JP5655725B2 - Steel plate for square steel pipe and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel plate for producing a square steel pipe and a method for producing the same, and more particularly to a steel plate for a square steel pipe excellent in surface layer deformability used for a building structure and a method for producing the same.

  A square steel pipe is generally manufactured by bending a steel plate to 90 ° by bending a portion that becomes a corner by cold working and welding the end of the steel plate. In recent years, the steel structure has been increased in size, and a higher strength square steel pipe has been demanded.

  When manufacturing a square steel pipe using a high-strength square steel pipe, particularly a high-strength steel sheet having a tensile strength of 530 MPa or more, there is a problem that surface flaws are likely to occur at the corners when bending 90 °. When surface flaws occur, it becomes a starting point for cracks, so it is necessary to carry out care and remove the wrinkles. However, when the care is performed, not only the cost increases, but also the board thickness required for the design cannot be secured, and there is a case where the scrap becomes scrap.

  In particular, surface flaws are likely to occur when a square steel pipe is manufactured using a thick steel plate having a thickness of 25 mm or more. This is considered to be because when the plate thickness is large, the steel plate surface layer is cured during the water cooling of the steel plate production, and the elongation is lowered.

  Further, the toughness deteriorates at the corners with 90 ° bending. Although a decrease in toughness due to cold working is inevitable, it is necessary to ensure a certain toughness even after the pipe is made. On the other hand, it is required to have a low yield ratio (yield strength / tensile strength) in order to ensure the seismic resistance of the structure at a location not subject to distortion due to bending.

  With respect to these problems, for example, in Patent Document 1 and Patent Document 2, a steel pipe manufacturing method is improved, and a square steel pipe is obtained by performing heat treatment on a cold-formed part (corner part).

Japanese Patent Laid-Open No. 10-60579 Japanese Patent Laid-Open No. 10-60580

  According to the inventions described in Patent Document 1 and Patent Document 2, it is possible to manufacture a square steel pipe having certain characteristics. However, performing heat treatment after forming a steel pipe leads to an increase in cost.

  Moreover, a square steel pipe is normally manufactured by a steel pipe maker bending and welding the steel plate manufactured by the steel plate maker, and forming a square steel pipe. It is extremely difficult to suppress surface defects generated in the steel pipe forming process, which is a lower process, only by improving the forming method. For this reason, there is a strong demand from steel pipe manufacturers for steel sheets that can easily produce rectangular steel pipes having certain characteristics.

  Therefore, the present inventors focused on the steel plate manufacturing process, which is the previous stage (upper process) of steel pipe manufacturing, not the steel pipe manufacturing process, and studied to solve the problem.

  The problem to be solved by the present invention is that even when a square steel pipe is produced from a steel plate, surface flaws do not occur, the corner has a toughness of a certain level or more, and the yield ratio is 80% outside the corner. It is to provide a high-strength steel sheet that is:

  As a result of various studies on the steel sheet for rectangular steel pipes that satisfies the above conditions, the present inventors have obtained the following knowledge (A) to (D).

  (A) In order to obtain a steel pipe having a certain level of toughness at the corner, it is necessary to design a steel plate (steel pipe material) in consideration of the toughness deterioration at the corner due to cold working (corner formation). Furthermore, in order to obtain the structural strength as a building structure, it is necessary to use a high-strength steel plate having a tensile strength of 530 MPa or more.

  (B) In order to ensure the strength of a steel sheet having a tensile strength of 530 MPa or more, the composition design of the steel is not sufficient, and quenching by water cooling immediately after rolling is essential. However, particularly in a thick steel plate having a thickness of 25 mm or more, the surface layer is hardened by water cooling immediately after rolling, and the plastic deformability deteriorates. This tendency becomes stronger as the plate thickness is increased, and as a result, a large number of surface flaws are generated during the square processing.

  (C) Therefore, the surface layer modification of the steel sheet was examined, and specifically, the surface layer state of the steel sheet was modified by tempering. However, tempering at high temperatures not only causes a decrease in tensile strength, but also increases the yield strength, so that it is required as a square steel pipe for use in building structures that require a yield ratio of 80% or less to ensure earthquake resistance. The performance may not be satisfied. Therefore, tempering was performed at a low temperature, the strength characteristics were satisfied, the surface layer state of the steel sheet was improved, and the tempering temperature was adjusted so that surface flaws were not generated even when cold worked.

