WO2008066194A1

WO2008066194A1 - Free-cutting steel excellent in manufacturability

Info

Publication number: WO2008066194A1
Application number: PCT/JP2007/073277
Authority: WO
Inventors: Masayuki Hashimura; Atsushi Mizuno; Kenichiro Miyamoto; Jun Aoki; Seiji Ito
Original assignee: Nippon Steel Corporation
Priority date: 2006-11-28
Filing date: 2007-11-27
Publication date: 2008-06-05
Also published as: EP2096186A1; EP2096186B1; AU2007326255B2; JPWO2008066194A1; EP2096186A4; CN101573463A; BRPI0719310A2; US20100054984A1; KR101118852B1; TW200840875A; AU2007326255A1; KR20090055648A; BRPI0719310B1; TWI363804B; JP5212111B2

Abstract

The invention provides a free-cutting steel which is excellent in machinability and little erodes a plate refractory of a sliding nozzle for continuous casting and which is protected from surface deterioration in hot rolling by virtue its exhibiting excellent ductility in hot rolling. A free-cutting steel which contains by mass C: 0.005 to 0.2%, Si: 0.001 to 0.5%, Mn:0.3 to 3.0%, P: 0.001 to 0.2%, S: 0.30 to 0.60%, B: 0.0003 to 0.015%, O: 0.005 to 0.012%, Ca: 0.0001 to 0.0010%, and Al: ≤ 0.01%, with the N content satisfying the relationships: N ≥ 0.0020% and 1.3×B - 0.0100 ≤ N ≤ 1.3×B + 0.0034 and with the balance being Fe and unavoidable impurities, characterized in that in a section of the steel perpendicular to the direction of rolling, the area of Mn0 of 0.5μm or above in diameter of the equivalent circle is at most 15% based on the total area of all the Mn-containing inclusions.

Description

Description Free-cutting steel with excellent manufacturability

The present invention relates to a low-carbon free-cutting steel that is used in automobiles and general machinery and requires machinability rather than strength characteristics, and in particular, in terms of tool life during cutting, finished surface roughness, and chip disposal. The present invention relates to a free-cutting steel with excellent machinability, low melting of a plate refractory of a sliding nozzle for continuous forging, excellent ductility in hot rolling, and excellent manufacturability. Background art

General machines and automobiles are manufactured by combining various parts, but these parts are often manufactured through a cutting process from the viewpoint of required accuracy and manufacturing efficiency. At that time, cost reduction and improvement in production efficiency are required, and steel is also required to improve machinability. In particular, low-carbon sulfur free-cutting steel SUM23 and low-carbon sulfur-lead composite free-cutting steel SUM24L have been disclosed with an emphasis on machinability. It has been known that adding machinability improving elements such as S and Pb is effective to improve machinability. However, some customers may avoid using Pb as an environmental burden, and the amount is being reduced.

In the past, when Pb is not added, a technique has been used to improve machinability by forming soft inclusions in a cutting environment such as sulfides mainly composed of MnS. However, SUM24L, a low-carbon sulfur-lead composite free-cutting steel, contains the same amount of S as the low-carbon sulfur free-cutting steel SUM23, so it is necessary to add more S than before. However, MnS is the main component when a large amount of S is added. It is not effective to improve the machinability simply by making the sulfide to be coarse, and the matrix cannot be made sufficiently brittle, and the finished surface due to falling off of the constituent edge and chip separation phenomenon Problems such as deterioration of roughness and poor chip disposal due to insufficient cutting of the chips occur. Furthermore, in production processes such as rolling and forging, sulfides mainly composed of coarse MnS become the point of destruction and cause many manufacturing problems such as rolling mills. Addition of elements other than S, such as Te, Bi, P, and N, can improve the machinability to some extent. However, when rolled, the surface properties such as cracking and flawing during hot forging can be improved. In order to cause deterioration, it is desirable to have as little as possible, and it is impossible to achieve both machinability and manufacturability.

In Japanese Patent Laid-Open Publication No. U-U, a sulfide of 20/2 m or more alone or a group of sulfides having a length of 20 2 m or more in which a plurality of sulfides are connected in a substantially series shape has a cross section in the rolling direction of 1 mm ^2. A method has been devised to improve chip disposal by having more than 30 in the field of view. However, the sub-m, which is the most effective for machinability in practice, is not mentioned for the dispersion of m-level sulfide, including the production method, and cannot be expected from its component system.

Examples of using inclusions other than sulfides to improve machinability have existed so far. For example, JP-A-9-840, JP-A-2001-329335, JP-A-2002- No. 3991, JP-A-2000-178683. This is a technique for improving machinability by using BN. However, these are not intended to improve the roughness of the finished surface. JP-A-9-117840, JP-A-2001-329335, and JP-A-2000-178683 aim to improve the tool life. Kai 2002-399 No. 1 public information aims to improve chip disposal. When applied to the chemical components within the range of the examples disclosed in these documents, sufficient effects cannot be obtained in improving the finished surface roughness. Specifically, matrix homogeneity due to fine dispersion in BN steel If it is not made, the effect for improving the finished surface roughness cannot be obtained, but the technology is not described in these patent documents.

