JP3009714B2 - Steel for hot press tools - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) For steel for hot press tools, for example, a hot slab after continuous casting is repeatedly pressed in the width direction using a press tool (hereinafter referred to as anvil). In addition, it is applied in anvils in continuous width presses developed to reduce the width of the slab from the leading end to the trailing end according to the billet size.

The present invention seeks to propose a steel for a hot press tool that is advantageously adapted for use as described above as a hot press tool.

(Prior art) Hot die steel used for press dies, forging dies, etc.
Along with cutting tools, impact-resistant tools and cold dies
There is a standard in JIS G4404, and SKD 6 has been mainly used as an anvil for continuous width pressing.

(Problems to be Solved by the Invention) Mold steel as described in the prior art shows sufficient resistance to normal hot working, but is suitable for use such as anvil for continuous width pressing. Sex is inadequate.

In other words, continuous width press anvils are large and are used continuously under the width pressure of a hot slab of 1200 ° C or more, and the surface of the anvil is the highest while heating and cooling are repeated in each width reduction cycle.
Although it reaches 850 ° C, the SKD 6 hot die steel as described above has a low transformation point (Ac, Ac ₃ ), and it has insufficient fire cracking resistance and thermal shock resistance. Since cracks are frequently generated on the contact surface with the slab and the occurrence rate of chipping flaws is high, there is a disadvantage that the anvil replacement cycle is short and the press productivity is reduced.

Accordingly, an object of the present invention is to provide a steel that is advantageously adapted to a hot press tool used under severe conditions such as a continuous width press.

(Means for solving the problem) SKD 6, which has been conventionally used, has a transformation point (Ac ₁ = 810 ° C, Ac ₃ = 880 ° C) because of the high C content.
Since the temperature is lower than 50 ° C. and the resistance to burning cracking and thermal shock resistance are insufficient, the surface of the anvil has a problem in that a change in the structure or a change in the material occurs and cracks are easily generated or propagated.

Therefore, in order to prolong the life of the anvil, it is important to raise the transformation point (Ac ₁ , Ac ₃ ) to a higher temperature, and to improve the resistance to fire cracking and impact resistance. Can be achieved. However, when the C content is reduced, the high-temperature strength becomes insufficient.
It is necessary to increase the amount and to include Nb. In addition, when the carbon content is low, delta ferrite is easily formed, and when the amount of delta ferrite is 10% or more, the high temperature strength and toughness are remarkably reduced. ) Must be reduced to 15 wt% or less to suppress the appearance of delta ferrite, and for that purpose, the inclusion of Co and N is effective.

In the present invention, the transformation point (Ac ₁ ,
The high temperature strength and toughness were improved by raising the temperature of Ac ₃ ), improving the resistance to fire cracking and thermal shock resistance, and suppressing the formation of delta ferrite by containing Co and N.

In addition, sulfides precipitate at the grain boundaries of the slab contact surface during the use of anvil, and there is a problem that the thermal shock resistance deteriorates. To improve this, one or two selected from Ca and REM are contained. And the parameter indicating the fixed amount of S: Is set to 20 or less, the sulfide precipitation at the crystal grain boundaries is prevented, and the thermal shock resistance is further improved.

That is, the present invention provides C: 0.05 to 0.16 wt% (hereinafter simply referred to as%
), Si: 0.80 to 1.20%, Mn: 0.10 to 0.60%, Cr: 7.0
0 to 9.00%, Mo: 1.00 to 1.50%, V: 0.10 to 0.60%, Nb: 0.0
10 ~ 0.100%, Co: 0.50 ~ 4.00% and N: 0.0300 ~ 0.100%
And one or two selected from Ca and REM
0.0005 to 0.1000%, Cr equivalent = Cr + 4Mo + 5Nb + 11V + 6Si-40C-2Mn-2Co-30N (wt%) is 15Tw% or less, and the balance is Fe and unavoidable impurities.

(Operation) The reason why the composition ranges of the respective components of the steel for a hot press tool of the present invention are limited as described above will be described.

