Steel material excellent in shearing workability

Abstract

Proposed is a steel material in which the occurrence of burrs is suppressed and which is excellent in shear processing. In the steel material that has a prescribed component composition, the maximum values [Cu]max, [Ni]max, and [Sn]max and the minimum values [Cu]min, [Ni]min, and [Sn]min of content of Cu, Ni, and Sn that are measured in five predetermined regions on the cross section of the steel material, and the maximum value wmax and the minimum value wmin of the half value width of 200 reflection peak of ferrite measured in the five predetermined regions satisfy the following formulas (1) and (2). ([Cu]max / [Cu]min + [Ni]max / [Ni]min + [Sn]max / [Sn]min) ≤ 3.45 … (1); (wmax-wmin) / (wmax + wmin) ≤ 0.30 … (2)

Description

Steel with excellent shear workability

[0001] The present invention relates to a steel material, and more particularly to a steel material that is subjected to shearing in a part manufacturing process.

[0002] In the manufacture of mechanical structural parts such as steel bolts, nuts, and screws used in automobiles, construction machinery, etc., the raw steel material is subjected to preliminary processing such as wire drawing, and then cut to the desired length for processing into the target part. The cut steel material is then processed into the desired shape by hot and / or cold forging and cutting, and finished into the part.

[0003] Cutting materials is generally done by shearing due to its cost and short processing time. However, cutting by shearing creates burrs on the cut edge. If the burrs are left unattended, they can not only reduce processing accuracy and product life, but can also lead to injuries to workers and end users. For this reason, a process for removing burrs after cutting is often included. If burrs are suppressed or only slight, it is possible to omit or simplify the deburring process, so in recent years there has been an increasing need to suppress burrs in order to shorten lead times and reduce costs in parts manufacturing.

[0004] Techniques proposed for suppressing burrs include, for example, Patent Documents 1 and 2. Patent Document 1 describes a steel material that suppresses the generation of burrs during deep hole drilling with minimal oil lubrication by optimizing the additive elements and controlling the metal structure.

[0005] Patent Document 2 describes a steel material that has excellent groove cutting workability and torsional fatigue strength after induction hardening due to the optimization of the chemical composition of the steel.

[0006] Patent No. 5018237 Specification Patent No. 5375212 Specification

[0007] Although Patent Documents 1 and 2 propose techniques for suppressing the generation of burrs during hole drilling using a drill or cutting using a cutting tool, there have been no examples of detailed studies of techniques for suppressing the generation of burrs during cutting using shearing.

[0008] The present invention has been made in view of the above circumstances, and aims to propose a steel material that suppresses the generation of burrs and is excellent in shearing workability.

[0009] The present inventors conducted research focusing on the segregation of alloy elements and the distribution of processing strain in steel materials in order to suppress the generation of burrs during shear cutting. As a result, they discovered that it is important to suppress the segregation of alloy elements that improve shear workability, and to control the processing strain so that the strain in the cross section of the steel material is uniform, which led to the completion of the present invention.

[0010] That is, the gist and configuration of the present invention are as follows.

[0011] 1. A steel material having a chemical composition containing, by mass%, C: 0.03 to 0.80%, Si: 0.010 to 1.800%, Mn: 0.010 to 1.500%, P: 0.0010 to 0.0500%, S: 0.0010 to 0.0500%, Ni: 0.010 to 1.000%, Cu: 0.010 to 0.500%, Cr: 0.010 to 1.500%, Sn: 0.0010 to 0.1000%, N: 0.0020 to 0.0250%, with the balance being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): ([Cu] ｍａｘ / [Cu] ｍｉｎ + [Ni] ｍａｘ / [Ni] ｍｉｎ + [Sn] ｍａｘ / [Sn] ｍｉｎ )≦3.45…(1) (w ｍａｘ -w ｍｉｎ ) / (w ｍａｘ +w ｍｉｎ ) ≦ 0.30 ... (2) where [Cu] ｍａｘ , [Ni] ｍａｘ , [Sn] ｍａｘ are the maximum values ​​of the Cu content, Ni content, and Sn content measured in the five regions shown below, respectively, [Cu] ｍｉｎ , [Ni] ｍｉｎ , [Sn] ｍｉｎare the minimum values ​​of the Cu content, Ni content, and Sn content measured in the five regions. The five regions are the center point of a cross section perpendicular to the longitudinal direction of the steel material, and four midpoints of four line segments drawn from the center point to the surface of the steel material at 90° intervals around an axis perpendicular to the cross section. ｍａｘ and w ｍｉｎ are the maximum and minimum values ​​of the half width at the 200 reflection peak of ferrite obtained by performing X-ray diffraction measurement in the above five regions.

