JPH0672258B2 - Method for producing rolled steel bar with excellent homogeneity - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a rolled steel bar having excellent homogeneity,
Specifically, it does not require normalizing treatment after rolling, and has a uniform fine ferrite / pearlite structure throughout the cross section as hot-rolled, and is equivalent to or better than that subjected to normalizing treatment. The present invention relates to a method for producing a rolled steel bar having excellent mechanical properties.

(Prior Art) Low-medium carbon steel bars and low-medium carbon low alloy steel bars are widely used as steels for machine structures for manufacturing shafts, bolts, nuts and the like. Conventionally, these steel bars are hot-rolled, then subjected to a normalizing treatment to make their structure uniform and fine, and to cut,
It is used for processing such as cutting and drawing, but in recent years, for the purpose of energy saving and process saving, in order to omit the normalizing process after hot rolling, rolling is performed at a low temperature and fine rolling is performed as it is. A method for producing a steel bar having a uniform structure has been developed. For example, Japanese Examined Patent Publication No. 58-9816 discloses that medium carbon steel and medium carbon low alloy steel are rolled at a temperature of 850 to 1050 ° C, and finish rolling is performed at a temperature of 850 to 950 ° C. A method of making a quality rolled steel bar is disclosed. According to such low temperature rolling, compared with the conventional ordinary rolled steel bar obtained by heating the billet to a temperature of 1100 to 1200 ° C and rolling at a temperature of 900 to 1100 ° C, it is somewhat improved. However, the crystal grains in the central portion are coarse and the homogeneity of the structure in the central portion and the surface layer portion is poor.

(Purpose of the invention) As a result of intensive research to solve the problems in the production of the conventional non-heat treated rolled steel bar as described above,
By cooling from a predetermined temperature to a predetermined temperature before finish rolling, and by setting the finish rolling end temperature to a predetermined temperature, there is no need for normalizing treatment and uniform fine ferrite is spread across the cross section. The present invention has been completed by finding that a homogeneous steel bar having a pearlite structure can be obtained. Accordingly, it is an object of the present invention to provide a method for producing a homogeneous steel bar having a uniform fine ferrite-pearlite structure across a cross section without requiring a normalizing treatment.

(Structure of the Invention) A method for producing a rolled steel bar excellent in homogeneity according to the present invention is
When hot rolling a steel slab consisting of C 0.10 to 0.60% by weight, Si 0.10 to 0.35%, Mn 0.30 to 1.80%, balance iron and unavoidable impurities at a temperature of 850 to 1000 ° C before finish rolling, 850 ° C
It is cooled to a temperature higher than the M _s point and the finish rolling end temperature is 700 to 900 ° C., and both the surface layer portion and the interior have a uniform fine ferrite grain size of 7 or more. It is characterized by having a pearlite structure.

First, the chemical composition of the steel used in the present invention will be described.

C is the required strength of the obtained steel bar, for example, 35 kgf / mm ²
It is necessary to add at least 0.10% in order to impart the above tensile strength and to form a ferrite / pearlite structure. However, if too much is added, the toughness of the steel will be reduced and it will not be possible to use it as it is rolled, so the upper limit of addition is set to 0.60%. A particularly preferred C additive is in the range of 0.30 to 0.60%.

Si needs to be added as a deoxidizing agent in an amount of 0.10% or more. However, addition of a large amount exceeding 0.35% not only saturates the deoxidizing effect, but is economically disadvantageous. The addition amount is in the range of 0.10 to 0.35%.

Mn suppresses the harmful effect of S as an impurity,
It has the effect of improving the strength and toughness of steel. It is an important element in the present invention for producing a rolled steel bar having high toughness as it is rolled, and it is necessary to add at least 0.30% in order to effectively obtain the above effects. However, when added in excess of 1.80%, a brittle metal structure is formed, and the steel becomes brittle, so the upper limit of the addition amount is 1.
8%

In the present invention, steel contains Nb, in addition to the above-mentioned elements.
At least one selected from the group consisting of V, Ti, Al and N
Seed elements may be added. All of these elements are effective in forming fine carbonitrides and refining the structure. Particularly, when at least one selected from Nb, V, Ti and Al is added together with N, It is preferable because the micronization of the structure is stably achieved.

In order to effectively obtain such effects, Nb is added in the range of 0.10% or less, V 0.20% or less, Ti 0.10% or less, Al 0.10% or less, and N 0.015% or less. Excessive addition beyond this range not only saturates the effect, but also increases inclusions that adversely affect the toughness, which is not preferable.

Further, in the present invention, Cr and / or Mo can be added to the steel together with the above-mentioned structure refining element or separately from them. Cr and Mo are both hardenability adjusting elements and are added when high strength toughness is obtained by quenching and tempering. However, the addition of an excessive amount, as a result of producing a hard and brittle bainite or martensite structure after rolling, it becomes difficult to use as-rolled, while avoiding such adverse effects, in order to effectively obtain the above effect, for Cr, 1.
50% or less, and Mo is added in the range of 0.30% or less. When Cr and / or Mo is added, the C content is
The range of 0.10 to 0.45% is preferable.

