JPH0995735A - Production of steel for reinforcing bar excellent in earthquake resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel for a concrete reinforcing bar excellent in earthquake resistance by subjecting a carbon steel having a specified compsn. to hot rough rolling, intermediate rolling and finish rolling under specified temp. conditions and thereafter executing accelerated cooling by water cooling. SOLUTION: A slab having a compsn. contg., by weight, 0.10 to 0.40% C, 0.05 to 0.60% Si, 0.60 to 2.00% Mn and specified amounts of Al, N, B, P, S, Cu, Ni, Mo and Ti or furthermore contg. one or more kinds of V and Nb is heated at 950 to 1,250 deg.C and is subjected to rough rolling. Successively, the treatment of rapidly cooling the surface of the steel to the temp. range of 600 to 700 deg.C by water cooling between the passes in the subsequent intermediate rolling and finish rolling is repeated for one to five times to form the structure of the surface of the steel into fine ferrite-bainite, furthermore, the finish rolling is finished at 750 to 950 deg.C, and after that, it is rapidly cooled to 550 to 400 deg.C at a cooling rate of 3 to 10 deg.C/sec. The objective steel for a reinforcing bar having >345MPa yield strength, <0.8 yield ratio, >1.4% yield elongation and >=27J/cm<2> Charpy impact value and excellent in earthquake resistance can be produced in a relatively easy way under high productivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a steel material for reinforcing bars which is excellent in earthquake resistance, and more specifically, a yield strength of 3
45 MPa or more, yield ratio 0.8 or less, yield elongation 1.
The present invention relates to a method for producing a steel material for concrete rebar, which has an excellent earthquake resistance of 4% or more and a Charpy impact value of 27 J / cm ² or more.

[0002]

BACKGROUND ART Conventionally, JIS G 31 has been used for reinforcing bars for concrete.
Steel materials with standardized chemical components and mechanical properties have been used as “bars for reinforcing steel concrete” in 12. At present, this standard steel material is used as a normal reinforcing bar for building construction.

However, the earthquake resistance standards have been reviewed in response to the occurrence of the huge earthquakes such as the Hyogoken Nanbu Earthquake mentioned above, and as a result, they have higher strength and higher toughness than conventional steel for reinforcing bars. The demand for steel for reinforcing bars is increasing.

Regarding the steel for high-strength reinforcing bars, for example, Japanese Patent Publication No. 7-26152 proposes a "method for producing steel for high-strength reinforcing bars having a large yield elongation". If it is manufactured by the method described in this publication, it is certainly possible to impart high yield strength and large yield elongation to steel for reinforcing bars. However, it is not a manufacturing method that considers the toughness of steel for reinforcing bars. Therefore, the steel for high-strength rebar produced by the method proposed in the above-mentioned publication is not necessarily sufficient in preparation for a large earthquake in which a large stress is applied to a building by impact.

Japanese Patent Publication No. 63-64494 proposes a "method for producing high-strength reinforcing steel having a large yield shelf ratio". Although the technique described in this publication takes into consideration the toughness of the high-strength steel for reinforcing steel, it attempts to secure the toughness by defining the so-called “yield shelf ratio”. For this reason, it is difficult to say that a high-strength steel for a reinforcing steel bar manufactured by the method proposed in the above-mentioned gazette is not always sufficiently prepared for a huge earthquake in which a complex and large stress is applied to a building by impact.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and its purpose is to provide a yield strength of 345 MPa.
As described above, the yield ratio is 0.8 or less, the yield elongation is 1.4% or more, and the 2 mmV notch Charpy impact value is 27.
An object of the present invention is to provide a method for producing a steel material for reinforcing bars having J / cm ² or more and excellent in earthquake resistance with high productivity. In particular, a method for manufacturing concrete rebar with excellent earthquake resistance that can significantly endure the earthquake resistance performance by satisfying the above characteristics and can withstand even a huge earthquake such as the Hyogoken Nanbu Earthquake mentioned above. The purpose is to provide.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the present inventor first carried out an impact test and a tensile test of a current JIS G 3112 standard steel material at room temperature, and examined problems in seismic performance. . As a result, it became clear that the JIS standard steel material has low mechanical properties required for the steel material for seismic reinforcing bar, especially toughness, tensile elongation and yield ratio, and is not so preferable for use as seismic reinforcing bar. Therefore, it was concluded that it is important to use high-strength reinforcing bars having excellent toughness and high tensile elongation and yield ratio in order to enhance the safety of the structure.

