JP2006104519A - High toughness hot tool steel and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high toughness hot tool steel to be used as a production material for a die for hot working, wherein even it is used as a large-sized die stock, a long die life is attained by giving high hardness, and further, danger that cracks are generated during the use of a die is reduced by securing high toughness and ductility. <P>SOLUTION: A hot tool steel is made into an ingot under the condition that the average cooling rate from a solidus temperature at the time of solidification to a liquidus temperature is ≥0.025°C/s, the ingot is heated to the temperature range of 1,240 to 1,360°C, and the crystallized carbides and carbonitrides are subjected to solution treatment, thus the one in which the density of the carbides and carbonitrides crystallized in the steel together with oxide-based inclusions is (1) ≤0.01 piece in the large-sized one having the diameter of the equivalent circle of >50 μm, and is (2) ≤100 pieces in the small-sized one having the diameter of the equivalent circle of 1 to 50 μm per mm<SP>2</SP>of the cross-section in the steel is obtained. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention is a hot tool steel used as a material for producing a mold such as a hot forging die, hot press die, die casting die, hot extrusion die, etc., and has high toughness even if the hardness is increased. The present invention relates to a hot work tool steel that is not low and therefore has a low risk of cracking and can obtain a long mold life.

Damage that limits the service life of hot molds is often caused by wear or heat check, and in order to reduce such phenomena, it is generally effective to increase the mold hardness. Efforts are being made to increase the hardness of steel. However, in recent years, what has been divided into multiple parts in the past has often been manufactured in an integrated manner with the aim of improving production efficiency. Increasing hardness has become difficult. This difficulty is based on the following two causes.

1) Due to the increase in size of the mold, a sufficient cooling rate cannot be secured when quenching the material, and the impact value is lower than that of a small mold even with the same hardness.
2) The material for manufacturing large molds must be a product with a large cross section, and in order to manufacture a material with a large cross section, it is necessary to cast it into a large steel ingot. The cooling rate during solidification decreases. When the cooling rate is decreased, carbides, particularly vanadium carbide VC, and carbonitrides, particularly vanadium carbonitride V (C, N), are crystallized, oxides such as Mn, Si, Al, etc. These composite oxides are also coarsened, and the hit value is greatly reduced. The carbides and carbonitrides in which crystallization is a problem are mainly those of vanadium, and therefore are represented by “VC” and “V (C, N)” in the following description, respectively.

For these reasons, in a large mold, if the hardness is to withstand wear and heat check, the impact value decreases, and a “large crack” tends to occur in the initial stage of mold use. For this reason, there is a limit to the hardness that can be realized.

As a means to solve the problem that it is difficult to achieve both hardness and toughness in such a large mold material, it is necessary to compensate for the insufficient cooling rate during quenching, that is, quench the mold material. It has been proposed to improve the deficiency, and as a means for that, it has been proposed to increase the amount of elements such as Mn and Mo that improve the hardenability. However, the effect is naturally limited.

On the other hand, as a measure to prevent coarsening of crystallized carbide VC, crystallized carbonitride V (C, N), and oxide inclusions with the increase in the size of the steel ingot, and the reduction in impact value caused by it. It has been proposed to regulate the upper limit of the content of V and O. However, the size of VC, V (C, N), and oxide inclusions that crystallize depends not only on the V and O content but also on the size of the steel ingot, that is, the cooling rate during solidification. Therefore, a sufficient toughness improvement effect cannot be expected by adjusting only the components.

About 1% of V is added to hot tool steel represented by JIS-SKD61 in order to prevent coarsening of grains during quenching heating and to improve resistance to temper softening. In the equilibrium diagram, this V should all dissolve when heated to 1200 ° C. or higher. However, in reality, very coarse VC or V (C, N) is present in the concentrated molten steel during solidification. However, they cannot be dissolved in an industrially feasible processing time by heating at about 1200 ° C. If VC and V (C, N) remain, the impact value is reduced by breaking from the starting point, and early cracking of the mold is likely to occur. Therefore, crystallization VC and V (C, N) Inviting serious problems.

In order to prevent a drop in impact value due to coarse crystallized VC, V (C, N), for the above reasons, heating is performed at a temperature higher than 1200 ° C. for a long time, so that the crystallized product is dissolved. Heat treatment is required. Furthermore, the size of the crystallization VC, V (C, N) increases as the size of the steel ingot increases, that is, as the cooling rate during solidification decreases, so that the larger the steel ingot size, the higher the temperature. Therefore, a longer time for solution heat treatment is required.