The technique disclosed in Japanese Patent Application Laid-Open No. 2004-Π 6 1 76 is also an example in which BN is used for improving machinability, considering the balance with the amount of N added. However, this technology completely suppresses the occurrence of rolling flaws, while reducing the balance of chemical components of steel materials to ensure machinability, which is a conflicting property, and the amount of B oxide that has high affinity with enzymes. There is no known method for increasing the amount of BN deposited. Japanese Laid-Open Patent Publication No. 5-345 951 is a technology for increasing the size of MnS by increasing the oxygen concentration in steel to improve machinability. However, this technology does not mention anything about the decrease in MnS and the resulting decrease in machinability due to the increase in oxygen, and further measures to prevent significant manufacturability degradation such as refractory erosion and surface flaws. Not touched.

In addition, JP-A-2001-329335 discloses that, in order to improve hot ductility, it suppresses grain boundary embrittlement due to grain boundary precipitation of BN, and in order to utilize the effect of preventing solid grain B embrittlement, A technique for limiting the amount of N added has also been proposed. However, since the amount of N is simply reduced, there is not enough consideration for the control of the amount of dissolved N in the BT heating to processing temperature range. Becomes insufficient. In addition, since the amount of N is lower than the stoichiometric composition, the amount of BN necessary to improve the finished surface roughness is insufficient, but it is good because there is no supplementation by other techniques to compensate for it. Finished surface roughness cannot be obtained.

Japanese Patent Application Laid-Open No. 2004-27297 proposes a technique for limiting the amount of oxygen in steel in order to reduce surface defects. However, there is no mention of how to control the amount of oxygen in the steel, and it is impossible to limit the amount of oxygen in the steel and prevent the occurrence of flaws in undeoxidized low-carbon free-cutting steel without special control. is there. This is an example of adding Ca to improve machinability in low-carbon free-cutting steel. It also exists in For example, Japanese Patent Laid-Open No. 2000-1 60284 does not describe a specific effect of improving machinability, and also has a wide range of Ca addition amount, and also describes an effective addition amount for improving machinability. In addition, there is a problem that the plate refractory of the sliding nozzle is easily melted when producing the low-carbon free-cutting steel with B added by the continuous forging method, but there is no prior literature that solves this problem. Disclosure of the invention

The present invention is a low-carbon free-cutting steel used in automobiles and general machinery, and is particularly excellent in tool life during cutting, finished surface roughness, and machinability of chip disposal, and for continuous forging. Sliding nozzle plate This provides free-cutting steel that has little refractory refractory loss, has excellent ductility in hot rolling, and can prevent deterioration of surface properties due to hot rolling.

Cutting is a destructive phenomenon that separates chips, and it is important to promote them. However, as already mentioned, there is a limit to simply increasing S. In order to achieve both machinability and manufacturability, it is necessary to consider the amount of element that improves machinability.

Therefore, in order to improve hot ductility, the ratio of B and N required to obtain BN necessary for machinability at room temperature where cutting is performed is controlled while controlling the amount of solute N in the rolling temperature range. They found that both hot ductility and machinability can be achieved. Here, solid solution N is the amount obtained by subtracting the amount of compound N from the total amount of N, and the amount of compound N indicates the amount of N that is substantially BN. This solute N is produced in large quantities because BN is dissolved by heating at a rolling temperature range of 800 to 110 ° C. In order to perform good rolling with less surface flaws, it is necessary to reduce the amount of solute N in this temperature range.

Furthermore, Mn is easily consumed as an oxide in molten steel. In order to improve the yield and the yield of B as BN to improve machinability and hot ductility, to improve machinability and to suppress the refractory damage of the plate refractory of the sliding nozzle for continuous forging, We found that it was necessary to reduce the amount of MnO produced in steel.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) By mass%

C: 0.005-0.2%

Si: 0.00 to 0.5%

Mn: 0.3-3.0%

P: 0.001 to 0.2%

S: 0.30 to 0.60%

B: 0.0003 to 0.015%

O: 0.005-0.012%

Ca: 0.0001 to 0.0010%

A1≤0.01%

N content is

N≥0.0020% and 1.3XB—0.0100≤ N≤1.3XB + 0.0034, the balance is Fe and unavoidable impurities, and MnO in steel is a cross section perpendicular to the rolling direction of steel. Free-cutting steel with excellent manufacturability, characterized in that the area of MnO with an equivalent circle diameter of 0.5 m or more is 15% or less of the area of all Mn inclusions.

(2) Regarding the sulfide described in (1), the abundance density of the sulfide whose main component is MnS is 10000 / mm ^{2 with an} equivalent circle diameter of 0.1 to 0.5 m in the cross section perpendicular to the rolling direction of the steel. Free-cutting steel with excellent manufacturability, characterized by the above.

(3) Furthermore, in mass%, V: 0.05-1.0%

Nb: 0.005-0.2%

Cr: 0.01-2.0%

Mo: 0.05-1.0%

W: 0.05-1.0%

Ni: 0.05 ~ 2.0%

Cu: 0.01 to 2.0%

Sn: 0.005-2.0%

Zn: 0.0005-0.5%

Ti: 0.0005-0.1%

Zr: 0.0005-0.1%

Mg: 0.0003 to 0.005%

Te: 0.0003-0.2%

Bi: 0.005-0.5%

Pb: 0.005-0.5%

The free-cutting steel excellent in manufacturability according to any one of (1) to (5), characterized by containing one or more of the following. Brief Description of Drawings

Fig. 1 is a conceptual diagram showing the plunge cutting test method. (A) is a bird's-eye view and (b) is a plan view.

