JP6055400B2 - Steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material that does not generate burning when heated before hot working or using high frequency, and a method for manufacturing the same.

  Prior to hot working such as hot forging or hot rolling, the steel slab is heated to a high temperature of, for example, 1200 ° C. or higher in order to reduce deformation resistance and facilitate hot working. Moreover, the temperature of a steel slab may become high temperature, for example, 1200 degreeC or more at the time of the heating using a high frequency. However, when the heating temperature exceeds the target temperature and is heated to a high temperature, burning occurs. According to JIS G0201 (2000), burning is an irreversible change in structure and properties caused by the fusion of crystal grain boundaries. In the subsequent heat treatment, machining, or a combination of processing and heat treatment, It is defined as a phenomenon in which the various properties that were possessed cannot be recovered. When burning occurs, the material becomes brittle.

  As a method for preventing the occurrence of burning, it is conceivable to reduce the segregation generated at the grain boundaries or to increase the solidus temperature. When segregation occurs at the grain boundaries, the solidification temperature locally decreases due to the segregated components, and thus burning tends to occur during heating. Further, when the solidus temperature is lowered, the solidification temperature is lowered, so that a part of the steel material is melted at the time of heating, and crystal grain boundaries are fused to cause burning. However, in order to prevent the occurrence of burning, a technique for adjusting the component composition of steel has not been studied.

  Therefore, in order to prevent the occurrence of burning, it is common in actual operations to lower the heating temperature and not overheat. That is, since burning occurs when the heating temperature approaches the solidus temperature (solidification temperature) of the steel material, the heating temperature is controlled so as not to exceed the solidus temperature of −130 ° C. for safety. However, when the heating temperature is lowered, the deformation resistance increases, so that hot working becomes difficult.

Although it is not the technique which prevents generation | occurrence | production of burning, the technique which paid its attention to the MnS type | system | group inclusion contained in steel materials is disclosed by patent document 1. FIG. The steel material disclosed in Patent Document 1 is a steel material for machine structure that can improve both chip disposal and tool life. In this steel material, MnS inclusions having an equivalent diameter of 4 μm or more existing in a cross section parallel to the processing direction satisfy the following expressions (i) and (ii). L and W in the following formula (i) represent the major axis and minor axis of the target MnS-based inclusion, respectively, and the aspect ratio is an aspect ratio as an average value. Further, n, n1, and n2 in the following formula (ii) each represent the number of the following MnS-based inclusions.
Aspect ratio (L / W) <3 (i)
(N1 + n2) /n≦0.2 (ii)
n: Number of MnS-based inclusions per 1 mm ² area n1: Number of MnS-based inclusions in which Ca is solid-solved in an amount of 0.5 mass% or more n2 per area of 1 mm ² n2: MnS-based inclusions Of these, the number per 1 mm ² of area containing different kinds of inclusions

JP 2008-57021 A

  In Patent Document 1, attention is focused on MnS inclusions in the steel material, but attention is not paid to the occurrence of burning.

  An object of the present invention is to provide a steel material excellent in burning resistance that does not generate burning when heated before hot working or heated using a high frequency, and a method for producing the same.

The steel materials according to the present invention that have been able to solve the above-mentioned problems are: C: 0.1 to 0.8% (meaning mass%; the same shall apply hereinafter), Si: 0.01 to 2.0%, Mn: 0 2 to 2.0%, P: 0.20% or less, S: 0.015 to 0.12%, Al: 0.002 to 0.1%, and Ca: 0.0001 to 0% 0.01%, Mg: 0.0001-0.01%, Zr: 0.01-0.5%, Te: 0.001-0.05%, and REM: 0.001-0.05% Including at least one selected from the group, the Mn and the S satisfy the relationship of the following formula (1), and the balance consists of iron and inevitable impurities. In the following formula (1), [] indicates the content (% by mass) of each element.
[Mn] / [S] ≧ 13.0 (1)
The steel material of the present invention has the following number of sulfide inclusions with an equivalent circle diameter of 4 μm or less contained in the steel material and the number of sulfide inclusions with a minor diameter of 2 μm or more contained in the steel material: The point is that the relationship of formula (2) is satisfied.
[Number of sulfide inclusions with equivalent circle diameter of 4 μm or less] / [Number of sulfide inclusions with minor axis of 2 μm or more] ≦ 0.30 (2)

