JPH0657380A - Mandrel-rolling core bar having long service life - Google Patents

Abstract

PURPOSE:To drastically prolong the useful life of a core bar by using tool steel obtd. by adding a specified small amt. of Nb to a Cr-Mo low alloy steel as a core bar material used at the time of producing a seamless steel pipe by mandrel rolling. CONSTITUTION:As the core bar used at the time of producing a seamless steel pipe by mandrel rolling, the Cr-Mo low alloy tool steel having a compsn. contg., by weight, 0.25 to 0.35% C, 0.8 to 1.2% Si, 0.2 to 0.6% Mn, 4.5 to 5.5% Cr, 0.8 to 1.2% Mo, 0.02 to 0.08% Nb, 0.1 to 0.3% V, 0.01 to 0.05% Al, less than 0.025% P and less than 0.015% S is used. By the addition of Nb, austenite in the core bar after hardening treatment is executed is made finely granular so that grain size is 20mum or smaller, by that, its thermal fatigue life can drastically be prolonged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to extending the service life of a cored bar used in mandrel rolling in the process of producing a seamless steel pipe.

[0002]

2. Description of the Related Art Mandrel rolling is one of the main processes in the production of seamless steel pipe. In the main rolling, a cored bar called mandrel bar (hereinafter referred to as cored bar) is used. This cored bar is inserted into a high temperature hollow shell, and then subjected to a large rolling load by a roll having a hole shape, Sliding friction occurs at the contact interface with the material to be rolled. Therefore, the core metal is subjected to extremely large heat load and friction. As a result, complex phenomena such as surface wear, rough skin, and thermal fatigue cracking occur individually or simultaneously, which is the rate-limiting condition for the useful life of the cored bar.

In order to withstand the harsh operating conditions as described above,
As the core metal, an alloy steel in which JIS standard steel SKD61 for hot tools is adjusted to have a hardness of about Hs = 55 by induction hardening and tempering has been applied.

A die for hot forging has a problem of abrasion resistance, heat crack resistance, and fracture resistance in a use environment relatively close to a core metal, and there is a die for hot forging. -67
Japanese Patent No. 543 is disclosed. That is, in place of SKD-61 and SKD-62 which are usually used for this purpose, C: 0.25 to 0.35, Si: 1.3 to
1.7, Mn: 0.1 to 0.7, Cr: 4.5 to 5.
5, Mo: 0.7 to 1.0, Nb: 0.03 to 0.1
5, P, S: 0.025 or less, and the remaining 5% Cr based steel consisting of Fe and unavoidable impurities is applied to a forging die. Features include (1) C and expensive Mo
And improved the heat crack resistance without causing the strength reduction by adding Si instead of (2) Nb was added to prevent coarsening at the time of quenching heating and toughness, that is, fracture resistance. It has been mentioned that it has improved the sex. However, since the normal temperature of the forging die is about 200 ° C. and the sliding friction condition is significantly different from that of the core metal, it is unclear how effective such a component improvement is for improving the life of the core metal. .

[0005]

The factors that determine the life of the cored bar include wear, rough skin, and thermal fatigue cracking. Of these, the former two gradually progress, so there is little adverse effect on the rolling operation. On the other hand, thermal fatigue cracking occurs irregularly after the core metal has been used for a certain period of time → propagation and growth occur, which may lead to breakage in some cases. Therefore, if the core becomes unusable due to thermal fatigue cracking, the accuracy of life prediction is poor and the life may suddenly end during rolling. Is a big problem. Further, generally, in thermal fatigue, the core metal often has a shorter life than that caused by wear or rough skin, which is one of the causes of an increase in manufacturing cost of the seamless steel pipe. The aim of the present invention is to overcome the short life of the core metal due to thermal fatigue cracking among the above problems, and to reduce the time loss due to misrolling and core metal replacement.

[0006]

Means for Solving the Problems The present inventors have repeatedly observed and considered in detail the thermal fatigue phenomenon occurring in the surface layer of the cored bar, and the thermal fatigue in this case occurs at the austenite grain boundaries in the initial process.・ Knowing that it propagates,
Furthermore, it was clarified that the generation and growth of thermal fatigue cracks can be significantly delayed by making the austenite grains finer. Then, it has been found that Nb addition is extremely effective as a concrete method for making the austenite grains finer with respect to the steel equivalent to SKD-61.

The present invention is based on the above series of technical knowledge, and in% by weight, C: 0.25 to 0.35, Si: 0.
8 to 1.2, Mn: 0.2 to 0.6, Cr: 4.5 to
5.5, Mo: 0.8 to 1.2, Nb: 0.02 to 0.
08, V: 0.1 to 0.3, Al: 0.01 to 0.0
5, including P: 0.025 or less, S: 0.015 or less,
The remainder is a mandrel rolling core bar characterized by a component consisting of Fe and inevitable impurities.

In the case of the present invention, since it is an improvement in composition,
Although it is not necessary to change the manufacturing process and conditions of the core metal as before, precipitation hardening due to Nb carbon / nitride is added, so the final firing for hardness adjustment when the same hardness is targeted. Only the return temperature will need to be slightly higher than before.
A high tempering temperature is effective in preventing surface softening due to sudden and excessive heat load.

[0009]

The function of the present invention is, as described above, that Nb is added to the alloy steel based on the steel for hot work SKD-61 for the purpose of reducing the austenite grain size.

FIG. 1 shows the relationship between the amount of Nb added in SKD-61 and the austenite grain size after quenching (air cooling treatment) after heating to 1050 ° C. by high frequency induction heating. Nb
It can be seen that when the addition amount is 0.02% or more, the austenite grain size is sharply reduced and reaches a saturation value at 0.05%. That is, by setting Nb ≧ 0.02%,
Austenite grains can be made finer than 20 μm.

