JPWO2001066814A1 - Steel with excellent forgeability and machinability - Google Patents

Abstract

(57) [Abstract] A steel with excellent forgeability and machinability, which suppresses the deterioration of mechanical properties in the weakest direction and improves forgeability, and which contains, by weight, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.0003 to 0.01%, with Al: 0.01% or less, total-0: 0.02% or less, and total-N: 0.02% or less, and has an average aspect ratio of MnS of 10 or less, a maximum aspect ratio of 30 or less, and the balance being Fe and unavoidable impurities.

Description

Detailed Description of the Invention

TECHNICAL FIELD The present invention relates to steels used in automobiles, general machinery, etc., and in particular to steels with excellent hot forging and machinability. BACKGROUND ART In recent years, while steels have become stronger, their workability has decreased, and therefore there is a growing need for steels that do not decrease forging or cutting efficiency. Until now, general measures for hot forging have been to reduce inclusions, add elements that increase high-temperature ductility, and reduce elements that inhibit high-temperature ductility. On the other hand, it is known that adding machinability-improving elements such as S and Pb is effective in improving machinability, but these elements that are effective in improving machinability also reduce high-temperature ductility, making it difficult to achieve both hot forging and machinability. Pb, B
Although it is believed that i improves machinability and has relatively little effect on forging, it is known to reduce high-temperature ductility. S improves machinability by forming inclusions that become soft in cutting environments, such as MnS, but the size of MnS is larger than particles such as Pb, making it more likely to become a source of stress concentration. When MnS is stretched, particularly through forging or rolling, it becomes anisotropic, becoming extremely weak in a specific direction. Furthermore, such anisotropy must be taken into consideration in design. Therefore, technology is needed to minimize the anisotropy of such free-machining elements. Furthermore, P is known to improve machinability, but it cannot be added in large amounts due to the tendency for cracks to form during hot casting, and there is a limit to its effectiveness in improving machinability. It has been claimed that the addition of Te eliminates anisotropy (see Japanese Patent Application Laid-Open No. 1975).
(5-41943), Te is prone to cracking during casting, rolling, and forging. Furthermore, a technology disclosed in JP 49-66522 A (1974-66522) aims to improve the machinability of steel over a wide range of cutting speeds, from low to high, by adding a deoxidizer containing Zr and Ca to the steel. However, this technology still does not solve the problem of fracture due to MnS elongated by rolling or forging. Therefore, further technological innovation is required to achieve both hot ductility and machinability. Disclosure of the Invention The present invention addresses the above-mentioned situation by providing a steel with excellent hot ductility and machinability. Generally, steel is processed by rolling or forging, and the plastic flow during this process causes anisotropy in its mechanical properties. During forging, cracking due to this anisotropy effectively indicates the forging limit. Therefore, to improve forgeability, it is effective to make the shape of inclusions such as MnS as spherical as possible and minimize anisotropy. Furthermore, even if anisotropy occurs, the influence of anisotropy can be reduced if the size of the inclusions is small. Therefore, it is desirable to use steel components that finely disperse MnS, which improves machinability, and maintain its spherical shape. The present invention is a steel with excellent forgeability and machinability, which was made based on the above findings,
The gist of the invention is as follows: (1) In mass %, the alloy contains C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, and Zr: 0.0003 to 0.01%, with Al being limited to 0.01% or less, total O being limited to 0.02% or less, and total N being limited to 0.02% or less, and the average aspect ratio of MnS being 10 or less and the maximum aspect ratio being 30 or less.
(2) A steel having excellent forgeability and machinability, characterized by containing, by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.0003 to 0.01%, and also containing Al: 0.01% or less, total-O: 0.02% or less, and total-N: 0.02% or less, and the average aspect ratio of MnS is 10 or less, and the maximum aspect ratio is 30.