C: 0.05 to 0.16% C is a component necessary for improving hardenability and maintaining strength after quenching and tempering. In addition, it combines with Cr, Mo and V to form a carbide, thereby improving wear resistance and tempering softening characteristics. In order to raise the transformation point (Ac ₁ , Ac ₃ ) to a higher temperature and to improve the sintering crack resistance and the thermal shock resistance, it is desirable to reduce the C content. The strength is insufficient due to the balance with the components. On the other hand, if the C content exceeds 0.16%, excellent toughness cannot be obtained, so the content is set in the range of 0.05 to 0.16%.

Si: 0.80 to 1.20% Si is contained for maintaining the oxidation resistance and maintaining the transformation point (Ac ₁ , Ac ₃ ), but if the Si content is less than 0.80%
There is a disadvantage that the Ac ₁ transformation point ≥850 ° C cannot be maintained, while 1.20
%, The inability to maintain the appearance of delta ferrite may not be maintained, so the range is 0.80 to 1.20%.

Mn: 0.10 to 0.60% Mn is necessary for improving hardenability and suppressing the appearance of delta ferrite. If the Mn content is less than 0.10%, its effect is poor, while if it exceeds 0.60%, the transformation point ( Ac ₁ , Ac
_In order to reduce ₃ ), the range was 0.10 to 0.60%.

Cr: 7.00 to 9.00%, Cr forms a carbonitride and is dispersed in the matrix to improve wear resistance. In addition, a part of the alloy forms a solid solution to improve the quenchability, thereby improving the high temperature strength. Further, it is an effective component for improving high-temperature oxidation resistance, increasing the Ac ₁ transformation point, and decreasing the coefficient of thermal expansion at high temperatures. However, if the Cr content is less than 7.00%, the effect is poor. %, The inability to suppress the appearance of the delta ferrite amount cannot be maintained. Therefore, the content is set in the range of 7.00 to 9.00%.

Mo: 1.00-1.50% Mo forms a solid solution in the matrix to improve hardenability and
Forms hard carbides and disperses in the matrix to improve wear resistance. In addition, it increases temper softening resistance and high-temperature strength. Mo is added in order to obtain the above-mentioned effects, but if the Mo content is less than 1.00%, a sufficient effect cannot be obtained. On the other hand, if the Mo content exceeds 1.50%, the toughness is deteriorated. %.

V: 0.10 to 0.60% V precipitates fine carbonitrides, increases temper softening resistance and high-temperature strength, and raises transformation points (Ac ₁ , Ac ₃ ). However, if the V content is less than 0.10%, a sufficient effect cannot be obtained. On the other hand, if the V content exceeds 0.60%, coarse carbides are formed and the toughness is reduced, so that the range is 0.10 to 0.60%.

Nb: 0.010 to 0.100% Nb has the effect of suppressing grain coarsening when quenching at a higher temperature in order to increase softening resistance and high-temperature strength, and obtains excellent high-temperature strength and toughness It is an important ingredient for. If the Nb content exceeds 0.100%, carbides which hardly form a solid solution are formed, and the toughness is reduced. On the other hand, if it is less than 0.010%, the effect is difficult to obtain, so that the content is set in the range of 0.010 to 0.100%.

Co: 0.50 to 4.00% Co plays an important role in preventing the formation of delta ferrite without significantly lowering the Ac ₁ transformation point, and is an important component for enabling the effects of Mo, V, and Nb to be sufficiently exerted. is there. Co
When the content is less than 0.50%, the effect is not sufficient. On the other hand, when the content exceeds 4.00%, there is a disadvantage that the toughness is reduced. Therefore, the content is set in the range of 0.50 to 4.00%.

N: 0.0300 to 0.100% N is contained in an amount of 0.0300% or more to prevent high-temperature strength and to prevent the formation of delta ferrite, but if it exceeds 0.100%, the toughness is significantly deteriorated, so the upper limit is made 0.100%.

