JP2000273590A - Cast steel for heat treatment, excellent in weldability and machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cast steel for heat treatment, capable of securing superior mechanical properties, particularly hardness and toughness, excellent in weldability and machinability, and having superior castability required of a casting. SOLUTION: The cast steel has a composition consisting of, by weight, 0.4-0.65% C, 0.3-1.4% Si, 0.01-0.4% S, 0.3-1.0% Mn, 5.5-9.5% Cr, Mo and W in amounts within the ranges satisfying (Mo+1/2W)=1.0 to 2.0%, 0.1-0.6% V, and the balance Fe with inevitable impurities. Further, the value Z of the eutectic value represented by equation Z=8*(C%)+0.6*(Cr%) is regulated to <=10.8, and the area ratio of M7C3 carbides at the time of solidification in the structure is regulated to <=2%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the weldability and machinability of steel plates used for automobiles, household appliances, agricultural equipment, etc., which are heat-treated with a die for punching, bending, drawing or trimming. It relates to cast steel for heat treatment having excellent heat resistance.

[0002]

2. Description of the Related Art Automakers and the like have been reducing costs in all fields in order to overcome price competition and secure profits. The field extends to the mold manufacturing field.To reduce costs, various measures such as shortening the manufacturing process of press-formed products, reducing the number of molds, and developing machining methods and tools are also underway. Has been implemented. Also,
With the die used for punching, bending, drawing or trimming of steel sheets, the advantage of reducing the processing allowance compared to steel in a die that shapes the shape of the punched product that changes three-dimensionally Taking advantage of this, cast steel products have also been adopted. Focusing on this point, there has recently been a growing need for improved quality of cast steel products.

[0003] Conventionally, such a mold material, particularly for cold working, contains a large amount of carbide for imparting abrasion resistance, and further has excellent quenching properties and a large content of Cr for securing toughness. Was sought. For example, as one example, a high C-high Cr steel such as SKD11 which is an alloy tool steel specified in JIS G 4404 is used.

[0004]

However, the tendency in recent years is to reduce the number of parts constituting a mold, to integrally mold, and to complicate the shape. However, the volume to be processed is enormous, which causes an increase in manufacturing cost. On the other hand, attempts have been made to reduce the machining allowance by casting with a material component equivalent to SKD11, but because of the large amount of carbides, there are problems with toughness, weldability, machinability, etc. At present, it has not been put to practical use.

The basic characteristics required for such a mold material include hardenability, wear resistance, toughness, weldability, machinability and the like. As technology has developed, it has become unnecessary to contain a large amount of hard brittle carbide in order to ensure wear resistance. Looking at recent trends, the start of the die machining process has been shortened, and the shape has been modified by a design change. Weldability for use has become important. Furthermore, in addition to shortening the start-up time of the die processing step, reduction of die processing cost has been pursued more, and improvement of machinability of the die has become important.

As described above, each of the conventional materials has advantages and disadvantages. Therefore, as a result of intensive studies, the present invention is to provide a mold material which is excellent in weldability and machinability without reducing the mechanical properties and quenching, and which is cast steel with a small processing amount.

[0007]

SUMMARY OF THE INVENTION The present inventor has reviewed the basic conditions required for improving the weldability, thereby reducing the C content as a basic component, and freeing S and Ca as free-cutting elements alone or separately. Even if the machinability is improved by adding a composite, good mechanical properties, particularly hardness and toughness, can be obtained, and the cast steel of the present invention has good castability without impairing the weldability even as a casting.

First, the cast steel for heat treatment according to the present invention comprises
C: 0.4 to 0.65%, Si: 0.3 to 1.4%,
S: 0.01 to 0.4%, Mn: 0.3 to 1.0%, C
r: 5.5-9.5%, Mo + 1 / 2W: 1.0-2.
0%, V: 0.1 to 0.6%, with the balance being Fe and unavoidable impurities.
Alternatively, C: 0.4 to 0.65% and Si: 0.
3-1.4%, S: 0.01-0.4%, Mn: 0.3
0.8%, Cr: 5.5-9.5%, Mo + 1/2
W: 1.0 to 2.0%, V: 0.1 to 0.6%, Ca:
The chemical component is 0.0005 to 0.004%, with the balance being Fe and unavoidable impurities.

The eutectic value Z is 10.8 or less, and the M7C3 carbide during solidification is 2% or less in area ratio. The eutectic value Z is a dangerous value for causing a casting defect to occur easily when the solid-liquid coexistence temperature width increases in the process of solidifying the molten metal of the cast steel, and Z = 8 × (C%) + 0.6 × ( Cr
%). Note that (C%) and (Cr
%) Is% by weight of the content. If the eutectic value Z exceeds 10.8, casting defects after solidification are likely to occur, which is not preferable. In addition, M7C3 carbide at the time of solidification is coarse and hard and brittle, which causes deterioration of machinability. For this reason, if M7C3 carbide at the time of solidification exceeds 2%, machinability will deteriorate remarkably. Diffusion annealing at 1100 ° C. or higher is performed to achieve a state in which the area ratio of the M7C3 carbide is 2% or less.

