JP2014177669A - Non-heat treated low yield ratio high tensile thick steel plate, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio high tensile thick steel plate for building construction member excellent in earthquake resistance and a manufacturing method therefor.SOLUTION: There is provided a thick steel plate containing a component composition consisting of, by mass%, C:0.05 to 0.16%, Si:0.05 to 0.45%, Mn:1.2 to 1.8%, P, S, Al:0.05% or less, N:0.0040% or less, Ti:0.005 to 0.020%, 4.0≥Ti/N≥2.0, Ceq:0.35 to 0.48, and one or more kind of Cu, Ni, Cr, V, Ca and REM as needed and the balance Fe with inevitable impurities, and a hard phase consisting of ferrite having an average crystal particle diameter of 4.0 to 18.0 μm, one or more kinds of pearlite, bainite and martensite, and having a micro structure having a ferrite area ratio of 45% to 70%, and an average hardness of a steel plate surface part of 225 HV or less and difference of hardness between the surface part and a plate thickness center part of 60 HV or less. The steel having the component composition is applied to a two-stage cooling after hot rolling.

Description

  The present invention relates to a non-tempered low yield ratio high-tensile steel plate suitable for building structural members that require earthquake resistance, and a method for manufacturing the same, and in particular, with a plate thickness of 19 mm or more, a circular steel pipe column or a square steel pipe. The present invention relates to a column and the like that are suitable for applications that are subjected to cold bending.

  In recent years, from the viewpoint of ensuring safety during an earthquake, it is required to use a steel plate (steel material) having excellent earthquake resistance as a material in a building structure or the like. In addition, research results so far have shown that steel plates with lower yield ratios have better earthquake resistance.

  For this reason, it is obliged to use low yield ratio steel with a yield ratio (YR) of 80% or less for building structures (constructed in 1981). Furthermore, recently, with the increase in the height and span of building structures, there are an increasing number of cases in which 550 MPa class high-tensile steel material having higher strength than before is applied to building structures.

  Conventionally, a high-tensile steel material of 550 MPa class or higher having a low yield ratio is generally manufactured by performing a heat treatment such as a two-phase region heat treatment or a tempering treatment. However, the heat treatment has a problem in that the process becomes complicated, the manufacturing period is prolonged, and the manufacturing cost is increased. For this reason, the examination of the non-tempered low yield ratio high-tensile steel material in which the above-described two-phase region heat treatment and tempering treatment are omitted has been advanced.

  For example, Patent Document 1 contains C: 0.02 to 0.04%, solute B: 0.0002 to 0.002%, and the formula CEN related to the alloy element content is 0.21 to 0. A 590 MPa class non-tempered low yield ratio high tensile strength steel sheet composed of a composition satisfying a range of 30% and a structure mainly composed of bainite and dispersed in an amount of 0.8 to 2.5% by volume of island martensite. Proposed. In the technique described in Patent Document 1, the production is performed only by controlled rolling. However, in the technique described in Patent Document 1, the carbon content of the steel sheet is reduced to 0.02 to 0.04%, so that a large amount of alloying element is required to ensure the desired high strength. Therefore, there is a problem that the manufacturing cost increases.

  Patent Document 2 includes C: 0.045 to 0.08%, Si: 0.05 to 0.50%, Mn: 0.6 to 2.0%, P, S, Al, N In addition, a composition containing Mo and / or W so as to satisfy a specific relational expression, a composition in which Pcm is 0.22% or less, and a structure in the central portion of the plate thickness is the main phase of ferrite. And a high-tensile thick steel plate having a low yield ratio, which is a composite structure containing a hard phase mainly composed of island-shaped martensite (MA phase) of 20% by volume or less. With such a structure, a desired low yield ratio can be realized. Further, in order to obtain such a structure, in the technique described in Patent Document 2, the steel material having the above composition is subjected to hot rolling in which the rolling end temperature is 800 to 950 ° C. at the surface temperature, and 0.5 It is said that it is preferable to sequentially perform a cooling process of accelerated cooling to a temperature range of 580 to 670 ° C. at an average cooling rate of ˜50 ° C./s. However, the technique described in Patent Document 2 has a problem that it requires expensive Mo and W to be contained, resulting in an increase in manufacturing cost.

  Patent Document 3 includes C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1%. The steel containing is heated, the rolling end temperature is set to a temperature in the range of 900 ° C. to Ar 3 transformation point, and the cumulative rolling reduction in the temperature range is set to less than 30%. Is subjected to accelerated cooling in which water cooling is started and cooling is stopped between 350 to 600 ° C. after reaching a temperature in the range of (Ar 3 transformation point −20 ° C.) to (Ar 3 transformation point −80 ° C.). A method for manufacturing steel is described.

  Patent Document 4 includes C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1%. The steel containing is heated, the rolling end temperature is set to a temperature in the range of 900 ° C. to Ar 3 transformation point, the hot rolling in which the cumulative rolling reduction in the temperature range is less than 30%, and air cooling after hot rolling, the surface Low yield after the temperature is in the range of (Ar3 transformation point-20 ° C) to (Ar3 transformation point-80 ° C), water cooling is started and accelerated cooling is performed until the temperature falls to 250 ° C or lower, followed by tempering heat treatment. A method for producing non-tempered steel is described.

  In Patent Document 5, C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.50 to 2.2%, Al: 0.001 to 0.1%, Nb: 0 0.003 to 0.030%, Ti: 0.005 to 0.020%, N: 0.006% or less of steel slabs, the cumulative reduction amount of 900 ° C or less is 30% or more and the finishing temperature is Ar3 + 100 ° C or less Perform hot rolling to be Ar3 or higher, air-cool the steel sheet to (Ar3-20 ° C) to (Ar3-100 ° C), start water cooling from this temperature, stop cooling in the range of 400-550 ° C, low A method for producing a yield ratio non-tempered steel is described.

  In the techniques described in Patent Documents 3 to 5, in order to reduce the addition amount of alloy elements, high strength is achieved by utilizing accelerated cooling, and both high strength and low yield ratio are achieved. In these technologies, after hot rolling is completed on the steel slab at the Ar3 transformation point or higher, before starting accelerated cooling, air cooling to the austenite + ferrite two-phase temperature range generates proeutectoid ferrite This is intended to reduce the yield ratio. However, in these techniques, it is difficult to refine the pro-eutectoid ferrite and hard second phase that are generated during air cooling, and there is a problem that the toughness of the surface layer portion where a large amount of pro-eutectoid ferrite is generated tends to decrease. . In addition, since the ferrite generation rate varies depending on a slight difference in cooling start temperature, there is a problem that material variations vary from steel plate to steel plate, making it difficult to produce a stable steel plate.

  In Patent Document 6, C: 0.01 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.1 %, N: In a steel slab containing 0.001 to 0.010%, rough rolling with a cumulative rolling reduction of 10 to 80% in the range up to 900 ° C., and after rough rolling, 2-40 ° C./s Accelerated cooling is performed from (Ar3 transformation point + 50 ° C.) to (Ar3 transformation point−50 ° C.) to supercool the austenite (γ) phase, and finish rolling with a cumulative rolling reduction of 30 to 90% is completed at 650 ° C. or more. Furthermore, a method for producing a low yield ratio high-tensile steel material that performs accelerated cooling at 5 to 40 ° C./s up to 250 to 450 ° C. is described.

  In the technique described in Patent Document 6, accelerated cooling is performed after rough rolling, and the γ phase is subcooled to the vicinity of the Ar3 temperature, and then finish rolling is performed, whereby fine ferrite is formed from the supercooled γ phase. (Α) is generated, and accelerated cooling is performed after finish rolling, so that the ferrite (α), which is a soft phase, is refined, and the ratio of the soft phase to the hard phase is appropriately controlled to achieve high toughness and a low yield ratio. It is said that it will be possible to improve the productivity and at the same time. According to this technology, it is said that a high yield steel material with a low yield ratio can be produced without requiring a large amount of expensive alloy elements or complicated heat treatment with low productivity.

