JP4396851B2 - High tensile steel with excellent plastic deformability after cold working and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel for a steel structure and a method for producing the same, which must take into account that plastic deformation such as seismic load occurs during use. This high-tensile steel has a strength class of 780 MPa or higher and is used as a steel material for welded structures.

  In recent years, the tendency to increase the size of welded structures has become more prominent, and the demand for higher strength of steel sheets used in these structures has increased accordingly. For example, thick 780 MPa class high-tensile steel plates have been used for hydraulic iron pipes of pumped-storage power plants, rack materials for jack-up type drilling rigs for offshore structures, and the like. Increasing the strength of thick steel plates not only reduces the weight of the structure, but also significantly reduces welding costs, and there is a strong demand for higher strength.

  The plastic deformability of steel materials used as structural members is an important performance especially in the area where the plasticization of members is allowed in the design of the building steel field, etc., but it is also important in other fields from the viewpoint of security against final fracture It is.

  Generally, the plastic deformability of a steel material is evaluated using a stress strain curve. The yield ratio is mainly used as a parameter of plastic deformability from the idea that the lower the yield ratio, which is the ratio of yield stress to tensile strength, the greater the energy that can be absorbed by the member. In addition, it is known that the plastic deformability of a material decreases when it receives an earthquake load or undergoes cold working at the time of molding. It is desirable to reduce the yield ratio.

  In general, the yield ratio of steel increases with increasing tensile strength. Up to a tensile strength of 490 MPa class, a soft ferrite phase is generated, so the yield ratio is sufficiently low. However, when the bainite or martensite single phase structure is formed and the tensile strength is increased to 590 to 780 MPa or higher, the yield ratio becomes high.

  Non-Patent Document 1 shows an example in which the yield ratio is 98% for a 950 MPa class high-strength steel for a hydraulic iron pipe, and a high yield ratio of 95% for a 780 MPa class high-tensile steel.

  From the viewpoint that the yield ratio can be reduced by increasing the ferrite fraction, a two-phase combination of a soft ferrite phase and a hard bainite phase or martensite phase as a technique for lowering the yield ratio of high strength materials. An organization is being tried.

  In the case of the 590 MPa class, for example, according to Non-Patent Document 2, the yield ratio remains high in the standard manufacturing method of direct quenching-tempering, but DL (Direct Lamellarizing) -T (Tempering), or DQ (Direct An example in which a low yield ratio is realized by controlling the area ratio of the ferrite phase by Quench) -LT treatment or the like is shown. In addition, even in the high strength steel of 780 MPa class or higher, which is the target of the present invention, the low yield ratio was achieved by using the DQ-LT method and the two phases of ferrite and martensite by the study of Ohashi et al. Examples are also introduced.

Nishiwaki "University of Tokyo Thesis" 2001,02 Ohashi et al. “Steel Research” 334 (1989), p.17 However, in the method using the multi-stage heat treatment as described above, the manufacturing cost increases and the manufacturing lead time also increases. Not a desirable solution.

  The present invention is directed to a structural steel material for welding having a tensile strength of 780 MPa or more, and an object of the present invention is to provide a steel material used for a steel structure that needs to be considered to undergo plastic deformation due to seismic load or the like during use. And Specifically, an object is to provide a high-strength steel having the following performances and a method for producing the high-strength steel that can be sufficiently realized from a commercial viewpoint.

  (1) Tensile strength: 780 MPa or more, (2) Yield ratio (YR): 90% or less, (3) Impact energy at 0 ° C .: 100 J or more, (4) “D (tube diameter) / t (sheet thickness) ) ”Is YR in a tensile test in the tube axis direction when the tube forming process of 12 is carried out: 95% or less.

  When performing a tensile test after cold working, a round bar test piece is used in the tube axis direction. Also, when calculating the yield ratio, the yield point is defined as the yield stress when there is a clear yield phenomenon, and the yield stress is defined as 0.2% proof stress when not accompanied.

  The present inventors have found that the following matters are important in order to give a steel material having a tensile strength of 780 MPa or more to have a low yield ratio without impairing commercial properties.

  (1) Assuming that the microstructure has two phases of “ferrite and bainite” or “ferrite and martensite” as in the conventional method, multi-stage heat treatment is required, which increases the manufacturing cost and increases the commercial cost. There is no sex.

