JPH0681083A - Forged steel operating roll material for hot rolling - Google Patents

Abstract

PURPOSE:To obtain a wear resistant forged steel operating roll material for hot rolling excellent in thermal shock cracking resistance. CONSTITUTION:The roll material has a chemical compsn. contg., by weight, 1.4 to 2% C, <=0.6% Si, 0.4 to 1% Mn, <=O.5% Ni, 2 to 3% Cr, 0.7 to 1.2% Mo, 4 to 7% V and <=1% W and, as necessary, contg. <=1% Co, and the balance Fe with inevitable impurities as well as satisfying the relational inequality of 0.7<{(%C)+(%Cr)}/(%V)<1. In this way, the wear resistant forged steel operating roll material in which C, Cr and V are suitably balanced and excellent in thermal shock cracking resistance can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wear-resistant forged steel working roll material for hot rolling, which is used for rolling steel and has excellent thermal shock crack resistance.

[0002]

2. Description of the Related Art Conventionally, working roll materials for hot rolling are
A roll made of high alloy cast iron, cast steel or forged steel that contains a large amount of C, Cr, Mo, V, W in a complex manner and disperses hard carbides of these elements in the structure to ensure high wear resistance. Wood is used. For example, in% by weight, C: 2-2.
5%, Cr: 4-6%, Mo: 2-4%, V: 5-8%,
A composite continuous casting high speed steel roll containing W: 2 to 4% is known.

[0003]

However, the conventional high-alloy roll material cannot be said to have sufficient resistance to thermal shock cracks, and there is a problem in improving it. Working rolls for hot rolling mills are used in an environment where thermal shocks are almost inevitable when rolling mills are turned on or off, or when rolling accidents such as narrowing accidents occur. Is an important property. The crack generated due to the above-mentioned situation or the like needs to be completely ground and removed before the next use, which causes the roll cost to be greatly deteriorated. In addition, the cracks that have formed may greatly propagate inside the roll, leading to fatal roll damage that makes subsequent use impossible.

As a result of investigating the cause of this crack,
It has been found that in the conventional material, a large amount of coarse eutectic carbide is generated in the grain boundary in a net shape, and cracks easily propagate through the carbide as a route. That is,
In the material with developed eutectic carbide net, a defect was found that deep thermal shock cracks occur in the carbide as a propagation path. The present invention has been made in view of the above circumstances and provides a forged steel working roll material for hot rolling which has improved problems of the conventional roll material and excellent in heat shock-cracking properties and high wear resistance. The purpose is.

[0005]

In order to solve the above-mentioned problems, the forged steel working roll material for hot rolling of the present invention is
C: 1.4 to 2%, Si: 0.6% or less, Mn:
0.4 to 1%, Ni: 0.5% or less, Cr: 2 to 3%,
Mo: 0.7 to 1.2%, V: 4 to 7%, W: 1% or less, optionally Co: 1% or less, the balance Fe and unavoidable impurities, and 0.7 <{ (% C) + (% Cr)} / (% V) <1, and is characterized by having a chemical composition satisfying the relational expression.

[0006]

According to the present invention, the thermal shock crack characteristics are improved by properly mixing C, Cr and V. Further, in the present invention, a large working roll having a diameter of 700 mm or more is also targeted, and this large roll causes a large thermal stress in the quenching heat treatment, resulting in a large risk of quench cracking, which is a manufacturing problem. Therefore, in order to reduce the risk and enable stable production, control of defects (forging effect), which are the origin of quenching cracks such as void defects and segregation during solidification, and quenching cracks are caused. Lower quenching heating temperatures are required to reduce the working thermal stress. In practice, the upper limit of the quenching heating temperature is 1050 ° C. depending on various conditions. In the material of the present invention,
The material is designed in consideration of not only the performance but also the manufacturability thereof.

The reasons for limiting the chemical components of the material of the present invention specified from these viewpoints will be specifically described below. C: 1.4 to 2% C not only gives the hardness of a predetermined matrix , but also forms an alloy carbide to improve wear resistance. Therefore, it is necessary that the amount of C that forms a solid solution in the matrix of the roll and the amount of C that forms carbides be appropriate amounts. In order to obtain a Shore hardness of 75 or more, which is a desired hardness as a roll material, under predetermined heat treatment conditions, as shown in FIG.
It is necessary to contain C by 1.4% or more. On the other hand, C is
Since the formation of net-like eutectic carbides in the solidified grain boundaries is remarkably promoted, when the content exceeds 2%, deep thermal shock cracks are generated as shown in FIG. Further, if a large amount of eutectic carbide is generated, the hot workability deteriorates, and it becomes difficult to stably manufacture the roll. Therefore, C
The content was limited to 1.4-2%.

Si: 0.6% or less Si acts effectively as a deoxidizing agent, but from the viewpoint of segregation, the smaller the content, the smaller the tendency for segregation to occur, which is desirable. In the material of the present invention, if the content exceeds 0.6%, it becomes difficult to secure the sound layer depth of the surface layer required for the roll (surface layer thickness without segregation). It was limited to 0.6% or less.