Further, in Patent Document 7, a steel piece or steel plate having a temperature of Ac3 or higher is cooled at least 2 ° C./s in a region (surface layer portion) of 1 mm to 30% of the product thickness in the thickness direction from the surface layer. Rapid cooling to the Ar1 point or less at a speed, rolling starts or resumes after the surface layer reaches a temperature of the Ar3 point or less, finishes rolling in the range of (Ac3-50 ° C.) to Ac3, and then reaches the Ac3 point or more. A method for producing a surface low yield strength steel sheet is described in which the surface layer is cooled to 1 ° C./s or more to the Ar1 point without reheating, and further retained for 3 minutes or more in the range of (Ac1-100 ° C.) to Ac1. Yes. As a result, the structure of the front and back layer portions of 1 mm to 30% of the steel plate thickness has a grain size that is three times or more the ferrite grain size inside the plate thickness, and the yield strength is greater than the yield strength of the inner thickness layer. It is said that the surface layer has a low yield strength steel plate having a lower yield of 5 kg / mm ² or more.

Japanese Patent Laid-Open No. 2000-219934 JP 2007-177325 A JP-A-63-219523 JP 63-223123 A JP-A-1-301818 JP-A-10-306316 Japanese Unexamined Patent Publication No. 6-49596

  By the way, in a building structure, there are many column-beam joints, column-diaphragm joints, etc., and many T joints and cross joints are formed. In such a T joint part and a cross joint part, when a deformation | transformation arises by the shake by an earthquake, a big distortion concentrates on steel plate surfaces, such as a weld toe part.

  Fig. 1 shows the situation where a joint (cross joint) between a column 1 and a diaphragm 2 using a press column (cold-formed square steel pipe) or a circular steel pipe breaks when subjected to repeated tensile and compression deformation due to an earthquake. This is shown schematically. When the joint is subjected to repeated tensile and compression deformations, a ductile crack usually occurs at the weld toe of the weld 3, and the ductile crack propagates (proliferates) toward the thickness center of the column 1 and finally breaks. To. Reference numeral 4 is a backing gold.

  For this reason, in order to increase the amount of deformation until breakage, it is important that the material in the vicinity of the surface layer of the column, that is, the ductility and toughness of the steel plate surface layer portion is excellent, but it is used in building structures. In the case of circular steel pipes and press columns formed by cold bending, the steel plate surface area is significantly hardened by the cold bending process, and the ductility and toughness near the surface layer are reduced compared to the case where the steel plate is used without being processed. It has become a state.

  Therefore, the design using a cold-formed square steel pipe as a column is operated by the “Cold-formed square steel pipe design and construction manual” (Architecture Center). In 2009, the “2008 cold-formed square steel pipe design and construction manual” As a design method compliant with (Building Center), structural regulations for using cold-formed square steel pipes for columns are announced.

  However, recently, building structures have become more important in design than safety, so ductility and toughness in the vicinity of the surface layer can be relaxed even after cold working, which can relax the above structural regulations. There is a demand for circular steel pipes and press columns using low yield ratio, high-tensile thick steel plates with little reduction.

  The techniques described in Patent Documents 1 to 6 are all mechanical properties (tensile properties, ductility) evaluated by full thickness tensile test pieces or round bar tensile test pieces at a thickness of 1/4 t or 1/2 t. , Toughness) is a technique aimed at making it a desired property, and the tensile properties by round bar tensile specimens at the thickness of 1 / 4t or 1 / 2t do not suggest properties near the surface layer of the steel plate. Because there is no, its characteristics are also unknown.

  Moreover, as shown in FIG. 2, for example, as shown in FIG. 2, the steel sheet by controlled rolling or accelerated cooling (TMCP technology) according to Patent Documents 1 to 6 has the highest surface layer hardness and the lowest center thickness. It has a hardness distribution. When cold bending is performed on a steel sheet having such a thickness distribution in the plate thickness direction, the hardness in the vicinity of the front and back surfaces is further increased, resulting in a hardness distribution like “after bending” in FIG. The difference in hardness between the thickness center portion and the surface layer portion is further enlarged.

  According to the technique described in Patent Document 7, the front and back layer portions of the steel sheet can have low yield strength, and the ductility of the steel sheet surface layer portion after cold bending can be improved. Since the ferrite grains are coarse, it cannot be said that the toughness is sufficient, and when incorporated as a member in a building structure, there is a concern that brittle fracture will occur from the member.

  Therefore, the present invention solves the above-mentioned problems of the prior art, does not perform heat treatment such as quenching and tempering, and normalization, and suppresses the alloy content to a minimum, and then press columns and circular steel pipes. Suitable for the building structure member used, even after cold bending, there is little increase in the hardness of the steel sheet surface layer part, and the steel sheet surface layer part is excellent in ductility and toughness, yield strength: 385 MPa or more, tensile strength: An object of the present invention is to provide a non-tempered low yield ratio high tensile steel plate having a yield ratio of 550 MPa or more and a yield ratio of 75% or less, and a method for producing the same.

In order to achieve the above object, the present inventors assume that a non-uniform hardness distribution in the thickness direction is allowed to some extent. The plastic strain due to cold bending is maximum at the front and back surfaces of the steel sheet, zero at the neutral point near the center of the plate thickness, and work hardening by cold bending is most noticeable at the surface layer of the steel sheet. The thickness distribution in the thickness direction is controlled, and first, the hardness near the surface layer is reduced. Policy 2. If the hardness of the surface layer is reduced while leaving the hardness at the center of the plate thickness as it is, the strength at the full thickness of the steel plate will decrease, so ensure a certain level of hardness (strength) at the center of the plate thickness. . Based on the above policy, intensive studies were conducted and the following findings were obtained.
1. In order to reduce the hardness in the vicinity of the surface layer portion and achieve a low yield ratio, at least in the microstructure of the surface layer portion, ferrite (preferably 50 area% or more) as a soft phase is precipitated, In addition, the ductility and toughness of the surface layer portion can be set within the desired range by adjusting the average grain size of ferrite in the surface layer portion within a desired appropriate range.
2. If it is a thick steel plate which satisfies the following (1)-(4), the deformation | transformation performance required as an object for building structure members can be secured even after cold bending.
(1) At least the surface layer portion (region of 1 to 5 mm in the plate thickness direction from the front surface and the back surface) of the steel sheet has a multiphase structure composed of ferrite and a hard phase.
(2) The average hardness of the steel sheet surface layer portion satisfies 225 HV or less.
(3) The hardness difference between the steel plate surface layer and the plate thickness center is 60HV or less.
(4) The average ferrite grain size of the steel sheet surface layer portion satisfies the range of 4.0 to 18.0 μm.
Here, the hard phase means a phase composed of one or more of pearlite, bainite, and martensite, and the steel sheet surface layer portion refers to a region of 1 to 5 mm in the sheet thickness direction from the steel sheet front and back surfaces. The plate thickness central portion refers to a plate thickness center of ± 2 mm. In addition, the reason for limiting the structure and hardness of the steel sheet surface layer part is that the influence of the surface layer part, which is an area of 1 to 5 mm in the thickness direction from the steel sheet front surface or the back surface, is great for the destruction of the welded structure. Based on finding.

  Excluding the outermost layer, which is less than 1 mm in the thickness direction from the front or back surface of the steel sheet, the outermost layer receives an extremely complicated thermal history due to rolling, accelerated cooling, etc., so the microstructure of the outermost layer is controlled. This is because it is often extremely difficult to do.

