JP2007262435A - Method for manufacturing low carbon sulfur free cutting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide low-carbon sulfur free-cutting steel which has superior machinability (finished surface roughness, in particular) despite its Pb-free state and can be manufactured with high productivity by a continuous casting process. <P>SOLUTION: At the time of tapping the low-carbon sulfur free cutting steel from a converter to a ladle and subjecting the steel to molten steel treatment in manufacturing the low-carbon sulfur free cutting steel, ≥70% of the FeMn alloy and FeS alloy to be added to the steel for the purpose of component adjustment are added to the steel during converter tapping and the ladle of ≤0.20% in the Si content and ≤0.30% in the Al content in the molten steel previously received therein or the ladle not subjected to the molten steel treatment at all after the execution of ladling is used, and the Si content in the molten steel after the molten steel treatment is controlled to ≤0.020% and the Al content to ≤0.0015%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a useful method for producing a low-carbon sulfur free-cutting steel that exhibits good cutting finish surface roughness without using Pb, which is harmful to the human body.

  Low-carbon sulfur free-cutting steel is widely used as a steel for small parts such as screws and printer shafts that do not require so much strength, in addition to hydraulic parts for automobile transmissions. Further, when further cutting finish surface roughness and chip disposal are required, lead-sulfur free-cutting steel obtained by adding lead (Pb) to the low-carbon sulfur free-cutting steel is used.

  Pb contained in free-cutting steel is an extremely effective element for improving machinability, but it has been pointed out to be harmful to the human body, and there are also problems in the treatment of lead fumes and cutting scraps during melting. In many cases, it is required to exhibit good machinability without adding Pb (Pb-free).

  Various techniques have been proposed so far in order to improve machinability with Pb-free in low-carbon sulfur free-cutting steel. For example, Patent Document 1 proposes a technique that improves machinability (finished surface roughness and chip disposal) by controlling the size of sulfide inclusions. Patent Document 2 shows that it is important to appropriately control oxygen in steel in order to control the size of sulfide inclusions. Furthermore, the technique which improved the machinability by prescribing the oxide type inclusion in steel is also proposed (for example, patent documents 3-5).

  On the other hand, techniques for improving machinability by appropriately defining the chemical composition of steel materials have also been proposed (for example, Patent Documents 6 to 9).

  All of the technologies proposed so far are useful from the viewpoint of improving the machinability of free-cutting steel, but in particular in terms of finished surface roughness in forming, the workability is as good as that of Pb-containing steel. The reality is that machinability has not been achieved.

In addition to the machinability as described above, it is also important that the productivity is good as a characteristic desired for Pb-free steel. From this point of view, it is also a necessary requirement that it can be manufactured by a continuous casting method, no surface flaws are generated, and that rolling can be easily performed. However, the continuous casting process is said to be disadvantageous for improving the machinability of steel materials, and it is also an important issue to be able to produce free-cutting steel excellent in machinability with a continuous casting process with high productivity. .
JP 2003-253390 A JP-A-9-31522 JP 7-173574 A JP-A-9-71838 Japanese Patent Laid-Open No. 10-158781 JP 2000-319753 A JP 2001-152281 A JP 2001-152282 A JP 2001-152283 A

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to exhibit good machinability (particularly finished surface roughness) even when Pb-free, and by a continuous casting method. An object of the present invention is to provide a method for producing a low-carbon sulfur free-cutting steel that can be produced with high productivity.

  The production method of the present invention that has been able to achieve the above-mentioned object is to add Fe for adjusting the components when producing low-carbon sulfur free-cutting steel by removing the steel from the converter to the ladle and processing the molten steel. -70% or more of Mn alloy and FeS alloy (meaning of mass%, the same applies hereinafter) is added at the time of steelmaking from the converter, and the ladle has a Si content of 0.20% or less in the molten steel previously received and Use a ladle with an Al content of 0.030% or less, or use a ladle that has not been subjected to any molten steel treatment since the construction of the ladle, and determine the Si content in the molten steel after the molten steel treatment. It has a gist in that it is controlled to 0.0020% or less and the Al content to 0.0015% or less.

