JPH0949021A - Production of steel material for low temperature reinforcement - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel material for large-diameter low temp. reinforcement, excellent in strength and toughness at low temp., at a low cost by subjecting a steel stock, containing specific amounts of C, Si, Mn, Al, and B, to specific hot rolling including rapid cooling and then to specific cooling. SOLUTION: A steel stock, which has a composition consisting of, by weight, 0.10-0.40% C, 0.05-0.60% Si, 0.60-2.00% Mn, 0.005-0.080% Al, <=0.0050% B, and the balance Fe with inevitable impurities and further containing, if necessary, either or both of 0.01-0.20% V and 0.01-0.10% Nb, is heated to 950-1250 deg.C and roughed. Subsequently, this roughed steel stock is rolled while repeating, 1 to 5 times, a treatment consisting of application of rapid cooling by water down to 600-700 deg.C surface temp. between the passes in the course of intermediate rolling and/or finish rolling. Further, rolling finishing temp. is controlled to 750-950 deg.C, and, after rolling is finished, accelerated cooling is carried out down to 550-400 deg.C at (3 to 10) deg.C/S cooling rate. By this method, the steel material, having >=400MPa yield strength at orddinary temp., <=-60 deg.C Charpy fracture appearance transition temp., and >= about 750MPa tensile strength at -60 deg.C, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel material for a low-temperature rebar, and more particularly to a method for producing a steel material for a low-temperature rebar which is used for a structural member of an LNG tank and has excellent strength and toughness at low temperature. .

[0002]

[Prior Art] Conventionally, reinforcing bars for concrete are JIS G 3112.
Steel materials with standardized chemical composition and mechanical properties have been used as "steel bars for reinforced concrete". Current,
This standard steel material is used not only as an ordinary reinforcing bar for building construction, but also as a reinforcing bar for low temperature environment such as a reinforcing bar for LNG tank or a reinforcing bar for reinforced concrete structure in cold regions.

However, the reinforcing bars standardized as "steel bars for reinforced concrete" in JIS G 3112 described above assume normal temperatures or higher temperatures as the temperature of the actual use environment, and are not necessarily the low temperatures as described above. It is not intended for use in the environment.

Therefore, Japanese Patent Publication No. 6-72260 proposes a "method for manufacturing a low temperature reinforcing bar" having a fracture surface transition temperature of V notch Charpy of -80 ° C or lower. Also,
Japanese Patent Publication No. 2-24904 discloses "reinforcing bar steel having excellent low temperature toughness and seawater resistance". However, the low-temperature reinforcing bars described in these publications all have Ni as an essential component in order to improve impact properties at low temperatures, and therefore have a problem in cost when used as reinforcing bars.

In Japanese Patent Publication No. 4-8486, a steel material having a specific chemical composition is subjected to controlled rolling and cooling to have a bainite + ferrite structure containing 20% or more of a bainite structure, "a steel bar having excellent low temperature toughness. Manufacturing method "has been proposed. However, since the C content of the steel used in the method proposed in this publication is 0.02 to 0.10%, in the case of a reinforced steel bar having a large diameter, the desired strength may not be achieved in the central region thereof. is there.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is low cost and sufficient strength and low temperature toughness to be used in a low temperature environment.
Specifically, it has a yield strength at room temperature of 400 MPa or more, a Charpy fracture surface transition temperature of -60 ° C or less, and a notch tensile strength of 750 MPa or more at -60 ° C, and particularly JIS D32.
It is intended to provide a method for manufacturing a steel material for a large diameter low temperature rebar having a size corresponding to ~ 51.

[0007]

In order to solve the above-mentioned problems, the present inventor first investigated the characteristics of the current JIS G 3112 standard steel material at low temperature, and the problems when this was used in a low-temperature environment. I examined the points. As a result, it has been clarified that the JIS standard steel material is fragile and easily broken when the use environment temperature becomes -10 ° C or less, the toughness (impact value) and the notch tensile strength are significantly reduced. This is a very fatal defect when the reinforcing bar is used at a low temperature.

Therefore, it was concluded that it is necessary to enhance the toughness and notch tensile strength at low temperature in order to enhance the safety of the structure.

Therefore, the present inventor next examined a structure capable of increasing the toughness and notch tensile strength at low temperature even in the case of a low alloy steel containing no expensive element Ni.

As a result, the following findings were obtained.

By forming a fine ferrite-pearlite structure in the vicinity of the surface of the steel material, the low temperature toughness and the notch tensile strength at low temperature can be enhanced.

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

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 to 10 ° C./s. The effect is great if cooled.

