JP2011174137A - Method for manufacturing steel material for reinforcing bar - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel material for a reinforcing bar having high strength and ductility under a superior balance, and further having tensile strength and ductility which have equal levels to those of the base metal when having been welded, at a low cost without adding an expensive alloy element. <P>SOLUTION: This manufacturing method includes: hot-rolling a steel raw material containing a predetermined composition at a heating temperature between the Ac<SB>3</SB>point and 1,100°C and a termination temperature between the Ar<SB>3</SB>point and 950°C; and subsequently cooling the material in a temperature range of 300-800°C at a rate of 2-20°C/s to adjust a bainite transformation starting temperature to 500°C or lower and a transformation completion temperature to a temperature higher than 300°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a high-strength steel for reinforcing bars used as a material for shear reinforcing bars used in reinforced concrete structures, for example.

  For example, in a reinforced concrete structure, a shear reinforcing bar is used as a reinforcing material in order to prevent its collapse. In a reinforced concrete structure using shear reinforcement, when the reinforced concrete structure undergoes shear deformation, the shear reinforcement extends and plastically deforms, so that the deformation energy of the reinforced concrete structure is absorbed by the shear reinforcement and the reinforced concrete structure Collapse is prevented. However, conventional shear reinforcements are not always sufficient in terms of elongation characteristics. That is, since the shear reinforcing bar is formed into a circular shape, a square shape, or the like by bending, if it has excellent elongation characteristics, bending becomes easy, which is a great merit from the viewpoint of workability.

In recent years, there has been an increasing demand for a welded closed type with good workability, in which a shear reinforcing bar is welded to reinforce a reinforced concrete structure. In this welded-type shear reinforcement, it is important not to lower the strength and ductility after welding, and the joint elongation at the welded portion is also an important characteristic. Usually, the welding of the shear reinforcement uses a high capacity and high productivity resistance welding called flash butt welding or upset butt welding.
Here, the flash butt welding means that two end faces of steel bars are brought into contact with each other, a large voltage is applied between the two end faces, and arc contact and short-circuiting are repeated to form a molten portion at the end. This is a welding method in which the molten part is discharged by upset (upsetting deformation) and a joining part is formed at the ends of two steel bars. Upset butt welding is a welding method in which a large voltage is applied between the end faces of two steel bars that are completely butted, the ends are upset by resistance heat generation, and a joint is formed at the ends of the two steel bars. It is.

  As a steel material for reinforcing bars used for such a shear reinforcing bar, for example, in Patent Document 1 and Patent Document 2, excellent strength and ductility are obtained without performing heat treatment such as quenching and tempering after rolling. Non-tempered steel for rebar having a tensile strength and ductility equivalent to that of the steel is disclosed.

JP-A-8-325637 Japanese Patent No. 2973909

  However, the steel material for non-tempered rebar described in Patent Document 1 has a problem of high cost because it requires the addition of Mo. Moreover, since the non-tempered steel material for high-strength reinforcing steel described in Patent Document 2 contains 0.003% or more of Ti, the toughness may be reduced due to the generation of TiN.

  Moreover, in these steel materials, since low temperature toughness is not taken into consideration, there is a possibility that cracking may occur when used in a cold region.

  Furthermore, all of the techniques described in these patent documents are aimed at achieving a high strength-ductility balance in the as-rolled state, but there is a large variation in characteristics due to variations in wire rod cooling history after hot rolling. In fact, it is difficult to stably obtain excellent characteristics.

  Therefore, the present invention has a high strength and ductility in an excellent balance, and when welded, a high-strength steel material for reinforcing steel having a tensile strength and ductility equivalent to that of the base material, and an expensive alloy element. It aims at proposing about the method for manufacturing at low cost, without adding.

