JPH0579733B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing steel materials for cold working, and more particularly, to a method for producing steel materials for cold working, in particular, a method that prevents forging cracks from occurring during cold forging of mechanical structural parts and has excellent machinability after forging. The present invention relates to a method for producing hot rolled steel bars and bar-in coils. (Conventional technology and problems to be solved) Conventionally, in order to improve the cold workability of steel materials, in addition to adjusting the internal structure of the steel material through heat treatment, it has also been necessary to reduce nonmetallic inclusions in the steel. method has been adopted. In particular, elongated sulfide-based inclusions (A ₁ -based inclusions specified in JIS G0555), which are often found in hot-rolled materials, are considered to promote cracking during cold working and are treated with various desulfurization methods. Therefore, the S content in the steel is kept as low as possible. However, although this method is effective in improving cold workability and mechanical properties, particularly anisotropy of toughness and ductility values, on the other hand, machinability significantly decreases as the S content decreases. Therefore, in order to satisfy both cold workability and machinability, Ca and Zr were added to remove MnS inclusions.
It has been proposed to change the composition to CaS, Ca-Mn-S, Zr-Mn-S, etc. to make nonmetallic inclusions difficult to stretch during hot rolling (Japanese Patent Publication No. 59-47024,
(See Special Publication No. 56-21345). However, it is difficult to adjust the composition of these nonmetallic inclusions in a considerable ratio, and there is a problem in that it is difficult to stably ensure cold forgeability of steel materials such as steel bars and bar-in coils. An object of the present invention is to provide a method for manufacturing hot rolled steel bars and bar-in coils that can eliminate the drawbacks of the above-mentioned conventional techniques and provide excellent cold forgeability with good machinability. It is something. (Means for Solving the Problems) In order to achieve the above object, the present inventor aimed to find a method that can improve cold forgeability without reducing the S content, which has a large effect on machinability. As a result of extensive research, it was discovered that this is possible by high temperature rolling of slabs and/or steel slabs with a controlled oxygen content in the steel at a high reduction rate, and the present invention was made based on this finding. be. That is, the present invention can be summarized as follows: S: 0.010~
0.030%, 0≦0.0025% and Ca≦ as necessary
When manufacturing slabs or steel ingots from molten steel containing 0.01% by continuous casting method or steel ingot method, rolling these slabs or steel ingots into steel slabs, and then hot rolling the steel slabs, rolling The starting temperature is less than 1100℃,
and a method for producing hot rolled steel bars and bar-in coils with excellent cold forgeability, characterized by hot rolling such that the total cross-sectional area reduction rate is 80% or more at a rolling temperature of 950°C or higher. This is a summary. The present invention will be explained in detail below based on examples. The present invention provides various carbon steels as so-called steel bars,
Regarding low-alloy steel, etc., the targets are steel bars that are cut to size after hot rolling, and bar-in coils that are coiled after hot rolling, and the reason for limiting the chemical composition of molten steel for this purpose is as follows. It is generally known that S promotes forging cracks during cold forging, and from the viewpoint of deformability it is better to have as little S as possible, but if it is too small, it may impede machinability after cold forging. Therefore, the lower limit of the content is set to 0.010%. Furthermore, if the content exceeds 0.030%, cold forging cracking will not be sufficiently reduced due to the elongation suppressing effect of sulfides caused by high-temperature rolling (described later). Therefore, the S content is 0.010-0.030%
shall be. O generates oxide inclusions that deteriorate the deformability during cold forging next to sulfide inclusions, so it is better to have less O, and if it is included in excess of 0.0025%, the cold forging cracking incidence will increase. . Therefore, O
The content shall be 0.0025% or less. In the molten steel targeted by the present invention, it is sufficient to restrict at least the contents of S and O, but Ca may be added as necessary. Ca is an element that changes the composition of sulfide-based inclusions or oxide-based inclusions, produces inclusions that are difficult to elongate during hot rolling, and improves deformability during cold forging. At the initial stage of addition, Ca acts as a deoxidizing or desulfurizing agent, floats up as inclusions, and is captured in the slag.
-It is necessary to continuously add a large amount of Ca to adjust the inclusion ratio to a considerable proportion with the composition of Ca-S, but when the Ca content exceeds 0.01%, giant inclusions of clusters occur. This makes it difficult to stably improve the deformability during cold forging. Therefore, the Ca content should be 0.01% or less.
Note that the Ca addition method is not particularly stipulated. In addition to the above-mentioned components, the content of the following elements may be regulated as necessary in view of the desired properties of the steel material. C is an element necessary for imparting the strength required for mechanical structural steel materials, and if it is less than 0.10%, its effect will be small, but if it exceeds 0.75%, it will become brittle, so it is preferably 0.10 to 0.75%. Si is an effective element for solid solution strengthening as well as a deoxidizing element, and for this purpose it can be contained up to 0.40%. Mn is 0.40 to 1.20%, especially when you want to increase hardenability and provide the necessary strength as a mechanical structural steel material.
It can be contained within the range of. in this case,