The steel material for rebar having a fine ferrite-pearlite structure having a yield strength at room temperature of 400 MPa or more, a Charpy fracture surface transition temperature of -60 ° C or less and a notch tensile strength of 750 MPa or more at -60 ° C is L
Sufficiently usable as a reinforcing bar for low temperature environment such as reinforcing bar for NG tank and reinforcing bar for reinforced concrete structure in cold regions.

If the above treatment is carried out, a low temperature rebar having a yield strength of 400 MPa or more, a Charpy fracture surface transition temperature of -60 ° C. or less, and a notch tensile strength of 750 MPa or more at -60 ° C. using Ni-free steel. Steel for use is obtained.

The gist of the present invention based on the above findings is the method for producing a steel material for low-temperature rebar shown in the following (1) and (2).

(1) A method for producing a steel material for low-temperature rebar, which comprises a rolling step, a rough rolling step, an intermediate rolling step, and a finish rolling step, wherein C: 0.10 to 0.40% and Si:
0.05-0.60%, Mn: 0.60-2.00%,
Al: 0.005-0.080%, B: 0.0050%
A steel material containing the following, the balance being 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 to obtain a steel surface. Is rapidly cooled to a temperature range of 600 to 700 ° C., which is repeated 1 to 5 times, and the rolling finish temperature is controlled to be in the range of 750 to 950 ° C. to finish the rolling, and then the temperature exceeds 3 ° C./s. A method for producing a steel material for low-temperature rebar, comprising accelerating cooling to a temperature in a temperature range of 550 to 400 ° C at a cooling rate of 10 ° C / s.

(2) A method for producing a steel material for low-temperature rebar, wherein the rolling step comprises rough rolling, intermediate rolling, and finish rolling, in addition to the components described in (1) above, a weight%
At 0.01 to 0.20% V and 0.01 to 0.10.
% Nb at least one kind, and the balance consisting of 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 cooling the surface of steel to 600-700
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 then the temperature exceeds 3 ° C./s and 10 ° C.
A method for manufacturing a steel material for low-temperature rebar, which comprises accelerating cooling to a temperature in a temperature range of 550 to 400 ° C. at a cooling rate of / s.

[0019]

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 and notch tensile strength of the product steel material do not reach the target values even by the production method of the present invention. Therefore, the content of C is set to 0.10 to 0.40%.

Si: Si has the effects of improving hardenability and preventing surface oxidation at high temperatures during hot heating and rolling. Further, it also has the effect of improving strength. However, if the content is less than 0.05%, the desired strength cannot be ensured and the oxidation resistance of the surface at high temperature deteriorates, and if it exceeds 0.60%, the toughness and notch tensile strength deteriorate. Therefore, the content of 0.05 to 0.
It was set to 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 the surface of the steel product is cracked during hot rolling. Therefore, the content of Al is 0.0
It was set to 05 to 0.080%.

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 0.0050%, 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 0.0050% or less.

The steel used in the present invention may further contain at least one 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 if necessary. However, in the case of V, if the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.20%, the above effect is not only saturated, but rather it causes an embrittlement phenomenon and rather causes toughness and cutting. Notch causes a decrease in tensile strength. On the other hand, also in the case of Nb, if the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.10%, the above effect is not only saturated, but rather it causes embrittlement and rather causes toughness. Notch causes reduction in tensile strength. Therefore, when one or more of these alloy elements are added, V: 0.01
.About.0.20% and Nb: 0.01 to 0.10% are preferable.

By the way, the steel used in the present invention may contain Cu, Ni, Cr, Mo, and Ti at usual impurity levels in addition to the above-mentioned constituent elements. That is, C
u, Ni and Cr are each 0.3% or less, and Mo is 0.1%.
% Or less and 0.01% or less of Ti may be contained. This is because there is no adverse effect on the characteristics of the steel material for low-temperature rebar obtained by the present invention.

(B) Hot Rolling (B-1) Heating It is desirable that the heating temperature in continuous hot rolling is low in order to prevent coarsening of austenite crystal grains. A large number of them may remain to cause cracking during rolling, and the rolling resistance becomes higher as the temperature lowers, resulting in excessive load on the rolling mill. 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 95
It was 0 to 1250 ° C.

(B-2) Interpass 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 repeat the rapid cooling to the temperature range of 00 to 700 ° C. for 1 to 5 times and to perform rolling. 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, and heat retained inside the steel material. It is necessary to make the final structure of the steel material a fine ferrite-pearlite structure by repeating the process of re-heating the material to reverse-transform it from ferrite-pearlite to austenite. It becomes possible to remarkably improve the low temperature toughness and strength of the steel material and the notch tensile strength only by making the surface of the steel material a fine ferrite-pearlite structure by the above treatment.

