CN1436875A - Low-carbon free-cutting steel - Google Patents

Abstract

A low-carbon sulfur free-cutting steel that does not contain lead is provided, which contains, in mass %, C: 0.05-0.19%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.03-0.30%, and Si: Below 1.0, P: 0.001-0.3%, Al: below 0.2%, O: 0.0010-0.050% and N: 0.0001-0.0200%, the rest is Fe and impurities, and the content of Ti and S satisfies the following formula (1), The atomic ratio of Mn/S satisfies the following formula (2), and the steel contains MnS in which Ti sulfide and/or Ti carbosulfide exists, Ti (mass %)/S (mass %)<1 (1) , Mn/S≥1 (2), in addition to the above-mentioned components, it may also contain more than one of the following two groups of elements. Se, Te, Bi, Sn, Zr, Ca, Mg, and rare earth element groups, and Cu, Ni, Cr, Mo, V, and Nb groups.

Description

Low carbon free cutting steel

field of invention

The present invention relates to a low-carbon free-cutting steel that does not contain Pb but has excellent cuttability and hot workability of conventional lead-combined free-cutting steels and composite free-cutting steels of lead and other free-cutting elements.

Background technique

In the past, for small soft objects that do not require strength, in order to improve production efficiency, steel with excellent cutting properties, so-called easy-cut steel, was used. The most well-known free-cutting steels are sulfur free-cutting steels with a large amount of S added, the cutting properties improved by MnS, lead free-cutting steels with Pb added, and composite free-cutting steels containing both S and Pb. In particular, free-cutting steel containing Pb has excellent machinability and prolongs tool life. There are also free-cutting steels containing Te (tellurium) and Bi (bismuth) to improve cuttability. Mainly automotive parts and mechanical parts around personal computers, these are widely used in various mechanical parts such as electrical equipment parts and molds.

In recent years, since the performance of cutting machines has been improved, it has been possible to increase the speed of cutting operations. At the same time, it is also desired to improve the machinability of steel materials forming the above-mentioned component materials during high-speed cutting.

As the machinability of steel materials, the machinability for prolonging the tool life and the splitting property of chips, that is, chip handling properties are regarded as important. In the automation of processing lines, it cannot be ignored. In order to improve production efficiency, it is necessary to pay attention to the handling of chips.

Lead free-cutting steel and composite free-cutting steel combined with lead and other machinability-improving elements have the best above-mentioned machinability. However, Pb-containing steel requires considerable exhaust equipment during its manufacturing process. In order to protect the environment and strengthen the control of Pb use, a free-cutting steel that improves work efficiency and does not contain Pb is strongly desired.

In order to meet the above requirements, a technical solution to improve the machinability is proposed. As a substitute for lead free-cutting steel, the content of S is increased in low-carbon sulfur free-cutting steel, and the amount of MnS in the steel is increased. However, an increase in the sulfur content leads to deterioration of the hot workability of the steel. Even high-S free-cutting steel lacks the effect of prolonging tool life when the cutting speed reaches 150m/min or more in high-speed cutting, and cannot obtain the machinability comparable to lead free-cutting steel.

JP-A-2000-319753 discloses a low-carbon sulfur-based free-cutting steel containing more than 0.4% of S and increasing the amount of MnS without adding Pb. It is considered that such steel improves the tool life to some extent, but the effect is small in high-speed cutting. In addition, this steel has not been improved in chip disposability, which is regarded as an important factor of machinability along with tool life, and the performance of conventional sulfur free-cutting steels has not been greatly changed.

Japanese Unexamined Patent Publication No. 50-20917 discloses a steel containing 0.5% or less of C, 0.3-0.75% of S, and 0.1-0.5% of Ti, and a sulfur free-cutting steel in which the amount of Ti does not exceed the amount of S. This steel mainly utilizes iron sulfide, in which Ti is added to make solid solution of Ti and Mn in iron sulfide to improve machinability. However, the C content of this steel is 0.24% or more as described in Examples. In the same gazette, there is no description at all of achieving better machinability by controlling the composition of sulfides in low carbon steel having a C content of 0.19% or less. Iron sulfide in which an appropriate amount of Ti and Mn are solid-dissolved can improve machinability, but it does not have sufficient machinability compared with low-carbon free-cutting steel and composite free-cutting steel of the invention steel described below. Furthermore, the steel disclosed in the above-mentioned gazette is difficult to control the composition of iron sulfide, and sufficient hot workability cannot be obtained, so it is difficult to manufacture it by continuous casting equipment, and lacks practicality.

Japanese Patent Application Publication No. 09-53147 discloses a free-cutting steel with excellent machinability for superhard tools, containing C: 0.01-0.2%, Si: 0.10-0.60%, Mn: 0.5-1.75%, P: 0.005-0.15%, S: 0.15-0.40%, O: 0.001-0.010%, Ti: 0.0005-0.20%, N: 0.003-0.03%. According to this composition range, a certain degree of improvement in tool life can be obtained. However, since the upper limit of the amount of Ti is seldom 0.02%, not only sufficient tool life cannot be obtained, but also excellent chip disposal properties important for tool life cannot be ensured.