Fig. 2 is a conceptual diagram showing the longitudinal turning test method and the quality of the finished surface. (A) is a plan view, (b) is an enlarged view of the finished surface (feed mark), and Fig. 3 is an MnO measurement by EPMA. It is an optical microscope photograph which shows an example. FIG. 4 shows (a) a TEM rebris photograph and (b) an optical microscope photograph of a sulfide containing MnS as a main component of the present invention. FIG. 5 shows (a) a TEM replication force photograph and (b) an optical microscope photograph of a sulfide containing MnS as a main component in a comparative example.

Figure 6 shows the change in machinability due to MnO in terms of the finished surface roughness in longitudinal turning after cutting 800 pieces.

FIG. 7 is a diagram showing a balance between finished surface roughness and hot ductility in longitudinal turning in the inventive example and the comparative example.

FIG. 8 is an explanatory view of the depth position of 1 Z4 of the thickness of the flange. BEST MODE FOR CARRYING OUT THE INVENTION

The present invention improves the machinability by adding B and precipitating BN in the low-carbon free-cutting steel, which requires machinability rather than strength characteristics, without adding Pb. With regard to the steel component composition, in particular, B and N are added so as to satisfy an appropriate relationship, thereby improving machinability and ductility during hot rolling and reducing MnO in the steel. The invention has been completed by improving the machinability and the life of the refractory for injection amount control in continuous fabrication. Furthermore, the present invention improves the machinability by finely dispersing MnS inclusions in steel. The component composition specified in the present invention and the reasons for limitation will be described below.

[C] 0.005 to 0.2%

Since c is related to the basic strength of steel and the amount of oxygen in the steel, it greatly affects the machinability. Increasing the strength by adding more C reduces the machinability, so the upper limit was made 0.2%. On the other hand, if the amount of C is simply reduced too much by blowing, not only will the cost be increased, but deoxidation by C will not be performed, so a large amount of oxygen will remain in the steel, causing problems such as pinholes. Become. Therefore, it is possible to easily prevent defects such as pinholes. The C content of 0.005% is set as the lower limit.

[Si] 0.00 卜 0.5% Excessive addition of Si produces hard oxides and reduces machinability, but appropriate addition softens the oxides and does not reduce machinability. The upper limit is 0.5%. Above this, hard oxides are formed. If it is less than 0.001%, it will be difficult to soften the oxide and it will be industrially expensive.

[Mn] 0.3 ～ 3 · 0%

Mn is necessary to fix and disperse sulfur in steel as MnS. It is also necessary to soften the oxides in steel and render them harmless. The effect depends on the amount of S to be added, but if it is less than 0.3%, the added S is sufficiently fixed with MnS, and the surface becomes scratched and S becomes FeS and becomes brittle. As the amount of Mn increases, the hardness of the substrate increases and the machinability and cold workability deteriorate, so 3.0% was made the upper limit.

[P] 0.001 ~ 0.2%

P increases the hardness of the substrate in the steel and lowers not only the cold workability but also the hot workability and the forging properties, so the upper limit must be 0.2%. On the other hand, the lower limit is set to 0.001% for elements that are effective in improving machinability.

[S] 0.30 ~ 0.60%

S combines with Mn and exists as a sulfide mainly composed of MnS. Sulfides mainly composed of MnS improve machinability, but sulfides mainly composed of expanded MnS are one of the causes of anisotropy during forging. Large MnS-based sulfides should be avoided, but a large amount is preferred from the viewpoint of improving machinability. Therefore, it is preferable to finely disperse the sulfide mainly composed of MnS. Addition of 0.30% or more is necessary to improve machinability without adding Pb. On the other hand, if too much S is added, not only the formation of sulfides mainly composed of coarse MnS is unavoidable, but also during manufacturing due to deterioration of forging characteristics and hot deformation characteristics due to FeS and the like. Cause this. Therefore, the upper limit was made 0.60%.

[B] 0.0003 to 0.015%

B precipitates as M and is effective in improving machinability. In particular, it becomes more conspicuous when it is combined with sulfides containing MnS as the main component and finely dispersed in the matrix. These effects are not noticeable at less than 0.0003%, and if added over 0.015%, the reaction with the refractory in the molten steel becomes intense, and the refractory melts at the time of forging, resulting in a significant loss of manufacturability. Therefore, the range is 0.0003% to 0.015%.

Since B tends to form oxides, if dissolved O is high in molten steel, it is consumed as oxides, which may reduce the amount of BN effective in improving machinability. Improving the yield of the amount of B, which is essentially BN, by adding B after lowering dissolved oxygen (free oxygen) to some extent by adding Ca is effective for improving machinability.

[O] 0.005-0.012%

If O is not an oxide and exists alone, it becomes bubbles during cooling and causes pinholes. The formation of hard oxides may cause deterioration of machinability and must be controlled. In addition, Mn and B added to improve machinability are consumed as oxides in the molten steel, and the amount of Mn that becomes Mn S and the amount of B that becomes BN is reduced, affecting the machinability. There is a case. If it is less than 0.005%, the machinability deteriorates due to the formation of sulfides mainly composed of MnS in the form of Sims type II. Furthermore, desulfurization S reaction is likely to occur in molten steel, and stable S addition cannot be performed. Therefore, 0.005% was set as the lower limit. 〇 If the amount exceeds 0.012%, oxides of Mn and B are likely to be formed in the molten steel, and Mn that becomes MnS and B that becomes M are reduced and machinability is deteriorated. A large amount of is generated and the amount of scratches increases. Furthermore, since the refractory material melts severely, the upper limit was made 0.012%. Addition of Ca is essential for O control. [Ca] 0.0001 to 0.0010%

Ca is a deoxidizing element that can control the amount of dissolved oxygen (free oxygen) in the steel, stabilize the yield of Mn and B, which easily form oxides, and suppress the formation of hard oxides. Can do. In addition, if it is a trace amount, it generates a soft oxide and improves the machinability. If it is less than 0.0001%, there is no effect at all, and if it exceeds 0.0010%, a large amount of soft oxide is generated and adheres to the cutting edge of the tool with unevenness. Oxides are also produced in large quantities, further reducing machinability and hot ductility. Therefore, the component range was defined as 0.0001 to 0.0010%.