In the end, there is a practical upper limit on the cooling rate that can be achieved during solidification, and it is assumed that it is within a certain limit, but the crystallization VC, V (C, N) corresponding to the cooling rate during solidification is large. Accordingly, it is practical to determine the heat treatment conditions for the solid solution, and the toughness of the toughness is not increased without performing the solution heat treatment for a relatively high temperature and for a long time, and thus without increasing the cost. The conclusion is reached that it is wise to obtain high hot tool steel.

From such a viewpoint, the inventors have found that the cooling rate during solidification (average cooling rate from the liquidus to the solidus) and the size of the crystallized carbide VC and carbonitride V (C, N). In addition, the relationship between the size of the crystallized products and the temperature and time of the solution heat treatment was investigated in detail, and the optimum solution heat treatment conditions were found according to the cooling rate during solidification. Based on this, we have established a hot work tool steel that can produce a high impact value compared to conventional products, even when a large mold material is manufactured, and its manufacturing method.

The object of the present invention is based on the knowledge of the above-described inventors, in hot tool steel as a material for manufacturing a hot working mold, by providing high hardness even as a large mold material. An object of the present invention is to provide a high toughness hot tool steel that enjoys a long mold life, secures high toughness and reduces the risk of cracking during use of the mold, and a method for producing the same.

In the high toughness hot work tool steel of the present invention, the abundance density of carbides and carbonitrides crystallized in the steel and oxide inclusions is 1) equivalent circle diameter per 1 mm ² of the cross-sectional area of the steel. The number of large ones exceeding 50 μm is 0.01 or less, and 2) the number of small ones having an equivalent circle diameter of 1 to 50 μm is 100 or less.

In the method of producing the high toughness hot tool steel of the present invention, in the ingoting, the average cooling rate from the solidus temperature during solidification to the liquidus temperature is 0.025 ° C./second or more, It is characterized by subjecting the crystallized carbides and carbonitrides to a solid solution treatment by heating to a temperature range of 1240 to 1360 ° C. with the steel ingot or after the forging.

The high toughness hot work tool steel of the present invention can be obtained by balancing the high hardness and high toughness realized in the small mold material even in the large mold material that has been manufactured so far. A mold made of steel has a low risk of cracking and can withstand a wear and heat check and enjoy a long life. The production method of the present invention makes it possible to reliably produce a high toughness hot tool steel having such characteristics.

As described above, coarse crystallized carbides and carbonitrides and oxide inclusions act as starting points for fracture, thereby greatly reducing the impact value of the mold. For this reason, it is preferable to reduce these coarse crystallization products as much as possible. In order to determine the threshold value at which the desired impact value can be obtained, the inventors observed the fracture surface of the impact test piece cut out from the mold with a low impact value, and confirmed the coarse crystallized product acting as a starting point. As a result, a large one having an equivalent circle diameter of more than 50 μm, or a relatively fine carbide or carbonitride of 1 to 50 μm, which is present in a cluster shape, is the starting point of destruction, Therefore, it was confirmed that the existence density should be strictly controlled, and a crystallized material having an equivalent circle diameter of less than 1 μm does not substantially act as a starting point of fracture and does not lower the impact value. Based on the above knowledge, the presence of crystallized carbonitrides and carbonitrides and oxide inclusions having an equivalent circle diameter of 50 μm or more and 1 to 50 μm so as not to lower the impact value of the mold the upper limit of the density, defined 0.01 pieces / mm ² respectively into 100 / mm ^2.

The typical composition of the alloy constituting the high toughness hot tool steel of the present invention is C: 0.3 to 0.5%, Si: 0.05 to 1.5%, and Mn: 0.00 on the basis of weight. 3-2%, Cr: 3-6%, Mo + 0.5W: 0.5-3.5%, V: 0.5-1.5%, Al: 0.001-0.025% and N: 0 . 005 to 0.025%, P: 0.05% or less, S: 0.015% or less, O: 0.0025% or less, and has an alloy composition consisting of the balance Fe and impurities.

In addition to the above alloy components, the high toughness hot work tool steel of the present invention is also Ni: 2% or less, Co: 5% or less, Cu: 1% or less, Ti: 0.2% or less, Zr: 0.00%. 1 type or 2 types or less of 2% or less and Nb: 0.2% or less can be contained.

The reason for selecting the above alloy composition will be described below.

C: 0.3-0.5%
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 hot tool steel, the presence of 0.3% or more of C is necessary. When it exceeds 0.5% and is added excessively, it causes a decrease in impact value due to an increase in carbides that do not dissolve at the time of quenching.

