Ferritic stainless steel bars with excellent shape retention after processing.

Abstract

To provide a ferritic stainless steel bar that can reduce surface irregularities during cutting by reducing variations in the orientation of the texture. [Solution] A ferritic stainless steel bar, in which X-ray diffraction measurements were taken using CoKα1 rays at 8 or more points along the entire circumference of a surface perpendicular to the longitudinal direction of the bar, A ferritic stainless steel bar in which the magnitude of variation of the ratio of the integrated intensity of peaks with a peak angle of 123-125° at 2θ to the integrated intensity of peaks with a peak angle of 76-78° at 2θ, divided by 1.5, is within 0.6 and the standard deviation is within 0.12.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel bar that is suitable for the material of a rotating member formed by cutting, has excellent machining accuracy when machining a steel bar, and can easily secure a desired shape.

Background Art

[0002] As steel for a rotating member formed by cutting, for example, in the case of steel for a motor for a hard disk drive (HDD motor), excellent corrosion resistance and machinability are required. Therefore, among stainless steels with high corrosion resistance, ferritic stainless steels with excellent machinability are generally used.

[0003] In a rotating member, high machining accuracy is also required from the viewpoint of preventing abnormal vibration during high-speed rotation. Therefore, machinability is particularly emphasized.

[0004] Therefore, in order to improve machinability, addition of various free-cutting elements such as S, Te, Se, Pb, Bi, and Ca has been studied. For example, addition of S is expected to generate inclusions such as MnS, which serve as stress concentration sources during cutting and promote crack propagation. Also, addition of Pb, which is a low-melting-point metal, is expected to melt between the tool and the chip to obtain a lubricating effect.

[0005] However, unless the sulfide can be controlled to an effective size and shape for machinability, the effect of machinability cannot be fully obtained. Also, if the total amount of free-cutting property-imparting elements increases, there is a problem of deteriorating hot workability. Furthermore, there are concerns that elements such as Pb and Se may be harmful to the human body.

[0006] As a ferritic free-cutting stainless steel with excellent machinability, its composition in mass% is as follows: C: 0.005~0.020%, Si: 0.10~0.50%, Mn: 0.05~0.50%, P: 0.005~0.10%, S: 0.20~0.35%, Cu: 0.01~0.60%, Ni: 0.01~0.60%, Cr: 17.0~25.0%, Mo: 0.01~1.0%, Pb: 0.03~0.30%, Te: 0.01~0.10%, B: 0.003~0.008%, O: 0.005~0.020%, N: 0.005~0.0 A steel has been proposed that contains 30% Nb: 0.03-0.50%, Al: 0.001-0.100%, satisfies the following equations: [Nb] ≥ 4 × ([C] + [N]), [Te] / [S] ≥ 0.05, 10 ≤ [S] / [O] ≤ 50 (the values ​​in brackets indicate the mass percentage of each element), and furthermore, the total amount of sulfides with an equivalent circle diameter of 2.0 μm or more and an aspect ratio of 10 or less is 0.50% or more by area percentage, and the Vickers hardness is 160 or less, with the remainder being substantially Fe and unavoidable impurities (see Patent Document 1). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2008-106306 [Overview of the project] [Problems that the invention aims to solve]

[0008] In their investigation of ferritic stainless steel bars suitable for rotating members, the inventors observed rotating members that had been turned to a desired shape. They found that when hot-rolled ferritic stainless steel bars are turned to produce rotating members, the turning process can sometimes result in irregularities appearing on the outer periphery of the surface perpendicular to the longitudinal direction of the bar, and that these irregularities can affect the accuracy of the machining to the desired shape.

[0009] Upon investigating the factors causing these irregularities, it was found that ferritic stainless steel is less prone to recrystallization, and during rolling, in the direction of rolling... <110> or <211> We discovered that the tendency to form a texture in specific crystal orientations results in unevenness during cutting, leading to a decrease in machining accuracy.

