KR101118852B1 - Free-cutting steel excellent in manufacturability - Google Patents

Abstract

The present invention is to provide a free-cutting steel which is excellent in machinability, has a low melting point of the plate refractory material of the sliding nozzle for continuous casting, is excellent in ductility due to hot rolling, and can prevent deterioration of surface properties due to hot rolling. %, 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%, Al ≦ 0.01%, N content is N ≧ 0.0020% and satisfies 1.3 × B-0.0100 ≦ N ≦ 1.3 × B + 0.0034, the balance being Fe and inevitable impurities The MnO having a circular equivalent diameter of 0.5 µm or more in a cross section perpendicular to the rolling direction of the steel with respect to MnO in the steel is 15% or less with respect to the area of all Mn-based inclusions.

Machinability, sliding nozzle, plate, ductile, surface appearance

Description

Free-cutting steel with excellent manufacturability {FREE-CUTTING STEEL EXCELLENT IN MANUFACTURABILITY}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to low-carbon free-cutting steel used in automobiles, general machinery, and the like, where machinability is required rather than strength characteristics. It is related with the free cutting steel excellent in the manufacturability which has little melt | dissolution loss of the plate refractory body of a rough sliding nozzle, and has favorable ductility in hot rolling.

General machines and automobiles are manufactured by combining a variety of parts, but the parts are manufactured through cutting processes in most cases in terms of required precision and manufacturing efficiency. In that case, cost reduction and the improvement of productive efficiency are calculated | required, and the machinability improvement is calculated | required also in steel. In particular, low carbon sulfur free cutting steel SUM23 and low carbon sulfur lead composite free cutting steel SUM24L were invented with importance in machinability. Until now, in order to improve machinability, adding machinability improvement elements, such as S and Pb, is known to be effective. However, depending on the demand price, the use of Pb as an environmental load may be avoided, and the amount of use thereof tends to be reduced.

Until now, when Pb is not added, the method which improves machinability by forming a soft inclusion in the cutting environment, such as a sulfide which has MnS as a main component, is used. However, the same amount of S as the low-carbon sulfur free-cutting steel SUM23 is added to the low-carbon lead-sulfur composite free cutting steel SUM24L. However, the addition of a large amount of S simply makes the sulfide mainly composed of MnS coarse, not effective for improving machinability, and not able to soften the matrix sufficiently, resulting in dropping of edges and separation of cutting chips. Problems such as deterioration of surface roughness and poor cutting chip processability due to insufficient division of the cutting chip occur. In production processes such as rolling and forging, sulfides having coarse MnS as a main component have a breaking point and cause many manufacturing problems such as rolling marks. In addition, addition of Te, Bi, P, N, etc., which are machinability enhancing elements other than S, may improve machinability to some extent, but may cause deterioration of surface properties such as cracking and traces during rolling or hot forging. Therefore, as few as possible is preferred, and machinability and manufacturability cannot be made compatible.

Japanese Unexamined Patent Application Publication No. 11-222646 discloses a cutting process in which 30 or more sulfides alone or 20 or more sulfide groups in which a plurality of sulfides are continuous in series form exist within a field of view of 1 mm 2 in the rolling direction. Ways to increase sex have been devised. However, the dispersion of the sulfide at the submicron level which is most effective for machinability is not mentioned, including the manufacturing method, and cannot be expected from the component system.

Examples of attempting to utilize inclusions other than sulfides for improving machinability still exist. For example, Japanese Patent Application Laid-Open No. 9-17840, Japanese Patent Application Laid-Open No. 2001-329335, and Japanese Patent Application Laid-Open 2002- It is the technique which aimed at the improvement of machinability using BN like 3991 and Unexamined-Japanese-Patent No. 2000-178683. However, these are not intended to improve the finish roughness, but Japanese Patent Application Laid-Open No. 9-17840, Japanese Patent Application Laid-Open No. 2001-329335, and Japanese Patent Application Laid-Open No. 2000-178683 aim at improving tool life. In addition, Japanese Patent Application Laid-open No. 2002-3991 aims at improving cutting chip treatability. With the application in the chemical component of the Example range disclosed in these, sufficient effect cannot be acquired in improving a finishing surface roughness. Specifically, if the matrix is not homogenized by fine dispersion in BN steel, the effect of improving the finish surface roughness is not obtained, but the technique is not described in these patent documents.

The technique disclosed in Japanese Patent Application Laid-Open No. 2004-176176 is also an example of utilizing BN for improving machinability, and is considering balance with N addition amount. However, in the present technology, while completely suppressing the occurrence of rolling marks, the amount of the oxide of B having a high affinity to the enzyme is suppressed and precipitated as BN for the balance of the steel chemical component which secures the machinability which is the opposite property. There is no mention of a method for increasing the amount to be made.

Japanese Patent Laid-Open No. 5-345951 discloses a technique for increasing MnS by increasing the oxygen concentration in steel to improve machinability. However, the present technology makes no mention of the decrease in MnS due to the increase in oxygen and the accompanying machinability deterioration, nor is it mentioned in terms of prevention of significant deterioration in manufacturability such as refractory loss or increased surface marks.

In addition, Japanese Patent Application Laid-Open No. 2001-329335 also proposes a technique for suppressing grain boundary embrittlement due to grain boundary precipitation of BN for improving hot ductility, and limiting the amount of N added to utilize grain boundary embrittlement prevention action of solid solution B. It is. However, since the amount of N is simply reduced, sufficient consideration is not given to the control of the amount of solid solution N in the BT heating or processing temperature range, so that the amount of solid solution N necessary for the prevention of marks is insufficient. In addition, the amount of BN necessary for improving the finish surface roughness is insufficient because the N content is lower than the stoichiometric composition. However, the supplemental surface roughness cannot be obtained because no supplementation by other techniques for replenishing it is planned.