  According to the present invention, the amount of SiMn and Al content in steel is reduced as much as possible by appropriately controlling the addition time and ratio of FeMn alloy and FeS alloy to be added for component adjustment. Even if oxygen is not increased (that is, even if the S concentration is high), the total oxygen concentration can be increased, and a large number of useful large-sized and spherical MnS that can form microcracks can be present. A low-carbon sulfur free-cutting steel with good surface roughness could be produced.

  The finished surface roughness of free-cutting steel is highly dependent on the generation, size, shape and uniformity of the constituent cutting edges. The component cutting edge is a phenomenon in which a part of the work material is deposited on the cutting edge of the tool, and in effect acts as a part of the tool (cutting edge). Depending on this generation behavior, the finished surface roughness is reduced. This constituent cutting edge is generated only under a certain condition, but the cutting conditions that are usually implemented are conditions that the constituent cutting edge can easily generate.

  Such a component cutting edge is supposed to cause a fatal defect due to the change in size, but it also has an effect of protecting the tool cutting edge and improving the tool life. Therefore, it is not a good idea to eliminate the constituent cutting edges completely, and it is necessary to stably generate the constituent cutting edges and make the size and shape uniform.

  In order to stably generate the constituent cutting edges and make the size and shape uniform, it is important to generate a large number of microcracks in the primary shear region and the secondary shear region in the portion to be cut. In order to generate a large number of such microcracks, it is necessary to introduce a large number of crack generation sites. And it is known that a MnS inclusion is useful as a micro crack generation site. However, not all MnS-based inclusions act as microcrack generation sites, and large, spherical (that is, wide) MnS works effectively. MnS is stretched in the primary shear region and the secondary shear region, but when it is stretched and becomes too thin, most of it becomes the same as the matrix and does not become a site for introducing microcracks. For these reasons, it is necessary to control the MnS-based inclusions of the work material to be large and spherical in advance.

  By the way, it is known that oxygen (total oxygen) in steel generally affects the size and spheroidization of MnS inclusions (for example, Patent Document 2), and the oxygen in steel increases. The sulfide diameter is said to increase. Therefore, in order to increase the size and spheroidization of MnS inclusions, it is necessary to increase the oxygen concentration in the steel to some extent. At the same time, in order to increase MnS inclusions that become microcrack generation sites, it is necessary to increase the Mn concentration and the S concentration as compared with conventional free-cutting steel (for example, JIS SUM23, SUM24L). However, when the Mn concentration and the S concentration are increased, these act as a deoxidizer, so that free oxygen is reduced and the total oxygen concentration is reduced. That is, raising the total oxygen in the steel and raising the Mn concentration and the S concentration are in a trade-off relationship, and it is difficult in principle to achieve both.

Under these circumstances, the present inventors examined effective means for making MnS inclusions large and spheroidized from various angles. As a result, the Si content in the steel was controlled to 0.0020% or less (20 ppm or less), and the Al content was controlled to 0.0015% or less (15 ppm or less), and the inclusion composition of the slab was MnO—SiO ₂ — The average composition when normalized with the MnS ternary system (that is, when the total of MnO, SiO ₂ and MnS is 100%) is MnS: 60% or less, SiO ₂ : 4% or less, MnO: 36% or more Since it was found that the total acid concentration can be increased without increasing free oxygen (that is, high Mn, high S concentration), and its technical significance was recognized. The application has been filed earlier (Japanese Patent Application No. 2005-301552).

  As described above, it is possible to stably generate the constituent cutting edges by making the MnS inclusions large and spheroidized, and it is found that the size and shape are made uniform, and as a result, the finished surface in the steel forming process The roughness was improved dramatically, and the same characteristics as Pb free-cutting steel could be exhibited.

  As described above, the low-carbon sulfur free-cutting steel targeted by the method of the present invention is important to control the contents of Si and Al to an appropriate range. The reasons for limiting these ranges are as follows. Street.

[Si: 0.0020% or less (excluding 0%)]
Si is an element effective for securing strength by solid solution strengthening, but basically acts as a deoxidizer to generate SiO ₂ . The inclusion composition becomes MnO—SiO ₂ —MnS based on this SiO ₂ , but when Si exceeds 0.0020%, the SiO ₂ concentration in the inclusion increases and the finished surface roughness Will deteriorate. From such a viewpoint, the Si content needs to be 0.0020% or less, and preferably 0.0010% or less.

