JPS6153406B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a sulfur free-cutting steel bar, and particularly to a method for producing a low-carbon sulfur free-cutting steel bar. Low-carbon sulfur free-cutting steel contains less than 0.15 [wt%] of carbon, and after being rolled into a sulfur free-cutting steel bar, it is processed into bolts, nuts, and other small parts by cutting. Often. With conventional sulfur free-cutting steel bars, it is not easy to exhibit sufficient hardness while maintaining machinability, and it is also difficult to finish the bar with sufficient dimensional accuracy by rolling alone. Therefore, in the past, sulfur free-cutting steel bars were subjected to cold drawing to ensure hardness and dimensional accuracy, but production efficiency could not be sufficiently increased. This invention was devised to solve these conventional problems, and aims to provide a method for manufacturing sulfur free-cutting steel bars that can ensure sufficient hardness and dimensional accuracy without cold drawing. purpose. The method for producing a sulfur free-cutting steel bar according to the present invention includes: (i) setting the carbon content to 0.12 to 0.25 [wt%];
Improve hardness while maintaining machinability; (ii) Minimize the adverse effects of silicon by keeping the silicon content below 0.10 [wt%]; (iii) Reduce the manganese content from 0.85 to 1.50 [wt%]. (iv) The sulfur content is set to 0.260 to 0.350 [wt%] to prevent hot brittleness and ensure good machinability, (v) The phosphorus content is 0.100 [wt%] or less,
Maintaining machinability while suppressing embrittlement tendency, and (iv) further improving machinability by containing at least one of lead at 0.30 [wt%] or less or calcium at 0.0100 [wt%] or less. For the final product of sulfur free-cutting steel, the error with respect to the intended diameter, intended diameter across sides, or intended diagonal diameter is within ±1 [%], and the bending for a length of 1 [m] is within 2 [mm]. Rolling is performed so that That is, in the present invention, steel consisting of the above components is heated to the extraction temperature.
Tension-free rolling is performed at 850 to 1050℃, and the final finishing temperature is in the temperature range of 800 to 950℃.Moreover, a rotatable roller is installed almost perpendicular to the exit of the final finishing roll, and the rolling reduction amount is 0.25 to 5.0%. It is characterized by performing shape correction rolling, cooling at a cooling rate of 0.3 to 5°C/S, that is, normal cooling, followed by cold straightening rolling. Next, an embodiment of the method for producing a sulfur free-cutting steel bar according to the present invention will be described. The components of the sulfur free-cutting steel materials in the first example and the second example are shown in Table 1 together with the components in the comparative example.