  Furthermore, the present inventors diligently studied various factors affecting the improvement of the toughness of the high heat input welding heat affected zone in addition to the improvement of the surface layer ductility and toughness after cold working. As a result, it has been found that the inclusion of Nb and Mo significantly deteriorates the toughness of the high heat input welding heat-affected zone. Nb and Mo are elements that improve the hardenability, greatly contribute to the formation of upper bainite including island martensite, and significantly deteriorate the toughness of the heat-affected zone with high heat input welding. Therefore, in order to improve the toughness of the high heat input welding heat-affected zone, in the present invention, it is necessary to strictly limit the content of Nb and Mo as impurities without adding Nb and Mo. Obtained.

The present invention has been completed with further studies based on this finding, that is, the present invention
1. In mass%, C: 0.05 to 0.16%, Si: 0.05 to 0.45%, Mn: 1.2 to 1.8%, P: 0.020% or less, S: 0.005 %: Al: 0.05% or less, Ti: 0.005-0.020%, N: 0.0040% or less, 4.0 ≧ Ti / N ≧ 2.0, and Nb, Mo as impurity elements Nb: 0.004% or less and Mo: less than 0.04%, and further, the carbon equivalent Ceq defined by the following formula (1) satisfies 0.35 to 0.48, and the balance Fe and Consists of component composition consisting of unavoidable impurities, and at least in the surface layer portion of the steel sheet, the ferrite has an average crystal grain size of 4.0 to 18.0 μm, and a hard phase composed of one or more of pearlite, bainite and martensite. Has a microstructure with a ferrite area ratio of 45% to 70% The average hardness of the surface layer portion of the steel sheet is 225 HV or less, and the difference in hardness between the surface layer portion and the plate thickness central portion is 60 HV or less, which is excellent in the ductility and toughness of the surface layer portion after cold working A non-tempered low yield ratio high tensile steel plate having a yield strength of 385 MPa or more, a tensile strength of 550 MPa or more, and a yield ratio of 75% or less.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
(Here, C, Mn, Cr, Mo, V, Cu, and Ni are set to 0 when not contained in the content (mass%) of each element.)
2. The component composition is further mass%, Cu: 0.05-0.50%, Ni: 0.05-0.80%, Cr: 0.05-0.60%, V: 0.01-0. The non-tempered low yield ratio high-tensile thick steel plate according to 1, which contains one or more of 05% and B: 0.0003 to 0.0030%.
3. 3. The component composition according to 1 or 2, further comprising, by mass%, one or two of Ca: 0.0005 to 0.0050% and REM: 0.0010 to 0.0050% Non-tempered, low yield ratio, high tensile thick steel plate.
4). Furthermore, ACR defined by the following formula (2) is 0.2 to 0.8, the non-tempered low yield ratio high-tensile thick steel plate according to 3.
ACR = [(Ca + 0.29 × REM) − {0.18 + 130 × (Ca + 0.29 × REM)} × O] / (1.25 × S) (2)
(Ca, REM, O, and S are the contents of each element (mass%))
5. After heating the steel material according to any one of the components 1 to 4 to 1050 to 1200 ° C., the cumulative reduction amount in the temperature range of 950 ° C. or less at the surface temperature is 30% or more, and the rolling end temperature is the surface The hot rolling is performed at a temperature of 900 ° C. or lower and the Ar3 transformation point or higher, and then the first stage cooling is performed, and the average cooling rate 2 at the 1/4 position of the sheet thickness (t) from the temperature of the Ar3 transformation point or higher at the surface temperature. Acceleration cooling until the surface temperature becomes 550 ° C. or higher at a temperature of not less than ℃ / s and (Ar3 transformation point−100 ° C.). ) From 600 ° C. or higher and when the surface temperature reaches the maximum value, the thickness (t) from the time t1 (second) that satisfies the formula (3) to the time t2 (second) that satisfies the formula (4) ) At an average cooling rate at 1/4 position of 2 ° C./s or higher, after cooling stop Non-heat treated low yield ratio method for manufacturing a high-tensile steel plates, characterized by accelerated cooling to a cooling stop temperature that the surface temperature by heat is 600 ° C. or less 400 ° C. or higher.

6). The first stage cooling starts from a temperature equal to or higher than the Ar3 transformation point at the surface temperature, is at an average cooling rate of 2 ° C./s or more at a 1/4 position of the plate thickness (t), and the cooling stop temperature is the surface temperature. The accelerated cooling at 550 ° C. or higher is repeated multiple times, and in the multiple times of accelerated cooling, the accelerated cooling at which the cooling stop temperature is not more than (Ar 3 transformation point−100 ° C.) at the surface temperature is 550 ° C. or higher at least once. 5. The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to 5, wherein
7). In the second stage cooling, the surface temperature is (Ar3 transformation point−20 ° C.) or lower and 600 ° C. or higher, and the time when the surface temperature takes the maximum value is equal to or longer than the time t1 (seconds) that satisfies Equation (3). 4) The cooling stop temperature at which the surface temperature becomes 400 ° C. or higher by reheating after cooling stop at an average cooling rate of 2 ° C./s or more at the 1/4 position of the plate thickness (t) from within the time t2 (seconds) that satisfies The cooling to be accelerated cooling to the cooling is repeated a plurality of times, and the accelerated cooling to the cooling stop temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by the recuperation after the cooling is stopped is the final cooling. The method for producing a non-tempered, low yield ratio, high-tensile thick steel plate according to 5 or 6, wherein

  According to the present invention, even after cold bending, there is little increase in hardness of the steel sheet surface layer part, excellent ductility and toughness of the steel sheet surface layer part, suitable for building structure members, yield strength: 385 MPa or more, Tensile strength: high strength of 550 MPa or higher and yield ratio: non-refined low yield ratio high tensile steel plate having a low yield ratio of 75% or less, without heat treatment, and without containing a large amount of alloy, It can be manufactured and has a remarkable industrial effect. Moreover, the non-tempered low yield ratio high-tensile thick steel plate according to the present invention also has an effect of greatly contributing to the weight reduction of the steel structure and the improvement of the earthquake resistance of the steel structure.

The schematic diagram explaining the destruction condition in a column-diaphragm junction part. The schematic diagram explaining the thickness direction hardness distribution of a non-tempered thick steel plate. The schematic diagram explaining the outline of the three-point bending test (column bending test) method of a press column-diaphragm junction part. The schematic diagram explaining the welding part vicinity in the test body of FIG. The schematic diagram explaining the point of a column bending test. The schematic diagram explaining the load-deformation amount hysteresis curve in a column bending test. The schematic diagram explaining the cooling conditions of the cooling process in this invention.

In the present invention, the component composition, microstructure, and hardness distribution in the thickness direction are defined.
[Component Composition] In the following description, “%” means “mass%”.
C: 0.05 to 0.16%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. Further, C has an effect of increasing the volume fraction of the hard phase and decreasing the yield ratio. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.16%, weldability and toughness are significantly reduced. For this reason, C was limited to the range of 0.05 to 0.16%. In addition, Preferably it is 0.06 to 0.15%.

Si: 0.05 to 0.45%
Si acts as a deoxidizer and dissolves in the steel to increase the strength of the steel material. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.45%, the toughness of the base metal is lowered and the toughness of the weld heat affected zone (also referred to as HAZ) is significantly lowered. For this reason, Si was limited to the range of 0.05 to 0.45%. In addition, Preferably, it is 0.05 to 0.35%.

Mn: 1.2 to 1.8%
Mn is an element that has the effect of increasing the strength of steel by solid solution and is inexpensive, and in the present invention, which aims to minimize the content of other expensive alloy elements, the desired high strength In order to ensure (tensile strength of 550 MPa or more), a content of 1.2% or more is required. On the other hand, if the content exceeds 1.8%, the toughness and the HAZ toughness of the base material are significantly reduced. For this reason, Mn was limited to 1.2 to 1.8% of range. In addition, Preferably it is 1.2 to 1.6%.