[Al: 0.0015% or less (excluding 0%)]
Al is an element useful for securing the strength by solid solution strengthening and deoxidation, but acts as a strong deoxidizer to form an oxide (Al ₂ O ₃ ). With this Al ₂ O ₃ , inclusions become MnO—Al ₂ O ₃ —MnS, but when the Al content exceeds 0.005%, the concentration of Al ₂ O ₃ in the inclusions increases. The finished surface roughness will deteriorate. The preferable upper limit is 0.0010%, and more preferably 0.0005% or less.

  In the low-carbon sulfur free-cutting steel targeted by the present invention, components other than Si and Al (C, Mn, P, S, etc.) are the same as those in ordinary low-carbon sulfur free-cutting steel (for example, JIS SUM12, SUM22). Although it may be a chemical component composition, preferred chemical component compositions are C: 0.02 to 0.15%, Mn: 0.6 to 3%, P: 0.02 to 0.2%, S: 0.00. What contains 2-1% of each is mentioned. Other than these components (including Si and Al) (remainder) is basically made of iron, but may contain trace components other than these. Further, the low-carbon sulfur free-cutting steel of the present invention inevitably contains impurities (for example, Cu, Sn, Ni, O, N, etc.), but they do not impair the effects of the present invention. Is acceptable.

  The present inventors examined the optimum conditions for producing the low-carbon sulfur free-cutting as described above. As a result, it was found that the Si content and Al content in the steel can be reduced as much as possible by appropriately controlling the addition timing and ratio of the FeMn alloy and the FeS alloy to be added for component adjustment, and the present invention was completed. did.

  Next, the reason why each requirement of the method of the present invention is specified will be described. In producing low-carbon sulfur free-cutting steel, various alloys are added to adjust the components when the steel is discharged from the converter to the ladle and processed with molten steel. In order to control the Si content and the Al content in the low-carbon sulfur free-cutting steel as described above, it is natural to limit their contents before treatment to a low concentration. It is also necessary to consider the contents of Si and Al contained therein.

  For additive elements including the molten steel processing stage, carburized material (carbon powder), FeMn alloy (ferromanganese), FeP alloy (ferrophosphorus), FeS (ferrosulfur), SiMn alloy (silicon manganese), and nitrogen addition source MnN (manganese nitride) and the like. These alloys contain Si and Al as impurities, and their contents are as shown in Table 1 below.

  In the method of the present invention, it is important to prevent Si and Al from being introduced (input) into the molten steel from the various additive alloys described above. Among the additive elements, at least the FeMn alloy and the FeS alloy are necessary for adjusting the components in the low carbon sulfur free-cutting steel of the present invention.

In the method of the present invention, in order to control the Si content and the Al content in the low-carbon sulfur free-cutting steel as described above, 70% or more of the required addition amount of the Fe-Mn alloy and the FeS alloy is converted into the converter steel. Sometimes it is necessary to add. These alloys contain Si and Al as impurities as described above, but by adding these to the high oxygen molten steel at the time of the steel leaving the converter, Si and Al are oxidized, and SiO ₂ and Al ₂ O ₃ In addition, these float and separate during the subsequent molten steel treatment and enter the slag, so that Si and Al remaining in the steel are reduced to a target concentration. In order to exert the above effect, it is preferable that the molten steel at the time when the steel is discharged from the converter is high oxygen. For that purpose, C is blown down in the converter, and the C concentration is 0.04% or less. It is preferable to create a situation where free oxygen (dissolved oxygen) is high (for example, free oxygen concentration: 500 ppm or more).

  The alloy for adjusting the components is generally added in the whole amount during the ladle molten steel treatment, but a part of the alloy may be added at the time of steel conversion from the converter. However, the addition amount is generally about 60 to less than 70% of the required addition amount from the viewpoint of rough component adjustment before the molten steel treatment. The required addition amount is the total amount of values calculated in consideration of the yield including the amount used for deoxidation, etc. for the molten steel components [shown in Table 2 below (input amount at steel output + time at steel output) Other than the input amount)].

  In the method of the present invention, 70% or more of the required total addition amount of FeMn alloy, FeS alloy, etc. to be added for component adjustment is added at the time of steel leaving the converter. As an alloy to be added, a SiMn alloy cannot be adopted including during molten steel processing. This is because, in this SiMn alloy, the Si content basically increases, so that it becomes difficult to reduce the Si content in the molten steel even if the addition time and the amount thereof are adjusted. The MnN alloy may be added as necessary when N is contained in the low-carbon sulfur free-cutting steel, but it is necessary to add 70% or more of the required addition amount at the time of steel conversion from the converter.

