JP2005336553A - Hot tool steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide hot tool steel having a low dependency of impact value on cooling rate and also having excellent toughness, hardness, machinability and heat check resistance. <P>SOLUTION: The hot tool steel has a composition consisting of, by weight, 0.3 to 0.5% C, 0.2 to 1.0% Si, 0.5 to 2.5% Mn, 0.001 to 0.05% P, 0.01 to 0.06% S, 3.0 to 6.0% Cr, 0.5 to 3.0% (Mo+1/2W), 0.3 to 1.5% V, 0.001 to 0.02% Al, 0.005 to 0.025% N, 0.001 to 0.01% O, 0.0001 to 0.01% Ca and the balance Fe with impurities and is refined to 38 to 52 HRC after quench-and-temper treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hot tool steel, and more specifically, various tools such as a hot forging die, a hot press die, a die cast die, a hot extrusion die, etc., after adjusting the hardness by quenching and tempering treatment. The present invention relates to hot work tool steel for finishing into a shape.

  Conventionally, 5% Cr-based hot tool steels such as JIS-SKD6, SKD61, and SKD62 have been used for applications such as hot forging dies, hot pressing dies, die casting dies, and hot extrusion dies. Conventionally, these hot work tool steels are generally processed by roughing an annealed material, adjusted to a desired hardness by quenching and tempering, and further processed into a predetermined mold shape by finishing.

  However, such a process has a problem that it takes a very long time to complete the mold. Therefore, in recent years, for the purpose of shortening the mold production period, shortening the delivery time, and reducing the production cost, a method of directly processing a pre-hardened material (pre-hardened material) into a mold is used. It is coming.

  Pre-hardened materials are very hard and poor in machinability compared to normalized materials. Therefore, there is a problem that the tool life is shortened when cutting is performed on a pre-hardened material made of a conventional material. As means for solving this problem, for example, a method is known in which a small amount of sulfur is added to hot tool steel such as JIS-SKD6, SKD61, and SKD62 to improve machinability.

  Further, Patent Document 1 discloses a high-strength pre-hardened steel material having a predetermined composition and having the martensite packet size adjusted to be equal to or larger than No. 8 evaluated by the austenite grain size number. This document describes that the machinability can be improved while maintaining the balance between strength and ductility by relatively increasing the martensite packet size.

  Furthermore, Patent Document 2 discloses a hot tool steel having a predetermined composition and having a martensite structure having a direction after heat treatment of 17 to 33%. In this document, by making the martensite structure isotropic, the cutting resistance and its fluctuation amount during cutting can be reduced, the tool life is improved, and fine inclusions and carbides are uniformly dispersed. This describes that a large number of martensite precipitation sites are formed, and the non-directional martensite structure can be realized.

JP 2000-054068 A JP 2003-268486 A

  In general, as the tool steel for hot working has a slower cooling rate at the time of quenching, the characteristic value representing the mold performance such as the impact value decreases. In the conventional method in which roughing is performed after annealing and quenching and tempering are performed, the surface used as a mold (cavity surface) directly touches the refrigerant during quenching, so the cooling rate is fast, and even large molds have high impact values. I was able to get it.

  However, since the pre-hardened material is quenched and tempered in advance, the cavity surface after the cutting process uses the inside having a low cooling rate during quenching. Therefore, the mold obtained by cutting the pre-hardened material has a lower impact value on the cavity surface and may cause large cracks at an early stage compared to a mold manufactured by the conventional method, and the mold life is short. There was a problem.

  Furthermore, since the mold used in the hot state undergoes a thermal cycle during use, the pre-hardened material is required to have heat check resistance in addition to hardness, toughness and machinability. However, there has never been proposed an example of a hot work tool steel that satisfies these conditions simultaneously.

  The problem to be solved by the present invention is to provide a hot work tool steel in which the dependence of the impact value on the cooling rate is small and high toughness can be obtained even inside the material. Another object of the present invention is to provide a hot work tool steel that is excellent in hardness, machinability and heat check resistance in addition to high toughness.

