JP2016145415A - Steel material for reinforcement and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for reinforcement having 0.2% bearing force:785 MPa or more, containing no expensive alloy element, small in variation of ductility, excellent in toughness and having tensile strength and ductility at similar levels as a base material even after weldment.SOLUTION: There is provided a steel material for reinforcement having a predetermined component composition and a metallic structure consisting of, by volume percentage, 85% or more of bainite, 2 to 9% of retained austenite and the balance martensite and having mechanical properties of 0.2% bearing force:785 MPa or more, elongation:8% or more, restriction:30% or more and elongation after weldment:5% or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel material for reinforcing steel used as a material such as a shear reinforcement used in a reinforced concrete structure and a method for manufacturing the same.

  In order to reinforce a reinforced concrete structure and prevent its collapse, it is common to use shear reinforcement in the structure. In the reinforced concrete structure using this shear reinforcement, when the reinforced concrete structure undergoes shear deformation, the shear reinforcement is stretched and plastically deformed, so that the deformation energy of the reinforced concrete structure is absorbed by the shear reinforcement, and the reinforced concrete structure Is prevented from collapsing. However, conventional shear reinforcements are not always sufficient in terms of ductility and toughness. Since the shear reinforcement is manufactured by bending a steel bar into a round shape or a square shape, if the steel used as the material is excellent in ductility and toughness, the bending work becomes easy. This is a great merit in terms of sex.

  Further, in recent years, there is an increasing demand for a welded closed type shear reinforcing bar having good workability, which reinforces a reinforced concrete structure by welding and installing the shear reinforcing bar. In this welded closed type reinforcing reinforcement, it is important that the strength and ductility of the steel material are not reduced by welding, and the joint elongation of the welded portion is also an important characteristic. In general, high capacity and high productivity resistance welding called flash butt welding or upset butt welding is applied to the welding of the shear reinforcement.

  Here, flash butt welding is a state where a large voltage is applied between two steel bars, the end faces of the steel bars are repeatedly brought into contact with each other, and an arc generated at that time is used to melt the molten steel at the steel bar ends. , And the molten part is finally discharged by upsetting (upsetting deformation), and a joining part is formed at the ends of the two steel bars. Upset butt welding is welding in which a large voltage is applied between the two 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. Is the law.

  As a steel material for reinforcing bars used for such a shear reinforcement, it is excellent in strength and ductility without being subjected to heat treatment such as quenching and tempering after rolling, and it is a non-adjustable material having tensile strength and ductility equivalent to that of the base material even if it is welded. Steel materials for quality reinforcing steel are known (for example, Patent Documents 1 to 5).

Japanese Patent No. 3930064 Japanese Patent No. 2973909 Japanese Patent No. 4715166 Japanese Patent No. 5205815 Japanese Patent No. 5205820

  However, the steel for non-tempered rebar described in Patent Document 1 has a problem of high cost because it requires the addition of Mo. Moreover, all the non-tempered steel materials for high-strength reinforcing bars described in Patent Documents 1 and 2 contain Ti. Ti is an element having a function of fixing solute nitrogen and reducing strain aging, and by adding Ti, ductility after welding can be improved. However, since Ti forms TiN after rolling, and TiN is hard at an acute angle and becomes a relatively large inclusion, the bending characteristics of the steel material may deteriorate.

  Furthermore, in the steel materials described in Patent Documents 1 and 2, the toughness of the steel material is not taken into consideration, and there is a possibility that the steel material may break during bending. In addition, these steel materials have large variations in characteristics due to variations in the wire cooling history after hot rolling, and it is difficult to stably obtain excellent properties.

  Moreover, the steel materials for non-tempered rebars described in Patent Documents 3 to 5 are excellent steel materials in which the addition amount of Ti is suppressed to less than 0.04% and can solve the problem of toughness deterioration. When the amount is reduced, the toughness is improved, but depending on the heat history during rolling, particularly the heating time before rolling, the solid solution B may be insufficient and the strength of the material may be insufficient.