[0010] Further, casting is performed using the vanishing model. As a casting model, a wooden mold used for a general green sand mold or a self-hardening sand mold may be used. The model production cost is lower when the disappearing model is used.

The present invention enables welding at a preheating temperature of at least 250 ° C. before welding. Generally, cast and forged steel products such as molds are welded for shape change or repair depending on the situation during manufacture or use, but alloy steel is preheated to high temperature to prevent cracking during welding. Will be implemented. In particular, when Cr or the like is contained, it is generally carried out after preheating to 450 to 550 ° C. or higher, but with the chemical components of the present invention, cracking does not occur even if welding is performed after preheating to at least 250 ° C. Low preheating temperature makes it economical and easy to work.

Next, after the post-heating after the welding is maintained at 450 ° C. for one hour, the cooling time to the room temperature can be cooled in at least three hours. The working time can be reduced and it is economical as compared with the conventional cooling method of cooling from 450 to 550 ° C. for 15 to 16 hours.

The main object of the cast steel for heat treatment of the present invention is to suppress the content of C and to contain an appropriate amount of free-cutting elements such as S and Ca so that the mechanical properties and hardenability are not reduced, and the weldability,
It is to produce cast steel excellent in machinability and castability. When used in mold materials, they have excellent weldability, so even if they break, crack, or wear during use, they can be repaired and reused. Also, since it is made of cast steel, the material can be cast in a shape close to the required final product shape, and the processing cost is small compared to rolled steel and the machinability is excellent, so the cost of processing is small. I'm done. Further, a rolling operation is not required as compared with a steel material, and the base structure after casting does not have the directivity like a rolled steel material, and solidification is completed. That is, since the structure is stretched in the rolled direction in the rolled steel material, a difference easily occurs in the mechanical properties between the rolling direction and a direction perpendicular to the rolling direction. Also,
When the heat treatment is performed, the degree of change in dimensions is different between the rolling direction and the direction perpendicular thereto. In the case of cast steel, there is no directionality in the structure in terms of mechanical properties and dimensional change after heat treatment, so that it has the property that stable properties can be obtained in any direction.

In addition, the cast steel for heat treatment according to the present invention does not necessarily need to be subjected to quenching / tempering in a heat treatment furnace. When used for a mold material or the like, flame quenching may be performed only at a site necessary for its function, and a heat treatment method for obtaining hardness may be selected in consideration of the number of manufacturing steps or required characteristics. .

Next, the reasons for limiting the components of the present invention will be described. 1) C is necessary for improving the hardenability and maintaining the hardness after the heat treatment. In order to ensure hardness after heat treatment of HRC 55 or more, both the content and the amount of solid solution must be 0.4% or more. If it is less than 0.4%, quenching hardness is insufficient and sufficient strength cannot be secured. C combines with Cr, Mo, V, and W to form a carbide, and improves wear resistance and tempering softening resistance. If the content is excessive, the toughness is reduced, and 0.65
%, The weldability deteriorates. Further, the temperature range of the solid-liquid coexistence becomes large, which may cause the risk of casting defects, that is, increase the eutectic value Z. Therefore, the content of C is 0.4 to 0.65.
%. 2) Si Si is added for the purpose of improving the castability with a deoxidizing agent, but at least 0.3% or more is required to obtain the effect. On the other hand, excessive addition causes the machinability and weldability to be impaired, and the segregation of the matrix components also becomes severe. Therefore, the content of Si is set to 0.3 to 1.4%. 3) Mn Mn is added for improving hardenability, but if it is less than 0.3%, it is insufficient to stably obtain hardenability. On the other hand, if the amount is too large, a large amount of retained austenite is generated, and the quenching hardness is saturated, or conversely, the quenching hardness is reduced. Compounds with S (sulfide compounds)
Is required to improve the machinability, so the content is set to 0.3 to 1.0%. 4) Cr Cr combines with C to form carbides, which has the effect of improving wear resistance and increasing hardenability. However, if the amount is too small, the effect is insufficient. If too much, Cr
The toughness decreases due to the increase in carbides. Further, similarly to the addition of C, the temperature range of coexistence of solid and liquid becomes large, which causes an increase in the risk of occurrence of casting defects (eutectic value Z). Therefore, the addition amount of Cr is set to 5.5 to 9.5%. 5) Mo, W Mo and W improve hardenability. Further, it combines with C to form a hard carbide and improves wear resistance. Since the atomic weight of W is about twice that of Mo, the content of Mo 1% is W2
%, And in the present invention, one or two of Mo and W can be contained, and (Mo + 1 /
The effect can be expressed in 2W) amount. (Mo +
Which component should be preferentially used in the amount of (Ｗ W) may be determined in consideration of economy. If the content of (Mo + 1 / 2W) is less than 1.0%, the effect is insufficient. On the other hand, if the content is excessive, a layered eutectic carbide that reduces toughness is generated, so the content was set to 1.0 to 2.0%. 6) VV is effective in suppressing the growth of retained austenite during quenching and ensuring toughness. To exhibit this effect, a content of 0.1% or more is required. Conversely, an excessive content crystallizes a huge V-based carbide at the time of solidification and causes a decrease in toughness. 7) S S becomes (Mn · Cr) S containing Cr to improve the machinability of the material, but if too much, the mechanical properties deteriorate, so the content was made 0.01 to 0.4%. 8) Ca Ca becomes a low melting point composite oxide composed of CaO, SiO 2, Al 2 O 3, and MnO to improve the machinability of the material. However, if too large, the melting point of the composite oxide increases and the free cutting effect is lost. 0.0005 to 0.004%.