[Al] A1≤0.01%

A1 is a deoxidizing element and forms A1 ₂ 0 ₃ and AIN in the steel. But Al ₂ 〇 ₃ cause tool damage during cutting so hard to promote wear. In addition, forming AIN reduces N for forming BN, which reduces machinability. Therefore, it was set to 0.01% or less, which does not generate a large amount of A1 ₂ 0 ₃ and AIN.

[N≥ 0.0020% and 1.3X B— 0. (contains N that satisfies H00≤ N≤ 1.3X B + 0.0034]

N combines with B to generate BN and improve machinability. BN is an inclusion that improves machinability, and is significantly improved by finely dispersing at high density. By mass ratio B: N = 10.8: 14 (= 1: 1.3) The stoichiometric ratio of B and N combines with each other to form BN. BN has a solubility in steel. As the steel temperature rises, the solubility increases and the amount of solute N increases. If there is a large amount of N dissolved in the rolling temperature range (800 to 1100 ° C), it will cause rolling flaws, so it is necessary to limit the amount of N to a certain level or less. It must be controlled according to the amount added. Therefore, the upper limit of N amount is combined with B without excess or deficiency. If the amount exceeds + 0.0034% with respect to the N content (1.3XB), the occurrence of rolling defects becomes significant, so 1.3XB + 0.0034 or less. On the other hand, when too little N is added, the amount of BN produced decreases. The lower limit of the amount of N relative to the amount of B is 1.3XB—0.010 0 because the amount of BN required to improve machinability cannot be obtained if it is less than 0.0100% of the amount of N (1.3XB) that binds to B without excess or deficiency It was above. In addition, if the N content is less than 0.0020%, the absolute amount of N is insufficient, and the diffusion distance to the location where B is present in the steel increases. It is not possible to generate enough BN. Therefore, it is necessary to secure 0.0020% or more. Based on the above, N content must satisfy N≥0.0020% and 1.3XB—0.0100≤N≤1.3XB + 0.0034 in order to achieve both manufacturability and machinability.

[MnO] The area of MnO with a circle equivalent diameter of 0.5 m or more is 15% or less than the area of all Mn inclusions

Mn is an element with a strong affinity for oxygen. In the presence of a certain amount of dissolved oxygen (free oxygen) in molten steel, the formation of MnO is inevitable. MnO is a relatively low melting point, inclusions soft, itself is not intended to cause deterioration of machinability significant tool wear or the like as a hard inclusions such as Al ₂ 〇 _3. However, when MnO increases, the amount of Mn that becomes MnS decreases and the fine dispersion of MnS is inhibited, so machinability deteriorates. Furthermore, in an environment where a large amount of MnO is generated, the dissolved oxygen (free oxygen) in the molten steel is high in concentration, so that the amount of B oxide generated is also increased, generating BN. The amount of B decreases and the machinability further deteriorates. Also, since Mn becomes MnS and Sn cannot be fixed at high temperatures, hot ductility deteriorates due to the formation of many FeS.

Furthermore, the refractory of the plate refractory of the sliding nozzle for continuous forging due to MnO in the molten steel becomes severe and the productivity is significantly deteriorated. steel If the area of MnO in steel with an equivalent circle diameter of 0.5 zm or more in the cross section perpendicular to the rolling direction of the material exceeds 15% of the total Mn inclusion area, machinability and manufacturability Deterioration becomes significant, so to obtain good machinability and manufacturability, MnO in the steel must be 15% or less of the total Mn inclusions.

If the equivalent diameter of MnO is 0.5 m or less, the area ratio is extremely small, and therefore the amount of Mn consumed by MnO is very small, so the amount of Mn S produced is not greatly affected. For this reason, it was specified for a circle equivalent diameter of 0.5 m or more.

Here, the identification method of Mn 2 O and the area measurement method referred to in the present invention will be described.

MnO is usually present alone in addition to MnO alone or in combination with other oxides, but in the present invention, the area measured by the following method is identified as MnO.

Figure 3 shows an example of MnO measurement by EPMA. A test piece cut from a depth of 1 Z 4 in the diameter of the cross section perpendicular to the rolling direction of the steel material, embedded in resin, and polished with an electronic probe microanalyzer (EPMA) Measure over 20 fields of view. Since Mn0 1 3 in the steel substrate 1 2 of steel is contained in the sulfide 14 containing Mn S as the main component, the portion where Mn and O overlap in the elemental surface analysis by EPMA And the area is calculated.

All Mn inclusions are a general term for all inclusions combined with Mn in steel. Sulfides mainly composed of Mn S, oxides of MnO alone, MnO and others All oxides to which these oxides are bonded are targeted. Since all Mn inclusions can be fixed by elemental surface analysis with EPMA and the area can be measured, the ratio of the measured MnO area to the measured area of all Mn inclusions is obtained. .