JP-A-2001-107182, JP-A-2001-152281, JP-152282, and JP-152283 each disclose a steel containing C: less than 0.05%, Mn: 0.1-4.0%, S: 0.15-0.5%, Cr: less than 0.5%, Ti: 0.003-0.3%, B: 0.0003-0.004%. In this steel, since B is segregated around the sulfide, the chip disposability is improved, and at the same time, the machinability is improved by making C less than 0.05%. However, if C is less than 0.05%, chip cracking occurs during cutting, the finished surface deteriorates, and sufficient machinability cannot be obtained.

JP-A-2001-294976 discloses a free-cutting steel containing C: 0.02-0.15%, Mn: 0.3-1.8%, S: 0.2-0.5%, and further contains Ti: 0.1-0.6% and Zr: 0.1 At least one of -0.6%, and Ti+Zr is 0.3-0.6%, (Ti+Zr)/S is 1.1-1.5. This steel, with the above composition, produces Ti and Zr sulfides with high deformation resistance during hot working, and improves mechanical anisotropy and machinability. However, for sulfides with high deformation resistance, it is difficult to obtain the approximate lubrication effect formed by sulfides during cutting, the cutting resistance increases, and the effect of improving machinability is limited.

Contents of the invention

The subject of the present invention is to provide a carbon-substituting free-cutting steel that does not contain lead and has a machinability higher than that of the current leaded free-cutting steel and the composite added free-cutting steel containing lead and improving machinability elements, and the thermal Processability is also excellent.

In order to improve the machinability of low-carbon sulfur free-cutting steel that does not actually contain Pb, the present inventors studied in detail the relationship between the form of the existing substance due to the addition of Ti and the machinability, and obtained the following new insights.

(1) The ideal C content is 0.05-0.19%.

(2) The atomic ratio of Mn and S contained in the steel with the above-mentioned C content satisfies the condition of Mn/S ≥ 1, and, within the range not exceeding the S content (mass %), when Ti is contained, most of the sulfidation compounds, which are neither Ti sulfides nor iron sulfides, but form MnS.

(3) In the composition defined in (2) above, Ti hardly dissolves in MnS, and Mn·Ti sulfide, that is, (Mn,Ti)S does not form. Thus, Ti sulfide and Ti carbide exist as separate phases with MnS. Most of such Ti-based inclusions (sulfides, carbides) exist in the form of inclusions in MnS.

(4) In the form of (3) above, a steel material containing MnS and Ti-based inclusions exhibits excellent machinability in high-speed cutting. That is, for example, when rotary cutting is performed at a high speed of 100 m/min or more, MnS adheres to the tool surface and simultaneously forms TiN in the form of a hard layer. Compared with JIS SUM 22L-24L composite free-cutting steel, which has the best machinability so far, this TiN-protected tool can obtain extremely excellent tool life. In addition, by adding Ti within the above-mentioned predetermined range, fine sulfides are generated and the number of them increases. These sulfides form a source of stress concentration during cutting and promote the propagation of cracks. Therefore, compared with current sulfur free-cutting steels and composite free-cutting steels with Pb, excellent chip handling properties are obtained at the same time. Furthermore, since this steel has no problem in terms of hot workability at all, it does not cause any trouble when it is produced by continuous casting equipment and the like, and it is practical.

Based on the above knowledge, the present invention has studied in detail the action and effect of components other than the above alloy components, and the gist is the following (1)-(4) free-cutting steel.

(1) A low-carbon sulfur free-cutting steel, characterized by mass %, containing C: 0.05-0.19%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.03-0.30%, Si : less than 1.0, P: 0.001-0.3%, Al: less than 0.2%, O: 0.0010-0.050% and N: 0.0001-0.0200%, the rest is Fe and impurities, formed, the content of Ti and S satisfies the following (1 ) formula, the atomic ratio of Mn and S satisfies the following formula (2), and the steel contains MnS in which Ti sulfide and/or Ti carbosulfide exist.

 Ti(mass%)/S(mass%)＜1……………(1)

Mn/S≥1……………(2)

(2) A low-carbon sulfur free-cutting steel characterized by containing, in addition to the ingredients described in (1) above, one or more kinds selected from the group consisting of Se: 0.001-0.01%, Te : 0.001-0.01%, Bi: 0.005-0.3%, Sn: 0.005-0.3%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, and rare earth elements: 0.0005-0.01%, and satisfy the above formula (1) And (2) type.

(3) A low-carbon sulfur free-cutting steel characterized by containing, in addition to the components described in (1) above, one or more of the following, namely, Cu: 0.01-1.0%, Ni: 0.01 -2.0%, Cr: 0.01-2.5%, Mo: 0.01-1.0%, V: 0.005-0.5%, and Nb: 0.005-0.1%, and satisfy the above formulas (1) and (2).

(4) A low-carbon sulfur free-cutting steel characterized by containing, in addition to the ingredients described in (1) above, one or more kinds selected from the following, namely, Se: 0.001-0.01%, Te : 0.001-0.01%, Bi: 0.005-0.3%, Sn: 0.005-0.3%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, and rare earth elements: 0.0005-0.01%, further containing selected from the following 1 or more, namely, Cu: 0.01-1.0%, Ni: 0.01-2.0%, Cr: 0.01-2.5%, Mo: 0.01-1.0%, V: 0.005-0.5%, and Nb: 0.005-0.1% , and satisfy the above formulas (1) and (2).

In the free-cutting steels (1)-(4) above, the Si content is preferably less than 0.1% by mass.

Description of drawings

Fig. 1 is a schematic diagram of EPMA analysis results for observing MnS with Ti sulfide and/or Ti carbosulfide inside the steel of the present invention.