Mn: 0.4 to 1% Mn has the effect of improving hardenability, but 0.4%
If the content is less than that, the effect is not clearly shown. Also,
If the content exceeds 1%, embrittlement occurs. Therefore, the Mn content is limited to 0.4 to 1%. Ni: 0.5% or less Ni improves the hardenability and the mechanical properties of the matrix. However, in the material of the present invention, if the content exceeds 0.5%, a large amount of retained austenite is generated during quenching. However, the quenching hardness is reduced, so the content is limited to 0.5% or less.

Cr: 2-3% Cr improves the hardenability and mechanical properties of the matrix, and also forms carbides to improve wear resistance. But,
It remarkably promotes the formation of net-like eutectic carbides at solidified grain boundaries. As shown in FIG. 3, when the content exceeds 3%, deep thermal shock cracks are generated. Further, in the material of the present invention, the effect of Cr on the wear resistance is small,
From the viewpoint of wear resistance, it is not necessary to contain a large amount of Cr, which exceeds 3%. On the other hand, Cr solid-dissolved in the matrix has an effect of improving the thermal shock crack resistance, and it is necessary to contain Cr in an amount of 2% or more to effectively obtain this effect. Therefore, the Cr content is limited to 2 to 3%.

Mo: 0.7 to 1.2% Mo has an important action for securing a surface hardened layer required as a roll material for improving hardenability and temper softening resistance. It also has the function of forming carbides and improving wear resistance. The Mo content is 0.
If it is less than 7%, the effect does not appear clearly. on the other hand,
If the content exceeds 1.2%, as shown in FIG. 4, the upper limit temperature during hot working is lowered and the forgeability is lowered. Therefore, the Mo content is limited to 0.7 to 1.2%.

V: 4 to 7% V is known to form a very hard carbide and to have an extremely large effect of improving wear resistance. In the present invention, while ensuring this high wear resistance due to V carbide, excellent thermal shock crack resistance was imparted by optimizing the form of the carbide. That is, V has a great influence on the morphology of eutectic carbides, and when its content is balanced with C and Cr in a certain relationship, the eutectic cells in which V-based carbides are dispersed in the solidified grains form carbides. The formation of coarse eutectic carbide, which is formed as a main component and has a net-like form, is reduced. That is, in the structure obtained in such an appropriate balance, excellent thermal shock crack resistance is obtained due to the reduction of coarse net-like eutectic carbide.

As described above, it is possible to suppress the development of net-like eutectic carbides and to generate eutectic cells of V-based carbides so that a necessary amount can be secured from the viewpoint of wear resistance. It is the main technical focus. For that purpose, it is necessary to contain 4% or more of V which makes the depth of the thermal shock crack generated as shown in FIG. 5 extremely shallow. On the other hand, when the content exceeds 7%, a eutectic carbide with good morphology is obtained, but the amount thereof is large, so the amount of C fixed as V-based carbide increases, and as shown in FIG. It becomes difficult to secure the amount of solute C in the matrix, which is required in terms of hardness, at a predetermined quenching heating temperature. Further, the segregation is remarkably generated, and the tendency that cracks are generated from the segregated portion where the carbides are densely formed during hot working or quenching is largely deteriorated. Therefore, the V content is limited to 4 to 7%.

0.7 <{(% C) + (% Cr)} / (% V) <1 In the present invention, the C and Cr solid solution amounts required in the matrix are secured and the eutectic carbide form is obtained. In order to optimize C, Cr and V contents are effectively combined as described above. Therefore, in addition to the reason for limiting the content described for each element in the material of the present invention,
As a combination range of these, 0.7 <{(% C) + (% C
By defining r)} / (% V) <1, excellent thermal shock crack resistance can be obtained. If this value deviates from either the upper limit or the lower limit, the balance is lost and good thermal shock crack resistance cannot be obtained.

W: 1% or less W forms a hard carbide to improve wear resistance.
On the other hand, if a large amount is contained, a net-like eutectic carbide is generated at the solidified grain boundaries and the hot workability is deteriorated, so the content is limited to 1% or less. Co: 1% or less Co is almost solid-solved in the matrix and has an effect of improving the temper softening resistance, and is effective for ensuring the hardness of the roll and improving the thermal shock crack resistance. On the other hand, if contained too much, the hardenability deteriorates, so the upper limit was made 1%.

[0016]

Examples 5 of the invention materials and comparative materials having the compositions shown in Table 1
Each 0 kg steel ingot was melted. The components of each steel ingot are shown in FIG.
Plotted on.