[0010] While the Young's modulus of iron exhibits a unique value for each crystal orientation, if each orientation is randomly distributed within the steel, the Young's modulus in the longitudinal direction of the steel bar will exhibit an average value based on those random orientations.

[0011] However, in a specific direction, for example, along the longitudinal direction of the steel bar <110> If the orientation is strongly expressed, the Young's modulus in the longitudinal direction of the steel bar will be close to that specific orientation. During cutting, the cutting surface receives compressive stress from the chip, and deformation recovery occurs by the amount of elastic deformation when the chip passes and the stress is removed. As a result, the region with a high Young's modulus will be concave compared to the region with a low Young's modulus.

[0012] Conventional ferritic stainless steels, such as the ferritic stainless steel proposed in Patent Document 1, have not taken into consideration the influence of the texture formed by rolling on the processed shape. Therefore, from the perspective of improving processing accuracy, they have not been sufficient.

[0013] Since the manufacturing of steel bars generally involves a rolling process, the formation of a texture in the steel with a dominant orientation in which crystal grains are arranged in parallel in a specific direction is unavoidable. Therefore, the inventors realized that to improve the processing accuracy when processing steel materials with such a texture into a desired shape, it is effective to reduce the variation in the orientation of the texture within the plane.

[0014] Therefore, the present invention aims to provide a ferritic stainless steel bar that can reduce surface irregularities during cutting by reducing variations in the orientation of the texture. [Means for solving the problem]

[0015] The present inventors have established a ferritic stainless steel bar and a method for manufacturing the same, which reduces surface irregularities during cutting caused by texture, by the following means.

[0016] The first means for solving the problems of the present invention is a ferritic stainless steel bar in which, when X-ray diffraction measurements are taken at eight or more points on the entire outer circumference of a surface perpendicular to the longitudinal direction of the bar using CoKα1 rays, the ratio of the integrated intensity of the peak with a peak angle of 2θ of 123 to 125° to the integrated intensity of the peak with a peak angle of 2θ of 76 to 78° at each measurement point, further divided by 1.5, has a difference of 0.6 or less between its maximum and minimum values, and the standard deviation of these values ​​is 0.12 or less.

[0017] In other words, when the value of A is defined as (integral intensity of the peak at the peak position at 2θ:123~125°) / (integral intensity of the peak at the peak position at 2θ:76~78°) / 1.5 in X-ray diffraction (XRD) measurements using specific CoKα1 X-rays, including Co tubes, the difference between the maximum and minimum values ​​of the A value, which is the upper and lower limits of the variation in the A value obtained by measuring the outer circumference of the cross-section perpendicular to the longitudinal direction of the steel bar at eight or more locations using XRD, is within 0.6, and the standard deviation of the A value is within 0.12, which is a ferritic stainless steel bar. The upper and lower limits of the variation can be determined by (maximum value of A) - (minimum value of A), so the variation of A in ferritic stainless steel bars is within the range of 0.1 < (maximum value of A) - (minimum value of A) ≪ 0.6. Here, if the standard deviation is within 0.12, the standard score of the value (mean of A + 0.30) will be 75 or higher, and the standard score of the value (mean of A - 0.30) will be 25 or lower.

[0018] The second means is a ferritic stainless steel bar described in the first means, which contains, by mass%, C: 0.100% or less, Si: 1.00% or less, Mn: 1.00% or less, S: 0.050 - 0.500%, Cr: 13.00 - 20.00%, Al: 0.011 - 0.500%, Te: 0.010 - 0.100%, and O: 0.020% or less, with the balance being Fe and inevitable impurities.

Advantages of the Invention

[0019] According to the means of the present invention, a ferritic stainless steel bar with small variations in local orientation within the bar and small variations in the orientation of the in-plane microstructure can be obtained. That is, if the upper and lower width of the variation in the A value of the ferritic stainless steel bar is within 0.6 and the standard deviation of the A value is within 0.12, the blur in the <110> orientation is small. As a result, the variation in the in-plane Young's modulus is small, and the formation of unevenness due to cutting is reduced. Therefore, the processing accuracy when obtaining the desired shape is excellent, and it is easy to ensure the intended processed shape. Thus, it becomes a ferritic stainless steel bar with excellent shapeability after processing.