In addition, Japanese Patent Application Laid-Open No. 2004-27297 discloses a technique for limiting the amount of oxygen in steel for reducing surface marks. However, it is not mentioned at all in the method of controlling the amount of oxygen in the steel, and in the low carbon free cutting steel of mithal acid, it is impossible to limit the amount of oxygen in the steel without special control to prevent the occurrence of marks.

In low carbon free cutting steel, the example which adds Ca for the improvement of machinability exists even now. For example, Japanese Patent Application Laid-Open No. 2000-160284 does not describe a specific effect of improving machinability, and also has a wide range of Ca addition amount, and does not describe an addition amount effective for improving machinability.

In addition, when the low-carbon high-cutting steel of B addition is manufactured by the continuous casting method, there is a problem that the refractory plate of the sliding nozzle is easily melted, but prior art documents which solve such a problem cannot be found.

The present invention is a low-carbon free-cutting steel used in automobiles, general machinery, and the like, and particularly has excellent machinability in cutting tool life, finish surface roughness, and cutting chip treatment, and has less loss of refractory plate of the sliding nozzle for continuous casting. It is excellent in the ductility by hot rolling, and provides the free cutting steel which can prevent deterioration of the surface property by hot rolling.

Cutting is a breaking phenomenon that separates the cutting chip, and promoting it becomes one point. However, as already described, there is a limit to simply increasing S. In addition, in order to make both machinability and manufacturability compatible, it is necessary to consider also the machinability improvement element amount.

In order to improve the hot ductility, the ratio of the amount of B and N necessary to obtain the BN required for machinability at the room temperature at which cutting is performed while controlling the amount of solid solution N in the rolling temperature range can be controlled to achieve both hot ductility and machinability. I figured it out. The solid solution N is the amount which subtracted the amount of compound N from the total amount of N, and the amount of compound N shows the amount of N which becomes substantially BN. This solid solution N is produced | generated in large quantities because it solidifies BN by the heating of the rolling temperature range 800-1100 degreeC. It is necessary to reduce the amount of solid solution N in this temperature range in order to produce a good rolling because the amount of surface marks is small.

In addition, the yield of Mn, which is easily consumed as an oxide in molten steel, as the MnS and the yield of B as BN are improved to improve the machinability and hot ductility, while improving the machinability and suppressing the melting loss of the refractory plate of the sliding nozzle for continuous casting. It was found that it is necessary to reduce the amount of MnO produced in the steel.

This invention is made | formed based on the above knowledge, The summary is as having described below.

(1) at 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%

Al≤0.01%

Containing N content,

N≥0.0020% and 1.3 x B + 0.0100≤N≤1.3 x B-0.0034, and the balance is made of Fe and unavoidable impurities, and in the cross section perpendicular to the rolling direction of the steel with respect to MnO in the steel. The area of MnO which is 0.5 micrometer or more in circular equivalent diameter is 15% or less with respect to the area of all Mn type inclusions, The free cutting steel excellent in the manufacturability characterized by the above-mentioned.

(2) The sulfide whose steel according to (1) has MnS as a main component has a presence density of 0.1 to 0.5 µm in a circular equivalent diameter in a cross section perpendicular to the rolling direction of the steel material, characterized in that 10000 pieces / mm 2 or more. Free cutting steel with excellent manufacturability.

(3) In addition, in mass%,

V: 0.05 to 1.0%

Nb: 0.005 to 0.2%

Cr: 0.01 to 2.0%

Mo: 0.05-1.0%

W: 0.05 to 1.0%

Ni: 0.05 to 2.0%

Cu: 0.01 to 2.0%

Sn: 0.005 to 2.0%

Zn: 0.0005 to 0.5%

Ti: 0.0005 to 0.1%

Zr: 0.0005 to 0.1%

Mg: 0.0003 to 0.005%

Te: 0.0003 to 0.2%

Bi: 0.005 to 0.5%

Pb: 0.005 to 0.5%

A free cutting steel excellent in the manufacturability according to any one of 1 to 5, comprising one kind or two or more kinds thereof.

1 is a conceptual diagram showing a flange cutting test method, (a) is a bird's eye view, (b) is a plan view.

2 is a conceptual diagram showing whether the longitudinal turning test method and the finish surface roughness is defective, (a) is a plan view, and (b) is an enlarged view of a finish surface (feed mark).

3 is an optical micrograph showing an example of MnO measurement by EPMA.

Fig. 4 is a (a) TEM replica photograph and (b) optical micrograph of a sulfide containing MnS as a main component of the present invention.

(A) TEM replica photograph and (b) optical micrograph of the sulfide which has MnS as a main component of a comparative example.

Fig. 6 is a diagram showing the change in machinability by MnO as a finish surface roughness by longitudinal turning after 800 cuts.

Fig. 7 is a diagram showing the finish surface roughness-hot ductility balance by longitudinal turning in the invention example and the comparative example.

8 is an explanatory view of a depth position of 1/4 of a cast piece thickness;

The present invention improves the machinability by adding B to precipitate BN even in a low-carbon free-cutting steel which requires machinability rather than strength characteristics. In particular, B and N are appropriately related to the steel component composition. The present invention has been completed by improving the machinability and ductility during hot rolling, and by reducing MnO in steel, thereby improving the life of the refractory for controlling the machinability and injection amount by continuous casting. In addition, the present invention is to finely disperse the MnS inclusions in the steel to improve the machinability. Below, the component composition prescribed | regulated by this invention and the reason for limitation are demonstrated.