and the maximum MnS particle size (μm) is 110×[S%]-15 or less, and
A steel having excellent forgeability and machinability, characterized in that the number of MnS per ^m2 is 3800 × [S%] - 150 or less, and the balance consisting of Fe and unavoidable impurities. (3) A steel containing, by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.0003 to 0.01%, with Al: 0.01% or less, total O: 0.02% or less, and total N: 0.02% or less, and further containing one or more of Cr: 0.01 to 2.0%, Ni: 0.05 to 2.0%, and Mo: 0.05 to 1.0%, and having an average aspect ratio of MnS of 10 or less and a maximum aspect ratio of 30 or less, with the balance being Fe and unavoidable impurities. (4) In mass%, it contains C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.0003 to 0.01%, and is limited to Al: 0.01% or less, total-O: 0.02% or less, total-N: 0.02% or less, and further contains one or more of Cr: 0.01 to 2.0%, Ni: 0.05 to 2.0%, and Mo: 0.05 to 1.0%, and the average aspect ratio of MnS is 10 or less and the maximum aspect ratio is 30 or less, and further, the maximum MnS particle size (μm) is 110×
[S%] -15 or less, the number of MnS per ^mm2 is 3800 × [S%] + 150
(5) A steel having excellent forgeability and machinability, characterized in that the steel according to any one of (1) to (4) above contains, by mass, at least one of V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, and Ti: 0.005 to 0.1%, with the balance consisting of Fe and unavoidable impurities. (6) A steel having excellent forgeability and machinability, characterized in that the steel according to any one of (1) to (5) above contains, by mass, one or more of Ca: 0.0002 to 0.005%, Mg: 0.0003 to 0.005%, and Te: 0.0003 to 0.005%, with the balance consisting of Fe and unavoidable impurities. (7) A steel with excellent forgeability and machinability, characterized in that the steel according to any one of (1) to (6) above contains, by mass, one or two of Bi: 0.05 to 0.5%, Pb: 0.01 to 0.5%, and the balance consisting of Fe and unavoidable impurities. (8) A steel with excellent forgeability and machinability, characterized in that the steel according to any one of (1) to (7) above contains, by mass, B: 0.0005% or more but less than 0.004%, and the balance consisting of Fe and unavoidable impurities. Best Mode for Carrying Out the Invention First, the steel composition according to the present invention will be explained. C is an element that has a significant effect on the basic strength of the steel, and is set to 0.1 to 0.85% to obtain sufficient strength. If the content is less than 0.1%, sufficient strength cannot be obtained, necessitating the addition of larger amounts of other alloying elements. If the content exceeds 0.85%, the steel approaches hypereutectoid, causing the precipitation of large amounts of hard carbides, significantly reducing machinability. Silicon is added as a deoxidizing element to strengthen ferrite and to provide resistance to temper softening. In the present invention, silicon is also necessary as a deoxidizing element.
If the content is less than 1%, the effect is not observed, and if the content exceeds 1.5%, embrittlement occurs and deformation resistance at high temperatures increases, so this was set as the upper limit. Mn is necessary to fix and disperse sulfur in the steel as MnS, and
It is necessary to dissolve in the matrix to improve hardenability and ensure strength after quenching. The lower limit is 0.05%; below this limit, S becomes FeS, resulting in embrittlement. As the Mn content increases, the hardness of the matrix increases, reducing cold workability, and its effects on strength and hardenability saturate, so the upper limit is set at 2.0%. P increases the hardness of the matrix in steel, reducing not only cold workability but also hot workability and castability, so its upper limit must be set at 0.2%. On the other hand, S is an element that has an effect on machinability, so its lower limit is set at 0.003%. S combines with Mn to form MnS inclusions. MnS improves machinability, but elongated MnS is one of the causes of anisotropy during forging. While the content should be adjusted depending on the degree of anisotropy and the required machinability, its upper limit is set at 0.5% because it is prone to cracking during hot and cold forging. The lower limit was set at 0.003%, which is the limit at which costs do not rise significantly at the current industrial production level. Zr is a deoxidizing element and forms _ZrO2 or oxides containing Zr (hereinafter referred to as Zr oxides). The oxide is considered to be _ZrO2 , and _ZrO2 acts as a nucleus for MnS precipitation, increasing the number of MnS precipitation sites and uniformly dispersing MnS. Zr also dissolves in MnS to form complex sulfides, reducing its deformability and suppressing the elongation of the MnS shape even during rolling or hot forging. Therefore, it is an effective element for reducing anisotropy. At less than 0.0003%, the effect is not significant, and adding 0.01% or more not only significantly reduces yield, but also increases the hardness of _ZrO2 and ZrS.
