JP2006200032A - Low-carbon sulfur free-cutting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-carbon sulfur free-cutting steel which can reliably acquire machinability and the roughness of a machined surface equal to those of low-carbon lead sulfur free-cutting steel, even though containing no lead. <P>SOLUTION: The low-carbon sulfur free-cutting steel includes fundamental components comprising, by wt.%, 0.02-0.15% C, 0.01% or less Si, 0.6-3.0% Mn, 0.2% or less P, 0.2-1.0% S, 0.005% or less Al and 20-500 ppm O; includes MnS particles having a thickness of 1 μm or larger and being extended in a rolling direction, of which the average thickness is 2.5 μm or larger, the total area is 1.5% or larger of the measured area, and the ratio Mn/S is 3 to 4.5; and includes oxides having sizes of 10 μm or larger in an amount of 30 pieces/g or less among oxides consisting of one or more oxides of Al<SB>2</SB>O<SB>3</SB>, SiO<SB>2</SB>, MgO and MnO; and further includes dissolved N of 0.0060 to 0.0200 wt.%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a low-carbon sulfur free-cutting steel that does not contain lead and has good surface finish.

  Low-carbon sulfur steel is widely used as a steel material for manufacturing hydraulic parts of automobile transmissions and small parts such as screws and printer shafts that do not particularly require strength by cutting. And when it is desired to improve the roughness of the cut surface, lead sulfur free cutting steel is used.

  However, in recent years, the harmfulness of lead to the human body has been pointed out, and there are many problems such as lead fume and cutting scrap disposal when melting steel. Lead-free free-cutting steel (so-called lead-free free-cutting steel) is a high-quality steel material. The demand for is getting higher.

  In response to such demands, several lead-free free-cutting steels having machinability equivalent to that of lead-sulfur free-cutting steel have been developed. Representative inventions (Patent Documents 1 to 6) will be described below.

  Although these are common in the point of trying to exclude lead, the inventions of Patent Documents 1 to 3 still allow the inclusion of lead, that is, only pursuing the machinability of steel, or of Patent Document 2 As described above, even though the improvement in melting is taken into consideration, the finished surface roughness at the time of cutting is not considered. Moreover, although patent documents 4-6 are the free-cutting steel as-rolled made into lead-free in order to improve the environmental surface at the time of manufacture, the improvement of a finished surface roughness is still left.

Thus, a low-carbon sulfur free-cutting steel excellent in machinability and finished surface roughness equivalent to that of lead-sulfur free-cutting steel without adding lead has not yet been provided.
JP 7-173574 A JP-A-9-31522 JP 2000-319753 A JP 2001-152281 A JP 2001-152282 A JP 2001-152283 A

  It is an object of the present invention to provide a low-carbon sulfur free-cutting steel that can ensure the same machinability and excellent finish surface roughness as lead-sulfur low-carbon free-cutting steel without adding lead. To do.

In order to achieve the above object, the present invention provides C: 0.02-0.15%, Si: ≦ 0.01%, Mn: 0.6-3.0%, P: ≦ 0 by weight%. .2%, S: 0.2 to 1.0%, Al: ≦ 0.005% (preferably <0.002%), and O: 20 to 500 ppm as a basic component, The average thickness of MnS stretched in the direction of 1 μm or more is 2.5 μm or more, the total area of MnS is 1.5% or more of the measurement area, and Mn / S is 3 to 4.5. In addition, among oxides composed of one or more of Al ₂ O ₃ , SiO ₂ , MnO and MgO, the number of oxides of 10 μm or more is 30 pieces / g or more, and the solid solution N is 0% by weight. It is a low carbon sulfur free-cutting steel characterized by being .0060-0.0200%.