  In order to produce non-tempered steel for reinforcing steel, the inventors firstly have excellent strength and ductility even if they are not tempered, and have the same level of tensile strength and ductility as the base metal even if they are welded. In addition, various experiments and research were conducted. At that time, without quenching and tempering, the yield strength is 785 MPa or more, the tensile strength is 930 MPa or more, the base material elongation (EL) is 8% or more, and the welded joint elongation is 5% or more. The goal was to produce a steel for non-tempered rebars with mechanical properties that had both strength and ductility, with no breakage during bending. Another target for low temperature toughness was that the Charpy impact value (uEO) of the base material at 0 ° C. was 80 J or more.

  As a result, it is effective to suppress softening of the weld heat-affected zone (HAZ) in the vicinity of the joint in order to prevent deterioration in strength and ductility after welding in non-heat treated steel. It was found that it is effective to reduce the content and prevent the deterioration of low temperature toughness by suppressing the formation of TiN. However, on the other hand, the ductility characteristics of manufactured steel materials, especially the “drawing value”, vary greatly, and bending may occur during bending, and studies were made to improve this point.

  The inventors conducted various investigations by paying attention to the relationship between residual hydrogen in steel and the tensile properties of steel. As a result, it was found that about 0.5 to 1.5 ppm of hydrogen remains in the conventional steel, which has a significant adverse effect on the ductility of the steel material. That is, it has been confirmed that when the residual hydrogen is reduced, the absolute value of the drawing value, which has a great influence on the tensile properties, particularly the bending workability, is increased, the variation is reduced, and the tensile properties are greatly improved. Then, the relationship between the hydrogen content in the steel and the drawing value during the tensile test was investigated, and the hydrogen content in the steel as rolled was controlled to 0.4 ppm or less, so that a high drawing of 35% or more during the tensile test. The value was achieved, and it was found that stable bending workability was obtained at a high level.

On the other hand, by holding for a predetermined time at a predetermined temperature after rolling the steel material, it is possible to reduce the hydrogen content in the steel material. It has also been found that if the processing capacity is insufficient compared to other manufacturing facilities, there may be restrictions on the production volume of the entire process.
Therefore, the inventors have further studied diligently, and found a way to obtain the desired low residual hydrogen steel as it is rolled without necessarily performing a special treatment for dehydrogenation. It came to complete.

Here, the mechanism by which the residual hydrogen of the steel is controlled to a low level as it is rolled is not necessarily clear, but is considered as follows.
First, when the structure is in the austenite state, the hydrogen solid solubility limit in the steel is extremely high, and a high concentration of hydrogen remains in the untransformed steel after rolling. During cooling after rolling, most of the hydrogen remaining in the austenite after the transformation to the structure of ferrite, pearlite, bainite, martensite, etc. is completed starts to be released into the atmosphere.
That is, from the viewpoint of dehydrogenation, it is advantageous that the transformation is completed at a higher temperature. On the other hand, when the transformation temperature becomes extremely high, it is difficult to achieve high strength, so that the bainite transformation start temperature is 500 ° C or lower and the bainite transformation completion temperature is higher than 300 ° C. It was newly found that it is effective for obtaining characteristics.

That is, the gist configuration of the present invention is as follows.
1. C: 0.15 mass% or more and 0.30 mass% or less,
Si: 0.05 mass% or more and 0.50 mass% or less,
Mn: 0.2% by mass or more and 2.0% by mass or less,
Cr: 0.1% by mass or more and 1.0% by mass or less,
Al: 0.01 mass% or more and 1.00 mass% or less,
Nb: 0.001 mass% or more and 0.300 mass% or less,
V: 0.01% by mass or more and 1.00% by mass or less,
Ti: less than 0.003 mass%,
Ni: less than 0.5% by mass,
Cu: less than 0.5% by mass,
N: less than 0.0060% by mass,
P: 0.03% by mass or less and S: 0.03% by mass or less, satisfying the following formula (1), the balance being Fe and inevitable impurities, heating temperature: Ac ₃ points or more and 1100 ° C or less And end temperature: Ar Hot rolling at ₃ points or more and 950 ° C or less, and then cooling at a temperature range of 800 ° C or less and 300 ° C or more at 2 ° C / s or more and 20 ° C / s or less. A method for producing a steel material for reinforcing steel, comprising adjusting a bainite transformation start temperature to 500 ° C. or less and a transformation completion temperature to more than 300 ° C.
2 [Si] +1 [Mn] +1.2 [Cr] ≦ 3.0 (1)
Here, [] is the content of the component in parentheses (% by mass)