In order to reduce the amount of MnO produced, the dissolved oxygen in the molten steel before LF Lee oxygen) achieved by reducing the concentration. The dissolved oxygen (free oxygen) concentration is preferably 200 ppm or less. However, if the amount is too low, desulfurization reaction proceeds between the metal and slag, and it becomes difficult to secure S in the steel to maintain machinability. . As a method for controlling dissolved oxygen (free oxygen), it is effective to perform pre-deoxidation before LF treatment. Addition of Ca is indispensable for controlling free oxygen, but addition of Si or Al, Ti, Zr, Mg, etc. alone or in combination is also effective.

[Sulfide dispersion mainly composed of MnS] The existence density of 0.1 to 0.5 m in the equivalent circle diameter is 10000 pieces Z mm ² or more

Sulfides containing MnS as a main component are inclusions that improve machinability, and are significantly improved by finely dispersing them at high density. In particular, in the case of a cutting method that progresses while forming a crest called a feed mark on the finished surface as in longitudinal turning, the presence or absence of peeling greatly affects the height of the crest, that is, the roughness of the finished surface. Sulfide containing MnS dispersed at high density as the main component can homogenize the steel material to improve the breakability of the steel material, reduce burrs, and improve the finished surface roughness. It is more effective in improving the finished surface roughness of parts that are cut by longitudinal turning, such as the shaft of office automation equipment. An abundance density of 10000 / mm ² or more is necessary to exert this effect, and the dimensions must be 0.1 to 0.5 m at the equivalent circle diameter. Usually, the sulfide distribution with MnS as the main component is observed with an optical microscope, and its dimensions and density are measured. Sulfides containing MnS of this dimension as the main component cannot be confirmed by observation with an optical microscope, and can only be observed with a transmission electron microscope (TEM). Even if there is no difference in size and density in optical microscope observation, this is a sulfide mainly composed of MnS with a size that can be clearly seen in TEM observation. In the present invention, this is controlled to quantify the existence form. The difference from the conventional technology It is intended to be separated. Existence of sulfides mainly composed of MnS exceeding this size with a density of 10000 mm ² or more requires the addition of a large amount of S exceeding the scope of the claims, but if a large amount is added, coarse MnS There is a high probability that there are many sulfides mainly composed of, and soot generation during hot rolling increases. If the amount of S added in the scope of claims exceeds MnS, the amount of sulfide containing MnS as the main component is insufficient, and the density required for improving the finished surface roughness cannot be maintained. . In addition, those with a minimum diameter of less than 0.1 μm do not substantially affect the machinability. The density of sulfides mainly comprised of MnS of 0. 1 to 0.5 m in circle equivalent diameter therefore has one 0000 or ZMM ² or more. This sulfide containing MnS as a main component becomes the precipitation nuclei of BN that are difficult to uniformly disperse in the matrix, thereby uniformly dispersing and improving the machinability of BN, especially the finish surface roughness. Can be more prominent.

Note that sulfides mainly composed of MnS include not only pure MnS but also sulfides such as Fe, Ca, Ti, Zr, Mg, and MM coexist in solid solution with or combined with MnS. In addition to inclusions such as MnTe, elements other than S forming a compound with Mn and coexisting with MnS in solid solution, and the inclusions deposited with oxide as a nucleus, (Mn, X) (S, Y) (where X is an element that forms a sulfide other than Mn, Y: an element that binds to Mn other than S) It is a collective term for interrelated substances.

In order to obtain the size and density of sulfides mainly composed of MnS, it is more effective to set the ratio MnZ S between Mn and S to 2 to 2.8.

In order to produce sulfides containing fine MnS as the main component more effectively, the solidification cooling rate range should be controlled. If the cooling rate is less than 10 ° C / min, solidification is too slow and crystallization of MnS-based sulfides becomes coarse, making it difficult to finely disperse. If the cooling rate exceeds 100 ° C / min, Generation The density of sulfides composed mainly of fine MnS is saturated, the hardness of the steel slab increases and the risk of cracking increases. Therefore, the cooling rate during fabrication should be 10-100 ° C / min. In order to obtain this cooling rate, it is easily obtained by controlling the size of the saddle cross section, the penetration rate, etc. to appropriate values. This can be applied to both continuous forging and ingot making.

As shown in FIG. 8, the solidification cooling rate here refers to the depth position of 1 Z 4 of the thickness (L) of the steel piece 16 in the cross section 17 of the steel piece 16 produced in the production direction 15 indicated by the arrow. This is the cooling rate from the liquidus temperature to the solidus temperature in Fig. 18 (see Fig. 8 (b)). The cooling rate is calculated by the following formula from the distance between the secondary dendritic arm of the solidified structure in the thickness direction of the flake after solidification.

X2 0.41

Rc =

770 Where Rc: Cooling rate CZiiiin), λ2: Secondary dendritic arm spacing (m)

In other words, the secondary dendrite arm spacing changes depending on the cooling conditions, and the controlled cooling rate was confirmed by measuring this.

Next, the reason for defining the optional additive element will be described.

[Steel reinforcement element]

[V] 0.05-1.0%

V forms carbonitride and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect in increasing the strength. If it is added in excess of 1.0%, a large amount of carbonitride precipitates, and on the contrary, the mechanical properties are impaired, so this was made the upper limit.

[Nb] 0.005-0.2% Nb also forms carbonitrides and can strengthen steel by secondary precipitation hardening. If it is less than 0.005%, there is no effect in increasing the strength, and if added over 0.2%, a large amount of carbonitride precipitates, and on the contrary, the mechanical properties are impaired, so this was made the upper limit.