Then, the present inventor next conducted a study on the relationship between the seismic performance and the mechanical properties, and further on the structure capable of increasing the toughness, the tensile elongation and the yield ratio.
I got the knowledge of.

When an impact is complicated and a large stress is applied, it is important for the toughness of the steel material to increase the impact value itself rather than to secure a so-called "yield shelf ratio".

The impact value, the tensile elongation and the yield ratio can be increased by forming a fine ferrite-pearlite structure near the surface of the steel material.

In order to obtain the above structure, the surface of the steel material is cooled to 600 to 600 by water cooling between the intermediate rolling and / or finish rolling passes.
It may be rapidly cooled to a temperature range of 700 ° C.

Following the above treatment, the rolling finishing temperature is controlled in the range of 750 to 950 ° C., and then accelerated to a temperature in the range of 550 to 400 ° C. at a cooling rate of more than 3 ° C./s and 10 ° C./s. The effect is great if cooled.

In order for a high-rise building and a bridge to withstand a huge earthquake of the Hyogo-ken Nanbu Earthquake class, (a) Yield strength of 345 M is necessary for at least concrete reinforcing steel.
Tensile properties of Pa or more, yield ratio of 0.8 or less, yield elongation of 1.4% or more, and (b) 2 mm V notch Charpy impact value 2
Both impact properties of 7 J / cm ² or more are required.

If the above process is performed, the yield strength is 345 M even if the size is larger than the nominal diameter D32.
Pa or more, yield ratio of 0.8 or less, yield elongation of 1.4% or more, 2 mm V notch Charpy impact value of 27 J / cm ²
A steel material for reinforcing bars having the above-mentioned fine ferrite-pearlite structure can be obtained.

The gist of the present invention based on the above findings is a method for manufacturing a steel material for reinforcing bars, which is excellent in earthquake resistance as shown in the following (1) and (2).

(1) A method for producing a steel material for reinforcing bars having excellent earthquake resistance, wherein the rolling step comprises rough rolling, intermediate rolling and finish rolling, in which C: 0.10 to 0.40 in% by weight.
%, Si: 0.05 to 0.60%, Mn: 0.60
2.00%, Al: 0.005-0.080%, N:
0.001-0.007%, B: 0-0.0080%,
P: 0.030% or less, S: 0.030% or less, Cu:
A steel material having a composition of 0.3% or less, Ni: 0.3% or less, Mo: 0.1% or less, Ti: 0.01% or less, the balance Fe and unavoidable impurities is placed in a temperature range of 950 to 1250 ° C. The steel material is heated to rough rolling, and then water-cooled between passes of intermediate rolling and / or finish rolling to make the surface of the steel material 600 to 700.
Rolling is performed by repeating rapid cooling to a temperature range of ℃ 1 to 5 times, and further, the rolling finishing temperature is controlled within a range of 750 to 950 ° C. to finish the rolling, and thereafter, the rolling finish temperature exceeds 3 ° C./s and exceeds 10 ° C.
A method for producing a steel material for reinforcing bars having excellent earthquake resistance, which comprises accelerating cooling to a temperature in a temperature range of 550 to 400 ° C at a cooling rate of up to ° C / s.

(2) A method for producing a steel material for reinforcing bars having excellent earthquake resistance, wherein the rolling step comprises rough rolling, intermediate rolling and finish rolling steps, wherein the components and weight% are the same as described in (1) above. V: 0.01 to 0.20% and Nb: 0.01
Steel material having a composition of 0.1 to 0.10% of balance Fe and unavoidable impurities is heated to a temperature range of 950 to 1250 ° C. for rough rolling, and then between intermediate rolling and / or finish rolling passes. Water-cooled the surface of steel to 600-700 ℃
It is rolled while repeating the rapid cooling to the temperature range of 1 to 5 times, and further, the rolling finish temperature is controlled in the range of 750 to 950 ° C. to finish the rolling, and then the rolling finish temperature exceeds 3 ° C./s and 10 ° C.
A method for producing a steel material for reinforcing bars having excellent earthquake resistance, which comprises accelerating cooling to a temperature in a temperature range of 550 to 400 ° C. at a cooling rate up to / s.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, “%” of the component content is “% by weight”.
Means

(A) Chemical Composition of Steel Material C: C is an element effective for increasing strength. But,
If its content is less than 0.10%, the effect of addition is poor and desired strength cannot be obtained. On the other hand, if it exceeds 0.40%, the toughness of the product steel material does not reach the target value even by the manufacturing method of the present invention. Therefore, the content of C is set to 0.10 to 0.40%.