  (D) A steel sheet produced by subjecting steel having a predetermined chemical composition to appropriate quenching and tempering treatment has a uniform elongation at the surface of a certain value or more, and as a result, surface flaws when processing into a square steel pipe. It has been found that the occurrence of can be suppressed.

The present invention has been completed based on the above findings, and the gist of the production how the RHS steel plate shown in the following (1) to (6).

(1) By mass%, C: 0.05 to 0.20%, Si: 0.10 to 0.40%, Mn: 1.20 to 1.50%, Al: 0.003 to 0.06% , Ti: 0.005 to 0.050%, a balance of Fe and impurities, and a steel material satisfying Ceq of 0.34 or more defined by the following formula is 900 to 1200 ° C (however, 1160 ° C except above. after heating to), starts rolling, after completion of rolling in Ar ₃ point or more, and water cooling from below the Ar ₃ point to Ar ₃ point -400 ° C. or less, then, be tempered at 500 ° C. or less The manufacturing method of the steel plate for square steel pipes characterized by these.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, each element symbol in the formula represents the content (% by mass) of each element contained in the welding material.

(2) By mass%, C: 0.05-0.20%, Si: 0.10-0.40%, Mn: 1.20-1.50%, Al: 0.003-0.06% , Ti: 0.005 to 0.050%, a balance of Fe and impurities, and a steel material satisfying Ceq of 0.34 or more defined by the following formula is 900 to 1200 ° C (however, 1160 ° C After heating to 3 ) or higher , the rolling is started at Ar ₃ point or higher, and then cooled from Ar ₃ point or lower with water, and Ar ₃ point -400 ° C. or higher and Ar ₃ point -200 ° C. or lower in the temperature range. A method for producing a steel plate for a square steel pipe, characterized in that the water cooling is terminated and then air cooling is performed.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, each element symbol in the formula represents the content (% by mass) of each element contained in the welding material.

  (3) The square shape as described in (1) or (2) above, wherein the chemical composition of the steel material includes, in place of a part of Fe, mass% and further contains Nb: 0.05% or less. A method for manufacturing a steel plate for steel pipes.

  (4) The chemical composition of the steel material is mass% instead of part of Fe, and further Cu: 0.40% or less, Ni: 1.0% or less, Cr: 0.50% or less, Mo: 0 The method for producing a steel plate for a square steel pipe according to any one of (1) to (3) above, wherein the steel plate contains one or more selected from 50% or less and V: 0.050% or less.

  (5) Any one of (1) to (4) above, wherein the chemical composition of the steel material includes, in place of a part of Fe, mass% and further contains Ca: 0.0050% or less. The manufacturing method of the steel plate for square steel pipes as described in 2.

  (6) Any one of (1) to (5) above, wherein the chemical composition of the steel material includes, in place of a part of Fe, mass% and further Sn: 0.50% or less. The manufacturing method of the steel plate for square steel pipes as described in 2.

  According to the present invention, it is possible to obtain a steel sheet that does not generate wrinkles on the surface even when it is bent, has good toughness at the bent portion, has high tensile strength, and has a low yield ratio. Therefore, the high-strength steel sheet of the present invention is optimal as a steel material for easily producing a square steel pipe used for a building structure.

It is the figure which showed typically the shape of the test piece used for a tension test.

1. Chemical composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.05-0.20%
C is an element that is extremely effective in increasing the strength of the steel material. In order to ensure the strength, it is necessary to contain 0.05% or more. On the other hand, if it exceeds 0.20%, a hardened structure is formed, surface flaws are easily generated during molding, and toughness is also deteriorated. Therefore, the C content is set to 0.05 to 0.20%.

Si: 0.10 to 0.40%
Si, together with Al, is an element that is effective as a deoxidizer during steelmaking, and is extremely effective in increasing the strength of steel. However, when the content is less than 0.10%, these effects cannot be obtained. On the other hand, if the content exceeds 0.40%, the toughness deteriorates. Therefore, the Si content is set to 0.10 to 0.40%.

Mn: 1.20 to 1.50%
Mn is an important element in improving hardenability and ensuring the strength and toughness of steel. However, if the content is less than 1.20%, the effect of improving toughness is low. On the other hand, if Mn is contained in excess of 1.50%, these effects are not only saturated, but also a main cause of center segregation during the production of slabs by continuous casting. Therefore, the Mn content is 1.20 to 1.50%.