P: 0.020% or less P is an element that has an effect of increasing the strength of steel, but is an element that lowers toughness, particularly the toughness of a welded portion. Since the reduction of the cost increases the refining cost and is economically disadvantageous, it is preferable to be about 0.005% or more. On the other hand, if the content exceeds 0.020%, the above-described adverse effects become remarkable, so P is limited to 0.020% or less. In addition, Preferably it is 0.015% or less.

S: 0.005% or less S is present in the steel as sulfide inclusions such as MnS, which deteriorates the toughness of the base metal and the welded portion, and is unevenly distributed in the center segregated portion of the slab. It makes it easier to generate defects in pieces. Such a tendency becomes remarkable when the content exceeds 0.005%. For this reason, S was limited to 0.005% or less. Preferably it is 0.003% or less. In addition, since excessive S reduction raises refining cost and becomes economically disadvantageous, it is desirable for S to be about 0.001% or more.

Al: 0.05% or less Al is an element that acts as a deoxidizer, and is most commonly used as a deoxidizer in a molten steel deoxidation process of high-strength steel. In order to obtain such an effect, it is desirable to contain 0.01% or more. However, if the content exceeds 0.05%, the toughness of the base material decreases, and the weld metal is mixed into the weld metal during welding. Reduce toughness. For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.010 to 0.045%.

N: 0.0040% or less N, when dissolved in steel, causes strain aging after cold working and deteriorates toughness. Therefore, it is desirable to reduce N as much as possible in the present invention. If the content exceeds 0.0040%, the toughness deteriorates remarkably. For this reason, N was limited to 0.0040% or less.

Ti: 0.005-0.020%
Ti is an element having a strong affinity for N, and precipitates as TiN during solidification, thereby reducing solid solution N in the steel and reducing the toughness deterioration due to strain aging of N after cold working. Ti also contributes to the improvement of HAZ toughness through the improvement of the HAZ structure. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.020%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to 0.005 to 0.020% of range. In addition, Preferably it is 0.007 to 0.015%.

4.0 ≧ Ti / N ≧ 2.0
In the present invention, an amount of Ti commensurate with the N content (mass%) is contained, and the solid solution N is fixed as TiN. Therefore, the ratio of Ti content (mass%) to N content (mass%), Ti content (mass%), N content (mass%) so that Ti / N satisfies 2.0 or more. ). When Ti / N is less than 2.0, the Ti content (mass%) is too small compared to the N content (mass%), so that a lot of N remains as solid solution N, and the HAZ toughness is lowered. Due to the occurrence of brittle fracture, the member deformation performance may deteriorate. For this reason, Ti / N was limited to 2.0 or more. On the other hand, when Ti / N exceeds 4.0, the TiN particles become coarse and the desired effect cannot be ensured. For this reason, Ti / N was limited to the range of 2.0-4.0. In addition, Preferably, it is the range of 2.5-3.5.

Nb: 0.004% or less, Mo: less than 0.04% Nb and Mo are elements that improve the hardenability and facilitate the formation of upper bainite containing island martensite. Reduce the toughness of the part. For this reason, Nb and Mo are not added in the present invention. When it is contained as an inevitable impurity, it is limited to Nb: 0.004% or less and Mo: less than 0.04%, and is reduced within the range allowed by the manufacturing cost.

  If Nb exceeds 0.004% or Mo exceeds 0.04% as an unavoidable impurity, the toughness of the high heat input weld heat affected zone is lowered. In order to satisfy Nb: 0.004% or less and Mo: less than 0.04%, it is important to use raw materials with a low content of Nb and Mo, smelting furnace refractories, and the like.

Ceq: 0.35-0.48
If Ceq is less than 0.35, the desired base metal strength cannot be ensured, and the softening of the weld heat affected zone cannot be suppressed within a desired allowable limit. On the other hand, when Ceq is higher than 0.48, the weldability is lowered and the base material toughness and the HAZ toughness are lowered. For this reason, Ceq was limited to the range of 0.35 to 0.48. In addition, Preferably it is 0.36-0.46. Ceq is based on the following equation.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (where C, Mn, Cr, Mo, V, Cu, Ni are 0 in the case of not containing each element content (mass%)) .)
The above-mentioned components are basic components and the balance is Fe and unavoidable impurities. Further, as selective elements, Cu: 0.05 to 0.50%, Ni: 0.05 to 0.80%, Cr: 0.05 ˜0.60%, V: 0.01 to 0.05%, B: one or more selected from 0.0003 to 0.0030%, and / or Ca: 0.0005 One or two selected from 0.0050% and REM: 0.0010 to 0.0050% can be contained.

Cu: 0.05 to 0.50%, Ni: 0.05 to 0.80%, Cr: 0.05 to 0.60%, V: 0.01 to 0.05%, B: 0.0003 to One or more selected from 0.0030% are Cu, Ni, Cr, and V, which are elements having an action of increasing the strength of steel, and can be selected and contained.

  Cu increases the strength of the steel sheet through solid solution strengthening and hardenability improvement, and contributes to increasing the strength of the thick steel sheet. In order to obtain such an effect, the content is preferably 0.05% or more. However, if the content exceeds 0.50%, the alloy cost increases and surface properties are deteriorated due to hot brittleness. For this reason, when it contains, it is preferable to limit Cu to 0.05 to 0.50% of range. In addition, More preferably, it is 0.10 to 0.40%.

  Ni is an element that increases the strength of the steel sheet with little deterioration in toughness, and has a small adverse effect on the HAZ toughness, and is an element useful for increasing the strength of thick steel sheets. In order to acquire such an effect, it is preferable to contain 0.05% or more, but when it contains a large amount exceeding 0.80%, Ni is an expensive element, which causes an increase in alloy cost. For this reason, when it contains, it is preferable to limit Ni to 0.05 to 0.80%. In addition, More preferably, it is 0.10 to 0.80%.

  Cr is an element that increases the strength of the base material through improvement in hardenability, and is an element useful for increasing the strength of thick steel plates. In order to acquire such an effect, it is preferable to contain 0.05% or more, but inclusion exceeding 0.60% causes an increase in alloy cost. For this reason, when it contains, it is preferable to limit Cr to 0.05 to 0.60% of range. In addition, More preferably, it is 0.10 to 0.60%.

  V is an element that increases the strength of the base metal through precipitation strengthening, and is a useful element for increasing the strength of the thick steel plate. In order to acquire such an effect, it is preferable to contain 0.01% or more, but inclusion exceeding 0.05% reduces the toughness of a base material and HAZ. For this reason, when it contains, it is preferable to limit V to 0.01 to 0.05% of range. In addition, More preferably, it is 0.02-0.04%.

  B is an element that contributes to an increase in the strength of steel through the improvement of hardenability. In order to obtain such an effect, the content is preferably 0.0003% or more. However, the content exceeding 0.0030% deteriorates the base material and the HAZ toughness. For this reason, when contained, B is preferably limited to a range of 0.0003% to 0.0030%. In addition, More preferably, it is 0.0006 to 0.0020%.

Ca: 0.0005-0.0050%, REM: 0.0010-0.0050% of 1 type or 2 types of Ca, REM are both improved toughness and ductility of the base metal through the control of the form of sulfide. Contribute to. Further, when fine sulfide particles are dispersed in the steel, it acts as a ferrite transformation nucleus, thereby contributing to improvement of HAZ toughness. In order to obtain these effects, Ca preferably contains at least 0.0005% and REM contains at least 0.010%. However, if both Ca and REM contain more than 0.0050%, excessive amounts are required. Inclusions are produced, and the toughness may be reduced. For this reason, when it contains, it is preferable to limit Ca to 0.0005 to 0.0050% and REM to 0.0010 to 0.0050%.

  When Ca and REM are contained, the carbon equivalent ACR defined by the following formula should be adjusted to satisfy 0.2 to 0.8 in order to ensure the sulfide morphology control action. Is preferred.