If it is less than 0.40%, hardenability is insufficient, and if it exceeds 1.20%, segregation tends to occur, impairing ductility and toughness. Cr can be added to increase hardenability and provide high strength and toughness, but if it exceeds 1.5%, it increases the softening resistance at high temperatures, so it should be kept at 1.5% or less. Like Cr, Mo can be added to increase hardenability and provide high strength and toughness, and has a greater effect than Cr, but even if it exceeds 0.50%, the effect is saturated and it is uneconomical. , 0.50% or less. In addition, alloying elements such as Ni and V can be included as appropriate depending on the required properties, and neither of them will inhibit the elongation-suppressing effect of the sulfide together with the above-mentioned optional components. Molten steel having the above chemical components is made into slabs by continuous forging or into steel ingots by steel ingot method, as in the past. These slabs or steel ingots are then rolled into billets and further hot rolled. In particular, in the latter hot rolling, ie, hot rolling of steel slabs, it is necessary to regulate the rolling start temperature and the total cross-sectional area reduction rate in a specific rolling temperature range as follows. First, the rolling start temperature during hot rolling of steel slabs is regulated to be less than 1100°C. If the rolling start temperature is 1100° C. or higher, the austenite crystal grain size becomes coarser and cold forgeability deteriorates, which is not preferable. Next, the rolling temperature in a specific rolling temperature range is
Below 950℃, the elongation suppressing effect of sulfide is small;
The sulfide elongates more than necessary, and the effect of improving deformability during cold forging becomes extremely insufficient. Therefore, the specific rolling temperature range is 950℃ or higher, and 1050℃
℃ or higher is preferable. Furthermore, it is necessary to control the total cross-sectional area reduction rate during rolling at this hot rolling temperature. If the total cross-sectional area reduction rate is less than 80%, the sulfides will be difficult to separate, and in the end, sulfides with large widths and lengths will remain, which will have a negative effect on cold forging deformability. , the total cross-sectional area reduction rate is 80% or more, preferably 95% or more. Therefore, when hot rolling the latter steel billet, the total cross-sectional area reduction rate at a rolling temperature of 950℃ or higher is
As a mode of hot rolling to achieve 80% or more,
The hot rolling end temperature is set at 950°C or higher, and the total cross-sectional area reduction rate is set at 80% or higher. Alternatively, even if the hot rolling end temperature is less than 950°C, the rolling temperature at any stage before the end of hot rolling is 950°C or more, and the total cross-sectional area reduction rate at that temperature is 80% or more. There are other methods. In addition, during the above-mentioned high-temperature rolling of the steel billet, cold forgeability can be further improved if the rolling conditions are regulated as necessary in the step of manufacturing the steel billet by rolling it from a slab or steel ingot. In other words, by setting the rolling temperature (rolling end temperature or rolling temperature at any stage before the end of rolling) at 950°C or higher when rolling a slab or steel ingot to a steel slab, sulfides accompanying steel billet rolling can be reduced. It is effective in suppressing the elongation of. However, the degree of the effect is secondary, and it is not expected that the effect will be as great as that obtained by hot rolling the steel slab in the next step under the above conditions, so it is preferable to carry out the combined use. (Example) Next, an example of the present invention will be shown. Manufacture steel slabs from molten steel having the chemical components shown in Table 1 by a continuous casting method, or manufacture steel ingots by a steel ingot method, hot-roll these slabs and steel ingots, and turn the steel slabs into steel slabs. was hot rolled under the conditions shown in the same table. At that time, the hot rolling start temperature of the steel billet rolled from the slab is 1030℃ (the rolling end temperature in this case is
975℃), 980℃ (in this case, the rolling end temperature is 900℃)
°C), and the hot rolling start temperature of the steel billet rolled from the steel ingot is 1080 °C (in this case, the rolling end temperature is 1050 °C).
℃), 970℃ (rolling end temperature in this case is 900℃)
And so. After hot rolling, a cold upsetting test was conducted, and the results are also shown in Table 1. The upsetting test was conducted in the following manner. A cylindrical test piece was taken by machining so that the height of the test piece was parallel to the rolling direction, and was then cold-upset using a V-grooved end face restraint type tool.
The deformability of the steel material was examined by visually determining cracks (Test Method A). In this test method A, the rolling reduction at which cracking did not occur was defined as the critical rolling reduction for cracking. In addition, a test piece similar to Test Method A was prepared from a bar-in-coil material rolled to 20φ, and an upsetting test was performed using a flat plate tool (Test Method B), at a reduction rate of 70%.
An upsetting test was conducted on 3,000 pieces, and cracks were investigated using an eddy current flaw detector. Furthermore, the shape of chips was determined by cutting the obtained steel material. The results are shown in Table 2. Note that chip shapes a and b shown in the table correspond to shapes a and b shown in FIG. 1, respectively. As is clear from Table 1, the steel bars and bar-in coils obtained according to the examples of the present invention both have excellent cold forgeability and machinability (chip disposability).
In particular, when the conditions for rolling from billet to billet are also regulated (B, H, M), the effect of improving cold forgeability is even greater. However, even when the rolling conditions are within the range of the present invention, Comparative Example L, which has a low amount of S, also has excellent cold forgeability but poor machinability. Furthermore, Comparative Example D with a large amount of S outside the scope of the present invention, Comparative Example J with a large amount of K and O,
Comparative Examples C, N, and Q, in which the rolling temperature of the steel billet is low, and Comparative Examples F, I, in which the total cross-sectional area reduction rate during rolling is low, all have good machinability but poor cold forgeability. There is.

