JPH04120244A - Steel for hot pressing tool - Google Patents

Abstract

PURPOSE:To obtain a steel for hot pressing tool having strength at high temp. and toughness and excellent in thermal shock resisting property and quenching crack resistance by specifying a composition consisting of C, Si, Mn, Cr, Mo, V, Nb, Co, N, Ca, REM, and Fe. CONSTITUTION:This steel is a steel for hot pressing tool having a composition consisting of, by weight, 0.05-0.16% C, 0.80-1.20% Si, 0.10-0.60% Mn, 7.00-9.00% Cr, 1.00-1.50% Mo, 0.10-0.60% V, 0.010-0.100% Nb, 0.50-4.00% Co, 0.0300-0.100% N, 0.0005-0.1000% Ca and/or REM, and the balance Fe with inevitable impurities, and this steel has high transformation point and is improved in strength at high temp. and toughness as well as in quenching crack resistance and thermal shock resisting property. Accordingly, this steel can be advantageously used, e.g. for anvil for continuous spreading press for hot slab.

Description

Detailed Description of the Invention (Field of Industrial Application) Steel for hot press tools is produced by repeatedly applying widthwise clamping pressure to a hot slab after continuous casting using a pressure tool (hereinafter referred to as an anvil). In addition, it is applied to an anvil in a continuous width press developed to reduce the width of a slab from the front end to the back end according to the billet size.

The present invention aims to propose a steel for hot press tools that is advantageously suitable for the above-mentioned uses as hot press tools.

(Prior art) Steel for hot molds used for press dies, forging dies, etc., as well as for cutting tools, shock-resistant tools, and cold molds, is classified by J.
There is a standard in IS G4404, and SKD 6 has been mainly used as an anvil for continuous width presses.

(Problems to be Solved by the Invention) Mold steels such as those listed in the prior art exhibit sufficient resistance to normal hot working, but are not suitable for uses such as anvils for continuous width presses. Sexuality is insufficient.

In other words, the continuous width press anvil is large and is used continuously for width reduction of hot slabs at temperatures of 1200°C or higher, and the anvil surface reaches a maximum temperature of 850°C while heating and cooling are repeated in each width reduction cycle. However, the above SK
D6 hot mold steel has a transformation point (Ac.

Due to the low Ac3), insufficient fire cracking resistance, and insufficient thermal shock resistance, cracks occur frequently on the contact surface with the slab, and the occurrence of cracks and spots is high, so the anvil replacement cycle is short and press This had the disadvantage of reducing productivity.

Therefore, an object of the present invention is to provide a sea bream that is advantageously suitable for hot press tools used under severe conditions in continuous width presses and the like.

(Means for solving the problem) The conventionally used SKD 6 has a high C content, so its transformation point (Ac+ = 810°C, AC3 = 880°C)
is lower than the maximum temperature of the anvil surface of 850°C, and the anvil surface is susceptible to structural changes and material changes, causing cracks to occur and propagate. There was a problem.

Therefore, in order to extend the life of the anvil, the transformation point (A
It is important to raise the temperature of the alloys (c, , Ac, ) to higher temperatures and to improve the quench cracking resistance and thermal shock resistance, and these can be achieved by reducing the amount of C. However, if the amount of C is reduced, the high temperature strength becomes insufficient, so in order to improve the high temperature strength, it is necessary to increase the amount of Cr and include Nb.

In addition, if the carbon content is low, delta ferrite is likely to be generated, and if the amount of delta ferrite exceeds 10%, high temperature strength and toughness will decrease significantly, so Cr equivalent = Cr + 4Mo + 5Nb + 11V + 6Si
It is necessary to suppress the appearance of delta ferrite by reducing 40C-2Mn-2Co-3ON (wtZ) to 15 wt% or less, and for that purpose, the inclusion of Co and N is effective.

In this invention, the transformation point (^
C1l AC3) to a higher temperature, and has resistance to sintering,
High-temperature strength and toughness were improved by improving thermal shock resistance and suppressing the formation of delta ferrite by containing Co and N.