In order to solve the above problems, the hot work tool steel according to the present invention is:
C: 0.3 to 0.5 wt%,
Si: 0.2 to 1.0 wt%
Mn: 0.5 to 2.5 wt%,
P: 0.001 to 0.05 wt%,
S: 0.01-0.06 wt%,
Cr: 3.0 to 6.0 wt%,
Mo + 1 / 2W: 0.5-3.0 wt%,
V: 0.3 to 1.5 wt%,
Al: 0.001 to 0.02 wt%,
N: 0.005-0.025 wt%
O: 0.001 to 0.01 wt%
Ca: 0.0001 to 0.01 wt%,
The balance consists of Fe and impurities,
It is summarized that it has been tempered to 38 to 52 HRC after quenching and tempering.

  Since the hot work tool steel according to the present invention increases the amount of Mn as compared with the conventional JIS steel, the impact value is less dependent on the cooling rate. Moreover, since the Si content is reduced as compared with the conventional JIS steel, it is possible to improve the heat check resistance which is lowered by the Mn content increase. Furthermore, since the contents of S and Ca, which are free-cutting elements, are optimized, the machinability can be improved without reducing the impact value and heat check resistance.

  Hereinafter, an embodiment of the present invention will be described in detail. The cold tool steel according to the present invention contains the following elements, with the balance being substantially composed of Fe and inevitable impurities. The kind of additive element, its component range, and the reason for limitation are as follows.

(1) C: 0.3 to 0.5 wt%.
C is an element necessary for securing hardness and wear resistance important for mold performance. In order to ensure sufficient hardness and wear resistance as a hot tool steel, C needs to be added in an amount of 0.3 wt% or more.
On the other hand, when C is added excessively, coarse eutectic carbides are produced during melting, and carbides that do not dissolve at the time of quenching increase, leading to a decrease in toughness and heat check resistance. Therefore, the amount of C is preferably 0.5 wt% or less.

(2) Si: 0.2 to 1.0 wt%.
Si is an element necessary as a deoxidizing element during steelmaking. Further, the machinability and temper softening resistance are improved by increasing the content thereof. In order to obtain good machinability, the Si amount is preferably 0.2 wt% or more.
However, when there is too much Si amount, toughness and heat check resistance will fall. Moreover, in this invention, although hardenability is ensured by Mn content increase, heat check resistance falls with the increase in Mn content. Therefore, in order to compensate for the decrease in heat check resistance accompanying an increase in the amount of Mn, the Si amount is preferably set lower than the composition range of normal JIS-SKD6, SKD61, and SKD62. Specifically, the Si amount is preferably 1.0 wt% or less.

(3) Mn: 0.5 to 2.5 wt%.
Mn is a component necessary for ensuring hardenability and hardness, and in order to obtain a structure having high toughness to the center during quenching and tempering of the material, the composition range of normal JIS-SKD6, SKD61, and SKD62 It is necessary to add a lot. Specifically, the amount of Mn is preferably 0.5 wt% or more.
Moreover, when Mn is added excessively, the heat check resistance is lowered, or the hardenability becomes too high, and a large amount of residual γ is generated during quenching. When a large amount of residual γ is generated, the impact value may be lowered due to transformation that occurs during tempering, or the hardness may not be lowered even by annealing. Therefore, the amount of Mn is preferably 2.5 wt% or less.

(4) P: 0.001 to 0.05 wt%.
P is an element that is preferably reduced because it lowers toughness and heat check resistance. When contained unavoidably, it is preferable to reduce the amount of P to 0.05 wt% or less.

(5) S: 0.01 to 0.06 wt%.
In general, S decreases the heat check resistance and toughness, and therefore the amount of S is generally kept extremely low. However, S is an element effective for improving machinability. In order to obtain good machinability, the amount of S is preferably 0.01 wt% or more.
However, when the amount of S is excessive, heat check resistance and toughness are greatly deteriorated. Therefore, the amount of S is preferably 0.06 wt% or less.