  When the ACR is adjusted within this range, fine Ca and / or REM sulfates (oxysulfide) are dispersed in the steel during rolling, and the inclusions (Ca and / or REM sulfates) surface during cooling after welding. MnS precipitates on the surface.

  Such composite inclusions / precipitates function as intragranular ferrite formation sites, preventing the microstructure near the weld bond portion from being occupied by low toughness upper bainite and improving toughness.

  If the ACR is less than 0.2, the amount of Ca and REM is insufficient, so that desired composite inclusions / precipitates cannot be generated, and MnS harmful to the base material and weld heat affected zone toughness increases. On the other hand, when the ACR exceeds 0.8, most of S is taken into the inclusions of Ca and / or REM, MnS precipitated on the inclusion surface is insufficient, and the inclusions are sufficient as intragranular ferrite formation sites. Stops functioning. For this reason, it is preferable to limit ACR to the range of 0.2-0.8.

ACR = [(Ca + 0.29 × REM) − {0.18 + 130 × (Ca + 0.29 × REM)} × O] / (1.25 × S)
(Ca, REM, O, and S are the contents of each element (mass%))
The balance other than the above components is composed of Fe and inevitable impurities. As an inevitable impurity, O: 0.005% or less is acceptable.

[Microstructure]
In the present invention, the microstructure of at least the surface layer portion of the steel sheet (in the region from 1 mm to 5 mm in the thickness direction from the steel sheet surface, hereinafter the same), the ferrite having an average crystal grain size of 4.0 to 18.0 μm, and hard The phase is composed of one or more of pearlite, bainite and martensite, and has a multiphase structure with a ferrite area ratio of 45% to 70%.

  Except for the outermost layer of steel plate (region less than 1mm from the steel plate surface to the plate thickness direction) in order to combine a low yield ratio of 75% or less with a yield strength of 385 MPa or higher and a tensile strength of 550 MPa or higher. A multiphase structure consisting of a soft phase of ferrite and a hard phase composed of one or more of pearlite, bainite and martensite.

  By forming a multiphase structure in which the soft phase ferrite and the hard phase are combined, it is possible to achieve both excellent ductility, desired high strength, and a low yield ratio. The ferrite area ratio is 45% or more in order to achieve YR of 75% or less, and 70% or less to ensure a yield strength of 385 MPa or more and a tensile strength of 550 MPa or more.

  The hard phase is one or more selected from pearlite, bainite, and martensite. The types of hard phases and their fractions can be appropriately selected depending on the desired strength and toughness, as well as chemical components and plate thickness.

  The average crystal grain size of ferrite greatly affects toughness, yield ratio and elongation. When the ferrite particle size is less than 4.0 μm, the yield ratio increases rapidly, the uniform elongation decreases, and the plastic deformability greatly decreases. On the other hand, when the ferrite grain size exceeds 18.0 μm and becomes coarse, the toughness is lowered and brittle fracture is likely to occur. Therefore, the ferrite grain size is limited to the range of 4.0 to 18.0 μm. In addition, Preferably it is 7.0-14.0 micrometers. The method for obtaining the average crystal grain size will be described in Examples.

  When the surface layer portion of the steel sheet has the above-described microstructure, the plastic deformability of the steel sheet surface layer portion is greatly improved, so that after the cold bending process, the deterioration of the toughness and ductility of the steel sheet surface layer portion is suppressed, and deformation due to earthquake Occurrence of ductile cracks from the weld toe is sometimes suppressed.

[Hardness distribution in the thickness direction]
The average hardness of the surface layer portion of the steel sheet is 225 HV or less, and the hardness difference between the surface layer portion and the plate thickness central portion (the region of ± 2 mm centered on the plate thickness center position is the same hereinafter) is 60 HV or less. The hardness distribution in the thickness direction.

  If the hardness of the steel sheet surface layer part exceeds 225 HV, after the cold bending process, the hardness of the steel sheet surface layer part further increases, the toughness and ductility of the steel sheet surface layer part significantly decrease, and the column of the building structure In members such as beam joints (T joints and cross joints), cracks are likely to occur from the surface layer parts such as weld toes at the time of deformation due to an earthquake.

  Further, in order to ensure a predetermined strength throughout the thickness, it is desirable that the difference in hardness in the thickness direction is as small as possible. When the hardness difference between the surface layer and the plate thickness center exceeds 60 HV, the deformation concentrates on the plate thickness center with relatively low strength depending on the shape of the welded joint during deformation due to an earthquake, etc. Destruction may occur. In order to prevent such destruction and ensure necessary member performance, the hardness difference between the surface layer portion and the plate thickness center portion is 60 HV or less, preferably, the hardness difference is 55 HV or less, more preferably 50 HV or less. The hardness at the center of the plate thickness is preferably 175 to 200 HV in order to ensure the desired high strength. The hardness test method will be described in Examples.

  The steel sheet according to the present invention is a steel sheet having the above composition, which is subjected to hot rolling to form a steel sheet, and subsequently to the rolling process, one-stage cooling and one-stage cooling are stopped, and two-stage cooling performed after reheating. And a cooling process for performing two-stage accelerated cooling consisting of:

  The manufacturing method of the steel material is not particularly limited, and any conventional melting method and casting method can be applied, but the molten steel having the above composition is melted in a converter, electric furnace, vacuum melting furnace or the like. The steel material (slab) is preferably obtained by a continuous casting method through a deoxidation treatment or a degassing process.

  The obtained steel material (slab) is first heated and subjected to a rolling process that is hot-rolled into a thick steel plate. In the rolling process, the steel material is heated to a heating temperature of 1050 to 1200 ° C., the cumulative reduction amount in the temperature range of 950 ° C. or less at the surface temperature is 30% or more, and the rolling end temperature is 900 ° C. or less at the surface temperature. Hot rolling at a point or more is performed to obtain a thick steel plate having a predetermined thickness.

Heating temperature: 1050-1200 ° C
If the heating temperature is less than 1050 ° C., the strength tends to decrease. On the other hand, if it exceeds 1200 ° C., the toughness obtained by coarsening the structure decreases, the hardenability increases excessively, and the surface hardness increases. It becomes easy. For this reason, the heating temperature of the steel material was limited to the range of 1050 ° C to 1200 ° C. In addition, Preferably it is 1080 to 1150 degreeC.

Cumulative reduction in surface temperature range of 950 ° C. or lower: 30% or more Control rolling is performed in a temperature range where the surface temperature of the steel sheet is 950 ° C. or lower in order to refine the microstructure appropriately. When the cumulative reduction amount in the temperature range is less than 30%, the structure becomes coarse and hardenability increases too much, so that desired toughness and surface hardness cannot be ensured. For this reason, the cumulative reduction amount in the temperature range of 950 ° C. or less at the surface temperature is limited to 30% or more. In addition, Preferably it is 35% or more.

Rolling end temperature: 900 ° C. or less at the surface temperature and Ar3 transformation point or higher If the rolling end temperature exceeds 900 ° C. at the surface temperature, the structure becomes coarse and hardenability increases, ensuring the desired toughness and surface hardness. become unable. On the other hand, if the rolling end temperature is less than the Ar3 transformation point at the surface temperature, ferrite is generated during rolling or immediately after rolling and becomes coarse, and the toughness of the surface layer portion decreases. For this reason, the rolling end temperature was limited to 900 ° C. or lower and Ar 3 temperature or higher as the surface temperature. In addition, Preferably it is 880-780 degreeC.

  As the Ar3 transformation point, a value calculated using the following formula is used.