[0016]

Next, embodiments of the present invention will be described in detail, but the present invention is not limited to these embodiments.

(Example 1) Table 1 shows an example of the chemical composition of a cast steel for heat treatment according to the present invention having excellent weldability and machinability and a comparative material thereof.

[0018]

[Table 1]

First, the material was melted using a 1-ton high-frequency furnace. Next, the molten metal was poured into a mold (Fran sand mold) at 1560 ° C. using a ladle. After solidification and cooling in the mold, the test piece material was removed from the mold. Subsequently, for the purpose of improving the casting structure and removing casting stress, annealing was carried out at 850 ° C. for 4 hours using a batch type heat treatment furnace. Then, after dropping the scale with shot blast,
The feeder and the runner were cut off. The shape of the test piece material used for the evaluation of the weldability was T-shaped, and the dimensions of the flange part were 200 width.
mm, length 100 mm, thickness 40 mm, and dimensions of the web portion were height 40 mm and thickness 15 mm. The dimensions of the test piece material used for evaluation of machinability were 100 mm in width and 200 m in length
m and height 100 mm.

Next, the cast surface of the test piece material was removed by machining to finish the shape of the test piece for evaluating weldability. Then, the test piece for welding evaluation was 102
After heating and holding at 5 ° C., gas cooling quenching was performed with an inert gas, and further, tempering was performed at 500 to 550 ° C. so that the target hardness of each test piece was 55 or more HRC. The weldability was evaluated under the conditions shown in Table 2 using the test pieces thus manufactured.

[0021]

[Table 2]

Table 3 shows an example of the hardness and weldability (presence or absence of cracks after welding) of the quenching and tempering heat treatment results.
In all cases, no cracking occurred in the invention material, but cracks occurred in the comparative material at preheating temperatures of 350 and 450 ° C.

[0023]

[Table 3]

Further, JIS was used for the tensile test from the test piece material.
A No. 4 test piece (14 mm in diameter) was cut out from a 10 mm square, 55 mm long, 2 mm central notch depth, and 10 mm radius test piece for Charpy impact test for toughness measurement, and subjected to mechanical property investigations. . Heat treatment is 102 in the vacuum heating path.
After heating to 5 ° C., gas cooling and quenching are performed with an inert gas, and further, 500 to 55 so that the target hardness is HRC 55 or more.
Tempering was performed at 0 ° C. Table 4 shows an example of the result. Comparative material 1 is a SKD11 rolled steel material and has mechanical properties in a direction perpendicular to the rolling direction. Comparative material 2 is a cast material whose chemical component is equivalent to SKD11. It can be seen that the invented material has higher toughness in the direction perpendicular to the rolling direction of the rolled steel material and more than twice the toughness of the conventional cast steel material.

[0025]

[Table 4]

Next, Table 5 shows an example of the result of investigation on the eutectic value Z and the occurrence of casting defects. No micro shrinkage cavities were observed in any of the inventive materials.

[0027]

[Table 5]

Next, the cast surface of the test piece material was removed by machining to evaluate the machinability, and the shape of the test piece for evaluating the machinability was finished. The machinability is evaluated for both the annealed material and the quenched / tempered material. The finished test piece was used as an annealed material. For the quenched / tempered material, after heating and holding the finished test piece at 1025 ° C. using a vacuum heating furnace,
Gas cooling quenching is performed with an inert gas, and further, 500 g of each test piece is set so that the target hardness is HRC 55 or more.
Tempering was performed at 550 ° C. Using the test pieces of the annealed material and the quenched / tempered material thus manufactured, Table 6
Evaluation of machinability was carried out under the conditions shown below.