[0012] 2. The steel material according to 1 above, wherein the composition further contains, in mass%, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, and B: 0.0100% or less.

[0013] According to the present invention, it is possible to provide a steel material that suppresses the generation of burrs and has excellent shearing properties, which makes it possible to simplify or even eliminate the deburring process that has been conventionally performed, thereby contributing to improving productivity and reducing the environmental impact and part manufacturing costs.

[0014] Hereinafter, an embodiment of the present invention will be described. First, the chemical composition of the steel material will be described. Note that "%" representing the chemical composition below means "mass %" unless otherwise specified. Furthermore, the expression "a to b" means a to b inclusive.

[0015] C: 0.03 to 0.80% C is added to ensure the strength required for machine components. If the C content is less than 0.03%, the strength required for machine structural components cannot be ensured, which is unsuitable. On the other hand, if the C content exceeds 0.80%, it has a negative effect on cold workability. For these reasons, the C content is set to 0.03 to 0.80%. The C content is preferably 0.05 to 0.60%, and more preferably 0.10 to 0.50%.

[0016] Si: 0.010 to 1.800% Si is an element necessary for deoxidation, and is also an element effective in imparting the necessary strength to steel by improving solid solution strengthening and hardenability. If the Si content is less than 0.010%, the above effects are insufficient. On the other hand, if the Si content exceeds 1.800%, the cold workability of the steel is reduced. For these reasons, the Si content is set to 0.010 to 1.800%.

[0017] Mn: 0.010 to 1.500% Mn is an element necessary for deoxidation, and is also an element effective in imparting the necessary strength to steel by improving hardenability. If the Mn content is less than 0.010%, the above effect is insufficient. On the other hand, if the Mn content exceeds 1.500%, the cold workability of the steel is reduced. For these reasons, the Mn content is set to 0.010 to 1.500%.

[0018] P: 0.0010 to 0.0500% P is an element effective in increasing the strength of steel. If the P content is less than 0.0010%, the above effect cannot be fully obtained. On the other hand, if the P content exceeds 0.0500%, P segregates at grain boundaries, reducing the toughness of the steel. For these reasons, the P content is set to 0.0010 to 0.0500%.

[0019] S: 0.0010 to 0.0500% S is an element that is effective in improving the machinability of steel by forming a compound with Mn in the steel to become MnS. If the S content is less than 0.0010%, the above effect cannot be fully achieved. On the other hand, if the S content exceeds 0.0500%, the large amount of MnS formed becomes the starting point for cracks during cold working, thereby reducing cold workability. For these reasons, the S content is set to 0.0010 to 0.0500%.

[0020] Ni: 0.010 to 1.000% Ni, like Cu, is an effective element for improving shear workability. If the Ni content is less than 0.010%, the above effect cannot be fully achieved. On the other hand, if the Ni content exceeds 1.000%, surface defects are likely to occur during steel production, increasing manufacturing costs due to maintenance. For these reasons, the Ni content is set to 0.010 to 1.000%.

[0021] Cu: 0.010 to 0.500% Cu is an element effective in improving shear workability. If the C content is less than 0.010%, the above effect cannot be fully obtained. On the other hand, if the Cu content exceeds 0.500%, surface defects are likely to occur during steel production, increasing manufacturing costs due to maintenance. For these reasons, the Cu content is set to 0.010 to 0.500%.