Brief Description of the Drawings

[0020] [Figure 1] FIG. 1 is a schematic diagram for explaining an example of the measurement location of X-ray diffraction of the bar steel. [Figure 2] FIG. 2 is a diagram showing an example of the XRD pattern of the measurement results of X-ray diffraction using a Co tube target for the bar steel of the present invention. [Figure 3] FIG. 3 is a diagram showing the variation in the degree of <110> orientation at each measurement point every 45 degrees in (a) Example 1 (Sample 1) which is the bar steel of the present invention and (b) the bar steel of Comparative Example 1 (Sample 6). [Figure 4] FIG. 4 is a diagram in which the surfaces for measuring the peak intensity of (a) Example 1 (Sample 1) which is the bar steel of the present invention and (b) the bar steel of the Comparative Example (Sample 6) are parallelized and cut, and the unevenness on the outer periphery is measured over the entire circumference.

Modes for Carrying Out the Invention

[0021] Before describing the embodiments of the bar steel of the present invention, the reason for defining the degree of variation of the A value based on the peak integral intensity ratio will be explained. First, the crystal structure of ferrite iron is generally a body-centered cubic structure (BCC). Therefore, when measuring ferrite iron with a Co tube sphere by XRD, from the Bragg condition of the CoKα1 line, a peak of (220), which is a plane perpendicular to <110>, appears at 2θ: 123 to 125°, and a peak of (200), which is a plane perpendicular to <100>, appears at 2θ: 76 to 78°. In ferrite iron, the peak intensities of these (220) and (200) planes are relatively close.

[0022] And the peak intensity ratio of the (220) and (200) planes in XRD, (220) / (200), is said to be 1.5 in a sample with a random crystal orientation. On the other hand, in a sample having a texture with a preferred direction in which crystal grains in the steel are aligned in a specific direction, the ratio of (220) / (200) becomes a relatively high value.

[0023] Therefore, when obtaining the ratio of (the integrated intensity of the peak at the position where 2θ is 123 to 125°) / (the integrated intensity of the peak at the position where 2θ is 76 to 78°) in the XRD measurement using a Co tube sphere and dividing this value by 1.5 corresponding to the ratio in a random sample, the A value becomes an index indicating the degree of orientation of the <110> texture.

[0024] When the orientation of the <110> texture of the rolled bar steel is large, when measuring the outer periphery of a plane perpendicular to the longitudinal direction of the bar steel at multiple locations by X-ray diffraction, the A value at each location varies greatly and is likely to vary. And the greater the variation in the A value, the greater the unevenness after cutting. A bar steel with a large variation in the A value, when cut into a desired shape by cutting, the surface of the processed surface becomes uneven, resulting in poor machining accuracy and poor shape retention after processing.

[0025] Therefore, from the standpoint of maintaining good processing accuracy of the steel bars, the A value was determined by measuring at least eight points on the outer circumference of the cross-section perpendicular to the longitudinal direction of the steel bar, almost evenly spaced, using XRD. Measuring at least eight points on the outer circumference at almost even positions is done to efficiently evaluate the variability when measuring the entire outer circumference of the steel bar, where the effects of rolling are likely to be strongly evident, thereby appropriately evaluating whether there is variability in the degree of orientation. For each obtained A value, the range of variation is kept small, that is, (maximum A value) - (minimum A value) is within 0.6.

[0026] Furthermore, when expressing the variance of these A values ​​in terms of standard deviation, we assume that the standard deviation is within 0.12. For example, if the standard deviation is 0.12, the standard score for a deviation of 0.3 above the mean corresponds to 75, and the standard score for a deviation of -0.3 from the mean corresponds to 25. Therefore, if the standard deviation is within approximately 0.12, it can be reasonably explained that the A values ​​will fall within the range of 0.6. If the standard deviation is even smaller, the standard score for a value that is 0.3 above the mean will exceed 75, meaning that such deviations will be extremely small. This results in less variation in the degree of orientation, making it less likely for irregularities to occur on the machined surface after processing, thus ensuring stable machining accuracy and excellent shape retention after processing.

