JP2501438B2 - Low carbon steel wire rod and steel bar manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a method for producing a low carbon steel wire rod and a steel bar which have low strength and are excellent in workability, which are subjected to wire drawing and cold forging. It is about.

[Prior Art] Low carbon steel wire rod and steel bar (hereinafter referred to as low carbon steel wire)
Is an ordinary carbon steel that normally contains 0.2% or less of carbon, and is used for the manufacture of many steel products after hot rolling, and through the so-called secondary processing steps such as wire drawing, cold forging, and annealing. Has been done. Needless to say, the characteristic feature of the low carbon rod is that it is soft and has excellent workability. In particular, low tensile strength and small work hardening at the cold working stage produce economical effects such as omission of the annealing process or improvement of tool life. Much research and development aimed at has been made.

The basic idea for softening low carbon rods is summarized as follows.

(1) Reduce the content of C, Si, Mn, P, etc. to obtain a composition close to pure iron.

(2) C and N solid-dissolved in ferrite are fixed in the form of carbide and nitride, respectively, to reduce age hardening.

(3) Enlarge ferrite crystal grains.

Regarding (1), it is stated in Japanese Examined Patent Publication No. 59-3535 that, although it is a low carbon steel, it is difficult to obtain a sound slab surface due to the small amount of deoxidizing elements such as Si, Mn and Al. There is a problem that the workability is poor because there are many metal inclusions.
Regarding (2), a technique of adding Al, B or the like as a means for preventing the aging caused by N is disclosed in JP-A-55-61319 and JP-A-55-61319.
56-158841, and JP-A-59-215463.
All of them are effective in suppressing the aging caused by the solid solution N, but cannot prevent the aging caused by the solid solution C, so that the effect of suppressing the aging is naturally limited. On the other hand, if Ti is added, as disclosed in "Iron and Steel (The Iron and Steel Institute of Japan)" Vol. 70, No. 5, page S535, aging can be prevented almost completely. This is because Ti is a strong carbonitride forming element. However, as described in the above publications, in order to perform non-aging with Ti, the stoichiometrically required amount of Ti is 2 to 3 times, that is, Ti is a component weight ratio Ti / (C + N). It is necessary to add 8-10. Therefore, in general, the production of the non-aged rod wire by adding Ti results in high cost, and thus the development of a more economical production method is required. Regarding (3), there is a method of causing abnormal grain growth of ferrite during annealing by appropriately selecting the ratio of Al and N contents, thereby achieving softening of the steel wire. 36618, "Method for manufacturing low strength mild steel wire"
Is shown in. However, the steel wire having such coarse crystal grains cannot be drawn, and during cold forging, remarkable unevenness (so-called orange peel) is generated on the surface of the material, so that its application is naturally limited.

In view of the above situation, the present inventors have repeatedly studied non-aging by adding Ti according to the idea of (2) above.

[Problems to be Solved by the Invention] The purpose of the present invention is to study the optimization of the steel composition mainly containing Ti and the hot rolling conditions thereof, thereby making it possible to obtain a low carbon rod wire which is soft and has little age hardening. The present invention is to provide a method of manufacturing the same.

[Means for Solving Problems] That is, according to the present invention, C: 0.003 to 0.04%, Si: 0.005 to 0.
35%, Mn: 0.10 to 0.60%, Al: 0.005 to 0.080%, containing Ti at a component weight ratio Ti / C of 4 to 8, with the balance being Fe and inevitable impurities at 700 to 950 ° C. It is a method for producing a low carbon steel wire rod and a steel bar, which comprises heating to a temperature range of 1, and then hot rolling.

[Operation] First, the reason for limiting the chemical components according to the present invention will be described.

Ti is a strong carbonitride forming element, and by forming precipitates of TiN and TiC, N and C which are solid-solved in ferrite are fixed and the steel is non-aged. But Ti
The precipitation temperature differs between N and TiC, and TiN precipitates near the solidification temperature of steel.
It can be considered that it is precipitated as TiN. On the other hand, T
Since iC starts to precipitate down to the normal hot rolling temperature, the description will be limited to TiC precipitation. Since the atomic weight ratio of Ti and C is 4, Ti required for complete non-aging
Is 4 times the C amount. In other words, the Ti / C component weight ratio Ti / C must be 4 or more. Therefore
It is desirable that the amount of C is small in order to reduce the amount of addition of Ti and economically perform non-aging. From this, the upper limit of C amount is 0.04
%. However, lowering the C content to less than 0.003% causes a sharp increase in smelting costs, so the lower limit is 0.003%.