[0022] Cr: 0.010 to 1.500% Cr is an element that is effective in imparting the necessary strength to steel by improving solid solution strengthening and hardenability. If the Cr content is less than 0.010%, the above effects cannot be fully obtained. On the other hand, if the Cr content exceeds 1.500%, the hardness of the steel increases and cold workability decreases. For these reasons, the Cr content is set to 0.010 to 1.500%.

[0023] Sn: 0.0010 to 0.1000% Sn, like Cu, is an element that is effective in improving shear workability. If the Sn content is less than 0.0010%, the above effect cannot be fully obtained. On the other hand, if the Sn content exceeds 0.1000%, the manufacturing cost increases due to maintenance. For these reasons, the Sn content is set to 0.0010 to 0.1000%.

[0024] N: 0.0020 to 0.0250% N is an element that forms nitrides with nitride-forming elements in the steel and acts as grain boundary pinning particles, thereby effectively preventing coarsening of crystal grains. If the N content is less than 0.0020%, the above effect cannot be fully achieved. On the other hand, if the N content exceeds 0.0250%, the solute N in the steel undergoes dynamic strain aging, making cracks more likely to occur during cold working. For these reasons, the N content is set to 0.0020 to 0.0250%.

[0025] The above-mentioned composition may contain one or more of the following elements as required.

[0026] Mo: 0.30% or less Mo can be added because it is an element that can significantly improve the hardenability of steel material with a small amount of addition and is effective in improving the strength of steel. To achieve this effect, the Mo content is preferably 0.15% or more. On the other hand, if the Mn content exceeds 0.30%, the hardenability becomes excessive and the cold workability deteriorates. Therefore, the Mo content is preferably 0.30% or less.

[0027] Al: 0.100% or less Al can be added because it is a deoxidizing element and also an element that is effective in refining crystal grains by bonding with N in the steel to form nitrides. To achieve this effect, the Al content is preferably 0.001% or more. On the other hand, if the Al content exceeds 0.100%, a large amount of Al oxide is generated in the steel, making it more likely to crack during cold working. Therefore, the Al content is preferably 0.100% or less.

[0028] Ti: 0.100% or less Ti, like Al, is an element that bonds with N in steel to form nitrides, thereby effectively refining crystal grains, and can therefore be added. To achieve this effect, the Ti content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.100%, a large amount of Ti-based inclusions is generated in the steel, making it more likely to crack during cold working. Therefore, the Ti content is preferably 0.100% or less.

[0029] V: 0.300% or less Like Al and Ti, V is an element that combines with N in steel to form nitrides, thereby effectively refining crystal grains, and therefore can be added. To achieve this effect, the V content is preferably 0.001% or more. On the other hand, if the V content exceeds 0.300%, a large amount of V-based precipitates will precipitate, making cracks more likely to occur during cold working. Therefore, the V content is preferably 0.300% or less.

[0030] Nb: 0.100% or less Nb can be added because it is an element that is effective in refining crystal grains by bonding with C in the steel to form carbides. To achieve this effect, the Nb content is preferably 0.001% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of Nb-based carbides is generated, making cracks more likely to occur during cold working. Therefore, the Nb content is preferably 0.100% or less.

[0031] B: 0.0100% or less B can be added because it is an element that can significantly improve the hardenability of steel material with a small amount of addition and is effective in improving the strength of steel. To obtain this effect, the B content is preferably 0.0005% or more. Furthermore, if the B content exceeds 0.0100%, the effect saturates. Therefore, the B content is preferably 0.0100% or less.

[0032] Fe and inevitable impurities The steel material according to the present invention has a composition containing the elements described above, with the balance being Fe and inevitable impurities. Fe is the main component of the steel material according to the present invention. In this specification, "unavoidable impurities" generally refer to impurities that are present in the raw materials of metal products or that are inevitably mixed in during the manufacturing process, and which are essentially unnecessary but are tolerated because they are present in trace amounts and do not affect the properties of the metal product.