One or two selected from Ca and REM are 0.0005 to 0.10
00% Both rare earth elements (REM) and Ca mainly composed of La and Ce have the effect of fixing the amount of S and preventing sulfides from precipitating at the crystal grain boundaries. Improve properties. However, a large amount is not necessary, but rather increases inclusions. Therefore, the upper limit is 0.1000% in both cases of single use and combined use, and the lower limit is a parameter indicating a fixed amount of S, from the viewpoint of [S] control. From 0.0005%.

In the present invention, in addition to the limitation of the component composition range, it is essential that Cr equivalent = Cr + 4Mo + 5Nb + 11V + 6Si-40C-2Mn-2Co-30N (wt%) is 15 wt% or less. Cr equivalent of 15wt%
If it exceeds 1, the amount of delta ferrite becomes 10% or more, and the high-temperature strength and toughness are remarkably reduced.

In addition, P and S were inevitably used as 0.01
It can be contained at 5% or less, 0.010% or less.

The steel for a hot press tool of the present invention is manufactured by the following manufacturing process.

Steelmaking-Forging-Heat treatment-Machining (Examples) Steels having the respective component compositions shown in Table 1 were melted in a converter, ingoted, forged into a shape of 410 x 440 mm square, and quenched (1040 to 1040).
80 ° C, quenched after 10h, and tempered (670-600 ° C, 10h, A.
C.), and then machined to an anvil of a predetermined size and subjected to an actual machine test.

Table 2 and Fig. 1 show the results obtained by examining the results of using anvils for each test steel thus obtained. Fig. 2 shows the relationship between the C content and the transformation points (Ac ₁ , Ac ₃ ). The strength at temperature, fracture toughness and coefficient of thermal expansion are shown in FIGS. 3 and 4, respectively.

In Comparative Examples 1 to 4, the transformation point (Ac ₁ ) is lower than the maximum temperature of 850 ° C. of the anvil surface, so that the structure change and the material change during use are likely to occur, and the fire cracking resistance and the thermal shock resistance are insufficient. Forging of 1500 hot slabs, crack depth 30-50mm
Cracks were generated and the chipping rate was as high as 18 to 36%.

On the other hand, invention steels 5-28 have a hot slab because the Ac ₁ transformation point is higher than the maximum temperature of 850 ° C. on the anvil surface, and the cracking resistance, thermal shock resistance, high temperature strength, and fracture toughness are improved. A crack depth of 10 mm or less has been achieved with 4,000 forgings, and the crack occurrence rate has been improved to 0%.

(Effects of the Invention) Thus, according to the present invention, the transformation points (Ac ₁ , Ac ₃ ), which are insufficient in the conventional hot die steel, increase in quenching resistance, thermal shock resistance, high-temperature strength, and fracture. It is possible to achieve improvement in toughness, and therefore, it is possible to provide a press tool that is advantageously suited for continuous width pressing of hot slabs and the like.

[Brief description of the drawings]

Figure 1 is a graph showing the relationship between the C content and the transformation point (Ac _1, Ac _3), FIG. 2 is a graph showing the relationship between the C content and the transformation point (Ac _1, Ac _3), FIG. 3 is a graph showing the strength and fracture toughness at each temperature, and FIG. 4 is a graph showing the thermal expansion coefficient at each temperature.

Claims (1)

(57) [Claims] C: 0.05 to 0.16 wt%, Si: 0.80 to 1.20 wt%, Mn: 0.10 to 0.60 wt%, Cr: 7.00 to 9.00 wt%, Mo: 1.00 to 1.50 wt%, V: 0.10 to 0.60 %, Nb: 0.010 to 0.100 wt%, Co: 0.50 to 4.00 wt%, and N: 0.0300 to 0.100 wt%, and one or two selected from Ca and REM are 0.0005 to 0.10.
A steel for hot press tooling, containing 00wt%, Cr equivalent = Cr + 4Mo + 5Nb + 11V + 6Si-40C-2Mn-2Co-30N (wt%) is 15wt% or less, and the balance is Fe and unavoidable impurities.