[0027] Next, preferred chemical components for the ferritic stainless steel of the present invention will be described. Note that the percentages for the chemical components are given in mass percentages.

[0028] C: 0.100% or less Carbon (C) is a useful component for increasing strength. However, if the C content is too high, it will reduce corrosion resistance and toughness, so it is preferable to keep the C content to 0.100% or less.

[0029] Si: 1.00% or less Si is an effective component for deoxidation. However, if the Si content is too high, the annealing hardness increases and the workability decreases, so it is preferable that the Si content be 1.00% or less.

[0030] Mn: 1.00% or less Mn, along with Cr, forms sulfides, which improve machinability. However, if the Mn content is too high, the Cr concentration in the sulfides decreases, leading to corrosion initiation and reduced corrosion resistance. Therefore, it is preferable that the Mn content be 1.00% or less.

[0031] S: 0.050~0.500% S forms sulfides, which act as stress concentration sources during cutting, thereby improving machinability. To achieve this effect, a content of 0.050% or more is preferable. On the other hand, if there is too much S, hot workability decreases, so it is preferable that the S content be 0.500% or less. Therefore, an S content of 0.050 to 0.500% is preferable.

[0032] Cr: 13.00~20.00% Cr is a fundamental component of stainless steel and is useful for ensuring corrosion resistance. To achieve sufficient corrosion resistance, it is preferable to have a Cr content of 13.00% or more. On the other hand, if Cr is too high, the material becomes expensive and its hot workability decreases, so it is preferable to have a Cr content of 20.00% or less. Therefore, a Cr content of 13.00% to 20.00% is preferable.

[0033] Al: 0.011~0.500% Al is a powerful deoxidizing agent. If the amount of Al is too low, it is likely to lead to insufficient deoxidation during manufacturing, resulting in a decrease in the cleanliness of inclusions. Therefore, it is preferable that the amount of Al be 0.011% or more. However, if the amount of Al is too high, hard oxides will be formed during solidification, hindering machinability. Therefore, it is preferable that the amount of Al be 0.500% or less. Thus, an Al content of 0.011 to 0.500% is preferred.

[0034] Te: 0.010~0.100% Te is a component that suppresses the spread of sulfides, reduces anisotropy, and improves machinability. To obtain this effect, it is preferable to have a Te content of 0.010% or more. However, if Te is too high, the hot workability decreases, so it is preferable that Te be 0.100% or less. Therefore, a Te content of 0.010 to 0.100% is preferable.

[0035] O: 0.020% or less If the amount of oxygen is high, it generates coarse oxides, which inhibits machinability and worsens surface finish. Therefore, the amount of oxygen is preferably 0.020% or less, and more preferably 0.0090% or less.

[0036] The ferritic stainless steel bars of the present invention will be described in detail below with reference to examples. First, Table 1 shows the chemical composition of the steels used in Examples 1 to 3 and Comparative Example 1 of the present invention. The units are in mass%, and the remainder is Fe and unavoidable impurities.

[0037] First, steel ingots averaging 460 mm square were prepared by ingot casting using the steel components of Example Steels 1-3 and Comparative Example Steel 1 as described in Table 1. These steel ingots were then subjected to the processing and measurements described below.