[C] 0.005 to 0.2%

C has a great influence on machinability since it is related to the basic strength of steel and the amount of oxygen in the steel. When a large amount of C was added to increase the strength, the machinability was lowered, so the upper limit thereof was 0.2%. On the other hand, if the amount of C is reduced too much by simply blowing, not only the cost increases but also the deoxidation by C does not occur, so that the amount of oxygen in the steel remains in a large amount and causes defects such as pinholes. Therefore, the amount of C 0.005% which can easily prevent defects, such as a pinhole, was made into the minimum.

[Si] 0.001 to 0.5%

Excessive addition of Si generates hard oxides to lower machinability, but proper addition softens oxides and does not lower machinability. The upper limit is 0.5%, and hard oxide is generated more than that. If it is less than 0.001%, oxide nitriding becomes difficult and industrially expensive.

[Mn] 0.3 to 3.0%

Mn is necessary to fix and disperse sulfur in steel as MnS. It is also necessary to soften oxides in the steel to make the oxides harmless. The effect is also dependent on the amount of S added, but if it is less than 0.3%, the addition (S) is sufficiently fixed with MnS, so that the scratch surface scratches and S become FeS and become soft. When the amount of Mn became large, the hardness of the base became large, and machinability and cold workability fell, and 3.0% was made into an upper limit.

[P] 0.001 to 0.2%

P has a high hardness in the base in steel, and not only cold workability but also hot workability and casting characteristics. Therefore, the upper limit should be 0.2%. On the other hand, it is an element which is effective in improving machinability, and the lower limit was made into 0.001%.

[S] 0.30 to 0.60%

S is present as a sulfide having MnS as a main component in combination with Mn. Sulfide containing MnS as a main component improves machinability, while sulfide containing elongated MnS as a main component is one of the causes of anisotropy in forging. Sulfide containing large MnS as a main component should be avoided, but a large amount of addition is preferable from the viewpoint of improving machinability. Therefore, it is preferable to finely disperse sulfide containing MnS as a main component. In order to improve machinability when Pb is not added, 0.30% or more of addition is required. On the other hand, when the amount of S added is too large, the formation of sulfides containing coarse MnS as a main component cannot be avoided, and cracks are generated during manufacturing due to deterioration of casting characteristics and hot deformation characteristics due to FeS or the like. Therefore, the upper limit was made into 0.60%.

[B] 0.0003 to 0.015%

When B precipitates as BN, it is effective in improving machinability. In particular, it becomes more remarkable by complex precipitation with sulfide containing MnS as a main component and fine dispersion in the matrix. These effects are not remarkable at less than 0.0003%, and when added at more than 0.015%, the reaction with the refractory in the molten steel is severe, and the melting of the refractory is increased during casting, which significantly impairs the manufacturability. Therefore, 0.0003% to 0.015% was made into the range.

Since B is easy to form an oxide, when dissolved O in molten steel is high, it may be consumed as an oxide and the amount of BN effective for improving machinability may decrease. It is effective to improve machinability by improving the yield of the amount of B which becomes real BN by adding B after lowering dissolved oxygen (free oxygen) to some extent by addition of Ca.

[O] 0.005 to 0.012%

When 0 does not become an oxide and exists alone, it becomes a bubble at the time of cooling, and causes pinholes. The generation of hard oxides may cause deterioration of machinability and marks, and control is necessary. In addition, Mn and B added to improve the machinability may be consumed as an oxide in molten steel, and the amount of Mn and BN to MnS may be reduced to affect machinability. If it is less than 0.005%, the machinability deteriorates because the sulfide which has MnS of the form called Sims type II as a main component is produced | generated. In addition, desulfurization S reaction tends to occur in molten steel, and stable S addition cannot be performed. Therefore, 0.005% was made into a minimum. When the amount of O exceeds 0.012%, oxides of Mn and B are easily formed in molten steel, Mn and BN which are substantially MnS are reduced to deteriorate machinability, and a large amount of hard oxide is formed to generate scratches. This increases. Moreover, since refractory loss also became severe, 0.012% was made into an upper limit. Addition of Ca is essential for the control of O.

[Ca] 0.0001 to 0.0010%

Ca is a deoxidation element, and can control the amount of dissolved oxygen (free oxygen) in steel materials, and can stabilize the yield of Mn and B which are easy to form an oxide, and can suppress formation of a hard oxide. In addition, when the amount is small, soft oxides are produced to improve machinability. If it is less than 0.0001%, there is no effect at all, and if it is more than 0.0010%, a large amount of soft oxide is produced, and it attaches to the tool edge with concavity and convexity, and as a result, not only the finishing roughness becomes extremely bad, but also a large amount of hard oxide is produced. In addition, machinability and hot ductility are reduced. Therefore, the component range was defined as 0.0001 to 0.0010%.

[Al] Al ≤ 0.01%

Al is a deoxidation element, which forms Al ₂ O ₃ or AIN in steel. However, Al ₂ O ₃ is hard, which causes tool damage during cutting and promotes wear. In addition, by forming AIN, N for forming BN is reduced, resulting in poor machinability. Therefore it decided to not more than 0.01% does not produce the Al ₂ O ₃ or AIN in a large amount.