The steel material of the present invention is still another element,
(A) at least one selected from the group consisting of Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 2% or less, and B: 0.01% or less,
(B) at least one selected from the group consisting of V: 0.5% or less, Ti: 0.5% or less, Nb: 0.5% or less, and W: 2% or less,
(C) at least one selected from the group consisting of Pb: 0.3% or less, Bi: 0.3% or less, Sn: 0.02% or less, and Sb: 0.02% or less,
May also be included, and
(D) N may be contained in a range of 0.02% or less.

  The steel material of the present invention can be produced by setting the average cooling rate at the center part of the slab from the start of casting to the completion of solidification to 7 ° C./min or less when casting molten steel satisfying the above component composition to produce a steel material. .

  According to the present invention, in order to appropriately adjust the component composition of the steel material and suppress the formation of fine sulfide inclusions having an equivalent circle diameter of 4 μm or less, heating is performed before hot working or using high frequency. However, it is possible to provide a steel material excellent in burning resistance that does not generate burning, and a method for producing the same.

FIG. 1 is a graph showing the relationship between the average cooling rate at the center of the slab surface during casting and the sulfide inclusion ratio. FIG. 2 shows No. 1 shown in Table 3 below. 1 is a drawing-substituting photograph obtained by photographing the surface of a test piece 1 with an optical microscope at a magnification of 400 times.

  The present inventor has intensively studied in order to prevent burning from occurring before hot working or when heated using high frequency. As a result, it became clear that sulfide inclusions (for example, MnS) are thought to affect the occurrence of burning. Specifically, among sulfide inclusions, fine sulfide inclusions having an equivalent circle diameter of 4 μm or less are decomposed during heating, and the generated S is combined with Fe to form sulfide sulfide inclusions. It was found that the formation of FeS having a lower melting point than that, and the formed FeS partially melts during heating, which is considered to cause burning.

Therefore, in the present invention, among the component compositions of the steel material, in particular, the steel material is adjusted so as to satisfy the following formula (2) after adjusting the Mn amount and the S amount so as to satisfy the relationship of the following formula (1). It is only necessary to suppress the formation of fine sulfide inclusions having an equivalent circle diameter of 4 μm or less, and the sulfur (S) in the sulfide inclusions is stabilized so as not to decompose during heating. Has found that it is sufficient to contain at least one selected from the group consisting of Ca, Mg, Zr, Te, and REM, thereby completing the present invention.
[Mn] / [S] ≧ 13.0 (1)
[Number of sulfide inclusions with equivalent circle diameter of 4 μm or less] / [Number of sulfide inclusions with minor axis of 2 μm or more] ≦ 0.30 (2)

  Hereinafter, the present invention will be described in detail.

[Sulfide inclusions]
The steel material of the present invention contains Mn in a range of 0.2 to 2.0% and S in a range of 0.015 to 0.12%.

  Mn is an element that combines with S to form sulfide inclusions such as MnS, suppresses the formation of FeS, and suppresses the occurrence of burning during heating. If Mn is less than 0.2%, the effect is insufficient. Therefore, in the present invention, the amount of Mn is 0.2% or more. The amount of Mn is preferably 0.5% or more, more preferably 0.7% or more. However, when the amount of Mn becomes excessive and exceeds 2.0%, there are many fine sulfide inclusions having an equivalent circle diameter of 4 μm or less in addition to coarse sulfide inclusions having an equivalent circle diameter exceeding 4 μm. Therefore, burning occurs during heating. Therefore, in the present invention, the amount of Mn is set to 2.0% or less. The amount of Mn is preferably 1.8% or less, more preferably 1.6% or less.