  Accordingly, the object of the present invention is to solve the above-mentioned problems of the prior art, 0.2% proof stress (yield stress): high strength steel for reinforcing bars of 785 MPa or more, does not contain expensive alloy elements, and is ductile. It is an object of the present invention to provide a steel material for reinforcing steel having a small variation in strength, excellent toughness, and having a tensile strength and ductility equivalent to those of a base material even after welding. Moreover, an object of this invention is to provide the manufacturing method of the steel material for reinforcing bars.

  The inventors have conducted various experiments and researches in order to produce a steel material for reinforcing steel having excellent strength and ductility, and having a tensile strength and ductility equivalent to those of the base metal even after welding. At that time, 0.2% proof stress: 785 MPa or more, tensile strength: 930 MPa or more, base material elongation (El): 8% or more, and cracking occurred at 180 ° bending in a state where quenching and tempering is not performed (as rolled). None, Elongation after welding: The objective was to produce a steel material for reinforcing bars that had the properties of 5% or more and had both strength and ductility.

As a result of the study, the present inventors have obtained the following knowledge regarding the characteristics of the steel material for reinforcing bars.
(1) In order to ensure bending workability after rolling, it is important that the volume ratio of bainite in the metal structure of the steel material is 85% or more.
(2) In order to ensure elongation properties, it is important to increase the ductility by allowing a small amount of retained austenite to exist.
(3) In order to prevent a decrease in ductility after welding, it is effective to suppress softening of the weld heat affected zone (HAZ) near the joint.
(4) In order to suppress the softening of the HAZ, in addition to precipitation strengthening by commonly used Ti, V carbonitride, etc., precipitation of a fine bainite structure by MnS is effective.
(5) When an appropriate amount of Ti is added, the strength and ductility after rolling and welding can be stably obtained regardless of temporal fluctuations in the heating conditions before rolling, but the toughness is lowered by TiN and bending workability may be inferior. Conceivable. However, the presence of a small amount of retained austenite ensures stable high toughness and improves bendability.
(6) Although retained austenite precipitates at the time of cooling after rolling, a desired amount of retained austenite can be stably obtained by adding an appropriate amount of Ti to the steel and controlling the production conditions.
Based on the above findings, the inventors have found that the above-described goal can be achieved, and have completed the present invention.

That is, the gist configuration of the present invention is as follows.
<1> By mass%
C: 0.15-0.30%,
Si: 0.05-1.00%,
Mn: 0.20 to 2.50%,
P: 0.030% or less,
S: 0.008 to 0.035%,
sol. Al: 0.010 to 0.100%,
Nb: 0.001 to 0.300%,
Ti: 0.004 to 0.080%,
N: 0.0080% or less, and B: 0.0005-0.0100%
Comprising the balance Fe and inevitable impurities and satisfying the following formulas (1) and (2):
It has a metal structure consisting of bainite of 85% or more by volume ratio, 2 to 9% of retained austenite, and the remaining martensite,
0.2% proof stress: 785 MPa or higher, elongation: 8% or higher, drawing: 30% or higher, and post-weld elongation: 5% or higher mechanical properties.
B (%) ≧ {N (%) / 14−Ti (%) / 48 + S (%) / 72} × 11 + 0.0005 (1)
S (%) ≧ (Ti (%) / 2 × 3) × 0.27 + 0.001 (2)

<2> The component composition is mass%,
Cr: 2.0% or less,
Mo: 1.0% or less,
V: 1.0% or less,
Ni: 1.0% or less,
The steel material for reinforcing bars according to <1>, further including one or more selected from Cu: 1.0% or less and Sb: 0.0100% or less.

<3> A method for producing a steel material for reinforcing steel as described in <1> or <2>,
After heating the steel material having the component composition to a temperature range of 800 ° C. or higher and 1350 ° C. or lower, the steel is subjected to hot working with an end temperature of 800 ° C. or higher, and then a cooling rate in a temperature range of 500 ° C. or higher and 800 ° C. or lower. The manufacturing method of the steel material for reinforcing steel which cools on the conditions of 3 degreeC / s or more and 10 degrees C / s or less, and also hold | maintains in the temperature range of 100 degreeC or more and 450 degrees C or less for 10 minutes or more.