[N ≥0.0020%, and also contains N satisfying 1.3 × B-0.0100 ≦ N ≦ 1.3 × B + 0.0034]

N is combined with B to produce BN to improve machinability. BN is an inclusion which improves machinability and is remarkably improved by dispersing finely at a high density. The stoichiometric ratio of B: N = 10.8: 14 (= 1: 1.3) by mass ratio enables BN to form by combining B and N without excess or deficiency. BN has solubility in steel, and solubility increases with steel temperature rise, and the amount of solid solution N increases. When the amount of N employed in the rolling temperature range (800 to 1100 ° C.) is large, it is necessary to limit the amount of N dissolved in solid solution to a certain amount or less, because it causes rolling marks, and the amount of N added to the steel is controlled according to the amount of B added. Should be. Therefore, when the upper limit of the amount of N exceeds + 0.0034% with respect to the amount of N (1.3xB) bonded to B without excess or deficiency, the occurrence of rolling marks becomes remarkable, so it is set to 1.3xB + 0.0034 or less. On the other hand, when the amount of N added is too small, the amount of BN produced will reduce. The lower limit of the amount of N relative to the amount of B was made 1.3 × B-0.0100 or more because the amount of BN necessary for improving machinability was not obtained at less than −0.0100% with respect to the amount of N (1.3 × B) bonded to B without excess or insufficient. . If the amount of N is less than 0.0020%, the absolute amount of N is insufficient, and the distance to the point where B is present in the steel becomes large, so that even if the amount of N added in the stoichiometric ratio is sufficient, BN cannot be produced. Therefore, it is necessary to ensure 0.0020% or more. As mentioned above, in order to make manufacturability and machinability compatible, it is necessary for N content to satisfy N≥0.0020% and 1.3xB-0.0100 <= N <= 1.3 * B + 0.0034.

[MnO] The area of MnO of 0.5 µm or more in circular equivalent diameter is 15% or less with respect to the area of all Mn-based inclusions.

Mn is an element having a strong affinity with oxygen, and formation of MnO is inevitable in the presence of a certain amount of dissolved oxygen (free oxygen) in molten steel. MnO is a relatively low melting point and soft inclusion, and itself is a hard inclusion such as Al ₂ O ₃ and does not cause machinability deterioration such as significant tool wear. However, when MnO increases, the amount of Mn which becomes MnS decreases, and the machinability deteriorates because fine dispersion of MnS is inhibited. Also, in an environment in which a large amount of MnO is produced, dissolved oxygen (free oxygen) in molten steel is high in concentration, thereby increasing the amount of B oxides produced, thereby reducing the amount of B produced as BN and further improving machinability. It will deteriorate. In addition, since Mn which becomes MnS decreases, it becomes impossible to fix S at high temperature, and many FeS is produced and hot ductility deteriorates.

Moreover, the melt of the plate refractory of the sliding nozzle for continuous casting by MnO in molten steel becomes severe, and remarkably degrades manufacturability. In the cross section perpendicular to the rolling direction of the steel, if the area of MnO in the steel equivalent to 0.5 µm or more in the original equivalent diameter exceeds 15% of the area of the entire Mn inclusions, the machinability and manufacturability deteriorate remarkably, thereby obtaining good machinability and manufacturability. In order to do this, MnO in steel needs to be 15% or less of all the Mn inclusions.

When MnO is 0.5 micrometer or less in circular equivalent diameter, the area ratio is extremely small, and therefore the amount of Mn consumed by MnO is also small, and thus does not significantly affect the amount of MnS produced. Therefore, it prescribed | regulated about 0.5 micrometer or more in circular equivalent diameter.

Here, the identification method of MnO and the measuring method of area which are said to this invention are demonstrated.

MnO is usually present alone in addition to MnO alone, but may be present in combination with other oxides. In the present invention, MnO is determined by MnO to determine the area.

An example of MnO measurement by EPMA is shown in FIG. 3. A test piece cut out from a depth position of 1/4 of the diameter of the cross section perpendicular to the rolling direction of the steel material, embedded in a resin and polished, was measured with an electronic probe microanalyzer (EPMA), and measured at 1 o'clock 200 μm × 200 μm at 20 o'clock or more. Do it. Since the MnO 13 in the steel base 12 of steel exists in the state contained in the sulfide 14 which has MnS as a main component, element area analysis by EPMA makes MnO the part which overlaps with MnO, and the area is made into MnO. To save.

All the Mn-based inclusions refer to all inclusions combined with Mn in steel, and are intended for all of sulfides having MnS as a main component described below, oxides of MnO alone, and oxides in which MnO and other oxides are combined. Since all the Mn inclusions can also be fixed by elemental surface analysis by EPMA, the area can be measured. Therefore, the ratio of the area of the measured MnO to the area of the total Mn inclusions thus measured is obtained.

In order to reduce MnO production amount, it is achieved by reducing the dissolved oxygen (free oxygen) concentration in molten steel before LF. It is preferable to make said dissolved oxygen (free oxygen) concentration into 200 ppm or less. However, if the reduction is too low, the desulfurization reaction proceeds between the metal and the slag, which makes it difficult to secure S in the steel for maintaining machinability, so that sufficient consideration is required and preferably 150 ppm or more. As a method for controlling dissolved oxygen (free oxygen), it is effective to perform deoxidation before LF treatment. Although addition of Ca is essential for the control of free oxygen, addition of single or complex addition of Si, Al, Ti, Zr, Mg, etc. is also effective.