[0038] [Table 1]

[0039] (rolling) Rolling has the effect of adjusting the product diameter and refining the microstructure. While texture is formed during plastic working such as rolling, this can be suppressed by recrystallization, which occurs more readily at higher temperatures. On the other hand, if the temperature is too high, the hot workability deteriorates, such as causing the steel bar to bend. From these viewpoints, the rolling temperature for the steel bar in this invention is preferably 1060°C to 1120°C. The diameter of the steel bar can be rolled to the desired diameter as long as it is within the range of 20 to 50 mm. In Example 1, steel ingots were extracted from a heating furnace at temperatures of 1100°C for sample 1, 1060°C for sample 2, and 1115°C for sample 3, and then rolled to a diameter of 30 mm. In Sample 4 of Example 2, the steel ingot was extracted from a heating furnace at a temperature of 1060°C and rolled to a diameter of 30 mm. In Sample 5 of Example 3, the steel ingot was extracted from a heating furnace at a temperature of 1060°C and rolled to a diameter of 30 mm. In Comparative Example 1, steel ingots were extracted from a heating furnace at temperatures of 1050°C for sample 6 and 1150°C for sample 5, and then rolled to a diameter of 30 mm.

[0040] Furthermore, the steel bar (sample 7) of Comparative Example 1, extracted at 1150°C, bent at the end of the steel billet in the furnace, making it difficult to transport and thus preventing rolling. As a result, subsequent procedures and evaluations were discontinued.

[0041] (Multi-roll / 2-roll orthodontics) In both the examples (samples 1-5) and the comparative example (sample 6), the rolled steel bars were straightened using multi-roll and two-roll methods to adjust their dimensions. Furthermore, two-roll straightening was also performed after the annealing process that followed.

[0042] (Annealing) In the as-rolled state, the microstructure is non-uniform and internal stresses remain, so annealing is performed to homogenize the microstructure and remove internal stresses. If the annealing temperature is too low, recrystallization and relaxation of residual stresses do not occur easily, and if it is too high, it becomes too soft, so 770 to 810°C is preferred. The steel bars of the Examples (Samples 1-5) and Comparative Example (Sample 6) were annealed at 795°C for 10 hours and then air-cooled.

[0043] (Peeling, polishing and straightening) After annealing, the steel bars of the examples (Samples 1-5) and comparative example (Sample 6) that underwent two-roll straightening were further dimensionally adjusted and had their surfaces smoothed by peeling and polishing.

[0044] ( <110> (Investigation of variability in the degree of orientation) To evaluate the orientation characteristics of each steel bar obtained using these procedures (Examples 1-5 and Comparative Example 6), X-ray diffraction measurements were performed using a Co-tube.

[0045] First, a surface perpendicular to the longitudinal direction of the steel bar was cut out, and as shown in Figure 1, the outer periphery of this surface was divided into eight regions. X-ray diffraction measurements were then performed at eight points approximately evenly spaced at 45-degree intervals around the entire circumference of the cross-section perpendicular to the longitudinal direction of the steel bar. An example of an XRD pattern from the measurement results is shown in Figure 2. In this example, eight measurements were taken at approximately equal intervals around the entire circumference, but the number of measurement points is not limited to this.

[0046] From the obtained XRD pattern, the integrated peak intensity (1) at the peak position 2θ:123~125° and the integrated peak intensity (2) at the peak position 2θ:76~78° were determined. The integrated intensity may also be calculated using XRD analysis software.

[0047] Next, we calculate the integral intensity ratio of (1) and (2), which is (1) / (2), and divide this ratio by 1.5, which corresponds to the integral intensity ratio for samples with random crystal orientations. <110> The A value, which indicates the degree of orientation of the texture, is calculated. Note that (1) corresponds to the integrated intensity of the (220) plane of BCC, and (2) corresponds to the integrated intensity of the (200) plane. In other words, the A value for each XRD pattern can be calculated using the formula A value = (1) / (2) / 1.5.

[0048] If the value of (1) / (2) / 1.5 (A value) is greater than 1.0, then compared to the case of random crystal orientation, <110> The orientation becomes stronger. On the other hand, when the A value is less than 1.0, compared to the case of random crystal orientation, <110> This means that the orientation is weak.