(6) Cr: 3.0 to 6.0 wt%.
Cr is necessary for forming carbides to improve the strengthening and wear resistance of the base and for ensuring hardenability. In order to obtain such an effect, Cr needs to be added at 0.5 wt% or more. Similarly to Mn, if the Cr content is increased, the hardenability is improved, and the toughness inside the material can be improved during quenching and tempering.
However, an increase in the Cr content lowers the temper softening resistance and lowers the mold performance. Therefore, the Cr amount is preferably 6.0 wt% or less.

(7) Mo + 1 / 2W: 0.5-3.0 wt%.
Both Mo and W are necessary for forming carbides and improving the strengthening and wear resistance of the base, and for ensuring hardenability. In order to obtain such an effect, it is necessary to add 0.5% by weight or more of Mo + 1 / 2W.
However, even if Mo and / or W is added excessively, the effect is saturated and economically disadvantageous. Therefore, the Mo + 1 / 2W amount is preferably 3.0 wt% or less.
Mo and W bring about the same effect. Since W has an atomic weight approximately twice that of Mo, it is defined by the Mo equivalent. Mo and W may be added alone or simultaneously.

(8) V: 0.3 to 1.5 wt%.
V is an element that improves the strengthening and wear resistance of the base by forming and precipitating carbides during tempering. Moreover, at the time of quenching heating, the formation of fine carbides has the effect of suppressing the coarsening of crystal grains and suppressing the decrease in toughness. In order to obtain such an effect, V needs to be added at 0.3 wt% or more.
On the other hand, when V is added excessively, coarse crystals are generated as carbonitride during solidification, and toughness is reduced. Therefore, the V amount is preferably 1.5 wt% or less.

(9) Al: 0.001 to 0.02 wt%.
In addition to acting as a deoxidizing element during steelmaking, Al is an element that binds to N in the steel, finely disperses as nitride, and suppresses crystal grain coarsening during quenching heating. In order to obtain such an effect, Al needs to be added in an amount of 0.001 wt% or more.
However, even if Al is added excessively, the effect is saturated. Accordingly, the Al content is preferably 0.02 wt% or less.

(10) N: 0.005 to 0.025 wt%.
N is an element that is effective for preventing coarsening and preventing toughness reduction by bonding with Al and V in steel to form nitrides and finely dispersing them to suppress coarsening of crystal grains during quenching heating. . In order to obtain such an effect, N needs to be added in an amount of 0.005 wt% or more.
However, even if N is added excessively, the effect is saturated. Therefore, the N amount is preferably 0.025 wt% or less.

(11) O: 0.001 to 0.01 wt%.
Usually, O is an element that is preferably reduced in order to form inclusions as oxides and reduce toughness and heat check resistance. However, in the present invention, an appropriate amount of oxygen is added in order to form a protective film called belag on the surface of the cutting tool during cutting. “Bellag” is a free-cutting inclusion (calcium oxysulfide) obtained by simultaneously adding Ca and S. In order to obtain such an effect, it is necessary to add 0.001 wt% or more of O.
However, the effect is saturated even if O is added excessively. In addition, excessive addition of O causes inclusions that reduce toughness and heat check resistance. Therefore, the amount of O is preferably 0.01 wt% or less.

(12) Ca: 0.0001 to 0.01 wt%.
Ca, together with S and O, is an element necessary for forming inclusions that improve free-cutting properties. When an appropriate amount of Ca is added, a protective coating (berag) is formed on the tool surface during cutting, and the effect of suppressing tool wear is obtained. In order to obtain such an effect, Ca needs to be added in an amount of 0.0001 wt% or more.
On the other hand, even if Ca is added excessively, the effect is saturated. Therefore, the Ca content is preferably 0.01 wt% or less.

  Moreover, in addition to the element mentioned above, the hot tool steel which concerns on this invention may further contain the following 1 type, or 2 or more types of elements. The component ranges of each element and the reasons for limitation are as follows.

(13) Ni: 2.0 wt% or less.
(14) Cu: 1.0 wt% or less.
Ni and Cu are both effective in enhancing hardenability and strengthening the base, and can be added as necessary. However, even if it is added excessively, the effect is saturated and it becomes economically disadvantageous. Therefore, the Ni amount is preferably 2.0 wt% or less, and the Cu amount is preferably 1.0 wt% or less.