In addition, there is a problem that sulfides precipitate at the grain boundaries of the contact surface of the slab during use of the anvil, reducing the thermal shock resistance.In order to improve this problem, one or two selected from Ca and REM are added. By setting the parameter representing the fixed amount of S to 20 or less, sulfide precipitation at grain boundaries was prevented and thermal shock resistance was further improved.

That is, in the present invention, C: 0.05 to 0.16 wt
Z (hereinafter referred to as single % test), Si: 0.80 to 1.20
%, Mn: 0.10-0.60%, Cr: 7.
00-9.00%, Mo: 1.00-1.50%,
V: 0.10-0.60%, Nb: 0.010
~0.100%, co=0.50-4.00% and N
: Contains 0.0300-0.100%, and Ca,
0.0005 for one or two types selected from REM
It is a hot press steel containing ~0.1000%, with the remainder consisting of Fe and unavoidable impurities.

(Function) Regarding the steel for hot press tools of the present invention, the reason why the composition range of each component is limited as described above will be explained.

C: 0.05 to 0.16% C is a component necessary to improve hardenability and maintain strength after quenching and tempering. It also combines with Cr, Mo and V to form carbides, improving wear resistance and temper softening properties. Metamorphosis point (Ac, .

It is desirable to reduce the amount of C in order to raise the temperature of Ac, ) to a higher temperature and improve the quench cracking resistance and thermal shock resistance, but if the amount of C is less than 0.05%, it is difficult to balance with other components. The strength becomes insufficient. On the other hand, the amount of C is 0.16
If it exceeds 0.0%, excellent toughness cannot be obtained.
It was made into the range of 5-0.16%.

Si: 0.80-1.20% Si maintains oxidation resistance and maintains transformation point (Ac, , AC
It is included to maintain 3), but the Si content is 0.8
If it is less than 0%, there is a disadvantage that Ac+transformation point ≧850°C cannot be maintained, while if it exceeds 1.20%, there is a disadvantage that it is not possible to maintain the suppression of the appearance of delta ferrite, so 0.80 to 1.20%. The range of

Mn: 0.10-0.60% Mn is necessary for improving hardenability and suppressing the appearance of delta ferrite, and if the Mn content is less than 0.10%, the effect will be poor; If it exceeds 60%, the transformation point (Ac, , Ac3) will be lowered, so 0.
The range was 10% to 0.60%.

Cr: 7.00 to 9.00%, Cr forms carbonitrides and is dispersed in the matrix to improve wear resistance. In addition, some of it is dissolved in solid solution to improve hardenability and high-temperature strength. Furthermore, it is an effective component for improving high-temperature oxidation resistance, raising Ac and transformation points, and lowering the coefficient of thermal expansion at high temperatures, but if the Cr content is less than 7.00%, its effects are poor. On the other hand, if it exceeds 9.00%, there will be an inconvenience that it will not be possible to maintain suppression of the appearance of delta ferrite, so it is set in the range of 7.00 to 9.00%.

Mo: 1.00 to 1.50%, Mo improves hardenability by solid solution in the matrix and also improves C
It combines with the metal to form a hard carbide, which is dispersed throughout the matrix and improves wear resistance. It also increases temper softening resistance and high temperature strength. Mo is added to obtain the above effects, but if the MO content is less than 1.00%, sufficient effects cannot be obtained, while if it exceeds 1.50%, the toughness may deteriorate. From 1.00 to 1.50%.

V: 0.10 to 0.60% (2) precipitates fine carbonitrides, increases temper softening resistance and high temperature strength, and also raises the transformation point (Ac, Ac3). However, if the content is less than 0.10%, sufficient effects cannot be obtained, while if it exceeds 0.60%, coarse carbides are formed and the toughness is reduced.
The range was 0.60%.

Nb: 0.010-0, ioo% Nb has the effect of suppressing grain coarsening when hardening is performed at a higher temperature in order to increase softening resistance and high-temperature strength, and has excellent high-temperature strength and dust. It is an important ingredient for obtaining sex. If the Nb content exceeds 0.100%, carbides that are difficult to dissolve in solid form are formed and the toughness decreases;
If it is less than 10%, it is difficult to obtain the effect, so 0.010
The range was 0.100%.