The method for producing a steel bar excellent in homogeneity according to the present invention is, when hot rolling a steel slab having the chemical composition as described above, from a temperature in the range of 850 to 1000 ° C. before finish rolling to 850 ° C.
Below, and, while cooling to a temperature in the range higher than the M _s point, the finish rolling end temperature is set to a temperature of 700 to 900 ° C., and both the surface layer portion and the inside have a uniform fine ferrite / pearlite structure. .

The heating temperature of the billet is the normal heating temperature of 850-120.
It is in the range of 0 ° C., and after rolling with a rough row and intermediate row rolling mill, preferably with a total reduction of 80% or more, in the method of the present invention, before the finish rolling, the cooling start temperature is 850 to 1000
The rolled material is so-called intermediate water-cooled at a temperature in the range of 0 ° C, a cooling stop temperature of 850 ° C or lower, and a temperature in the range higher than the M _s point. In this intermediate water cooling, the lower limit of the cooling start temperature is usually defined as 850 ° C. as the limit temperature at which rolling is possible, while the intermediate water cooling causes both the surface layer portion and the inside of the rolled material to be 850 ° C. The upper limit of the cooling start temperature is specified as 1000 ° C so that the temperature becomes equal to or lower than ℃. Further, the upper limit of the cooling stop temperature is defined as the limit temperature for obtaining a fine structure by finish rolling, while, when the cooling stop temperature is below the M _s point, the structure becomes a martensite structure. Since the finish rolling becomes impossible, the lower limit temperature of cooling stop is regulated to a temperature higher than the M _s point.

Then, in the method of the present invention, the intermediate water-cooled rolled material at a temperature in the range of 700 to 900 ° C. is usually set to 20% or more in the finish rolling, and finish rolling is completed to obtain a uniform fine grain. Obtain a ferrite / pearlite structure. If the finish rolling finish temperature exceeds 900 ° C, a coarse ferrite / pearlite structure is formed, while if the finish rolling finish temperature is lower than 700 ° C, needle-like ferrite and bainite are formed. In any case, it is difficult to obtain a uniform fine ferrite / pearlite structure. Particularly, in the method of the present invention, when the finish rolling end temperature is high, the temperature from the finish rolling end temperature to 600 ° C. is 5
By cooling at a rate of ℃ / sec or more, coarsening of the structure due to recrystallization due to cooling after rolling can be prevented, and thus stable and fine ferrite / pearlite can be obtained. You can get a tissue.

(Effect of the invention) As described above, according to the method of the present invention, intermediate water cooling is performed before finish rolling and finish rolling is performed at a predetermined low temperature, so that the billet heating temperature and the rough row rolling heating temperature are lowered. Therefore, it is not necessary to reduce the rolling speed, and thus rolled steel bar can be manufactured with high productivity. Moreover, the steel bar obtained by the method of the present invention has a uniform fine ferrite-pearlite structure having a grain size number 7 or more over the cross-section in both the surface layer portion and the inside, and its mechanical properties are also normal. It is equal to or higher than the rolled steel that has been subjected to the heat treatment.

(Example) Hereinafter, the present invention will be described with reference to examples, but the present invention is
The embodiments are not limited in any way.

Example 1 A 155 mm square steel billet having the chemical composition shown in Table 1, Table No. 1 was rolled into a 40 mm diameter steel bar under the conditions shown in the drawings. The grain sizes of the surface layer portion, D / 4 portion (D is the steel bar diameter) and D / 2 portion of these steel bars are shown in the drawings. Also, from the D / 4 part of these steel bars to JIS 4
No. 1 test piece was taken and its mechanical properties were measured. The results are shown in Table 2.

For comparison, the same steel slab as described above was rolled under the conditions shown, and the grain size and the mechanical properties of the D / 4 portion in the cross section were similarly measured. The results are shown in the drawings and Table 2.

The steel bars A to C produced by the method of the present invention have a uniform fine ferrite / pearlite structure having a grain size of 7 or more in both the surface layer portion and the inside thereof, and also have excellent mechanical properties. Steel bars D to G have a temperature before finish rolling.
Since it exceeds 850 ° C and the finishing rolling temperature exceeds 900 ° C, it is not possible to obtain the desired steel bar having a homogeneous fine structure. Particularly, in Comparative Steels D, E and F, the internal structure is rougher than in the surface layer portion. Conventional steel is a normalized product, and the steel bar of the present invention has mechanical properties almost the same as the normalized product.

Example 2 A 155 mm square steel piece having the chemical composition shown in Table 1, Steel No. 2 was 8
The steel was heated to 70 ° C., water-cooled from 860 ° C. to 740 ° C. before finishing rolling, finishing rolling was finished at 770 ° C., and rolling was performed on a steel bar having a diameter of 25 mm to obtain a steel bar I of the present invention.