The steel surface temperature when water-cooled between passes is 70
If the temperature exceeds 0 ° C, the desired structure cannot be obtained because the transformation from austenite to ferrite and pearlite does not occur sufficiently. If the temperature is lower than 600 ° C, reheating due to the recuperation due to the retained heat inside the steel is insufficient. The reverse transformation from ferrite / pearlite 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 form a fine ferrite-pearlite structure on the surface of the steel material by performing water cooling between passes at least once, but the effect of refining the ferrite-pearlite structure at 6 times or more is also obtained. Saturate. 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 depth, the so-called “surface portion” has a fine structure,
By performing accelerated cooling under the conditions described below after finish rolling, the yield strength at room temperature is 400 MPa or more, the Charpy fracture surface transition temperature is -60 ° C or less, and the notch tensile strength at -60 ° C is 75.
This is because the necessary properties of 0 MPa or more can be imparted to the reinforcing bar for a low temperature environment. 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) Accelerated Cooling After Rolling After the completion of rolling, it is necessary to accelerate cooling 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 nonuniform, resulting in toughness and cutting. Notch causes deterioration of tensile strength. On the other hand, at a cooling rate of 3 ° C./s or less, the structure in the central portion becomes a coarse ferrite / pearlite structure, and desired mechanical properties (yield strength, toughness, notch tensile strength) may not be obtained. Therefore, the accelerated cooling rate after rolling was set to more than 3 ° C / s and up to 10 ° C / s.

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 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, a yield strength at room temperature of 400 MPa or more, It is possible to manufacture a steel material for low-temperature rebar having a Charpy fracture surface transition temperature of -60 ° C or lower and a notched tensile strength of 750 MPa or higher at -60 ° C, which is a characteristic required for a reinforcing bar for a low-temperature environment.

[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 content range specified in the present invention, and Steels D to G 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 subjected to continuous rolling and cooling under the conditions shown in Tables 2 to 7, and the diameters were 32, 35, and
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.

From the surface of the steel bar, JIS No. 4 tensile test piece, JIS No. 4 Charpy impact test piece and notched tensile test piece (smooth portion diameter: 7 mm, notched portion diameter: 5 mm, notched bottom radius: 0.05 mm, Notch angle: 60 degrees)
Were collected, and the tensile properties at room temperature, the Charpy fracture surface transition temperature, and the low temperature notch tensile strength at -60 ° C were investigated.

Tables 8 to 13 show examples of test results. Table 8
According to Nos. 13 to 13, it is possible to obtain desired yield strength, toughness and notch tensile strength by "hot rolling-accelerated cooling" under the conditions specified in the present invention, for the steel having the chemical composition specified in the present invention. Is clear.

[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]

As described above, according to the method for producing a steel material for a low-temperature rebar of the present invention, the low-temperature rebar having a higher yield strength and toughness and notch tensile strength at a low temperature at a lower cost than before. Since it is possible to manufacture a steel material for use, and it is possible to provide a structural reinforcing bar having high safety even when used in a low temperature environment, the industrial effect is extremely large.

Claims (2)

[Claims] 1. A method for producing a steel material for low-temperature rebar, comprising a rolling step of rough rolling, intermediate rolling, and finish rolling, wherein C: 0.10 to 0.40% and Si: 0 by weight. ．
05-0.60%, Mn: 0.60-2.00%, A
1: 0.005 to 0.080%, B: 0.0050% or less, with the balance being steel consisting of Fe and unavoidable impurities, heated to a temperature range of 950 to 1250 ° C. to perform rough rolling, and then Rolling is performed by repeating water-cooling between the intermediate rolling and / or finish rolling to rapidly cool the surface of the steel material to a temperature range of 600 to 700 ° C. 1 to 5 times, and further rolling at a finishing temperature of 750 to 950 ° C. Manufacture of a steel material for low-temperature rebar characterized by accelerating cooling to a temperature range of 550 to 400 ° C. at a cooling rate of more than 3 ° C./s and 10 ° C./s after the rolling is controlled within a range. Method. 2. A method for producing a steel material for low-temperature reinforcing bars, wherein the rolling step comprises steps of rough rolling, intermediate rolling and finish rolling, and in addition to the components described in claim 1, in a weight percentage,
A steel material containing 0.01 to 0.20% of V and 0.01 to 0.10% of Nb, and the balance of Fe and inevitable impurities is heated to a temperature range of 950 to 1250 ° C. Rough rolling, then water cooling between passes of intermediate rolling and / or finish rolling to rapidly cool the surface of the steel material to a temperature range of 600 to 700 ° C. by repeating 1 to 5 times, and further rolling finishing. The temperature is controlled in the range of 750 to 950 ° C. to finish the rolling, and then the temperature exceeds 3 ° C./s and 10 ° C./s.
A method for manufacturing a steel material for low-temperature rebar, which comprises accelerating cooling to a temperature in a temperature range of 550 to 400 ° C. at a cooling rate of up to s.