(2) In martensite produced when quenching low-carbon steel, it is presumed that cementite (Fe ₃ C) precipitation is suppressed and the concentration of mobile dislocations is increased by lowering the subsequent tempering temperature. . In this case, the yield ratio of the steel material can be kept low.

Therefore, in order to quantify the amount of cementite that is the key point, we investigated using the extraction residue method. The extraction residue method is a method in which precipitates and inclusions in steel are melted and separated together with a matrix, and composition analysis is performed on the residue by emission spectroscopic analysis. In this method, in order to quantify cementite, the ratio of Fe in the residue may be obtained. Precisely, the amount of Fe in the residue is counted by precipitates containing other elements in addition to cementite (Fe ₂₃ (C, B) ₆ etc.). It is safe to assume that the whole amount is cementite. In addition, other precipitates, as well as cementite, are obstacles to dislocation movement during tensile loading, which will be described in detail later, and are therefore considered to have a function of raising the yield point. For these reasons, the amount of Fe in the precipitates in martensite was taken as a parameter for controlling the yield ratio.

  As a result of the above investigation, the following became clear.

  (3) The yield ratio can be lowered efficiently when the number of precipitate particles in the martensite structure is small. In this case, since Nb and Ti carbonitrides can be mentioned as typical precipitates, it is desirable to reduce Nb and Ti.

  Hereinafter, it will be described in detail how these factors lead to a decrease in the yield ratio.

(a) About the function of precipitates in the martensite structure Whether they are cementite or Nb or Ti carbonitrides, both precipitate in the martensite phase. In the martensite structure, a number of dislocations are entangled from the transformation mechanism, and it is reported that the dislocation density is 104 to 107 times that of the annealed ferrite. This is the cause of the high strength of the martensite structure, but when precipitate particles with a size much larger than the dislocations are formed in them, the movement of the dislocations is hindered, resulting in macro plastic deformation. Stress, i.e., yield point, rises. Therefore, in order to lower the yield point, it is important to prevent generation of precipitate particles, that is, cementite, Nb and Ti carbonitrides as much as possible.

(b) Movable dislocation density of martensite structure When tempering a low carbon martensite structure, the structure is changed according to the tempering temperature. In particular, when tempering is performed at a temperature exceeding 500 ° C., precipitation of cementite becomes remarkable, and a phenomenon occurs in which the ferrite changes to a low carbon concentration. At the same time, rearrangement of the dislocations that have been introduced occurs, and it is considered that the number of movable dislocations that cause deformation at low loads is reduced. Since an important point for realizing a low yield ratio is a high dislocation density, it is considered that the yield ratio naturally increases when decomposition into ferrite occurs. Since the decrease in the movable dislocation density is exactly a change caused by discharging C as cementite, the necessary condition for reducing the yield ratio is to secure a martensite structure and to reduce the amount of the associated cementite. it is conceivable that.

  As described above, the present inventor has achieved low low by indirectly specifying the mobile dislocation density by suppressing the amount of precipitates such as cementite in the martensite structure without adopting a high-cost manufacturing method such as multistage heat treatment. We succeeded in realizing the yield ratio.

  The gist of the present invention is the following high-strength steel and method for producing the same.

(1) By mass%, C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.4 to 2.5%, Cr: 0.1 to 1%, Mo : 0.1 to 1%, Ti: 0.01 to 0.035%, B: 0.0003 to 0.005%, Al: 0.001 to 0.1%, with the balance being Fe and impurities, Among impurities, P is 0.05% or less, S is 0.008% or less, N is 0.01% or less, and a value represented by the following formula (a) is 85 or less, and the martensite ratio is an area. der 80% at the rate is, high-tensile steel tensile strength is characterized in der Rukoto least 780 MPa.

44.6 + 1086 × Nb (%) + 773 × Ti (%) + 2.44 × (R _Fe / C (%)) ... (a)
However, the element symbol in the formula (a) is the content (mass%) of the element, and R _Fe is the Fe quantity (mass%) present as a precipitate measured by the extraction residue method.

  (2) In place of a part of Fe, at least one of Nb: 0.1% or less, Cu: 2% or less, Ni: 3% or less, and V: 0.1% or less is contained in mass%. The high-tensile steel according to (1) above, which is contained.