Si: 0.05 to 1.5%
Si is necessary as a deoxidizing element during steelmaking, and increasing its content also has the benefit of improving machinability and temper softening resistance, so at least 0.05% Si is added. However, since the toughness decreases as the amount added increases, the upper limit is 1.5%.

Mn: 0.3-2%
Mn is a component necessary for ensuring hardenability and hardness, and for this purpose, the addition amount is set to 0.3% or more. If added in excess, the hardenability becomes too high, and a large amount of residual γ phase is produced during quenching, and the impact value may decrease, or the hardness may not decrease even after annealing. 2%.

P: 0.05% or less, preferably 0.015% or less P is an element that is generally preferably reduced because it lowers the impact value, but is unavoidably present. 0.05% is an allowable limit, and preferably 0.015% or less.

S: 0.015% or less Since S forms MnS and lowers the impact value, it is preferably reduced, but it inevitably enters steel. Also when it contains, it is preferable to reduce to 0.015% or less.

Cr: 3-6%
Cr forms carbides and carbonitrides to strengthen the matrix and improve wear resistance. In order to ensure hardenability, Cr is necessary. In order to obtain such an effect, addition of 3% or more is necessary. However, an increase in Cr content weakens the temper softening resistance and lowers the mold performance. For this reason, the upper limit of the Cr amount is set to 6%.

Mo + 0.5W: 0.5-3.5%
Both Mo and W form carbides and carbonitrides to strengthen the base and improve wear resistance. It is a necessary component for ensuring hardenability. Either one of them may be added alone or in combination. Since Mo and W have the same effect and the atomic weight of W is about twice that of Mo, as is well known, the addition amount of both is defined by Mo equivalent (Mo + 1 / 2W). . In order to obtain the above effect, it is necessary to add 0.5% or more in terms of Mo equivalent. Even if it is added excessively, the effect is saturated and economically disadvantageous, so 3.5% was set as the upper limit.

Since the abundance density of crystallization carbides and carbonitrides and oxide inclusions in hot tool steel increases with the increase in the V content, which is a carbonitride-forming element, and the O content (in addition to V Ti, Nb and Zr also contribute to the formation of carbides and carbonitrides, so their content is also a problem, but what is important is the V content.) It is necessary to define the content of V and O is there. Moreover, the range was prescribed | regulated about the other alloy content for the following reasons.

V: 0.5 to 1.5%
V is an element useful for strengthening the base and improving wear resistance by forming and precipitating carbides and carbonitrides during tempering. In addition, the formation of fine carbides and carbonitrides during quenching heating has the effect of suppressing the coarsening of the crystal grains and suppressing the reduction in impact value. In order to obtain such an effect, it is necessary to add 0.5% or more of V. On the other hand, when an excess amount exceeding the upper limit of 1.5% is added, as described again, coarse crystallized products are generated as solidified carbides and carbonitrides during solidification, and the above-described (crystallized carbide / carbonitride + The limiting condition regarding the density of oxide inclusions) cannot be satisfied, and the toughness is lowered.

Al: 0.001 to 0.025%
In addition to acting as a deoxidizing element during steelmaking, Al combines with N in the steel to form a nitride and finely disperse, thereby suppressing the grain coarsening during quenching heating. In order to obtain such an effect, it is necessary to add at least 0.001% Al. Even if added in a large amount, the effect is saturated, so the upper limit was made 0.025%.

N: 0.005 to 0.025%
N combines with Al and V in the steel to form a nitride, which finely disperses to suppress coarsening of the crystal grains during quenching heating and is effective in preventing a drop in impact value It is. In order to obtain such an effect, addition of 0.005% or more is necessary. Even if added in a large amount, the effect is saturated, so the addition is limited to within the upper limit of 0.025%.

O: 0.0025% or less O forms oxide inclusions and lowers the impact value. In order to satisfy the above-described condition of the density of existence of (crystallized carbide / carbonitride + oxide inclusion) and to suppress a decrease in impact low, the O content needs to be 0.0025% or less. .

Ni: 2% or less, Cu: 1% or less Both Ni and Cu are effective for enhancing the hardenability and strengthening the base, and can be added as necessary. Even if added excessively, the effect is saturated and disadvantageous economically, so the upper limit of each was made 2% and 1%.

Co: An element that improves the strength by solid solution strengthening of 5% or less, and can be added as necessary. Even if excessively added, the effect is saturated and economically disadvantageous, so an upper limit of 5% was set.