  S is an element inevitably contained in the steel material, but is an element that effectively acts on improvement of machinability when sulfide inclusions such as MnS are formed. If the amount of S is less than 0.015%, such an effect is insufficient. Therefore, in the present invention, the S content is 0.015% or more. The amount of S is preferably 0.017% or more, more preferably 0.019% or more. However, if it is excessively contained, fine sulfide inclusions (for example, MnS etc.) having an equivalent circle diameter of 4 μm or less are likely to be generated, and FeS that causes burning is formed. Therefore, in the present invention, the S amount is 0.12% or less. The amount of S is preferably 0.115% or less, more preferably 0.110% or less.

  The steel material of the present invention must contain Mn and S in the above range, and the amount of Mn and the amount of S must satisfy the relationship of the above formula (1).

  When the value of [Mn] / [S] is less than 13.0, the amount of S becomes excessive compared to the amount of Mn in the steel material, S is not fixed by Mn, and a large amount of FeS having a low melting point is generated. As a result, partial melting occurs during heating, and burning occurs. Therefore, in the present invention, [Mn] / [S] is set to 13.0 or more. [Mn] / [S] is preferably 13.5 or more, more preferably 14.0 or more. The upper limit of the value of [Mn] / [S] is not particularly limited, and as will be described later, the upper limit of the amount of Mn that can be contained in the steel material is 2.0%, and the lower limit of the amount of S is 0.015%. , [Mn] / [S] is 133.3. [Mn] / [S] is preferably 100 or less, more preferably 80 or less, and still more preferably 50 or less.

  In the steel material of the present invention, the number of sulfide inclusions having an equivalent circle diameter of 4 μm or less contained in the steel material and the number of sulfide inclusions having a minor diameter of 2 μm or more contained in the steel material are expressed by the above formula (2 It is also important to satisfy the relationship. Hereinafter, the value on the left side of the above formula (2) may be referred to as a sulfide inclusion ratio.

  If fine sulfide inclusions having an equivalent circle diameter of 4 μm or less are excessively generated, they are decomposed during heating, and S combines with Fe to generate FeS having a melting point lower than that of MnS. If FeS having a low melting point is excessively produced, partial melting occurs and burning occurs. Therefore, in the present invention, the sulfide inclusion ratio is set to 0.30 or less. The sulfide inclusion ratio is preferably 0.20 or less, more preferably 0.15 or less. Although the minimum of the said sulfide type inclusion ratio is not specifically limited, For example, what is necessary is just 0.01 or more, More preferably, it is 0.02 or more.

  The sulfide inclusion is an inclusion containing 25% by mass or more of S when the total amount of alloy elements excluding iron (Fe) is 100% by mass when the component composition of the inclusion is analyzed. Means a thing. For the method of measuring the number of sulfide inclusions having an equivalent circle diameter of 4 μm or less contained in the steel material and the number of sulfide inclusions having a minor diameter of 2 μm or more contained in the steel material, refer to the section of Examples. explain.

  Next, components other than Mn and S contained in the steel material of the present invention will be described.

[C: 0.1 to 0.8%]
C is an element necessary for ensuring the strength of the steel material (strength of the final product). If it is less than 0.1%, the strength of the steel material is too low to be used for machine structures. Therefore, in the present invention, the C amount is 0.1% or more. The C content is preferably 0.12% or more, more preferably 0.15% or more. However, when C is contained excessively, the strength becomes too high and the workability is lowered. Therefore, in the present invention, the C content is 0.8% or less. The C content is preferably 0.75% or less, more preferably 0.70% or less.

[Si: 0.01 to 2.0%]
Si is an element necessary for increasing the strength of the final product by solid solution hardening. In the present invention, Si is 0.01% or more. The amount of Si is preferably 0.05% or more, and more preferably 0.10% or more. However, when the amount of Si becomes excessive and exceeds 2.0%, an Si-containing oxide is excessively generated and the solidus temperature is lowered, so that burning tends to occur during heating. Therefore, in the present invention, the Si amount is set to 2.0% or less. The amount of Si is preferably 1.5% or less, more preferably 1.3% or less.

[P: 0.20% or less (excluding 0%)]
P is an element inevitably contained in the steel material, and contributes to improving machinability. However, if P is contained excessively exceeding 0.20%, it segregates at the grain boundary to lower the solidus temperature, and burning tends to occur during heating. Therefore, in the present invention, the P amount is 0.20% or less. The amount of P is preferably 0.15% or less, more preferably 0.12% or less.

