JP6114118B2 - Method for producing S-containing steel - Google Patents

Description

  The present invention relates to a method for producing S (sulfur) -containing steel, and proposes a refining method for accurately controlling the S concentration.

  S-containing stainless steel represented by SUS303 is a stainless steel with improved free-cutting properties and is used for machining precision parts. This steel type contains S in a high concentration of 0.15 to 0.25% by mass, and forms MnS particles to improve machinability when grinding with a tool. Several chemical components of austenitic stainless steel with improved machinability or machinability are disclosed.

  By containing 0.15 to 0.50% by mass of sulfur and adjusting the O concentration to 80 to 200 ppm, the shape of sulfide after hot working or hot / cold working is controlled to a granular type, An austenitic stainless steel with improved machinability is disclosed (for example, see Patent Document 1). This technique shows that it is important to make Si—Mn oxide in the oxide precipitated in the sulfide.

In addition, sulfur-added austenitic stainless steel used for high-speed cutting and threading is shown (for example, see Patent Document 2). Sulfur: 0.10 to 0.55 mass%, copper: 1 to 5 mass%, calcium> 35 × 10 ⁻⁴ mass%, oxygen> 70 × 10 ⁻⁴ mass%, ratio of calcium content to oxygen content Therefore, the machinability is improved by having a weight composition of 0.2 to 0.6.

  As described above, since the S concentration is high, there are some difficulties in the steel making process. One is the high cracking susceptibility, so there is a problem that the slab is easily broken during casting, and some techniques have been shown (for example, see Non-Patent Document 1).

  There has been disclosed a rolling method for easily and reliably preventing tip cracks during hot rolling of difficult-to-machine steel pieces such as sulfur-containing free-cutting stainless steel (see, for example, Patent Document 3). This is a method in which a dummy member having better hot workability than the steel slab of the material to be rolled is welded so as to cover the end face of the steel slab before rolling, and rolling is started from the end face side where the dummy member is welded.

  Sulfur free-cutting stainless steel is cast with a horizontal continuous casting machine, the slab surface layer is rapidly cooled to the solidus temperature, and slowly cooled from the solidus temperature on the slab surface to 1000 ° C during casting. Moreover, by limiting the ratio of the cross-sectional area of the slab to the cross-sectional area of the product to 500 or less, the breakdown process can be omitted during hot working of the sulfur free-cutting stainless steel slab, and further machinability is improved. The manufacturing method which can be performed is disclosed (for example, refer patent document 4).

  In addition, a technique for controlling nitrogen contained in molten steel so as not to decrease during the melting of SUS303 is shown. That is, in the RH degassing process at the time of melting SUS303, by introducing the S component into the molten steel during the initial period of 5 to 15 minutes after the start of the RH degassing process, the RH degassing process is performed. A method for suppressing denitrification in the RH degassing process by increasing the proportion of time during which the S concentration is high after charging is disclosed (for example, see Patent Document 5).

Japanese Patent Publication No. 8-260102 Japanese Patent Publication No. 9-137254 Japanese Patent Publication No. 2007-98436 Japanese Patent Publication No. 5-50199 Japanese Patent Publication No. 2009-235550

Toshio Kishida, Atsushi Shinagawa, Hisao Ishizuka, Masatoshi Ozawa, Shigenori Hayakawa: Iron and Steel, Vol. 60 (1974) No. 7, p. 1052-1062

  As described above, there is an invention relating to chemical components of austenitic stainless steel with improved machinability or machinability, a refining method for suppressing denitrification, and a manufacturing method for improving hot workability, but the S concentration itself is also disclosed. The invention to be precisely controlled is not shown.

  However, at the time of refining, the amount of S added is large and the yield in the molten steel is too high, or even if the amount added is appropriate, the yield shifts excessively into the slag and the yield in the molten steel is too low. There has been a problem that the S concentration falls outside the target range.

  An object of the present invention relates to a method for producing S-containing steel, and is to propose a refining method for accurately controlling the S concentration.