[Sulfide Dispersion with MnS as a Main Component] Abundance of 0.1 to 0.5 µm in a Round Equivalent Diameter of 10000 Pieces / mm 2 or More

Sulfide containing MnS as a main component is an inclusion to improve machinability and is remarkably improved by finely dispersing it at a high density. Especially in the case of the cutting method in which the acid called transfer mark is formed on the finishing surface, such as longitudinal turning, the presence or absence of burrs greatly affects the level of the acid, that is, the finishing surface roughness. The sulfide which is a main component can make the fracture property of steel materials favorable by homogenizing steel materials, can reduce abrasion, and can improve the finish surface roughness. It is more effective for improving the finish surface roughness of parts to be cut by longitudinal turning such as shafts of OA machines. In order to exert the effect, an existing density of 10000 pieces / mm 2 or more is required, and the dimension thereof should be 0.1 to 0.5 μm in the equivalent diameter. Usually, the sulfide distribution which has MnS as a main component is observed with an optical microscope, and the dimension and density are measured. The sulfide containing MnS of the said dimension as a main component cannot be confirmed by observation by an optical microscope, and can only be observed by a transmission electron microscope (TEM). It is a sulfide containing MnS whose main component is a dimension whose density is clearly observed in TEM observation even though there is no difference in the dimension and density by optical microscopy. In the present invention, the present invention is controlled to quantify the present form to differentiate it from the prior art. In order to have sulfides containing MnS exceeding this dimension as a main component at a density of 10000 / mm2 or more, a large amount of S exceeding the scope of the claims is required, but a large number of sulfides containing coarse MnS as a main component are also added. The probability of existence increases, and the occurrence of marks on hot rolling increases. If the sulfide containing MnS as the main component exceeds the dimension in the amount of S added in the claims, the amount of sulfide containing MnS as the main component is insufficient, and the density necessary for improving the finish surface roughness cannot be maintained. In addition, the minimum diameter of less than 0.1㎛ does not affect the machinability substantially. Therefore, the existence density of the sulfide which has MnS of 0.1-0.5 micrometer as a main equivalent diameter as a main component was 10000 piece / mm <2> or more. The sulfide containing MnS as a main component becomes a precipitation nucleus of BN which is difficult to uniformly finely disperse in the matrix, thereby uniformly finely dispersing BN, thereby making the effect of improving the machinability of BN, in particular, the finish roughness more remarkable.

In addition, sulfides containing MnS as a main component include not only pure MnS but also inclusions in which sulfides such as Fe, Ca, Ti, Zr, Mg, and REM are dissolved or combined with MnS, and elements other than S, such as MnTe, Inclusions that form a compound with Mn and coexist by solid-solution-coupling with MnS, or the inclusions in which oxides are precipitated as nuclei, that is, in the chemical formula, (Mn, X) (S, Y) (where X: sulfides other than Mn Inclusions which can be described as a forming element, an element which combines with Mn other than Y: S), and refers to Mn sulfide type inclusions generically.

In order to obtain the size and density of the sulfide containing MnS as a main component, the ratio Mn / S of Mn and S contained is set to 1.2 to 2.8, which is more effective.

In order to more effectively produce sulfides based on fine MnS, the solidification cooling rate range may be controlled. When the cooling rate is less than 10 ° C / min, the coagulation is too slow, and the sulfide containing MnS as a main component is coarsened, and it is difficult to finely disperse. When the cooling rate is higher than 100 ° C / min, the produced fine MnS is used as the main component. The density of sulfides is saturated and the hardness of the slabs is increased, increasing the risk of cracking. Therefore, the cooling rate at the time of casting is 10-100 degreeC / min is good. In order to obtain this cooling rate, it can be easily obtained by controlling the size of the mold cross section, the injection speed and the like to an appropriate value. This is applicable to both the continuous casting method and the ingot method.

As shown in Fig. 8, the solidification cooling rate refers to 1/4 of the thickness L of the casting piece in the cross section 17 of the casting piece 16 cast in the casting direction 15 indicated by the arrow. It refers to the speed at the time of cooling from liquidus temperature to solidus temperature in depth position 18 (refer FIG.8 (b)). The cooling rate is calculated | required by calculation by the following formula from the space | interval of the secondary dendrite arm of the casting piece thickness direction solidification structure after solidification.

Where Rc is the cooling rate (° C./min), and λ2 is the spacing of the secondary dendrite arms (μm).

That is, since secondary dendrite arm spacing changes with cooling conditions, the cooling rate controlled by this was confirmed.

Next, the reason for the definition of the optional addition optional element will be described.

[Steel reinforcement element]

[V] 0.05 to 1.0%

V forms carbonitride and can strengthen steel by secondary precipitation hardening. If it is less than 0.05%, it is ineffective for a high strength, and when it adds exceeding 1.0%, many carbonitrides will precipitate and rather it will damage mechanical property, and made it the upper limit.

[Nb] 0.005 to 0.2%

Nb also forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.005%, it is ineffective in high strength, and when it adds exceeding 0.2%, many carbonitrides will precipitate and rather it will damage mechanical property, and made it the upper limit.

[Cr] 0.01 to 2.0%

Cr is an element which improves hardenability and imparts tempering softening resistance. Therefore, it is added to the steel which needs high strength. In that case, 0.01% or more of addition is required. However, when it adds in large quantities, Cr carbide produces | generates and embrittles, and made 2.0% an upper limit.

[Mo] 0.05 to 1.0%

Mo is an element which gives a temper softening resistance and improves hardenability. If it is less than 0.05%, the effect is not confirmed and even if it adds exceeding 1.0%, since the effect is saturated, 0.05 to 1.0% was made into the addition range.