Si: Si is an element effective for improving hardenability and strength. However, the content is 0.05
If it is less than 0.1%, the desired strength cannot be ensured, and if it exceeds 0.60%, the toughness is deteriorated. Therefore, the content is set to 0.05 to 0.60%.

Mn: Mn is an element effective for improving the hardenability and hot ductility of steel. However, when its content is 0.1.
If it is less than 60%, sufficient hardenability cannot be obtained, and 2.00%
If it is contained in excess of 1.0, segregation will occur, and conversely the hot ductility will decrease. Therefore, if the Mn content is 0.60
˜2.00%.

Al: Al acts to stabilize and homogenize deoxidation of steel. Further, it also has the effect of forming fine AlN by combining with N to make the crystal grains fine and improve the toughness and strength.

However, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.080%, the above effect is not only saturated, but rather cracks occur on the surface of the steel during hot rolling. Therefore, the content of Al is 0.0
It was set to 05 to 0.080%.

N: N has a function of forming fine AlN by combining with Al and making crystal grains fine to improve toughness and strength. However, if the content is less than 0.007%, an amount of AlN effective for refining crystal grains is not generated, while if it exceeds 0.007%, toughness is deteriorated. Therefore, the content of N is 0.001 to 0.007%
And

B: B may not be added. If added, it has the effect of enhancing hardenability. In order to surely obtain this effect, it is desirable that the content of B be 0.0003% or more. However, if the content exceeds 0.0080%, the above effect may be saturated, and in addition, coarsening of crystal grains may occur, resulting in deterioration of toughness. Therefore, the content of B is set to 0 to 0.0080%.

P, S, Cu, Ni, Mo and Ti: The steel used in the present invention contains P, S, C in addition to the above-mentioned constituent elements.
Even if u, Ni, Mo, and Ti are contained at the usual impurity levels, there is no effect on the characteristics of the steel material for reinforcing bars which is excellent in earthquake resistance obtained by the present invention. Therefore, as an impurity element, P: 0.030% or less, S: 0.030%
Below, Cu: 0.3% or less, Ni: 0.3% or less, M
o: 0.1% or less and Ti: 0.01% or less. If the toughness of the steel for reinforcing bars is increased, the safety of the concrete structure will be great even if a complicated and large stress is applied in shock, so a larger value of the 2mmV notch Charpy impact value is secured. Therefore, especially P as an impurity element is 0.015% or less, and S is 0.0
It is preferable to regulate it to 15% or less.

The composition of the steel according to the present invention may further contain one or more of V and Nb in addition to the above components. The effects and desirable contents of these alloy elements are as follows.

V, Nb: V and Nb have the effects of improving the hardenability of the steel and forming carbides in the steel to refine the crystal grains to improve the toughness and strength. Therefore, one or both of V and Nb may be added as required. However, in the case of V, if the content is less than 0.01%, the desired effect is not obtained, and if it exceeds 0.20%, the above effect is not only saturated, but rather the embrittlement phenomenon is caused and the room temperature impact value is rather increased. Result in a large decrease. On the other hand, in the case of Nb as well, if the content is less than 0.01%, the desired effect cannot be obtained, and if it is more than 0.10%, the above effect is not only saturated, but rather causes an embrittlement phenomenon and causes toughness. Cause decline. Therefore, when one or more of these alloy elements are added, V: 0.01 to 0.20%, Nb: 0.01
It is preferable to set the content to 0.10%.

(B) Hot rolling (B-1) Heating It is desirable that the heating temperature during continuous hot rolling is low in order to prevent coarsening of austenite crystal grains, but if it is less than 950 ° C. during rolling. There is a risk of cracking, and the rolling resistance increases as the temperature decreases, and the rolling mill is overloaded. On the other hand, when the heating temperature exceeds 1250 ° C., the surface of the rolled material is significantly oxidized and surface cracking occurs during rolling. Therefore, the heating temperature is set to 950 to 1250 ° C.

(B-2) Inter-pass water cooling of intermediate rolling and / or finish rolling The hot continuous rolling step comprises three steps of rough rolling, intermediate rolling and finish rolling. Of these, intermediate rolling and / or finish rolling are performed. Water cooling between the passes of the
It is important to roll while repeating rapid cooling to a temperature range of 0 to 700 ° C. 1 to 5 times. That is, water cooling is performed between passes of intermediate rolling and / or finish rolling to rapidly cool the surface of the steel material to 700 ° C. or lower below the Ar ₁ point to transform austenite into ferrite and pearlite. It is necessary to make the final microstructure of the steel material a fine ferrite-pearlite structure by repeating the process of reconverting the ferrite pearlite to austenite by recuperating the retained heat. The toughness and strength of the steel material can be remarkably improved only after the surface of the steel material is made into a fine ferrite-pearlite structure by the above treatment.