  (3) Further, in place of a part of Fe, by mass%, Ca: 0.004% or less, Mg: 0.002% or less, REM: 0.002% or less, and Zr: 0.02% or less The high-tensile steel according to (1) or (2) above, which contains one or more kinds.

(4) A steel slab having the chemical composition according to any one of (1) to (3) above is heated to 1000 to 1200 ° C. and hot-rolled, then air-cooled, and the quenching temperature is set to 950 ° C. or less to produce water. Quenching, the water cooling stop temperature is 300 ° C. or less, and the maximum temperature during recuperation is 500 ° C. or less. Production of high-tensile steel according to any one of (1) to (3) above Method.

(5) A steel slab having the chemical composition according to any one of (1) to (3) above is heated to 1000 to 1200 ° C. and hot-rolled, and then quenched by cooling from a temperature of ₃ or more points of Ar. The method for producing high-tensile steel according to any one of (1) to (3) above , wherein the water cooling stop temperature is 300 ° C. or lower and the maximum temperature during recuperation is 500 ° C. or lower. .

  (6) The method for producing high-tensile steel according to (4) or (5), wherein tempering is performed at 500 ° C. or lower after quenching.

1. First, the reason why the chemical composition of the high-strength steel of the present invention is determined as described above will be described. In addition, in the following description,% regarding component content means "mass%".

C: 0.02-0.2%
C is added to ensure the strength of the steel. If the content is less than 0.02%, the hardenability is insufficient, it is difficult to ensure a tensile strength of 780 MPa, and the toughness is not sufficient. On the other hand, if it exceeds 0.2%, not only the toughness of the base metal and the brittle crack propagation stopping performance will be lowered, but also the hardness of the HAZ (welding heat affected zone) will be increased, and the weld cold cracking susceptibility will be increased. Not suitable for use.

Si: 0.01 to 0.5%
Si is added for the purpose of improving the yield of Al in the final deoxidation because of its deoxidation action. In the steel of the present invention, “Si contained in steel” refers to Si remaining in the steel in excess of the amount worked for deoxidation. If the amount is less than 0.01%, the required strength cannot be ensured. The Si remaining in the steel is effective for increasing the strength, but if it exceeds 0.5%, the toughness of the base metal and the HAZ is reduced.

Mn: 0.4 to 2.5%
Mn is added to improve the hardenability of the steel and increase the strength. If the content is less than 0.4%, it is difficult to ensure the strength. On the other hand, if it exceeds 2.5%, the toughness of both the base material and the HAZ is lowered.

Cr: 0.1 to 1%
Cr improves hardenability and improves strength and toughness by precipitation hardening during tempering. If it is less than 0.1%, the effect is not sufficient. On the other hand, if it exceeds 1%, the strength is excessively increased and the toughness of the base material and the HAZ is impaired. A more desirable upper limit is 0.5%.

Mo: 0.1 to 1%
Mo has a larger hardenability improving effect and precipitation hardening than Cr compared with the same amount, and particularly when coexisting with B, the hardenability improving effect is remarkable. If it is less than 0.1%, it is not sufficient to “fire” to the center of the thick steel plate and to obtain a tensile strength of 780 MPa or more. On the other hand, if it exceeds 1%, “fire” will enter the surface layer. The toughness of the surface layer part deteriorates too much.

Ti: 0.01-0.035%
Ti is mainly used as a deoxidizing element, but forms an oxide phase together with Al and Mn. In order to form this oxide phase in steel, Ti in the steel needs to be 0.01% or more. A more preferable Ti content is an amount exceeding 0.012%, and an even more preferable amount is exceeding 0.015%. On the other hand, if the Ti content exceeds 0.1%, the formed oxide becomes Ti oxide or Ti-Al oxide, and the dispersion density decreases, particularly in the heat affected zone of the small heat input weld. The ability to refine the tissue is lost. Ti is an element that has a remarkable ability to produce not only oxides but also carbonitrides, and the produced carbonitrides cause an increase in yield ratio. For this reason, the Ti content must be 0.035% or less.

B: 0.0003 to 0.005%
B is an important element for achieving both weldability and high strength. B has the effect of improving the hardenability and increasing the strength. In order to reliably obtain this effect, the B content is preferably 0.0003% or more. When the content exceeds 0.005%, the effect of increasing the strength is saturated, and the tendency of deterioration of toughness becomes remarkable in both the base material and HAZ. Therefore, the upper limit of the B content is set to 0.005% or less.