  The inventors analyzed various effects on the S concentration of S-containing stainless steel based on actual machine data. In particular, we focused on the relationship with the slag composition in AOD or VOD, and the relationship with the operation time in ladle refining. As a result, the basicity of slag is 1 to 1.5, the addition amount of S is 0.3 to 0.6% by mass, and the addition amount of S and the basicity of slag are defined by a predetermined function. It was confirmed that the target S concentration range can be obtained in the S-containing stainless steel.

  Furthermore, after Ar gas stirring for 40 minutes or more in ladle refining, it was clarified that by controlling the target S concentration of 0.15 to 0.2% by mass by producing a slab by continuous casting. .

That is, the present invention forms a CaO—SiO ₂ slag in which the basicity CaO / SiO ₂ of slag is controlled to 1 to 1.5, and S is added in an amount of 0.3% to 0.6% by mass. The basicity of slag and the amount of S added satisfy the following relationship:
(2/3) × (CaO / SiO ₂ ) −0.5 ≦ S addition amount (mass%) ≦ (2/3) × (CaO / SiO ₂ ) − (4/15)
Furthermore, after making S transfer to slag, the S concentration in molten steel is finally controlled to 0.15-0.25 mass%, and it is a manufacturing method of S containing steel characterized by manufacturing a slab.

  More preferably, the steel contains C: 0.30 mass% or less, Si: 1 mass% or less, and Mn: 2 mass% or less.

  More preferably, the steel is stainless steel.

  More preferably, the steel includes Cr: 15-20% by mass and Ni: 5-10% by mass.

  More preferably, the steel contains C: 0.30 mass% or less, Si: 1 mass% or less, Mn: 2 mass% or less, Cr: 15-20 mass%, Ni: 5-10 mass%, the balance Fe and Consists of inevitable impurities.

More preferably, in refining the steel, the raw materials are first melted, then decarburized by AOD or VOD, Cr reduced using an FeSi alloy, limestone is added, and the slag basicity CaO / SiO ₂ is changed. Control within the range.

  More preferably, after the S concentration in the molten steel is finally controlled to 0.15 to 0.25% by mass, the slab is manufactured by continuous casting.

  More preferably, refining is performed by ladle refining before continuous casting, and Ar concentration is stirred for 40 minutes or more from the start of ladle refining, so that the S concentration is controlled to 0.15 to 0.20% by mass, and then continuous. It is to produce a slab by casting.

It is a graph which shows the relationship between Ar stirring time and S concentration in molten steel in the ladle refining of this invention. It is a graph which shows the relationship between slag basicity (C / S) and the amount of S addition in the Example (invention example and comparative example) of this invention.

  In the refining method of the present invention, scrubbing, ferronickel, ferrochrome, etc. are first melted in an electric furnace to obtain a crude stainless steel melt. Thereafter, this crude molten steel is subjected to oxygen blowing (oxidation refining) to remove C in AOD or VOD, and Cr reduction by FeSi alloy is performed. Thereafter, limestone is added to adjust the slag composition as described later.

Thereafter, refining for the purpose of adjusting the components is performed, and the S content is adjusted (S addition). Basically, S in the range of 0.3 to 0.6 mass% is added, and the basicity CaO / SiO ₂ of the slag is adjusted to 1 to 1.5, and finally 0.15 to 0.25. Stainless steel containing mass% S is produced. The principle and reasons for limitation will be described below.

The AOD or VOD refining is not particularly limited, but the S content is usually adjusted using a FeS alloy. This FeS is preferably FeS having a pure S content of 20 to 60% by mass. The added amount of S once enters the molten steel. Thereafter, S in the molten steel is distributed into the slag by the reaction of the formula (1).
S = (S) (1)
Here, the underline indicates that it is included in the molten steel, and () indicates that it is included in the slag.

S distribution ratio = (mass% S) / [mass% S] (2)
Here, () represents the S concentration in the slag, and [] represents the S concentration in the molten steel.

In producing S-containing stainless steel, it is preferable to operate so that S is effectively retained in the metal. That is, it is important to keep the S distribution ratio in the equation (2) low. In order to keep the S distribution ratio low, it is desirable to lower the activity of CaO and increase the activity of SiO ₂ . That is, it was found that CaO / SiO ₂ should be controlled to be low.