Ar3 transformation point (° C) = 900-332C + 6Si-77Mn-20Cu-50Ni-18Cr-68Mo
(In the formula, C, Si, Mn, Cu, Ni, Cr, Mo: The content (mass%) of each element, and when the element described in the above formula is not contained, the element is calculated as 0 Suppose)
After rolling, a cooling step for accelerated cooling is performed. The cooling step includes first-stage cooling, a process of stopping cooling and returning to heat, and second-stage cooling. In the first stage cooling, the surface layer part is supercooled and then reheated, and the time until the start of the second stage cooling (cooling stop time) is used to advance the ferrite transformation of the surface layer part to achieve the desired surface layer microstructure. Get. The second stage cooling is carried out after the first stage cooling so that the untransformed portion becomes a hard phase such as pearlite, bainite, martensite or the like. By making the untransformed portion a hard phase, the final structure can be (ferrite + (pearlite, bainite and / or martensite)), and a desired high strength and low yield ratio can be realized.

[First stage cooling]
In the first stage cooling, cooling is started from a temperature equal to or higher than the Ar3 transformation point at the surface temperature, and accelerated cooling is performed at a cooling rate of 2 ° C./s or higher at an average cooling rate at 1/4 position of the plate thickness (t). Accelerated cooling is stopped when the temperature reaches (Ar3 transformation point−100 ° C.) or less, 550 ° C. or more, and preferably the temperature difference between the surface and the plate thickness center position is 150 ° C. or more.

First stage cooling start temperature: Ar3 transformation point or higher at the surface temperature If the first stage cooling start temperature is less than the Ar3 transformation point, ferrite is generated and coarsened before the start of accelerated cooling. Cannot be achieved. For this reason, the start temperature of the first stage cooling is limited to the Ar3 transformation point or higher.

Cooling rate of the first stage cooling: 2 ° C./s or more at an average cooling rate of 1/4 position of the plate thickness (t) When the cooling rate is less than 2 ° C./s, the cooling is slow and the ferrite is coarse and low toughness during cooling. Grain may form. For this reason, the cooling rate of the first stage cooling was limited to 2 ° C./s or more as the average cooling rate at the 1/4 position of the plate thickness (t). The upper limit of the cooling rate of the first stage cooling is not particularly limited, and is almost determined by the plate thickness and the capacity of the cooling device, and is about 5 ° C./s or more when the plate thickness is 60 mm. The “average cooling rate at the 1/4 position of the plate thickness (t)” refers to the average cooling rate from the start to the end of the accelerated cooling at the 1/4 position of the plate thickness (t).

Cooling stop temperature of the first stage cooling: (Ar3 transformation point −100 ° C.) or less at the surface temperature 550 ° C. or more In the first stage cooling in the present invention, the cooling is performed so that the temperature difference between the surface layer portion and the inside becomes larger. Then, ferrite is generated in the surface layer portion during the recuperation after stopping the first stage cooling and the time after the recuperation until starting the second stage cooling (hereinafter referred to as holding time).

  When the cooling stop temperature exceeds the surface temperature (Ar3 transformation point −100 ° C.), the subsequent recuperation temperature is too high, and ferrite formation in the surface layer becomes insufficient. On the other hand, if the cooling stop temperature is less than 550 ° C., the temperature of the surface layer portion becomes too low, and phase transformation is almost completed during cooling after recuperation, and the surface layer portion is composed of a structure mainly composed of a hard phase such as bainite and martensite. Become. For this reason, the cooling stop temperature of the first stage cooling is set to the surface temperature (Ar3 transformation point−100 ° C.) or lower and 550 ° C. or higher. The cooling stop temperature of the first stage cooling is preferably 650 to 550 ° C.

[Recuperation and second stage cooling]
After stopping the first stage cooling, the second stage cooling is started after reheating to generate ferrite. The recuperation is performed until the steel sheet surface temperature is (Ar3 transformation point−20 ° C.) or lower and 600 ° C. or higher after the reheating and a predetermined holding time is satisfied.

  If the surface temperature of the steel sheet is less than 600 ° C., acicular ferrite and bainite having a relatively high strength and yield ratio are formed in the surface layer portion, resulting in a decrease in elongation of the surface layer portion and an increase in yield ratio, resulting in a decrease in deformability. Invite. On the other hand, if the surface temperature exceeds (Ar3 transformation point −20 ° C.), phase transformation does not proceed after recuperation, and ferrite formation in the surface layer portion becomes insufficient, so (Ar3 transformation point −20 ° C.) or lower 600 ° C. Hold until above.

  The holding time (the time from when the surface temperature of the steel sheet takes the maximum value after recuperation until the start of the second stage cooling) is equal to or longer than the time t1 (seconds) that satisfies the following equation (3), and the following equation (4): Within a time t2 (seconds) that satisfies

  When the holding time is less than t1 (seconds), the ferrite precipitated on the surface layer portion of the steel sheet is less than 50%, and the surface layer softening of the steel sheet and YR ≦ 80% cannot be achieved. If the holding time exceeds t2 (seconds), the ferrite precipitated on the surface layer of the steel sheet exceeds 70%, the steel sheet softens, and the TS strength becomes less than 550 MPa, so the holding time is t1 (seconds) or more. Within t2 (seconds).

  Thus, when reheating is performed, the temperature difference between the surface and the center of the plate thickness is 60 ° C. or less at the start of the second stage cooling, and the ferrite area fraction of the surface layer portion is 45 to 70 at the end of the second stage cooling. %, The difference in the amount of ferrite produced in the thickness direction between the surface and the center of the plate thickness can be reduced, and the hardness difference between the surface layer portion and the plate thickness center portion can be reduced to 60 HV or less.

  The second stage cooling is an average cooling rate of 2 ° C./s or more at the 1/4 position of the plate thickness (t), and the surface temperature is 600 ° C. or less and 400 ° C. or more by recuperation after stopping the second stage cooling. Accelerate cooling to the cooling stop temperature.

Cooling rate of the second stage cooling: 2 ° C./s or more at an average cooling rate at 1/4 position of the thickness (t) In order to make the untransformed part a hard phase, 2 ° C./s or more in the second stage cooling The cooling is preferably performed at 8 ° C./s or more. If the cooling rate is less than 2 ° C./s, the amount of transformation to the hard phase decreases, and the desired high strength and low yield ratio cannot be realized.

Cooling stop temperature of the second stage cooling: Temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by the recuperation after stopping the cooling. The cooling stop temperature of the second stage cooling is the recovery temperature after the cooling stop of the second stage cooling. At a temperature at which the surface temperature exceeds 600 ° C. due to heat, the amount of transformation to the hard phase decreases, or the strength decreases due to self-tempering, making it impossible to ensure the desired high strength. On the other hand, at the cooling stop temperature at which the surface temperature is less than 400 ° C. due to recuperation, the hard phase hardness becomes too high and the toughness is lowered. For this reason, the cooling stop temperature of the second stage cooling is limited to a temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by reheating after cooling is stopped. Since the temperature after recuperation depends on the temperature at the half position of the plate thickness (t) when the accelerated cooling is stopped, it can be predicted from various heat transfer calculations.

  After the second stage cooling, a tempering step may be performed for the purpose of adjusting strength and toughness. Tempering is preferably performed at a temperature of 400 ° C. or higher and 700 ° C. or lower. If the tempering temperature is less than 400 ° C., the desired effect cannot be expected. On the other hand, when the temperature exceeds 700 ° C., the strength is significantly reduced.

  In the present invention, both the first-stage cooling and the second-stage cooling, or only one of them, are replaced with cooling consisting of one accelerated cooling, and cooling is repeated a plurality of times with a cooling stop and subsequent recuperation. It is good.

  By dividing the accelerated cooling into a plurality of times, it is possible to cool to the target temperature without excessively increasing the temperature difference between the surface layer and the inside. In addition, since it is sufficient to obtain a desired cooling effect while it is repeated a plurality of times, options for cooling temperature control can be expanded, and the accuracy of cooling temperature control can be improved. FIG. 7 schematically shows an example of the history of the steel sheet temperature when such cooling is performed. In the drawing, the holding time indicates the holding time of the above-described recuperation.

  When the first stage cooling is performed by a plurality of times of cooling, the starting temperature of the first accelerated cooling (first cooling) is set to the Ar3 transformation point or more at the surface temperature for the same reason as in the case of one time.