[Al: 0.002 to 0.1%]
Al is an element that acts as a deoxidizing element. When the amount of Al is less than 0.002%, an oxide having a low melting point is generated, partial melting is promoted during heating, and burning occurs. Therefore, in the present invention, the Al content is 0.002% or more. The amount of Al is preferably 0.003% or more. However, if excessively contained, Al ₂ O ₃ is often generated, the workability is deteriorated. Therefore, in the present invention, the Al content is 0.1% or less. The amount of Al is preferably 0.075% or less, and more preferably 0.060% or less.

[Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Zr: 0.01 to 0.5%, Te: 0.001 to 0.05%, and REM (rare earth element ): At least one selected from the group consisting of 0.001 to 0.05%]
Ca, Mg, Zr, Te, and REM all stabilize S in sulfide inclusions (for example, MnS, etc.) and coarsen sulfide inclusions to cause burning during heating. It is an element that suppresses this. In order to exert such an effect, Ca is 0.0001% or more, preferably 0.0003% or more, more preferably 0.0004% or more. Mg is made 0.0001% or more, preferably 0.0003% or more, more preferably 0.0004% or more. Zr is 0.01% or more, preferably 0.03% or more, more preferably 0.04% or more. Te is set to 0.001% or more, preferably 0.002% or more. The REM is set to 0.001% or more, preferably 0.002% or more, more preferably 0.003% or more. However, even if contained excessively, the effect of addition is saturated and the cost is increased. Therefore, in the present invention, Ca is 0.01% or less, preferably 0.0095% or less, more preferably 0.0090% or less. Mg is set to 0.01% or less, preferably 0.0095% or less, more preferably 0.0090% or less. Zr is 0.5% or less, preferably 0.45% or less, more preferably 0.40% or less. Te is 0.05% or less, preferably 0.045% or less. REM is made 0.05% or less, preferably 0.04% or less. The above elements may contain two or more selected alone or arbitrarily.

  The REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium), and among these elements, it is selected from the group consisting of La, Ce and Y. It is preferable to contain at least one element, more preferably at least one element selected from the group consisting of La and Ce.

  The steel material of the present invention contains the above elements, and the balance is iron and inevitable impurities.

The steel material of the present invention further includes other elements,
(A) at least one selected from the group consisting of Cu, Ni, Cr, Mo, and B;
(B) at least one selected from the group consisting of V, Ti, Nb, and W;
(C) at least one selected from the group consisting of Pb, Bi, Sn, and Sb,
May contain
(D) N may be contained in a range of 0.02% or less.

  The preferable range of each element is as follows.

[(A) Cu: 1% or less (not including 0%), Ni: 2% or less (not including 0%), Cr: 2% or less (not including 0%), Mo: 2% or less (0 %), And B: at least one selected from the group consisting of 0.01% or less (not including 0%)]
Cu, Ni, Cr, Mo, and B are all elements that act to improve the hardenability of the steel material and increase the strength of the final product, and contain two or more selected alone or arbitrarily. May be. In order to exhibit such an action effectively, Cu is preferably 0.1% or more, more preferably 0.2% or more. Ni is preferably 0.2% or more, more preferably 0.5% or more, and further preferably 1.0% or more. Cr is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.5% or more. Mo is preferably 0.2% or more, and more preferably 0.3% or more. B is preferably 0.0005% or more, more preferably 0.0010% or more. However, when the content of these elements is excessive, the strength of the steel material becomes too high, and the workability may be deteriorated. Therefore, in the present invention, Cu is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Ni is preferably 2% or less, more preferably 1.8% or less, and still more preferably 1.7% or less. Cr is preferably 2% or less, more preferably 1.9% or less, and still more preferably 1.8% or less. Mo is preferably 2% or less, more preferably 1.5% or less, and still more preferably 1.0% or less. B is preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.005% or less.