Co: 0.5 (1-4,00% Co plays the role of preventing the formation of delta ferrite without significantly lowering the Ac and transformation point, and allows the effects of Mo, ■, and Nb to be fully exhibited. If the Co content is less than 0.50%, the effect will not be sufficient, while if it exceeds 4.00%, it will cause the disadvantage of decreasing toughness. 4.00%
The range of

N: 0.0300-0.100% N is contained in an amount of 0.0300% or more to prevent high-temperature strength and delta ferrite formation, but if it exceeds 0.100%, the toughness will deteriorate significantly, so the upper limit is set. 0.10
It was set to 0%.

One or two selected from Ca and REH are added to 0.
0005~o, 1ooo% Rare earth elements (REM) mainly composed of La and Ce and Ca have the effect of fixing the amount of S and preventing sulfide from precipitating at grain boundaries, and this effect improves slab contact. Improves the thermal shock resistance of surfaces. However, it is not necessary to include a large amount of S, and rather it will lead to an increase in inclusions, so the upper limit is set at 0.1000% whether used alone or in combination, and the lower limit is set by a parameter indicating a fixed amount of S, [S] control. From this point of view, it was set at 0.0005%.

In addition, P and S are each 0.0 as unavoidable impurity components.
It may be contained in an amount of 15% or less, 0.010% or less.

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

Steelmaking - Forging - Heat treatment - Machining (Example) Steels having the respective compositions shown in Table 1 were melted in a converter, and after forming ingots, they were forged into a shape of 410 x 440 m square, and quenched (
After performing heat treatment consisting of 1040 to 1080 °C, 10 h, quenching) and annealing at 670 to 700 °C, 10 h, A, C,), it was machined into an anvil of predetermined dimensions and subjected to actual machine testing. .

Table 2 and Figure 1 show the results of the investigation on the anvil usage history of each test steel obtained in this way, and the C content and transformation point (A
c, , Ac3) is shown in FIG. 2, and the strength, fracture resistance, and coefficient of thermal expansion at each temperature are shown in FIG. 3 and FIG. 4, respectively.

Comparative Examples 1 to 4 have transformation points (Act) lower than the maximum surface temperature of the anvil of 850°C, so they tend to undergo structural changes and material changes during use, and their quench cracking resistance and thermal shock resistance are insufficient. Therefore, in the forging of 1,500 hot slabs, hexagonal cracks with a depth of 30 to 50 m occurred, and the occurrence rate of chips and spots was as high as 18 to 36%.

On the other hand, invention fI! 45 to 28 are Ac, have a transformation point higher than the maximum anvil surface temperature of 850°C, and have improved quench cracking resistance, thermal shock resistance, high temperature strength, and fracture resistance, so they do not crack after 4000 hot slabs are forged. A depth of 10 mm or less has been achieved, and the occurrence rate of defects has been improved to 0%.

(Effects of the Invention) Thus, according to the present invention, the transformation point (Act + AC3) and the quench cracking resistance, thermal shock resistance, high temperature strength, and fracture resistance, which were lacking in conventional steels for heat-related molds, can be increased. improvements can be achieved and thus the provision of a pressing tool advantageously adapted for continuous width pressing of hot slabs etc. can be realized.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between C content and transformation point (Am, Ac, ), Figure 2 is a graph showing the relationship between C content and transformation point (Act, AC3), Figure 3 is a graph showing the relationship between C content and transformation point (Act, AC3). is a graph showing strength and fracture resistance at each temperature,
゛Figure 4 is a graph showing the coefficient of thermal expansion at each temperature. Fig. 2 ci*#(wtz> Fig. 3

Claims (1)

[Claims] 1. 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 wt%, Nb: 0.010 to 0.100 wt%, Co: 0.50 to 4.00 wt%, and N: 0. 0300 to 0.100wt%, and 0.0% of one or two selected from Ca and REM.
005 to 0.1000 wt%, with the remainder consisting of Fe and unavoidable impurities.