[Cr] 0.01 ～ 2.0%

Cr is an element that improves hardenability and imparts temper softening resistance. Therefore, it is added to steel that requires high strength. In that case, addition of 0.01% or more is required. However, if added in a large amount, Cr carbide is formed and embrittled, so 2.0% was made the upper limit.

[Mo] 0.05-1.0%

Mo is an element that imparts resistance to temper softening and improves hardenability. If less than 0.05%, the effect is not observed, and even if added over 1.0%, the effect is saturated, so 0.05% to 1.0% was added.

[W] 0.05-1.0%

W forms carbonitride and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect in increasing the strength. If it is added in excess of 1.0%, a large amount of carbonitride precipitates, and on the contrary, the mechanical properties are impaired, so this was made the upper limit.

[Nil 0.05-2.0%

Ni strengthens ferrite and is effective in improving ductility and improving hardenability and corrosion resistance. If less than 0.05%, the effect is not recognized, and even if added over 2.0%, the effect is saturated in terms of mechanical properties, so this was made the upper limit.

[Cu] 0.01-2.0%

Cu strengthens ferritic iron and is effective in improving hardenability and corrosion resistance. If it is less than 0.01%, the effect is not recognized, and it exceeds 2.0%. Even so, the effect is saturated in terms of mechanical properties, so we set this as the upper limit. In particular, it is preferable to add at the same time as Ni because it reduces hot ductility and tends to cause defects during rolling.

[Machinability improving element due to embrittlement]

CSn] 0.005-2.0%

Sn embrittles ferrite and prolongs tool life and improves surface roughness. If less than 0.005%, the effect is not recognized, and even if added over 2.0%, the effect is saturated, so this was made the upper limit.

[Zn] 0.0005-0.5%

Zn embrittles ferrite and prolongs tool life and improves surface roughness. If less than 0.0005%, the effect is not recognized, and even if added over 0.5%, the effect is saturated, so this was made the upper limit.

[Machinability improving element by adjusting deoxidation]

[ΤΠ 0.0005-0.1%

Ti is a deoxidizing element, and can control the amount of oxygen in the steel, and can stabilize the yield of Mn and B, which easily form oxides. If the amount is very small, soft oxides are produced, and the machinability is improved. If it is less than 0.0005%, there is no effect, and if it is 0.1% or more, a large amount of hard oxide is produced in large quantities, and Ti that forms a solid solution without forming an oxide combines with N to form hard TiN. Reduces machinability. Therefore, the component range was defined as 0.0005 to 0.1%. Ti consumes N necessary for BN formation by forming TiN. Therefore, it is desirable that the Ti content be 0.01% or less.

[Zr] 0.0005〜0 · 1%

Zr is a deoxidizing element and can control the amount of oxygen in the steel. It is possible to stabilize the yield of Mn and B that are easy to form. If the amount is very small, soft oxides are produced, and the machinability is improved. If it is less than 0.005%, there is no effect, and if it is 0.1% or more, a large amount of soft oxide is generated and adheres to the tool edge with unevenness, so that the finished surface roughness is not only extremely deteriorated, Hard oxides are also produced in large quantities, further reducing machinability. Therefore, the component range was defined as 0.0005 to 0.1%.

[Mg] 0.0003-0.005%

Mg is a deoxidizing element that can control the amount of oxygen in the steel and can stabilize the yield of Mn and B, which easily form oxides. If the amount is very small, soft oxides are produced, and the machinability is improved. If it is less than 0.0003%, there is no effect, and if it is 0.005% or more, a large amount of soft oxide is formed and adheres unevenly to the tool edge, resulting in an extremely poor finished surface roughness. In addition, a large amount of hard oxide is generated, further reducing the machinability. Therefore, the component range was defined as 0.003 to 0.005%.

[Machinability improving element by sulfide form control and lubrication between tool and steel

]

[Te] Te: 0.0003-0.2%

Te is a machinability improving element. It also has the function of controlling the distraction of the MnS shape by generating MnTe and coexisting with MnS to reduce the deformability of MnS. Therefore, it is an effective element for reducing anisotropy. This effect is not observed at less than 0.0003%, and if it exceeds 0.23%, the effect is not only saturated, but the hot ductility is lowered and is likely to cause flaws.

[B i] 0.005 to 0.5%

B i is a machinability improving element. The effect is not observed at less than 0.005%, and adding more than 0.5% only saturates the machinability improvement effect. In addition, the hot ductility is likely to decrease and cause wrinkles.

[Pb] 0.005-0.5%

Pb is a machinability improving element. The effect is not observed at less than 0.005%, and adding more than 0.5% not only saturates the machinability improvement effect but also tends to cause flaws due to a decrease in hot ductility. Example

The effects of the present invention will be described with reference to examples. The steels of the inventive examples of Examples 1 to 72 shown in Tables 1 to 4 were forged in a 270 t converter so that the solidification cooling rate was 4 to 18 ° C / min. Among them, the solidification cooling rate of the steel type of claim 1 of Examples 1 to 8 and the steel type of Claim 6 of 62 to 72 is 1 to 7 ° C / min, Claims 2 to 6 of Examples 9 to 61 The solidification cooling rate of each steel grade was forged and sorted so as to be 12 to 85 ° C / min. The steels of comparative examples of Examples 73 to 102 shown in Tables 5 to 6 were forged in a 270 t converter so that the solidification cooling rate was 4 to 7 ° C / miii. In both the inventive example and the comparative example, the 270-t converter material was rolled into billets and then rolled to Φ9.5. This Φ9.5mm rolled material was drawn to Φ8mm and used as a test material. Tensile specimens were taken from the billet and 18 Omm square forged material for hot ductility evaluation before rolling. The solidification cooling rate was adjusted by controlling the squeezing rate if the size of the vertical cross section was large.