  The cooling rate of accelerated cooling is the average cooling rate at 1/4 position of the plate thickness (t) from the first cooling to the last cooling in the first stage cooling, and it is 2 ° C./s or more for the same reason as in the case of one time. And

  The average cooling rate at the 1/4 position of the plate thickness (t) is the average cooling rate from the start to the end of accelerated cooling at the 1/4 position of the plate thickness (t), and the average from point A to point B in FIG. Cooling rate. Point A is the time when the temperature at the plate thickness 1 / 4t position becomes equal to the surface cooling start temperature, and point B is the time when the last accelerated cooling in the first stage cooling is stopped.

  In the multiple times of accelerated cooling in the first stage cooling, when the cooling stop temperature is less than 550 ° C. in the surface layer part, bainite and martensite transformation occurs during cooling, and the surface layer part becomes hard, so all accelerated cooling The cooling stop temperature is set to 550 ° C. or higher.

  The first stage cooling is performed so that a temperature difference between the surface layer part and the inside is generated to some extent, and the purpose is to generate ferrite in the surface layer part during the recuperation after the cooling stop and the holding time. In all of the multiple times of accelerated cooling, if the cooling stop temperature exceeds the surface temperature (Ar3 transformation point −100 ° C.), the steel sheet temperature becomes too high at the time of subsequent reheating, and ferrite formation at the surface layer portion is not possible. It will be enough. For this reason, at least one cooling stop temperature is set to (Ar3 transformation point−100 ° C.) or lower among a plurality of times of accelerated cooling.

  When the cooling of the second stage cooling is repeated a plurality of times with the cooling stop and the subsequent recuperation interposed therebetween, the first accelerated cooling (first cooling) is performed after the recuperation as in the case of one time. Start after satisfying temperature and time.

  The cooling rate of the multiple times of the accelerated cooling that constitutes the second stage cooling is the average cooling rate at the position of 1 / 4t thickness until the first cooling and the last cooling in the first stage cooling, and the same reason as the one-time cooling To 2 ° C./s or more.

  The average cooling rate at the 1/4 position of the plate thickness (t) is the average cooling rate from the start to the end of the accelerated cooling at the 1/4 position of the plate thickness (t), and the average from point C to point D in FIG. Cooling rate. Point C is the time when the temperature at the 1/4 position of the plate thickness (t) becomes equal to the surface cooling start temperature, and point D is the time when the last accelerated cooling in the second stage cooling is stopped.

  The final cooling of the second stage cooling is performed for the same reason as in the case of one time, and is cooled to a cooling stop temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by recuperation after cooling stop.

  Depending on the accuracy of desired cooling temperature control, both the first-stage cooling and the second-stage cooling, or only one of them, is repeated multiple times. Hereinafter, the present invention will be described in more detail with reference to examples.

  The steel material having the composition shown in Table 1 was subjected to the rolling step and the cooling step shown in Table 2 to obtain a thick steel plate having a plate thickness of 40 mm. In the cooling step, the second stage cooling was performed after the cooling stop of the first stage cooling-recuperation. The steel plate temperature in each step was measured by measuring the surface temperature with an infrared radiation thermometer, and calculating the temperature at the plate thickness ¼ t position and the plate thickness central temperature using various heat transfer calculation methods.

The obtained thick steel plate was subjected to structure observation, hardness test, tensile test, and impact test. The test method was as follows.
(1) Microstructure observation The cross section in the L direction of the specimen for structural observation of the full thickness of the plate is polished, and after nital corrosion, the surface layer portion and the central portion of the plate thickness are optical microscope (magnification: 400 times) or scanning electron microscope (magnification: 2000 times), the microstructure was observed for three or more visual fields, imaged, and image analysis was performed to determine the type of structure and the structure fraction (area ratio%) of ferrite.

For the surface layer portion, the average crystal grain size of ferrite (sometimes referred to as a nominal grain size) was determined. The nominal grain size of the ferrite was determined as the square root of the average area of the crystal grains obtained by calculating the average area of the crystal grains.
(2) Hardness test Collect a test piece for hardness measurement of the full thickness of the plate, and measure the hardness of the cross section in the plate thickness direction using a Vickers hardness tester in accordance with the provisions of JIS Z 2244. It was. The measurement position was a surface layer portion and a plate thickness central portion, and four or more points were measured at a pitch of 1 mm in the plate thickness direction in each region. The test load (test force) was 1 kg (9.8 kN). The obtained hardness HV was arithmetically averaged to obtain the average hardness HV in that region.
(3) Tensile test JIS No. 5 full-thickness tensile test specimens are collected in accordance with the provisions of JIS Z 2201 so that the tensile direction is the L direction, and the tensile tests are performed in accordance with the provisions of JIS Z 2241. The tensile properties (yield strength YS, tensile strength TS) were determined. Moreover, the yield ratio YR (= YS / TS × 100%) was calculated from the obtained measured values.
(4) Impact test V-notch impact test specimens were collected from ¼ position of the plate thickness (t) and 1 mm below the surface (test specimen central position was 6 mm below the surface) according to JIS Z 2242, and Charpy. An impact test was performed to determine the fracture surface transition temperature vTrs (° C.). In addition, the case where vTrs is -40 degrees C or less was considered to be excellent in toughness.

(5) Column bending test Using the obtained thick steel plate, a square steel pipe (press column) was produced by cold pressing. The cross-sectional dimensions of the square steel pipe (press column) were 500 × 500 (mm), the length was 3250 (mm), and the seam welding was submerged arc welding with one layer on each side.

  FIG. 3 shows a specimen 6 for the column bending test. Square steel pipes (press columns) 1a and 1a were attached to the left and right sides of the four-sided BOX column 3a through SN490 steel plate through diaphragms (plate thickness 40 mm) 2a and 2a. Welding between each member was carbon dioxide welding.

  By making the strength and rigidity of the four-sided BOX column sufficiently higher than that of the press column, plastic deformation was prevented from occurring outside the press column during the test. FIG. 4 shows an enlarged view of the vicinity of the welded portion of the press column 1a, the diaphragm 2a, and the four-sided BOX column 3a in the test body 6. In the figure, reference numerals 4a and 5a denote welds. A column bending test was performed using the illustrated specimen 6 in the following manner.

  As shown in FIG. 5, a three-point repeated bending test (column bending test) was performed in which positive and negative loads were repeatedly applied in the vertical direction to the center of the test body 6 as shown in FIG. The load P and the deformation amount (rotation angle) θ were measured, and a load (moment, M) -deformation amount (rotation angle, θ) hysteresis curve as shown in FIG. 6 was created.

When the load (moment) decreases by 5% from the maximum value due to local buckling or brittle fracture, the specimen is considered to be fractured, and the total plastic rotation angle (cumulative plastic rotation angle Σθpl) of the specimen is calculated and tested. The cumulative plastic deformation magnification η was obtained as an index of the plastic deformability of the body, and when it was 30 or more, it was assumed that the structural member was excellent in earthquake resistance (plastic deformation performance). Η is calculated from the following equation.
η = Σθpl / θp
However, θp = (Pp / 2) L ² / (3 · E · I) + Pp / 2 / (G · Aw)
Here, Pp: Total plastic load (N) = Mp / L,
L: Column cantilever length (distance from diaphragm to column end support point, 3250 mm)
E: Young's modulus 205000 (MPa), G: shear rigidity 79000 (MPa),
Mp: Total plastic moment of the column.

I: secondary moment of section of column, σy: yield strength of steel (MPa)
Here, I is according to the following equation.

D: Column diameter (mm), t: Column plate thickness (mm),
r: bending radius of the inner surface of the column corner, R = r + t
Aw: Shear area (mm ² ) according to the following formula.