  In addition to the basic composition of steel required as a low carbon sulfur free-cutting steel as described above, the present invention includes the characteristics of MnS and Mn / S ratio, the number of oxides such as Al2O3, By defining the specifications of the solute N amount within a predetermined numerical range, it is possible not only to ensure the machinability but also to ensure the finished surface roughness equivalent to that of lead-containing free-cutting steel. be able to.

  The present invention comprises the following basic components as a lead-free low carbon sulfur free-cutting steel (expressed in% by weight).

C: 0.02-0.15%
C needs to be added in an amount of 0.02% or more in order to ensure the strength of the steel and improve the roughness of the finished surface, but if it exceeds 0.15%, the tool wear increases and the machinability decreases. CO gas is generated during casting to induce soot.

Si: 0.01% or less Si is necessary for securing strength by solid solution strengthening and as a deoxidizing element, but the SiO2 oxide of the deoxidation product decreases the tool life, so 0.01% or less To do.

Mn: 0.6 to 3.0%
Mn is necessary for improving the hardenability and promoting the formation of a bainite structure, improving the machinability of the steel, and ensuring the strength. Further, the machinability is improved by forming MnS or MnO-MnS composite inclusions. For this reason, 0.6% or more is necessary, but if it exceeds 3.0%, the strength becomes excessive and the machinability deteriorates.

S: 0.2 to 1.0%
S should be 0.2% or more as an effective element that MnS generated by combining with Mn in steel becomes a stress concentration source during cutting and facilitates chip separation to improve machinability. . However, 1.0% or more may not reduce the hot workability of steel.

P: 0.2% or less
P improves the finish surface roughness with N described later, and also facilitates the propagation of cracks in the chips, and remarkably improves its processability. However, when it is 0.2% or more, it decreases the hot workability. .

Al: 0.005% or less Al is effective in securing strength by solid solution strengthening and deoxidation, but since deacidification is strong, hard Al2O3 suppresses tool wear, so 0.005% or less And 0.002% or less is preferable.

O: 20 to 500 ppm
O combines with Mn to form MnO, and MnS is formed on it, forming MnO-MnS composite inclusions, which are difficult to stretch during rolling of steel and exist in a relatively spherical shape. It acts as a stress concentration source during cutting. For this reason, O is positively added, but the effect is small if the amount is less than 20 ppm, and addition exceeding 500 ppm is not good because it causes internal defects due to CO gas in the steel ingot.

Solid solution N: 0.0060 to 0.0200%
The solid solution N is an element that affects the generation amount of the constituent cutting edge, and hence the roughness of the finished surface. If it is less than 0.0060%, the generation amount of the constituent cutting edge increases and the finish surface roughness deteriorates. Further, N has a property of being easily segregated on dislocations in the steel structure, which promotes the propagation of cracks generated by embrittlement of the base metal, and as a result, improves the fracture property of the chips. Therefore, N needs to be 0.0060% or more. However, if it exceeds 0.0200%, bubbles are generated during casting, which easily induces internal defects and surface defects in the ingot.

  The present invention is based on the above-mentioned basic components for the purpose of providing a lead-free low-carbon sulfur free-cutting steel that has the same machinability as a lead-added free-cutting steel as it is rolled. Further, in order to guarantee the object, the present invention provides the following four factors: MnS form (size, shape and number) in steel, Mn / S ratio, composition and amount of harmful oxide, and solid solution. Clarified how N should be.

  First, the larger the size of MnS, the smaller the aspect ratio, the smaller the aspect ratio, that is, the larger the thickness, the more advantageous the machinability and surface finish. In the present invention, the average thickness of MnS having a thickness of 1 μm or more stretched in the rolling direction is 2.5 μm or more, and the total area of MnS is 1.5% or more of the measurement area, thereby ensuring the above effect. To do.

  Further, the size of the MnS increases as the Mn / S ratio decreases, but on the other hand, the hot rollability is reduced, and cracks and wrinkles are generated. Conversely, when the Mn / S ratio is increased, MnS decreases and it is difficult to obtain large spherical MnS. In order to make this compatible, it has been confirmed that the Mn / S ratio should be controlled within the range of 3 to 4.5, and more preferably 3 to 4.