Al: 0.001 to 0.1%
Al is added as a deoxidizer and remains 0.001% or more in the steel. The remaining Al is combined with N after solidification to form AlN, or becomes solid solution Al. If the Al content exceeds 0.1%, the toughness tends to deteriorate particularly in HAZ. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. Therefore, the Al content is limited to 0.1% or less.

  One of the high-strength steels of the present invention is one in which the balance consists of Fe and impurities in addition to the above components. Among impurities, P, S and N need to regulate their contents as follows.

P: 0.05% or less P is unavoidably present in steel as an impurity. If the amount exceeds 0.05%, it not only segregates at the grain boundary and lowers toughness, but also causes hot cracking during welding, so it is necessary to make it 0.05% or less.

S: 0.008% or less S is unavoidably present in steel as an impurity. If the amount is too large, center segregation is promoted or stretched MnS is produced in a large amount, so that the mechanical properties of the base material and the HAZ are deteriorated. Therefore, the upper limit is made 0.008%. The smaller the S, the better.

N: 0.01% or less N is an unavoidable impurity, and its content is preferably as small as possible. If it exceeds 0.01%, the toughness of the base material and HAZ will be significantly reduced, so the content is made 0.01% or less.

  Another one of the high-tensile steels of the present invention is a steel containing at least one component selected from at least one of the following first group and second group.

First group: Nb of 0.1% or less, Cu of 2% or less, Ni of 3% or less, and V of 0.1% or less
Second group: 0.004% or less of Ca, 0.002% or less of Mg, 0.002% or less of REM, and 0.02% or less of Zr
Hereinafter, the effect of these components and the reason for limiting the content will be described.

Nb: 0.1% or less Nb is not particularly required to be added. However, when a small amount is added, a fine Nb carbonitride is formed in a low temperature region of austenite, thereby refining austenite grains and forming a fine martensite structure. Is formed from the surface layer portion to the center portion of the thick steel plate, thereby improving the toughness and brittle fracture propagation stopping characteristics of the high-strength steel. Therefore, you may add especially when improving these performances of a surface layer part. In order to obtain a desired effect, the content is preferably 0.01% or more. However, if the Nb content exceeds 0.1%, carbonitride that not only causes transverse cracks in the weld metal during welding but also hinders the cold plastic workability that is the main focus of the present invention. Generation becomes remarkable. Therefore, even when added, the content is 0.1% or less. Desirable is 0.02% or less.

Cu: 2% or less Cu need not be particularly added. However, since Cu has an effect of improving hardenability, Cu is added to obtain this effect. In that case, the content is preferably 0.1% or more. However, if it exceeds 2%, not only the toughness of the base metal and the HAZ is impaired, but also the hot ductility is greatly reduced.

Ni: 3% or less Ni is not particularly required to be added. However, Ni improves the low-temperature toughness, brittle fracture propagation stoppage performance and weldability of high-strength thick-walled steel. In that case, the content is desirably 0.1% or more. On the other hand, if it exceeds 3%, the improvement in effect is small for the cost increase.

V: 0.1% or less V may not be particularly added. However, V is an element that increases hardenability and contributes to high strength. Therefore, when this effect is desired, it may be added. In that case, the content is preferably 0.01% or more. However, if it exceeds 0.1%, precipitates are generated during slab cooling, and the toughness and yield ratio are impaired.

Ca: 0.004% or less Ca reacts with S in steel to form an acid / sulfide (oxysulfide) in molten steel. Unlike MnS and the like, this acid / sulfide does not extend in the rolling direction by rolling and remains spherical after rolling. Therefore, weld cracks and hydrogen-induced cracks starting from the ends of the elongated inclusions are prevented. In order to obtain such an effect, Ca is added. In that case, the content is preferably 0.0002% or more. However, if its content exceeds 0.004%, toughness may be deteriorated.

Mg: 0.002% or less Mg forms an oxide, and is particularly effective for refining the metal structure of the weld heat affected zone. It is added when this effect is desired. When added, the content is preferably 0.0002% or more. On the other hand, if it exceeds 0.002%, the amount of oxide becomes excessive and ductility is lowered.