The refining method according to the present invention will be described in detail below.
In stainless steel, S is an element that combines with Mn in stainless steel to form MnS particles and improve the machinability with a tool. The effect is not sufficiently exhibited when it is less than 0.15% by mass. On the other hand, even if it is added excessively exceeding 0.25% by mass, the effect of improving the machinability is saturated, so that it is not economical and crack sensitivity is deteriorated. Therefore, the S content in the stainless steel is 0.15% to 0.25% by mass. Preferably, it is 0.15 mass%-0.20 mass%.

  Further, the steel type targeted by the present invention is not particularly limited, but preferably C: 0.30 mass% or less, Si: 1 mass% or less, Mn: 2 mass% or less, Cr: 15-20 mass%, Ni : Stainless steel consisting of 5 to 10% by mass, the balance Fe and inevitable impurities. The reason for limiting to this component will be described.

C: 0.30% by mass or less C is an element useful for maintaining strength, but if it is too high, it causes sensitization and reduces corrosion resistance. Therefore, 0.30 mass% or less is preferable.

Si: 1% by mass or less Si is a useful element because it contributes to deoxidation, but if it is too high, it causes embrittlement. Therefore, it is preferable to set it as 1 mass% or less.

Mn: 2% by mass or less Mn is an important element for bonding with S to form MnS and maintaining machinability. However, if it is too high, the hot workability is lowered.

Cr: 15-20% by mass
Cr is an element necessary for obtaining corrosion resistance as an austenitic stainless steel. However, if it exceeds 20% by mass, the balance of δ / γ structure is impaired and hot workability is reduced. Therefore, it is preferable to set it as 15-20 mass%.

Ni: 5 to 10% by mass
It is an indispensable element for austenitic stainless steel and an element that stabilizes the austenitic phase. If it is low, δ ferrite increases rapidly, the hot workability is impaired and the austenite phase is not stable, so the lower limit was made 5 mass%. However, since Ni is an expensive element, the upper limit is preferably 10% by mass.

In the above-described refining process of stainless steel in AOD or VOD, in order to obtain the above-mentioned S concentration, FeS alloy corresponding to 0.3% to 0.6% by mass of S is added. It is an extremely important operation to adjust the CaO / SiO _{2 of} slag generated during Cr reduction or deoxidation accompanied by slag to 1 to 1.5. From the analysis of operation data, it was found that the S distribution ratio can be kept as low as 5 to 50 by controlling CaO / SiO ₂ to 1 to 1.5. Here, although not particularly limited, in normal refining, when the metal weight is 50 to 70 t, it is desirable to maintain the weight of the slag at about 4 to 6 t.

Furthermore, when CaO / SiO ₂ is less than 1, the fluidity of the slag is deteriorated, resulting in a problem that it cannot be finally discharged, the deoxidation state is deteriorated, and the value of the oxygen concentration exceeds 100 ppm. End up. If the value of the oxygen concentration exceeds 100 ppm, there will be problems such as cracking in the plate during hot rolling. In consideration of this reason, CaO / SiO ₂ was set to 1 or more.

If CaO / SiO ₂ is higher than 1.5, the S distribution ratio is increased, and the yield of S in the molten steel is reduced. That is, even when CaO / SiO ₂ exceeds 1.5, if S is added exceeding 0.6 mass%, S of 0.15% or more can be obtained, but this means is an effective resource. This shifts the slag to the slag for discarding S, which not only wastes S resources such as FeS but also increases the cost. Therefore, in this invention, the basicity of slag was prescribed | regulated as 1.5 or less. At the same time, the upper limit of the addition amount of S was defined as 0.6% by mass.

  Here, although not particularly limited, it is desirable to control the S concentration in the slag to 1 to 7% by mass.

Here, the S amount 0.3 wt% to 0.6 wt%, CaO / SiO ₂ 1 to 1.5, yet, illustrating the reason for limiting to satisfy the following.
(2/3) × (CaO / SiO ₂ ) −0.5 ≦ S addition amount (mass%) ≦ (2/3) × (CaO / SiO ₂ ) − (4/15)

Basically, when the slag basicity (CaO / SiO ₂ ) increases, the S distribution ratio increases, so that S tends to migrate into the slag. As a result of analyzing data with an actual machine, it became clear that it is necessary to increase the amount of S addition with a slope of 2/3 with respect to the slag basicity. In addition, it has been clarified that the above formula needs to be satisfied in order to satisfy the lower limit of 0.15 mass% and the upper limit of 0.25 mass% of the S concentration. Therefore, the reason for limiting to the scope of the present invention is as follows.