This leads to the formation of large amounts of oxides such as Zn, which in turn reduces mechanical properties such as machinability, impact strength, and fatigue resistance. Therefore, the element range was specified as 0.0003 to 0.01%. It has been known that adding Zr causes MnS to become spheroidized. However, in "Iron and Steel," Vol. 7, 1976, p. 893, it was noted that the formation of MnS _{- Zr3S4} eutectic inclusions reduces the deformability of MnS and inhibits its elongation, and that a content of 0.02% or more is required for 0.07% S. This knowledge suggests that the formation of complex sulfides is important to inhibit the deformability of MnS, and therefore the addition of large amounts of Zr is necessary. However, excessive Zr generates hard inclusions and clusters other than oxides, such as Zr-based nitrides and sulfides, which degrade mechanical properties and machinability. In other words, reducing MnS deformability through the addition of large amounts of Zr is accompanied by the adverse effects of hard inclusions and clusters. On the other hand, the present inventors focused on the role of Zr-based oxides as nuclei for precipitation of MnS rather than the deformability of MnS. They developed free-cutting steel on the basis that if MnS is finely dispersed in steel, even if MnS is elongated by rolling or forging, it will not become a fatal defect in the steel. As a result of their investigation, they found that the Zr generated by adding 0.01% or less Zr
We discovered that Zr-based oxides can be finely dispersed and easily serve as nuclei for MnS precipitation. By actively utilizing this, we developed a steel with finely dispersed MnS, which has excellent mechanical properties and machinability. In this invention, Zr exists as an oxide alone or in combination with other oxides, and its distribution is finely dispersed, making it easy to serve as nuclei for MnS precipitation in the steel. Furthermore, simply finely dispersing Zr-based oxides as MnS nuclei does not require the addition of excess Zr relative to S. Therefore, excess Zr does not produce hard inclusions other than oxides, such as Zr-based nitrides and sulfides, and their clusters. This avoids the adverse effects of adding large amounts of Zr, namely, a decrease in mechanical properties such as impact value and machinability. Al is a deoxidizing element and forms _Al2O3 in steel. Because _Al2O3 is hard _, it can cause tool damage and promote wear during cutting. Furthermore, the addition of Al reduces oxygen, making it difficult for Zr oxides to form. Furthermore _, it is preferable not to add Al to achieve uniform dispersion of fine _ZrO2 . This influence significantly affects the amount of Zr added, yield, and the distribution and shape of MnS. In this invention, the _Zr content is limited to 0.01% or less to suppress hard _Al2O3 and to finely and uniformly disperse Zr oxide. This significantly reduces the amount of Zr added, enhancing the effect of Zr as a precipitation nucleus and its compounding effect with MnS. When free O is present, it forms bubbles during cooling, causing pinholes. Furthermore, when it combines with Si, Al, Zr, etc., it forms hard oxides, so it is necessary to limit its content. In this steel, the upper limit is set at 0.02%, at which point the fine dispersion effect of Zr is lost. When N is dissolved, it hardens the steel. In particular, during cutting, it hardens near the cutting edge due to dynamic strain aging, shortening tool life. Furthermore, when present as nitrides of Ti, Al, V, etc., it suppresses austenite grain growth, so it must be limited. TiN and ZrN form, especially at high temperatures. Even when nitrides are not formed, bubbles form during casting, causing defects. In the present invention, the upper limit is set at 0.02%, at which point the adverse effects become significant. Cr is an element that improves hardenability and imparts temper softening resistance. Therefore, it is added to steels that require high strength. In such cases, an addition of 0.01% or more is necessary. However, adding large amounts generates Cr carbides and causes embrittlement, so the upper limit is set at 2.0%. Ni strengthens ferrite, improves ductility, and is effective in improving hardenability and corrosion resistance. This effect is not observed at less than 0.05%, and the effect saturates at more than 2.0%, so this is the upper limit. Mo is an element that imparts temper softening resistance and improves hardenability. This effect is not observed at less than 0.05%, and the effect saturates at more than 1.0%, so the addition range is set at 0.05 to 1.0%. B is effective in strengthening grain boundaries and hardenability when dissolved, and precipitates as BN, which improves machinability. These effects are not significant when added in an amount of less than 0.0005%, and the effects are saturated when added in an amount of 0.004% or more. If too much BN precipitates, the mechanical properties of the steel are impaired.