[(B) V: 0.5% or less (not including 0%), Ti: 0.5% or less (not including 0%), Nb: 0.5% or less (not including 0%), and W: at least one selected from the group consisting of 2% or less (excluding 0%)]
V, Ti, Nb, and W are elements that combine with C to form carbides or combine with N to form nitrides and contribute to improving the strength of the steel, and can be selected alone or arbitrarily. Two or more types may be contained. In order to exhibit such an action effectively, V is preferably 0.05% or more, more preferably 0.10% or more. Ti is preferably 0.005% or more, more preferably 0.010%, and still more preferably 0.015% or more. Nb is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. W is preferably 0.2% or more, more preferably 0.5% or more, and still more preferably 0.7% or more. However, when these elements are contained excessively, the formed carbides and nitrides may increase the deformation resistance of the steel material and may deteriorate the workability. Therefore, in the present invention, V is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.30% or less. Ti is preferably 0.5% or less, more preferably 0.3% or less, and still more preferably 0.1% or less. Nb is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less. W is preferably 2% or less, more preferably 1.5% or less, and still more preferably 1.0% or less.

[(C) Pb: 0.3% or less (not including 0%), Bi: 0.3% or less (not including 0%), Sn: 0.02% or less (not including 0%), and Sb: at least one selected from the group consisting of 0.02% or less (excluding 0%)]
Pb, Bi, Sn, and Sb are all elements that contribute to the improvement of machinability, and may contain two or more selected alone or arbitrarily. In order to effectively exhibit such an action, Pb is preferably 0.03% or more, more preferably 0.1% or more, and further preferably 0.15% or more. Bi is preferably 0.03% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. Sn is preferably 0.002% or more, more preferably 0.005% or more, and still more preferably 0.010% or more. Sb is preferably 0.002% or more, more preferably 0.004% or more, and still more preferably 0.006% or more. However, when these elements are contained excessively, strength and workability may be lowered. Therefore, in the present invention, Pb is preferably 0.3% or less, more preferably 0.28% or less, and still more preferably 0.25% or less. Bi is preferably 0.3% or less, more preferably 0.25% or less, and still more preferably 0.20% or less. Sn is preferably 0.02% or less, more preferably 0.018% or less, and still more preferably 0.015% or less. Sb is preferably 0.02% or less, more preferably 0.015% or less, and still more preferably 0.010% or less.

[(D) N: 0.02% or less (excluding 0%)]
N is an element inevitably contained in the steel material. If the amount of solute N in the steel material is excessively increased, the hardness may be increased by strain aging or the ductility may be decreased, and the workability may be deteriorated. Therefore, in the present invention, the N content is preferably 0.02% or less. The N amount is more preferably 0.018% or less, still more preferably 0.015% or less, and most preferably 0.010% or less.

[Production conditions]
Next, the manufacturing method of the steel material which concerns on this invention is demonstrated.

  In producing the steel material by casting the molten steel satisfying the above component composition, it is important for the steel material of the present invention that the average cooling rate at the center of the slab from the start of casting to the completion of solidification is 7 ° C./min or less. is there. By suppressing the average cooling rate to 7 ° C./min or less, sulfide inclusions can be grown, and the sulfide inclusion ratio can be reduced to 0.30 or less. As a result, burning can be prevented from occurring during heating. The average cooling rate is preferably 6 ° C./min or less.

  The average cooling rate can be controlled, for example, by adjusting the size of the mold used during casting. That is, the larger the inner diameter of the mold, the lower the average cooling rate at the center of the slab and the larger the sulfide inclusions. On the other hand, the smaller the inner diameter of the mold, the higher the average casting speed at the center of the slab, and the finer the sulfide inclusions. For example, the inner diameter of the mold is preferably 350 to 1100 mm, and more preferably 450 to 1000 mm.

  What is necessary is just to control the said average cooling rate to 7 degrees C / min or less at least in the average cooling rate in the surface of a slab. If the average cooling rate at least on the surface of the slab is controlled to 7 ° C./min or less, the average cooling rate at the center of the slab becomes 7 ° C./min or less. The surface of the slab is, for example, the central portion of the surface of the slab. What is necessary is just to measure the average cooling rate in the center part of slab surface in the following procedure, for example. That is, a thermocouple for temperature measurement inserted in a protective tube made of alumina is installed near the center of the surface of the feeder, based on the temperature from the start of casting to the completion of solidification and the time required for cooling. Thus, the average cooling rate can be obtained. The average cooling rate may be obtained by calculating the average cooling rate at the center of the slab, and the average cooling rate may be controlled as described above.

