristics, suppress cracking after use of the dies, and contribute to life improvement, machinability, heat treatment sizing characteristics, and impact characteristics. To do.

The gist of the invention is that
(1) By mass%, C: 0.80 to 0.89%, Si: 1.0 to less than 1.4%, Mn: 0.1 to 1.0%, S: 0.030 to 0.070 %, Cr: 7.5 to 8.5%, Ni: 0.05 to 0.2%, one or two of Mo and W being Mo + 1 / 2W: 0.9 to 1.6%, V One or two of Nb and V + 1 / 2Nb: 0.03 to 0.3% are contained, and the balance is excellent in machinability, heat treatment sizing characteristics, and impact characteristics comprising Fe and inevitable impurities. Steel for cold press dies.

(2) In addition to the steel described in (1) above, Cr / C: 8.5 to 10.5, (Mo + 1 / 2W / (V + 1 / 2Nb): 5.5 to 15.5 Steel for cold press dies with excellent machinability, heat treatment sizing characteristics, and impact characteristics.
(3) In addition to the steel described in (1) or (2) above, (Si + S × 100) / [(V + ½Nb) × 10]: 2.5 to 10.0 Steel for cold press dies with excellent machinability, heat treatment sizing characteristics, and impact characteristics.

(4) The steel according to any one of (1) to (3) above is subjected to forging at 1075 to 1150 ° C. at 1075 to 1150 ° C. by subjecting the steel according to any one of the steel ingot states to forging by a forging from the steel ingot state in the ingot width direction A steel for cold press dies excellent in machinability, heat treatment sizing characteristics, and impact characteristics, characterized by being forged or rolled after being kept soaked.
(5) A cold press die that uses the steel according to any one of (1) to (4) and is subjected to heat treatment or surface treatment after cutting.

  As described above, according to the present invention, the cutting time at the time of manufacturing the automobile press die, the reduction of the cutting tool, the reduction of the reworking time due to the size change at the time of the heat treatment, the reduction of the maintenance cost by the improvement of the die life, and the Mo , It is possible to reduce the alloy cost by reducing the V alloy element, and to achieve an extremely excellent effect.

Hereinafter, the reasons for limiting the component composition according to the present invention will be described.
C: 0.80 to 0.89%
C is an element for ensuring the hardness by dissolving in the matrix during heat treatment, and 0.80% or more is necessary to obtain the effect. Moreover, it combines with alloy elements such as Cr, Mo, V, W and Nb to form carbides and contributes to the improvement of secondary hardening and wear resistance. However, if added too much, the carbides become coarse and the impact characteristics are remarkably increased. Since it deteriorates, the upper limit was made 0.89%.

Si: 1.0 to less than 1.4% Si is added as a deoxidizer during steelmaking and is also effective for hardenability. Furthermore, in the present invention, the machinability is improved by improving the cutting tool life by suppressing adhesion by the Belag effect that embrittles the base and lowers the cutting resistance, and makes the constituent cutting edge (oxide layer) in the cutting tool edge. For that purpose, 1.0% or more is necessary. However, if it exceeds 1.4%, the impact characteristics are remarkably lowered, so the upper limit was made less than 1.4%.

Mn: 0.1 to 1.0%
Mn is added as a deoxidizer in the same manner as Si, and is also effective for hardenability. Furthermore, in the present invention, sulfide MnS is formed to improve machinability, and it is necessary to add 0.1% or more in order to obtain a necessary amount of sulfide. However, if added too much, the machinability and impact properties are degraded, so the upper limit was made 1.0%.

S: 0.030 to 0.070%
S is required to be added in an amount of 0.030% or more in order to form MnS and improve machinability. However, if added too much, impact characteristics deteriorate, so the upper limit was made 0.060%.
Ni: 0.05-0.2%
Since Ni dissolves in the matrix and improves toughness, it is necessary to add 0.05% or more. However, in order to inhibit the machinability in the annealed state in order to increase the annealing hardness, the upper limit is made 0.2%.

Cr: 7.5 to 8.5%
Cr is required to be 7.5% or more in order to improve the hardenability by forming a solid solution in the base, and to form carbides and ensure wear resistance. However, Cr is likely to bond with C during solidification to form coarse primary carbide, which significantly deteriorates impact characteristics and machinability, so the upper limit was made 8.5%.