The machinability of the material was evaluated by three typical cutting methods: drill drilling test with conditions shown in Table 7, plunge cutting test with conditions shown in Table 8, and longitudinal turning test with conditions shown in Table 9. The drilling test is a method of evaluating machinability at the maximum cutting speed (so-called VL1000, unit: m / min) that can cut to a cumulative hole depth of 1000 mm. The plunge cutting test is a method for evaluating the roughness of the finished surface by transferring the tool shape (configuration edge shape) with a high-speed steel parting tool. Figure 1 shows an overview of this experimental method. In the experiment, the finished surface roughness when 200 grooves were machined was measured with a stylus type roughness meter. Set. It was used as an index indicating the finished surface roughness of 10-point surface roughness Rz (unit: m). Longitudinal turning test is a cutting method that cuts the outer circumference of the test piece 2 in the cutting direction 3 while feeding the cemented carbide tool 1 in the longitudinal direction.Similar to plunge cutting, the surface roughness measurement surface 4 is the finished surface of the tool shape transfer 4 This is a method of repeatedly measuring roughness and evaluating it. Figure 2 shows an overview of this experimental method. In this method, the test piece 2 is rotated while the carbide tool 1 is fed along the test piece 2 (0.05 mni / rev) and cutting is performed with a predetermined depth of cut 6 (1 mm) (cutting speed 80m Z min). This is a cutting method in which a crest called feed mark 5 is formed on the finished surface 7 and progressed to form the surface roughness measuring surface 8. The roughness of the stripped surface (theoretical roughness + stripping) is 10. In other words, it becomes the finished surface roughness, which greatly affects the surface roughness (theoretical roughness) 11 (see Fig. 2 (b)). If there is no irregularity, the value will be close to the theoretical roughness, but if irregularity occurs, the roughness will decrease (deteriorate) accordingly. Sulfide containing MnS finely dispersed at a high density as a main component can reduce burrs and improve the finished surface roughness by homogenizing the steel material. Therefore, MnS dispersed at a high density can be used as the main component. This is a method capable of remarkably expressing the effect of sulfide. In addition, this method can also show the quality of the finished surface roughness due to the transfer of tool irregularities due to tool wear after heavy cutting, so the experiment evaluated the difference in machinability in the state where tool wear progressed. Possible to evaluate the finished surface roughness after cutting 800 pieces. The finished surface roughness was measured with a stylus-type roughness meter, and the 10-point surface roughness Rz (unit: m) was used as an indicator of the finished surface roughness. With regard to chip disposal, a chip with a small radius at the time of chip curl or a piece that is divided is preferable, and is marked as “O”. A sample with a small radius of curvature at most turns or a sample with a large radius of curvature that did not reach a chip length of 10 Omm was rated as ◯. Chips that exceed 20 mm and with a radius of curvature of 3 or more rolls are curled continuously and defective And x.

Regarding MnO in steel, the area ratio of 0.5 m or more in a circle equivalent system in the cross section perpendicular to the rolling direction of the steel material is measured on the cross section perpendicular to the rolling / drawing direction after φ 8 mm wire drawing. A test piece cut out from a depth of 1 Z 4 in diameter, embedded in resin, and polished was performed with an electronic probe microanalyzer (EPMA). The measurement was performed with 20 fields of view at least 200 ^ m × 200 zm, and the area ratio was determined by taking the MnO area in the inclusions measured by elemental analysis as the ratio to the total Mn inclusion area. Since MnO in steel exists in the state of being contained in MnS, the area where Mn and 〇 overlapped was identified as MnO by the EPMA analysis. The superposition of Mn and O was performed by image processing. An example of MnO measurement by EPMA is shown in Fig. 3. The sulfide density measurement with a major equivalent of MnS with the maximum equivalent circle diameter of 0.5 m and the minimum diameter of 0.1 μm is shown in Fig. 3. The sample was extracted by the extraction replica method from the depth position of 1 Z 4 with a diameter of a section perpendicular to the rolling / drawing direction, and was performed with a scanning electron microscope. The measurement was performed at a magnification of 10,000 and 40 visual fields of 80 / im ² per field were obtained, and this was converted to the number of sulfides containing MnS as the main component per mm ² .

The hot ductility was evaluated by the drawing value of the hot tensile test at 1000 ° C. If the drawing is 50% or more, good rolling is possible, but if the drawing is less than 80%, surface flaws occur frequently, the flaw removal care area after rolling increases, and high-grade varieties with severe surface properties are required. Not applicable. If an aperture value of 80% or more is obtained, the occurrence of surface flaws is remarkably reduced, and it can be used without maintenance, making it applicable to high-grade varieties. In addition, maintenance costs can be reduced. Therefore, the hot ductility was marked as ◯ when the aperture was 80% or more, and X was marked when it was less than 80%.