  After casting, hot working or the like may be performed according to a conventional method. For example, after heating to about 850-1250 degreeC, after making it into a desired wire diameter by hot processing, it may cool by standing_to_cool or air cooling. The average cooling rate at the time of cooling may be, for example, 3 ° C./second or less.

  The steel material of the present invention thus obtained does not generate burning even when it is heated to high temperature using hot forging or hot working or using high frequency. The high temperature is, for example, a range of a solidus temperature of −150 ° C. or more and a solidus temperature of −100 ° C. or less, and particularly a range of a solidus temperature of −130 ° C. or more and a solidus temperature of −100 ° C. or less. is there.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Molten steel was cast to produce steel materials having the component compositions shown in Tables 1 and 2 below (the balance being iron and inevitable impurities). Based on the amount of Mn and the amount of S shown in Tables 1 and 2 below, the ratio of the amount of Mn to the amount of S ([Mn] / [S]) was calculated, and the results are shown in Tables 1 and 2 below.

  The average cooling rate at the center of the slab surface from the start of casting to the completion of solidification during casting was measured by the following procedure. That is, a thermocouple for temperature measurement inserted in a protective tube made of alumina is installed near the center of the surface of the feeder part, based on the temperature from the start of casting to the completion of solidification and the time required for cooling The average cooling rate was calculated.

  The inner diameter of the mold used at the time of casting is 200 mm, 300 mm, 450 mm, or 1000 mm. The average cooling rate at the center of the slab surface when using a mold having an inner diameter of 200 mm was 30 ° C./min (No. 39 in Table 3 below). The average cooling rate at the center of the slab surface when using a mold having an inner diameter of 300 mm was 20 ° C./min (No. 38 in Table 3 below). The average cooling rate at the center of the slab surface when using a mold having an inner diameter of 450 mm was 5 ° C./min (Nos. 1-36 in Table 3 below). The average cooling rate at the center of the slab surface when using a mold having an inner diameter of 1000 mm was 1 ° C./min (No. 37 in Table 3 below).

Moreover, based on the component composition shown in the following Table 1 and Table 2, and the following formula (a), the solidus temperature was calculated, and the result is shown in Table 3 below. In the following formula (a), [] indicates the content (% by mass) of each element.
Solidus temperature (° C.) = 1536− (170 × [C] + 12.3 × [Si] + 6.8 × [Mn] + 124.5 × [P] + 183.9 × [S]) (a )

  Next, the slab obtained by casting was hot-rolled to produce a test piece, and the ratio of sulfide inclusions contained in the test piece was calculated. That is, the obtained slab was heated to 900 ° C. or higher, then hot-rolled at 900 to 1200 ° C. so that the forging ratio was about 20, and allowed to cool to prepare a test piece. The forging ratio is a value obtained by dividing the cross section of the test piece before hot rolling by the cross section of the test piece after hot rolling. Specifically, the slab obtained using a mold having an inner diameter of 200 mm was hot-rolled to φ45 mm. The forging ratio in the axial direction at this time was 20. The slab obtained using a mold having an inner diameter of 300 mm was hot-rolled to φ70 mm. The forging ratio in the axial direction at this time was 18. The slab obtained using a mold having an inner diameter of 450 mm was hot-rolled to φ95 mm. The forging ratio in the axial direction at this time was 22. The slab obtained using a mold having an inner diameter of 1000 mm was hot-rolled to φ230 mm. The forging ratio in the axial direction at this time was 19.