The lower limit of the Ti / C component weight ratio Ti / C is equal to the above theoretical value, but in practice it is necessary to take into account the TiC precipitation driving force and the Ti / C diffusion time. The above is necessary. As will be described later, the inventors of the present invention will discuss the solid solution Ti (TiC that does not form TiC) that affects the heating temperature of the steel piece or cast piece in the heating furnace and the Ti addition amount and the material of the rod wire necessary for non-aging as described below.
The effect of i) was investigated. As a result, the heating temperature is 700 to 950.
If the Ti / C ratio is 8 or less, it can be almost completely non-aged if the temperature is selected to be ℃, and if the Ti / C ratio exceeds 8, it forms a solid solution in ferrite.
Since the amount of Ti becomes excessive, we have obtained a new finding that recrystallization is delayed during annealing after cold working and softening is hindered. Taken together, these results
It is understood that the Ti / C ratio is preferably 4-8. Si
Is an element necessary for deoxidation, so 0.005% or more is added.
The upper limit is 35%.

Mn is added as a deoxidizing element and is added in an amount of 0.10% or more in order to prevent hot embrittlement due to S. Mn also has a solid solution strengthening action, so the upper limit is 0.60%. Al is added in an amount of 0.005% or more in terms of deoxidation. However, Al has a solid solution strengthening effect like other elements, so the content is made 0.080% or less.

In order to minimize the amount of Ti required for non-aging,
It is necessary to set the heating temperature of the steel slab or the slab in hot rolling within an appropriate range.

The present inventors conducted many basic experiments on the precipitation of TiC from the solution state, and as a result, found that the temperature at which the precipitation rate of TiC was maximum was 800 to 850 ° C. Therefore, if the steel slab or cast is heated in this temperature range and then hot-rolled, precipitation of TiC proceeds during heating, and the hot-rolling and cooling conditions thereafter will be sufficient. It is possible to obtain such a non-aging effect. Therefore, it is desirable that the heating temperature of the steel slab or the cast slab is 700 to 950 ° C. At higher or lower temperatures, TiC
Precipitation takes a long time, and hot rolling is difficult at temperatures below 700 ° C.
℃ and 950 ℃. A method of causing precipitation of TiC during hot rolling or during cooling after hot rolling is also conceivable, but in ordinary continuous hot rolling, the rolling time is short and the cooling rate is high. Precipitation is insufficient and its non-aging effect is small compared to the present invention.

Regarding elements other than the above-mentioned constituent elements, P
Has a solid solution strengthening effect, and S forms TiS and is an effective T
Since the amount of i is reduced, it is desirable to reduce it as much as possible. The same thing can be said about N, because N produces TiN and consumes Ti, so it is as small as possible, preferably 0.0050.
% Or less is preferable.

[Example] Low carbon steel was melted in a 250 ton converter, and decarburization and composition adjustment were performed using a degassing facility. Table 1 shows the chemical composition of the test materials.

300 x 500 mm ingots were made by a continuous casting facility, and 122 mm square cross-section steel pieces were produced by slabbing. After heating this in a heating furnace at a wire rod rolling plant, a diameter of 5.
It was rolled into a 5 mm wire rod and cooled with Stelmore. Table 1 shows the wire rolling conditions and the characteristics of the wire.

Since the aging characteristics of the wire are governed by the amounts of C and N dissolved in ferrite, the amount of solid solution (C + N) was measured. The measurement was carried out by the method of determining the aging index described below. That is, after applying a strain of 8% in advance with a tensile tester, aging treatment was performed at 100 ° C. for 1 hour, and then the tensile test was performed again to obtain the aging index shown by the following formula.

Aging index = (yield strength after aging treatment)-(deformation strength before aging treatment, equivalent to 8% prestrain) The unit of the aging index is kgf / mm ² , the smaller the value is solid solution ( The amount of C + N) is small, and therefore the amount of age hardening is small.

In Table 1, No. 1 to No. 5 affect the Ti / C ratio, and No. 6 to No.
No. 10 shows the effect of the billet heating temperature. Incidentally, No. 11 and No. 12 is a conventional method, but No. 11 when performing rolling under normal rolling conditions, No. 12 uses steel of the same chemical composition as No. 11, during rolling. By water cooling the material
This is the case when low-temperature rolling is performed in the temperature range of 700 to 900 ° C.
It can be seen from Table 1 that the wire produced according to the method of the present invention has a low strength and a small aging index as compared with any conventional method.

[Effects of the Invention] As described above, the low carbon steel wire rod manufactured according to the method of the present invention is much softer than that manufactured by the conventional method, and further solid solution (C + N) Since it is almost completely fixed, work hardening during wire drawing is small, and it is possible to manufacture a significantly softened steel wire.

Although the wire rod and the steel bar have been described, a steel plate having low strength and excellent workability can be manufactured by manufacturing the steel plate by the same method.

Claims (1)

(57) [Claims] 1. C: 0.003 to 0.04%, Si: 0.005 to 0.35% Mn: 0.10 to 0.60%, Al: 0.005 to 0.080% and 4 to 8 of Ti in the component weight ratio Ti / C, the balance being Fe and A method for producing a low-carbon steel wire rod and a steel bar, which comprises heating a steel slab or a slab containing unavoidable impurities to a temperature range of 700 to 950 ° C, and then hot rolling the steel.