Al: 0.003 to 0.06%
Al is an effective element as a deoxidizer during steel making, and it is necessary to contain 0.003% or more. On the other hand, if the content exceeds 0.06%, the toughness is deteriorated. Therefore, the Al content is set to 0.003 to 0.06%.

Ti: 0.005 to 0.050%
Ti combines with N and precipitates finely in the slab as TiN, which suppresses the coarsening of the austenite grains during heating, and is therefore effective for refining the rolling structure. Further, when TiN is present in the steel, the heat-affected zone is not coarsened during welding. For this reason, Ti is an element necessary for improving the toughness of the base material and the welded portion. These effects are insufficient when the content is less than 0.005%. However, if the content exceeds 0.050%, the low temperature toughness of the weld is deteriorated. Therefore, the Ti content is set to 0.005 to 0.050%.

  The steel material of the present invention contains the above-described elements from C to Ti, and the remainder has a chemical composition consisting of Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

  The steel material of the present invention can contain one or more selected from Nb, Cu, Ni, Cr, Mo, V, Ca and Sn in the following amounts in place of a part of Fe.

Nb: 0.05% or less Nb expands the non-recrystallization temperature range during hot rolling, facilitates controlled rolling, and improves strength and toughness. Therefore, Nb may be contained as necessary. However, if the content exceeds 0.05%, the toughness of the weld heat affected zone is deteriorated, so the upper limit when Nb is contained is set to 0.05%. The above effect becomes prominent when Nb is contained in an amount of 0.005% or more.

Cu: 0.40% or less Cu has an effect of increasing the strength of the base material, and may be contained as necessary. However, if the content exceeds 0.40%, the surface properties of the slab are deteriorated, so the upper limit when Cu is contained is set to 0.40%. The above effect becomes remarkable when 0.05% or more of Cu is contained.

Ni: 1.0% or less Ni has a function of increasing the strength by solid solution in steel to improve toughness and improve hardenability, and may be contained as necessary. However, if the content exceeds 1.0%, the hardenability becomes excessive and the weld heat-affected zone toughness deteriorates, so the upper limit when Ni is contained is set to 1.0%. The above effect becomes remarkable when 0.05% or more of Ni is contained.

Cr. : 0.50% or less Cr can increase the hardenability at low cost and has an effect of increasing the strength, so it may be contained if necessary. However, if the content exceeds 0.50%, the weld heat affected zone toughness deteriorates, so the upper limit in the case of containing Cr is preferably 0.50%. The above effect becomes remarkable when Cr is contained by 0.03% or more.

Mo: 0.50% or less Mo has an effect of enhancing the hardenability and improving the strength, and may be contained as necessary. However, if the content exceeds 0.50%, the weld heat affected zone toughness deteriorates, so the upper limit in the case of containing Mo is preferably 0.50% or less. Said effect becomes remarkable when Mo is contained 0.03% or more.

V: 0.050% or less V has an effect of improving strength by precipitating carbonitride, and may be contained as necessary. However, if the content exceeds 0.050%, the weld heat affected zone toughness deteriorates, so the upper limit when V is contained is preferably 0.05%. The above effect becomes remarkable when V is contained by 0.005% or more.

Ca: 0.0050% or less Ca has an effect of adjusting the shape of inclusions to improve the toughness of the welded portion, and may be contained as necessary. However, since the effect is saturated when the content exceeds 0.0050%, the upper limit when Ca is contained is preferably 0.0050%. The above effect becomes significant when Ca is contained in an amount of 0.0005% or more.

Sn: 0.50% or less Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, by reducing Fe ³⁺ quickly and reducing the Fe ³⁺ concentration as an oxidant, the corrosion promoting action of Fe ³⁺ is suppressed, so that the weather resistance in a high flying salinity environment is improved. Moreover, Sn has the effect | action which suppresses the anodic dissolution reaction of steel and improves corrosion resistance. Therefore, you may contain Sn as needed. However, since these effects are saturated when the content exceeds 0.50%, the upper limit when Sn is contained is preferably 0.50%. The upper limit of the Sn content is more preferably 0.40%. The above effect becomes remarkable when Sn is contained by 0.01% or more. It is more desirable to contain Sn at 0.10% or more.

Ceq: 0.34 or more Even if it has the above chemical composition, the yield strength may increase. Therefore, in the steel material of the present invention, Ceq represented by the following formula is 0.34 or more. As such, the chemical composition must be adjusted. A low yield ratio can be achieved by mixing a steel structure with ferrite as a soft phase and bainite martensite as a hard phase to form a multiphase structure. By setting Ceq to 0.34 or more, a multiphase structure can be easily obtained, the yield ratio can be suppressed, and the plastic deformability can be improved. The following formula is the same as the “carbon equivalent” formula defined in JIS G3136.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, each element symbol in the formula represents the content (% by mass) of each element contained in the welding material.