  According to the present invention, 0.2% proof stress: a steel material for high-strength reinforcing steel with a strength of 785 MPa or more, with small variation in ductility, excellent toughness, and even after welding, the same level of tensile strength and ductility as the base material It is possible to obtain a steel material for reinforcing bars having Further, the steel material for reinforcing bars of the present invention is not only excellent in performance but also advantageous in terms of production cost because it does not contain expensive alloy elements.

It is a figure which shows the welding procedure of a bar steel, and the cross-sectional hardness profile after welding.

Next, a method for carrying out the present invention will be specifically described.
In the present invention, the steel material for reinforcing bars has a metal structure composed of 85% or more of bainite, 2 to 9% of retained austenite, and the balance of martensite, 0.2% proof stress: 785 MPa or more, elongation: It is important to have mechanical properties of 8% or more, drawing: 30% or more, and elongation after welding: 5% or more. Hereinafter, the metal structure of the steel material in the present invention and the reasons for limiting the mechanical properties will be described. Note that the “%” display regarding the metal structure means the volume fraction unless otherwise specified.

[Metal structure]
-Bainite: 85% or more In the present invention, in order to ensure the balance between the strength and ductility of the steel material, the volume fraction of bainite in the metal structure is set to 85% or more. By containing 85% or more of bainite, high strength can be obtained, and a 30% or more aperture can be realized as described later. The volume fraction of bainite is preferably 87% or more, more preferably 89% or more, and further preferably 91% or more. On the other hand, although the upper limit of the volume fraction of bainite is not particularly limited, it is less than 98% because 2 to 9% of retained austenite exists as will be described later.

-Residual austenite: 2-9%
In the present invention, in order to secure the elongation characteristics of the steel material, the volume fraction of retained austenite in the metal structure is set to 2 to 9%. Residual austenite is a strain-induced transformation, that is, a structure having a function of improving ductility by preventing strain concentration by transforming the strained portion into a martensite phase when the material is deformed and transforming into a martensite phase. is there. Therefore, by setting the volume fraction of retained austenite to 2% or more, it is possible to increase the ductility of the steel for reinforcing bars and ensure the elongation characteristics. On the other hand, the retained austenite phase has a high C concentration and is hard, so if it exists in excess, the elongation characteristics are rather lowered. Therefore, the volume fraction of retained austenite is set to 9% or less. The volume fraction of retained austenite is preferably 7% or less.

-Martensite The remaining structure other than bainite and retained austenite is martensite.
If the balance becomes a soft ferrite structure, the variation in hardness becomes large, and sufficient strength cannot be obtained.

[Mechanical properties]
In the present invention, it is important that the steel for reinforcing bars has mechanical properties of 0.2% proof stress: 785 MPa or more, elongation: 8% or more, drawing: 30% or more, and elongation after welding: 5% or more. 0.2% yield strength, elongation, and drawing are all characteristic values measured by a tensile test. 0.2% yield strength reflects the strength of the steel material, and elongation and drawing reflect the ductility of the steel material. . Reinforced steel materials used as shear reinforcement for reinforced concrete structures have high proof stress (yield stress) that can prevent initial deformation and plastic deformation to prevent shear deformation of reinforced concrete structures. It is required to have excellent elongation for absorbing the energy. Moreover, in order to prevent the fracture | rupture by the bending process at the time of construction, the outstanding local ductility represented by the drawing value is needed. Therefore, in this invention, the mechanical characteristic of the steel material for reinforcing bars is made into the said range. In addition, upset welding or flash butt welding is used for the welding performed prior to measuring the weld elongation, but the welding conditions for these welding are not particularly defined. When these weldings are performed, it is sufficient that the butt portion generates heat and a melting state is obtained, and if there is no welding failure such as separation of the joining portion, the elongation after welding does not depend on the welding conditions and depends on the characteristics of the material. Because it depends.