(15) Co: 5.0 wt% or less.
Co is an element that improves the strength by solid solution strengthening, and can be added as necessary. However, even if it adds excessively, the effect will be saturated and it will become economically disadvantageous. Therefore, the amount of Co is preferably 5.0 wt% or less.

(16) Ti: 0.2 wt% or less.
(17) Zr: 0.2 wt% or less.
(18) Nb: 0.2 wt% or less.
Ti, Zr, and Nb are all elements that form carbonitrides and finely precipitate and prevent crystal grain coarsening during quenching heating. Necessary when it is desired to refine crystal grains and ensure toughness. It can be added depending on. However, if added in excess, it will crystallize out as coarse carbonitride during solidification, which in turn reduces toughness. Accordingly, the content of these elements is preferably 0.2 wt% or less.

  The hot work tool steel according to the present invention is used in a state of being tempered to 38 to 52 HRC by quenching and tempering the steel material having the above composition. Quenching conditions and tempering conditions are not particularly limited, and optimum conditions are selected according to the composition so that the desired hardness, toughness, machinability and the like can be obtained.

  Next, the operation of the hot tool steel according to the present invention will be described. In the conventional hot work tool steel, the characteristic value representing the mold performance such as the impact value decreases as the cooling rate during quenching becomes slower. This is because, since the hardenability is low, the lower the cooling rate during quenching, the coarser the lath martensite during cooling, or the bainite transformation occurs during cooling. Therefore, when a method of directly processing a mold from a pre-hardened material is applied to a conventional hot tool steel, a surface having a low impact value is exposed on the cavity surface, and the mold life is shortened.

  On the other hand, when the amount of Mn is increased as compared with the conventional hot work tool steel, the hardenability is improved and the dependence of the impact value on the cooling rate is reduced. As a result, a high impact value can be obtained even at the center of the material. This is because as the amount of Mn increases, the Ms point decreases and the bainite transformation moves to the long time side. For this reason, even if a method of directly processing a pre-hardened material into a mold is used, the impact value of the cavity surface can be kept high.

  Si is an element that improves machinability, but if added excessively, heat check resistance is lowered. On the other hand, when the amount of Mn is increased, the hardenability is improved, but the heat check resistance is lowered. In the present invention, since the Si content is reduced from the conventional hot tool steel to such an extent that the machinability is not significantly reduced, it is possible to suppress a decrease in heat check resistance due to an increase in the Mn content. . Furthermore, since appropriate amounts of S and Ca are contained, the machinability can be improved without lowering the impact value and heat check resistance.

  Steels having compositions shown in Table 1 (Examples 1 to 10, Comparative Examples a to q, conventional materials) were melted in a vacuum high-frequency induction furnace, and then cast into a 150 kg steel ingot. The steel ingot was soaked at 1230 ° C. for 10 hours, and then forged into a 60 mm × 60 mm square at a heating temperature of 1200 ° C. After forging, annealing was performed by holding at 870 ° C. for 3 hours and then gradually cooling. Various sample pieces such as a machinability test piece, a Charpy test piece, and a heat check test piece were roughly processed from the obtained square material.

After roughing, each steel material was quenched and subjected to tempering twice (600 ° C. to 650 ° C. × 1 h → air cooling) × 2. The quenching conditions were 1030 ° C. × 30 min → oil quenching and 1030 ° C. × 30 min → air cooling. The air-cooled material simulates the state where the cooling rate is slowed assuming the cavity surface of the pre-hardened material.
Various test pieces were machined from the tempered square bar and then subjected to the test. The oil-quenched material was subjected to all tests, and the air-cooled material was only subjected to hardness measurement and impact test. Details of various test conditions are as follows. Table 2 shows the test results.