REM: 0.002% or less REM contributes to refinement of the structure of the weld heat affected zone and fixation of S. It is added when this effect is desired. In that case, the content is preferably 0.0002% or more. However, excess REM becomes inclusions and reduces cleanliness. Inclusions formed by the addition of REM have a relatively small effect on toughness deterioration, so that if the content is 0.002% or less, a decrease in toughness of the base material is acceptable. Note that REM is a generic name for 17 elements obtained by adding Y and Sc to 15 elements from La to Lu. One or more of these are added in combination.

Zr: 0.02% or less Zr has the effect of improving the strength by finely dispersing and precipitating nitrides in steel. It is added when this effect is desired. In that case, the content is desirably 0.001% or more. However, if it exceeds 0.02%, coarse precipitates are formed and the toughness is deteriorated.

  Next, the index represented by the following equation (a) will be described.

44.6 + 1086 × Nb (%) + 773 × Ti (%) + 2.44 × (R _Fe / C (%)) ... (a)
The index shown in this equation (a) must be 85 or less. The reason is as follows.

As described above, R _Fe in the formula (a) is the amount (% by mass) of Fe present in the steel as precipitates (including those called inclusions) measured by the extraction residue method. Equation (a) is an index representing the cold workability of the material, that is, the yield ratio, and the yield ratio increases as the value increases. That is, if the amount of Nb or Ti carbonitride forming elements is large, the structure will contain a large amount of carbonitride particles that cause an increase in the yield ratio. Further, if the ratio of the precipitated Fe amount to the contained C amount is large, a large amount of particles that similarly increase the yield ratio will be included.

Therefore, in order to reliably reduce the yield ratio, it is not sufficient to specify the upper limit value of each element, and an upper limit is set for an index composed of Nb, Ti, and R _Fe / C (%). -ing If this index exceeds 85, the yield ratio increases, and the plastic deformability after cold working cannot be ensured. Therefore, it was defined as 85 or less.

Furthermore, it is indispensable for the martensite ratio to be 80% or more in terms of area ratio in order to ensure high strength. If the martensite ratio is less than 80% in area ratio, the strength of 780 MPa or more, which is the object of the present invention, cannot be ensured. Therefore, the martensite ratio was defined as 80% or more in terms of area ratio .

2. About the manufacturing method of this invention The manufacturing method of this invention has the characteristics in the conditions of rolling and heat processing as above-mentioned. The reasons for limitation will be described below.

(1) Rolling and heat treatment conditions Heating temperature:
A steel slab (slab) having the above-described chemical composition is heated and hot-rolled. If the heating temperature of the steel slab is less than 1000 ° C., the austenitization is insufficient, so that sufficient property improvement cannot be achieved even if the conditions of rolling and heat treatment are changed later. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains are not refined, and the base material toughness of the steel material is significantly reduced. Therefore, the slab heating temperature before rolling shall be 1000-1200 degreeC.

(2) Heat treatment after rolling After reducing the thickness to the desired thickness by hot rolling, it may be air-cooled and re-heated and quenched, or may be directly quenched from the rolling end temperature. .

  In the case of performing reheating and quenching after air cooling, the reheating and quenching temperature needs to be a temperature that sufficiently austenizes the whole, but when the temperature is higher than 950 ° C., the austenite becomes extremely coarse, The quenching temperature is set to 950 ° C. or lower because it causes the toughness of the product to be impaired. In addition, air-cooling once after completion | finish of rolling is for, for example, in order to perform a dehydrogenation process, and to avoid the direct hardening method in which rolling efficiency falls.

When performing direct quenching, it is necessary to quench from a single austenite phase. That is, the quenching start temperature needs to be Ar ₃ point or higher. Therefore, the rolling end temperature is set to Ar ₃ point or higher.

  In any of the above methods, cooling from the quenching temperature is water cooling. And in order to suppress that a precipitate is produced | generated by the phenomenon called an auto temper after the cooling stop at the time of hardening, it is necessary to cool a precipitation temperature range with a big cooling rate. Therefore, the water cooling stop temperature was set to 300 ° C. or lower.

  Further, in order to suppress the formation of precipitate particles such as cementite, it is necessary to suppress the recuperation after quenching as much as possible. For this reason, the maximum temperature at the time of recuperation after cooling needs to be 500 degreeC. Desirable is 400 ° C. or lower.