[0033] Examples of elements corresponding to inevitable impurities include O (oxygen), Ca, Bi, Sb, etc. Even if the steel material according to the present invention inevitably contains, by mass percentage, 0.0100% or less of O (oxygen), 0.01% or less of Ca, 0.01% or less of Bi, and 0.03% or less of Sb, the contents of these elements are so small that they do not affect the cold forgeability. It is permissible in the present invention for the steel material to contain trace amounts of elements other than the exemplified O (oxygen), Ca, Bi, and Sb, as long as they do not affect the effects of the present invention.

[0034] ([Cu] ｍａｘ / [Cu] ｍｉｎ + [Ni] ｍａｘ / [Ni] ｍｉｎ + [Sn] ｍａｘ / [Sn] ｍｉｎ )≦3.45 ... (1) The above formula (1) defines the degree of segregation of Cu, Ni, and Sn in steel. As described above, Cu, Ni, and Sn effectively act to improve shear workability. If the above formula (1) is not satisfied, even if other requirements are met, differences will occur in the shear workability within the cross section of the steel, and the effect of suppressing burrs will not be achieved. For these reasons, the above formula (1) is defined.

[0035] In the present invention, the maximum value of the Cu content, Ni content and Sn content [Cu] ｍａｘ , [Ni] ｍａｘ , [Sn] ｍａｘ and minimum value [Cu] ｍｉｎ , [Ni] ｍｉｎ , [Sn] ｍｉｎ is measured in five specified regions. That is, the five regions are the center point of a cross section perpendicular to the longitudinal direction of the steel material, and four midpoints of four line segments drawn from the center point to the surface of the steel material, passing through the center point and spaced at 90° angles around an axis perpendicular to the cross section. In the present invention, the shape of the cross section perpendicular to the longitudinal direction of the steel material is circular, elliptical, square, or rectangular. If the cross section is circular, first draw one line segment at an arbitrary position, and then draw the remaining three line segments spaced at 90° angles around an axis perpendicular to the cross section, thereby drawing four line segments. If the cross section is elliptical, draw four line segments so that two of the four line segments form the minor axis of the ellipse and the remaining two form the major axis. If the cross section is square, draw four line segments so that the four line segments form the diagonals of the square. If the cross section is rectangular, draw four line segments from the center of the rectangle perpendicular to each face. If the shape of the rectangle is very close to a square, the way the lines are drawn will be different, but in such cases the way the lines are drawn does not have a significant effect on the results, so either method is acceptable.

[0036] (w ｍａｘ -w ｍｉｎ ) / (w ｍａｘ +w ｍｉｎ)≦0.30 ... (2) The above formula (2) defines the bias of strain in the steel material. The processing strain at the measurement position can be evaluated by the half-width of the X-ray reflection peak. If formula (2) is not satisfied, there is bias of strain within the cross section of the steel material, and burrs are generated as the shearing process is not uniform. For these reasons, the above formula (2) was defined.

[0037] The half-value width of the 200 reflection peak of ferrite can be obtained by X-ray diffraction after appropriately cutting and polishing the steel material. After mirror-polishing the cut surface perpendicular to the longitudinal direction of the steel material, an X-ray spectrum of the measurement region is obtained using an X-ray diffractometer. The half-value of the 200 reflection peak of ferrite in the obtained X-ray spectrum is half the maximum value of that peak. The value of the half-value width can be obtained by measuring the width of the peak at half value.

[0038] The half width of the 200 reflection peak of ferrite is measured for the above-mentioned five regions, as well as the maximum and minimum values ​​of the Cu content, Ni content, and Sn content. ｍａｘ , the minimum value is w ｍｉｎ is.