  In carrying out the method of the present invention, in order to control the Si content and the Al content in the molten steel after the molten steel treatment as described above, it is necessary to consider the molten steel treatment history in the ladle for the molten steel treatment. It is also important that the ladle used in the present invention has a Si content of 0.20% or less and an Al content of 0.030% or less in the previously received molten steel. If a ladle that has received molten steel whose content is higher than the respective upper limit is used, it will be affected by impurities in the molten steel that was previously processed, and these will be input into the molten steel, and the desired content will be reduced. Can no longer be achieved. However, as a ladle used in the present invention, a ladle that has not been subjected to any molten steel treatment after the ladle has been installed may be used. In such a ladle, Si or Al is input into the molten steel during the molten steel treatment. There is no fear.

  In the method of the present invention, it is only necessary to appropriately control the Si and Al contents in the molten steel by performing the above-described treatment, and this method is basically applied to a continuous casting method to increase productivity. be able to. However, the method of the present invention can be applied not only to the continuous casting method but also to the ingot-making method.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, but is implemented with modifications within a range that can meet the purpose described above and below. Of course, it is also possible and they are all included in the technical scope of the present invention.

  Using molten steel processing equipment with 100t converter and ladle, the target C concentration (blown C amount) at the end of converter blowing is 0.04%, and dissolved oxygen (free oxygen) concentration at blowing is Melting was carried out at 485 to 600 ppm with varying conditions for the amount of FeMn alloy and FeS alloy added at the time of steel output, the steel receiving method of the ladle, and the like.

  Free oxygen at the time of blowing is measured using a free oxygen probe (trade name “HYOP10A-C150” manufactured by Heraeus Electro Knight Co., Ltd.). Sampling was performed for chemical analysis.

  The additive alloys at the time of steel production are basically FeMn alloys and FeS alloys, but the types of additive alloys including the molten steel processing stage are enumerated in addition to the above, as carburizing materials, FeP, and nitrogen addition sources. (For the Si content and the Al content, see Table 1 above). These additive alloys were appropriately contained to adjust the components of the molten steel. Further, in the comparative steel, a SiMn alloy (silicon manganese) was further added as a Mn source (Test Nos. 9 to 33), but SiMn was not used as a Mn source in the present invention (Test Nos. 1 to 8). FeMn is used.

  In the ladle receiving method, the ladle receiving steel is repeated, and in the facility that continuously performs production activities, what is the Si content and Al content in the molten steel that the ladle received last time? It was measured and recorded whether it was a thing. These results are shown in Table 2 below together with the operating conditions.

  The molten steel thus obtained was subjected to component adjustment and temperature adjustment in a molten steel processing facility, and then cast in a Bloom continuous casting machine having a cross section of 300 mm × 430 mm. About the obtained slab, it sampled, the chemical analysis was implemented, and the component composition was measured. The results are shown in Table 3 below together with the Si content and Al content input from the additive alloy.

  The obtained slab was heated at 1250 ° C. for 1 hour and then subjected to block rolling (cross-sectional size: 155 mm × 155 mm), then rolled to 25 mmφ and pickled to obtain a 22 mmφ polishing rod, which was subjected to a cutting test. At this time, the rolling was performed at 1000 ° C., and the average cooling rate from 800 ° C. to 500 ° C. was set to about 1.5 ° C./second by forced cooling. The steel material temperature was measured with a radiation thermometer.

A cutting test was performed on each steel material. The cutting test conditions are as follows. Moreover, the evaluation criteria of the finished surface after the cutting test are as follows.
[Cutting test conditions]
Tool: High-speed tool steel SKH4A
Cutting speed: 100 m / min Feed: 0.01 mm / rev
Cutting depth: 0.5mm
Cutting oil: Chlorine-based water-insoluble cutting fluid Cutting length: 500 m
[Evaluation criteria]
Finished surface evaluation: The surface roughness was evaluated by the maximum height Rz based on JIS B 0601 (2001).