[Component composition of steel materials]
In the present invention, it is further important that the steel material for reinforcing bars has a predetermined component composition. Then, next, the reason which limits the component composition of steel materials in the present invention as described above will be described. Note that “%” in relation to the component composition means “mass%” unless otherwise specified.

C: 0.15-0.30%
C is an element having an effect of increasing the strength of steel by forming a precipitate with an element such as Ti. In order to ensure the strength expected in the present invention, the C content needs to be 0.15% or more. However, if added over 0.30%, the weldability and ductility deteriorate, so in the present invention the C content is made 0.30% or less.

Si: 0.05-1.00%
Si is an element added for deoxidation and strengthening of steel. In order to acquire the said effect, Si content shall be 0.05% or more in this invention. On the other hand, when the Si content exceeds 1.00%, the joint bendability deteriorates. Therefore, in the present invention, the Si content is set to 1.00% or less.

Mn: 0.20 to 2.50%
Mn is an element having an effect of improving the strength and hardenability of steel. In order to obtain the desired hardenability and strength, the Mn content needs to be 0.20% or more. On the other hand, if the Mn content exceeds 2.50%, ductility and weldability deteriorate, so the Mn content is set to 2.50% or less in the present invention.

P: 0.030% or less P is an inevitable impurity element, and has a property of embrittlement of steel and deterioration of ductility and low-temperature toughness. Therefore, in the present invention, the P content is 0.030% or less. In addition, although it does not limit about a minimum, it is over 0% industrially. Further, excessively low P causes an increase in refining time and cost, so 0.001% or more is preferable.

S: 0.008 to 0.035%
S combines with Mn in the steel to crystallize MnS. MnS is remelted by heating during welding and finely dispersed and precipitated. Since the bainite is refined by the fine MnS, a decrease in ductility due to welding is suppressed. In order to acquire the said effect, S content shall be 0.008% or more in this invention. On the other hand, if S is present excessively, coarse sulfides are formed and ductility and low-temperature toughness are reduced. However, if the S content is 0.035% or less, such deterioration in properties does not pose a problem. Therefore, in the present invention, the S content is set to 0.035% or less.

sol. Al: 0.010 to 0.100%
Al is an element added as a deoxidizer. sol. If Al (acid-soluble aluminum) is less than 0.01%, a sufficient deoxidizing effect cannot be obtained. Al is made 0.010% or more. However, if added over 0.100%, the joint bendability deteriorates. Al is made 0.100% or less.

Nb: 0.001 to 0.300%
Nb is an element that forms fine carbonitrides in steel, increases the strength of the base material, and is effective in suppressing softening of the weld heat affected zone. Since the precipitated Nb carbonitride is finer than TiN, the adverse effect on toughness 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 toughness deterioration of the weld heat-affected zone becomes remarkable even with Nb carbonitride. Therefore, the Nb content is 0.001 to 0.300%.

Ti: 0.004 to 0.080%
Ti fixes N and produces nitrides (TiN), and has an effect of suppressing strength reduction by carbonitride precipitation during welding. It also has the effect of controlling the amount of retained austenite of the rolled material. Addition of 0.004% or more is necessary to obtain the above action. However, if Ti is excessively present, bending breakage occurs due to toughness deterioration. Therefore, the content of Ti is set to 0.004 to 0.080%. More preferably, it is 0.004 to 0.050%.

N: 0.0080% or less N is an unavoidable impurity. When 0.0080% or more is contained, coarse precipitates such as TiN and VN are formed during welding, and the tensile strength and bendability of the welded joint. Decreases. Therefore, the N content is 0.0080% or less. In addition, although it does not limit about a minimum, it is over 0% industrially. Further, excessively low N causes an increase in refining time and cost, so 0.0005% or more is preferable.

B: 0.0005 to 0.0100%
B is an element that improves the hardenability, and the strength of the base material can be improved by adding B. In order to acquire the said effect, B content shall be 0.0005% or more. On the other hand, even if added over 0.0100%, the effect of improving the hardenability is saturated, but the weldability deteriorates, so the B content is set to 0.0100% or less.