2. 2. The method for producing a steel material for reinforcing steel as described in 1 above, wherein the steel material further contains B in a range satisfying the following formula (2).
Record
0.0100 ≧ B (mass%) ≧ {[N] / 14− [Ti] / 27} × 11 + 0.0005 (2)
Here, [] is the content of the component in parentheses (% by mass)

3. The steel material is further
Mo: 0.01 mass% or more and 1.0 mass% or less is contained, The manufacturing method of the steel material for reinforcing bars of said 1 or 2 characterized by the above-mentioned.

  According to the present invention, a high-strength alloy element having a high balance between ductility during cold bending and a high balance of final strength, small variation in ductility characteristics, and excellent base material elongation and weld joint elongation when welded is an expensive alloy element. Can be produced at low cost without adding. In addition, a high-strength steel material for rebar excellent in low-temperature toughness can be produced.

It is a figure which shows the reinforcing bar faced mutually and the cross-sectional hardness profile after the welding.

Hereinafter, the manufacturing method of the steel material for reinforcing bars of the present invention will be described in detail. First, it demonstrates in order from the reason for the component limitation of the steel raw material in the method of this invention. In the following description, the unit indicated by “%” means “mass%” unless otherwise specified.
C: 0.15% or more and 0.30% or less C is required to be 0.15% or more in order to ensure the intended strength. However, if added over 0.30%, the weldability and ductility, particularly when hydrogen remains in the steel, is significantly deteriorated, so the content is made 0.30% or less.

Si: 0.05% or more and 0.50% or less
Si is added at 0.05% or more for deoxidation and strengthening of steel. On the other hand, if added over 0.50%, the finally formed parent phase structure is embrittled and the bainite transformation completion temperature is lowered. As a result, the bendability of the welded joint is lowered.

Mn: 0.2% or more and 2.0% or less
Mn needs to be added in an amount of 0.2% or more in order to ensure hardenability and obtain the target strength. However, if added over 2.0%, ductility and weldability are deteriorated, and the bainite transformation completion temperature is lowered to increase the residual amount of hydrogen in the steel after rolling.

Cr: 0.1% to 1.0%
Cr is an element that enhances hardenability and needs to be added in an amount of 0.1% or more in order to increase the strength. However, if added over 1.0%, the hardenability becomes excessive and the ductility and weldability are deteriorated, and the bainite transformation completion temperature is lowered to increase the residual amount of hydrogen in the steel after rolling, so the amount is made 1.0% or less. .

2 [Si] +1 [Mn] +1.2 [Cr] ≦ 3.0 (1)
In the present invention, the above-described Si, Mn and Cr are additive elements essential for achieving the target strength, but at the same time, all of them lower the completion temperature of the bainite transformation. Even if the respective addition amounts are within the range of the present invention, when the above formula (1) is not satisfied, the bainite transformation completion temperature is lowered, and hydrogen remains in the as-rolled steel.

Nb: 0.001% or more and 0.300% or less
Nb is an element that forms fine carbonitrides in steel and is effective in suppressing softening of the weld heat affected zone as the strength of the base material increases. Since the precipitated carbonitride is finer than TiN, the adverse effect on toughness is small. Furthermore, the influence on the transformation completion temperature is small. However, if the addition is less than 0.001%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.300%, the Nb content is 0.001% or more because the toughness deterioration of the weld heat affected zone becomes remarkable even with Nb carbonitride. 0.300% or less.