Fig. 2 is a schematic diagram showing the relationship between chip disposal and tool life of steels of the present invention (steel Nos. 1-29) and comparative steels (steel Nos. 30-47).

Fig. 3 is a schematic diagram showing the relationship between the reduction of area and the tool life in the thermal tensile test of steels of the present invention (steel No. 1-29) and comparative steels (steel No. 30-47).

Detailed ways

1. Regarding MnS with Ti sulfide and/or Ti carbosulfide inside

One of the greatest features of the free-cutting steel of the present invention is that it contains "MnS with Ti sulfide and/or Ti carbosulfide inside".

Ti can be dissolved in a small amount in MnS and exists as (Mn, Si)S, but since the amount of Ti dissolved in MnS is extremely small, this sulfide is actually MnS. However, it is clear that its composition is different from such MnS, and Ti sulfide or Ti carbosulfide represented by the following specific formula of TiS or Ti ₄ C ₂ S ₂ exists. The presence of most of these in MnS is a clear phase separation from MnS formation.

Surface analysis and quantitative analysis of micro test pieces cut from steel materials using EPMA (Electron Micrometer) and EDX (Energy Dispersive X-ray Analyzer) will confirm the presence of vulcanization in the above form with greater certainty. things.

Fig. 1 is the result of surface analysis of sulfides in No. 3 steel in Table 1 below by EPAM. (a) shows one internal substance, and (b)-(d) show that Ti, Mn, and S exist in the internal substance.

As is clear from these figures, Ti sulfide or Ti carbon sulfide exists in the vicinity of the periphery of one sulfide, exists in a form surrounding MnS, and exists in various forms. The Ti sulfide and/or Ti carbon sulfide that exists simultaneously with such a MnS form a phase separation, and the area ratio occupied by MnS in one sulfide is more than 50%, such a sulfide is defined in the present invention It is "MnS with Ti sulfide and/or Ti carbosulfide inside".

The composition and area ratio of Ti sulfide and Ti carbosulfide contained in one piece of MnS can be confirmed by the above-mentioned EPMA or EDX, and "MnS containing Ti sulfide and/or Ti carbosulfide inside" in steel is also It can be confirmed by the same method, and its number can also be measured. When the number measured in multiple visible regions is converted into the number per 1 mm ² , if the average value is 10 or more per 1 mm ² , excellent machinability is obtained.

When cutting steel containing MnS with Ti sulfide and/or Ti carbon sulfide inside, the soft MnS forms a similar lubrication effect between the contact surface of the cut material and the tool, and forms TiN on the tool surface, while protecting tool. That is to say, during cutting, MnS, and Ti sulfide or Ti carbon sulfide are attached to the surface of the tool in contact with the material to be cut. Furthermore, the temperature rises due to friction during cutting, and these Ti-based sulfides interact with N in the environment. Nitrogen reacts to form hard TiN in layers ranging in thickness from several μm to tens of μm. After cutting, the presence of TiN can be confirmed by surface analysis and point analysis using AES (Auger Electron Spectroscopy) and EPMA on the tool surface from which carbon-based contamination (oil, etc.) has been removed by Ar gas jet or the like.

When researched by the above method, the surface area of the existing layered TiN film is roughly 10-80% of the contact area between the material to be cut and the tool, and the rest is attached to MnS and Fe, or the original tool surface without attachments. In this way, the hard TiN film formed on the working surface has a great effect on protecting the tool, improving the wear resistance of the tool and prolonging its service life. The effect of improving tool life is much greater than that of sulfur free-cutting steel and Pb-containing composite free-cutting steel.

In the steel of the present invention, in addition to "MnS in which Ti sulfide and/or Ti carbosulfide exists inside", MnS, Ti sulfide, and Ti carbosulfide exist as fine internal substances. That is, the number of all the inclusions is quite large, and these function as stress concentration points in the chips generated during cutting, and promote the propagation of cracks, so the splittability of chips is also improved.

By adjusting the composition of the steel as described above, "MnS in which Ti sulfide and/or Ti carbosulfide exists" can exist in the steel. For the stable existence of such MnS, it is preferable to heat to a relatively high temperature of 1000° C. or higher after casting, and to forge with sufficient maintenance, or to apply so-called normalizing heat treatment at the same temperature.

2. Reasons for limiting chemical composition

The reasons for limiting the chemical composition in the present invention will be described below. % with respect to component content means mass %.

C: 0.05-0.19%

C is an important element that greatly affects the machinability of steel. In steel applications where machinability is important, if the C content exceeds 0.19%, the strength of the steel increases and the machinability deteriorates. However, when the C content is lower than 0.05%, the steel material becomes excessively soft and cracks occur during cutting, which accelerates tool wear on the contrary and deteriorates chip handling properties. Therefore, C is limited to the range of 0.05-0.19%. Furthermore, in order to obtain good machinability, the more suitable range of C content is 0.05-0.17%.

Mn: 0.40-2.0%

Mn and S form sulfide-based inclusions and are important elements that greatly affect machinability. When it is less than 0.40%, the absolute amount of sulfide is insufficient, and satisfactory machinability cannot be obtained. On the other hand, when it exceeds 2.0%, the cutting resistance increases due to the increase in the strength of the steel material, and the tool life decreases. Furthermore, in order to reduce cutting resistance, improve tool life, improve chip disposability, and improve hot workability, the relationship with the amount of S is also important. That is, the amount must maintain the relationship of Mn/S≧1 in atomic ratio. In order to securely obtain these properties, the Mn content is preferably 0.6-1.8%.

