JPS63157817A - Manufacture of low carbon steel wire rod and bar - Google Patents

Abstract

PURPOSE:To manufacture a low carbon steel wire rod or bar having low strength and superior workability, by heating a steel billet contg. specified percentages of C, Si, Mn, Al and Ti in Fe to a specified temp. and by hot rolling the billet. CONSTITUTION:A steel billet consisting of, by weight, 0.003-0.04% C, 0.005-0.35% Si, 0.10-0.60% Mn, 0.005-0.080% Al, Ti in 4-8 weight ratio of Ti to C and the balance Fe with inevitable impurities is heated to 700-950 deg.C and hot rolled to manufacture a steel wire rod or bar. Thus, the soft low carbon steel wire rod or bar hardly causing age hardening is economically manufactured.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a method for producing low-carbon steel wire rods and steel bars that have low strength and excellent workability when subjected to wire drawing or cold forging. It is related to.

[Prior art] Low-carbon steel wire rods and bars (hereinafter referred to as low-carbon steel bars) are ordinary carbon steels that usually contain 0.2% or less carbon, and are processed through a so-called secondary processing process after hot rolling. 1) It is used to manufacture many steel products through processes such as wire drawing, cold forging, and annealing. Needless to say, the material characteristics of low carbon rods and wires are that they are soft and have excellent workability. In particular, low tensile strength and low work hardening during cold working produce economic effects such as omitting the annealing process and improving tool life, so conventionally, low carbon rods have been made softer. Much research and development has been carried out with the aim of

The basic idea for softening low-carbon rods and wires can be summarized as follows.

(1) Reduce the amount of components such as C, SL, Mn, P, etc.
The composition is close to that of pure iron.

(2) C and N dissolved in ferrite are fixed in the form of carbides and nitrides, respectively, to reduce age hardening.

(3) Enlarge ferrite crystal grains.

Regarding (1), it is stated in Japanese Patent Publication No. 59-3535 that although it is a low carbon steel, it is difficult to obtain a healthy surface due to the lack of deoxidizing elements such as Si, Mn, and AΩ. There is a problem in that workability is poor due to the large number of nonmetallic inclusions. Regarding (2), the technology of adding AQ, B, etc. as a means to prevent aging caused by N was disclosed in Japanese Patent Application Laid-Open No. 55
-61319, JP-A-56-158841, and JP-A-59-215463. All of these are effective in suppressing aging caused by solute N, but solute C
Since aging due to aging cannot be prevented, there is a limit to the aging suppression effect.

In contrast, “Iron and Steel (Japan Iron and Steel Association)” Vol. 70, No. 5
If Ti is added as disclosed in No. 1, page 5535, aging can be completely prevented. This is because Ti is a strong carbonitride forming element. However, as described in the above publication, Ti
In order to perform non-aging by
2 to 3 times the amount of Ti, that is, the component weight ratio Ti/(C
It is necessary to add 8 to 10 N). Therefore, in general, the production of non-aged rods by adding Ti is expensive, so there is a need to develop a more economical production method.

Regarding (3), JP-A No. 6 discloses a method of causing abnormal grain growth of ferrite during annealing by appropriately selecting the ratio of AQ and N contents, thereby softening the steel wire.
0-36618, "Production method of low-strength mild steel wire". However, steel wires having such coarse grains cannot be drawn, and their use is naturally limited because they produce significant irregularities (so-called orange peel) on the material surface during cold forging.

In view of the above situation, the present inventors have repeatedly investigated anti-aging by adding Ti in accordance with the above-mentioned idea (2).

[Problems to be Solved by the Invention] The purpose of the present invention is to study the optimization of the Ti-based steel composition and its hot rolling conditions, and thereby to economically produce low-carbon rods and wires that are soft and have little age hardening. The objective is to provide a method for manufacturing the same.

[Means for solving the problems] That is, the present invention provides C: 0.003 to 0.04% r
si: 0.005-0.35%, Mn: 0.1
0-0.60%, AQ: 0.005-0.080% NiT
Contains 4 to 8 i in component weight ratio Ti/c.

This is a method for producing low carbon steel wire rods and steel bars, which is characterized in that a steel slab or slab, the balance of which is Fe and unavoidable impurities, is heated to a temperature range of 700 to 950°C, and then hot rolled.

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

Ti is a strong carbonitride forming element, and TiN and T
By forming a precipitate called iC, N and C dissolved in solid solution in ferrite are fixed and the steel is non-aged. However, since TiN and TiC have different precipitation temperatures, and TiN precipitates near the solidification temperature of rust, it can be considered that most of the N in the slab after continuous casting is precipitated as TiN. On the other hand, since TiC starts to precipitate when the temperature drops to the normal hot rolling temperature, the following explanation will be limited to the precipitation of TiC. Since the atomic weight ratio of Ti and C is 4, the theoretical amount of Ti required for complete non-aging is four times the amount of C. In other words, the component weight ratio Ti/C of Ti and C must be 4 or more.

Therefore, in order to economically perform non-aging by reducing the amount of Ti added, it is desirable that the amount of C be small.

From this, the upper limit of the amount of C is set to 0.04%. However, lowering the C content to less than 0.003% causes a rapid increase in smelting costs, so 0.003% is set as the lower limit.