When S addition amount is less than 0.3 mass% or lower than (2/3) × (CaO / SiO ₂ ) −0.5, S is excessively distributed in the slag, and the S content in the stainless steel Is less than 0.15% by mass and less than the scope of the present invention.

Even if S is distributed to the slag when the S addition amount exceeds 0.6 mass% or higher than (2/3) × (CaO / SiO ₂ ) − (4/15), S in the stainless steel Content will become high exceeding 0.25 mass%.

  After the refining process in the above-described AOD or VOD, the temperature is adjusted while stirring Ar by ladle refining. The amount of S added in the refining process of AOD or VOD once enters all of the molten steel, but is distributed into the slag by the reaction of formula (1). FIG. 1 shows the transition of the molten steel S concentration after the AOD refining process under the conditions of a metal amount of 60 tons, a slag amount of 5 tons, an S addition amount of 0.5 mass%, and C / S = 1.3.

  From this figure, it can be seen that immediately after the start of the ladle refining (30 minutes), the S distribution from the molten steel into the slag is not completed, and is 0.3 mass%, which is a high state exceeding the range of the present invention. . It can be seen that at 50 minutes, they are within 0.21% by mass and 0.25% by mass or less, but have not yet reached the equilibrium value of 0.17%. Thus, when Ar gas is stirred for 40 minutes or more in the ladle refining process, the S concentration in the metal reaches an equilibrium value, and it is possible to obtain a stable S concentration in the slab. By this operation, it becomes possible to control the S concentration to 0.15 to 0.2 mass%. Therefore, a more preferable mode of production is to stir Ar gas for 40 minutes or more in the ladle refining process. Although not particularly limited, the flow rate of Ar gas is preferably 40 to 200 NL / min.

Next, examples will be presented to clarify the configuration and operational effects of the present invention. However, the present invention is not limited to the following examples.
First, the raw material was melted in an electric furnace, and then oxygen blowing (oxidation refining) for removing C in AOD or VOD was performed, and Cr reduction or deoxidation was performed using an FeSi alloy. Thereafter, limestone was added to adjust the slag composition. Here, refining was performed for the purpose of adjusting the components, and S was added. In this operation, Fe S containing 30% by mass of S was used as the S source. After the refining process at AOD or VOD, the temperature was adjusted by ladle refining while stirring Ar. Finally, the slab was manufactured by continuous casting. The molten steel weight was 50 to 70 tons, and the slag weight was 4 to 6 tons.

  The manufactured slab was 1000 mm wide × 154 mm thick × 6000 to 8000 mm long. The surface of the slab was ground, heated to 1200 ° C. and hot-rolled to produce a 6 mm thick tropics. Thereafter, annealing and pickling were performed to remove the scale on the surface.

  Table 1 shows the chemical composition of the obtained stainless steel alloy, the slag composition at the end of AOD or VOD refining, the amount of S added, and the ladle refining time. The chemical composition and slag composition of the alloy were quantitatively analyzed using a fluorescent X-ray analyzer, and the oxygen concentration of the alloy was quantitatively analyzed by an inert gas impulse melting infrared absorption method.

The total analysis value of slag is 95 to 99% by mass because it contains Al ₂ O ₃ , FeO, Cr ₂ O ₃ , NiO and the like as unavoidable impurities.

  The hot workability was evaluated by inspecting the tropics after the hot rolling step by full length visual inspection. The case where no cracks were observed over the entire length was marked as ◎. A case where the occurrence of mildness, that is, the case where there were less than 10 ear cracks, was rated as ◯. When 10 or more ear cracks were confirmed, it was evaluated as x. In addition, the crack length of the ear crack was about 10-30 mm when it generate | occur | produced.