The steel surface temperature when water-cooled between passes is 70
If the temperature is higher than 0 ° C, the transformation from austenite to ferrite and pearlite does not occur sufficiently and the desired structure cannot be obtained. If the temperature is lower than 600 ° C, reheating by recuperation due to the heat retained inside the steel is sufficient. Not a ferrite power
The reverse transformation from light to austenite is insufficient and the desired structure cannot be obtained. Therefore, the temperature at which the surface of the steel material is rapidly cooled in the case of performing water cooling between passes is 600 to
The temperature range must be 700 ° C.

It is possible to make the surface of the steel material into a fine ferrite-pearlite structure by performing water cooling between passes at least once, but the ferrite-pearlite structure can be made fine even if it is repeated 6 times or more. The effect is saturated. Therefore, the water cooling between passes was repeated 1 to 5 times.

By the way, the "steel material surface" that is water-cooled between passes is not limited to the surface of the steel material, and the radius ratio of the steel material surface from the steel surface is 0.
It may be up to a depth of 3. When the portion rapidly cooled to a temperature range of 600 to 700 ° C. by water cooling between passes is up to the above-mentioned depth, the so-called “surface portion” has a fine structure, yield strength of 345 MPa or more, yield ratio of 0.8 or less, and yield. This is because elongation of 1.4% or more and a 2 mm V notch Charpy impact value of 27 J / cm ² or more can impart necessary characteristics to the steel material for earthquake-proof reinforcing bars. On the other hand, when the depth exceeds the depth of 0.3 from the surface of the steel material in terms of the radius ratio, the internal retained heat amount becomes small and reheating due to recuperation does not sufficiently occur and the desired structure cannot be obtained, Deformation resistance increases during rolling after quenching, resulting in excessive load on the rolling mill.

(B-3) Rolling Finishing Temperature A lower rolling finishing temperature is more effective for grain refinement, but if the rolling finishing temperature is lower than 750 ° C., the load on the rolling mill becomes excessive and the steel material is On the other hand, when surface cracking occurs, on the other hand, when the temperature exceeds 950 ° C., the crystal grains become coarse and a desired fine structure cannot be obtained, so the rolling finishing temperature was set to 750 to 950 ° C. This rolling finish temperature can be ensured by the recuperation of the rolled steel material itself and the heat generated during rolling.

(C) Cooling after rolling After the completion of rolling, it is necessary to accelerate and cool the steel material to a temperature in the range of 550 to 400 ° C at a cooling rate of more than 3 ° C / s and 10 ° C / s. When accelerated cooling is performed at a cooling rate of more than 10 ° C./s, the surface layer portion becomes a so-called “low-temperature transformation structure” with quenching, and the inside becomes a ferrite-pearlite structure, and the structure becomes non-uniform, resulting in toughness. In addition, the tensile properties are deteriorated. On the other hand, at a cooling rate of 3 ° C./s or less, the structure of the central portion becomes a coarse ferrite-pearlite structure, and desired mechanical properties (toughness and tensile properties) cannot be obtained.
Therefore, the cooling rate after rolling exceeds 3 ° C / s and is 10 ° C / s.
Up to

When the temperature for accelerated cooling exceeds 550 ° C., even if accelerated cooling is performed at a cooling rate of more than 3 ° C./s and up to 10 ° C./s, a desired structure is not obtained, and therefore desired mechanical properties are not obtained. I can't get it. On the other hand, the temperature for accelerated cooling is 40
If the temperature is lower than 0 ° C, the inside of the steel material may be hardened and the desired mechanical properties may not be obtained. Therefore, the temperature for accelerated cooling at a cooling rate of more than 3 ° C./s to 10 ° C./s is set to a temperature in the temperature range of 550 to 400 ° C. After performing this accelerated cooling, it may be allowed to cool.

The cooling rate referred to here is the cooling rate on the surface of the steel material.

By subjecting the steel material having the component composition shown in (A) above to controlled rolling and accelerated cooling under the conditions shown in (B) and (C) above, the yield strength is 345.
MPa or more, yield ratio 0.8 or less, yield elongation 1.4%
Above, 2mmV notch Charpy impact value is 27J / cm
It is possible to manufacture high-strength, high-toughness rebars that have characteristics of ² or more and are excellent in earthquake resistance.