2. Manufacturing Method Even when a steel plate for a square steel pipe is manufactured using a steel material having the above chemical composition, the performance intended by the present invention may not be satisfied. Therefore, when a square steel pipe is manufactured from a steel plate, surface flaws do not occur, the toughness of a certain level or more is provided at the corner, and the yield ratio is 80% or less at other than the corner. The method will be described below.

(I) Heating process In a heating process, the slab which has the above-mentioned chemical composition as a rolling raw material of the steel plate for square steel pipes is heated to a temperature range of 900-1200 degreeC. The reason why the slab is heated to 900 ° C. or more is to transform it into an austenite to form a uniform structure. On the other hand, the reason why the slab heating temperature is set to 1200 ° C. or less is to prevent coarsening of crystal grains and ensure toughness.

  In addition, in order to make temperature uniform to the center part of a slab, it is preferable that the heating time of the slab in the said temperature range shall be 3 hours or more. However, the upper limit of the heating time is preferably 12 hours for cost reduction.

(Ii) Rolling process After heating the slab, rolling the slab is started to a predetermined dimension. At this time, if rolling is performed at a steel sheet surface temperature of Ar ₃ or less, the generated ferrite is processed, yield stress is increased, and as a result, the yield ratio is increased. Therefore, rolling is performed at Ar ₃ or more. It needs to be terminated. Ar ₃ points are obtained by the following formula.
Ar ₃ points = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb
However, each element symbol in the formula represents the content (% by mass) of each element contained in the welding material.

The water cooling process subsequent to (iii) water cooling step rolling process, the steel sheet surface temperature makes the water cooling from below the Ar ₃ point to Ar ₃ point -400 ° C. or less. In the rolling process described above, the rolling is completed at Ar ₃ points or more. Therefore, after the rolling, the steel sheet is air-cooled until the steel sheet surface temperature becomes a temperature of Ar ₃ points or less, and the water cooling process is started.

  By performing water cooling as described above, the steel structure should be a multiphase structure of ferrite, which is a soft phase, and bainite-martensite, which is a hard phase, to obtain a low yield ratio and at the same time to ensure the plastic deformability of the surface layer. Can do.

(Iv) Tempering step After the water cooling step, tempering is further performed in order to improve the plastic deformability of the surface layer and to secure a certain level of toughness. At this time, tempering at a high temperature is not preferable because it causes an increase in yield strength. Therefore, the steel sheet surface temperature is tempered at a temperature of 500 ° C. or lower.

Moreover, you may give the same function as tempering with what is called an auto temper. In this case, in the above-described water cooling step, the same effect as tempering can be obtained by cooling with water from Ar ₃ points or less, stopping water cooling in the temperature range of Ar ₃ points -400 ° C. to Ar ₃ points -200 ° C., and then air cooling. Therefore, the plastic deformation ability of the surface layer can be secured, and at the same time, a low yield ratio and toughness can be secured.

3. Mechanical properties of the steel sheet The steel sheet manufactured by the above manufacturing method has a uniform elongation of 8% or more even in a steel sheet having a thickness of 25 mm or more. If the uniform elongation of the surface layer is 8% or more, the deformability of the surface layer is excellent, and surface flaws do not occur even when manufacturing a square steel pipe.

  Further, since the steel sheet has a multiphase structure composed of a soft phase and a hard phase, the tensile strength can be set to 530 MPa or more and the yield ratio can be set to 80% or less. Further, when processed into a square steel pipe, although the toughness is reduced at the corner, the Charpy absorbed energy at 0 ° C. can be maintained at 70 J or more. Therefore, the steel plate which concerns on this invention can be used suitably as a steel plate for square steel pipes.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and the obtained 180 kg slab was rolled with a small rolling mill to obtain a steel material.

Table 2 shows the manufacturing conditions. Among the production conditions, Ar ₃ points are calculated values of the above formula, heating temperature is the set temperature of the heating furnace, rolling end temperature, water cooling start temperature, water cooling stop temperature and tempering temperature are measured on the surface temperature of the steel sheet. Value.

