JP2001073080A - High tensile strength steel excellent in delayed fracture characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce PC steel excellent in delayed fracture resistance and to provide a method for producing it. SOLUTION: As a high tensile strength steel bar or steel wire excellent in delayed fracture characteristics and a method for producing the same, a wire rod obtd. by subjecting a billet contg., by weight, 0.10 to 0.40% C, 0.10 to 2.5% Si, 0.3 to 1.0% Mn, <=0.03% P, <=0.02% S, <=0.005% Al, and the balance iron with inevitable impurities and cast by a billet continuous casting method to hot rolling or cold drawing after the hot rolling is heated to the tamp. range of 850 to 1,050 deg.C, is thereafter quenched, is then rapidly heated to the temp. region of 350 to 550 deg.C and is thereafter tempered, by which the maximum grain size of inclusions in the C direction is controlled to <=300 μm, and also, the number of inclusions of >30 mm is controlled to <=30 pieces/mm2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PC (prestressed concrete) steel excellent in spot weldability and a method for producing the same, and particularly to a tensile strength of 120.
The present invention relates to a high-tensile steel rod or steel wire having high ductility and delayed fracture resistance at a strength level of 0 MPa or more, and a method for producing the same.

[0002]

2. Description of the Related Art Among concrete piles, concrete piles which are reinforced by applying compression to concrete for the purpose of improving rigidity and bending strength and preventing cracking of concrete are called PC piles and are manufactured by the following method. First, after a mild steel wire is spirally wound around a PC steel material arranged in parallel on a circumference (hereinafter referred to as a spiral muscle), an intersection of the PC steel material and the spiral muscle is fixed, and a cylindrical cage piece type reinforcing body ( (Hereinafter abbreviated as reinforcement). Next, the reinforcing body is introduced into a mold, and both ends of the PC steel material constituting the reinforcing body are fixed and tension is applied with a stress of about 70% of the tensile strength. After the concrete poured into the form is solidified, the tension of the PC steel material is removed, and at the same time, a compressive force is applied to the concrete to produce a PC pile. During this manufacturing process, in order to automate the assembling of the reinforcing member, a heat-treated reinforced PC steel material of low-medium carbon steel having good weldability is used, and fixing of the helical muscle to the PC steel material is performed by spot welding.

[0003] In recent years, with the enlargement of steel structures, the steel materials used for the members have tended to have higher strength. For example, PC used as the main reinforcement in concrete pile
The steel rod has a tensile strength of 1420 MPa or more as specified in JIS G3109. After the refining of the converter is completed, the steel produced as wire rod steel is mainly cast into a billet by a continuous casting method. Free oxygen contained in molten steel at the completion of refining is removed as an oxide by adding a deoxidizing material prior to casting. A typical example of the deoxidizing material is deoxidizing using Al. Al ₂ O ₃ as a deoxidation product generated as a result of deoxidation is mostly floated and separated in molten steel, but a part thereof remains in the steel, and remains in the steel after continuous casting. The remaining coarse inclusions increase the hydrogen sensitivity after heat treatment.

[0004] For this reason, the aperture value of the tensile test decreases with the decrease in the delayed fracture characteristic required as a PC steel material.

[0005]

SUMMARY OF THE INVENTION Concrete poles,
In the case of PC steel materials used for concrete structures such as piles, generally, a strength of about 1200 MPa or more is required. In order to satisfy both characteristics of the drawing value and the delayed fracture of the tensile test with such a high-strength steel, the inventors conducted a detailed study of the effects of manufacturing conditions on the morphology of the inclusions and the drawing value. The following has been found.

[0006] The aperture value of the tensile test is highly dependent on the particle size of inclusions. When the particle size of the inclusions is large, ductile fracture does not occur, but hydrogenic fracture occurs, which significantly reduces the aperture value. The area of hydrogen destruction by inclusions is about 3
It is 10 times. When the particle size of the inclusions is thus coarsened, white spots are generated due to hydrogen embrittlement starting from the inclusions, and the aperture value is reduced.