S: 0.21-1.0%

S and Mn or Ti form sulfide or carbon sulfide, which is an effective element that must be added to improve machinability. In particular, the effect of improving the machinability by MnS increases with the amount of its generation. However, if it is less than 0.21%, a sufficient amount of sulfide-based inclusions cannot be obtained, and satisfactory machinability cannot be obtained. Generally, when the S content exceeds 0.35%, the hot workability of the steel deteriorates, S segregation occurs at the center of the steel block, and fracture is induced during forging. However, if the composition defined in the present invention is maintained, the upper limit of the S content may be as high as 1.0% in order to eliminate such hazards. Considering the pass rate at the time of manufacture, the optimum upper limit of the S content is 0.70%.

Ti: 0.03-0.30%

Ti and S or C form Ti sulfide or Ti carbosulfide, which exist in MnS in an intrinsic form, which can improve the machinability and hot workability of steel. Therefore, it is an important element required in the steel of the present invention. Compared with Mn, Ti is also a powerful element for forming sulfide. If the content is more than 0.03%, Ti sulfide or Ti carbosulfide is formed. Since it exists in MnS in an internal form, the effect of improving machinability can be fully obtained. . If it is less than 0.03%, the effect is insufficient. On the other hand, when Ti exceeds 0.30%, sulfides are formed, a large amount of hard Ti sulfides or Ti carbosulfides are formed, the cutting resistance becomes high, and the machinability deteriorates. The optimum upper limit of the Ti content is 0.10%.

Si: 1.0% or less

Si is used as a deoxidizing element to adjust the amount of oxygen in steel. However, when its content exceeds 1.0%, the hot workability of the steel deteriorates because the solid solution strengthens the steel oxide body, the cutting resistance increases, and the machinability is impaired. Therefore, the upper limit of the Si content is taken as 1.0%, but it is more preferably controlled to be less than 0.1%. For deoxidation, the Si content is preferably 0.001% or more, actually 0 (zero)%. With the following addition of Al, etc., the oxygen content in the steel is adjusted within an appropriate range, and the machinability will not deteriorate.

P: 0.001-0.3%

When P exceeds 0.3%, the segregation of the steel block will be promoted, and the hot workability will be deteriorated. Therefore, the upper limit content is 0.3%. On the other hand, P is an element having an effect of improving machinability, and in order to obtain this effect, the lower limit is made 0.001%. The optimal content of P is 0.01-0.15%.

Al: less than 0.2%

Al is used as a deoxidizing element and is contained up to 0.2%. However, oxides formed by deoxidation are hard. When the Al content exceeds 0.2%, a large amount of hard oxides are formed, resulting in deterioration of machinability. More preferably, it is less than 0.1%. In addition, when sufficient deoxidation can be achieved by the aforementioned Si, it is not necessary to add Al, and its content is actually 0%.

O (oxygen): 0.0010-0.05%

When the steel contains an appropriate amount of oxygen, the oxygen is dissolved in MnS, which can prevent the extension of MnS caused by rolling, and the anisotropy of mechanical properties is very small. Furthermore, the machinability and hot workability can be improved, and deflection of S can be effectively prevented. Therefore, the oxygen content can be above 0.0010%. However, if it exceeds 0.05%, there will be disadvantages such as damage to the refractory during melting. Therefore, the upper limit is 0.05%, and in order to obtain the above effects, the optimum range is 0.005-0.02%.

N: 0.0001-0.0200%

N forms hard nitrides with Al or Ti, and these nitrides have the effect of making crystal grains finer. This effect can be produced when the N content is 0.0001% or more. When these nitrides exist in a large amount, the machinability is deteriorated, and the wear of the cutting tool is increased. The steel of the present invention can protect the tool due to the formation of TiN on the surface of the tool during cutting, even if there is a certain amount of TiN in the steel. Nitride does not lower the machinability. However, when the amount of N exceeds 0.0200%, the effect becomes weak. In order to obtain longer tool life, it is better to be below 0.0150%. When it is desired to further extend the tool life, it is preferably 0.0100% or less.

One of the steels of the present invention is formed by, in addition to the above-mentioned components, a balance of Fe and impurities.

Still another steel of the present invention contains, in addition to the above components, one or more of the following group 1 elements and/or group 2 elements.

Group 1 elements include Se, Te, Bi, Sn, Ca, Mg and rare earth elements, which can further improve the machinability of steel. The second element includes Cu, Ni, Cr, Mo, V and Nb, which can improve the mechanical properties of steel.

Se: 0.001-0.01%, Te: 0.001-0.01%

Se and Te, together with Mn(S, Se) or Mn(S, Te), are effective elements for improving machinability. When these are less than 0.001%, respectively, the effect will not show. On the other hand, when both Se and Te exceed 0.01%, the effect becomes saturated, and it is economically uneconomical, and the hot workability is deteriorated.

Bi: 0.005-0.03%, Sn: 0.005-0.3%

Bi and Sn, as metal inclusions with a low melting point, exhibit a lubricating effect during cutting and improve machinability. Respectively above 0.005%, the effect is remarkable. However, when the content exceeds 0.3%, not only the effect becomes saturated, but also the hot workability deteriorates.