  Table 3 shows the test results of (1) to (5). In all of the inventive examples (thick steel plates No. 2, 4, 5, 10, 11, 15, 16, 17), the yield strength YS: 385 MPa or more, the tensile strength TS: 550 MPa or more, the yield ratio YR: 75% A high-strength, high-toughness, non-tempered, low-yield-ratio, high-tensile steel plate having the following:

  Further, all of the examples of the present invention have a thickness distribution in the thickness direction in which the average hardness of the surface layer portion is 225 HV or less, and the hardness difference between the surface layer portion and the thickness center portion is 60 HV or less, and cold bending is performed. When a press column-diaphragm joint structural member is formed by processing into a press column, the cumulative plastic deformation ratio in the three-point bending test of the press column-diaphragm joint is 30 or more, and the seismic performance (plastic deformation performance) is improved. It can be set as the excellent structural member.

  On the other hand, comparative examples (thick steel plates No. 1, 3, 6-9, 12-14, 18-24) whose component composition and / or microstructure are outside the scope of the present invention are insufficient in strength, yield ratio, and toughness. In other words, the ductility and toughness of the surface layer portion after cold working are reduced, and the cumulative plastic deformation ratio η as a structural member is low.

  Steel No. shown in Table 1 A-No. Using the steel material having the composition E, in the cooling process, the first stage cooling is accelerated cooling in which the cooling stop and the subsequent recuperation are repeated one or more times, and the second stage cooling is performed in the cooling stop and the subsequent recovery. A plate thickness: 40 mm thick steel plate was manufactured as accelerated cooling in which heat was repeated once or more. The obtained thick steel plate was subjected to structure observation, hardness test, tensile test, impact test and column bending test under the same conditions as in Example 1.

  Table 4 shows the manufacturing conditions, and Table 5 shows the test results. Examples of the present invention (thick steel plates No. A1 to A5, A8) all have a yield strength YS: 385 MPa or more, a tensile strength TS: 550 MPa or more, and a yield ratio YR: 75% or less. This is a high-strength, high-toughness, non-tempered, low yield ratio, high-tensile steel plate with a vTrs at a position of 1/4 t in the thickness direction of −40 ° C. or less.

  Each of the examples of the present invention has a thickness distribution in the thickness direction in which the average hardness of the surface layer portion is 225 HV or less, and the hardness difference between the surface layer portion and the plate thickness central portion is 60 HV or less. When a press column-diaphragm joint structural member is formed, the structure has excellent seismic performance (plastic deformation performance) with a cumulative plastic deformation ratio of 30 or more in a three-point bending test of the press column-diaphragm joint. It can be a member. On the other hand, in the comparative examples (thick steel plates No. A6, A7) outside the scope of the present invention, the desired tensile strength or yield strength, the desired yield ratio cannot be ensured, or the desired thickness distribution in the thickness direction is ensured. The cumulative plastic deformation magnification η as a structural member is low.

1 Column 2 Through Diaphragm 3 Welded Part 4 Backing Metal 1a Press Column 2a Diaphragm (Through Diaphragm)
3a 4-sided BOX column 4a, 5a Welded part 6 Specimen

Claims (7)

In mass%, C: 0.05 to 0.16%, Si: 0.05 to 0.45%, Mn: 1.2 to 1.8%, P: 0.020% or less, S: 0.005 %: Al: 0.05% or less, Ti: 0.005-0.020%, N: 0.0040% or less, 4.0 ≧ Ti / N ≧ 2.0, and Nb, Mo as impurity elements Is limited to Nb: 0.004% or less and Mo: less than 0.04%, and Ceq defined by the following formula (1) satisfies 0.35 to 0.48, and the balance Fe and unavoidable Component composition consisting of impurities, at least in the surface layer portion of the steel sheet, consisting of a ferrite having an average crystal grain size of 4.0 to 18.0 μm and a hard phase consisting of one or more of pearlite, bainite and martensite, It has a microstructure with a ferrite area ratio of 45% to 70%, steel The yield strength is excellent in the ductility and toughness of the surface layer part after cold working, characterized in that the average hardness of the surface layer part is 225 HV or less and the hardness difference between the surface layer part and the plate thickness central part is 60 HV or less. A non-tempered low yield ratio high tensile steel plate having a thickness of 385 MPa or more, a tensile strength of 550 MPa or more, and a yield ratio of 75% or less.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
(Here, C, Mn, Cr, Mo, V, Cu, and Ni are set to 0 when not contained in the content (mass%) of each element.) The component composition is further mass%, Cu: 0.05-0.50%, Ni: 0.05-0.80%, Cr: 0.05-0.60%, V: 0.01-0. The non-tempered low yield ratio high-tensile thick steel plate according to claim 1, containing one or more of 05% and B: 0.0003 to 0.0030%.   The component composition further includes one or two of Ca: 0.0005 to 0.0050% and REM: 0.0010 to 0.0050% in terms of mass%. Non-tempered low yield ratio high tensile steel plate. Furthermore, ACR defined by the following formula (2) is 0.2 to 0.8, the non-tempered low yield ratio high tensile thick steel plate according to claim 3.
ACR = [(Ca + 0.29 × REM) − {0.18 + 130 × (Ca + 0.29 × REM)} × O] / (1.25 × S) (2)
(Here, Ca, REM, O, and S are set to 0 when not included in the content (mass%) of each element.) After heating the steel material according to any one of claims 1 to 4 to 1050 to 1200 ° C, the cumulative reduction amount in the temperature range of 950 ° C or less at the surface temperature is 30% or more, and the rolling finish temperature Is subjected to hot rolling at a surface temperature of 900 ° C. or lower and an Ar3 transformation point or higher, and then, as the first stage cooling, from the temperature at the surface temperature of the Ar3 transformation point or higher, the average cooling at 1/4 position of the sheet thickness (t) At a rate of 2 ° C./s or more, accelerated cooling is performed until the surface temperature becomes (Ar 3 transformation point−100 ° C.) or less, 550 ° C. or more, recooling is performed after cooling is stopped, and the surface temperature is (Ar 3 transformation point− 20 ° C. or less, 600 ° C. or more, and from the time when the surface temperature takes the maximum value, from the time t1 (second) that satisfies the following formula (3) or more, and within the time t2 (second) that satisfies the following formula (4), Average cooling rate at 1/4 position of thickness (t) is 2 ° C / s or more The method of non-heat treated low yield ratio high-strength thick steel plate surface temperature at the recuperator after cooling stops characterized by accelerated cooling to a cooling stop temperature becomes 600 ° C. or less 400 ° C. or higher.

  The first stage cooling starts from a temperature equal to or higher than the Ar3 transformation point at the surface temperature, is at an average cooling rate of 2 ° C./s or more at a 1/4 position of the plate thickness (t), and the cooling stop temperature is the surface temperature. The accelerated cooling at 550 ° C. or higher is repeated multiple times, and in the multiple times of accelerated cooling, the accelerated cooling at which the cooling stop temperature is not more than (Ar 3 transformation point−100 ° C.) at the surface temperature is 550 ° C. or higher at least once. The manufacturing method of the non-tempered low yield ratio high-tensile thick steel plate of Claim 5 characterized by the above-mentioned.   The second stage cooling is performed at a time t1 (seconds) or more satisfying the expression (3) from the time when the surface temperature is (Ar3 transformation point −20 ° C.) or lower and 600 ° C. or higher and the surface temperature takes the maximum value. ) The cooling stop temperature at which the surface temperature becomes 400 ° C. or higher by reheating after cooling stop at an average cooling rate of 2 ° C./s or more at the 1/4 position of the plate thickness (t) from the time t2 (seconds) satisfying the formula The cooling to be accelerated cooling to the cooling is repeated a plurality of times, and the accelerated cooling to the cooling stop temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by the recuperation after the cooling is stopped is the final cooling. The method for producing a non-tempered low yield ratio high tension thick steel plate according to claim 5 or 6.

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