[0017]

[Table 1]

After forging these steel ingots at a forging ratio of 2.0, a predetermined heat treatment is applied to produce thermal shock test pieces of HS76 to 78, which are subjected to a thermal shock test using a tester shown in FIG. I served. The thermal shock tester drops the weight 2 on the rack 1,
The pinion 3 is rotated, and the mild steel rod 4 attached to the pinion 3 is rubbed on the surface of the test piece 5 fixed to the tester, and the friction heat at that time and the composition deformation heat of the mild steel rod 4 cause the test piece 5 to move. It gives a thermal shock. With respect to the test piece after the test, the thermal shock cracking property of the material was evaluated by observing the cross section of the heat shock portion with an optical microscope and comparing the maximum depths of the generated cracks. The thermal shock test results evaluated in this way are shown in FIG. As is clear from the figure, the invention material has shallower cracks than the comparative material, and excellent thermal shock crack characteristics are obtained.

[0019]

As described above, according to the forged steel working roll material for hot rolling of the present invention, C: 1.4% by weight%.
2%, Si: 0.6% or less, Mn: 0.4 to 1%, Ni
: 0.5% or less, Cr: 2-3%, Mo: 0.7-
1.2%, V: 4-7%, W: 1% or less, optionally C
o: Containing 1% or less, consisting of the balance Fe and inevitable impurities, and satisfying the relational expression of 0.7 <{(% C) + (% Cr)} / (% V) <1, C, Cr, and V are appropriately balanced, the formation of net-like eutectic carbides in the solidified grain boundaries, which is the path of thermal shock crack propagation, is suppressed, and the amount of V-based carbides necessary for wear resistance is reduced. It is possible to obtain a forged steel working roll material for rolling having high wear resistance, which is secured as a eutectic cell and is excellent in thermal shock crack characteristics.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between hardness and C content obtained by a predetermined heat treatment.

FIG. 2 is a graph showing the effect of C content on the thermal shock cracking characteristics.

FIG. 3 is a graph showing the effect of Cr content on thermal shock cracking characteristics.

FIG. 4 is a graph showing the effect of Mo on the upper limit temperature of hot working.

FIG. 5 is a graph showing the effect of V content on the thermal shock cracking characteristics.

FIG. 6 is a graph showing a relationship between hardness and V content obtained by a predetermined heat treatment.

FIG. 7 is a graph showing the relationship between the C, Cr and V contents of each steel ingot shown in the example.

FIG. 8 is a schematic view of a tester used for a thermal shock test of an example.

FIG. 9 is a graph showing the thermal shock test results of the invention material and the comparative material.

[Explanation of symbols]

 1 rack 2 drop weight 3 pinion 5 test piece

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[Procedure amendment]

[Submission date] March 24, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between hardness and C content obtained by a predetermined heat treatment.

FIG. 2 is a graph showing the effect of C content on thermal shock cracking characteristics.

FIG. 3 is a graph showing the influence of Cr on thermal shock cracking characteristics.
It is a graph which shows the influence of content.

[Fig. 4] Fig. 4 is a graph showing the effect of Mo on the upper limit temperature of hot working.
It is a graph which shows the influence of.

FIG. 5 is a graph showing the effect of V content on the thermal shock crack characteristics.

FIG. 6 is a graph showing a relationship between hardness and V content obtained by a predetermined heat treatment.

FIG. 7 is a graph showing the relationship among the C, Cr and V contents of each steel ingot shown in the examples.

FIG. 8 is a schematic diagram of a testing machine used for a thermal shock test of an example.

FIG. 9 is a graph showing the thermal shock test results of the invention material and the comparative material.

[Explanation of symbols] 1 rack 2 drop weight 3 pinion 5 test piece

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

[0019]

As described above, according to the forged steel working roll material for hot rolling of the present invention, C: 1.4% by weight%.
2%, Si: 0.6% or less, Mn: 0.4 to 1%, Ni
: 0.5% or less, Cr: 2-3%, Mo: 0.7-
1.2%, V: 4-7%, W: 1% or less, optionally C
o: Containing 1% or less, consisting of the balance Fe and inevitable impurities, and satisfying the relational expression of 0.7 <{(% C) + (% Cr)} / (% V) <1, C, Cr, and V are appropriately balanced, the formation of net-like eutectic carbides in the solidified grain boundaries, which is the path of thermal shock crack propagation, is suppressed, and the amount of V-based carbides necessary for wear resistance is reduced. It is possible to obtain a forged steel working roll material for rolling having high wear resistance, which is secured as a eutectic cell and is excellent in thermal shock crack characteristics.

Claims (2)

[Claims] 1. By weight%, C: 1.4 to 2%, Si:
0.6% or less, Mn: 0.4 to 1%, Ni: 0.5% or less, Cr: 2 to 3%, Mo: 0.7 to 1.2%, V: 4
˜7%, W: 1% or less, balance Fe and unavoidable impurities, and 0.7 <{(% C) + (% Cr)} / (% V) <1 Forged Steel Working Rolls for Hot Rolling with Satisfactory Chemical Composition 2. In addition to the composition of claim 1, in% by weight Co:
Forged steel working roll material for hot rolling characterized by containing 1% or less