KR101259271B1

KR101259271B1 - Method for manufacturing groove-rolling free cutting steel billet

Info

Publication number: KR101259271B1
Application number: KR1020100133187A
Authority: KR
Inventors: 손재승; 황훈
Original assignee: 주식회사 포스코
Priority date: 2010-12-23
Filing date: 2010-12-23
Publication date: 2013-04-29
Also published as: KR20120071586A

Abstract

In one aspect of the present invention, there is provided a method of rolling a free cutting steel piece comprising at least one of Bi and Sn, wherein left and right corner rounding values of the steel strip are set to 35 to 45 at the time of right rolling, By providing a method of casting a free-cutting steel piece having a reduction ratio of 15 to 25% for each stand rolling,
It is possible to prevent the occurrence of cracks and breakage at the left and right corners of the steel strip during the rolling of the free-cutting steel strip, and at the same time, the reduction in productivity can be prevented by reducing the burden on the reduction ratio during rough rolling, It is possible to prevent the productivity drop and to secure the quality of the surface of the steel strip.

Description

METHOD FOR MANUFACTURING GROOVE-ROLLING FREE CUTTING STEEL BILLET [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of rolling a free-machining steel piece, and more particularly, to a method of rolling a free-machining steel piece by controlling left and right corner rounding values of the steel piece during left- .

Rolling of the slabs consists of rough rolling and finishing rolling. Rough rolling is reversible box cast rolling, and hot rolling is irreversible diamond-square rolling. The present invention relates to a finishing rolling method in the above-mentioned rolling process, and diamond-square forming is advantageous in minimizing a cross-sectional deviation of a steel piece by steel type.

However, a problem is that when the hot rolling property of the billet to be subjected to the above-mentioned rolling process is inferior, a crack can easily occur in the corner during the rolling process. In order to improve machinability, Pb-free free-cutting steels are added with low melting point elements such as Bi and Sn to improve cutting performance. In order to improve machinability, Pb-free free- Have the property that the hot rolling property is very hot.

Therefore, in the case of the free cutting steel to which Bi or Sn is added, stress is concentrated on the left and right corners at the time of finish rolling, and cracks are easily generated.

In order to solve such a problem, a method which has been conventionally used is to reduce the reduction rate per pass so as to minimize the stress applied to the right and left corners at the time of finishing rolling. Particularly, in order to accomplish this, there is a problem in that the number of passes at the time of rough rolling is inevitably increased and the productivity is lowered because the reduction in the roughness must be further performed in the rough rolling before the finish rolling.

In other words, for example, in the past, free-cutting steel slabs were rolled from 240 to 160 angles using a general steel roll at the time of rolling. In order to do this, a large amount of stress was applied to the right and left corners at the time of rolling. . Therefore, in order to solve this problem, conventionally, the reduction ratio in the rough rolling has been increased. On the contrary, in the rolling process, the reduction ratio has been reduced to minimize the cracks occurring at the right and left corners of the steel strip by using the rolling method from 190 to 160 .

However, in the conventional technique, the number of passes is increased from about 8 passes to 14 passes at the time of rough rolling, and productivity of free cutting steel is greatly lowered. In order to perform the above-mentioned rolling, Of the total population.

Therefore, a study on the rolling technique of free-machining steel strips that can reduce the burden on the rough rolling of the free-cutting steel and reduce the productivity and minimize the cracks in the right and left corners of the steel strip by using the general steel roll It is a very urgent time.

One aspect of the present invention is to prevent cracks and breakage occurring in the right and left corners of the steel strip during finish rolling of free-machined steel strips. In addition, the reduction in productivity can be prevented by reducing the burden on the reduction ratio during rough rolling, There is provided a method of rolling a free-machining steel piece which can use a general steel roll instead of a roll.

In one aspect of the present invention, there is provided a method of rolling a free cutting steel piece comprising at least one of Bi and Sn, wherein left and right corner rounding values of the steel strip are set to 35 to 45 at the time of right rolling, A method for rolling a free-machining steel piece with a reduction ratio of 15 to 25% for each stand rolling.

In this case, the free cutting steel slab may contain 0.03 to 0.1% of C, 0.01 to 0.3% of Si, 0.8 to 2.0% of Mn, 0.02 to 0.1% of P, 0.25 to 0.5% of S, 0.005 to 0.2% of Sn, 0.002 to 0.01% of B, 0.003 to 0.01% of N, 0.002 to 0.025% of T [O], and the balance of Fe and other unavoidable impurities.

Further, it is more preferable that the rolling is performed by four stand rolling, the first and third stands are rolled up and down, and the second and fourth stands are rolled by right and left rolling.

In this case, the upper and lower corner rounding values of the first and third stands are preferably 20 to 27, and the left and right corner rounding values of the fourth and fifth stands are preferably 20 to 27.

In addition, it is preferable that the maximum strain of the free cutting steel piece after the final stand rolling is 0.45 or less.

One aspect of the present invention is to prevent cracks and breakage occurring in the right and left corners of the steel strip during finish rolling of free-machined steel strips. In addition, the reduction in productivity can be prevented by reducing the burden on the reduction ratio during rough rolling, It is possible to prevent the productivity drop and ensure the quality of the surface of the steel strip while using a general steel roll rather than a roll.

FIG. 1 is a photograph of a piece of a billet having a break phenomenon at left and right corners according to an example of the conventional rolling of a free cutting steel piece.
FIG. 2 is a photograph showing strain distribution of a piece of a steel strip according to an example of the conventional rolling of a free-machining steel piece.

First, a steel strip to be applied to the present invention is a free cutting steel containing at least one of Bi and Sn. Free cutting steel is not added with Pb in order to secure eco-friendliness. Therefore, securing of cutting ability has become a technical problem. Currently, free cutting steel technology in which Bi or Sn is added is being developed for securing the cutting ability.

However, although Bi or Sn can contribute to improvement in cutting performance, there is a problem that the hot rolling property is adversely affected, causing surface cracks particularly at the right and left corners during rolling of the steel strip, and further, cornering phenomenon occurs.

The present invention relates to a composition of a free cutting steel strip having improved cutting performance by the addition of Bi or Sn, which comprises 0.03 to 0.1% of C, 0.01 to 0.3% of Si, 0.8 to 2.0% of Mn, 0.02 to 0.1% of P, 0.005 to 0.2% of Sn, 0.002 to 0.01% of B, 0.003 to 0.01% of N, 0.002 to 0.025% of T [O], and the balance of Fe and other unavoidable impurities Cracks on the left and right corners of the free-machining steel piece are mainly problematic.

FIG. 1 is a photograph of left and right corners of a steel billet in a finish rolling process of free-cutting steel billet having the above composition. In the case of a free-cutting steel billet having a hot rolling property, the productivity and free- There are many problems in ensuring quality.

As a method for solving these problems, a technique for minimizing the cracks in the right and left corners by reducing the reduction in the reduction of rolling during rough rolling has been studied instead of increasing the reduction ratio during rough rolling. However, There is a problem that the productivity is deteriorated due to the difference in the size of the steel strips applied to the rolling.

Therefore, in order to solve the above problems, the present invention has been made to solve the above problems, and it is an object of the present invention to provide a rolling mill for a rolling mill, which does not particularly increase the reduction ratio of rough rolling, The strain of the right and left corners is reduced to effectively prevent the occurrence of cracks or popping of the left and right corners.

Specifically, it is preferable to control the reduction ratio of each stand rolling at the time of finishing rolling to 15% or more in order to prevent productivity from being lowered by keeping the reduction ratio at a high level during the finishing rolling so as not to burden the rough rolling. However, it is not desirable to increase the reduction ratio of each of the above-mentioned stand rolling in order to prevent an excessive load from being applied to the stand rolling mill. Therefore, it is necessary to limit the upper limit to 25%.

That is, in order to suppress the cracks and the bursting phenomenon at the right and left corners of the free-machining steel strip while maintaining the reduction ratio of each stand rolling at 15 to 25% at the time of rolling, all of the stands are rolled before the final stand rolling, If the degree of bending of the left and right corners is increased by raising the value of the left and right corner rounds, the strain of the right and left corners of the steel strip obtained after the final stand rolling can be reduced.

It is necessary to control the corner round value to 35 or more at the time of the left and right rolling so as to increase the degree of bending of the corner in order to sufficiently attain the strain reduction effect of the left and right corners. However, when the corner round value is excessively large at the time of the left-right rolling, the degree of bending of the right and left corners becomes too large, so that a stress is applied to the right and left corners and the deformation rate of the left and right corners increases. Value is preferably controlled to be in the range of 35 to 45.

At this time, it is more preferable that the rolling is performed by four stand rolling, the first and third stands are rolled up and down, and the second and fourth stands are rolled by right and left rolling. This is a preferable example for rolling free-cutting steel strips. In general, it is preferable to roll the steel strips uniformly in the left and right rolls after the first rolling, desirable.

In this case, the upper and lower corner rounding values of the first and third stands are preferably 20 to 27, and the left and right corner rounding values of the fourth and fifth stands are preferably 20 to 27. It is very common to keep the corner round value in the range of 20 to 27 in the case of the above-mentioned four stand rolling, and in the case of the rolling of the upper and lower sides and the rolling of the left and right sides, the effect which affects the left and right corner deformation rates of the finally obtained steel strip 2 stand rolling, it is preferable to use the same as the general round value of 20 to 27 at the time of rolling the remaining first, third, and fourth stands, in order to secure the rolling efficiency and shape of the steel strip.

FIG. 2 schematically shows a change in the strain distribution of the steel strip during normal finishing rolling. First, (a) shows a change in the strain distribution of the steel strip after being subjected to the upper and lower rolling at a reduction ratio of 20% It can be confirmed that stress is applied to upper and lower corners at the upper and lower corners and deformation rate is lower at the right and left corners.

(B) shows the strain distribution of the steel strip after left-right rolling with a reduction ratio of 18% and a left and right corner rounding value of 18% in the second stand. The stress applied to the upper and lower corners is considerably loosened, .

(C) is a strain distribution of the steel strip after being subjected to the upper and lower rolling at the reduction ratio of 18% and the upper and lower corner rounding values of 18% in the third stand. The stress applied to the right and left corners by the upper and lower rolling is considerably loosened, It did not grow very bad.

(d) shows the strain distribution of the steel strip after left-right rolling with a reduction ratio of 18% and a left and right corner roundness value of 18% in the fourth stand. As a result, stress was concentrated at the right and left corners.

In other words, it is a conventional problem that the surface quality of the left and right corners is considerably reduced during the finishing rolling by the above-described principle. However, when the present invention is applied, Thereby relieving the concentration of stress on the left and right corners after the fourth stand is rolled, thereby preventing the corners from breaking.

The maximum strain of the free cutting steel piece after the final stand rolling can be controlled to be 0.45 or less by controlling the left and right corner round values at the time of left and right rolling before the final stand rolling, It is possible to effectively prevent the occurrence of a crack or a burst phenomenon in the left and right corners.

Hereinafter, the present invention will be described in detail by way of examples, which are intended to be more fully understood of the present invention, and the scope of the present invention is not limited by the following specific embodiments.

( Example )

The inventors of the present invention have found that, in a finishing mill having four stands which are continuous in up / down / left / right / up / down / left / right rolling, the reduction ratio is 20% and the up / down corner round value is 25 for the first stand rolling, The reduction ratio is set to 18% and the up and down corner round value is set to 25, the reduction ratio is set to 18% and the left and right corner round values are set to 22 when the fourth stand is rolled, The rolling test was carried out five times with 25 (Experiment 1), 30 (Experiment 2), 35 (Experiment 3), 40 (Experiment 4) and 45 (Experiment 5).

Then, the maximum strain per each stand was measured, and the surface condition of the left and right corners of the steel strip was evaluated and shown in Table 1 below. For the sake of reference, the above experiment was carried out in the same manner as in Example 1, except that C: 0.05%, Si: 0.03%, Mn: 1.2%, P: 0.07%, S: 0.3%, Bi: 0.1%, Sn: , T [O]: 0.02%, and the balance Fe and other unavoidable impurities.

division Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 Maximum strain after first stand rolling 0.703 0.703 0.703 0.703 0.703 Maximum strain after second stand rolling 0.516 0.545 0.642 0.748 0.835 Maximum Strain Rate after Third Stand Rolling 0.365 0.403 0.468 0.541 0.605 Maximum Strain Rate after 4th Stand Rolling 0.463 0.455 0.405 0.390 0.425 Left and right corner
Surface condition Outbreak Cracking Good Good Good

Experiments 3, 4 and 5 show that the maximum deformation rate after the fourth stand rolling tends to decrease compared to the maximum deformation rate after the third stand rolling in the case of Experiment 3, 4 and 5 in which the left and right corner round values are controlled to 35 to 45 The maximum deformation rate was 0.425 or less, indicating that the stress concentration was considerably alleviated. Further, the surface states of the right and left corners also showed good appearance without cracks or cracks. According to the experiment, it can be confirmed that the stress concentrating on the right and left corners can be most effectively alleviated when the corner round value is about 40 when the second stand is rolled.

On the other hand, in the case of Experiments 1 and 2, it was confirmed that the left and right corner rounds were less than 35 at the second stand rolling, and the maximum strain value exceeded 0.45 after the fourth stand rolling, have.

Claims

A method for rolling a free cutting steel piece comprising at least one of Bi and Sn,
A method for rolling a free-machining steel strip in which the left and right corner rounding values of the steel strip are 35 to 45 and the reduction ratio of each stand rolling is 15 to 25% during left-to-right rolling during stand rolling before final stand rolling.

The method according to claim 1,
The free-machining steel slabs according to claim 1, wherein the free-machined steel slabs contain 0.03 to 0.1% of C, 0.01 to 0.3% of Si, 0.8 to 2.0% of Mn, 0.02 to 0.1% of P, 0.25 to 0.5% of S, 0.04 to 0.2% 0.005 to 0.2% of Sn, 0.002 to 0.01% of B, 0.003 to 0.01% of N, 0.002 to 0.025% of T [O], and the balance Fe and other unavoidable impurities.

The method according to claim 1 or 2,
The rolling method according to claim 1, wherein the rolling is performed by four stand-rolling, the first and third stands are rolled up and down, and the second and fourth stands are rolled by left-right rolling.

The method of claim 2,
Wherein the upper and lower corner rounding values of the billet during the first and third stand rolling are 20 to 27 and the left and right corner round values of the billet during the fourth stand rolling are 20 to 27.

The method of claim 3,
Wherein the maximum strain of the free cutting steel piece after the final stand rolling is set to 0.45 or less.