The range is set to less than 0.004%. V forms carbonitrides and can strengthen steel through secondary precipitation hardening.
If V is added in an amount of 0.05% or less, there is no effect in increasing strength, and if V is added in an amount of more than 1.0%, a large amount of carbonitrides precipitates, which impairs the mechanical properties.
The addition of V is preferably more than 0.2%. Elements such as V, Nb, and Ti form nitrides, carbides, and carbonitrides in steel. They are often used as pinning particles to suppress austenite grain growth and control the austenite grain size when heated above the transformation point during forging or heat treatment. Although their precipitation temperatures vary, considering the precision of temperature control in industrially performed heat treatments, it is necessary to exert the pinning effect over as wide a temperature range as possible to control the austenite grain size. In particular, in hot forging, the cooling temperature varies significantly depending on the shape and position within the part. While Nb and Ti form precipitates at relatively high temperatures, V precipitates carbides at lower temperatures. Therefore, adding V is preferred. However, when adding V alone, the effect can be achieved by limiting the V content to more than 0.2% and not more than 1.0%. Furthermore, by using V in combination with either or both Nb and Ti, precipitates of optimal size as pinning particles can be uniformly dispersed in steel. When several elements are used in combination, the austenite grain size can be controlled even with a smaller amount of addition than when each element is added alone, and the effect can be seen even with a lower limit of 0.05% V. Therefore, when one or both of Nb and Ti are added simultaneously with V, the lower limit of V is set at 0.05%. Nb also forms carbonitrides and can strengthen steel through secondary precipitation hardening.
Below 0.005%, there is no effect on increasing strength, and above 0.2%, many carbonitrides precipitate, impairing mechanical properties, so this upper limit was set. Ti also forms carbonitrides and strengthens steel. It is also a deoxidizing element, and can improve machinability by forming soft oxides. Below 0.005%, this effect is not observed, and even if added above 0.1%, the effect saturates. Ti also forms nitrides even at high temperatures, suppressing the growth of austenite grains. Therefore, the upper limit was set at 0.1%. Ca is a deoxidizing element that not only forms soft oxides and improves machinability, but also dissolves in MnS, reducing its deformability and suppressing the elongation of the MnS shape during rolling and hot forging. Therefore, it is an effective element for reducing anisotropy. 0.
If the content is less than 0.002%, the effect is not significant, and if it exceeds 0.005%, not only does the yield deteriorate significantly, but a large amount of hard CaO is formed, which actually reduces machinability. Therefore, the component range is specified as 0.0002 to 0.005%. Mg is a deoxidizing element and forms oxides. The oxides act as nuclei for the precipitation of MnS, and M
Te is effective in finely and uniformly dispersing nS. Therefore, it is an effective element in reducing anisotropy. If it is added less than 0.0003%, the effect is not significant, and if it is added more than 0.005%, the yield will be extremely poor and the effect will saturate. Therefore, the component range is specified as 0.0003 to 0.005%. Te is an element that improves machinability. It also produces MnTe and, by coexisting with MnS, reduces the deformability of MnS and inhibits the elongation of the MnS shape. Therefore, it is an effective element in reducing anisotropy. This effect is not observed if it is less than 0.0003%, and if it exceeds 0.005%, it is likely to cause cracks during casting. Bi and Pb are elements that are effective in improving machinability. This effect is not observed if it is added less than 0.0003%, and if it is added more than 0.005%, it is likely to cause cracks during casting.