In order to obtain the effect of improving hardenability by B, B needs to be dissolved in steel. When solid solution N is present in the steel, B in the steel is consumed for the formation of BN, and when it becomes BN, it does not contribute to the improvement of hardenability. Therefore, when adding B, it is necessary to add more than is consumed for the formation of BN, and the relationship between the required amount of B and the amount of N in steel is expressed by the following formula (1). Can do.
B (%) ≧ {N (%) / 14−Ti (%) / 48 + S (%) / 72} × 11 + 0.0005 (1)

Moreover, in order to precipitate S as fine MnS after welding and refine the bainite structure of the weld heat affected zone, it is necessary that a certain amount or more of S that can be combined with Mn exists. The relationship between the necessary amount of S and the amount of Ti in steel can be expressed by the following equation (2).
S (%) ≧ (Ti (%) / 2 × 3) × 0.27 + 0.001 (2)
In addition, each element symbol of the said (1) and (2) formula represents content of each element in the mass%.

  Furthermore, since the following components are effective in improving the balance between strength and ductility of the steel material, one or more kinds can be selected and added as necessary.

Cr: 2.0% or less Cr is an element having an effect of improving hardenability. In order to obtain a sufficient strength improvement effect, the Cr addition amount is preferably 0.1% or more. However, if added over 2.0%, the hardenability becomes excessive and the ductility and weldability deteriorate, so the added amount of Cr is made 2.0% or less.

Mo: 1.0% or less Mo is an element having an effect of improving hardenability and improving the structure to improve ductility. In order to acquire the said effect, it is preferable to make Mo addition amount 0.01% or more. However, if added over 1.0%, the cost increases and weldability deteriorates, so the Mo addition amount is made 1.0% or less.

V: 1.0% or less V is an element having an action of improving the hardenability of the steel material. Moreover, V has the effect | action which suppresses the softening of a welding heat affected zone while improving the intensity | strength of a base material by forming a carbonitride. In order to acquire the said effect, it is preferable that V addition amount shall be 0.01% or more. On the other hand, if the addition amount exceeds 1.0%, the toughness of the weld heat affected zone is remarkably lowered, so the V addition amount is 1.0% or less.

Ni: 1.0% or less Ni is an element that improves hardenability. Although it can be added when it is necessary to ensure strength, the amount of Ni added is 1.0% or less because it is expensive and deteriorates weldability if added excessively. From the viewpoint of improving hardenability, when added, it is preferable to add 0.01% or more.

Cu: 1.0% or less Cu is an element that improves strength by enhancing hardenability. Although it can be added when it is necessary to ensure the strength, the effect is manifested when the addition amount is 0.01% or more, and therefore it is preferably 0.01% or more. Moreover, when it exceeds 1.0%, hot workability and weldability will fall. Therefore, the Cu addition amount is set to 1.0% or less.

Sb: 0.0100% or less Sb has the effect of suppressing γ grain size coarsening during heating before hot rolling and suppressing surface decarburization during heating, and heating during hot rolling. It can be added when the temperature is increased. However, if added over 0.0100%, the effect is saturated and hot workability and low-temperature toughness are reduced, so the amount of Sb added is 0.0100% or less. In order to effectively express the above action, Sb is preferably added in an amount of 0.0010% or more.

  The balance other than the above consists of Fe and inevitable impurities other than the above.

[Production method]
Next, a method for producing the steel for reinforcing bars of the present invention will be described.
The steel material for reinforcing bars of the present invention can be obtained by heating and hot-working a steel material having the above-mentioned predetermined component composition, and then cooling and holding it under predetermined conditions. In that case, the heating temperature before hot working is 800 ° C. or higher and 1350 ° C. or lower, the end temperature of hot working is 800 ° C. or higher, and cooling in the temperature range of 500 ° C. or higher and 800 ° C. or lower in cooling after hot working. It is important that the speed is 3 ° C./s or more and 10 ° C./s or less, and that the heat treatment conditions are such that the temperature is maintained at 100 ° C. or more and 450 ° C. or less for 10 minutes or more. As the hot working, any hot working method including hot rolling can be used.