Ca: 0.0005-0.01%, Mg: 0.0005-0.01%

Ca and Mg have great affinity for S and oxygen in steel, form sulfides or oxides with these, and at the same time solid dissolve in MnS, existing as (Mn, Mg)S. These oxides serve as formation nuclei to precipitate MnS crystals, so they have an effect of suppressing the extension of MnS. In this way, Ca and Mg suppress the form of sulfide and improve the machinability, so they may be added as needed. When it is really desired to obtain this effect, Ca and Mg may each be contained in an amount of 0.0005% or more. However, beyond 0.01%, even more, the effect is saturated. Addition of Ca or Mg will also reduce the pass rate, so in order to increase the content, a large amount can be added, but from the perspective of manufacturing components, it is not ideal, so the upper limit of the content is 0.01%.

Rare earth elements: 0.0005-0.01%

Rare earth elements are a group of elements classified as lanthanides. When these elements are added, mixed rare earths or the like mainly composed of them are usually used. In the present invention, the content of rare earth elements is represented by the total content of one or more elements in the rare earth elements. Rare earth elements form sulfides or oxides together with S and oxygen, and at the same time, can control the form of sulfides and improve machinability. In order to securely obtain this effect, it is preferable to contain 0.0005% or more. However, if the content exceeds 0.01%, not only the effect becomes saturated, but also like Ca and Mg, even if a large amount is added, the yield decreases and it is not economical.

Cu: 0.01-1.0%

Cu can improve the hardenability of steel. When this effect is desired, 0.01% or more may be contained. However, when the content exceeds 1.0%, the hot workability of the steel deteriorates and the machinability also decreases.

Ni: 0.01-2.0%

Ni has the effect of increasing the strength of steel through solid solution strengthening, and also has the effect of improving hardenability and toughness. In order to surely obtain this effect, its content is preferably 0.01% or more. However, when it exceeds 2.0%, machinability will deteriorate, and hot workability will also deteriorate.

Cr: 0.01-2.5%

Cr has the effect of improving the hardenability of steel, and to obtain this effect, it is preferable to contain 0.01% or more, and if it exceeds 2.5%, the machinability will deteriorate.

Mo: 0.01-1.0%

Mo has the effect of making the steel structure finer and improving the toughness. In order to surely obtain this effect, the content is preferably 0.01% or more. However, when it exceeds 1.0%, the effect will be saturated, and the manufacturing cost of steel will increase.

V: 0.005-0.5%, Nb: 0.005-0.1%

V and Nb are precipitated as fine nitrides and carbon oxides, which can increase the strength of steel. In order to surely obtain this effect, the respective contents are preferably above 0.005%. However, when V exceeds 0.5% and Nb exceeds 0.1%, the above-mentioned effects are not only saturated, but also excessive nitrides and carbides are formed, resulting in deterioration of machinability.

3. Regarding (1) and (2) formulas

The reason why the Ti content and the S content must satisfy the formula (1) is as follows.

As mentioned above, Ti forms Ti sulfide or Ti carbosulfide with C or S. Its tendency can greatly tend to the formation of Mn sulfide. The effect of Ti, as mentioned above, is that TiN is formed on the surface of the tool during cutting by the Ti-based inclusions, thereby improving the tool life. Therefore, Ti sulfide and Ti carbosulfide are internal substances having a large hardness against deformation compared with MnS. Therefore, in a composition where the Ti content is higher than the S content, the amount of MnS produced is small, and Ti sulfide and Ti carbosulfide become the main components. During cutting, no approximation of sulfide formation can be obtained between the tool and the workpiece. Due to the effect of lubrication, the cutting resistance rises sharply. When the cutting resistance rises, not only the tool life is shortened, but also adverse phenomena such as vibration caused by the material to be cut will occur when cutting small-diameter materials.

As above-mentioned formula (1) is satisfied, that is, by adjusting "Ti (mass %)/S (mass %)" to less than 1, Ti sulfide and Ti carbosulfide will not be formed as main sulfides, The formed sulfides will be dominated by MnS. In this case, as mentioned above, when Ti sulfide and Ti carbosulfide are formed as main sulfides, there will be no problems such as an increase in cutting resistance, and the tool life and chip treatability can be improved.

The reason why the atomic ratio of Mn and S must satisfy the formula (2) is as follows.

S is an element that induces cracks during hot forging. In terms of atomic ratio, the composition of Mn/S≥1 is maintained. S will be crystallized as Mn sulfide and will not have a bad influence on hot workability.

Even if Mn/S is less than 1, if the content of Ti and S is adjusted so that the above formula (1) is not satisfied, Ti-based sulfides can still be formed to improve hot workability. However, in this case, as described above, problems such as increased cutting resistance and shortened tool life may occur. Furthermore, Mn/S is less than 1, and when Ti is contained within the range not exceeding the S content, that is, when the composition satisfies the above formula (1) and does not satisfy the formula (2), the main content of the content is MnS and TiS A large amount of sulfides of FeS are solid-dissolved. Since a large amount of FeS is dissolved in these sulfides, the hot workability of steel deteriorates, and it is difficult to control the operating conditions when manufacturing by continuous casting or the like.