【table】

【table】

[Table] (Effects of the Invention) As detailed above, according to the present invention, in the production of hot rolled steel bars and bar-in coils for cold forging, chemical components are regulated, and hot rolling of steel slabs is Since the rolling start temperature and the total cross-sectional area reduction rate in a specific rolling temperature range are regulated, the resulting steel material has extremely few cracks due to cold forging and has excellent machinability, combining both of these properties. can be done.

[Brief explanation of the drawing]

FIGS. 1a and 1b are diagrams showing chip shapes in cutting tests, respectively.

Claims (1)

[Claims] 1% by weight (the same applies hereinafter), S: 0.010 to 0.030
%, from molten steel regulated to 0≦0.0025%, produce slabs or steel ingots by continuous casting method or steel ingot method, roll these slabs or steel ingots into steel slabs, and then hot-roll the steel slabs. In this process, hot rolling is carried out at a rolling start temperature of less than 1100°C and at a rolling temperature of 950°C or higher, the total cross-sectional area reduction rate is 80% or more. Method for manufacturing hot rolled steel bars and bar-in coils. 2 S: 0.010 to 0.030%, 0≦0.0025% regulated molten steel to produce slabs or steel ingots by continuous casting method or steel ingot method, and these slabs or steel ingots are rolled at rolling temperature.
When rolling a steel billet at 950°C or higher and then hot rolling the steel billet, the rolling start temperature is less than 1100°C, and the total cross-sectional area reduction rate is 80% or more at the rolling temperature of 950°C or higher. The method according to claim 1, characterized in that hot rolling is carried out so as to obtain the following properties. 3 S: 0.010-0.030%, regulated to 0≦0.0025%,
When producing slabs or steel ingots from molten steel containing Ca≦0.01% by a continuous casting method or a steel ingot method, rolling these slabs or steel ingots into steel slabs, and then hot rolling the steel slabs, Hot rolling with excellent cold forgeability, characterized by hot rolling with a rolling start temperature of less than 1100°C and a total cross-sectional area reduction rate of 80% or more at a rolling temperature of 950°C or higher. Method for manufacturing rolled steel bars and bar-in coils. 4 S: 0.010-0.030%, regulated to 0≦0.025%,
Producing slabs or steel ingots from molten steel containing Ca≦0.01% by a continuous casting method or a steel ingot method, and rolling these slabs or steel ingots into steel slabs at a rolling temperature of 950°C or higher,
Then, when hot rolling the steel billet, the rolling start temperature is less than 1100°C, and the rolling temperature is 950°C.
4. The method according to claim 3, wherein hot rolling is performed so that the total cross-sectional area reduction rate is 80% or more.