V: 0.01% or more and 1.00% or less V is an element that improves the hardenability of the steel material and is effective in suppressing softening of the carbonitriding and welding heat affected zone. If the addition is less than 0.01%, a sufficient effect cannot be obtained. If the addition exceeds 1.0%, the toughness of the weld heat affected zone is remarkably deteriorated, so 0.01% or more and 1.00% or less.

Al: 0.01% or more and 1.00% or less
Al is added for deoxidation of steel, but if less than 0.01%, the effect is small, so 0.01% or more is added. However, if added over 1.00%, the bendability of the welded joint is lowered, so the content is made 1.00% or less.

Ti: Less than 0.003%
Since Ti fixes N, produces | generates coarse nitride (TiN) and promotes the fall of toughness, it is desirable not to add fundamentally. The acceptable content is less than 0.003%.

P: 0.03% or less P is basically desirably not contained in order to embrittle the steel material and deteriorate the ductility and low temperature toughness after welding with the base material. The upper limit of the content acceptable as an inevitable impurity is 0.03%.

S: 0.03% or less S is desirably basically not contained because it combines with metals such as Mn to form coarse sulfides in steel and deteriorates the base metal and ductility after welding and low-temperature toughness. . The upper limit of the content acceptable as an inevitable impurity is 0.03%.

N: Less than 0.0060% N is an unavoidable impurity. When it exceeds 0.0060%, coarse precipitates such as TiN and VN are formed during welding, and the tensile strength and bendability of welded joints are reduced. Therefore, the content is less than 0.0060%.

Ni: Less than 0.5%
Ni is effective for increasing the strength of steel as it is rolled, but significantly lowers the bainite transformation completion temperature. Therefore, it is desirable not to add in the present invention, but the content up to 0.5% is acceptable.

Cu: Less than 0.5%
Cu, as well as Ni, is effective in increasing the strength of rolled steel, but on the other hand, the bainite transformation completion temperature is significantly reduced. Therefore, it is desirable not to add in the present invention, but the content up to 0.5% is acceptable.

Moreover, B can be added to the steel material as necessary.
B is an element that improves hardenability, and it is effective to add it when it is particularly necessary to increase the strength of the base material. On the other hand, even if added over 0.0100%, the effect of improving hardenability is saturated and the weldability is deteriorated. Moreover, in order to obtain the strength increasing effect, B needs to be dissolved in the steel. However, if solid solution N is present in the steel, B in the steel is consumed for the formation of BN. Thus, when B is present in the steel as BN, B does not contribute to the improvement of hardenability. However, when Ti is present, N in the steel is fixed as TiN according to the amount of Ti present, and N that becomes TiN does not contribute to the formation of BN. Therefore, when adding B, it is necessary to add more than is consumed for the formation of BN. For that purpose, the following formula (2) is added between the B content, N content and Ti content in the steel. It is necessary that the relationship indicated by is established. In order to obtain the effect of increasing the strength, it is preferable to add at 0.0020% or more.
0.0100 ≧ B (mass%) ≧ {[N] / 14− [Ti] / 27} × 11 + 0.0005 (2)
Here, [] is the content of the component in parentheses (% by mass)

Furthermore, Mo can be added as needed for the purpose of improving the strength-ductility balance of the steel material. That is, Mo increases the hardenability while having little influence on the transformation completion temperature, so it can be added to improve the structure and improve ductility. When adding, it is preferable to set it as 0.01% or more. However, if adding over 1.00%, the cost increases, and excessive addition deteriorates the weldability and causes a decrease in the transformation completion temperature. Therefore, it is preferably made 1.00% or less.
The balance other than the above components is composed of Fe and inevitable impurities other than those described above.