[0029]

[Table 6]

Table 7 shows the evaluation results of the machinability. here,
The end mill edge wear amount is indicated by a wear index when SKD11 is set to 10. A larger wear index indicates a larger end mill edge wear amount. Invention materials 9, 10,
In No. 11, it was recognized that the machinability of both the annealed material and the quenched and tempered material was significantly improved as compared with Comparative Materials 1, 12, and 13.

[0031]

[Table 7]

Example 2 A test piece was cast in the same manner as in Example 1, except that a vanishing model was used as a test piece material model.
The shape and size of the test piece material were T-shaped and the same as in Example 1. Subsequently, using a batch type heat treatment furnace at 850 ° C.
Time holding annealing was performed. Thereafter, the scale was dropped by shot blasting, and the feeder and the runner were cut and removed. next,
The cast surface of the test piece material was removed by machining, heated and maintained at 1025 ° C. in a vacuum heating furnace, and then quenched with an inert gas. The target hardness after tempering is HRC 55 or more.
The welding conditions were the same as in Example 1, except that the preheating temperature before welding was set at three levels of 250 ° C., 350 ° C., and 450 ° C., and the post-heating after welding was maintained at 450 ° C. for 1 hour, and then over 7 hours. It was cooled to room temperature.

Table 8 shows one example of a weldability test of the cast steel for heat treatment of the present invention and a comparative material. In the case of inventive materials 1 to 6, welding cracks did not occur in any case when the preheating temperature was 250 ° C or 350 ° C. On the other hand, Comparative Materials 1, 6, and 11 were preheated at 450 ° C., and Comparative Material 5 was performed at 350 ° C., but all of them had weld cracks.

[0034]

[Table 8]

Example 3 Next, the effects of the preheating temperature before welding and the cooling time after welding on the weldability were investigated. One example of the results is shown in Table 9. After welding, the post-heat was maintained at 450 ° C. for 1 hour, and then cooled to room temperature over 3 hours or 7 hours. The invention material did not crack even when the cooling time was 3 hours, but the comparative material cracked after 7 hours.

[0036]

[Table 9]

[0037]

As described above, according to the present invention, as compared with the SKD11 rolled steel material, the C content as a basic component is reduced, and S and Ca as free-cutting elements are added singly or in combination, that is, in a composite content. As a result, even if the machinability is improved, good mechanical properties, in particular, hardness and toughness can be secured, and a cast steel having good castability can be provided without impairing weldability even as a casting.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Kunichika Kubota 2107-2 Yasugi-cho, Yasugi-shi, Shimane Hitachi Metals Research Institute Metallurgy Co., Ltd. (72) Miki Yamaoka 2107-2 Yasugi-cho, Yasugi-shi, Shimane Hitachi Metals Inside Yasugi Plant

Claims (6)

[Claims] 1. C: 0.4 to 0.65% by weight, S:
i: 0.3 to 1.4%, S: 0.01 to 0.4%, M
n: 0.3 to 1.0%, Cr: 5.5 to 9.5%, Mo
+ 1 / 2W: 1.0 to 2.0%, V: 0.1 to 0.6
%, With the balance being Fe and unavoidable impurities, having a eutectic value Z of 10.8 or less and an M7C3 carbide at the time of solidification of 2% or less in area ratio. Cast steel for heat treatment with excellent machinability. 2. C: 0.4 to 0.65% by weight, S
i: 0.3 to 1.4%, S: 0.01 to 0.4%, M
n: 0.3 to 0.8%, Cr: 5.5 to 9.5%, Mo
+ 1 / 2W: 1.0 to 2.0%, V: 0.1 to 0.6
%, Ca: 0.0005 to 0.004%, and the balance consists of Fe and unavoidable impurities, the eutectic value Z is 10.8 or less, and the M7C3 carbide at the time of solidification is 2% in area ratio. A cast steel for heat treatment excellent in weldability and machinability, characterized in that: 3. The method according to claim 1, wherein diffusion annealing at a temperature of 1100 ° C. or more is performed in order to realize an area ratio of M7C3 carbide of 2% or less. Cast steel for heat treatment with excellent weldability and machinability. 4. The heat-treated cast steel having excellent weldability and machinability according to claim 1, wherein the cast steel is cast using a vanishing model. 5. The preheating temperature before welding is at least 250 ° C.
5. The welding according to claim 1, wherein the welding is possible.
The cast steel for heat treatment excellent in weldability and machinability according to any one of the above. 6. The welding according to claim 1, wherein after the post-weld heat is maintained at 450 ° C. for one hour, the cooling time to room temperature can be cooled in at least three hours. Cast steel for heat treatment with excellent heat resistance.