JP2005171305A - Prehardend die steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide die steel having satisfactory die workability in a prehardend state in which the stabilization of a die life made compatible with workability can be attained by reducing its crack sensitivity. <P>SOLUTION: The prehardend die steel is obtained by subjecting steel comprising, by mass, 0.1 to 0.6% C, 0.1 to 1.5% Si, 0.1 to 0.7% Mn, 1.0 to 6.0% Cr, 0.05 to 1.2% Ni, 0.005 to 0.06% S, 0.5 to 2.5% Mo+1/2W and 0.1 to 2.0% V+1/2Nb, and the balance Fe with inevitable impurities to hot rolling at a forging ratio (s) of 4 to 10 and thereafter subjecting the same to hardening so as to be cooled in such a manner that the cooling rate in the range from the quenching temperature to a temperature lower than that by 500°C is controlled to ≥3°C/min. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a mold steel that is processed into a mold in a pre-hardened state.

  Conventionally, heat check resistance and crack resistance are important characteristics in a mold that is used hot. In order to improve these characteristics, it is essential to improve toughness. On the other hand, recently, improvement in machinability has been demanded from the viewpoint of mold cost reduction and shortening of production period, but toughness and machinability are generally contradictory properties, and both are hot Development of tool steel has been desired. Therefore, for example, as disclosed in JP-A-53-16315 (Patent Document 1), by adding S and Zr in combination, the machinability is improved more than the effect of adding S alone, and the extension of sulfide is suppressed. Is disclosed. Further, as in JP-A-10-60585 (Patent Document 2), by adding appropriate amounts of S, Te and Ca, sulfide inclusions are refined and spheroidized, and the heat check resistance is prevented from lowering. A technique of drawing is disclosed.

JP-A-53-16315 Japanese Patent Laid-Open No. 10-60585

In the case of Patent Document 1 described above, the improvement of machinability and the suppression of the extension of sulfides are surely made to exceed the effects of the addition of S alone, but problems remain in terms of crack sensitivity. In the case of Patent Document 2, as in the case of Patent Document 1, a decrease in heat check resistance is suppressed, but there remains a problem in terms of crack sensitivity.

  In order to solve the above-described problems, the inventors have intensively developed, and as a result, by controlling the amount of S added, the machinability is improved due to the stress concentration effect of sulfide, and the forging ratio and Pre-hardening that can reduce the susceptibility to cracking and stabilize the mold life with both mold life and workability due to the synergistic effect of cooling rate control (breakage of solidification structure and refinement of grain structure) Therefore, the present invention provides a mold steel having good mold workability.

The gist of the invention is that
(1) By mass%, C: 0.1-0.6%, Si: 0.1-1.5%, Mn: 0.1-0.7%, Cr: 1.0-6.0% Ni: 0.05-1.2%, S: 0.005-0.06%, Mo + 1 / 2W: 0.5-2.5%, and V + 1 / 2Nb: 0.1-2.0% The cooling rate in the range from the quenching temperature to a temperature lower by 500 ° C. than the quenching temperature after hot working the steel composed of Fe and unavoidable impurities at a forging ratio (s): 4 to 10 Pre-hardened mold steel that is cooled at 3 ° C / min or higher.
(2) In addition to the component composition described in (1) above, one or two of Ca: 0.0003 to 0.1% and Mg: 0.0003 to 0.1% are added. Prehardened mold steel.

  As described above, the machinability is improved by the stress concentration effect of the sulfide according to the present invention, and the hardness and heat resistance check are achieved by reducing the crack sensitivity by the synergistic effect of the forging ratio and the cooling rate control. It is possible to provide a mold steel capable of reducing crack susceptibility excellent in pre-hardened workability without degrading various properties such as property, and exhibiting an extremely excellent effect.

Hereinafter, the reasons for limiting the chemical components according to the present invention will be described.
C: 0.1 to 0.6%
C is a basic element that imparts hardness and wear resistance. In order to ensure the hardness at the time of quenching and tempering, 0.1% is necessary. However, addition exceeding 0.6% lowers the toughness, so the range was made 0.1 to 0.6%. Desirably, the content is 0.2 to 0.4%.

Si: 0.1 to 1.5%
Si is an element for securing a deoxidizer, oxidation resistance, and machinability. However, if it is less than 0.1%, the effect cannot be obtained, and if it exceeds 1.5%, the workability and toughness deteriorate, so the range is made 0.1 to 1.5%. Desirably, it is 0.3 to 1.0%.
Mn: 0.1 to 0.7%
Mn is an element necessary for a deoxidizer and hardenability. However, if it is less than 0.1%, the effect cannot be obtained, and if it exceeds 0.7%, the toughness due to sulfide formation (MnS) decreases, so the range is 0.1 to 0.7%. To do.

Cr: 1.0-6.0%
Cr is an element that forms hardenability and hard carbide and improves wear resistance. However, if it is less than 1.0%, the effect cannot be obtained, and addition over 6.0% is Cr carbide. In addition, the high temperature softening resistance is lowered and the toughness is lowered. Therefore, the range is set to 1.0 to 6.0%. Desirably, the content is 3.0 to 5.0%.

Ni: 0.05-1.2%
Ni is an element that improves toughness. However, if it is less than 0.05%, the effect cannot be obtained, and if it exceeds 1.2%, the machinability deteriorates. Therefore, the range is made 0.05 to 1.2%. Desirably, it is 0.1 to 0.8%.
S: 0.005-0.06%
S is an element that forms sulfides and improves machinability. However, if it is less than 0.005%, the effect cannot be obtained, and if it exceeds 0.06%, the toughness decreases. Therefore, the range is made 0.005 to 0.06%. Desirably, the content is 0.01 to 0.04%.