The steel material having the above component composition is hot-rolled at a heating temperature: Ac ₃ points to 1100 ° C. and an end temperature: Ar ₃ points to 950 ° C. Thereafter, cooling is performed at a temperature range of 800 ° C. or lower and 300 ° C. or higher at 2 ° C./s or higher and 20 ° C./s or lower. As a result, a high-strength steel material for reinforcing steel having a structure with a volume ratio of bainite of 90% or more, a residual hydrogen concentration of 0.3 ppm or less, excellent balance between strength and ductility, and small variation in properties is manufactured. be able to.

  That is, when the heating temperature in hot rolling exceeds 1100 ° C., the initial γ grain size becomes coarse and the transformation during cooling is delayed. As a result, the transformation completion temperature is lowered. Similarly, even when the end temperature of hot rolling exceeds 950 ° C., the γ grain size before transformation also becomes coarse and the transformation at the time of cooling is delayed, resulting in a decrease in transformation completion temperature.

On the other hand, when the heating temperature in the hot rolling is less than Ac ₃ , the load on the mill increases due to an increase in deformation resistance, making it difficult to form by rolling. Also, when the end temperature of hot rolling is less than Ar ₃ point, similarly, rolling becomes difficult due to an increase in deformation resistance, and strain after rolling remains after cooling to room temperature, which adversely affects the ductility of the steel material. Effect.

  Next, after hot rolling, cooling is performed at a temperature range of 800 ° C. or lower and 300 ° C. or higher at 2 ° C./s or higher and 20 ° C./s or lower. First, when the cooling rate is lower than 2 ° C./s, the ferrite fraction increases, and a bainite structure of 90% or more cannot be obtained under the above-described component composition, and sufficient steel strength cannot be obtained. On the other hand, when the cooling rate exceeds 20 ° C./s, the transformation completion temperature becomes 300 ° C. or lower, resulting in residual hydrogen after rolling, increasing the martensite structure fraction, and worsening the strength-ductility balance.

  In addition, the reason why the temperature range defining the cooling rate is set to 800 ° C. or lower and 300 ° C. or higher is because ferrite transformation and bainite transformation proceed in this temperature region, and ferrite formation in this temperature region is suppressed. This is because temperature history control in this temperature region is important in order to keep the bainite transformation completion temperature within the specified range.

  The steel as-cooled after rolling obtained through the above steps substantially consists of a bainite structure. Here, being substantially composed of a bainite structure means that a structure containing a structure other than bainite is included in the scope of the present invention unless the effects of the present invention are lost. In addition, when a structure other than bainite is contained, the strength-ductility balance is deteriorated. However, since the influence can be ignored when the proportion of the structure other than bainite is low, the volume fraction of bainite may be 90% or more. When a structure other than bainite is contained, it is composed of ferrite and / or martensite. However, in order to achieve the desired strength, it is desirable that the ferrite content is low. On the other hand, the martensite content is still low in order to ensure the ductility of the steel material, especially to avoid the low ductility caused by hydrogen. Is desirable. Therefore, when martensite and ferrite are contained, it is desirable that the ratio of island martensite and ferrite is less than 5% in the total volume ratio.

  Further, the steel material after cooling has a residual hydrogen concentration of 0.4 ppm or less. Here, the purpose of setting the residual hydrogen concentration to 0.4 ppm or less is to suppress a reduction in ductility caused by the residual hydrogen and to enable bending.

  As described above, Ti fixes N and generates coarse nitride TiN to promote a decrease in toughness. Therefore, in the present invention, it is basically desirable not to contain it, but it is acceptable if it is less than 0.003%. The Even in a small amount of Ti, N in the steel is fixed as TiN. From the viewpoint of suppressing a decrease in toughness, it is desirable that the particle size of the TiN to be bent out is 10 μm or less. By making Ti less than 0.003%, the particle size of TiN can be made 10 μm or less.