For comparison, the same steel slab was heated to 960 ° C., finishing rolling was completed at 980 ° C. without intermediate water cooling, and rolling was performed on a steel bar having a diameter of 25 mm to obtain a comparative steel bar J.

Table 3 shows the grain size and mechanical properties over the cross section of each of the steel bars thus obtained. The steel bar according to the present invention has a uniform fine ferrite-pearlite structure having a grain size number 7 or more both in the surface layer portion and inside.
It is also clear that the mechanical properties are excellent.

Example 3 A 150 mm square steel piece having the chemical composition shown in Table 1, Steel No. 3 was 8
After heating to 75 ° C. and cooling from 880 ° C. to 725 ° C. before finish rolling, finish rolling was set to 808 ° C. to obtain rolled steel bar K of the present invention. The grain size and mechanical properties of the cross section are shown in Table 3.

Example 4 A 150 mm square steel piece having the chemical composition shown in Table 1, Steel No. 4 was used.
After heating to 83 ° C. and cooling from 853 ° C. to 759 ° C. before finish rolling, finish rolling was set to 781 ° C. to obtain rolled steel bar L of the present invention. The grain size and mechanical properties of the cross section are shown in Table 3.

Example 5 A 150 mm square steel piece having the chemical composition shown in Table 1, Steel No. 5 was used.
After heating to 80 ° C. and cooling from 950 ° C. to 825 ° C. before finish rolling, finish rolling was set to 805 ° C. to obtain rolled steel bar M of the present invention. The grain size and mechanical properties of the cross section are shown in Table 3.

[Brief description of drawings]

The drawings are graphs showing rolling conditions in the method of the present invention and the method as a comparative example, and the grain sizes of the surface layer portion and the inside of the obtained steel bar.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mitsuru Moritaka 3-9-1, Fukaehoncho, Higashinada-ku, Kobe-shi, Hyogo Prefecture 3-1-1 (72) Inventor Yukio Wada 5-7-7, Kamiya, Suma-ku, Kobe-shi, Hyogo (56) ) Reference JP 54-143716 (JP, A) JP 60-245722 (JP, A)

Claims (4)

[Claims] 1. When hot rolling a steel slab consisting of C 0.10 to 0.60% by weight, Si 0.10 to 0.35%, Mn 0.30 to 1.80%, balance iron and unavoidable impurities, and 850 to 1000 before finish rolling. From the temperature of ℃, 850 ℃
It is cooled to a temperature higher than the M _s point and the finish rolling end temperature is 700 to 900 ° C., and both the surface layer portion and the interior have a uniform fine ferrite grain size of 7 or more. A method for producing a rolled steel bar having excellent homogeneity, which has a pearlite structure. 2. In weight%, (a) C 0.10 to 0.60%, Si 0.10 to 0.35% and Mn 0.30 to 1.80% are contained, and (b) Nb 0.10% or less, V 0.20% or less, Ti 0.10% or less. , Al 0.10% or less, and N 0.015% or less, and at least one element selected from the group consisting of balance iron and unavoidable impurities, and 850 to 1000 before finish rolling during hot rolling. From the temperature of ℃, 850 ℃
It is cooled to a temperature higher than the M _s point and the finish rolling end temperature is 700 to 900 ° C., and both the surface layer portion and the interior have a uniform fine ferrite grain size of 7 or more. A method for producing a rolled steel bar having excellent homogeneity, which has a pearlite structure. 3. From the group consisting of (a) C 0.10 to 0.60%, Si 0.10 to 0.35% and Mn 0.30 to 1.80% by weight, and (b) Cr 1.50% or less and Mo 0.30% or less. At the time of hot rolling a steel slab containing at least one selected element and the balance of iron and unavoidable impurities, the temperature from 850 to 1000 ℃ to 850 ℃ before finish rolling.
It is cooled to a temperature higher than the M _s point and the finish rolling end temperature is 700 to 900 ° C., and both the surface layer portion and the interior have a uniform fine ferrite grain size of 7 or more. A method for producing a rolled steel bar having excellent homogeneity, which has a pearlite structure. 4. In weight%, (a) C 0.10 to 0.60%, Si 0.10 to 0.35%, Mn 0.30 to 1.80% are contained, and (b) Nb 0.10% or less, V 0.20% or less, Ti 0.10% Hereinafter, at least one element selected from the group consisting of Al 0.10% or less and N 0.015% or less, and (c) at least one element selected from the group consisting of Cr 1.50% or less and Mo 0.30% or less. When hot rolling a steel slab containing iron and the balance iron and unavoidable impurities, from 850 to 1000 ℃ before finish rolling to 850 ℃
It is cooled to a temperature higher than the M _s point and the finish rolling end temperature is 700 to 900 ° C., and both the surface layer portion and the interior have a uniform fine ferrite grain size of 7 or more. A method for producing a rolled steel bar having excellent homogeneity, which has a pearlite structure.