FIG. 2 shows the results of measuring the thermal fatigue life of a cored bar having an austenite grain size changed by adding Nb and subjecting it to mandrel rolling on an actual line. Since this test was carried out at the opportunity of rolling a large lot material for proper manufacturing,
The rolling conditions are almost constant throughout, and the surface hardness of the cored bar is as constant as possible (Hs value of about 52-
The heat treatment conditions were selected according to the chemical composition so as to be 55). The thermal fatigue life of the cored bar was evaluated by visually judging the number of rolling until the occurrence of cracking and the number of rolling when the maximum cracking depth measured by a crack meter reached 1 mm. The experimental results show that the smaller the austenite size, the more the thermal fatigue life is gradually improved. By setting the austenite grain size to 20 μm or less, the thermal fatigue life can be extended to twice as long as that of the Nb-free material.

From the above, if an appropriate amount of Nb is added to the core metal of SKD-61 steel, fine austenite grains are secured after quenching, and as a result, thermal fatigue life (cracking depth becomes a constant value). It is obvious that the number of rollings per core bar) can be extended until the above.

The reasons for limiting the core metal component range in the present invention will be described below. C: At least 0.25% or more is necessary to secure the hardness (strength) required for the core metal after quenching and tempering, and 0.3
If it exceeds 5%, the risk of quenching cracks increases, and at the same time, the toughness decreases greatly, and the risk of breakage accidents of the entire core metal originating from thermal fatigue cracking increases.

Si: S from the viewpoint of thermal fatigue cracking resistance and securing of strength.
The standard range of KD-61 is an appropriate level. 0.2% or more is required to fix S as Mn: MnS and prevent grain boundary embrittlement due to S. If it exceeds 0.6%, the effect is saturated and temper embrittlement becomes significant. .

Cr: An element effective from the viewpoint of ensuring high-temperature strength and hardenability and for producing a dense scale during heat treatment, and if less than 4.5%, its effect is insufficient. On the other hand, if it exceeds 5.5%, the effect is saturated. The upper and lower limits are selected for the same reason as Mo: Cr. Further, if it exceeds 1.2%, the toughness deteriorates significantly.

Al: An element used as a deoxidizing element. If 0.01% or more is sufficient for deoxidizing steel, 0.0
If it exceeds 5%, the effect is saturated and the amount of alumina-based inclusions increases.

Nb: Austenite grain refinement and normal temperature,
It is an essential element for suppressing the occurrence of thermal fatigue cracks through maintaining high-temperature strength. If it is less than 0.02%, the effect is insufficient. On the contrary, if it exceeds 0.08%, the effect is saturated and the toughness is greatly reduced. Therefore, the addition range is 0.02 to 0.08%.

V: Although less effective than Nb, charcoal
It is an element that enhances strength by nitride precipitation, and is added aiming at the complementary effect of Nb by adding Nb together.
In this case, unless 0.1%, the effect is insufficient,
If added over 0.3%, the toughness decreases.

P and S: Both are present as impurities in the steel, and if P is not less than 0.025%, embrittlement of the grain boundaries is caused and thermal fatigue cracking is promoted, and S is 0. 01
If it is not less than 5%, the amount of A-type inclusions that are the origin of thermal fatigue cracking increases.

[0020]

EXAMPLES Examples of the present invention will be described in detail below. The core metal material was manufactured by melting it in a large melting furnace, casting it into a square slab, and rolling it with a round bar. Then, it was machined to a predetermined size, and finally subjected to a quenching + tempering treatment so as to adjust the hardness so that the surface hardness was Hs of about 53, and then subjected to an actual mandrel rolling test. In mandrel rolling, the steel pipe size of the rolled material is the nominal outer diameter 195 mmφ and the nominal wall thickness 10.3.
It was carried out under a constant condition of mm, and only those whose life had expired due to the formation of thermal fatigue cracks were evaluated. (Excludes wear and rough skin that are rate limiting)

Table 1 shows the results,
The core metal (number: 1 to 6) of the component of the present invention is approximately 300
It is shown that the service life is improved to 1300 or more as compared with the book. No. 9 is a comparative material added with 0.11% Nb, which has a useful life of 1483 pieces, but was broken during rolling of the 1484th piece due to a decrease in toughness due to too much Nb, making it impossible to downsize. It is a thing.

[0022]

[Table 1]

[0023]

As described above, as a result of the application test of the present invention to the actual rolling, the effect that the life of the core metal in the mandrel rolling is significantly extended was verified. This effect is remarkable in terms of manufacturing stability and cost reduction of the seamless steel pipe, and it can be said that the industrial value of the present invention is great.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing changes in the size of austenite crystal grains due to Nb addition in a 5% Cr-based tool steel based on SKD-61. Austenite grain size is 10
It is an average value measured by a cutting method with a microscope after quenching after induction heating at 50 ° C.

FIG. 2 is an explanatory diagram showing a service life when a core metal in which the size of austenite grains is changed by adding Nb is subjected to mandrel rolling under substantially the same conditions. Line A in the figure is
The number of rollings when cracking is observed at the visual level, and the line B is the number of rollings when the cracks propagate up to 1 mm.

Claims (1)

[Claims] 1. By weight%, C: 0.25 to 0.35 Si: 0.8 to 1.2 Mn: 0.2 to 0.6 Cr: 4.5 to 5.5 Mo: 0.8 to 1.2 Nb: 0.02 to 0.08 V: 0.1 to 0.3 Al: 0.01 to 0.05 P: 0.025 or less S: 0.015 or less is contained, and the rest is inevitable with Fe. Core metal for mandrel rolling, which is characterized by being comprised of specific impurities.