If the content is less than 0.5%, this is not observed, and if added in excess of 0.5%, not only does the effect of improving machinability saturate, but hot casting properties deteriorate, making it more likely to cause defects. Next, in the present invention, in addition to the above-mentioned composition, the average aspect ratio and maximum aspect ratio of MnS, as well as the maximum MnS grain size and the number of MnS per unit area (1 mm ² ) are important factors, and it is necessary for the average aspect ratio of MnS to be 10 or less, the maximum aspect ratio to be 30 or less, the maximum MnS grain size (μm) to be 110×[S%]+15 or less, and the number of MnS per 1 mm ² to be 3800×[S%]+150 or less. The reason for specifying the average aspect ratio to be 10 or less and the maximum aspect ratio to be 30 or less is as shown in FIG. 8 (
As shown in Figure 9, the aspect ratio tends to increase as the initial MnS grain size increases. As shown in the examples, a large aspect ratio promotes anisotropy of the material, which reduces the fatigue strength of the impact value in the cross-sectional direction. In addition, various deformations are applied during forging, so the elongated MnS
These often become fracture initiation sites. Therefore, when the average aspect ratio of MnS is less than 20 or more, the degradation of fracture properties due to elongated MnS becomes significant. Furthermore, when the maximum aspect ratio of MnS exceeds 30, the degradation of fracture properties due to MnS becomes significant. The reason for limiting the maximum MnS particle size (μm) to 110 × [S%] + 15 or less and the number of MnS particles ^per mm2 to 3800 × [S%] + 150 or less is as follows: MnS is known to be a stress concentration source and therefore prone to become fracture initiation sites, and its size has a particularly strong effect. On the other hand, while machinability improves in proportion to the S content, we found that the effect of MnS size is not as significant. Therefore, when comparing steels with the same S content, steels with small, highly dispersed MnS particles have superior fracture properties and forgeability to steels with large, less dispersed MnS particles, even if they have equivalent machinability. The effect is affected by the amount of S, but as shown in Figures 8(a) and 9, when the maximum MnS particle size (μm) is less than 110
× [S%] + 15 and the number of MnS per ¹ mm2 > 3800 × [S%] + 150
It was found that if the MnS content is less than 110×(S%), the deterioration of forgeability and fracture properties can be minimized while ensuring machinability equivalent to the amount of added S.
] + 15 or the number of MnS per ^mm2 < 3800 × [S%] + 150, the fracture properties and forgeability are inferior. MnS-based inclusions are extracted using an image processing device, and the following items are calculated for each MnS. The image processing device digitizes the optically captured image using a CCD camera, making it possible to measure the size and occupied area of MnS. The measurement field is 500x magnification, and 50 fields are repeatedly measured, with each field being 9000 ^μm2 . The objects of this measurement are the circle equivalent diameter (R), rolling direction length (L), and radial thickness (
The maximum and average values of these measurements for individual MnS can be calculated, and the average aspect ratio is the average aspect ratio of the individual MnS.
The aspect ratio is the average value of the aspect ratios of the measured individual aspect ratios, and the largest aspect ratio among the individual measured aspect ratios is referred to as the maximum aspect ratio. Furthermore, the particle size of MnS was measured using an image processing device, and the diameter when the measured area of MnS is converted into a circle, i.e., the so-called circle equivalent diameter. The number of MnS per ^mm2 is the value obtained by dividing the number of MnS contained in the measured area by the measured area. Examples The effects of the present invention will be explained using examples. The test materials shown in Table 1 were melted in a 2-ton vacuum melting furnace, disassembled and rolled into billets, and further rolled to a diameter of 60 mm. After rolling, hot upset test pieces for evaluating hot workability and cold upset test pieces for evaluating cold workability were cut out and subjected to upset tests. Furthermore, some were heat-treated by heating to 1200°C, then allowed to cool, and then subjected to cutting tests. Regarding the method for analyzing Zr in steel, samples were treated in the same manner as in JIS G 1237-1997 Appendix 3, and the Zr content in the steel was measured using IC
The measurement was performed by inductively coupled plasma atomic emission spectroscopy (ICP). However, the sample used for the measurement in the examples of the present invention was 2 g per steel type, and the calibration curve for ICP was also based on a trace amount of Z
The measurement was performed by setting the Zr standard solution to be suitable for the Zr concentration. That is, the Zr standard solution was diluted to prepare solutions with different Zr concentrations so that the Zr concentration was 1 to 200 ppm, and the Zr amount was measured to prepare a calibration curve. The common method for these ICPs is described in JIS K 0116-1995 (General rules for optical emission spectroscopy analysis) and JIS K 0116-1995.