Mo + 1 / 2W: 0.9-1.6%
The reason why Mo + 1 / 2W is regulated is 0.9% or more in order to ensure hardenability, secure quenching and tempering hardness by secondary curing, and improve wear resistance by precipitated carbides. However, if it is added too much, it segregates during solidification and causes heat treatment size change and distortion, and the carbide becomes coarse and toughness and machinability deteriorate. In addition, a large amount of fine carbides are precipitated by secondary curing, and the amount of C in the matrix is reduced. As a result, martensitic transformation of the retained austenite is promoted, and the amount of heat treatment change during high temperature tempering is increased. 1.6%.

V + 1 / 2Nb: 0.03-0.3%
V + 1 / 2Nb was regulated because of improved wear resistance due to the dispersion and precipitation of fine and hard carbides, as well as crystal grains during diffusion and penetration surface treatment (TD treatment, etc.) applied to press dies at a high temperature for a long time. In order to suppress coarsening and maintain impact characteristics, 0.03% or more is necessary. However, if it is added too much, it segregates during solidification and causes heat treatment size change and distortion, and the carbide becomes coarse and toughness and machinability deteriorate. Furthermore, a large amount of fine carbides precipitates in the secondary curing, and the heat treatment dimensional change during high-temperature tempering increases, so the upper limit was made 0.3%.

Cr / C: 8.5 to 10.5
Cr / C was regulated by optimizing both the C-Cr amount and regulating the size of primary carbides that contribute to impact resistance, machinability and wear resistance, and the solid solution amount of carbon in the matrix. In addition, in order to combine the machinability, heat treatment sizing characteristics, and impact resistance by regulating the amount of precipitated carbides such as Mo and V that contribute to heat treatment sizing and wear resistance, 8.5 or more is required. is there. However, if it exceeds 10.5, the carbide is coarsened and the machinability and impact characteristics are remarkably deteriorated, so the range is set to 8.5 to 10.5.

(Mo + 1 / 2W) / (V + 1 / 2Nb): 5.5 to 15.5
By reducing Mo + 1 / 2W and V + 1 / 2Nb, carbide precipitation during high-temperature tempering is reduced, and decomposition of retained austenite is suppressed, so that expansion due to heat treatment size change can be reduced. Further, the forging and stretching heat treatment is intended to reduce the segregation of components and improve the heat treatment sizing characteristics and impact characteristics by making the carbide fine and uniform, and is achieved in a balance between the two. In order to obtain these characteristics, 5.5 or more is necessary. However, if it exceeds 15.5, it segregates at the time of solidification, causing heat treatment size change and distortion, and carbides become coarse and toughness and machinability deteriorate. In addition, a large amount of fine carbide precipitates by secondary curing, martensitic transformation is promoted, and the amount of heat treatment change during high temperature tempering is increased, so the upper limit was made 15.5.

(Si + S × 100) / [(V + 1 / 2Nb) × 10]: 2.5 to 10.0
Although machinability is improved by increasing Si and S, excessive addition deteriorates impact characteristics. Further, increasing V and Nb suppresses coarsening of crystal grains and improves impact characteristics, but machinability is remarkably deteriorated by hard fine precipitated carbides. By optimizing these balances, both machinability and impact properties are improved. That is, in order to obtain these characteristics, 2.1 or higher is required. However, if it exceeds 13.0, the toughness decreases.

The forging reduction area ratio is 25% or more. The forging reduction area ratio is calculated by (average cross-sectional area of forged products) / (average cross-section area of steel ingot) × 100%. The forging reduction rate was regulated by 25% in order to reduce component segregation that causes heat treatment size change, block internal defects in steel ingots, and disperse crystallized carbides that contribute to machinability and impact value. This is necessary. That is, by introducing a strain to the positive segregation part such as V segregation, columnar crystal zone and equiaxed crystal zone of the steel ingot, diffusion in the segregation part is promoted in the subsequent soaking, Is uniformly dispersed, so that the heat treatment size change is improved.