The cutting test results are shown in Table 4 below in comparison with the final component content in the molten steel of Si and Al. In Table 4, the basic classification is as follows.
[Test No. 1-8]
Invention examples satisfying the requirements specified in the present invention [Test No. 9-14]
Comparative example that does not satisfy the requirement of “addition amount of FeMn alloy and FeS alloy at 70% or more when steel is released” (Comparative Example I: Test Nos. 9, 12, and 14 have a free oxygen concentration of 500 ppm at the time of blowing) Less than)
[Experiment No. 15-21]
Although the requirement “addition amount of FeMn alloy and FeS alloy at the time of steel production is 70% or more” is satisfied, “the Al content in the molten steel received last time is 0.030% or less and the Si content is 0%. Comparative Example (Comparative Example II) using ". Ladle not less than 20%"
[Experiment No. 22-24]
Although the requirement “addition amount of FeMn alloy and FeS alloy at the time of steel production is 70% or more” is satisfied, “Al content in molten steel received last time is not 0.030% or less, and Si content is 0 .. Comparative Example (Comparative Example III) using a ladle not less than 20%
[Experiment No. 25-33]
“The amount of FeMn alloy and FeS alloy added at the time of steel production is less than 70%”, and “the Al content in the molten steel received last time is not 0.030% or less, and the Si content is 0.20%. Comparative example using "Ladle not below" (Comparative Example IV)

  As is apparent from these results, those satisfying the requirements defined in the present invention <Test No. 1-8> shows that the finished surface roughness (maximum height Rz) is fine, and good machinability can be exhibited.

  On the other hand, one lacking any of the requirements defined in the present invention (in Test Nos. 9 to 33, it can be seen that any of the characteristics is degraded.

  Also, based on the above results, the relationship between the FeMn addition rate during steel output and the Al content in the molten steel is shown in FIG. 1, the relationship between the FeMn addition rate during steel output and the Si content in the molten steel is shown in FIG. Fig. 3 shows the relationship between the Al content in molten steel and the Al content in molten steel, Fig. 4 shows the relationship between the FeS addition rate during steelmaking and the Si content in molten steel, and the relationship between the previous Si content in the molten steel and the final component Si content in the molten steel. Fig. 5 shows the relationship between the previous Al content in the molten steel and the final component Al content in the molten steel. Fig. 6 shows the relationship between the final component Si content in the molten steel and the finished surface roughness (maximum height Rz). Fig. 7 shows the relationship between the final component Al content in the molten steel and the finished surface roughness (maximum height Rz). Fig. 8 shows the free acid concentration at the time of blowing and the finished surface roughness (maximum height Rz). 9 (Test Nos. 1 to 3 and Test Nos. 9, 12, and 14) are shown in FIG.

It is a graph which shows the relationship between the FeMn addition rate at the time of steel extraction, and Al content in molten steel. It is a graph which shows the relationship between the FeMn addition rate at the time of steel extraction, and Si content in molten steel. It is a graph which shows the relationship between the FeS addition rate at the time of steel extraction, and Al content in molten steel. It is a graph which shows the relationship between the FeS addition rate at the time of steel extraction, and Si content in molten steel. It is a graph which shows the relationship between Si content in last time steel receiving molten steel, and final component Si content in molten steel. It is a graph which shows the relationship between Al content in molten steel last time, and final component Al content in molten steel. It is a graph which shows the relationship between final component Si content in molten steel, and a cutting finish surface roughness (maximum height Rz). It is a graph which shows the relationship between final Al content in molten steel, and a cutting finish surface roughness (maximum height Rz). It is a graph which shows the relationship between the free acid density | concentration at the time of blowing, and the cutting finish surface roughness (maximum height Rz).

Claims (1)

When manufacturing low-carbon sulfur free-cutting steel, it is 70% or more of FeMn alloy and FeS alloy to be added to adjust the ingredients when the steel is processed from the converter to the ladle and processed into molten steel (meaning by mass%, the same applies hereinafter) ) Is added at the time of converter steelmaking, and the ladle uses a ladle having a Si content of 0.20% or less and an Al content of 0.030% or less in the previously received molten steel, or Uses a ladle that has not been subjected to any molten steel treatment since the ladle was constructed, and controls the Si content in the molten steel after the molten steel treatment to 0.0020% or less and the Al content to 0.0015% or less. A method for producing a low-carbon sulfur free-cutting steel excellent in machinability, characterized by that.

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