  Furthermore, you may temper depending on the case. However, since tempering at a temperature exceeding 500 ° C. is accompanied by the formation of significant precipitates and a decrease in the density of mobile dislocations, the tempering temperature is defined as 500 ° C. or lower. Desirable is 400 ° C. or lower. Tempering is performed when it is necessary to adjust the balance between strength and toughness.

  Tables 1 and 2 are lists representing the chemical compositions of the steels used for the implementation and comparison of the present invention. These steels were melted in a 70-ton converter and cast in an Ar atmosphere to form a steel ingot having a thickness of 600 mm × 1000 mm width × 2500 mm.

  The columns of “Production conditions” in Table 3 and Table 4 show the conditions for rolling and heat treatment applied to these steel ingots. Thick steel plates having a thickness of 25 mm were produced by rolling and heat treatment shown in this table, and test pieces were cut out from the respective steel plates to evaluate mechanical properties.

  Tensile properties were evaluated by taking a test piece (JISZ2201-4 test piece) in the rolling direction and carrying out the test according to JISZ2241. As described above, with respect to the yield point, when a clear yield phenomenon is observed, the falling yield point is adopted, and when the clear yield phenomenon is not observed, the 0.2% proof stress is set as the yield stress.

  Toughness was evaluated by the absorbed energy at 0 ° C. in a Charpy impact test (JISZ2242: test piece JISZ2202-V notch test piece).

  The tensile test and Charpy impact test of the base material were evaluated at (1/4) t part (1/4 position of the plate thickness). The structure | tissue was observed using the transmission electron microscope whose acceleration voltage using an optical microscope and a thin film method is 100-200 kV. Measurement of the area ratio of martensite was carried out by observation of the structure with a transmission electron microscope by preparing a thin film at each position of the surface layer, (1/4) t, and (1/2) t. The value was the martensite ratio.

  Furthermore, the amount of Fe contained in the steel as precipitates / inclusions was evaluated using the extraction residue method, and the details are as follows.

  (1) A round bar test piece of 15 mmφ × 70 mmL is taken in the rolling direction from the position of (1/4) t of the steel material. In that case, the scale of the test piece surface is dropped.

  (2) Cleaning of test piece with petroleum benzine.

  (3) Weighing specimen (weight before electrolysis, this is V1).

  (4) Electrolysis. The electrolyte is tetramethylammonium chloride (TMAC) 1% -acetylacetone 10% -methanol solution. A current of 20 mA is applied per 1 cm 2 of the surface area of the test piece.

  (5) A residue is obtained by filtration. The filter mesh is 0.2 μm.

  (6) Weighing test piece (weight after electrolysis, this is V2).

  (7) Acid decomposition of the residue. The acid used is a mixed solution of 10 ml of nitric acid, 5 ml of perchloric acid, and 15 ml of mixed acid (5 ml of water + 5 ml of sulfuric acid + 5 ml of phosphoric acid).

  (8) White smoke treatment. Heat to remove organic matter.

  (9) Add 10 ml of tartaric acid (20%). Prevents precipitation of Nb and the like.

  (10) Add 5 ml of yttrium solution (1 mg / ml). For correcting time-varying factors during ICP analysis.

  (11) Make the total amount of the remaining solution 100 ml.

  (12) The amount (Xn) of Fe element contained in the residual solution is measured by high frequency inductively coupled plasma optical emission spectrometry (ICP).

  (13) The amount of Fe is obtained by “Xn / (V1-V2)”.

  Cold forming was performed on a steel pipe having an outer diameter D = 300 mm using the test material shown above. Forming was performed by submerged arc welding (SAW) as a seam welding with a heat input of X groove of 45 kJ / cm by a press bend method.

  A tensile test piece (JISZ2201-4 No. test piece) is collected from the (1/4) t part on the outer surface side in the pipe axis direction from the base material part of the above pipe, and a smooth round bar test piece ( Parallel part 4 mmφ-parallel part length 30 mm-gripping part diameter 8 mm) were collected and evaluated for tensile properties. Note that the entire circumferential tensile test piece may not be collected from the tube. In that case, the grip portion was appropriately added by electron beam welding.

  Specimen Nos. 1-41 are within the scope of the present invention, YR of the material satisfies the target of 90% or less, and YR after cold working is also evaluated in a severe circumferential direction. Satisfies 95% or less.