Ti: 0.2% or less, Zr: 0.2% or less, Nb: 0.2% or less All of these components are carbides such as TiC, ZrC and NbC, and Ti (C, N), Zr. Carbonitrides such as (C, N) and Nb (C, N), and their composite carbides or carbonitrides are formed and finely precipitated to prevent grain coarsening during quenching heating. Therefore, when it is desired to obtain a high effect of ensuring the toughness by refining the crystal grains, it is preferable to add it. However, excessive addition causes crystallization as coarse carbides and carbonitrides during solidification, making it impossible to comply with the above-mentioned limiting conditions for the existence density of (crystallized carbides / carbonitrides + oxide inclusions). In order to reduce the impact value, the upper limit was made 0.2%. When two or more components are added in combination, it is preferable that the total amount does not exceed 0.5%.

In carrying out the manufacturing method of the high toughness hot tool steel of the present invention, the average cooling rate v (° C./second) from the solidus temperature to the liquidus temperature during solidification and the crystallization carbide and carbonitride It is preferable to carry out so that the temperature T (absolute temperature K) and the time t (second) of the solution treatment satisfy the condition of the following formula (1).
0 ≦ {2 × 10 ⁻¹⁴ × exp (0.019 × T) × t ^0.54 } − {117 × exp (−35 × v)}
... (1)

This condition is important for obtaining a preferable distribution of crystallized carbides and carbonitrides. If the cooling rate during solidification, here the average cooling rate from the solidus to the liquidus temperature is 0.025 ° C./sec or more, the size of the crystallized carbide / carbonitride is said to be coarse. However, it becomes a thing which can be made to solid-solve with the heating temperature of 1240-1360 degreeC. If the heating temperature is further increased, the crystallized carbide / carbonitride can be dissolved in a steel ingot cooled at a slower cooling rate, but the heating temperature of 1360 ° C. is processed industrially continuously. It is the limit temperature that can be achieved, and heating to higher temperatures is economically disadvantageous. On the other hand, at a heating temperature of 1240 ° C. or lower, even if the cooling rate is 0.025 ° C./sec or higher, it takes a very long heating time to dissolve the coarse crystallized carbide / carbonitride. . For this reason, the cooling rate at the time of solidification was specified to be 0.025 ° C./second or more. Here, by setting the cooling rate at the time of solidification to 0.025 ° C./second or more, it is possible to ensure the prescribed distribution state of the oxide-based crystallized substance as well as the crystallized carbide / carbonitride. is there.

When manufacturing a die material for a large product, the steel ingot size is inevitably increased, and it may be difficult to obtain a cooling rate of 0.025 ° C./second or more by a normal ingot forming method. In such a case, it is desirable to perform a secondary melting method such as ESR or VAR after normal ingot forming to ensure a predetermined fast cooling rate. Since ESR and VAR also have a refining effect, they have an effect of greatly reducing oxide-based crystallized substances that cannot be eliminated by solution treatment.

Steel having the composition shown in Table 1 was melted and solidified at various cooling rates using a unidirectional solidification furnace to prepare a columnar steel ingot of Φ100 × 80 mm. These ingots were heated to a temperature in the range of 1200 to 1320 ° C., and the solution treatment was performed by changing the holding time. After the solution treatment, it was heated to 1180 ° C., placed by 1/2 upset, and processed into a disk of Φ140 × 40 mm. This was further heated to 1180 ° C. and hot forged into a 60 × 60 × 170 mm rectangular parallelepiped. This forged material was subjected to a normalizing treatment of 970 ° C. × 1 hour / air cooling, then subjected to low-temperature annealing of 750 ° C. × 1 hour / air cooling, further maintained at 870 ° C. for 2 hours, and then cooled to 600 ° C. at a cooling rate of 15 ° C. Spheroidizing annealing was performed, cooling at a time.

From the core of this spheroidized annealing material, a material for 11 × 11 × 55 mm JIS No. 3 Charpy impact test piece (notch direction is T direction) and a material for observing the structure with an optical microscope were cut out. Material for tissue observation, the mirror surface by performing the polishing and buffing by emery paper up to # 1500, and photographed the range of 10 mm ² at imaging magnification 400 times, crystallization carbide-carbonitrides present in this region All of the inclusions and oxide inclusions were image analyzed. Specifically, by measuring the area of individual carbides / carbonitrides and oxide inclusions, calculating the diameter of a circle having the same area and setting it as the “equivalent circle diameter”, The existence density of carbides, carbonitrides and oxide inclusions was investigated.