[0039]

Example A steel having a chemical composition shown in Table 1 was smelted in a 70 t converter by a usual method. In Table 1, steels A to C are comparative steels in which any of the components is out of the range of the content specified in the present invention, and steels D to H are steels of the present invention (hereinafter referred to as steels of the present invention). is there.

Next, these steels were made into billets by a continuous casting method, and further slab-rolled into 3t billets by a usual method.

Thereafter, the 3t billet was continuously rolled and cooled under the conditions shown in Tables 2 to 8 to have diameters of 32, 35,
Steel bars of 38, 41 and 51 mm were produced.

From the steel bar thus obtained, a test piece for microstructure observation having a diameter of 30 mm in length with controlled rolling and cooling was cut out, and the microstructure of a portion having a depth of 0.3 in radius ratio from the surface was observed by an optical microscope. I observed.

Further, JIS No. 4 tensile test pieces and JIS No. 4 impact test pieces (2 mm V notch Charpy impact test pieces) were sampled from the surface of the steel bar, and the impact properties and tensile properties at room temperature (20 ° C.) were investigated.

Tables 9 to 15 show examples of test results. Table 9
According to 15 to 15, if the steel having the chemical composition specified in the present invention is "hot rolled-accelerated cooled" under the conditions specified in the present invention, desired yield strength, yield ratio, yield elongation and Charpy impact value can be obtained. It is clear that it can be obtained.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

[Table 6]

[0051]

[Table 7]

[0052]

[Table 8]

[0053]

[Table 9]

[0054]

[Table 10]

[0055]

[Table 11]

[0056]

[Table 12]

[0057]

[Table 13]

[0058]

[Table 14]

[0059]

[Table 15]

[0060]

EFFECTS OF THE INVENTION According to the method for producing a steel material for reinforcing bars having excellent earthquake resistance of the present invention, the yield strength is 345 MPa or more, the yield ratio is 0.8 or less, and the yield ratio is relatively easy and with high productivity. It is possible to manufacture a steel material for reinforcing bars having an elongation of 1.4% or more and a 2 mmV notch Charpy impact value of 27 J / cm ² or more and excellent in earthquake resistance. Therefore, it is possible to provide a structural reinforcing bar that is highly safe and has excellent seismic resistance even when it is used for reinforced concrete such as a building.

Claims (2)

[Claims] 1. A method for producing a steel material for reinforcing bars having excellent earthquake resistance, wherein the rolling step comprises rough rolling, intermediate rolling and finish rolling, in which C: 0.10 to 0.40% by weight, S
i: 0.05 to 0.60%, Mn: 0.60 to 2.00
%, Al: 0.005-0.080%, N: 0.001
~ 0.007%, B: 0 to 0.0080%, P: 0.0
30% or less, S: 0.030% or less, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.1% or less, Ti:
A steel material having a composition of 0.01% or less and the balance of Fe and unavoidable impurities is heated to a temperature range of 950 to 1250 ° C. to perform rough rolling, and then water cooled between passes of intermediate rolling and / or finish rolling. The surface of the steel sheet is rapidly cooled to a temperature range of 600 to 700 ° C. while repeating 1 to 5 times, and further, the rolling finishing temperature is controlled in the range of 750 to 950 ° C. to finish the rolling, and then 3 ° C. / A method for producing a steel material for reinforcing bars having excellent earthquake resistance, which comprises accelerating cooling to a temperature in a temperature range of 550 to 400 ° C at a cooling rate of more than s and 10 ° C / s. 2. A method for producing a steel material for reinforcing bars having excellent earthquake resistance, wherein the rolling step comprises rough rolling, intermediate rolling and finish rolling steps, wherein the components and weight% are as set forth in claim 1.
V: 0.01 to 0.20% and Nb: 0.01 to 0.1
A steel material having a composition of 0% of one kind or more, the balance of Fe and unavoidable impurities is heated to a temperature range of 950 to 1250 ° C. to perform rough rolling, and then water-cooled between passes of intermediate rolling and / or finish rolling. Rolling is performed by repeating 1 to 5 times of rapidly cooling the surface of the steel material to a temperature range of 600 to 700 ° C., and further, rolling finish temperature is controlled in a range of 750 to 950 ° C. to finish rolling, and then 3 ° C. / S, a method for producing a steel material for reinforcing bars having excellent earthquake resistance, which comprises performing accelerated cooling to a temperature in a temperature range of 550 to 400 ° C at a cooling rate of 10 ° C / s to 10 ° C / s.