In order to diffuse the alloy components in the segregation part at 1075 to 1150 ° C. for 8 hours or more, it is necessary to maintain the soaking temperature at 1075 ° C. or more for 8 hours or more. However, if the holding temperature is too high, overheating occurs during rolling and forging, and internal defects are generated.

  As described above, in order to improve machinability, in addition to the conventional S addition, by increasing the amount of Si, the cutting resistance is reduced, the life of the cutting tool is improved by the Belag effect, and the heat treatment size change characteristics are improved. Therefore, by reducing Mo and V, carbide precipitation is reduced and decomposition of residual austenite is suppressed by high-temperature tempering, and component segregation is reduced by forge-stretching heat treatment. In addition, in order to improve the impact characteristics, carbide, Mo, and V are reduced by optimizing the amount of C—Cr.

Hereinafter, the present invention will be specifically described with reference to examples.
Steel of the composition shown in Table 1 was melted in a 100 kg vacuum induction melting furnace and then ingot in a mold, and this ingot was heated to 1100 ° C. and forged at a reduction in area of 28% in the steel ingot width direction. A flat bar material having a thickness of 35 mm and a width of 150 mm was formed by forging after holding at 1100 ° C. for 10 hours, and then a round bar material having a diameter of 32 mm was prepared as a test material. Table 1 shows the forge elongation reduction ratio and the temperature-time as heating conditions for each chemical component. Table 2 shows the heat treatment sizing ratio, quenching and tempering hardness (HRC), residual austenite amount, machinability evaluation, and Charpy impact value at each tempering temperature.

For heat treatment sizing evaluation, a test piece having a thickness of 25 mm, a width of 120 mm, and a length of 150 mm was used, followed by vacuum heat treatment (quenching: 1030 ° C., holding for 1 hour followed by pressurized gas cooling, tempering: 480 ° C., 500 ° C. Gas cooling after holding at 520 ° C. and 540 ° C. for 1 hour), and the heat treatment change rate is calculated from the measured dimensions before and after the heat treatment by the following formula.
Heat treatment change rate (%) = [(size after heat treatment−size before heat treatment) / size before heat treatment] × 100
In the present invention, the evaluation was performed based on the longitudinal change rate.

  As an evaluation of the quenching and tempering hardness (HRC), a block having a thickness of 10 mm, a width of 10 mm, and a length of 15 mm was treated under each heat treatment condition, then subjected to planar polishing, and the hardness was measured with a Rockwell C scale. The amount of retained austenite was measured by an X-ray diffraction method using a hardness measurement specimen.

As the machinability evaluation, a heat treatment sized test piece (520 ° C. tempered material) is used, and a drill workability test is performed on the quenched and tempered material. The test was performed using a cutting tool: KOBELCO carbide drill VC-SSS, diameter 1.5, cutting depth: 4.5 mm (3D), cutting speed 10 m / min (drilling speed 2122 rpm, feed rate 0.01 mm / rev.). Cutting oil: Continuous drilling was performed with an emulsion (Yushiloken EC50), and the number of drilled holes before drill breakage was evaluated.
In addition, the Charpy impact test was performed by collecting and producing a Charpy test piece (thickness 10 mm, width 10 mm, length 55 mm, 10R2 mmC notch) from the center of the 32 mm diameter material in the rolling direction, and vacuum heat treatment (quenching: 1030 ° C., After holding for 1 hour, cooling with pressurized gas and tempering: 500 ° C., after holding for 1 hour with gas cooling), a Charpy impact test was performed.

表１に示すように、Ｎｏ．１〜６は本発明例であり、Ｎｏ．７〜１３は比較例である。

As shown in Table 1, no. Nos. 1 to 6 are examples of the present invention. 7 to 13 are comparative examples.

  Comparative Example No. 7 has a high C and Cr content, a low Si and S content, and a short time as a heating condition. Therefore, the heat treatment dimensions at tempering temperatures of 520 ° C. and 540 ° C. are large, and the amount of retained austenite is high. Less, poor machinability and impact characteristics. Comparative Example No. No. 8 has a high C, Ni, Cr, and Mo + 1 / 2W content, a low Si content, and a high forge elongation reduction ratio, so that the heat treatment dimensions at tempering temperatures of 520 ° C. and 540 ° C. are large, and The amount of retained austenite is small, and the machinability, particularly the impact properties, is very poor.