[W] 0.05-1.0%

W forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on high strength, and when it adds exceeding 1.0%, many carbonitrides will precipitate and rather damage mechanical property, and this was made into an upper limit.

[Ni] 0.05 to 2.0%

Ni is also effective in improving quenching and ductility by strengthening ferrite and improving hardenability and corrosion resistance. If it is less than 0.05%, the effect is not recognized but even if it adds exceeding 2.0%, since an effect is saturated in terms of mechanical property, this was made into an upper limit.

[Cu] 0.01 to 2.0%

Cu is also effective in improving the hardenability and improving the corrosion resistance by reinforcing ferrite. If it is less than 0.01%, the effect is not recognized but even if it adds more than 2.0%, since an effect is saturated in terms of mechanical property, this was made into an upper limit. In particular, since hot ductility is lowered and it is easy to cause marks during rolling, it is preferable to add Ni at the same time.

[Machinability Enhancement Element by Embrittlement]

[Sn] 0.005 to 2.0%

Sn embrittles the ferrite and extends tool life and is effective in improving surface roughness. If it is less than 0.005%, the effect is not recognized but even if it adds exceeding 2.0%, since the effect is saturated, this was made into an upper limit.

[Zn] 0.0005 to 0.5%

Zn embrittles ferrite, extending tool life and improving surface roughness. If it is less than 0.0005%, the effect is not recognized but even if it adds exceeding 0.5%, since the effect is saturated, this was made into an upper limit.

[Machinability Improvement Element by Deoxidation Adjustment]

[Ti] 0.0005 to 0.1%

Ti is a deoxidation element, and the amount of oxygen in the steel can be controlled to stabilize the yield of Mn and B, which tend to form oxides. In addition, when the amount is small, soft oxides are produced to improve machinability. If it is less than 0.0005%, there is no effect at all, At 0.1% or more, a large amount of hard oxide is produced | generated in large quantities, and Ti which solidifies without forming oxide combines with N, forms hard TiN, and 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, 0.01% or less of Ti addition amount is preferable.

[Zr] 0.0005 to 0.1%

Zr is a deoxidation element and can control the amount of oxygen in the steel, and can stabilize the yield of Mn and B, which tend to form oxides. Moreover, when it is a trace amount, it has a function which produces | generates a soft oxide and improves machinability. If it is less than 0.0005%, there is no effect at all, At 0.1% or more, a large amount of soft oxide is produced, and it adheres to the tool edge with unevenness, and as a result, finishing surface roughness becomes very bad and hard oxide is produced in large quantities, and machinability Further lowers. Therefore, the component range was defined as 0.0005 to 0.1%.

[Mg] 0.0003 to 0.005%

Mg is a deoxidation element, can control the amount of oxygen in steel, and can stabilize the yield of Mn and B which are easy to form an oxide. In addition, when the amount is small, soft oxides are formed to improve machinability. If it is less than 0.0003%, there is no effect at all, and if it is 0.005% or more, a large amount of soft oxide is produced, and it adheres to the tool edge with unevenness, and as a result, not only the surface roughness becomes extremely bad but also a large amount of hard oxide is produced. The machinability is further lowered. Therefore, the component range was defined as 0.0003 to 0.005%.

[Machinability Enhancement Elements by Sulfide Form Control and Lubrication between Tool and Steel]

[Te] Te: 0.0003 to 0.2%

Te is a machinability improving element. In addition, by producing MnTe or coexisting with MnS, there is an action of decreasing the deformation ability of MnS to control the stretching of the MnS shape. Therefore, it is an element effective for reducing anisotropy. This effect is not confirmed at less than 0.0003%, and if it is more than 0.2%, the effect is not only saturated, but hot ductility is lowered, which is likely to cause marks.

[Bi] 0.005 to 0.5%

Bi is a machinability improving element. The effect is not confirmed at less than 0.005%, and the addition of more than 0.5% not only saturates the machinability improving effect, but also decreases hot ductility and is likely to cause marks.

[Pb] 0.005 to 0.5%

Pb is a machinability improving element. The effect is not confirmed at less than 0.005%, and addition of more than 0.5% not only saturates the machinability improving effect, but also decreases hot ductility, which is likely to cause marks.

<Examples>

The effect of this invention is demonstrated by an Example. The steel of the invention example of Example 1-Example 72 shown in Table 1-Table 4 was cast so that solidification cooling rate might be 4-18 degreeC / min after a solvent in the 270t converter. Among these, the solidification cooling rate of the steel grade of the first embodiment of the first to eighth embodiments and the steel grade of the sixth embodiment of the sixty-second to seventy-eighth embodiments is 1 to 7 ° C / min and the ninth to 61st embodiments. The solidification cooling rate of the steel grades of Claims 2 to 6 was classified and cast so as to be 12 to 85 ° C / min. The steel of the comparative example of Examples 73-102 shown in Tables 5-6 was cast so that solidification cooling rate might be 4-7 degreeC / min after a solvent in the 270t converter. Inventive example and comparative example, the 270t converter was rolled by (phi) 9.5 after the pressure of ingot by the billet. This φ9.5 mm rolled material was drawn to φ 8 mm to obtain a test material. Tensile test pieces were taken from a billet and a rectangular cast material of 180 mm on one side before rolling for hot ductility evaluation. In addition, adjustment of the solidification cooling rate was performed by control of the size of a mold cross section, and the injection speed.