The test specimens were cut from the cutting position 1 shown in Figure 1(a) and the cutting direction of the upset test specimens was such that the MnS2 in the steel was aligned longitudinally. The hot upset test specimen 3 and the cold upset test specimen 4 with a notch 5 shown in Figures 1(b) and 1(c) were cut from the cutting position 1 shown in Figure 1(a) and the cutting direction of the upset test specimens was such that the MnS2 in the steel was aligned longitudinally. Figure 2 is a diagram illustrating the location of cracks occurring during the upset test. In the upset test, as shown in Figure 2, when a load 6 is applied to the test specimen, deformation 7 occurs at the outer periphery, generating circumferential tensile stress. In many cases, MnS in the steel acts as a fracture source, resulting in cracks 8. Upset tests of test specimens cut in this manner can be used to evaluate forging workability. The hot upsetting test specimens were φ20 mm x 30 mm and equipped with thermocouples. They were heated to 1000°C by high frequency and upset forged within 3 seconds. Forging was performed with various strains, and as shown in Figure 3, the specimens were measured 9 mm before deformation and 1 mm after deformation.
The strain at which cracking of 0 occurs was measured as the limit strain.
1), which is the so-called nominal strain. ε = (H _o -H)/H _o Formula (1) where ε is strain, H _o is the height of the test piece before deformation, and H is the height of the test piece after deformation. Table 1 shows examples in which workability was evaluated. Table 1 Examples 1 to 5 are steels based on S45C, with varying amounts of S. As comparative examples, Examples 6 to 10 are steels to which no Zr has been added. Examples (comparative examples) 11 and 12 are steels to which a large amount of Al has been added, no Zr has been added, and Pb has been added. Examples (comparative examples) 13 and 14 are steels to which Zr has been added.
Although a large amount of Al was added, the amount of S was changed.
No. 5 is a comparative example in which a large amount of Al was added and no Zr was added.
Examples 11 and 12, in which Pb was added, were inferior in hot forgeability.
Inventive Examples 2 to 5, in which Zr was added, were superior to Comparative Examples 7 to 10. Furthermore, when the amount of S was large, regardless of the presence or absence of Zr, when the amount of Al was large, the hot workability was inferior to that of the inventive examples, as in Examples 14 and 15. Figure 4 shows the effect of S content on hot forgeability for the examples in Table 1. A cold upsetting test was also conducted to evaluate cold workability. The material cut out as shown in Figure 1 was quenched from 850°C and then spheroidized at 700°C for 12 hours. Then, cold upsetting test pieces with a 2mm notch and a diameter of 7mm x 14mm were machined. Figure 5 shows the results of limit strain measurement during cold working for Examples 1 to 15. The definition of strain is the same as in Equation 1. Similarly, Table 2 shows an example in which V was added to S45C to refine the austenite grain size and improve strength. Figure 6 shows the results of hot forgeability evaluation at 1000°C for the examples in Table 2. In this case, too, increasing the S content reduced hot forgeability, but when compared at the same S content, Examples 17 to 20 (inventive examples) showed significantly better results than Examples 22 to 24.