The lower limit of the component weight ratio Ti/C of Ti and C is equal to the above-mentioned theoretical value, but in reality it is necessary to take into account the precipitation driving force of TiC and the diffusion time of Ti and C. The above is necessary. The present inventors have determined the heating temperature of steel slabs or cast slabs in a heating furnace and the T required for non-aging, as described below.
The effect of solid solution Ti (TiC) on the J addition amount and the rod material
The influence of Ti (which does not form) was investigated. As a result, if the heating temperature is selected between 700 and 950°C, complete non-aging can be achieved when the Ti/C ratio is 8 or less, and when the Ti/C ratio exceeds 8, the amount of Ti dissolved in the ferrite becomes excessive. As a result, we have obtained new knowledge that recrystallization is delayed during annealing after cold working, and as a result, softening is inhibited. Combining these results, the Ti/C ratio is 4~
It can be seen that it is preferable to set the value to 8. SL is an element necessary for deoxidation, so it is added at 0.005% or more, while Si has a large solid solution strengthening effect on ferrite, so it is added at 0.35%.
The upper limit is %.

Mn is added in an amount of 0.10% or more as a deoxidizing element and to prevent hot embrittlement caused by S, but since Mn also has a solid solution strengthening effect, the upper limit is 0.60%.

AQ is added in an amount of 0.005% or more for deoxidation, but like other elements, AQ also has a solid solution strengthening effect, so o, og.

% or less.

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

The present inventors conducted many basic experiments regarding the precipitation of TiC from a solution state, and as a result, found that the temperature at which the precipitation rate of TiC becomes maximum is 800 to 850°C. Therefore, if a steel billet or slab is heated in this temperature range and hot rolled under tension, Ti
Precipitation of C progresses, and it is possible to obtain a sufficient non-aging effect regardless of the subsequent hot rolling and cooling conditions. From this, the heating temperature of the steel slab or slab is 700~9
The temperature is preferably 50°C. At temperatures higher or lower than this, it takes a long time for TiC to precipitate, and
Since hot rolling becomes difficult below 00°C, the lower and upper limits are set to 700°C and 950°C, respectively.

It is possible to consider a method in which TiC is precipitated during hot rolling or during cooling after hot rolling, but since the rolling time is short and the cooling rate is high in normal continuous hot rolling, TiC precipitation is not possible. Precipitation is insufficient and its non-aging effect is small compared to the present invention.

Regarding elements other than the above-mentioned component elements, P has a solid solution strengthening effect, and S produces TiS and reduces the effective amount of Ti, so it is desirable to reduce it as much as possible.

The same thing can be said about N, and since N generates TiN and consumes Ti, it is preferably as small as possible, preferably 0.0050% or less.

[Example] Low carbon steel was melted in a 250-ton converter, and decarburized and component adjusted using degassing equipment. Table 1 shows the chemical composition of the sample materials.

A piece of 300 x 500 mm was produced using continuous casting equipment, and a steel piece with a square cross section of 122 mm was produced by blooming rolling.

This was heated in a heating furnace at a wire rod rolling factory, then rolled into a wire rod with a diameter of 5.5 mm using a continuous rolling mill, and subjected to stelmor cooling. Table 1 shows the wire rolling conditions and wire properties.

Since the aging characteristics of the wire rod are controlled by the amount of C and N dissolved in the ferrite, the amount of solid solution (C+N) was measured. The measurements were carried out using the method described below to determine the aging index.6 In other words, after applying a pre-strain of 8% using a tensile testing machine, aging treatment was performed at 100°C for 1 hour, and then the tensile test was performed again. The aging index expressed by the formula was determined.

Aging index = (Yield strength after aging treatment) - (Before aging treatment,
Deformation strength equivalent to 8% pre-strain) Aging index unit is k
gf/mm'', the smaller the value, the smaller the amount of solid solution (C+N), and therefore the smaller the amount of age hardening.

In Table 1, No. 1 to No. 5 represent the influence of the Ti/C ratio, and No. 16 to No. 10 represent the influence of the billet heating temperature. Note that N o ,
Nos. 11 and 12 are conventional methods, but Nos. 11 and 12 are conventional methods when rolling is performed under normal rolling conditions.
, No. 11 is used, and low-temperature rolling is performed in the temperature range of 700 to 900° C. by water-cooling the material being rolled.

From Table 1, it can be seen that the wire produced according to the method of the present invention has lower strength and aging index than any of the conventional methods.

[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 furthermore, the solid solution (C + N) is Because it is completely fixed, there is little work hardening during wire drawing, making it possible to produce a significantly softened steel wire.

Although we have talked about wire rods and steel bars, steel plates can also be manufactured using the same method, resulting in lower strength and lower strength.
Moreover, it is possible to manufacture a steel plate with excellent workability.

Claims (1)

[Claims] C: 0.003 to 0.04%, Si: 0.005 to 0.04%.
35% Mn: 0.10-0.60%, Al: 0.005-0.
080% contains Ti at a component weight ratio Ti/C of 4 to 8, and the remainder is Fe.
70 pieces of steel or slabs containing unavoidable impurities.
A method for producing low carbon steel wire and steel bars, which comprises heating to a temperature range of 0 to 950°C and then hot rolling.