[0039] Next, a method for manufacturing a steel material according to the present invention will be described. The typical manufacturing steps up to the production of a steel material are as follows: (1) a casting step in which molten steel having a predetermined chemical composition is poured and cooled to obtain a steel ingot, and (2) a hot rolling step in which the steel ingot produced in the previous step is heated, rolled, and cooled.

[0040] In the present invention, by manufacturing the steel sheet so that predetermined conditions are satisfied in the above steps, it is possible to control the strain distribution and component segregation before shearing within predetermined ranges.

[0041] [Casting Process] In the casting process, molten steel having a predetermined composition is prepared, and an intermediate steel ingot is produced. The steel ingot is produced by casting such that the aspect ratio of the cross section perpendicular to the longitudinal direction (the value obtained by dividing the long side by the short side) is 1.00 or more and 2.00 or less, the casting temperature is in the range of the melting point of the component to the melting point + 100°C, and the casting speed is in the range of 0.1 to 3.0 m / min. By satisfying these conditions of the cross-sectional shape of the steel ingot, the casting temperature, and the casting speed, Cu, Ni, and Sn are evenly distributed throughout the steel material, and after subsequent hot rolling, an as-rolled material that satisfies formula (1) can be obtained.

[0042] [Hot Rolling Process] In the hot rolling process, the steel ingot produced as described above is held in a heating furnace at 1000 to 1250°C, and then hot rolled at a finish temperature of 800°C or higher with a cross-sectional area reduction of 20% or more. The steel ingot is then cooled from 800°C to 300°C at an average cooling rate of 30°C / sec or less. Note that the above temperature is the surface temperature of the steel plate. This allows for the production of a steel product that satisfies formula (2). Note that the cross-sectional area reduction is preferably 50% or more, and more preferably 80% or more.

[0043] The constitution and effects of the present invention will be specifically explained below with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the gist of the present invention, and all such modifications are included in the technical scope of the present invention.

[0044] Steels having the chemical compositions shown in Table 1 were melted and hot rolled into round bars with a diameter of 20 mm. Samples were taken from the as-rolled steels for a burr generation evaluation test, a Cu, Ni, and Sn segregation state evaluation test, and an X-ray measurement test.

[0045]

[0046] The burr generation evaluation was carried out as follows. That is, a hot-rolled round bar having a diameter of 20 mm was used as a sample, and it was cut into 50 mm lengths by shear cutting (cutting by shearing processing) perpendicular to the longitudinal direction, and the length of the burrs generated on the cut surface was measured. Five cuts were made for each sample, and the burr suppression ability was evaluated based on the maximum length of the burrs generated. In this test, samples with a maximum burr length of 0.06 mm or less were evaluated as having excellent burr suppression ability.

[0047] The segregation states of Cu, Ni, and Sn were investigated as follows. Specifically, a 20 mm diameter round bar after hot pressing was cut perpendicular to the longitudinal direction to obtain a circular cross-sectional sample. Measurement positions were the center point of the circle and four D / 4 positions (D is the diameter of the round bar). For each of the four line segments connecting each D / 4 position to the center point, the four D / 4 positions were determined so that two adjacent line segments were offset by 90° around an axis passing through the center point and perpendicular to the cross section. In other words, the four D / 4 positions were determined so that the four line segments were positioned at 90° intervals around the axis. Quantitative analysis of Cu, Ni, and Sn was performed at these five measurement positions using an electron probe microanalyzer (EPMA). The beam diameter was 10 μm, the acceleration voltage was 20 kV, the measurement time per point was 10 ms, and the measurement intervals were 10 μm in both the X-axis and Y-axis directions. A 100 μm square was analyzed. The average value of the area analyzed was taken as the measurement value of the measurement area, and the maximum and minimum values ​​of the Cu concentration of the measurement values ​​in the five areas were taken as [Cu] ｍａｘ and [Cu] ｍｉｎ , the maximum and minimum values ​​of Ni concentration are [Ni] ｍａｘ and [Ni] ｍｉｎ , the maximum and minimum values ​​of Sn concentration are [Sn] ｍａｘ and [Sn] ｍｉｎ , and the value of equation (1) was calculated.