[0049] To evaluate the degree of orientation variation at the eight measured locations, the difference between the maximum and minimum A values ​​was calculated. That is, <110> The degree of variation in orientation can be calculated by subtracting the minimum value of A from the maximum value of A.

[0050] Table 2 summarizes the manufacturing conditions and various characteristics of Examples 1-3 and Comparative Example 1. Furthermore, Figure 3(a) shows the XRD measurement results for the steel bar of Example 1 (Sample 1), and Figure 3(b) shows the results for the steel bar of Comparative Example 1 (Sample 6), at each measurement position at 45-degree intervals. <110> The degree of variation in orientation is illustrated in the diagram.

[0051] [Table 2]

[0052] As shown in Figure 3(a), the steel bar of Example 1 (Sample 1) rolled at 1100°C <110> Looking at the degree of orientation, the A value is between 2.2 and 2.6, indicating that a texture has developed due to rolling. Nevertheless, <110> The variation in the degree of orientation is stable, with the difference between the maximum and minimum values ​​remaining within a range of 0.3, and the fluctuation range being below 0.6. <110> When calculating the variance of the A value for the degree of orientation, the standard deviation is 0.097, which is below 0.12, indicating that the surface variability is small.

[0053] Next, Figure 4(a) shows the results of measuring the surface of Example 1 (Sample 1), which is a steel bar of the present invention, after the surface on which the peak strength was measured was straightened and cut, and the irregularities of the outer circumference were measured around the entire circumference. <110> The surface is concave at positions with a high degree of orientation. <110> The surface is convex at positions with a low degree of orientation, <110> Because the variation in the degree of orientation is small, the width of the surface irregularities remains at 1 μm.

[0054] Therefore, when this steel bar is cut to produce a rotating member, the difference in surface irregularities after cutting is small, and it was confirmed that the desired shape can be easily obtained by cutting. Consequently, when a rotating member was produced using the steel bar of the example, the processing accuracy was excellent, and no abnormal vibrations were observed during high-speed rotation.

[0055] On the other hand, as shown in Figure 3(b), the steel bar of Comparative Example 1 (sample 6) rolled at 1050°C had an A value, i.e. <110> The degree of orientation ranged from 2.1 to 2.9, resulting in a large variation in values, with a difference of 0.8 between the maximum and minimum values. Furthermore, the standard deviation, which indicates the variance of the A value (indicating the degree of orientation), was 0.28. This significantly exceeds the guideline of a standard deviation of 0.12, indicating a large degree of variability.

[0056] Furthermore, as shown in Figure 4(b), in the steel bar of Comparative Example 1 (sample 6) rolled at 1050°C, after cutting... <110> The degree of orientation corresponded to the resulting unevenness ranging from -1.2 to 0.7.

[0057] When a rotating member is manufactured by cutting the steel bar of Comparative Example 1, a large difference in the unevenness after cutting becomes apparent. Therefore, it is difficult to secure the desired shape even when processing the steel bar of Comparative Example 1, and the processing accuracy is poor. As a result, if used in a rotating member, this steel bar is prone to causing abnormal vibrations during high-speed rotation.

Claims

1. Ferritic stainless steel bar, Eight or more points on the outer circumference of the surface perpendicular to the longitudinal direction of the steel bar are marked with CoKα 1 When X-ray diffraction measurements are performed using a line, The ratio of the integrated intensity of peaks with a peak angle of 2θ between 123 and 125° at each measurement point to the integrated intensity of peaks with a peak angle of 2θ between 76 and 78°, further divided by 1.5, must have a difference of 0.6 or less between its maximum and minimum values, and a standard deviation of 0.12 or less between these values. Ferritic stainless steel bars.

2. The ferritic stainless steel bar according to claim 1, containing, by mass%, C: 0.100% or less, Si: 1.00% or less, Mn: 1.00% or less, S: 0.050 to 0.500%, Cr: 13.00 to 20.00%, Al: 0.011 to 0.500%, Te: 0.010 to 0.100%, and O: 0.020% or less, with the remainder being Fe and unavoidable impurities.

Images