The machinability of the material was evaluated by three typical cutting methods: a drill drilling test showing conditions in Table 7, a flange cutting test showing conditions in Table 8, and a longitudinal turning test showing conditions in Table 9. The drill drilling test is a method of evaluating machinability at the highest cutting speed (so-called VL1000, unit: m / min) capable of cutting up to a accumulated hole depth of 1000 mm. The flange cutting test is a method of transferring a tool shape (constituent blade shape) by a high speed steel grooving tool to evaluate the finish surface roughness. The outline of this experimental method is shown in FIG. In the experiment, the finish surface roughness in the case of 200 grooving was measured by a needle contact roughness measuring instrument. It was set as the index which shows the finish surface roughness of ten-point surface roughness Rz (unit: micrometer). Longitudinal turning test is a cutting method in which the steel outer periphery of the test piece 2 is cut in the cutting direction 3 while sending the carbide tool 1 in the longitudinal direction. It is a method of measuring and evaluating the finishing surface roughness of (4) repeatedly. The outline of this experimental method is shown in FIG. This method sends the carbide tool 1 along the test piece 2 (0.05 mm / rev) while rotating the test piece 2, and cuts (cutting speed 80 m / min) by the predetermined cutting amount 6 (1 mm). It is a cutting method of advancing while forming the acid called the transfer mark 5 on the finishing surface 7 to form the surface roughness measuring surface 8, and the presence or absence of deterioration 9 due to the burr becomes the level of the acid It becomes the roughness (theoretical roughness + grudge) 10 of the surface which generate | occur | produced. That is, it becomes a finish surface roughness and greatly affects the roughness (theoretical roughness) 11 of a favorable surface (refer FIG. 2 (b)). If there is no grind, the value becomes close to the theoretical roughness, but if grind occurs, the roughness is reduced (deteriorated) by that amount. Since sulfides containing MnS as a main component finely dispersed at high density can reduce the rubbing by homogenizing steels to improve the finish surface roughness, a method capable of remarkably exhibiting the effect of sulfides containing MnS as a main component dispersed at high density to be. In addition, this method can also remarkably show a poor finish surface roughness due to transfer of tool irregularities due to tool wear after a large amount of cutting. It evaluated as finishing surface roughness. The finish surface roughness was measured by the needle contact roughness measuring instrument, and 10-point surface roughness Rz (unit: micrometer) was made into the index which shows the finish surface roughness. Regarding the cutting chip processability, it is preferable that the radius at the time of cutting chip curl is small or divided, and is indicated by ○. Even if the number of turns was large, it was good that the radius of curvature was small or the length of the cutting chip did not reach 100 mm even if the radius of curvature was large. Cutting chips made by curling continuously for three or more turns with a radius of curvature exceeding 20 mm were made defective and marked with x.

The measurement of the area ratio of 0.5 m or more in a circular equivalence in the cross section perpendicular to the rolling direction of the steel with respect to MnO in the steel is cut out from the depth position of 1/4 of the diameter of the cross section perpendicular to the rolling and extension line direction after φ8 mm extension. And the test piece which was embedded in the resin and polished was carried out by an electron probe fine analyzer (EPMA). The measurement was carried out at 1 o'clock 200 micrometers x 200 micrometers at 20 o'clock or more, and the area ratio was calculated | required from the MnO area in the interference | inclusion measured by element surface analysis as a ratio with respect to the whole Mn type interference | inclusion area. Since MnO in the steel is present in the state contained in MnS, the area where Mn and O overlap by MnO was identified as MnS by analysis by EPMA. The overlap of Mn and O was performed by image processing. An example of MnO measurement by EPMA is shown in FIG. 3.

The measurement of the sulfide density mainly composed of MnS having the largest diameter of 0.5 µm and the minimum diameter of 0.1 µm with a circular equivalent diameter is carried out from a depth position of 1/4 of the diameter of the cross section perpendicular to the rolling and stretching direction after φ8 mm drawing. It collected by the method and performed with the transmission electron microscope. The measurement was performed at 1 000 times 80 micrometers ² or more at 40 000 times, and it calculated and converted it into the sulfide number which has MnS per 1 mm <^2> as a main component.

Hot ductility was evaluated by the value of the drawing of the high temperature tensile test at 1000 degreeC. If the drawing is 50% or more, good rolling is possible, but if it is less than 80%, the surface marks are frequently bunched, and the mark removal area after rolling becomes large and cannot be applied to the high-quality varieties with strict surface properties. When a value of 80% or more of the drawing is obtained, the occurrence of surface marks is significantly reduced, so that it can be used in a clean state, and can be applied to high-quality varieties. It can also reduce the cost of care. Therefore, hot ductility was made into (circle) and 80% or more of drawing, and less than 80% was made into x.