  When the diameter of the specimen is D (mm), the longitudinal section at the D / 4 position of the obtained specimen is observed with an electron probe microanalyser (EPMA) at a magnification of 200 times. The component composition of all inclusions having a (width) of 2 μm or more was analyzed. Observation was performed in a plurality of fields of view so that the number of inclusions analyzed for the component composition was 300 or more. Regarding the component composition of the inclusions analyzed, when the amount of alloy elements excluding iron (Fe) is 100% by mass, inclusions having an S content of 25% by mass or more are indicated as “sulfurization having a minor axis of 2 μm or more. The number was measured as “material inclusions”. Further, among the “sulfide inclusions having a minor axis of 2 μm or more”, inclusions having an equivalent circle diameter of 4 μm or less were designated as “sulfide inclusions having an equivalent circle diameter of 4 μm or less”, and the number thereof was measured. .

  Ratio of the number of sulfide inclusions having an equivalent circle diameter of 4 μm or less to the number of sulfide inclusions having a minor axis of 2 μm or more (number of sulfide inclusions having an equivalent circle diameter of 4 μm or less / short) The number of sulfide inclusions having a diameter of 2 μm or more; the ratio of sulfide inclusions) was calculated, and the results are shown in Table 3 below.

  Here, the relationship between the average cooling rate at the center of the slab surface during casting and the sulfide inclusion ratio is shown in FIG. In FIG. 1, as a representative example of an example in which a mold having an inner diameter of 450 mm is used and the average cooling rate at the center of the slab is 5 ° C./min. As a result of No. 3, the average cooling rate was 1 ° C./min. 37, the average cooling rate was 20 ° C./min. No. 38 and No. 38 with an average cooling rate of 30 ° C./min. 39 results were shown.

  Next, a cast piece obtained by casting was hot-rolled to produce a test piece, heated to a predetermined temperature and held for 30 seconds, and then the presence or absence of burning was evaluated. The specific evaluation procedure is as follows.

  The slab obtained by casting is heated at a heating temperature of 900 ° C. or higher, then hot-rolled at 900 to 1200 ° C. so that the forging ratio is about 400 to 600, and allowed to cool to produce a test piece. did. In order to evaluate the presence or absence of the occurrence of burning, the heating temperature was set to a solidus temperature of −100 ° C. to a solidus temperature of −130 ° C. calculated based on the component composition. Specifically, no. 1-26 and 31-36 are similar when the C content is about 0.4%, so the heating temperature was uniformly 1330 ° C. No. 27 and 28 are similar when the C content is about 0.15%, and the heating temperature was uniformly 1360 ° C. No. 29 and 30 are similar when the C content is about 0.7%, so the heating temperature was uniformly 1300 ° C. Table 3 below shows the heating temperature (° C.).

  Specifically, the hot rolling was performed under the following conditions. A slab obtained using a mold having an inner diameter of 200 mm was hot-rolled to 9 mm. The forging ratio in the axial direction at this time was 494. The slab obtained using a mold having an inner diameter of 300 mm was hot-rolled to 15 mm. The forging ratio in the axial direction at this time was 400. The slab obtained using a mold having an inner diameter of 450 mm was hot-rolled to φ18 mm. The forging ratio in the axial direction at this time was 625. The slab obtained using a mold having an inner diameter of 1000 mm was hot-rolled to 45 mm. The forging ratio in the axial direction at this time was 494.

  The obtained test piece was heated to the temperature shown in Table 3 below, held at this heating temperature for 30 seconds, and then cooled. The surface of the test piece after cooling was observed using an optical microscope at a magnification of 400 times, and the presence or absence of melting marks at the grain boundaries was examined. The melting mark was evaluated based on whether or not vacancies or oxides existed at the crystal grain boundaries. That is, the case where there were no oxides and pores at the crystal grain boundaries was evaluated as being excellent in burning resistance, and is shown as acceptable in Table 3 below. A case where any one of an oxide and a hole was present at the crystal grain boundary was evaluated as being inferior in burning resistance, and is shown as “Fail” in Table 3 below.

  In addition, No. shown in Table 3 below. FIG. 2 shows a photo, which substitutes for a drawing, of the surface of the test piece 1 taken at a magnification of 400 times using an optical microscope. In FIG. 2, a is a void | hole and b shows an oxide.

  It can consider as follows from the said Table 1-3. No. 3 to 26, 28, 30, and 37 are examples that satisfy the requirements defined in the present invention, the composition of the steel material, the relationship between the amount of Mn and S contained in the steel material, and the sulfide inclusion ratio Can be controlled appropriately, so burning does not occur during heating.