Next, if a large amount of oxides in steel, that is, Al ₂ O ₃ , SiO ₂ , MnO, and MgO, is present in a large amount, the tool is likely to be worn, resulting in deterioration of the finished surface roughness. And when the size becomes 10 μm or more, it has been confirmed that it has a bad influence on the tool life, so this is controlled to 30 pieces / g or less to protect the tool life, and a good surface finish is achieved. Obtainable.

  Further, when solid solution N is present as the fourth factor, the constituent cutting edge is reduced in size to improve the surface roughness, and this effect can be ensured in the range of 0.0060 to 0.0200%.

  Although the actions and effects of the above four factors have been clarified, each factor cannot exert its intended effect on its own, and by satisfying all of these factors simultaneously, the machinability and finish equivalent to lead-added free-cutting steel are achieved. Surface roughness and manufacturability can be secured.

  The lead-free low-carbon sulfur free-cutting steel of the present invention can be easily obtained by manufacturing as follows.

  First, in order to control O which affects the form of MnS, C is blown down in the converter and 0.04% or less is created to create a situation where free oxygen (dissolved oxygen) is high. When the molten steel is discharged, a peroxidized converter slag, which is normally prevented from entering the ladle, is actively added to the molten steel, or MnO 2 and scale are added from the outside. At this time, necessary alloy materials such as Fe—Mn, Fe—P, and Fe—S are added.

  Thereafter, in the molten steel treatment step, heating and component adjustment are performed in a state where a slag having a high FetO, MnO, and MnS concentration is formed on the upper portion, and a slab is manufactured by continuous casting or ingot forming. In this way, by forming slag with high oxidation and MnS concentration at the top, the yield of Mn and S is improved, and Mn and S sources are not supplied more than necessary, so that it is possible to prepare components in a short time. Free oxygen can be maintained high. In addition, when free oxygen is high, CO gas is generated and N2 gas is also generated from the increased N, so that blowholes are generated and may be trapped by the solidification interface and become wrinkled. In such a case, the strength of the electromagnetic stirring in the mold in the continuous casting mold is increased, and the operation is performed under casting conditions such that the blowhole comes out upward without being caught by the solidification interface. By this operation, both the increase of free oxygen and nitrogen and the suppression of blowholes can be achieved.

The alloy material to be used should be adjusted to reduce the amount of oxides such as Al ₂ O ₃ , SiO ₂ , MnO, and MgO in the molten metal by selecting raw materials containing low concentrations of Al, Si, and Mg. Can do. The number of these oxides can be adjusted by increasing the amount of free oxygen described above.

(Example)
Using a 3t-scale induction furnace, a 100t converter and a ladle processing facility consisting of a ladle, two groups of the steel of the present invention and the comparative steel are melted while changing the compounding amounts of Si, Mn, S, Al, N, etc. Made. In addition, about Si and Al, Mn to add, Si and Al content in a Fe-S alloy were changed, and each content in steel was changed.

  Immediately before casting each molten steel into a predetermined mold, free oxygen was measured using a free oxygen probe (manufactured by Heraeus). Molten steel was cast using a bloom continuous casting machine having a cross section of 430 mm × 300 mm and a 3 t scale induction furnace (cast iron mold of the same size) designed to have a cooling rate equivalent to that of bloom continuous casting.

  Sampling was performed from a quenching portion near the surface of each ingot, and chemical analysis was performed. Table 1 shows the analysis results for each steel. In the table, Ot represents the total oxygen amount, and Of represents the free oxygen amount.

  Next, each slab was subjected to ingot rolling, then rolled to φ25 mm, pickled, and used as a grinding rod of φ22 mm and subjected to a cutting test. The rolling was performed at 1000 ° C. and forced air cooling from 800 ° C. to 500 ° C. at an average cooling rate of 1.5 ° C. per second. Steel material temperature is measured with a radiation thermometer.