Hereinafter, the reasons for limiting the manufacturing conditions will be described.
First, the heating temperature of the steel material is set to 800 ° C. or more and 1350 ° C. or less because, when the heating temperature is less than 800 ° C., in addition to deterioration of workability in the subsequent rolling after heating, This is because the expanded ferrite remains and the elongation decreases. For the same reason, the end temperature of hot working is set to 800 ° C. or higher. On the other hand, when heating in a temperature range exceeding 1350 ° C., the austenite grains become coarser, the ductility and toughness are lowered, and the fuel consumption rate is increased. Therefore, heating temperature shall be 1350 degrees C or less.

  In cooling after hot rolling, the cooling rate in the temperature range of 500 ° C. to 800 ° C. is set to 3 ° C./s to 10 ° C./s. When the cooling rate in the temperature range is less than 3 ° C./s, ferrite increases in the structure. On the other hand, when the cooling rate in the temperature range exceeds 10 ° C./s, the volume fraction of island martensite increases. In either case, the balance between strength and elongation deteriorates.

Then, it hold | maintains for 10 minutes or more in the temperature range of 100 to 450 degreeC. During this time, the transformation of steel into a bainite structure proceeds. Thereby, the hardness variation of the microstructure level in the steel is reduced, and it is possible to improve the aperture value and reduce the variation. If the holding time is less than 10 minutes, the above-described transformation does not proceed sufficiently, and it becomes difficult to stably achieve the target characteristics, particularly the reduction in aperture variation. Therefore, in this invention, it hold | maintains for 10 minutes or more in the temperature range of 100 to 450 degreeC.
A manufacturing process other than the above is not particularly limited, and can follow a normal reinforcing bar manufacturing method.

  Next, the present invention will be described more specifically based on examples. The following examples show preferred examples of the present invention, and the present invention is not limited to the examples.

Example 1
Steels having the composition shown in Table 1 (steel Nos. 1 to 60) were cast after being melted to form billets, and hot rolled to produce deformed steel bars. Table 2 shows the heating temperature before hot rolling, the rolling end temperature, the cooling rate in the temperature range of 500 to 800 ° C, and the holding time at 100 to 450 ° C. Further, the dimensions and shape of the deformed steel bar were as defined in JIS G3112 as the name D13.

  Each deformed steel bar No. About 1-60, the metal structure was investigated by microscope observation. The amount of retained austenite (γ) was measured using X-rays. Further, in order to investigate the mechanical properties of the base material, a tensile test was performed using a JIS No. 2 test piece, and 0.2% yield strength, tensile strength (TS), elongation (El), and drawing were measured. The tensile test is No. 20 test pieces were prepared for each of the conditions 1 to 60, and the average of the measured values was adopted. Furthermore, a 180 ° bending test was performed on each of the 10 test pieces, with the bending radius being 1.5 times the effective diameter (12.7 mm), and the rate of occurrence of fracture during the test was determined. The measurement results of the metal structure and mechanical properties are as shown in Table 2. In addition, regarding the diaphragm, the variation (3σ) of measured values in 20 test pieces is also shown.

  Next, as shown in FIG. 1, two deformed steel bars 10 and 20 each having joints 10a and 20a are upset butt welded to produce a welded joint 1, which is subjected to a tensile test and stretched after welding (welding) The elongation at the time of tensile test of the steel bar itself including the part) was measured, and the fracture position was confirmed. The fracture position was determined by measuring the Vickers hardness at a pitch of 0.5 mm in the vicinity of the welded portion, obtaining a hardness profile in the length direction, as shown in FIG. The portion having a hardness smaller than the base material hardness was used as a softened portion, and the base material, the welded portion, or the softened portion was examined. The measurement results of the elongation after welding and the fracture position were as shown in Table 2.

  No. 2 in which both the composition of steel materials and the production conditions satisfy the conditions of the present invention. 1 to 34 all have excellent properties of 0.2% proof stress: 785 MPa or more, elongation: 8% or more, drawing: 30% or more, and elongation after welding: 5% or more, 0.2 % Proof stress and tensile strength were excellent, and no breakage occurred in the 180 ° bending test.