  On the other hand, in the test material number 101, C exceeds the specified upper limit. The yield ratio satisfies the target but has poor Charpy impact properties. In the specimen number 102, Si exceeds the specified upper limit, and this also has poor Charpy impact characteristics. In the material number 103, Si exceeds the specified upper limit, and the Charpy impact property is not good. In the specimen number 104, Cr exceeds the specified upper limit value, and the Charpy impact characteristics are also poor. In the specimen number 105, Mo exceeds the specified upper limit value, and the Charpy impact property is poor.

  In the specimen number 106, Nb exceeds the specified upper limit value, and the yield ratio deviates from the target range for both the material and after pipe production. In the specimen number 107, Ti exceeds the specified upper limit, the Charpy impact characteristics are poor, and the yield ratio deviates from the target range both in the raw material and after the pipe production.

In the specimen number 108, N exceeds the specified upper limit value, and the Charpy impact characteristics are inferior. In test material No. 109, the chemical composition is within the specified range, but the maximum temperature at the time of recuperation after cooling deviates from the specified range, R _Fe increases, and the yield ratio is both for the material and after pipe production. Out of target range. In the specimen No. 110, the chemical composition is within the specified range, but the tempering temperature deviates from the specified range, the R _Fe increases, and the yield ratio deviates from the target range for both the material and after pipe making. Yes. In the specimen number 111, both the chemical composition and the manufacturing method are within the specified range, but the index value defined by the equation (a) is out of the specified range. Yield ratio deviates from the target range for both materials and pipes. Sample No. 112 has poor hardenability and a martensite ratio after quenching of 60%. In this example, the strength does not reach the target of 780 MPa.

According to the method of the present invention, the tensile strength is 780 MPa or more, the yield ratio of the material is 90% or less, the impact energy at 0 ° C. is 100 J or more, and the yield ratio is even after pipe forming with D / t = 12. A steel material suppressed to 95% or less is obtained. This steel material can be manufactured at low cost by the manufacturing method of the present invention. As a result, a highly reliable high-strength steel structure can be manufactured.

Claims (6)

In mass%, C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.4 to 2.5%, Cr: 0.1 to 1%, Mo: 0.00. 1 to 1%, Ti: 0.01 to 0.035%, B: 0.0003 to 0.005%, Al: 0.001 to 0.1%, the balance being Fe and impurities, P is 0.05% or less, S is 0.008% or less, N is 0.01% or less, the value represented by the following formula (a) is 85 or less, and the martensite ratio is 80% in area ratio. more der is, high-tensile steel tensile strength is characterized in der Rukoto least 780 MPa.
44.6 + 1086 × Nb (%) + 773 × Ti (%) + 2.44 × (R _Fe /C(%))...(a)
However, the element symbol in the formula (a) is the content (mass%) of the element, and R _Fe is the Fe quantity (mass%) present as a precipitate measured by the extraction residue method.   Further, in place of a part of Fe, by mass%, Nb: 0.1% or less, Cu: 2% or less, Ni: 3% or less, and V: 0.1% or less The high-tensile steel according to claim 1.   Further, in place of a part of Fe, by mass%, one or more of Ca: 0.004% or less, Mg: 0.002% or less, REM: 0.002% or less, and Zr: 0.02% or less The high-tensile steel according to claim 1 or 2, characterized by comprising: A steel slab having the chemical composition according to any one of claims 1 to 3 is heated to 1000 to 1200 ° C and hot-rolled, and then air-cooled, water-quenched at a quenching temperature of 950 ° C or lower, and water-cooled. The method for producing high-tensile steel according to any one of claims 1 to 3 , wherein the stop temperature is 300 ° C or lower and the maximum temperature during recuperation is 500 ° C or lower. After the steel slab having the chemical composition according to any one of claims 1 to 3 is heated to 1000 to 1200 ° C and hot-rolled, quenching is subsequently performed by water cooling from a temperature of ₃ or more points of Ar, and the water cooling is stopped. The method for producing high-tensile steel according to any one of claims 1 to 3 , wherein the temperature is 300 ° C or lower and the maximum temperature during recuperation is 500 ° C or lower.   The method for producing high-tensile steel according to claim 4 or 5, wherein tempering is performed at 500 ° C or lower after quenching.