  In contrast, no. 1, 2, 27, 29, 31 to 36, 38, 39 are all comparative examples that do not satisfy the requirements defined in the present invention. Details will be described below.

  No. 1 and 27 are examples in which at least one element selected from the group consisting of Ca, Mg, Zr, Te, and REM is not included, and S contained in the sulfide inclusion cannot be stabilized. As a result, the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

  No. 29 is the above-mentioned No. 29. 1 is an example not containing at least one element selected from the group consisting of Ca, Mg, Zr, Te, and REM, and S contained in sulfide inclusions cannot be stabilized. Further, the ratio of the amount of Mn and the amount of S contained in the steel material is also outside the range defined in the present invention. As a result, the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

  No. 2 is an example which contains S excessively and the ratio of the amount of Mn contained in steel materials and the amount of S deviates from the range specified in the present invention. Further, this is an example that does not contain at least one element selected from the group consisting of Ca, Mg, Zr, Te, and REM, and S contained in sulfide inclusions cannot be stabilized. As a result, the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

  No. 31 is an example which contains S excessively and the ratio of the amount of Mn contained in steel materials and the amount of S remove | deviates from the range prescribed | regulated by this invention. As a result, the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

  No. No. 33 is an example containing excessive Mn, and the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

  No. 36 is an example in which the ratio of the amount of Mn and the amount of S contained in the steel material is outside the range defined in the present invention. As a result, the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

  No. No. 32 is an example containing excessive Si. No. 34 is an example of excessive P. 35 is an example in which the amount of Al is too small, and in all cases, burning occurred.

  No. Nos. 38 and 39 are examples in which the average cooling rate during casting was too large. The sulfide inclusions became coarse, the ratio of sulfide inclusions deviated from the requirement defined in the present invention, and burning occurred.

a hole b oxide

Claims (6)

C: 0.38 to 0.8% (meaning mass%, the same shall apply hereinafter),
Si: 0.01 to 2.0%,
Mn: 0.2 to 2.0%,
P: 0.20% or less,
S: 0.015-0.12%,
Al: 0.002 to 0.1% is contained,
Furthermore,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Zr: 0.01 to 0.5%,
Including at least one selected from the group consisting of Te: 0.001-0.05%, and REM: 0.001-0.05%,
The Mn and the S satisfy the relationship of the following formula (1),
The balance is steel consisting of iron and inevitable impurities,
Of the sulfide inclusions having a minor axis of 2 μm or more included in the steel material, the number of sulfide inclusions having an equivalent circle diameter of 4 μm or less and the sulfide inclusion having a minor axis of 2 μm or more included in the steel material A steel material characterized in that the number of objects satisfies the relationship of the following formula (2).
[Mn] / [S] ≧ 13.0 (1)
In the above formula (1), [] indicates the content (% by mass) of each element.
[Number of sulfide inclusions having an equivalent circle diameter of 4 μm or less among sulfide inclusions having a minor axis of 2 μm or more] / [Number of sulfide inclusions having a minor axis of 2 μm or more] ≦ 0.30 ... (2) As other elements,
Cu: 1% or less,
Ni: 2% or less,
Cr: 2% or less, and Mo: steel according to claim 1 containing at least one selected from 2% hereinafter by Li Cheng group. As other elements,
V: 0.5% or less,
Ti: 0.5% or less,
The steel material according to claim 1 or 2, containing at least one selected from the group consisting of Nb: 0.5% or less and W: 2% or less. As other elements,
Pb: 0.3% or less,
Bi: 0.3% or less,
The steel material according to any one of claims 1 to 3, comprising at least one selected from the group consisting of Sn: 0.02% or less and Sb: 0.02% or less. As other elements,
N: The steel material in any one of Claims 1-4 containing 0.02% or less. A method for producing the steel material according to any one of claims 1 to 5,
In casting a molten steel satisfying the component composition according to any one of claims 1 to 5 to produce a steel material, an average cooling rate of a slab central portion from the start of casting to the completion of solidification is set to 7 ° C / min or less. A method for producing a steel material.