In the billet continuous casting method, inclusions are finely dispersed, so that hydrogen embrittlement hardly occurs. However, large inclusions such as Al ₂ O ₃ which remain in the steel among the inclusions significantly reduce the aperture value. Also, even when the size is not large, if the number of inclusions increases, the aperture value may decrease, and the form control (size, number) of the inclusions is important.
That is, there is a correlation between the size of the inclusion and the critical diffusible hydrogen that causes brittle fracture due to hydrogen, and the larger the size of the inclusion, the lower the critical diffusible hydrogen. Therefore, it is important to control the size of the inclusions, and the particle size of the inclusions is 300
When the thickness exceeds μm, hydrogen embrittlement fracture starts from inclusions and white spots are generated. Although no white spot is generated, grain boundary fracture due to hydrogen embrittlement starts from inclusions of 30 μm or more, so that if the number of inclusions of 30 μm or more increases, the aperture value decreases.
Inclusions of 30 μm or more are 30 pieces / mm in C area distribution.
^If it is not less than ^{2, the} aperture value will be significantly reduced. It is preferable that these inclusions are as small as possible.

[0008]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems, and the gist thereof is as follows. (1) By weight%, C: 0.1 to 0.4%, Si: 0.1
-2.5%, Mn: 0.3-1.0%, P: 0.03%
Hereinafter, S: 0.02% or less, Al: 0.005% or less,
, The balance consisting of iron and unavoidable impurities, the maximum inclusion particle size of the C section is 300 μm or less, and 30 μm
A high-tensile steel material having excellent delayed fracture characteristics, characterized in that the number of extra inclusions is 30 or less / mm ² . (2) Further, as a steel material component, Nb: 0.0% by weight.
05-0.05%, Ti: 0.005-0.05%,
The high-tensile steel material having excellent delayed fracture characteristics according to claim 1, wherein one or more kinds of V: 0.005 to 0.060% are contained. (3) Further, as a steel material component, Cu: 0.0
5 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.
05 to 1.0%, Mo: 0.05 to 0.35%, B:
The high-tensile steel material excellent in delayed fracture characteristics according to claim 1 or 2, which contains one or more of 0.0005 to 0.005%. (4) Further, as a steel material component, Ca: 0.0
005-0.005%, REM: 0.0005-0.0
5%, Mg: 0.0005% to 0.007%, and one or more of these are contained.
A high tensile strength steel excellent in delayed fracture characteristics according to any one of the above items. (5) A slab containing the steel material component according to any one of claims 1 to 4 is cast by a billet continuous casting method,
After hot-rolling or hot-rolling and then cold-drawing a steel rod or wire rod, heating to a temperature range of 850 to 1050 ° C., then quenching, and then rapidly heating to a temperature range of 350 to 550 ° C., followed by tempering. A method for producing high-strength steel with excellent delayed fracture characteristics.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the chemical components specified in the present invention will be described. C: C is added together with increasing the strength of martensite, but less than 0.1% has little effect. On the other hand, if C is added in an excessive amount, the hardness of the spot-welded portion increases and the susceptibility to weld cracking increases, so the upper limit is made 0.4%. Si: Si is an element necessary for securing strength and relaxation characteristics, and if less than 0.1, there is no effect. On the other hand, if the Si content exceeds 2.5%, a large amount of scale harmful to spot welding is generated, so the upper limit was set to 2.5%. Mn: Mn is necessary for improving uniform elongation and hardenability. If less than 0.3%, the effect is not obtained. Even if added over 1.0%, the strength improving effect is saturated. In addition, micro-martensite is generated in the center segregation part, and the stretchability is reduced. M
The n amount is in the range of 0.3 to 1.0. P: P is required to be 0.03% or less in order to segregate at the grain boundaries and easily cause grain boundary embrittlement. It is desirable that P, which is an impurity element, be reduced as much as possible. S: S is also required to be 0.02% or less because S also segregates at the grain boundary similarly to P and easily causes grain boundary embrittlement. It is desirable that S, which is an impurity element, be reduced as much as possible. Al: In the billet continuous casting method, it is necessary to reduce Al as much as possible. If the content of Al exceeds 0.005%, the composition of the generated inclusions is mainly composed of Al ₂ O ₃ and is likely to be coarse.
In addition, it is likely to cause nozzle blockage. Therefore, the upper limit is made 0.005%.