  The steel material thus obtained has a yield strength of 785 MPa or more, a tensile strength of 930 MPa or more, a base material elongation (EL) of 8% or more, a welded joint elongation of 5% or more, and no breakage during bending. It has mechanical properties that have both strength and ductility.

In addition, although this invention does not necessarily require the heat processing after hot rolling, it was further excellent in ductility by passing through the process hold | maintained at the temperature and time which satisfy | fill following (3) Formula after hot rolling and cooling. It is also possible to obtain steel.
T × log t ≧ 1700 (3)
Where T: holding temperature (K)
t: Retention time (seconds)
However, if the holding temperature is set to 450 ° C. or higher, tempering of bainite and martensite proceeds excessively and the strength decreases, so T in the above formula (3) is set to 450 ° C. or lower. Preferably it is 400 degrees C or less.

Incidentally, in hot rolling, it is customary to roll into a round bar or a deformed shape to form a steel bar for a reinforcing steel bar or a deformed bar steel.
Moreover, the manufacturing process other than the above is not particularly limited, and a normal reinforcing bar manufacturing process can be used.

  Steel of chemical composition (steel types A to N) shown in Table 1 is melted and cast into billets, heated to the temperatures shown in Table 2, and hot rolled to complete at the temperatures shown in the table, The steel bars No. 1-19 having a diameter of 13 mm were manufactured by cooling the temperature range of 300 ° C. to 800 ° C. at the cooling rate shown in Table 2.

  A test piece was collected from each manufactured steel bar within 1 h after hot rolling and cooling, and the residual hydrogen concentration in the steel material was measured for the test piece. Moreover, the cooling rate after rolling measured at the time of manufacture was experimentally reproduced, and the start temperature of bainite transformation and the transformation completion temperature were measured from the thermal expansion behavior at that time.

  About each manufactured steel bar, the structure | tissue and its volume ratio were investigated by microscope observation. In addition, a tensile test was conducted to investigate the characteristics of the base material, and the yield strength (YS), tensile strength (TS), and base material elongation (EL) were measured. Further, in the tensile test, the drawing value was measured at 20 locations for each bar, and the standard deviation of the drawing value was obtained.

  Next, as shown in FIG. 1, two deformed steel bars 10 and 20 having nodes 10a and 20a, respectively, are upset butt welded to produce a welded joint, which is subjected to a tensile test and welded joint elongation (welded) The value of the total elongation when the steel bar itself including the part was subjected to a tensile test) was measured, and the breaking position was confirmed. The fracture position is determined by measuring the Vickers hardness at a pitch of 0.5 mm in the vicinity of the weld and obtaining a longitudinal hardness profile as shown in FIG. The part whose hardness is smaller than the hardness was regarded as a softened part, and it was evaluated which part the fracture position was.

  Next, in order to examine the bending characteristics of the base metal, the deformed steel bar is cut to a length of 500 mm, bent to 180 ° with a bending diameter that is one time the nominal diameter, and then bent back to 90 °. The bending workability was evaluated by performing a bending return test and calculating the ratio (breakage rate) of the number of breaks in 20 deformed bars.

  Further, the Charpy impact value (uE0) of the base material at 0 ° C. was measured as the low temperature toughness. The results are also shown in Table 2. In Table 2, the minimum hardness in the above-described hardness profile is also shown as HAZ Vickers hardness.

  Steel with small variation in ductility, yield strength is 785 MPa or more, tensile strength is 930 MPa or more, base material elongation (EL) is 8% or more, base material drawing value average is 40% or more, drawing value standard deviation is 10 or less. As evaluated. The weld joint elongation of 5% or more and the bending loss rate of 0% were the characteristics required for the steel material of the present invention. The Charpy impact value (uE0) was 80 J or more.