The evaluation method and conditions of each test piece are as follows.
Cutting conditions (forming)
Tool: High-speed tool steel SKH4A
Cutting speed: 35 m / min
Feed: 0.01mm / rev
Cutting depth: 0.5mm
Cutting oil: Chlorine-based water-insoluble cutting fluid Cutting length: 500 m
Finished Surface Roughness Measurement Method Evaluation by JIS Rz MnS Form Measurement Method The MnS stretched in the rolling direction was measured for those having a thickness of 1 μm or more by the image analysis method.
Oxide Measurement Method A sample was cut out from a φ25 mm steel bar, oxide inclusions were extracted by the acid dissolution method, the composition and number were investigated by EPMA, and converted to the number per 1 g of steel. In addition, as a pretreatment for acid dissolution, solution treatment was performed at 1000 ° C. to prevent the formation of carbides. Thereafter, a 10 g sample was cut out, an 800 cc solution was prepared at a ratio of reagent grade hydrochloric acid 1: pure water 3 and the sample was dissolved at 80 ° C. for 5 to 6 hours. Then, it filtered with the filter of the hole diameter of 10 micrometers, and analyzed the size and composition by EPMA.
Method for measuring solid solution N The amount of solid solution N was determined by the difference between the total N amount by the inert gas melting heat conduction method and the compound type N amount by the following method. That is, electrolytic extraction with 10% acetylacetone + 1% tetramethylammonium chloride + methanol solution, collection with a 1 μm filter → indophenol spectrophotometry.

  Table 2 shows the measurement values and evaluation results of the test pieces measured as described above.

  The following is understood from Table 2. That is, since there is no flaw due to cutting in both the steel of the present invention and the comparative steel, it is clear that there is no problem in free-cutting properties, but there is a significant difference between the two groups in the finished surface roughness. . And as Table 2 shows, when it sees about 4 factors, ie, the difference by the average thickness and area ratio of MnS, Mn / S ratio, the number of harmful oxides, and the amount of solute N, a part of factors are of this invention. Even if it is controlled within the specified condition range, it is clear that the finished surface roughness is not improved. Compared to these, all of the steels of the present invention exhibit a finished surface roughness of about 5 to 6 μm, which is equivalent to that observed for lead-containing free-cutting steels.

  That is, in order to ensure the quality with a finished surface roughness Rz of 6 μm or less after cutting 500 m, (1) the average thickness of MnS having a thickness of 1 μm or more stretched in the rolling direction is 2.5 μm or more, and (2) the same MnS. (3) Mn% / S% <2.7, (4) One or more oxides of Al2O3, SiO2, MnO, and MgO having a total area of 10% or more. Is less than 30 pieces / g, and (5) only the steel of the present invention which satisfies all the conditions of solute N of 0.0060 to 0.0200% at the same time, has no flaws and is as good as lead sulfur free cutting steel It can be seen that the finished surface roughness can be obtained reliably.

Claims (2)

C: 0.02-0.15%, Si: ≦ 0.01%, Mn: 0.6-3.0%, P: ≦ 0.2%, S: 0.2-1. A steel containing 0%, Al: ≦ 0.005%, and O: 20 to 500 ppm as basic components, and the average thickness of MnS having a thickness of 1 μm or more stretched in the rolling direction is 2.5 μm or more. The total area of MnS is 1.5% or more of the measurement area, Mn / S is 3 to 4.5, and one or more of Al ₂ O ₃ , SiO ₂ , MnO and MgO. A low carbon sulfur free cutting steel characterized in that the number of oxides of 10 μm or more is 30 pieces / g or more and the solid solution N is 0.0060 to 0.0200% by weight. . The low-carbon sulfur free-cutting steel according to claim 1, wherein Al: <0.002%.