  Table 3 shows the mechanical properties (tensile strength, yield ratio, uniform elongation) of the steel sheet, the surface observation result of the steel visually after pipe making, and the Charpy absorbed energy.

  The mechanical properties of the steel sheet were evaluated by collecting test pieces having the shapes shown in FIG. 1 and Table 4 from the steel sheet and conducting a tensile test. In the measurement of uniform elongation, a test piece having a thickness of 6 mm was cut out and tested in order to correctly evaluate the characteristics of the surface layer. Here, it was determined whether the steel sheet was suitable based on the tensile strength of 530 MPa or more, the yield ratio of 80% or less, and the uniform elongation of 8% or more.

  In addition, steel plates are processed into square steel pipes, and each steel pipe is visually checked for flaws, and a JIS No. 4 Charpy test piece (10 mm x 10 mm x 55 mm) with a 2 mmV notch from the bottom surface of the corner to the pipe axis direction. The Charpy test was conducted at 0 ° C. Here, in the Charpy test, a steel sheet having an absorption energy of 70 J or more was judged as a steel sheet having sufficient toughness.

From Table 3, the steel sheet of the present invention and the steel sheet of the reference example manufactured under the conditions specified in the present invention both satisfy the above-mentioned standard of mechanical properties, and there is no generation of flaws even when processed into a square steel pipe. Also, the Charpy absorbed energy at the corners was as high as 70 J or more.

On the other hand, as a comparative example, a steel plate (Mark 17-25) that does not satisfy the composition conditions defined in the present invention and a steel plate (Mark 3-7) that satisfies the composition but does not satisfy the manufacturing conditions, Any of yield ratio, uniform elongation, Charpy absorbed energy at the corner after square steel pipe processing does not satisfy the criteria of the present invention, or surface flaws remain after pipe making and are not suitable as a steel plate for square steel pipe processing Met.

  The high-strength steel sheet according to the present invention does not generate wrinkles on the surface even when it is bent, has good toughness at the bent portion, and has high tensile strength and low yield ratio. It is optimal as a steel material for easily producing a rectangular steel pipe to be used.

Claims (6)

In mass%, C: 0.05-0.20%, Si: 0.10-0.40%, Mn: 1.20-1.50%, Al: 0.003-0.06%, Ti: A steel material containing 0.005 to 0.050%, the balance being Fe and impurities, and satisfying Ceq defined by the following formula of 0.34 or more is 900 to 1200 ° C. (excluding 1160 ° C. or more) after heating to.), it starts rolling, after completion of rolling in Ar ₃ point or more, and water cooling from below the Ar ₃ point to Ar ₃ point -400 ° C. or less, then the feature to tempering at 500 ° C. or less A method of manufacturing a steel plate for a rectangular steel pipe.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, each element symbol in the formula represents the content (% by mass) of each element contained in the welding material. In mass%, C: 0.05-0.20%, Si: 0.10-0.40%, Mn: 1.20-1.50%, Al: 0.003-0.06%, Ti: A steel material containing 0.005 to 0.050%, the balance being Fe and impurities, and satisfying Ceq defined by the following formula of 0.34 or more is 900 to 1200 ° C. (excluding 1160 ° C. or more) )) , Then rolling is started, after rolling is completed at Ar ₃ points or more, water cooling is performed from Ar ₃ points or less, and water cooling is finished at a temperature range of Ar ₃ points to 400 ° C. or more and Ar ₃ points to 200 ° C. or less. And then cooling with air, a method for producing a steel plate for a square steel pipe.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, each element symbol in the formula represents the content (% by mass) of each element contained in the welding material.   3. The steel sheet for a square steel pipe according to claim 1, wherein the chemical composition of the steel material includes, in place of a part of Fe, mass% and further contains Nb: 0.05% or less. Production method.   The chemical composition of the steel material is mass% instead of part of Fe, and Cu: 0.40% or less, Ni: 1.0% or less, Cr: 0.50% or less, Mo: 0.50% The manufacturing method of the steel plate for square steel pipes in any one of Claim 1 to 3 containing 1 or more types selected from below and V: 0.050% or less.   5. The square according to claim 1, wherein the chemical composition of the steel material includes, in place of a part of Fe, mass% and further contains Ca: 0.0050% or less. A method for manufacturing a steel plate for steel pipes.   6. The square according to claim 1, wherein the chemical composition of the steel material contains, in place of a part of Fe, mass% and further Sn: 0.50% or less. A method for manufacturing a steel plate for steel pipes.

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