  The machinability was evaluated by a drill drilling test. The test conditions were as follows: drill diameter: φ10 mm, thrust: 400 N, peripheral speed: 10 m / min, and the drilling time until a tropical plate with a thickness of 6 mm penetrated was measured. If it was 10 seconds or less, it was evaluated as ◯, and if it exceeded 10 seconds, it was marked as x.

  FIG. 2 shows the distribution of C / S and S addition amount for the examples. The range enclosed by the solid line is the scope of the present invention. It turns out that the invention examples 1-10 satisfy | fill the range, and the comparative example has remove | deviated from the range. Details will be described below.

  In Examples 1 to 7, the ladle refining time was as long as 40 minutes or more and the range of the present invention was satisfied for the components, so the S concentration could be controlled to 0.15 to 0.2% by mass. Although 8-10 of the invention example was satisfying the range of this invention about the component, since the ladle refining time was as short as less than 40 minutes, it was controlled by the range of 0.2-0.25 mass%.

On the other hand, since the basicity of Comparative Example 11 was as low as less than 1, the S concentration was higher than 0.25% by mass. Therefore, the hot workability was lowered and the ear cracks were generated. In Example 12, since the basicity was higher than 1.5, the S concentration was as low as less than 0.15% by mass. In Example 13, since the addition amount was lower than 0.3% by mass, the S concentration was as low as less than 0.15% by mass. Therefore, sufficient machinability was not obtained. In Example 14, since the addition amount was higher than 0.6% by mass, the S concentration was higher than 0.25% by mass. Therefore, the hot workability was lowered and the ear cracks were generated. In Examples 15 to 17, since the addition amount of S was higher than the value of the relational expression (2/3) × (CaO / SiO ₂ ) − (4/15), the S concentration was 0.25% by mass. It was much higher. Therefore, the hot workability was lowered and the ear cracks were generated. Since Examples 18 to 20 were less than the value of the relational expression (2/3) × (CaO / SiO ₂ ) −0.5, the S concentration was as low as less than 0.15% by mass. Therefore, sufficient machinability was not obtained.

Since the S content can be controlled with high accuracy, it is promising in the production of S-containing stainless steel represented by SUS303.

Claims (8)

A slag basicity CaO / SiO ₂ is controlled to 1 to 1.5 to form a CaO-SiO ₂ slag, and 0.3% to 0.6% by mass of S is added. The addition amount of S satisfies the following relationship,
(2/3) × (CaO / SiO ₂ ) −0.5 ≦ S addition amount (mass%) ≦ (2/3) × (CaO / SiO ₂ ) − (4/15)
Furthermore, after making S transfer to slag, S concentration in molten steel is finally controlled to 0.15-0.25 mass%, and the manufacturing method of S containing steel characterized by manufacturing slab.   The said steel contains C: 0.30 mass% or less, Si: 1 mass% or less, Mn: 2 mass% or less, The manufacturing method of the S content steel of Claim 1 characterized by the above-mentioned.   The said steel is stainless steel, The manufacturing method of the S content steel of Claim 2 characterized by the above-mentioned.   The said steel contains Cr: 15-20 mass%, Ni: 5-10 mass%, The manufacturing method of the S containing steel in any one of Claims 1-3 characterized by the above-mentioned. The steel is composed of C: 0.30% by mass or less, Si: 1% by mass or less, Mn: 2% by mass or less, Cr: 15-20% by mass, Ni: 5-10% by mass, balance Fe and inevitable impurities. The method for producing S-containing steel according to claim 2, wherein:
In refining the steel, the raw materials are first melted, then decarburized with AOD or VOD, Cr reduced using FeSi alloy, limestone is added, and the basicity of slag CaO / SiO ₂ is controlled within the above range. The method for producing S-containing steel according to any one of claims 1 to 5, wherein:   The S-containing steel according to any one of claims 1 to 6, wherein the slab is produced by continuous casting after finally controlling the S concentration in the molten steel to 0.15 to 0.25 mass%. Production method. Before the continuous casting, the steel is refined by ladle refining, and Ar concentration is controlled for 40 minutes or more from the start of ladle refining to control the S concentration to 0.15 to 0.20% by mass. The manufacturing method of the S content steel in any one of Claims 1-7 characterized by manufacturing.

Images