[Table] Here, in the examples of the present invention, (i) the carbon content is set to 0.12 to 0.25 [wt%] while machinability is maintained, and the hardness after hot rolling is set to 150 to 150.
It has been increased to 200HB. That is, the carbon content
If it is less than 0.11 [%], hot rolling alone will not work.
It has been experimentally revealed that when the hardness of 150HB or higher cannot be maintained and the carbon content exceeds 0.25 [wt%], the shape of the MnS crystals precipitated in the steel becomes flat, resulting in a significant decrease in machinability. ing. (ii) The silicon content is kept to 0.10 [wt%] or less to minimize the adverse effects of silicon. That is, it is desirable that the silicon content be as low as possible, but due to metallurgical reaction constraints, the silicon content should be 0.10 [wt%] or less as a practical content. (iii) The manganese content is set to 0.85 to 1.50 [wt%], and hot embrittlement is suppressed by the minimum necessary manganese content. In other words, manganese is an essential element for suppressing hot brittleness, but when the content is lower than 0.85 [wt%], the effect of suppressing hot brittleness is significantly reduced, and when it exceeds 1.50 [wt%], The increase in tensile strength increases,
The manganese content was set at 0.85 to 1.50% to prevent deterioration of machinability. (iv) The phosphorus content is 0.100 [wt%] or less,
Maintains machinability while suppressing embrittlement tendency.
In other words, phosphorus improves machinability by dissolving in ferrite and promoting ferrite embrittlement, but experiments have shown that when the content exceeds 0.100 [wt%], the tendency of steel to become brittle becomes more pronounced. It is becoming clear that (v) The sulfur content is 0.260 to 0.350 [wt%] to prevent red brittleness and ensure good machinability. In other words, sulfur and manganese are MnS
However, when the sulfur content is lower than 0.260 [wt%],
Machinability decreases because MnS is not sufficiently extracted, and when the sulfur content exceeds 0.350 [wt%]
A large amount of eutectic structure of FeS and Fe precipitates, and red brittleness becomes noticeable. (vi) 0.30 [wt%] or less of lead is added to improve machinability in low-speed cutting and to improve the shape of chips. Here the lead content is 0.30
There is no difference in these effects even if the lead content is increased by more than [wt%], and therefore machinability and chip shape are improved by adding a minimum amount of lead. (vii) Calcium is added in an amount of 0.0100 [wt%] or less to promote the dispersion of lead and ensure the effect of lead addition.It also improves machinability in high-speed cutting, and also improves the machinability of non-metallic materials in low-speed cutting. The shape of inclusions has been improved to improve machinability. Calcium content here is 0.0100
There is no difference in these effects even if the calcium content is increased by [wt%], and therefore, the above effects were obtained with the minimum addition of calcium. (viii) Similar to the comparative example, in the first and second examples, a significant amount of nitrogen is added to promote work hardening. As shown by 〇 and △ marks in Figure 1,
The hardness of sulfur free-cutting steel that is tension-free rolled is the same as that of conventional sulfur free-cutting steel (marked with ● in the figure).
It is much higher than 100 [HB], and when this steel material is subjected to cold straightening rolling, it becomes 150 to
A hardness of about 200 [HB] can be obtained. That is, the present invention leaves room for securing a hardness of 150 to 200 [HB] by performing cold straightening rolling at the time of tensionless rolling. Therefore, the sulfur free-cutting steel material according to the present invention has a sufficient hardness of 150 to 200 [HB] by hot stressless rolling→shape correction rolling→cold straightening rolling. In addition, due to the above rolling, the error with respect to the intended diameter, intended diameter across opposite sides, or intended diagonal diameter is ±1.
[%] or less, and sufficient dimensional accuracy can be obtained in that the bending for a length of 1 [m] is within 2 [mm]. In the hot tensionless rolling, the rotation speed of the rolls is adjusted so as not to apply tension to the material to be rolled, thereby maintaining the dimensional accuracy of the shoulder and width parts of the roll hole shape. . The shape correction rolling involves rolling down the shoulder and width of the groove of the final roll in hot tensionless rolling using a rotatable roller that is substantially perpendicular to the final roll in hot tensionless rolling. It is. In this shape correction rolling, shaping rolling is performed with a reduction amount of 0.25 to 5.0 [%] of the intended diameter, intended diameter across sides, or intended diagonal diameter, and a reduction amount of at least 0.1 [mm], and In order to fully exhibit the shape modification effect of shape modification rolling, rolling is performed so that the groove of the final rolling roll in hot tensionless rolling tends to fill the shoulder and width regions. The cold straightening process involves rolling the sulfur free-cutting steel bar after it has been cooled on a cooling bed using a drum straightening machine or a multistage straightening machine to suppress the bending of the sulfur free-cutting steel bar within a predetermined range. During this rolling, the dimensional accuracy of the diameter, opposite side diameter, or diagonal diameter of the sulfur free-cutting steel bar is also improved. Descaling, which is performed prior to cold straightening and straightening, is done by chemical treatments such as pickling or mechanical treatments such as shot blasting, which significantly improves the surface properties of sulfur free-cutting steel bars and Improve work efficiency and yield in the process. Hot tensionless rolling and shape correction rolling are performed continuously using a series of equipment, and the rolling conditions are as follows:
Tension-free rolling should be started at a relatively low extraction temperature of 850-1050 [°C], and the final finishing temperature in shape correction rolling should be 800-950 [°C]. If the finish rolling temperature is higher than 950 [℃], austenite grains will grow after rolling and the hardness distribution will become uneven.
If the temperature is lower than 800 [°C], rolling becomes difficult. As shown in Table 2, the Examples were rolled under these rolling conditions, and the Comparative Examples were subjected to cold drawing in addition to similar rolling.

[Table] That is, the hot rolling has the same effect as normalizing the material to be rolled, and can produce a sulfur free-cutting steel bar with uniform hardness. The cooling rate after such rolling is
Normal natural cooling at 0.3 to 3 [°C/sec] is appropriate. Table 3 shows the effects of the cold straightening rolling.
As is clear from Table 3, dimensional accuracy is improved by cold straightening rolling, and the effect becomes more pronounced as the number of passes of straightening rolling increases and as the amount of rolling in one pass increases. The results in Table 3 are divided into diameter accuracy and bending and are shown in Figures 2 and 3, respectively.

【table】

[Table] The machinability of the sulfur free-cutting steel bars obtained in this way is compared by the resultant force generated in the automatic lathe cutting tool. (Table 4)

[Table] Here, the specifications of cutting conditions A, B, and C shown in Table 4 are shown in Tables 5 and 6.

【table】

[Table] As is clear from Table 4, the sulfur free-cutting steel bar according to the present invention exhibits free-cutting properties similar to those of the comparative example. The tool bits used for the machinability test were made of SKH4A material.
Blade width 3 [mm], rake angle 15 [deg], front relief angle 6
[deg], and the side clearance angle was 2 [deg]. As described above, the present invention has the following effects: improved production efficiency by omitting cold drawing, post-processing by improving surface properties, improved production efficiency by improving yield, and improved dimensional accuracy equivalent to cold drawing. be.

[Brief explanation of the drawing]

Fig. 1 is a graph comparing the hardness of an example of the present invention and a comparative example, Fig. 2 is a graph showing improvement in diameter dimensional accuracy by cold straightening rolling of the present invention, and Fig. 3 is a graph showing bending improvement in the same rolling. This is a graph showing the situation.

Claims (1)

[Claims] 1% by weight, including C: 0.12 to 0.25% Si: 0.10% or less Mn: 0.85 to 1.50% S: 0.260 to 0.350% P: 0.100% or less, and further includes Pb: 0.30% or less Ca: Contains one or two of the following: 0.01% or less; the remainder is substantially
A steel billet containing Fe and unavoidable impurities is subjected to tensionless rolling at an extraction temperature of 850 to 1050°C, the temperature is controlled so that the final finishing temperature is in the temperature range of 800 to 950°C, and the exit of the final finishing roll is performed. 1. A method for producing a sulfur free-cutting steel bar, which comprises shape-correcting rolling with a reduction of 0.25 to 5.0% using rotatable rollers installed in a sulfur free-cutting steel bar, cooling, and then cold straightening rolling.