The present invention may further contain one or more of the following components in order to improve the properties in addition to the above elements. Nb: Nb refines austenite grains and improves ductility by the pinning effect of Nb precipitates. For that purpose, 0.005% or more must be added. However,
Addition of more than 0.05% increases the hardness of the spot weld and increases the susceptibility to weld cracking. Therefore, the appropriate range of Nb is set to 0.005 to 0.05%. Ti: Ti refines the structure similarly to Nb due to the pinning effect of Ti precipitates. For that purpose, 0.005% or more must be added. However, if the addition exceeds 0.05%, a large amount of coarse TiN precipitates, thereby deteriorating the material properties. Therefore, the upper limit is set to 0.05%. V: V precipitates carbonitrides, refines γ grains, and improves strength and ductility. For this purpose, 0.005% or more must be added, and the lower limit is set to 0.005%. However, the effect is saturated with a large amount of addition, so the upper limit was made 0.060%. Cu: If Cu is less than 0.05%, the hardenability is not sufficiently improved, so 0.05% was made the lower limit. However, 1.0
% Causes hot cracking, so the upper limit is set to 1.0.
%. Ni: If Ni is less than 0.05%, the hardenability is not sufficiently improved, so 0.05% was made the lower limit. However, 1.0
%, The effect is saturated, so the upper limit is set to 1.0%. Cr: Cr increases the strength of the steel for solid solution strengthening and hardenability, but the effect is insufficient at 0.05% or less. However, if the content exceeds 1.0%, the effect is saturated, so the upper limit is set to 1.0%. Mo: Mo increases the strength of the steel to improve the hardenability, but the effect is insufficient at 0.05% or less. Also,
Addition of more than 0.35% increases the crack susceptibility of the spot weld. Therefore, the component range of Mo is set to 0.05 to 0.3.
Up to 5%. B: If B is less than 0.0005%, the hardenability is not sufficiently improved, so 0.0005% was made the lower limit. But,
If the content exceeds 0.005%, the effect is saturated, so the upper limit is set to 0.005%.

Next, the reasons for limiting the heat treatment conditions will be described. Ca: Ca is an element effective for refining the structure. If it is less than 0.0005, there is no effect.
% Was defined as the lower limit. However, when the content exceeds 0.005%, the cleanliness decreases and the inclusions become coarse, so the upper limit value is set to 0.005%. REM: REM is also an effective element for refining the structure similarly to Ca. If it is less than 0.0005, there is no effect, so 0.0005% is set as the lower limit. However, 0.005
%, The cleanliness decreases and the inclusions become coarse, so the upper limit was made 0.005%. Mg: Mg is added in an amount of 0.0005% or more because steel forms fine oxides in the steel and makes austenite fine. However, if Mg is added in excess of 0.007%, the oxide becomes coarse and the wire drawing workability is reduced. Therefore, the upper limit is made 0.007%.

Next, the reason for defining the inclusion will be described below. In order to suppress a decrease in the aperture value in the tensile test, it is necessary to reduce the maximum inclusion particle size in the C section. 300 μm
If there is even one inclusion exceeding, the aperture value is significantly reduced. Further, if there is an inclusion exceeding 30 μm, a hydrogen embrittlement region starts to be generated around the inclusion, so that it is desirable to reduce the number as much as possible. When the number of inclusions exceeding 30 μm is 30 / mm ² or more, the aperture value is significantly reduced.

A method for producing a steel material using the above-described component system to control the particle size of inclusions is as follows. Inclusions can be refined by casting the slab by billet continuous casting. The slab is hot-rolled or hot-rolled and then cold-drawn, and the wire is quenched in a temperature range of 850 to 1050 ° C. Unless heated to 850 ° C. or higher, an untransformed structure remains and a predetermined strength cannot be obtained. On the other hand, when heating is performed at a temperature exceeding 1050 ° C., γ grains are coarsened and the delayed fracture resistance is reduced.