5 (comparative example) showed better hot forgeability. The results of evaluating the machinability of the Examples shown in Table 1 are shown in Figure 7. The machinability evaluation was carried out by a drilling test, and the cutting conditions are shown in Table 3. Cumulative hole depth 1000 mm
Machinability was evaluated at the highest cutting speed possible (so-called VL1000). As shown in Figure 7, the machinability improves as the S content increases. However, when comparing at the same S content, when a large amount of Al is added (Examples 13 to 15), the machinability is inferior to when the Al content is limited to the specified range. When the Al content is within the specified range, and when comparing with and without Zr, the machinability is equivalent regardless of the S content. Also, Examples 11 and 12, which added Pb,
In comparison, Examples 2 and 11 had comparable machinability, but Example 2 had superior hot workability, as shown in Figure 4. Similarly, in a comparison of Examples 3 and 12, despite comparable machinability, Example 3 (invention example) had superior hot workability. Thus, the present invention is effective in achieving both hot workability and machinability. A similar effect was observed when increasing strength by adding V. Table 2 shows the numerical results of machinability evaluation. When compared at the same S content, the invention example had similar machinability to the comparative example. Therefore, by using the present invention, both forgeability and machinability can be achieved even with high strength. Table 4 shows examples in which the Zr content was varied. Examples 2 and 3 were added to the examples in Table 4 to examine the relationship between mechanical properties and Zr content. Figure 8(a) shows the impact value, sulfide aspect ratio, and number of sulfides per unit area for each Zr content. The impact test specimens were cut as shown in Figure 8(b), with L representing the longitudinal cut and C representing the cross-sectional cut. When Zr is not added, the impact value in the longitudinal direction of rolling is excellent, but the impact value in the cross-sectional direction is extremely low. This tendency becomes more pronounced as the amount of S increases. However, when Zr is added, the impact value in the longitudinal direction decreases slightly, but the impact value in the cross-sectional direction improves significantly. This is thought to be due to the fine dispersion of sulfides and the improvement of the aspect ratio. In particular, when the number of sulfides increases and they are finely dispersed, even if sulfides with a large aspect ratio are included, their small size is thought to reduce the effect on mechanical properties. Furthermore, Table 5 shows examples in which the Al content was changed. As already mentioned, an increase in the Al content reduces machinability. In order to clarify the effect of the Al content, Examples 2 and 27 were added to the examples in Table 5, and the effect of the Al content on the sulfide shape is shown in Figure 9.
When trace amounts of r were added, the number of sulfides decreased and the aspect ratio increased when the Al content exceeded 0.01%. In this case, the limiting strain in the hot upsetting test decreased. Furthermore, the machinability AL1000 clearly decreased with increasing Al content. For this reason, the Al content was specified to be 0.01% or less in this invention. Table 6 shows examples in which the effects of other elements were investigated. The manufacturing method and the methods for evaluating hot workability and machinability were the same as those of the examples shown in Table 1. Table 6, Table 6-1, Table 6
Tables 6-2 and 6-3 show the hot limit strain and machinability of Example Nos. 41 to 72 when various synthetic elements are added. Although the differences in machinability are small, the comparative examples in these tables are significantly inferior in terms of hot limit strain. Furthermore, when the basic strength is changed by the amount of C, as shown in Example Nos. 73 to 78 in these tables, the invention examples are also superior to the comparative examples. Example Nos. 79 and 80 in Tables 6-1 and 6-3 are comparative examples in which the total O and total N amounts are outside the ranges of the invention. Compared to Example No. 2, these were inferior in both hot limit strain and machinability. Thus, the examples included in the present invention are comparative examples in which the same S content is used.
When compared by amount, it can be seen that good hot workability and machinability are both achieved. FIG. 10 shows the adverse effect on machinability as a function of VL1000 (10
The cumulative hole depth of 1000 mm was evaluated at the maximum cutting speed at which drilling was possible.
It can be seen that the addition of a large amount of Zr reduces the machinability. Also, in the impact values shown in Figure 8, the addition of excessive Zr results in an excellent aspect ratio of MnS, but the addition of ZrN and Zr
It can be seen that clusters of sulfur and other elements are formed, resulting in a decrease in impact value. In Figures 4 to 10, the subscripts in the figures indicate Example Nos. Industrial Applicability Based on the above, it is possible to provide steels that combine hot workability, mechanical properties, and machinability. In particular, the technology of the present invention is not significantly affected by heat treatment or microstructure, and is based on shape control of sulfides, so there is no need to distinguish between tempered and non-tempered steels. Furthermore, with regard to processing, the technology is effective not only for hot forging but also for cold forging, and is effective for a wide range of steels that require forgeability, mechanical properties, and machinability.