Example

Use a high-frequency induction furnace to melt the test steel with the composition shown in Table 1 and Table 2, and make a steel block with a diameter of 220 mm and a weight of 150 kg. In order to stably generate "MnS with Ti sulfide and/or Ti carbon sulfide inside", these ingots are heated to a high temperature of 1200°C and kept for more than 2 hours, and then refined and forged at a temperature above 1000°C, after air cooling (AC) A round bar with a diameter of 65 mm was obtained. This round bar was held at 950° C. for 1 hour, and normalized by air cooling (AC). Table 1 Steel No. Chemical Composition (Mass%) Remaining Fe and Impurities Mn/S (atomic ratio) Ti/S (mass% ratio) C Si mn P S Al Ti N o other 1 0.08 - 0.96 0.017 0.33 - 0.13 0.0035 0.0060 - 1.70 0.39 2 0.06 0.01 0.92 0.017 0.49 0.001 0.07 0.0050 0.0210 - 1.10 0.15 3 0.07 - 0.91 0.017 0.47 - 0.12 0.0065 0.0100 - 1.13 0.26 4 0.06 0.06 0.86 0.020 0.49 0.001 0.25 0.0066 0.0170 - 1.02 0.51 5 0.07 0.02 0.86 0.075 0.46 0.001 0.26 0.0054 0.0110 - 1.09 0.57 6 0.06 0.02 0.87 0.018 0.50 0.001 0.13 0.0064 0.0190 - 1.02 0.26 7 0.07 0.17 0.98 0.018 0.50 0.001 0.14 0.0104 0.0150 - 1.14 0.28 8 0.06 0.05 0.92 0.019 0.50 0.014 0.14 0.0097 0.0160 - 1.07 0.28 9 0.19 0.29 0.40 0.016 0.22 0.003 0.10 0.0024 0.0020 - 1.06 0.45 10 0.10 0.01 1.02 0.022 0.34 0.003 0.05 0.0043 0.0110 - 1.75 0.15 11 0.09 0.02 1.17 0.023 0.49 0.005 0.05 0.0054 0.0180 - 1.39 0.11 12 0.05 - 0.93 0.018 0.33 - 0.08 0.0087 0.0091 - 1.62 0.25 13 0.15 0.01 1.00 0.024 0.39 0.001 0.13 0.0063 0.0158 - 1.49 0.34 14 0.10 - 0.58 0.017 0.25 - 0.06 0.0088 0.0179 - 1.35 0.24 15 0.07 - 1.55 0.018 0.69 - 0.25 0.0074 0.0166 - 1.31 0.51 16 0.08 - 0.95 0.020 0.35 - 0.10 0.0048 0.0135 Te: 0.0038 1.58 0.29 17 0.08 - 0.98 0.018 0.45 - 0.09 0.0055 0.0140 Se: 0.0020 1.27 0.20 18 0.06 0.01 0.88 0.017 0.49 0.001 0.11 0.0062 0.0210 Ca: 0.0031 1.05 0.22 19 0.07 - 0.91 0.015 0.48 - 0.15 0.0111 0.0148 Ca: 0.0025, Mg: 0.0021 1.11 0.31 20 0.07 - 0.91 0.016 0.46 0.001 0.13 0.0042 0.0145 REM: 0.0035 1.16 0.27 twenty one 0.06 - 0.95 0.018 0.50 0.001 0.13 0.0078 0.0190 Bi: 0.07 1.11 0.26 twenty two 0.07 0.17 0.89 0.018 0.47 0.001 0.11 0.0056 0.0158 Sn: 0.18 1.10 0.24 twenty three 0.08 - 0.86 0.016 0.36 - 0.08 0.0063 0.0139 Cu: 0.38 1.41 0.24 twenty four 0.09 0.01 0.90 0.017 0.38 0.002 0.13 0.0087 0.0188 Ni: 0.15 1.40 0.34 25 0.05 - 0.89 0.015 0.46 - 0.09 0.0054 0.0096 Cr: 0.55 1.11 0.20 26 0.09 0.15 0.94 0.019 0.50 0.001 0.10 0.0047 0.0075 Mo: 0.20 1.10 0.20 27 0.05 - 0.83 0.018 0.47 - 0.06 0.0085 0.0112 V: 0.10 1.03 0.13 28 0.08 - 0.94 0.015 0.45 0.001 0.20 0.0075 0.0148 Nb: 0.03 1.22 0.44 29 0.07 - 0.89 0.019 0.49 0.003 0.19 0.0026 0.0088 Cr: 0.50, Mo: 0.18 1.05 0.39