(1) Charpy test A specimen was taken from the width direction (T direction, perpendicular to the forging line) of the forged steel material, and the Charpy impact value was evaluated according to "JIS Z 2242".
(2) Heat check test First, a disk-shaped sample was cut out from the forged steel material. Next, using a high-frequency heating and water-cooled heat check tester, 700 ° C. heating and water cooling of the surface layer portion were repeated 1000 times. After completion of the test, the disk-shaped sample was cut parallel to the upper and lower surfaces, the depth and number of cracks generated on the sample surface (side surface of the disk-shaped sample) were measured, and the average crack length was calculated.
(3) Machinability test An end mill cutting test was performed under the following conditions. The cutting distance at which the flank wear became 0.2 mm was defined as the tool life, and the machinability was expressed as a relative value when the tool life of JIS-SKD61 (conventional material) was 100.
Tool: φ10mm carbide end mill (TiAlN coating, 4 blades)
Cutting speed: 150 m / min
Feed: 0.1 mm / blade Axial cut: 10 mm
Radial notch: 0.5mm
Coolant: None, air blow

Since Comparative Example a has a low C content, the required hardness after quenching and tempering cannot be obtained. On the other hand, since the comparative example b has a high C content, the impact value is low.
In Comparative Example c, the machinability is poor because the Si content is too low. On the other hand, Comparative Example d has poor heat check resistance because the Si content is too high.
Comparative Example e has a low impact value because the Mn content is too high. Similarly, Comparative Example f has a low impact value because the P content is too high.
Comparative Example g has poor machinability because the S content is too low. On the other hand, Comparative Example h has a low impact value because the S content is too high.
In Comparative Example i, since the Cr content is too low, the impact value is low with the air-cooled material. On the other hand, in Comparative Example j, since the Cr content is too high, the temper softening resistance is lowered and the heat check resistance is low.
Since comparative example k has too low Mo content, heat check resistance is low.
In Comparative Example 1, since the V content was too low, the crystal grains became coarse during quenching heating, and the heat check resistance was lowered. On the other hand, in Comparative Example m, since the V content is too high, a large amount of VC crystallized matter is present and the impact value is low.
In Comparative Example n, since Al was not added, the crystal grains became coarse during quenching heating, and the impact value decreased. Similarly, in Comparative Example o, since the N content was too low, the crystal grains became coarse during quenching heating, and the impact value decreased.
Furthermore, in Comparative Example p, since the O content is too low, a composite oxide of Ca and S that improves machinability is not generated, and machinability is poor. Similarly, in Comparative Example q, since Ca is not added, a composite oxide of Ca and S that improves machinability is not generated, and machinability is poor.

  In contrast, in Examples 1 to 10, both the oil-quenched material and the air-cooled material could be tempered to 45 HRC ± 1 by quenching and tempering. Moreover, the oil-quenched materials of Examples 1 to 10 had impact values and heat check resistance equivalent to or higher than those of SKD61 (conventional material), and further had machinability superior to that of SKD61. Furthermore, the air-cooled material that simulates the center of the pre-hardened material can maintain an impact value comparable to that of the oil-quenched material without reducing the impact value as compared with the oil-quenched material as in the case of SKD61. It was.

  The embodiment of the present invention has been described in detail above, but the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention.

The hot tool steel according to the present invention can be used as various hot working dies and various hot working tools.

Claims (3)

C: 0.3 to 0.5 wt%,
Si: 0.2 to 1.0 wt%
Mn: 0.5 to 2.5 wt%,
P: 0.001 to 0.05 wt%,
S: 0.01-0.06 wt%,
Cr: 3.0 to 6.0 wt%,
Mo + 1 / 2W: 0.5-3.0 wt%,
V: 0.3 to 1.5 wt%,
Al: 0.001 to 0.02 wt%,
N: 0.005-0.025 wt%
O: 0.001 to 0.01 wt%
Ca: 0.0001 to 0.01 wt%,
The balance consists of Fe and impurities,
Hot tool steel tempered to 38-52 HRC after quenching and tempering. Ni: 2.0 wt% or less,
Co: 5.0 wt% or less, and
Cu: 1.0 wt% or less,
The hot tool steel according to claim 1, further comprising one or more elements selected from the group consisting of: Ti: 0.2 wt% or less,
Zr: 0.2 wt% or less, and
Nb: 0.2 wt% or less,
The hot work tool steel according to claim 1 or 2, further comprising one or more elements selected from the group consisting of: