JP2000336454A - BISMUTH (Bi)-SULFUR (S) FREE-CUTTING STEEL EXCELLENT IN HIGH TEMPERATURE DUCTILITY AND ITS PRODUCTION - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce bismuth(Bi)-sulfur(S) free cutting steel excellent in high temp. ductility and free from the generation of surface defects even at low rolling temp. and to provide a method for producing it. SOLUTION: This Bi-S free cutting steel has a compsn. composed of, by weight, 0.05 to 0.15% carbon, 0.5 to 2.0% manganese, 0.15 to 0.40% sulfur, 0.01 to 0.10% phosphorus, 0.003 to 0.020% oxygen, 0.03 to 0.30% bismuth, <=0.01% silicon and <=0.0009% aluminum, and the balance Fe with Mns as inevitable impurities and MnS inclusions in which bismuth(Bi) has been adsorbed and metallic bismuth(Bi) inclusions. The steel is incorporated with 0.02 to 0.1% titanium and 0.003 to 0.01% nitrogen. Moreover, the steel is incorporated with 0.005 to 0.015% B and <=0.007% N.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a bismuth (Bi)-
The present invention relates to a free-cutting steel based on sulfur (S) and a method for producing the same, and more particularly, to a free-cutting steel excellent in high-temperature ductility and having no surface defects even at a low rolling temperature, and a method for producing the same. is there.

[0002]

2. Description of the Related Art Free-cutting steel is a material used for manufacturing small parts such as cameras and watches requiring excellent machinability, and is required for precision workability. With the increase in production of cameras and other products, lead-sulfur-based free-cutting steel to which elements such as lead that improve machinability have been added has been developed and put into practical use as a steel material suitable for high-speed and automatic cutting. Have been. However, since lead is toxic to the human body, a bismuth (Bi) -sulfur (S) free cutting steel has been developed to solve this problem.

[0003] Generally, bismuth (Bi) -sulfur (S) free-cutting steel is prepared by adding non-metallic or metallic inclusions to steel,
Machining performance has been improved and improved. A typical nonmetallic inclusion is MnS, and the metallic inclusion is:
It is a low melting point metal such as bismuth (Bi) which has almost no solid solubility in steel. Such inclusions act at the center of stress concentration during cutting, and the inclusions and matrix
It facilitates void formation and crack growth at the interface of the tructure, reducing the force required for cutting. Softened or melted by cutting heat, inserts and cutting tools
Acts as a lubricant at the interface of the surface, suppresses tool wear, and reduces the force required for cutting.

[0004] Sulfur added to form MnS becomes supersaturated even if the content of Mn is insufficient, or even if the content of Mn is sufficient, solidification or cooling occurs suddenly. During continuous casting, a slab has a structure in which the surface is rapidly solidified by a certain thickness, and then a columnar crystal is formed therein, and MnS is crystallized between the columnar crystals. However, in a structure formed by rapid solidification called chill crystal, sulfur cannot form MnS and is supersaturated.
Thus, part of the supersaturated sulfur precipitates as MnS during cooling or reheating, and partly precipitates as FeS, which causes hot embrittlement at crystal grain boundaries. In this steel, not only sulfur but also gas components with almost no solid solubility are supersaturated, so those gas components exceeding the solid solubility in the solid remain as fine pin holes. I will be.

On the other hand, bismuth (Bi) useful for cutting work
Has little solid solubility in steel and low melting point,
In general carbon steel that does not intentionally generate MnS inclusions,
It mainly precipitates at the grain boundaries in the form of a thin film (flim) and causes grain boundary embrittlement. On the other hand, in a free-cutting steel in which MnS inclusions are intentionally generated, bismuth (Bi) is adsorbed by the MnS inclusions and crystallized, so that the austenite interface or austenite grain boundary between the MnS inclusions and the supporting iron. And induces hot embrittlement. The liquid bismuth (Bi) existing at the interface between the MnS inclusions and the austenite of the supporting iron causes a decrease in ductility at approximately 900 to 1000 ° C., and the liquid bismuth (Bi) existing at the austenite crystal grain boundary is formed. ) Reduces ductility at about 800-900 ° C. As described above, when the ductility sharply decreases during hot rolling, defects occur on the surface of the hot-rolled wire. This problem occurs particularly remarkably when ductility is lowered at 800 to 900 ° C.

[0006] Such surface defects can be roughly classified into two types. One is large defects caused by scabs (scabs), whose length is 5-1.
It is about 0 mm and its distribution interval is not uniform, but is usually several mm. Further, other surface defects are fine cracks generated in the length direction of the wire, and the length is 1 mm or less,
The distribution interval is several μm, and there is some regularity. These defects are caused by a liquid bismuth (B) that has been extended from a large number of fine cracks with part of the fine cracks as a starting point.
It is formed in a pin hole or on a crystal grain boundary at the interface between MnS adsorbing i) and the matrix structure.

[0007] Therefore, conventionally, the rolling temperature is usually set at 1000 to suppress the generation of fine cracks which are the starting points of defects or to suppress further growth even if fine cracks are generated. The generation of defects was suppressed by increasing the temperature to at least ℃ and improving the ductility of the matrix structure. However, raising the rolling temperature causes problems such as difficulty in maintaining the rolling equipment and a decrease in productivity. Therefore, bismuth that can ensure excellent ductility in which generation of surface defects is suppressed even at a relatively low rolling temperature. There is a demand for (Bi) -sulfur (S) -based free-cutting steel.

The inventor of the present application has proposed to control the growth of crystal grains by controlling the reheating temperature low, or to obtain fine precipitates which do not form a solid solution at a high temperature even if the reheating temperature is increased by adding Ti and N. It has been confirmed that the presence of a material and the use of these materials can hinder the movement of the crystal grain boundaries, refine the crystal grains, and improve the high-temperature ductility. In addition, as a result of studying the segregation behavior of S and Bi at the austenite grain boundaries of the bismuth (Bi) -sulfur (S) -based free-cutting steel, the inventor of the present application has found that B has a higher diffusion rate to S and Bi. When an appropriate amount of
It has been confirmed that hot embrittlement due to eS and liquid Bi can be effectively suppressed, and that high-temperature ductility can be improved, and the present invention has been completed.

[0009]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to reduce the growth of crystal grains by controlling the reheating temperature to be low, to refine the crystal grains, and to reduce surface defects even in a relatively low rolling temperature range. It is an object of the present invention to provide a bismuth (Bi) -sulfur (S) -based free-cutting steel excellent in high-temperature ductility that can prevent generation of cracks and a method for producing the same.

Another object of the present invention is to provide a method for controlling the movement of crystal grain boundaries by utilizing fine precipitates which do not form a solid solution at a high temperature even when Ti and N are added and the reheating temperature is high. Obstruct,
An object of the present invention is to provide a bismuth (Bi) -sulfur (S) -based free-cutting steel excellent in high-temperature ductility that can refine crystal grains and prevent generation of surface defects even in a relatively low rolling temperature range, and a method for producing the same. Another object of the present invention is to provide B and N
To preferentially segregate boron having a faster diffusion rate than S and Bi at the austenite grain boundaries, and to prevent the occurrence of surface defects caused by FeS, molten Bi, etc. in a relatively low rolling temperature range. Bismuth with excellent ductility (B
(i) To provide a free-sulfur (S) -based free-cutting steel and a method for producing the same.

[0011]

In order to achieve the above object, in the present invention, carbon (C): 0.0% by weight.
5-0.15%, manganese (Mn): 0.5-2.0
%, Sulfur (S): 0.15 to 0.4%, phosphorus (P): 0.
01-0.1%, oxygen (O): 0.003-0.02
%, Bismuth (Bi): 0.03-0.3%, silicon (Si): 0.01% or less, aluminum (Al):
0.0009% or less, with the balance being iron (Fe) and unavoidable impurities, and containing a MnS inclusion adsorbing MnS and bismuth and a metallic bismuth inclusion, and a Bi-S system excellent in high-temperature ductility. Free-cutting steel.

Further, the present invention provides a method for producing a bloom having the same composition as described above, wherein the bloom is reheated at a reheating temperature of 850-1000 ° C. so that the austenite grain size does not exceed 60 μm, and billets are rolled. The obtained rolled billet (bi
llet) is reheated at a reheating temperature of 850-1000 ° C. to maintain the austenite grain size not to exceed 60 μm, and then to perform wire rolling.
System for producing free-cutting steel.

[0013] The present invention also relates to a carbon (C):
0.05-0.15%, manganese (Mn): 0.5-
2.0%, sulfur (S): 0.15-0.4%, phosphorus (P): 0.01-0.1%, oxygen (O): 0.003
-0.02%, bismuth (Bi): 0.03-0.3
%, Silicon (Si): 0.01% or less, aluminum (Al): 0.0009% or less, titanium (Ti):
0.02-0.1%, nitrogen (N): 0.003-0.0
A Bi-S-based free-cutting steel excellent in high-temperature ductility having MnS inclusions adsorbing bismuth and MnS inclusions adsorbed with bismuth and metallic bismuth inclusions, the balance being 1% and iron (Fe) and unavoidable impurities.

Further, according to the present invention, a bloom having the same composition as described above is reheated at a normal temperature to 850-120.
Billet rolling at a temperature of 0 ° C., reheating of the obtained billet (billet), and Bi-S-based free-cutting steel excellent in high-temperature ductility constituted by including wire rod rolling at a temperature of 850-1200 ° C. Manufacturing method. In addition, the present invention relates to a method for preparing C: 0.05-% by weight.
0.15%, Mn: 0.5-2.0%, S: 0.15-
0.4%, P: 0.01-0.1%, O: 0.003-
0.02%, Bi: 0.02-0.3%, Si: 0.0
1% or less, N: 0.007% or less, B: 0.005-
A Bi-S-based free-cutting steel which is excellent in high-temperature ductility and contains MnS and bismuth-adsorbed MnS inclusions and metallic bismuth inclusions, which is composed of 0.015% and impurities inevitably containing Fe and the balance. .

Further, the present invention provides a method for reheating a bloom having the same composition as described above, rolling the billet at a temperature of 800 to 1200 ° C., reheating the obtained billet, A method for producing a Bi-S-based free-cutting steel excellent in high-temperature ductility, characterized by performing wire rolling at a temperature of -1200 ° C.

[0016]

Embodiments of the present invention will be described below in detail. Bi-S free-cutting steel can be hot-rolled at 950 ° C. or higher, but hot rolling at a high temperature of 1000 ° C. or higher is generally performed in order to secure high-temperature ductility of 60% or more and prevent surface defects. It is a target. That is, when the rolling temperature is 100
When the temperature is reduced to 0 ° C. or less, the high-temperature ductility is reduced to 60% or less, so that a surface defect may be generated during hot rolling. Therefore, the present invention is characterized by providing a free-cutting steel capable of ensuring excellent high-temperature ductility even at a relatively low temperature of 1000 ° C. or less and preventing surface defects.

First, such high-temperature ductility can be achieved by refining crystal grains. In other words, the increase in the grain boundary area due to the refinement of the crystal grains can reduce the fraction of the grain boundary area formed by the segregation of bismuth relatively, and can suppress the grain boundary embrittlement due to bismuth. Instead, the size of the voids generated at the triple point of the crystal grain boundary is reduced and the recrystallization is facilitated only by miniaturization of the crystal grain itself, so that the ductility can be improved. As described above, the crystal grain refinement for improving the high-temperature ductility makes it possible to suppress the growth of crystal grains during reheating.

Therefore, the present invention provides a reheating temperature of 85
Control the growth of crystal grains by controlling as low as 0-1000 ° C,
Austenite grain size is reduced to 6 by hindering the movement of grain boundaries.
A free-cutting steel excellent in high-temperature ductility during hot rolling by controlling so as to be maintained at 0 um or less. Further, since the present invention does not form a solid solution at a high temperature even if the reheating temperature is high, the movement of the grain boundaries is hindered by using fine precipitates such as TiN to control the austenite grain size to 60 μm or less. The present invention also provides a free-cutting steel having excellent hot ductility at the time of hot rolling and containing Ti and N. Next, such high-temperature ductility can be achieved by adding boron (B). That is, the present invention preferentially segregates boron (B), which has a higher diffusion rate than S and Bi at the austenite grain boundaries, to improve the reduction in high-temperature ductility caused by FeS, molten Bi, and the like. Provide N-added free-cutting steel.

Next, the reasons for limiting the component composition of the free-cutting steel material and the like in the present invention will be described. C should be added in an amount of 0.05% or more to ensure the surface roughness and mechanical properties of the processed product. However, if it exceeds 0.15%, the pearlite structure increases and the machinability decreases. So 0.05-0.15
% Is desirably added.

The above-mentioned Mn should be added in an amount of 0.5% or more in order to secure a necessary amount of MnS inclusions and to suppress hot embrittlement due to generation of FeS at grain boundaries during hot rolling.
If it exceeds 2.0% or more, the hardness of the steel increases, leading to a decrease in machinability. Therefore, it is desirable to add it in the range of 0.5-2.0%. P should be added in an amount of 0.01% or more in order to improve the surface roughness of the workpiece, but if it exceeds 0.1%, mechanical properties and cold workability are ensured. Is difficult, so 0.01-0.1%
It is desirable to add in the range of.

The above S is Mn for improving the surface roughness of the workpiece by suppressing the growth of BUE (Built Up Edge).
Although 0.15% or more should be added to form S inclusions, if 0.4% or more is added, it is difficult to secure hot workability and cold workability. It is desirable to add in the range of 4%. The above O is at the time of hot rolling,
0.003% or more should be added in order to prevent the machinability from being lowered by the stretching of the MnS-based inclusions.
If it exceeds 02%, it is difficult to secure the plastic deformability of MnS inclusions during cutting, so that 0.003-0.02%
It is desirable to add in the range of.

The above-mentioned Bi is contained in steel alone with bismuth or M
Since it is present by being adsorbed by nS, it acts at the center of stress concentration during cutting, thereby reducing the radius of curvature of the chip and improving the processability of the chip. In addition, bismuth has the effect of increasing the area of MnS inclusions on the surface of the cutting tool and improving the surface roughness of the workpiece. In addition, since bismuth is melted by cutting heat, the frictional force between the chip and the cutting tool is reduced, and the wear of the cutting tool is suppressed. Therefore, if the content is less than 0.02%, the cutting effect is reduced. However, when the content is 0.3% or more, the hot workability becomes very poor.

Si forms SiO2 which forms a complex inclusion with the MnS inclusion. Since the plastic deformability of such a composite inclusion is very poor, the formation of a MnS-based inclusion layer at the end of the tool is hindered and BUE is easily generated, thereby adversely affecting the surface roughness of the workpiece. Therefore, the silicon content should be as low as possible, not exceeding 0.001%. Al easily forms Al2O3 to form a composite inclusion with the MnS inclusion. Since such a composite inclusion has a very poor plastic deformation ability like the MnS SiO2 composite inclusion, it interferes with the formation of the MnS-based inclusion layer at the end of the tool and facilitates the formation of a BUE. Has an adverse effect on the surface roughness. In addition, Al2O3 is very hard and promotes wear of the cutting tool. Therefore, it is desirable that the content of aluminum does not exceed 0.0009%.

The titanium (Ti) forms a solid solution in steel and combines with nitrogen and carbon to form TiN, Ti (CN).
And TiC and the like are deposited. In this case, TiN, Ti (C
N) and TiC are deposited at a higher temperature in this order. In other words,
TiN re-dissolves at the highest temperature. Although the exact re-solid solution temperature of TiN is determined by the solubility of Ti and N, it is known that it usually remains at a temperature of 1200 ° C. or higher. Therefore, at the time of reheating, in order to suppress the growth of austenite crystal grains, it is advantageous to use TiN which is stable even at a higher temperature. When titanium is added to 0.02% or less, the re-solidification temperature of TiN is low, which is effective in suppressing the growth of crystal grains. However, when 0.1% or more is added, crystal growth by TiN is effected. Even if effective for suppression, TiN hardens crystal grains and relatively weakens grain boundaries, deteriorating high-temperature ductility.

The purpose of adding B to steel is usually to segregate at austenite crystal grain boundaries to delay the ferrite generation rate and improve the strength. Therefore,
In general steel in which MnS does not exist as much as free-cutting steel, B in an amount capable of segregating at crystal grain boundaries is 0.0020-0.0%.
When about 0.3% is added, the strength can be improved. However, in a Bi-S-based free-cutting steel having MnS having a cross-sectional fraction of about 1.6 to 1.8% by weight, B is also segregated outside the austenite grain boundaries and at the interface between MnS and the matrix structure. Therefore, the Bi-S-based free-cutting steel usually needs to add a larger amount of boron than the steel. That is, if added at 0.005% or less, the amount of boron segregated at the crystal grain boundaries is too small.
The segregation of S and the like at the austenite crystal grain boundaries cannot be effectively and suppressed, and if too much is added in an amount of 0.015% or more, boron precipitates are formed at the austenite crystal grain boundaries. Even if S, S, etc. are not segregated, the austenite grain boundaries become brittle, so that the high-temperature ductility decreases.

The nitrogen (N) precipitates in the form of TiN or Ti (CN) in the steel to which titanium has been added, refines the crystal grains at high temperatures, and precipitates and hardens at normal temperatures. Therefore, in order to use TiN to refine the crystal grains, an appropriate amount of nitrogen concentration is required. If too much is added, age hardening occurs due to interaction with potential,
Cold embrittlement can be induced. Therefore, from 0.003
It is necessary to limit the range to 0.01%. On the other hand, N added in the boron-added steel is precipitated by BN or the like, so that the amount of effective boron segregated at the grain boundaries is reduced. Therefore, it must be limited to 0.007% or less.

MnS inclusions adsorbing MnS and Bi and metallic Bi are formed in the steel having the above composition. At this time, the cross-sectional fraction of the inclusions is determined by first adsorbing MnS and Bi. The cross-sectional fraction of the MnS inclusions
Desirably, it is 2.0%. The reason is that if the cross-sectional fraction of the inclusions is 0.5% or less, the machinability decreases,
Also, high temperature embrittlement shifts from interfacial embrittlement between supporting iron and MnS to grain boundary embrittlement, and if it is 2.0% or more, hot workability is reduced.

It is desirable that the metallic Bi inclusions have a cross-sectional fraction of 0.03-0.3%, because if the cross-sectional fraction is 0.03% or less, the machinability decreases. However, if it is 0.3% or more, the hot workability decreases. Each of the above two inclusions has a length of 5-20 μm.
And the width is desirably 1 to 10 μm. The reason is that if the inclusions are too fine, the role as the center of stress concentration is reduced and the cutting effect is lost, and if the inclusions are too coarse, machinability and hot workability are reduced. is there. If the shape ratio (length / width) of the inclusions or the like is too small, the machinability decreases, and if the shape ratio is too large, the material may tear in the direction of hot rolling during high-temperature deformation. Because of the increase, it is desirable that the appropriate value of the shape ratio is 1-2.

Next, a method for producing free-cutting steel and the like according to the present invention will be described. First, a method for producing the free-cutting steel of the present invention by adjusting the reheating temperature will be described. As described above, the reheating temperature is limited to a range of 850 to 1000 ° C. for rolling by using a free cutting steel bloom having a composition to which Ti, B and N are not added. This is because if the reheating temperature cannot be set to 850 ° C., the rolling temperature is too low, the ductility decreases, and
If the temperature exceeds 000 ° C., crystal grains grow. By controlling the reheating temperature, the austenite grain size does not exceed 60 μm. Along with this, 60%
Since the above-described excellent high-temperature ductility can be ensured, it is possible to prevent the occurrence of surface defects during the subsequent billet rolling and wire rod rolling. On the other hand, the maintenance time at the reheating temperature is desirably 1 minute or more.

Next, a method for producing the free-cutting steel of the present invention to which Ti and N are added will be described. As described above, titanium 0.
Reheat at normal temperature using a free cutting steel bloom containing 0-0.1%, nitrogen 0.003-0.01%. At this time, even at a high reheating temperature of 1000 ° C. or higher, the austenite crystal grain size can be adjusted to 60 μm or less by the action of the same precipitate as insoluble TiN, so that high-temperature ductility can be secured to 60% or more. Along with this, the subsequent billet rolling and wire rod rolling temperature range is 850-1200 ° C.
Can be extended to 1000 ° C. or less. At this time, the fact that the range of the rolling temperature was 850-200 ° C. means that the rolling temperature was 85 ° C.
If the temperature is lower than 0 ° C., surface defects may be generated during hot rolling. If the temperature is higher than 1200 ° C., the crystal grain boundary is partially melted, so that the crystal grain boundary is embrittled. This is because surface defects occur.

Next, a method for producing the free-cutting steel of the present invention to which B and N are added will be described. 0.005 boron together with the above
After reheating a free-cutting steel bloom containing 0.0015% or less and 0.007% or less of nitrogen by a normal reheating method, billet rolling is performed at a temperature of 800 to 1200 ° C.
At this time, after the obtained billet is reheated, if the wire is rolled at a temperature of 800 to 1200 ° C., a free-cutting steel wire having excellent high-temperature ductility and no surface defects can be obtained. Here, both the billet rolling and wire rod rolling temperatures were 800-1
200 ° C. The reason is that if the rolling temperature is lower than 800 ° C., there is a possibility that surface defects may occur during hot rolling, the rolling load is large, and a large-capacity rolling mill is required. If the temperature is higher than 1200 ° C., the crystal grain boundaries are partially melted, and surface defects due to embrittlement of the crystal grain boundaries occur. That is, according to the present invention, the range of the hot rolling temperature is extended to 1000 ° C. or less.

Hereinafter, the present invention will be described in detail through embodiments. [Example 1] A test piece having the same components as in Table 1 below was prepared. At this time, the cross-sectional fraction of MnS inclusions adsorbing MnS and bismuth contains 1.5-2.0%, and the cross-sectional fraction of only bismuth inclusions is 0.15-0.2%. Also,
The length of this inclusion is 5-15 um and the width is 5-15 um, and the ratio of the length to the width is about 1-2. The cross-sectional fraction of the bismuth inclusions was calculated from a scanning electron microscope (SEM) image (200 magnification).

[0033]

[Table 1]

A test piece using steel type A shown in Table 1 was heated under the same reheating conditions as in Table 2 below, and a tensile test was performed. The fracture surface reduction rate at each temperature was determined, and the results were shown in Table 2 below in terms of high-temperature ductility. The austenite grain size was measured, and the results were also shown in Table 2 below.

[0035]

[Table 2]

As can be seen from Table 2, when the austenite crystal grain size is 121 μm, the fracture surface reduction rate is 60% or less at a temperature of 900 ° C. or less. Therefore, surface defects occur when hot rolling is performed at 900 ° C. Become like However, if the austenite grain size is reduced to 60 μm or less by controlling the reheating temperature, the fracture surface reduction rate increases to 60% or more, and surface defects occur even when hot-rolled at 900 ° C. do not do. That is, by controlling the austenite grain size according to the present invention, it becomes possible to perform hot rolling while preventing the occurrence of surface defects even at a relatively low temperature around 900 ° C.

In the above embodiment of the present invention, the fracture surface reduction rate was used as a criterion for quantitative evaluation of high-temperature ductility. The reason is as follows. When the fracture surface reduction rate of a high-temperature tensile test specimen was measured to predict hot embrittlement, when a fractured material having a fracture surface reduction rate of 60% or more was observed, the interface between the MnS inclusions adsorbing bismuth and the iron support was observed. Then, a void is generated, and this void is accompanied by a plastic deformation rate, and a ductile fracture occurs in which the void is united with another grown adjacent void and fractured. On the other hand, a fractured material having a fracture surface reduction rate of 60% or less is made of Bi-adsorbed MnS.
Voids are formed at the interface between the inclusion and the iron support and at the austenite grain boundaries.The pores do not grow with plastic deformation but eventually develop into cracks and merge with other adjacent cracks. And brittle fracture occurs. In addition, the area reduction rate is 6
The grain boundary fracture rate increases as the fracture surface reduction rate decreases with a fractured material of 0% or less. Therefore, the temperature at which the fracture surface reduction rate of the tensile test specimen appears at 60% can be determined as the standard of hot embrittlement in which surface defects occur during hot rolling. Also,
The fracture surface reduction rate can be used as a standard for quantitative evaluation of hot ductility.

Example 2 Test pieces having the same components as in Table 3 below were prepared. At this time, the MnS inclusion containing MnS and bismuth contained 1.5 to 2.0% of the cross-sectional fraction, and the cross-sectional fraction of only the bismuth inclusion was 0.15 to 0.2%. The length of this inclusion is 5 to 15 μm and the width is 5
−15 μm, and the ratio of the length to the width was about 1-2. The sectional fraction of the bismuth inclusions was calculated from an image (200 magnification) of a scanning electron microscope (SEM).

A test piece using the steel types shown in Table 3 below was heated under the same reheating conditions as in Table 4 below, and a tensile test was performed. The austenitic grain size and the material having each austenite grain size were subjected to a tensile test to determine the fracture surface reduction rate, and the results are shown in Table 3 below. As can be seen from Table 4 below, even when the reheating temperature is as high as 1250 ° C., the type of steel (titanium type T1, T2,
In T4), since the TiN suppresses the growth of the austenite grain size at the time of reheating, the austenite grain size is 60 μm or less, and the fracture surface reduction rate at 850 to 1000 ° C. of this steel type is 60% or more, and hot rolling is performed. There is no fear that a surface defect sometimes occurs. However, even with the type of steel to which titanium is added, the type of steel having a low nitrogen concentration (type of steel T3)
Since TiN, which suppresses the growth of austenite grain size, is not precipitated, when the austenite grain size is 85 μm and the fracture surface reduction rate at 850 to 1000 ° C. is 60% or less and hot rolling is performed at this temperature, surface defects are reduced. appear.

Even if the nitrogen concentration is adjusted, the type of steel to which titanium is excessively added (type of steel T5) is TiN.
Even if the austenite crystal grain size is reduced to about 54 μm, the strength of the crystal grains at the grain boundaries due to excessive precipitation of TiN is so large that 850-10
Since the fracture surface reduction rate at 00 ° C. is 60% or less, surface defects occur during hot rolling. Also, in the case of steel type (T0) to which Ti and N are not added, since the fracture surface reduction rate is 60% or less at a temperature of 850 to 950 ° C, surface defects occur during hot rolling. Therefore, not only the type of steel (T1, T2, and T4) to which titanium and nitrogen are added in a predetermined amount but also the temperature range of 1000 ° C. or more,
It can be seen that it has excellent high-temperature ductility even at a relatively low temperature of 00 ° C.

[0041]

[Table 3]

[0042]

[Table 4]

Example 3 After a tensile test was performed at a high temperature on a material having the composition shown in Table 5 below and the fracture surface reduction rate was measured,
The results are shown in Table 6 below. As shown in the following Tables 5 and 6, in the case of the invention steel (1-3), which is a type of steel composed according to the present invention, boron is first segregated at the crystal grain boundaries to suppress segregation of Bi, S and the like. 800-10
When the fracture surface reduction rate at 00 ° C. is 70% or more, it is understood that the high-temperature ductility is excellent. Therefore, there is no possibility that a surface defect occurs. On the other hand, comparative steel without boron addition (1)
In the case of, the fracture surface reduction rate is 60% at 800 to 950 ° C.
In the following, hot embrittlement occurred, high-temperature ductility became embrittled, and in the case of the comparative steel (2-3) having a low boron concentration even in the type of steel to which boron was added, the steel was segregated to austenite grain boundaries. Due to the small amount of boron, a high-temperature ductility that suppresses hot embrittlement at 800-900 ° C. and a reduction rate of fracture surface of 60% or less could not be secured.

Further, in the case of the comparative steel (4) having a high boron concentration, even at a high temperature of 900 ° C. or higher where a large amount of boron is precipitated at the crystal grain boundaries, the fracture surface reduction rate is 60% or less, deteriorating the high-temperature ductility. Therefore, the steel of the present invention to which a predetermined amount of boron is added is 800 to 10% less than the comparative steel to which boron is not added.
It can be seen that it has excellent high-temperature ductility in the temperature range of 00 ° C. and no surface defects occur during hot rolling.

On the other hand, the steel of the present invention to which boron is added (1 to 1)
Comparing the comparative steel (3) with the comparative steel (1) to which boron is not added, the invention steels (1 to 3) have much higher hot ductility than the comparative steel (1) even at a relatively high rolling temperature. You can see that there is. For example, in the rolling temperature range of 1000 to 1100 ° C., the reduction ratio of the fracture surface of the invention steels (1 to 3) is 90% or less as compared with 90% or more. Therefore, the inventive steel to which boron is added has not only excellent high-temperature ductility under a high rolling temperature of 1000 ° C. or more but also a relatively low rolling temperature of 800 to 1000 ° C., as compared with the comparative steel to which boron is not added. It can be seen that the high temperature ductility is excellent even below.

[0046]

[Table 5]

[0047]

[Table 6]

Example 4 A steel slab having the composition shown in Table 5 of Example 3 was reheated at a temperature of 1250 ° C. for 3 hours, and then hot-rolled at a temperature shown in Table 6 below. The degree of occurrence of surface defects was confirmed, and the results are shown in Table 7 below. As shown in Table 7 below, the comparative steel (1-2) was 80
When hot rolling was performed at 0 to 950 ° C., surface defects were found. However, in the case of the steel (1-3) of the present invention, no surface defects occurred even at a low rolling temperature of about 800 ° C.

[0049]

[Table 7]

[0050]

As described above, according to the present invention,
A bismuth (Bi) -sulfur (S) -based free-cutting steel excellent in high-temperature ductility can be realized, and this free-cutting steel can be rolled even at a low temperature. It has an excellent effect of improving the properties.

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[Procedure amendment]

[Submission date] June 7, 2000 (2000.6.7)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Full text

[Correction method] Change

[Correction contents]

[Document Name] Statement

Title of the Invention Bismuth (Bi)-excellent in high-temperature ductility
Sulfur (S) -based free-cutting steel and method for producing the same

[Claims]

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a bismuth (Bi)-
The present invention relates to a free-cutting steel based on sulfur (S) and a method for producing the same, and more particularly, to a free-cutting steel excellent in high-temperature ductility and having no surface defects even at a low rolling temperature, and a method for producing the same. is there.

[0002]

2. Description of the Related Art Free-cutting steel is a material used for manufacturing small parts such as cameras and watches requiring excellent machinability, and is required for precision workability. With the increase in production of cameras and other products, lead-sulfur-based free-cutting steel to which elements such as lead that improve machinability have been added has been developed and put into practical use as a steel material suitable for high-speed and automatic cutting. Have been. However, since lead is toxic to the human body, a bismuth (Bi) -sulfur (S) free cutting steel has been developed to solve this problem.

[0003] Generally, bismuth (Bi) -sulfur (S) free-cutting steel is prepared by adding non-metallic or metallic inclusions to steel,
Machining performance has been improved and improved. A typical nonmetallic inclusion is MnS, and the metallic inclusion is:
It is a low melting point metal such as bismuth (Bi) which has almost no solid solubility in steel. Such inclusions act at the center of stress concentration during cutting, and the inclusions and support iron (matrix)
facilitates void generation and crack growth at the interface of the structure, reducing the force required for cutting. Cutting heat softens or melts, and inserts and cutting tools (too
l) acts as a lubricant at the interface, suppresses tool wear, and reduces the force required for cutting.

[0004] Sulfur added to form MnS becomes supersaturated even if the content of Mn is insufficient, or even if the content of Mn is sufficient, solidification or cooling occurs suddenly. During continuous casting, a slab has a structure in which the surface is rapidly solidified by a certain thickness, and then a columnar crystal is formed therein, and MnS is crystallized between the columnar crystals. However, in a structure formed by rapid solidification called a chill crystal, sulfur cannot form MnS and is supersaturated.
Thus, part of the supersaturated sulfur precipitates as MnS during cooling or reheating, and partly precipitates as FeS, which causes hot embrittlement at crystal grain boundaries. In this steel, not only sulfur but also gas components with almost no solid solubility are supersaturated, so those gas components exceeding the solid solubility in the solid remain as fine pin holes. I will be.

On the other hand, bismuth (Bi) useful for cutting work
Has little solid solubility in steel and low melting point,
In general carbon steel that does not intentionally generate MnS inclusions,
It mainly precipitates at the grain boundaries in the form of a thin film (flim) and causes grain boundary embrittlement. On the other hand, in a free-cutting steel in which MnS inclusions are intentionally generated, bismuth (Bi) is adsorbed by the MnS inclusions and crystallized, so that the austenite interface or austenite grain boundary between the MnS inclusions and the supporting iron. And induces hot embrittlement. The liquid bismuth (Bi) existing at the interface between the MnS inclusions and the austenite of the supporting iron causes a decrease in ductility at approximately 900 to 1000 ° C., and the liquid bismuth (Bi) existing at the austenite crystal grain boundary is formed. ) Reduces ductility at about 800-900 ° C. As described above, when the ductility sharply decreases during hot rolling, defects occur on the surface of the hot-rolled wire. This problem occurs particularly remarkably when ductility is lowered at 800 to 900 ° C.

[0006] Such surface defects can be roughly classified into two types. One is large defects caused by scabs (scabs), whose length is 5-1.
It is about 0 mm and its distribution interval is not uniform, but is usually several mm. Further, other surface defects are fine cracks generated in the length direction of the wire, and the length is 1 mm or less,
The distribution interval is several μm, and there is some regularity. These defects are caused by a liquid bismuth (B) that has been extended from a large number of fine cracks with part of the fine cracks as a starting point.
It is formed in a pin hole or on a crystal grain boundary at the interface between MnS adsorbing i) and the matrix structure.

[0007] Therefore, conventionally, the rolling temperature is usually set at 1000 to suppress the generation of fine cracks which are the starting points of defects or to suppress further growth even if fine cracks are generated. The generation of defects was suppressed by increasing the temperature to at least ℃ and improving the ductility of the matrix structure. However, raising the rolling temperature causes problems such as difficulty in maintaining the rolling equipment and a decrease in productivity. Therefore, bismuth that can ensure excellent ductility in which generation of surface defects is suppressed even at a relatively low rolling temperature. There is a demand for (Bi) -sulfur (S) -based free-cutting steel.

[0008]

OBJECTS OF THE INVENTION It is an object of present invention thus is refining crystal grains by suppressing the growth of crystal grain, it is possible to prevent the occurrence of surface defects in the range of relatively low rolling temperature Bismuth (B
(i) To provide a free-sulfur (S) -based free-cutting steel and a method for producing the same.

Another object of the present invention, even at high reheating temperature by adding Ti and N, the movement to the grain boundary utilizing the precipitates present finely not redissolved at high temperatures Obstruct,
An object of the present invention is to provide a bismuth (Bi) -sulfur (S) -based free-cutting steel excellent in high-temperature ductility that can refine crystal grains and prevent generation of surface defects even in a relatively low rolling temperature range, and a method for producing the same. Another object of the present invention is to provide B and N
To preferentially segregate boron having a faster diffusion rate than S and Bi at the austenite grain boundaries, and to prevent the occurrence of surface defects caused by FeS, molten Bi, etc. in a relatively low rolling temperature range. Bismuth with excellent ductility (B
(i) To provide a free-sulfur (S) -based free-cutting steel and a method for producing the same.

[0010]

In order to achieve the above object, in the present invention, carbon (C) is 0.05% by weight.
-0.15%, manganese (Mn): 0.5-2.0%,
Sulfur (S): 0.15-0.4%, phosphorus (P): 0.01
-0.1%, oxygen (O): 0.003-0.02%, bismuth (Bi): 0.03-0.3%, silicon (S
i): 0.01% or less, aluminum (Al): 0.0
009% or less, the balance being iron (Fe) and unavoidable impurities, and MnS or bismuth-adsorbed MnS
MnS containing inclusions and metallic bismuth inclusions
And the cross-sectional fraction of MnS inclusions adsorbing bismuth is
0.5% -2.0% with metallic bismuth
The cross-sectional fraction of the object is 0.03% -0.3%.
Are each 5 to 20 μm in length and 1-1 in width.
Bi- excellent in high-temperature ductility characterized by being 0 μm.
S-based free cutting steel.

Further , the present invention provides a method for preparing carbon (C) by weight%:
0.05-0.15%, manganese (Mn): 0.5-
2.0%, sulfur (S): 0.15-0.4%, phosphorus (P): 0.01-0.1%, oxygen (O): 0.003
-0.02%, bismuth (Bi): 0.03-0.3
%, Silicon (Si): 0.01% or less, aluminum (Al): 0.0009% or less, titanium (Ti):
0.02-0.1%, nitrogen (N): 0.003-0.0
MnS inclusions adsorbing MnS and bismuth composed of iron (Fe) and unavoidable impurities, and metallic bismuth inclusions, and the MnS and bismuth inclusion
Is 0.5% -2.
0%, and the cross-sectional fraction of the metallic bismuth inclusion is
0.03% -0.3%, and the above two inclusions are
Its length is 5-20 μm and its width is 1-10 μm.
And a Bi-S-based free-cutting steel having excellent hot ductility.

In the present invention, C: 0.05% by weight.
-0.15%, Mn: 0.5-2.0%, S: 0.15
-0.4%, P: 0.01-0.1%, O: 0.003
-0.02%, Bi: 0.02-0.3%, Si: 0.
01% or less, Al: 0.0009% or less, N: 0.00
7% or less, B: 0.0050-0.015%, with the balance being
Made of iron (Fe) and unavoidable impurities, MnS or bis
Mass adsorbed MnS inclusions and metallic bismuth inclusions
Bi-S system excellent in high-temperature ductility characterized by containing
Free-cutting steel.

Further, the present invention provides the above-mentioned MnS and bis
The cross-sectional fraction of the MnS inclusion adsorbing the mass is 0.5%-
2.0% and the cross-sectional area of metallic bismuth inclusions
Rate is 0.03% -0.3%, and the above two inclusions
Each has a length of 5-20 μm and a width of 1-10 μm.
Bi-S based free-cutting steel with excellent hot ductility
And

Further, the present invention relates to a method for preparing C: 0.05% by weight.
-0.15%, Mn: 0.5-2.0%, S: 0.15
-0.4%, P: 0.01-0.1%, O: 0.003
-0.02%, Bi: 0.02-0.3%, Si: 0.
01% or less, Al: 0.0009% or less, N: 0.00
7% or less, B: 0.005 to 0.015%, with the balance being iron
Bloom consisting of (Fe) and unavoidable impurities
Is reheated and billet-rolled at a temperature of 800-1200 ° C.,
The resulting billet was reheated to 800-1200
High temperature ductility characterized by rolling wire at a temperature of ℃
An excellent method for producing Bi-S free-cutting steel.

Further, the present invention relates to the above-described billet rolling.
And wire rolling at a temperature of 800 to 1000 ° C.
High temperature characterized by preventing the occurrence of surface defects
A method for producing a Bi-S-based free-cutting steel having excellent ductility.

Further, the present invention relates to the above-described billet rolling.
And wire rolling in a temperature range of 1000 to 1100 ° C.
And characterized in that the fracture surface reduction rate is 90% or more.
A method for producing a Bi-S-based free-cutting steel having excellent high-temperature ductility.

[0017]

Embodiments of the present invention will be described below in detail. Bi-S free-cutting steel can be hot-rolled at 950 ° C. or higher, but hot rolling at a high temperature of 1000 ° C. or higher is generally performed in order to secure high-temperature ductility of 60% or more and prevent surface defects. It is a target. That is, when the rolling temperature is 100
When the temperature is reduced to 0 ° C. or less, the high-temperature ductility is reduced to 60% or less, so that a surface defect may be generated during hot rolling. Therefore, the present invention is characterized by providing a free-cutting steel capable of ensuring excellent high-temperature ductility even at a relatively low temperature of 1000 ° C. or less and preventing surface defects.

[0018] First, such a high temperature ductility can逹成by grain refinement. In other words, the increase in the grain boundary area due to the refinement of the crystal grains can reduce the fraction of the grain boundary area formed by the segregation of bismuth relatively, and can suppress the grain boundary embrittlement due to bismuth. Instead, the size of the voids generated at the triple point of the crystal grain boundary is reduced and the recrystallization is facilitated only by miniaturization of the crystal grain itself, so that the ductility can be improved. As described above, the crystal grain refinement for improving the high-temperature ductility makes it possible to suppress the growth of crystal grains during reheating.

[0019] Accordingly, the present invention is to suppress the growth of crystal grain, high-temperature ductility to the control to the time of hot rolling so as to interfere with the movement of grain boundaries to maintain the austenite grain size below 60um is excellent Provides free-cutting steel. Further, since the present invention does not form a solid solution again at a high temperature even if the reheating temperature is high, the movement of the grain boundaries is hindered by utilizing fine precipitates such as TiN to control the austenite grain size to 60 μm or less. The present invention also provides a free-cutting steel having excellent hot ductility at the time of hot rolling and containing Ti and N. Next, such high-temperature ductility can be achieved by adding boron (B). That is, the present invention preferentially segregates boron (B), which has a higher diffusion rate than S and Bi at the austenite grain boundaries, to improve the reduction in high-temperature ductility caused by FeS, molten Bi, and the like. Provide N-added free-cutting steel.

[0020] Next, the reasons for limiting the component composition such as free-cutting steel in this invention. C should be added in an amount of 0.05% or more to ensure the surface roughness and mechanical properties of the processed product. However, if it exceeds 0.15%, the pearlite structure increases and the machinability decreases. So 0.05-0.15
% Is desirably added.

The above-mentioned Mn should be added in an amount of 0.5% or more in order to secure a necessary amount of MnS inclusions and to suppress hot embrittlement due to generation of FeS at grain boundaries during hot rolling.
If it exceeds 2.0% or more, the hardness of the steel increases, leading to a decrease in machinability. Therefore, it is desirable to add it in the range of 0.5-2.0%. P should be added in an amount of 0.01% or more in order to improve the surface roughness of the workpiece, but if it exceeds 0.1%, mechanical properties and cold workability are ensured. Is difficult, so 0.01-0.1%
It is desirable to add in the range of.

The above S is Mn for improving the surface roughness of the workpiece by suppressing the growth of BUE (Built Up Edge).
Although 0.15% or more should be added to form S inclusions, if 0.4% or more is added, it is difficult to secure hot workability and cold workability. It is desirable to add in the range of 4%. The above O is at the time of hot rolling,
0.003% or more should be added in order to prevent the machinability from being lowered by the stretching of the MnS-based inclusions.
If it exceeds 02%, it is difficult to secure the plastic deformability of MnS inclusions during cutting, so that 0.003-0.02%
It is desirable to add in the range of.

The above Bi is composed of bismuth alone or M in steel.
Since it is present by being adsorbed by nS, it acts at the center of stress concentration during cutting, thereby reducing the radius of curvature of the chip and improving the processability of the chip. In addition, bismuth has the effect of increasing the area of MnS inclusions on the surface of the cutting tool and improving the surface roughness of the workpiece. In addition, since bismuth is melted by cutting heat, the frictional force between the chip and the cutting tool is reduced, and the wear of the cutting tool is suppressed. Therefore, if it is smaller than 0.02%, the cutting effect is reduced. However, when the content is 0.3% or more, the hot workability becomes very poor.

[0024] Si is to produce a SiO ₂ to form the MnS inclusions compound inclusions. Since the plastic deformability of such a composite inclusion is very poor, the formation of a MnS-based inclusion layer at the end of the tool is hindered and BUE is easily generated, thereby adversely affecting the surface roughness of the workpiece. Therefore, the silicon content should be as low as possible, not exceeding 0.001%. Al easily forms Al ₂ O ₃ to form a composite inclusion with the MnS inclusion. Since such a composite inclusion has very poor plastic deformation ability like the SiO ₂ composite inclusion of MnS, it interferes with the formation of the MnS-based inclusion layer at the end of the tool and the BUE is easily formed. This has an adverse effect on the surface roughness of the product. In addition, Al ₂ O ₃ is very hard, so that the wear of the cutting tool is promoted. Therefore, it is desirable that the content of aluminum does not exceed 0.0009%.

[0025] The titanium (Ti) combines such as nitrogen and carbon are dissolved in the steel, TiN, Ti (CN)
And TiC and the like are deposited. In this case, TiN, Ti (C
N) and TiC are deposited at a higher temperature in this order. In other words,
TiN re-dissolves at the highest temperature. Although the exact re-solid solution temperature of TiN is determined by the solubility of Ti and N, it is known that it usually remains at a temperature of 1200 ° C. or higher. Therefore, at the time of reheating, in order to suppress the growth of austenite crystal grains, it is advantageous to use TiN which is stable even at a higher temperature. When titanium is added to 0.02% or less, the re-solidification temperature of TiN is low, which is effective in suppressing the growth of crystal grains. However, when 0.1% or more is added, crystal growth by TiN is effected. Even if effective for suppression, TiN hardens crystal grains and relatively weakens grain boundaries, deteriorating high-temperature ductility.

The purpose of adding B to steel is usually to segregate at austenite crystal grain boundaries to delay the ferrite generation rate and improve the strength. Therefore,
In general steel in which MnS does not exist as much as free-cutting steel, B in an amount capable of segregating at crystal grain boundaries is 0.0020-0.0%.
When about 0.3% is added, the strength can be improved. However, in a Bi-S-based free-cutting steel having MnS having a cross-sectional fraction of about 1.6 to 1.8% by weight, B is also segregated outside the austenite grain boundaries and at the interface between MnS and the matrix structure. Therefore, the Bi-S-based free-cutting steel usually needs to add a larger amount of boron than the steel. That is, if added at 0.005% or less, the amount of boron segregated at the crystal grain boundaries is too small.
The segregation of S and the like at the austenite crystal grain boundaries cannot be effectively and suppressed, and if too much is added in an amount of 0.015% or more, boron precipitates are formed at the austenite crystal grain boundaries. Even if S, S, etc. are not segregated, the austenite grain boundaries become brittle, so that the high-temperature ductility decreases.

[0027] The nitrogen (N), in the steel with the addition of titanium, deposited in the form of TiN and Ti (CN), at elevated temperatures to refining crystal grains and precipitation hardening at room temperature. Therefore, in order to use TiN to refine the crystal grains, an appropriate amount of nitrogen concentration is required. If too much is added, age hardening occurs due to interaction with potential,
Cold embrittlement can be induced. Therefore, from 0.003
It is necessary to limit the range to 0.01%. On the other hand, N added in the boron-added steel is precipitated by BN or the like, so that the amount of effective boron segregated at the grain boundaries is reduced. Therefore, it must be limited to 0.007% or less.

[0028] the steel is a composition together with the above, the MnS inclusions with adsorbed MnS and Bi, and the metal of Bi is formed, cross-sectional fraction of this time, inclusions, firstly adsorb MnS and Bi The cross-sectional fraction of the MnS inclusions
Desirably, it is 2.0%. The reason is that if the cross-sectional fraction of the inclusions is 0.5% or less, the machinability decreases,
Also, high temperature embrittlement shifts from interfacial embrittlement between supporting iron and MnS to grain boundary embrittlement, and if it is 2.0% or more, hot workability is reduced.

[0029] Then, it is desirable sectional fraction of metallic Bi inclusions is 0.03-0.3%, because, if cross-sectional fraction of 0.03% or less, decrease cutting ability However, if it is 0.3% or more, the hot workability decreases. Each of the two inclusions has a length of 5-20 μm.
m and a width of 1-10 μm. The reason is that if the inclusions are too fine, the role as the center of stress concentration is reduced and the cutting effect is lost, and if the inclusions are too coarse, machinability and hot workability are reduced. is there. If the shape ratio (length / width) of the inclusions or the like is too small, the machinability decreases, and if the shape ratio is too large, the material may tear in the direction of hot rolling during high-temperature deformation. Because of the increase, it is desirable that the appropriate value of the shape ratio is 1-2.

[0030] Next, a method for manufacturing such a free-cutting steel present invention. First, a method for producing the free-cutting steel of the present invention by adjusting the reheating temperature will be described. As described above, the reheating temperature is in the range of 850-1000 ° C. for rolling by using a free-cutting steel bloom having a composition to which Ti, B and N are not added.
I do. This is because if the reheating temperature cannot be set to 850 ° C., the rolling temperature is too low, the ductility decreases, and
If the temperature exceeds 0 ° C., crystal grains grow.
By controlling the reheating temperature, the austenite grain size does not exceed 60 μm. Along with this, it is possible to secure an excellent high-temperature ductility of 60% or more, so that it is possible to prevent the occurrence of surface defects during the subsequent billet rolling and wire rod rolling. On the other hand, the maintenance time at the reheating temperature is desirably 1 minute or more.

[0031] Next, a method for manufacturing a free-cutting steel of the present invention that the addition of Ti and N. As described above, titanium 0.
Reheat at normal temperature using a free cutting steel bloom containing 0-0.1%, nitrogen 0.003-0.01%. At this time, even at a high reheating temperature of 1000 ° C. or higher, the austenite crystal grain size can be adjusted to 60 μm or less by the action of the same precipitate as insoluble TiN, so that high-temperature ductility can be secured to 60% or more. Along with this, the subsequent billet rolling and wire rod rolling temperature range is 850-1200 ° C.
Can be extended to 1000 ° C. or less. At this time, the fact that the range of the rolling temperature was 850-200 ° C. means that the rolling temperature was 85 ° C.
If the temperature is lower than 0 ° C., surface defects may be generated during hot rolling. If the temperature is higher than 1200 ° C., the crystal grain boundary is partially melted, so that the crystal grain boundary is embrittled. This is because surface defects occur.

[0032] Next, a method for manufacturing a free-cutting steel of the present invention that the addition of B and N. 0.005 boron together with the above
After reheating a free-cutting steel bloom containing 0.0015% or less and 0.007% or less of nitrogen by a normal reheating method, billet rolling is performed at a temperature of 800 to 1200 ° C.
At this time, after the obtained billet is reheated, if the wire is rolled at a temperature of 800 to 1200 ° C., a free-cutting steel wire having excellent high-temperature ductility and no surface defects can be obtained. Here, both the billet rolling and wire rod rolling temperatures were 800-1
200 ° C. The reason is that if the rolling temperature is lower than 800 ° C., there is a possibility that surface defects may occur during hot rolling, the rolling load is large, and a large-capacity rolling mill is required. If the temperature is higher than 1200 ° C., the crystal grain boundaries are partially melted, and surface defects due to embrittlement of the crystal grain boundaries occur. That is, according to the present invention, the range of the hot rolling temperature is extended to 1000 ° C. or less.

[0033] will be specifically described below through embodiments of the present invention. [Example 1] A test piece having the same components as in Table 1 below was prepared. At this time, the cross-sectional fraction of MnS inclusions adsorbing MnS and bismuth contains 1.5-2.0%, and the cross-sectional fraction of only bismuth inclusions is 0.15-0.2%. Also,
The length of this inclusion is 5-15 um and the width is 5-15 um, and the ratio of the length to the width is about 1-2. The cross-sectional fraction of the bismuth inclusions was calculated from a scanning electron microscope (SEM) image (200 magnification).

[0034]

[Table 1]

[0035] by heating the specimen using the type A of the steel shown in Table 1 in the same reheating jaw matter and Table 2 below, was subjected to a tensile test. The fracture surface reduction rate at each temperature was determined, and the results were shown in Table 2 below in terms of high-temperature ductility. The austenite grain size was measured, and the results were also shown in Table 2 below.

[0036]

[Table 2]

As can be seen from Table 2, when the austenite grain size is 121 μm, the fracture area reduction rate is 60% or less at a temperature of 900 ° C. or less. Therefore, surface defects occur when hot rolling is performed at 900 ° C. Become like However, if the austenite grain size is reduced to 60 μm or less by controlling the reheating temperature, the fracture surface reduction rate increases to 60% or more, and surface defects occur even when hot-rolled at 900 ° C. do not do. That is, by controlling the austenite grain size according to the present invention, it becomes possible to perform hot rolling while preventing the occurrence of surface defects even at a relatively low temperature around 900 ° C.

In the above embodiment of the present invention, the fracture surface reduction rate was used as a standard for quantitative evaluation of high-temperature ductility. The reason is as follows. When the fracture surface reduction rate of a high-temperature tensile test specimen was measured to predict hot embrittlement, when a fractured material having a fracture surface reduction rate of 60% or more was observed, the interface between the MnS inclusions adsorbing bismuth and the iron support was observed. Then, a void is generated, and this void is accompanied by a plastic deformation rate, and a ductile fracture occurs in which the void is united with another grown adjacent void and fractured. On the other hand, a fractured material having a fracture surface reduction rate of 60% or less is made of Bi-adsorbed MnS.
Voids are formed at the interface between the inclusion and the iron support and at the austenite grain boundaries.The pores do not grow with plastic deformation but eventually develop into cracks and merge with other adjacent cracks. And brittle fracture occurs. In addition, the area reduction rate is 6
The grain boundary fracture rate increases as the fracture surface reduction rate decreases with a fractured material of 0% or less. Therefore, the temperature at which the fracture surface reduction rate of the tensile test specimen appears at 60% can be determined as the standard of hot embrittlement in which surface defects occur during hot rolling. Also,
The fracture surface reduction rate can be used as a standard for quantitative evaluation of hot ductility.

[0039] were prepared specimens having the same ingredients as Example 2] Table 3 below. At this time, the MnS inclusion containing MnS and bismuth contained 1.5 to 2.0% of the cross-sectional fraction, and the cross-sectional fraction of only the bismuth inclusion was 0.15 to 0.2%. The length of this inclusion is 5 to 15 μm and the width is 5
−15 μm, and the ratio of the length to the width was about 1-2. The sectional fraction of the bismuth inclusions was calculated from an image (200 magnification) of a scanning electron microscope (SEM).

[0040] The test piece using a type of steel shown in Table 3 below and heating at the same reheating jaw matter and Table 4 below, was subjected to a tensile test. The austenitic grain size and the material having each austenite grain size were subjected to a tensile test to determine the fracture surface reduction rate, and the results are shown in Table 3 below. As can be seen from Table 4 below, even when the reheating temperature is as high as 1250 ° C., the type of steel (titanium type T1, T2,
In T4), since the TiN suppresses the growth of the austenite grain size at the time of reheating, the austenite grain size is 60 μm or less, and the fracture surface reduction rate at 850 to 1000 ° C. of this steel type is 60% or more, and hot rolling is performed. There is no fear that a surface defect sometimes occurs. However, even with the type of steel to which titanium is added, the type of steel having a low nitrogen concentration (type of steel T3)
Since TiN, which suppresses the growth of austenite grain size, is not precipitated, when the austenite grain size is 85 μm and the fracture surface reduction rate at 850 to 1000 ° C. is 60% or less and hot rolling is performed at this temperature, surface defects are reduced. appear.

Even if the nitrogen concentration is adjusted, the type of steel to which titanium is excessively added (type of steel T5) is TiN.
Even if the austenite crystal grain size is reduced to about 54 μm, the strength of the crystal grains at the grain boundaries due to excessive precipitation of TiN is so large that 850-10
Since the fracture surface reduction rate at 00 ° C. is 60% or less, surface defects occur during hot rolling. Also, in the case of steel type (T0) to which Ti and N are not added, since the fracture surface reduction rate is 60% or less at a temperature of 850 to 950 ° C, surface defects occur during hot rolling. Therefore, not only the type of steel (T1, T2 and T4) to which a predetermined amount of titanium and nitrogen are added but also the temperature range of 1000 ° C. or more,
It can be seen that it has excellent high-temperature ductility even at a relatively low temperature of 0 ° C.

[0042]

[Table 3]

[0043]

[Table 4]

[0044] After the material that constituted with Example 3 Table 5 subjected to tensile tests at elevated temperatures, to measure the fracture surface reduction rate,
The results are shown in Table 6 below. As shown in the following Tables 5 and 6, in the case of the invention steel (1-3), which is a type of steel composed according to the present invention, boron is first segregated at the crystal grain boundaries to suppress segregation of Bi, S and the like. 800-10
When the fracture surface reduction rate at 00 ° C. is 70% or more, it is understood that the high-temperature ductility is excellent. Therefore, there is no possibility that a surface defect occurs. On the other hand, comparative steel without boron addition (1)
In the case of, the fracture surface reduction rate is 60% at 800 to 950 ° C.
In the following, hot embrittlement occurred, high-temperature ductility became embrittled, and even in the case of a steel to which boron was added, in the case of the comparative steel (2-3) having a low boron concentration, the steel was segregated at austenite grain boundaries. Due to the small amount of boron, a high-temperature ductility that suppresses hot embrittlement at 800-900 ° C. and a reduction rate of fracture surface of 60% or less could not be secured.

In the case of the comparative steel (4) having a high boron concentration, even at a high temperature of 900 ° C. or more where a large amount of boron is precipitated at the crystal grain boundaries, the fracture surface reduction rate is 60% or less, deteriorating the high-temperature ductility. Therefore, the steel of the present invention to which a predetermined amount of boron is added is 800 to 10% less than the comparative steel to which boron is not added.
It can be seen that it has excellent high-temperature ductility in the temperature range of 00 ° C. and no surface defects occur during hot rolling.

Meanwhile, the present invention steels the addition of boron (1-
Comparing the comparative steel (3) with the comparative steel (1) to which boron is not added, the invention steels (1 to 3) have much higher hot ductility than the comparative steel (1) even at a relatively high rolling temperature. You can see that there is. For example, in the rolling temperature range of 1000 to 1100 ° C., the reduction ratio of the fracture surface of the invention steels (1 to 3) is 90% or less as compared with 90% or more. Therefore, the inventive steel to which boron is added has not only excellent high-temperature ductility under a high rolling temperature of 1000 ° C. or more but also a relatively low rolling temperature of 800 to 1000 ° C., as compared with the comparative steel to which boron is not added. It can be seen that the high temperature ductility is excellent even below.

[0047]

[Table 5]

[0048]

[Table 6]

[0049] [Example 4] 3 hours slab that constituted with Table 5 at a temperature of 1250 ° C. Example 3, after reheating, after the hot-wire rolling at a temperature appearing in Table 6 The degree of occurrence of surface defects was confirmed, and the results are shown in Table 7 below. As shown in Table 7 below, the comparative steel (1-2) was 8
Hot rolling at 00-950 ° C. revealed surface defects. However, in the case of the steel (1-3) of the present invention, no surface defects occurred even at a low rolling temperature of about 800 ° C.

[0050]

[Table 7]

[0051]

As described above, according to the present invention,
A bismuth (Bi) -sulfur (S) -based free-cutting steel excellent in high-temperature ductility can be realized, and this free-cutting steel can be rolled even at a low temperature. It has an excellent effect of improving the performance.

Claims (12)

[Claims] 1. Carbon (C): 0.05-0.
15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.4%, phosphorus (P): 0.01-
0.1%, oxygen (O): 0.003-0.02%, bismuth (Bi): 0.03-0.3%, silicon (S
i): 0.01% or less, aluminum (Al): 0.0
009% or less, the balance being iron (Fe) and unavoidable impurities, and MnS or bismuth-adsorbed MnS
A Bi-S free-cutting steel excellent in high-temperature ductility, characterized by containing inclusions and metallic bismuth inclusions. 2. A method according to claim 1, wherein the MnS inclusion adsorbing MnS and bismuth has a cross-sectional fraction of 0.5%-
2.0%, the cross-sectional fraction of the metallic bismuth inclusions is 0.03% -0.3%, and each of the two inclusions has a length of 5-20 μm and a width of 1 Bi-S free-cutting steel excellent in high-temperature ductility, characterized in that it is -10 µm. 3. Carbon (C): 0.05-0.
15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.4%, phosphorus (P): 0.01-
0.1%, oxygen (O): 0.003-0.02%, bismuth (Bi): 0.03-0.3%, silicon (S
i): 0.01% or less, aluminum (Al): 0.0
A bloom consisting of iron (Fe) and unavoidable impurities is prepared at 850-1000 ° C. so that the austenite grain size does not exceed 60 μm.
To form a rolled billet obtained by rolling the billet, and rolling the billet to 850-100.
A method for producing a Bi-S-based free-cutting steel excellent in high-temperature ductility, characterized by reheating at a heating temperature of 0 ° C, maintaining an austenite crystal grain size not exceeding 60 μm, and performing wire rolling. 4. Carbon (C): 0.05-0.
15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.4%, phosphorus (P): 0.01-
0.1%, oxygen (O): 0.003-0.02%, bismuth (Bi): 0.03-0.3%, silicon (S
i): 0.01% or less, aluminum (Al): 0.0
009% or less, titanium (Ti): 0.02-0.1
%, Nitrogen (N): 0.003-0.01%, the balance being iron (Fe) and unavoidable impurities, containing MnS and Mn inclusions adsorbing bismuth and metallic bismuth inclusions Bi-S-based free-cutting steel with excellent hot ductility characterized by the following. 5. The method according to claim 4, wherein the MnS inclusion adsorbing MnS and bismuth has a sectional fraction of 0.5%-.
2.0%, the cross-sectional fraction of metallic bismuth inclusions is 0.0
Bi-S based free cutting excellent in high-temperature ductility, characterized in that each of the two inclusions has a length of 5-20 um and a width of 1-10 um. steel. 6. Carbon (C): 0.05-0.
15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.4%, phosphorus (P): 0.01-
0.1%, oxygen (O): 0.003-0.02%, bismuth (Bi): 0.03-0.3%, silicon (S
i): 0.01% or less, aluminum (Al): 0.0
009% or less, titanium (Ti): 0.02-0.1
%, Nitrogen (N): 0.003-0.01%, and the balance is iron (Fe) and unavoidable impurities.
Is reheated at normal temperature, billet rolled at a temperature of 850-1200 ° C., and the resulting billet is reheated to 85
A method for producing a Bi-S-based free-cutting steel excellent in high-temperature ductility, wherein the wire is rolled at a temperature of 0 to 1200 ° C. 7. The Bi- excellent in high-temperature ductility according to claim 6, wherein said billet rolling and wire rod rolling are performed at a temperature of 850 to 1000 ° C. to prevent generation of surface defects.
Method for producing S-based free-cutting steel. 8. C: 0.05-0.15% by weight%,
Mn: 0.5-2.0%, S: 0.15-0.4%,
P: 0.01-0.1%, O: 0.003-0.02
%, Bi: 0.02-0.3%, Si: 0.01% or less, Al: 0.0009% or less, N: 0.007%,
B: 0.0050-0.015%, with the balance being iron (Fe)
A Bi-S-based free-cutting steel excellent in high-temperature ductility, characterized by containing MnS or bismuth-adsorbed MnS inclusions and metallic bismuth inclusions, comprising unavoidable impurities. 9. The method according to claim 8, wherein the MnS and Bi
The cross-sectional fraction of the MnS inclusions adsorbed with 0.5 to 2.0%
And the cross-sectional fraction of the metallic Bi inclusion is 0.03-0.
3%, and each of the two inclusions has a length of 5-
A Bi-S free-cutting steel excellent in high-temperature ductility, characterized in that it is 20 um and the width is 1-10 um. 10. C: 0.05-0.15 by weight%
%, Mn: 0.5-2.0%, S: 0.15-0.4
%, P: 0.01-0.1%, O: 0.003-0.0
2%, Bi: 0.02-0.3%, Si: 0.01% or less, Al: 0.0009% or less, N: 0.007% or less, B: 0.005-0.015%, The rest is iron (F
e) and a bloom consisting of unavoidable impurities is reheated, billets are rolled at a temperature of 800-1200 ° C., and the resulting billet is reheated to a wire rod at a temperature of 800-1200 ° C. A method for producing a Bi-S-based free-cutting steel excellent in high-temperature ductility, characterized by rolling. 11. The Bi-S according to claim 10, wherein the billet rolling and the wire rod rolling are performed at a temperature of 800 to 1000 ° C. to prevent the occurrence of surface defects. Manufacturing method of free cutting steel. 12. The method according to claim 10, wherein the billet rolling and the wire rolling are performed in a temperature range of 1000 to 1100 ° C.
A method for producing a Bi-S-based free-cutting steel excellent in high-temperature ductility, wherein a fracture surface reduction rate is 90% or more.