Table 2 Steel No. Chemical Composition (Mass%) Remaining Fe and Impurities Mn/S (atomic ratio) Ti/S (mass% ratio) C Si mn P S Al Ti N o other 30 0.08 - 0.95 0.055 0.33 0.002 - ^** 0.0048 0.0185 Pb: 0.31 ^** 1.68 - 31 0.07 0.01 1.10 0.060 0.32 0.002 - ^** 0.0090 0.0150 Pb: 0.27 ^** 2.01 - 32 0.08 - 1.02 0.067 0.33 0.002 - ^** 0.0066 0.0150 - 1.80 - 33 0.06 0.01 0.84 0.018 0.47 0.001 - ^** 0.0083 0.0210 - 1.04 - 34 0.07 0.01 1.26 0.018 0.62 0.001 - ^** 0.0059 0.0140 - 1.19 - 35 0.08 - 1.00 0.020 0.35 - 0.01 ^** 0.0076 0.0175 - 1.67 0.03 36 0.19 0.29 0.4 0.016 0.25 0.003 0.35 ^** 0.0061 0.0020 - 1.14 1.40 ^** 37 0.52 ^** 0.17 0.54 0.016 0.21 - 0.05 0.0079 0.0180 - 1.50 0.24 38 0.35 ^** - 0.50 0.019 0.23 0.002 0.08 0.0046 0.0058 - 1.27 0.35 39 0.01 ^** - 0.89 0.013 0.33 0.002 0.06 0.0047 0.0148 - 1.57 0.18 40 0.06 - 0.49 0.014 0.15 ^** - 0.05 0.0095 0.0193 - 1.91 0.33 41 0.07 0.01 1.53 0.018 1.05 ^** 0.001 0.10 0.0072 0.0210 - 0.85 ^** 0.09 42 0.06 - 0.18 0.018 0.34 0.001 0.14 0.0074 0.0090 - 0.31 ^** 0.4 43 0.08 0.01 2.53 ^** 0.018 0.65 0.001 0.15 0.0066 0.0210 - 2.27 0.23 44 0.07 - 0.88 0.4 ^** 0.20 - 0.09 0.0079 0.0176 - 2.57 0.44 45 0.09 0.19 0.82 0.015 0.47 - 0.12 0.0098 0.0064 Cr: 4.0 ^** 1.01 0.24 46 0.06 - 0.93 0.019 0.45 0.004 0.14 0.0085 0.0116 V: 2.5 ^** 1.19 0.31 47 0.08 - 0.91 0.013 0.46 0.002 0.08 0.0076 0.0063 Cr: 2.8 ^** , Mo: 1.5 ^** 1.14 0.18

^** represents conditions beyond the range specified in the present invention

(1) Research on the composition and morphology of internal substances

A test piece for microscopic observation was cut out from the longitudinal section corresponding to Df/4 (Df is the diameter of the forged elongated material) of the above-mentioned forged elongated material, and after grinding, surface analysis and quantitative analysis were carried out by EPMA and EDX. As a result, in the steels No. 1 to No. 29, the presence of MnS in which Ti sulfide and/or Ti carbosulfide were present on average was confirmed to be 10 or more per mm ² .

(2) Machinability research

The round bar obtained by forging was cut from the outside to Φ60mm, and used for cutting test. Because the hot workability is very poor, if cracks occur due to forging, when the cracks occur, keep the original shape at 950°C for 1 hour, carry out normalizing, and after air cooling (AC), use external cutting to reach Φ60mm, and use it as a test material .

The machinability test was carried out using JIS P type superhard tools not subjected to TiN coating treatment. Cutting is dry (no lubricating oil) rotary cutting. The conditions are: cutting speed: 150m/min, feeding amount: 0.10mm/rev, cutting amount: 2.0mm.

Under the above conditions, after 30 minutes of rotary cutting, the average void wear (VB) of the cutting tool was measured. For the test materials whose average void wear amount reaches 200 μm or more within 30 minutes, measure the reaching time and the average void wear amount (VB) at this time. In addition, the time taken for the average void wear amount (VB) to reach 100 μm was evaluated as a standard of tool life. For those with excellent wear resistance during the test, because the wear rate is extremely small and the test materials are insufficient, from the curve of rotary cutting time-tool wear, using the regression method, the average wear of the empty cutter face (VB) is calculated to reach 100µm time. To evaluate chip handling properties, 200 or more representative samples were collected from the discharged chips, their weight was measured, and the number of chips per unit weight was calculated.

(3) Evaluation of hot workability

The hot workability evaluation was performed as follows. That is, in order to simulate the manufacturing conditions using continuous casting equipment, a steel block of 150 kg is made as above, and the position of Di/8 (Di is the diameter of the steel block) near the surface part is taken as the center, and the diameter is taken from the height direction of the steel block. 10mm, high temperature tensile test bar with a length of 130mm. Take the fixed distance as 110mm, heat it directly to 1250°C, keep it for 5 minutes, then cool it to 1100°C at a cooling rate of 10°C/s, and keep it for 10 seconds, then use a strain rate of 10 ^-3 /s Do a tensile test. The reduction of area of the fractured portion at this time was measured to evaluate hot workability.

The above test results are shown in Table 3 and Table 4. Fig. 2 shows the relationship between chip handling and tool life, and Fig. 3 shows the relationship between the shrinkage of area value of the hot tensile test and the tool life.

table 3 Steel No. rate of reduction in area(%) Tool wear after 30 minutes (μm) Time to reach VB=100μm (minutes) Chip disposal (number/g) Remark 1 65.8 twenty one 179 ^* 15 2 55.2 35 110 12 3 54.8 32 181 ^* 16 4 56.9 35 117 17 5 58.8 29 119 twenty one 6 52.7 38 109 15 7 56.7 35 116 14 8 55.8 41 101 18 9 57.7 44 94 13 10 61.0 42 108 12 11 57.8 38 111 11 12 63.2 34 121 15 13 61.5 39 96 14 14 60.6 39 94 13 15 52.1 27 128 twenty three 16 56.9 25 156 11 17 52.0 26 135 13 18 59.7 29 139 19 19 53.4 twenty four 155 16 20 55.9 28 130 17 twenty one 52.1 42 102 15 twenty two 50.4 35 113 15 twenty three 63.6 44 91 14 twenty four 65.8 38 97 13 25 54.9 45 90 18 26 58.8 40 93 17 27 53.8 39 95 14 28 57.6 36 102 14 29 54.3 39 90 18

^* Calculated by regression method from the tool wear curve due to insufficient samples