As described above, the continuous billet casting method can make inclusions fine and γ grains fine. In the present invention, the heating conditions during quenching are not specified, but rapid heating is desirable in order to further refine the γ grains.
C / s is good. The tempering temperature is in the range of 350 to 550 ° C. If the temperature is lower than 350 ° C., the strength becomes too high, and delayed fracture easily occurs. On the other hand, if the temperature is 550 ° C. or higher, the strength decreases, and it is not possible to secure predetermined material properties as PC steel.

[0015]

Embodiments of the present invention will be described below. <Example 1> Table 1 shows the chemical components, production conditions and material results of the test steel. A 125 mm square billet was cast, and the hot rolled wire was quenched and tempered. In the conventional method using comparative steel, a bloom cast material was machined into a 125 mm square and then heat-treated. The maximum inclusion particle size and the number of inclusions were determined by microscopic observation of the C rod in five fields of view and SEM observation of the fracture surface after five tensile tests. The steels 1 to 9 of the present invention satisfied the material characteristics.

[0016] Since steel 10 is not an appropriate steel component, mechanical properties or weldability could not be ensured. Steel 10 is Al
The inclusion amount was large, so that inclusions could not be controlled and the aperture value and the like were reduced. Steels 11 to 13 do not have proper production conditions and cannot obtain material properties. In Steel 11, since the billet casting method was not performed, the gamma grains were coarsened and the delayed fracture resistance was reduced. In Steel 12, since the quenching temperature was high, the γ grains were coarsened and the delayed fracture resistance was reduced. In Steel 13, the tempering temperature was low, so that the structure was hardened and the delayed fracture resistance was reduced. With steels 14 and 15, proper inclusion control is not performed, so that material properties cannot be obtained. Since the maximum inclusions were large in steel 14, the inclusions of steel 15 had more than 30 μm, so the aperture value was significantly reduced.

[0017]

[Table 1]

[0018]

According to the present invention, high strength P excellent in weldability is obtained.
A steel material for a C steel wire can be provided.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/54 C22C 38/54 (72) Inventor Satoshi Sugimaru 1 Kimitsu, Kimitsu City, Chiba Prefecture Nippon Steel Corporation Inside Kimitsu Works (72) Inventor Hiroshi Oba 1 Kimitsu, Kimitsu-shi, Chiba Prefecture AA35 AA36 AA40 BA02 CA05 CH00 CH05 CH06 CL03 4K042 AA14 BA01 BA02 CA02 CA03 CA05 CA06 CA08 CA09 CA10 CA12 CA13 CA14 DA01 DA02

Claims (5)

[Claims] C .: 0.1 to 0.4%, Si: 0.1 to 2.5%, Mn: 0.3 to 1.0%, P: 0.03% or less, by weight% S: 0.02% or less, Al: 0.005% or less, the balance being iron and unavoidable impurities, and the maximum inclusion grain size of the C section is 300 μm.
A high tensile strength steel excellent in delayed fracture characteristics, characterized in that the number of inclusions of not more than 30 m and not less than 30 μm is not more than 30 / mm ² . 2. As a steel material component, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, V: 0.005 to 0.060% by weight%. 2. The high-tensile steel material having excellent delayed fracture characteristics according to claim 1, wherein the steel material contains two or more kinds. 3. Further, as a steel material component, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.05 to 1.0%, Mo: 0 by weight%. 3. A high-tensile steel material having excellent delayed fracture characteristics according to claim 1 or 2, wherein the steel material contains one or more of the following: 0.05 to 0.35%, B: 0.0005 to 0.005%. . 4. Further, as a steel material component, one of the following by weight: Ca: 0.0005 to 0.005%, REM: 0.0005 to 0.005%, Mg: 0.0005 to 0.007%. The high tensile strength steel excellent in delayed fracture characteristics according to any one of claims 1 to 3, which contains two or more types. 5. A steel obtained by casting a slab containing the steel material component according to any one of claims 1 to 4 by a billet continuous casting method, hot rolling or hot rolling and then cold drawing. Production of high tensile strength steel having excellent delayed fracture characteristics, characterized in that a rod or a wire is heated to a temperature range of 850 to 1050 ° C, then quenched, then rapidly heated to a temperature range of 350 to 550 ° C and then tempered. Method.