  On the other hand, the component composition of the steel material does not satisfy the conditions of the present invention. 35 to 60, although the rolling conditions satisfy the conditions of the present invention, either 0.2% proof stress, tensile strength, elongation, drawing, elongation after welding, or a plurality are inferior, or are broken in a bending test. Was generated and the mechanical properties were inferior.

(Example 2)
Steel No. 1 in Table 1 as the material No. 30 was used to change the manufacturing conditions as shown in Table 3 to produce deformed steel bars. As a result, A, B, C, D, G, in which any of the heating temperature, the rolling end temperature, the cooling rate at 500 to 800 ° C., and the holding time between 100 ° C. and 450 ° C. does not satisfy the conditions of the present invention. In H, I, J, K, and L, the metal structure did not satisfy the conditions of the present invention, and the mechanical properties were inferior. On the other hand, E and F satisfying the production conditions of the present invention range were excellent in mechanical properties while the metal structure satisfied the conditions of the present invention.

(Example 3)
As a material, steel No. 1 that satisfies the conditions of the present invention is used. 31, 34 (Table 1-1), and comparative steel No. which does not satisfy the conditions of the present invention. 60 (Table 1-2) was used to produce deformed steel bars in the same procedure as in Example 1. As shown in Table 4, the production conditions all satisfy the conditions of the present invention, but the holding time at the heating temperature before rolling was changed between 1.5 and 3 hours.

  The structure and mechanical properties of the obtained deformed steel bar were evaluated in the same manner as in Example 1. The results are as shown in Table 4. Steel No. whose component composition satisfies the conditions of the present invention. 31, no. In Invention Examples M to T using No. 34, good results are obtained regardless of the holding time. On the other hand, a steel No. having a Ti content lower than the range of the present invention. In Comparative Examples U to X using No. 60, good results were obtained up to 2 hours, but the metal structure was out of the scope of the present invention when the retention time was 3 hours or more. The 0.2% proof stress was out of the scope of the present invention.

1 Welded joints 10 and 20 Deformed steel bars 10a and 20a

Claims (3)

% By mass
C: 0.15-0.30%,
Si: 0.05-1.00%,
Mn: 0.20 to 2.50%,
P: 0.030% or less,
S: 0.008 to 0.035%,
sol. Al: 0.010 to 0.100%,
Nb: 0.001 to 0.300%,
Ti: 0.004 to 0.080%,
N: 0.0080% or less, and B: 0.0005-0.0100%
Comprising the balance Fe and inevitable impurities and satisfying the following formulas (1) and (2):
It has a metal structure consisting of bainite of 85% or more by volume ratio, 2 to 9% of retained austenite, and the remaining martensite,
0.2% proof stress: 785 MPa or higher, elongation: 8% or higher, drawing: 30% or higher, and post-weld elongation: 5% or higher mechanical properties.
B (%) ≧ {N (%) / 14−Ti (%) / 48 + S (%) / 72} × 11 + 0.0005 (1)
S (%) ≧ (Ti (%) / 2 × 3) × 0.27 + 0.001 (2) The component composition is mass%,
Cr: 2.0% or less,
Mo: 1.0% or less,
V: 1.0% or less,
Ni: 1.0% or less,
The steel material for reinforcing bars according to claim 1, further comprising one or more selected from Cu: 1.0% or less and Sb: 0.0100% or less. It is a manufacturing method of the steel material for reinforcing bars according to claim 1 or 2,
After heating the steel material having the component composition to a temperature range of 800 ° C. or higher and 1350 ° C. or lower, the steel is subjected to hot working with an end temperature of 800 ° C. or higher, and then a cooling rate in a temperature range of 500 ° C. or higher and 800 ° C. or lower. A method for producing a steel material for reinforcing steel, wherein cooling is performed under conditions of 3 ° C./s or more and 10 ° C./s or less, and heat treatment is further performed in a temperature range of 100 ° C. or more and 450 ° C. or less for 10 minutes or more.

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