[0048] X-ray measurements were carried out as follows. That is, a hot-rolled round bar with a diameter of 20 mm was cut perpendicular to the longitudinal direction to obtain a sample with a circular cross section. The measurement positions were the center point of the circle and four D / 4 positions (D is the diameter of the round bar). For each of the four line segments connecting each D / 4 position to the center point, the four D / 4 positions were determined so that two adjacent line segments pass through the center point and are shifted by 90° around an axis perpendicular to the cross section. X-ray spectra were obtained in the above five measurement regions using an X-ray diffractometer. The voltage was 40 kV, the current was 40 mA, and the X-ray target was Cr, and its K α１ The half width of the 200 reflection peak of the ferrite in the acquired X-ray spectrum was measured. Three measurements were taken at each measurement position under the same conditions, and the average value was used as the measured value. The maximum value among the measured values ​​of the half width of the 200 reflection peak in the five measurement regions was taken as w ｍａｘ , the minimum value is w ｍｉｎ The value of equation (2) was calculated as follows.

[0049] The above evaluation results are shown in Table 2.

[0050]

[0051] Example Nos. 1 to 93 are examples made using suitable steel and an appropriate manufacturing method. Example Nos. 94 to 119 (excluding Nos. 104, 106, 108, and 110) are comparative examples made using an appropriate manufacturing method and have no problems with burr suppression, but have poor other properties such as steel strength and machinability. Example Nos. 120 to 130 are comparative examples made outside the range of appropriate manufacturing methods and do not suppress burrs. That is, Example Nos. 120 to 123, 128, and 129 did not satisfy formula (1) because the manufacturing conditions in the casting process were outside the range, and Example Nos. 124 to 128 and 130 did not satisfy formula (2) because the manufacturing conditions in the hot rolling process after the casting process were outside the range.

[0052] According to the present invention, it is possible to provide a steel material that suppresses the generation of burrs and has excellent shearing properties, which makes it possible to simplify or even eliminate the deburring process that has been conventionally performed, thereby contributing to improved productivity, reduced environmental impact, and reduced component manufacturing costs.

Claims

1. A steel material having a chemical composition containing, by mass%, C: 0.03 to 0.80%, Si: 0.010 to 1.800%, Mn: 0.010 to 1.500%, P: 0.0010 to 0.0500%, S: 0.0010 to 0.0500%, Ni: 0.010 to 1.000%, Cu: 0.010 to 0.500%, Cr: 0.010 to 1.500%, Sn: 0.0010 to 0.1000%, N: 0.0020 to 0.0250%, with the balance being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): ([Cu] ｍａｘ / [Cu] ｍｉｎ + [Ni] ｍａｘ / [Ni] ｍｉｎ + [Sn] ｍａｘ / [Sn] ｍｉｎ )≦3.45…(1) (w ｍａｘ -w ｍｉｎ ) / (w ｍａｘ +w ｍｉｎ ) ≦ 0.30 ... (2) where [Cu] ｍａｘ , [Ni] ｍａｘ , [Sn] ｍａｘ are the maximum values ​​of the Cu content, Ni content, and Sn content measured in the five regions shown below, respectively, [Cu] ｍｉｎ , [Ni] ｍｉｎ , [Sn] ｍｉｎ are the minimum values ​​of the Cu content, Ni content, and Sn content measured in the five regions. The five regions are the center point of a cross section perpendicular to the longitudinal direction of the steel material, and four midpoints of four line segments drawn from the center point to the surface of the steel material at 90° intervals around an axis perpendicular to the cross section. ｍａｘ and w ｍｉｎ are the maximum and minimum values ​​of the half width at the 200 reflection peak of ferrite obtained by performing X-ray diffraction measurement in the above five regions.

2. The steel material according to claim 1, wherein the composition further contains, in mass %, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, and B: 0.0100% or less.

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