Table 4 Steel No. rate of reduction in area(%) Tool wear after 30 minutes (μm) Time to reach VB=100μm (minutes) Chip disposal (number/g) Remark 30 47.8 97 36 9 31 49.6 99 30 8 32 55.4 165 17 1 33 51.8 210(20min) 9 6 34 45.4 68 72 10 35 54.4 90 39 2 36 64.2 72 69 15 37 52.8 93 36 12 38 67.9 89 38 14 39 57.5 104 29 7 40 65.3 138 20 5 41 5.1 39 102 5 forging crack 42 4.3 48 61 9 forging crack 43 65.7 205(20min) 8 10 44 13.6 103 28 18 forging crack 45 52.0 245(15min) 7 13 46 55.3 205(20min) 9 17 47 54.7 275(15min) 6 10

Steel Nos.30 and 31 in Table 2 are composite free-cutting steels, and No.32 is sulfur free-cutting steel, which is currently the steel with the best machinability (equivalent to JIS SUM23L or SUM23 materials). As can be seen from Table 3, Table 4 and FIG. 2 , even compared with these, the steel of the present invention has a superior effect of suppressing tool wear. Furthermore, Steel No. 1-29 of the present invention steel does not cause cracks at all during forging, and the reduction of area formed by high-temperature tensile test in simulation and practical production with continuous casting equipment is shown in Table 3. Same as composite free-cutting steel and sulfur free-cutting steel, practically no problem.

On the other hand, Steel No. 30-47, even if it is one of the conditions specified in the present invention, as long as it exceeds the conditions, at least one of the characteristics of hot rolling, tool life, and chip disposal is inferior to the steel of the present invention. . In Steel Nos. 4 and 42, since Mn and S did not satisfy the above formula (2), hot workability deteriorated.

Although the free-cutting steel of the present invention does not contain Pb, it has machinability superior to any of conventional lead free-cutting steels and composite free-cutting steels. Such steel is excellent in hot workability and can be produced at low cost by continuous casting, so it is most suitable as a material for various machine parts.

Claims (5)

1. A low-carbon sulfur free-cutting steel, characterized by mass %, containing C: 0.05-0.19%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.03-0.30%, Si: Below 1.0, P: 0.001-0.3%, Al: below 0.2%, O: 0.0010-0.050% and N: 0.0001-0.0200%, the rest are Fe and impurities, and the content of Ti and S satisfies the following formula (1), Mn The atomic ratio of /S satisfies the following formula (2), and the steel contains MnS with Ti sulfide and/or Ti carbosulfide inside,  Ti(mass%)/S(mass%)＜1……………(1) Mn/S≥1………………(2). 2. A low-carbon sulfur free-cutting steel, characterized by mass %, containing C: 0.05-0.19%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.03-0.30%, Si: Below 1.0, P: 0.001-0.3%, Al: below 0.2%, O: 0.0010-0.050%, N: 0.0001-0.0200%, and Se: 0.001-0.01%, Te: 0.001-0.01%, Bi: 0.005-0.3 %, Sn: 0.005-0.3%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, and rare earth elements: 0.0005-0.01%, one or more of them, and the rest are Fe and impurities, Ti and S The content satisfies the following (1) formula, the atomic ratio of Mn and S satisfies the following (2) formula, and contains MnS with Ti sulfide and/or Ti carbon sulfide inside,  Ti(mass%)/S(mass%)＜1……………(1) Mn/S≥1……………(2). 3. A low-carbon sulfur free-cutting steel, characterized by mass %, containing C: 0.05-0.19%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.03-0.30%, Si: 1.0 or less, P: 0.001-0.3%, Al: 0.2% or less, O: 0.0010-0.050% and N: 0.0001-0.0200%, and Cu: 0.01-1.0%, Ni: 0.01-2.0%, Cr: 0.01-2.5 %, Mo: 0.01-1.0%, V: 0.005-0.5%, and Nb: 0.005-0.1%, one or two or more, the rest are Fe and impurities, and the content of Ti and S satisfies the following formula (1) , the atomic ratio of Mn and S satisfies the following (2) formula, and contains MnS with Ti sulfide and/or Ti carbon sulfide inside,  Ti(mass%)/S(mass%)＜1……………(1) Mn/S≥1……………(2). 4. A low-carbon sulfur free-cutting steel, characterized by mass %, containing C: 0.05-0.19%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.03-0.30%, Si: Below 1.0, P: 0.001-0.3%, Al: below 0.2%, O: 0.0010-0.050%, N: 0.0001-0.0200%, and Se: 0.001-0.01%, Te: 0.001-0.01%, Bi: 0.005-0.3 %, Sn: 0.005-0.3%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, and rare earth elements: 0.0005-0.01%, one or more of them, and Cu: 0.01-1.0%, Ni : 0.01-2.0%, Cr: 0.01-2.5%, Mo: 0.01-1.0%, V: 0.005-0.5%, and Nb: 0.005-0.1%, one or more, the rest is Fe and impurities, Ti and the content of S satisfies the following (1) formula, the atomic ratio of Mn and S satisfies the following (2) formula, and contains MnS with Ti sulfide and/or Ti carbon sulfide inside, Ti(mass%)/S(mass%)＜1……………(1) Mn/S≥1……………(2). 5. The low-carbon sulfur free-cutting steel according to any one of claims 1-4, wherein the Si content is less than 0.1% by mass.