The refractory status of the plate refractory of the sliding nozzle for continuous forging , MgO-C protein as the material of Suraidi ring nozzle plate (Mg0 = 87%, Al ₂ 〇 _{3 = 10%, C = 3} %) was used to evaluate the melting rate. When the area of MnO of 0.5 m or more is 15% with respect to the area of all Mn inclusions, the refractory ratio of refractory is assumed to be 1 and the erosion ratio is quantified. Value. When the melting loss ratio exceeds 1, the refractory melting damage becomes severe. Therefore, when the melting loss ratio was 1 or less, it was evaluated as ◯, and over 1 was evaluated as X. The invention examples of Examples 1 to 72 all have a good drilling tool life, finished surface roughness in plunge cutting and longitudinal turning, and hot ductility values of 80% or more compared to Comparative Examples of Examples 73 to 102. Thus, good manufacturability with a low melting loss ratio could be obtained. For example, when the MnO area ratio is low by controlling the amount of N by a balanced addition amount of B and N and controlling the amount of O by adding Ca as in the inventive examples of Examples 1 to 8, It was possible to obtain a high hot ductility value and a low melting rate without degrading the machinability. In addition, very good machinability could be obtained due to the balanced addition of B and N and the low MnO area ratio. When the density of the sulfide mainly composed of fine MnS satisfies Claim 2 as in Examples 9 to 18 and 56 to 59, the finished surface roughness, particularly the value during longitudinal turning is further increased. It is getting better. It can be seen that excellent finished surface roughness and manufacturability were also obtained in Examples 19 to 55 and those to which the optional addition optional elements of claims 3 to 6 of 60 to 72 were added. Among them, Examples 47, 52, 60, 62 to 67 with a small amount of Pb known as a free-cutting element Examples 45, 48, 50, 53, 6 with a small amount of Te also known as a free-cutting element It can be seen that good hot ductility and machinability are also obtained with 1, 68, 69, and 55 and 70-72 with both elements of Pb and Te added.

On the other hand, since all of the comparative examples were manufactured at a low solidification cooling rate, the density of sulfides mainly composed of fine MnS was reduced, and overall machinability, especially finishing by longitudinal turning. The surface roughness is a bad value Thus, even for the inventive examples of claims 1 to 8 manufactured at the same solidification cooling rate of the same level, the chemical components are out of the scope of the present invention, and thus show bad values. For example, as in the comparative example of Example 76, when the MnO area ratio is high, the finished surface roughness becomes poor due to the decrease in the MnS content and the BN content, and the melting rate is large. In the comparative example of Example 80, the MnO area ratio is less than 15%, but the hot ductility is poor because the S and Ca contents are off. As in the comparative example of Example 81, in the case where Ca was not added, O could not be controlled. Show. Furthermore, Examples 90 and 91 are comparative examples in which the N content is outside the lower limit, but the increase in solid solution B causes an increase in hardness, and the hot ductility is low. Further, Example 93 is a comparative example in which the amounts of S and N are outside the upper limits, and the hot ductility reduction shows a bad value due to the increase in solute N. Example 102 is a comparative example in which MnO is high, and both the finished surface roughness and the erosion index are poor.

Fig. 4 shows (a) a TEM replication force photograph and (b) an optical microscope photograph of the sulfide containing MnS as the main component of the present invention. Figure 5 shows (a) a TEM replica photograph and (b) an optical microscope photograph of a sulfide mainly composed of MnS in the comparative example. Thus, in the example of the invention and the comparative example, the size and density of the sulfide mainly composed of MnS are not much different in the observation with the optical microscope of (b), but both the size and density are observed in the observation of the TEM replica of (a). A clear difference can be seen. Fig. 6 shows the change in machinability due to the area ratio of MnO as an example of the finished surface roughness in longitudinal turning after cutting 800 pieces. Since the progress of tool wear during heavy cutting becomes significant when the area ratio of Mn 0> 15%, the superiority or inferiority of the finished surface roughness that is affected by the transfer of irregularities due to tool wear appears remarkably here. Figure 7 shows the finish surface roughness and hot ductility in longitudinal turning in the invention and comparative examples. Show balance. Inventive examples have good finished surface roughness and a hot ductility of 80% or more. In the comparative example, it is in a region where both the finished surface roughness and hot ductility are inferior, or the finished surface roughness is poor even though the hot ductility is good.

From this, it can be seen that the invention example in which the B amount and the N amount are balanced and the MnO amount can be controlled has good manufacturability and machinability.

table

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 9

Industrial applicability

According to the present invention, the tool life at the time of cutting, the finished surface roughness, and the machinability of chip disposal are excellent, and further, the refractory of the plate refractory of the sliding nozzle for continuous forging is small, and hot rolling Free-cutting steel with good ductility and excellent manufacturability can be provided.

Claims

1. In mass%

C: 0.005 to 0 · 2%

Si: 0.0 (H to 0.5%

Mn: 0.3 to 3% 0%

Contract

：: 0.001-0.2%

S: 0.30 to 0.60%

B: 0.0003 to 0.015%

○: 0.005-0.012%

Ca: 0.0001 to 0.0010% 匪

A1≤0.01%

N content is

N≥ 0.0020% and 1.3XB— 0.0100≤ N≤ 1.3XB + 0.0034, the balance is Fe and unavoidable impurities, and MnO in steel is equivalent to the circle diameter in the cross section perpendicular to the rolling direction of the steel. Free-cutting steel with excellent manufacturability, characterized in that the area of MnO of 0.5 m or more is 15% or less of the total Mn inclusion area.

2. The steel according to claim 1 is about 0.1 to 0 in terms of equivalent circle diameter in a cross section perpendicular to the rolling direction of the steel material with respect to a sulfide mainly composed of MnS.

A free-cutting steel with excellent manufacturability, characterized by a density of 10000 pcs Zmiii ² or more.

3. Furthermore, in mass%,

V: 0.05 to 1.0%