Melting conditions of the plate refractories of continuous casting sliding nozzles, and as the material of the sliding nozzle plate using MgO-C quality _{(Mg0 = 87%, Al 2} O 3 = 10%, C = 3%) were evaluated for melting ratio . The melting loss ratio is a value obtained by quantifying the melting loss ratio by setting the melting loss ratio of the refractory to 1 when the area of MnO of 0.5 µm or more is 15% with respect to the area of all Mn inclusions. When the melting loss ratio exceeded 1, the refractory melting loss became severe. Therefore, when the melting loss ratio was 1 or less, it was evaluated as (circle) and over 1, x. The invention examples of the first to the seventy-eighth embodiments all have good finish surface roughness in drill tool life, flange cutting, and longitudinal turning, and hot ductility in comparison with the comparative examples of the seventy-third to thirty-second embodiments. Good manufacturability of a value of 80% or more and a low melting loss ratio could be obtained. For example, as in the invention examples of the first to eighth embodiments, the machinability deteriorates when the amount of N is controlled by the balanced amounts of B and N and the amount of MnO is low due to the amount of O controlled by Ca addition. Higher hot ductility values and lower melt loss ratios could be obtained. In addition, very good machinability was obtained by the addition amount with the balance of B and N and the low MnO area ratio. As in the ninth to eighteenth and fifty-fifth to fifty-ninth embodiments, when the sulfide density composed of fine MnS as a main component satisfies claim 2, the finish surface roughness, in particular in the longitudinal turning, Better. Also in the case where the optional addition elements of Claims 3 to 6 of the 19th to 55th Examples and the 60th to 72th Examples were added, it can be seen that good finish surface roughness and manufacturability are obtained. Among them, the forty-ninth, fifty-second, sixty-fifth, and sixty-fifth embodiments in which trace amounts of Pb as a free cutting element were added, and the forty-fifth embodiment in which trace amounts of Te, also known as free cutting element, were added. For example, forty-eighth, fifty, fifty-fifth, fifty-third, fifty-fifth, fifty-fifth, fifty-fifth, fifty-fifth, fifty-fifth, fifty-fifth, fifty-fifth, fifty-fifth, fifty-fifth, or fifty-fifth; It can be seen that also in the examples to the seventy-eighth, good hot ductility and machinability are obtained.

On the other hand, since all of the comparative examples are cast at a small solidification cooling rate, sulfide densities mainly composed of fine MnS have become smaller, and overall machinability, in particular, finish surface roughness due to longitudinal turning shows poor values. The inventive example of claim 1 of the first to eighth examples cast at the same level of small solidification cooling rate also exhibits a bad value because the chemical component is outside the scope of the present invention. For example, when the MnO area ratio is high as in the comparative example of Example 76, the finish surface roughness becomes a bad value due to the decrease of the MnS amount and the BN amount, and the melting loss ratio is a large value. In the comparative example of the eighty-eighth embodiment, the MnO area ratio was 15% or less, but the amount of S and Ca was out of order, resulting in poor hot ductility. In the case of no Ca addition, as in the comparative example of the 81st example, O was not able to be controlled, and exhibited poor manufacturability with hot ductility of less than 80% and large melt loss ratio due to a large amount of MnO or hard oxide produced. The ninety-ninth and ninety-ninth examples are comparative examples in which the amount of N deviated from the lower limit, but the hardness was increased due to an increase in the solid solution B, and the hot ductility was low. Also, Example 93 is a comparative example in which the amounts of S and N deviated from the upper limit, and the decrease in hot ductility due to the increase in the solid solution N indicates a bad value. Example 102 is a comparative example with a high MnO, and shows poor values in both finish surface roughness and melt loss index.

4 shows (a) TEM replica photographs and (b) optical micrographs of sulfides containing MnS of the present invention as main components. The (a) TEM replica photograph and (b) optical micrograph of the sulfide which has MnS of a comparative example as a main component in FIG. 5 are shown. Thus, in the invention example and the comparative example, the difference in size and density of sulfide mainly containing MnS without major difference in the observation by the optical microscope of (b) and the dimension and density are observed in the observation of the TEM replica of (a).

The change of machinability by MnO area ratio is shown in FIG. 6 as an example of the finishing surface roughness by longitudinal turning after 800 cutting. Since the progress of tool wear during heavy cutting becomes remarkable at an MnO area ratio of> 15%, the superiority of the finish surface roughness, which is influenced by the transfer of irregularities due to tool wear, is remarkably shown as a boundary.

Fig. 7 shows the finish surface roughness-hot ductility balance by longitudinal turning in the invention example and the comparative example. Invention example exists in the favorable area | region which finish surface roughness is favorable and hot ductility is 80% or more. In the comparative example, both the finish surface roughness and the hot ductility are in poor regions, or the finish surface roughness is poor even though the hot ductility is good.

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

According to the present invention, free-cutting steel is excellent in the machinability of the tool life at the time of cutting, the finish surface roughness and the cutting chip processing property, and the manufacturability which is good in ductility by hot rolling because there is little melting of the refractory plate of the sliding nozzle for continuous casting. Can be provided.

Claims (3)

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% Al: more than 0% and 0.01% or less Containing N content, In the cross section which satisfies N≥0.0020% and 1.3xB-0.0100≤N≤1.3xB + 0.0034, remainder consists of Fe and an unavoidable impurity, and is perpendicular to the rolling direction of steel materials with respect to MnO in steel. A free-cutting steel excellent in manufacturability, wherein an area of MnO having a circular equivalent diameter of 0.5 µm or more is more than 0% and 15% or less with respect to the area of all Mn-based inclusions. The sulfide containing MnS has a presence density of 0.1 to 0.5 µm in a circular equivalent diameter in a cross section perpendicular to the rolling direction of the steel material, wherein the presence density is 10000 pieces / mm 2 or more. Excellent free cutting steel. The mass% according to claim 1 or 2, V: 0.05 to 1.0% Nb: 0.005 to 0.2% Cr: 0.01 to 2.0% Mo: 0.05-1.0% W: 0.05 to 1.0% Ni: 0.05 to 2.0% Cu: 0.01 to 2.0% Sn: 0.005 to 2.0% Zn: 0.0005 to 0.5% Ti: 0.0005 to 0.1% Zr: 0.0005 to 0.1% Mg: 0.0003 to 0.005% Te: 0.0003 to 0.2% Bi: 0.005 to 0.5% Pb: 0.005 to 0.5% A free cutting steel excellent in manufacturability, further comprising one kind or two or more kinds thereof.

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