  Comparative Example No. No. 9 has a high Ni, Mo + 1 / 2W, V + 1 / 2Nb content, a low forging elongation reduction ratio, and a short heating time, so that the heat treatment dimensions at tempering temperatures of 520 ° C. and 540 ° C. are large. In addition, the amount of retained austenite is small, and the machinability and impact properties are inferior. Comparative Example No. No. 10 has a high C, Mn, Cr content, a low Si, S, Mo + 1 / 2W content, and a low forge elongation reduction ratio, so that the heat treatment dimensions at tempering temperatures of 520 ° C. and 540 ° C. It is large and the amount of retained austenite is small, and the machinability, particularly the impact property, is poor.

  Comparative Example No. No. 11 has a high Ni, Mo + 1/2 W content and a low temperature as a heating condition, and therefore has a large heat treatment ratio at tempering temperatures of 520 ° C. and 540 ° C., and residual austenite at a tempering temperature of 520 ° C. The amount is small and inferior in machinability and impact characteristics. Comparative Example No. No. 12 has a low Ni and Cr content, a high V + 1 / 2Nb content, and a short heating time, resulting in a large heat treatment size at tempering temperatures of 520 ° C. and 540 ° C., and a residual austenite amount. Since there are many, it is inferior to an impact characteristic.

  Comparative Example No. No. 13 has a high C and Cr content and a short heating time, so the heat treatment size at tempering temperatures of 520 ° C. and 540 ° C. is large, and especially the amount of retained austenite at 540 ° C. is large. In addition, the impact characteristics are inferior. On the other hand, No. which is an example of the present invention. Since all of Nos. 1 to 6 satisfy the conditions of the present invention, it can be seen that they are excellent in characteristics such as heat treatment size change rate, machinability, impact characteristics, etc. by high-temperature heat treatment.

As described above, by increasing the Si content, the base is embrittled, an oxide layer is generated on the surface of the cutting tool, and adhesion of the work material is suppressed. Further, by reducing Mo and V, carbide precipitation at the time of high-temperature tempering can be reduced, and decomposition of residual austenite can be suppressed, so that expansion due to heat treatment deformation can be reduced. Furthermore, it has an extremely excellent effect of improving heat treatment sizing characteristics and impact characteristics by reducing component segregation by forge-stretching heat treatment and making the carbides fine and uniform.


Patent Applicant Sanyo Special Steel Co., Ltd.
Attorney: Attorney Shiina

Claims (5)

% By mass
C: 0.80 to 0.89%,
Si: 1.0 to less than 1.4%,
Mn: 0.1 to 1.0%,
S: 0.030 to 0.070%,
Cr: 7.5 to 8.5%
Ni: 0.05-0.2%
One or two of Mo and W are Mo + 1 / 2W: 0.9 to 1.6%, and one or two of V and Nb are V + 1 / 2Nb: 0.03 to 0.3% A steel for cold press dies that contains excellent balance of machinability, heat treatment sizing characteristics, and impact characteristics. In addition to the steel according to claim 1, Cr / C: 8.5 to 10.5, (Mo + 1 / 2W) / (V + 1 / 2Nb): 5.5 to 15.5 Steel for cold press dies with excellent machinability, heat treatment sizing characteristics, and impact characteristics. In addition to the steel according to claim 1 or 2, machinability and heat treatment size change, wherein (Si + S × 100) / [(V + 1 / 2Nb) × 10]: 2.5 to 10.0 Steel for cold press dies with excellent properties and impact properties. The steel according to any one of claims 1 to 3 is subjected to soaking at 1075 to 1150 ° C for 8 hours or more by applying a reduction of a reduction in area of 25% or more in the width direction of the ingot by forging from the ingot state. A steel for cold press dies excellent in machinability, heat treatment sizing characteristics and impact characteristics, characterized by being forged or rolled later. A cold press die using the steel according to any one of claims 1 to 3 and subjected to heat treatment or surface treatment after cutting.