The material for the impact test piece was tempered twice at 630 ° C. for 2 hours and air cooling after quenching at 1030 ° C. for 1 hour and oil cooling, then processed into a JIS No. 3 impact test piece and tested at room temperature Provided. Six tests were carried out under each condition, and the lowest value among them was adopted. The results are shown in Table 2.

The reason why the impact value is low in each of the steels of Comparative Examples 1-11 to 15 is that the density of existence (crystallized carbide, carbonitride, and oxide inclusions) does not satisfy the conditions of the present invention. The reasons are as follows.
1-11 steel: V content is too high.
1-12 steel: O content is too high.
1-13 steel: Ti content is too high.
1-14 steel: Zr content is too much.
1-15 steel: Nb content is too much.

A JIS-SKD61 having the composition shown in Table 3 was melted and solidified by a unidirectional solidification apparatus, and the cooling rate and the solution treatment conditions were changed (crystallized carbide, carbonitride and oxide system). Inclusion density) and impact value were measured. The results are shown in Table 4. The test method is the same as in Example 1.

The reason why the impact value was low in the comparative example of Table 4 is as follows.
Condition of Comparative Example 2-11: Since the average cooling rate during solidification was slow, a predetermined carbide / carbonitride distribution could not be obtained even if the carbide solution treatment was performed for a long time at a high temperature.
Condition of Comparative Example 2-12: Since the retention temperature of the solution treatment was too low, a predetermined carbide / carbonitride distribution could not be obtained.
Conditions of Comparative Examples 2-13 and 14: Since the retention time of the solution treatment was too short, a predetermined carbide / carbonitride distribution could not be obtained.

Steels having the alloy compositions shown in Table 5 were primarily melted, then remelted using an ESR apparatus, and cast into a steel ingot of Φ1000 × 800 mm. The steel ingot was subjected to carbide / carbonitride solution treatment at 1280 ° C. × 20 h, and then forged at 1180 ° C. to obtain a square member having a side of 400 mm. From the central part of this square, a material for observing the structure and a material for impact test are cut out, and in the same way as described above, the existence density and impact of (crystallized carbide / carbonitride and oxide inclusions) The value was measured. The results are shown in Table 6. The cooling rate during solidification of the ESR steel ingot was calculated from the interval between the secondary dendrite arms after collecting a test piece for observing the structure with an optical microscope from the steel ingot under the same operating conditions.

Claims (6)

The existence density of carbides and carbonitrides crystallized in steel, and oxide inclusions is 1 mm ² per 1 mm ² of cross-sectional area of steel. High-toughness hot tool steel, characterized in that the number is 01 or less, and 2) the number of small ones having an equivalent circle diameter of 1 to 50 μm is 100 or less. Based on weight, C: 0.3-0.5%, Si: 0.05-1.5%, Mn: 0.3-2%, Cr: 3-6%, Mo + 0.5W: 0.5- 3.5%, V: 0.5-1.5%, Al: 0.001-0.025% and N: 0.005-0.025%, P: 0.05% or less, S The high toughness hot tool steel according to claim 1, having an alloy composition of: 0.015% or less, O: 0.0025% or less, the balance being Fe and impurities. In addition to the alloy components defined in claim 2, Ni: 2% or less, Co: 5% or less, Cu: 1% or less, Ti: 0.2% or less, Zr: 0.2% or less, and Nb: 0.00%. The high toughness hot work tool steel according to claim 1, containing 1% or 2% or less of 2% or less. A method for producing the high toughness hot tool steel according to claims 1 to 3, wherein the average cooling rate from the solidus temperature during solidification to the liquidus temperature is 0.025 ° C / second or more when ingot-making. There is a production characterized by subjecting a solidified treatment of crystallized carbides and carbonitrides by heating to a temperature range of 1240 to 1360 ° C. after ingot forming, in the form of steel ingots or after partial forging. Method. Average cooling rate v (° C./second) from solidus temperature to liquidus temperature during solidification, temperature T (absolute temperature K) and time t (second) of crystallizing carbide and carbonitride solution treatment 5) The manufacturing method according to claim 4, wherein the relationship is satisfied so as to satisfy the following formula.
0 ≦ {2 × 10 ⁻¹⁴ × exp (0.019 × T) × t ^0.54 } − {117 × exp (−35 × v)} The production method according to claim 4 or 5, comprising a step of performing ingot formation by secondary melting by ESR or VAR.