Mo + 1 / 2W: 0.5-2.5
Mo and W are elements that form hard carbides, improve wear resistance, and improve hardenability, high-temperature softening resistance, and high-temperature strength. However, if the Mo equivalent is less than 0.5, the effect cannot be obtained, and if it exceeds 2.5, the toughness decreases, so the range is set to 0.5 to 2.5. Desirably, it is set to 1.0 to 2.0.

V + 1 / 2Nb: 0.1-2.0
V and Nb are elements that have the same effects as Mo and W, form hard carbides, improve wear resistance, and play a role of crystal grain refinement. However, if the V / Nb equivalent is less than 0.1, the effect cannot be obtained, and if it exceeds 2.0, the toughness decreases and the heat treatment strain increases. And Desirably 0.3 to 1.5.

Ca: 0.0003 to 0.1%
Ca enhances the spheroidization of sulfides. However, if it is less than 0.0003%, the effect cannot be obtained, and if it exceeds 0.1%, the toughness is lowered, so the range is made 0.0003 to 0.1%.
Mg: 0.0003 to 0.1%
Mg, like Ca, enhances spheroidization of sulfides. However, if it is less than 0.0003%, the effect cannot be obtained, and if it exceeds 0.1%, the toughness is lowered, so the range is made 0.0003 to 0.1%.

Training ratio (s): 4-10
Hot work is performed with a forging ratio determined, and the toughness is improved by fracture of the solidified structure. However, if the forging ratio (s) is less than 4, the solidified structure is not sufficiently destroyed, so the lower limit is set to 4. Moreover, since sulfide will be refined | miniaturized excessively when 10 is exceeded, the upper limit was set to 10.
FIG. 1 is a diagram illustrating a relationship between a forging ratio, an impact value, and a cooling rate after quenching. As shown in FIG. 1, even within the range of the forging ratio (s) according to the present invention, when the cooling rate after quenching is less than 3 ° C./min, the impact value is poor. Moreover, FIG. 2 is a figure which shows the relationship between forge ratio and machinability. As shown in FIG. 2, it can be seen that the machinability is inferior when the training ratio is greater than 10.

Cooling rate after quenching: 3 ° C./min or more The reason for determining the cooling rate after quenching in the range from the quenching temperature to a temperature lower than the quenching temperature by 500 ° C. is to refine the grain structure. It is. However, if the cooling rate at the center is less than 3 ° C./min, the effect cannot be obtained, so the lower limit was set at 3 ° C./min at the center. Desirably, the temperature is 10 ° C./min at the center. The cooling rate according to the present invention is determined as the cooling rate at the center of the steel to be cooled. Moreover, as quenching temperature which concerns on this invention, 950-1150 degreeC and tempering temperature shall be 550-670 degreeC.

Hereinafter, the present invention will be specifically described with reference to examples.
Steels having the composition shown in Table 1 were melted in a 1 t vacuum induction melting furnace and cast into ingots. After heating to 1200 ° C., forging to 50 mm × 50 to 100 mm × 200 mm square, annealing and roughing of the test piece, quenching and tempering (quenching temperature: 1030 ° C., tempering temperature: 550 to 650 ° C.), The test piece was finished. The results are shown in Table 2. The machinability test results were evaluated by the time required for drilling a SKH51 φ8 drill and a depth of 10 mm as cutting conditions. Moreover, as evaluation of the Charpy impact test as a crack sensitivity evaluation method, the test piece is L direction (forging direction), T direction (vertical direction of L direction), 10 mm × 10 mm × 55 mm, 10R2 mm U notch test piece, The test was conducted at room temperature. Moreover, as a heat check test, it heated to test piece (phi) 40 * 100mm and 600 degreeC, and heating and cooling at room temperature were repeated 2000 times, and it evaluated by the average length of the crack which arose on the test piece surface.

  As shown in Table 2, no. Nos. 1 to 6 are examples of the present invention. 7 to 10 are comparative examples. Comparative Example No. Since No. 7 has a low training ratio (s), the L direction forging and stretching at the impact value is poor, and the impact value in the T direction (perpendicular to the L direction) is also poor. Comparative Example No. No. 8 has a low cooling rate after quenching, so the forging in the L direction at the impact value is poor, and the impact value in the T direction (perpendicular to the L direction) is also poor. Moreover, heat check resistance is also inferior. Comparative Example No. No. 9 has poor machinability due to its high C content and low S addition. Comparative Example No. No. 10 has a high Ni content and a high S addition amount, so that the forging in the L direction at the impact value is poor, the impact value in the T direction (perpendicular to the L direction) is also poor, and heat check resistance Is also inferior. In contrast to this, No. It turns out that 1-6 are excellent also about any performance.

It is a figure which shows the relationship between a training ratio, an impact value, and the cooling rate after hardening. It is a figure which shows the relationship between a training ratio and machinability. Patent applicant Sanyo Special Steel Co., Ltd. Attorney Attorney Shiina

Claims (2)

% By mass
C: 0.1-0.6%
Si: 0.1 to 1.5%,
Mn: 0.1 to 0.7%,
Cr: 1.0-6.0%,
Ni: 0.05-1.2%,
S: 0.005-0.06%,
Mo + 1 / 2W: 0.5-2.5%, and V + 1 / 2Nb: 0.1-2.0%, the steel composed of the balance Fe and inevitable impurities is heated at a forging ratio (s): 4-10. Pre-hardened mold steel which is cooled at a cooling rate of 3 ° C./min or more in the range from the quenching temperature to a temperature lower by 500 ° C. than the quenching temperature after quenching. A pre-hardened mold characterized by adding one or two of Ca: 0.0003 to 0.1% and Mg: 0.0003 to 0.1% in addition to the component composition of claim 1 Steel.

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