  As shown in Tables 1 and 2, although the chemical components are within the scope of the present invention, the cooling rate after rolling is No. 7, which is lower than 2 ° C./s, the N amount is as high as 0.0070%, and the B amount is the above No. 15 which does not satisfy the formula (2), No. 14 which does not contain V, and No. in which the amounts of C, Si and Mn do not satisfy the scope of the present invention. Nos. 17, 18, and 19 each have a high ferrite content in the microstructure in the steel and do not satisfy bainite of 90% or more by volume ratio as defined in the present invention. Therefore, as shown in Table 2, the yield strength (YS) and the tensile strength (TS) do not reach the target values, respectively.

  On the other hand, No. 8 whose cooling rate after rolling is higher than that of the present invention, and No. 9 and 10 whose heating temperature before rolling and rolling completion temperature are higher than those of the present invention include martensite in the microstructure in the steel. The rate is high, there are samples that break during bending, and the low temperature toughness does not reach the target value.

  Although the individual additive element amounts satisfy the provisions of the present invention, No. 11 in which the value of the above formula (1) exceeds 3 and No. 12 in which the Ni amount is outside the scope of the present invention are obtained by cooling after rolling. Although the microstructure to be satisfied the provisions of the present invention, the transformation completion temperature during cooling is lower than the provisions of the present invention, the amount of residual hydrogen after cooling is high, the drawing value during the tensile test is low, and the bending test There were samples that sometimes broke.

  In No. 13, where the amount of Ti is outside the scope of the present invention, although the target transformation temperature and microstructure were both obtained, breakage occurred during the bending test, and the Charpy impact value was low. Indicated.

  In No.16 where the amount of C is outside the limits of the present invention, in the tensile test after welding, the weld joint surface is broken and sufficient elongation cannot be obtained, and breakage occurs in all steel materials during the bending test. did. On the other hand, in No. 1 to 5 satisfying the provisions of the present invention, YS, TS, base material elongation, drawing value (average value, standard deviation), welding longitudinal elongation, bending breakage rate, The target values were obtained, respectively, and there were no weld cracks. Moreover, the low temperature toughness was also good.

10, 20 deformed steel bar
Sections 10a and 20a

Claims (3)

C: 0.15 mass% or more and 0.30 mass% or less,
Si: 0.05 mass% or more and 0.50 mass% or less,
Mn: 0.2% by mass or more and 2.0% by mass or less,
Cr: 0.1% by mass or more and 1.0% by mass or less,
Al: 0.01 mass% or more and 1.00 mass% or less,
Nb: 0.001 mass% or more and 0.300 mass% or less,
V: 0.01% by mass or more and 1.00% by mass or less,
Ti: less than 0.003 mass%,
Ni: less than 0.5% by mass,
Cu: less than 0.5% by mass,
N: less than 0.0060% by mass,
P: 0.03% by mass or less and S: 0.03% by mass or less, satisfying the following formula (1), the balance being Fe and inevitable impurities, heating temperature: Ac ₃ points or more and 1100 ° C or less And end temperature: Ar Hot rolling at ₃ points or more and 950 ° C or less, and then cooling at a temperature range of 800 ° C or less and 300 ° C or more at 2 ° C / s or more and 20 ° C / s or less. A method for producing a steel material for reinforcing steel, comprising adjusting a bainite transformation start temperature to 500 ° C. or less and a transformation completion temperature to more than 300 ° C.
2 [Si] +1 [Mn] +1.2 [Cr] ≦ 3.0 (1)
Here, [] is the content of the component in parentheses (% by mass) The said steel material contains B in the range which satisfies following formula (2) further, The manufacturing method of the steel material for reinforcing bars of Claim 1 characterized by the above-mentioned.
Record
0.0100 ≧ B (mass%) ≧ {[N] / 14− [Ti] / 27} × 11 + 0.0005 (2)
Here, [] is the content of the component in parentheses (% by mass) The steel material is further
Mo: 0.01 mass% or more and 1.0 mass% or less is contained, The manufacturing method of the steel material for reinforcing bars of Claim 1 or 2 characterized by the above-mentioned.

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