[Brief explanation of the drawings]

Figures 1(a), 1(b), and 1(c) are diagrams illustrating the cutting positions and test specimen shapes for evaluating forgeability (hot and cold). Figure 2 is a diagram illustrating the location of cracks during an upsetting test. Figure 3 is a diagram illustrating the definition of strain during forgeability evaluation (upsetting test). Figure 4 is a diagram illustrating the effect of S content on hot forgeability for the Examples in Table 1. Figure 5 is a diagram illustrating the effect of S content on cold forgeability for the Examples in Table 1. Figure 6 is a diagram illustrating the effect of S content on hot workability for the Examples in Table 2. Figure 7 is a diagram illustrating the effect of S content on machinability for the Examples in Table 1. Figure 8(a) is a diagram illustrating the effect of Zr content on impact value, sulfide shape, and sulfide number, and Figure 8(b) is a diagram illustrating the test specimen cutting positions. Figure 9 is a diagram illustrating the effect of Al content on sulfide shape, number, hot forgeability, and machinability. Figure 10 is a diagram illustrating the effect of Zr content on tool life.

───────────────────────────────────────────────────── Continued from the front page (72) Inventor: Kenichiro Naito 12 Nakamachi, Muroran, Hokkaido, Japan Muroran Steel Works, Nippon Steel Corporation (72) Inventor: Kenji Fukuyasu 6-3 Otemachi 2-chome, Chiyoda-ku, Tokyo, Japan (Note) This publication is based on the publication published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act) and is unrelated to this publication.

Claims (8)

[Claims] Claim 1: A steel sheet containing, by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, and Zr: 0.0003 to 0.01%, with Al being limited to 0.01% or less, total-O: 0.02% or less, and total-N: 0.02% or less, and with an average aspect ratio of MnS of 10 or less and a maximum aspect ratio of 30 or less.
A steel having excellent forgeability and machinability, characterized by having the following: Claim 2: A steel sheet containing, by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, and Zr: 0.0003 to 0.01%, with Al being limited to 0.01% or less, total-O: being 0.02% or less, and total-N: being 0.02% or less, and with an average aspect ratio of MnS being 10 or less and a maximum aspect ratio of 30 or less.
and the maximum MnS particle size (μm) is 110 × [S%] + 15 or less,
A steel having excellent forgeability and machinability, characterized in that the number of MnS per ^m2 is 3800 × [S%] + 150 or less, and the balance consisting of Fe and unavoidable impurities. [Claim 3] A steel with excellent forgeability and machinability, containing, by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.0003 to 0.01%, with Al: 0.01% or less, total O: 0.02% or less, and total N: 0.02% or less, and further containing one or more of Cr: 0.01 to 2.0%, Ni: 0.05 to 2.0%, and Mo: 0.05 to 1.0%, and having an average aspect ratio of MnS of 10 or less and a maximum aspect ratio of 30 or less, with the balance being Fe and unavoidable impurities. [Claim 4] A steel sheet containing, by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.0003 to 0.01%, with Al: 0.01% or less, total-O: 0.02% or less, total-N: 0.02% or less, and further containing one or more of Cr: 0.01 to 2.0%, Ni: 0.05 to 2.0%, and Mo: 0.05 to 1.0%, and having an average aspect ratio of MnS of 10 or less and a maximum aspect ratio of 30 or less, and further having a maximum MnS particle size (μm) of 110×
[S%] + 15 or less, the number of MnS per 1 ^mm2 is 3800 × [S%] + 150
A steel having excellent forgeability and machinability, characterized by having the following: [Claim 5] A steel having excellent forgeability and machinability, characterized in that the steel according to any one of claims 1 to 4 contains, in mass %, at least one of V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, and Ti: 0.005 to 0.1%, with the balance consisting of Fe and unavoidable impurities. [Claim 6] A steel with excellent forgeability and machinability, characterized in that the steel according to any one of claims 1 to 5 contains, in mass %, one or more of the following: Ca: 0.0002 to 0.005%, Mg: 0.0003 to 0.005%, Te: 0.0003 to 0.005%, with the balance consisting of Fe and unavoidable impurities. [Claim 7] A steel having excellent forgeability and machinability, characterized in that the steel according to any one of claims 1 to 6 contains, in mass %, one or two of Bi: 0.05 to 0.5%, Pb: 0.01 to 0.5%, and the balance consisting of Fe and unavoidable impurities. 8. The steel according to any one of claims 1 to 7, wherein the steel contains, in mass %, B: 0.
A steel having excellent forgeability and machinability, characterized in that it contains 0.0005% or more but less than 0.004%, with the balance consisting of Fe and unavoidable impurities.