JP2004300567A - High tensile steel sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a 590 MPa class steel sheet having various characteristics such as low sound anisotropy, base metal toughness and weldability (especially, HAZ toughness in high-heat input) even in non-heat-treated state and to provide its manufacturing method. <P>SOLUTION: The high tensile steel sheet has a specific chemical composition, and 90 vol.% or more of the steel structure consists of bainite, and old γ particles have an average aspect ratio of 1.8 or less and an average equivalent circular diameter of 100 μm or less, and the steel sheet has a tensile strength of not less than 590 MPa and less than 780 MPa. The high tensile steel sheet can be manufactured by using partial recrystallization phenomenon. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is a steel plate having a low acoustic anisotropy, weldability [weld heat affected zone (HAZ) toughness] and proof stress (for example, proof strength at 0.2% elongation) of 590 MPa or more and less than 780 MPa (hereinafter simply referred to as “steel plate”). And a manufacturing method thereof. The high-tensile steel plate of the present invention is particularly suitably used for large structures such as building structures and bridges.

  For example, in steel plates for construction and bridges (thick steel plates), if there is a defect in the welded part, this part is likely to be the starting point of fracture occurrence, so the presence or absence of the defective part is investigated by ultrasonic testing. When it exists, the work of repairing the part is generally performed. However, in a steel plate whose sound speed changes remarkably depending on the flaw detection direction, the exact position of the weld defect cannot be detected by an ultrasonic flaw detection test. Small is required.

  In addition, steel sheets used for construction and bridges are required to have various properties such as base metal strength, toughness, weldability, etc., and from the standpoint of reducing manufacturing costs, quenching and tempering are not performed. Even in the so-called non-tempered product, it is required that such characteristics can be sufficiently secured.

  For example, Patent Document 1 discloses that in ultra-low carbon bainitic steel, base material strength (tensile strength) is obtained by finely dispersing more diffusive αq (Quasi-Polygonal α) in αB (Granular Bainitic α) structure. ), An extremely thick steel plate that ensures toughness in the thickness direction and low acoustic anisotropy is disclosed.

  Patent Document 2 and Patent Document 3 have a structure of one or both of martensite and bainite, the aspect ratio of prior austenite (γ) grains is 1.5 or less, and the average value of the short diameters of prior γ grains. Has been proposed in which the content of Ti, N and S and the average value of the minor axis of the old γ grains have a specific relationship.

  In these Patent Documents 1 to 3, although low acoustic anisotropy has been achieved to some extent, there have been cases where it is insufficient in terms of, for example, weldability and base material toughness.

On the other hand, Patent Document 4 discloses a high-tensile steel plate having excellent weldability (high heat input HAZ toughness and weld crack resistance) by making the chemical composition specific. Although the high-tensile steel sheet disclosed in Patent Document 4 has very excellent characteristics, no consideration is given to acoustic anisotropy, and low acoustic anisotropy is not necessarily achieved. There was still room for improvement in this point.
Japanese Patent Laid-Open No. 11-193445 JP 2000-178645 A JP 2001-3137 A JP 2002-47532 A

  The present invention has been made in view of the above circumstances, and its purpose is to achieve various characteristics such as low acoustic anisotropy, base material toughness, and weldability (particularly, high heat input HAZ toughness) even if it is not tempered. An object of the present invention is to provide an excellent 590 MPa grade steel sheet and a method for producing the same.

  The high-tensile steel sheet of the present invention that has achieved the above-mentioned object has C: 0.010 to 0.06% (meaning of mass%, the same applies hereinafter), Mn: 1.25 to 2.5%, Cr: 0.00. 1 to 2.0%, Mo: 0.005 to 1.5%, V: 0.04% or less (including 0%), Nb: 0.001 to 0.04%, Ti: 0.005 to 0 .03%, B: 0.0006-0.005%, N: 0.0020-0.010% steel satisfying 2.4 ≦ KP ≦ 4.5 and KV ≦ 0.040, respectively. In addition, 90% by volume or more of the steel structure is bainite, the old γ grain (former austenite grain) has an average aspect ratio of 1.8 or less, an average equivalent circle diameter of 100 μm or less, and a tensile strength of It has a gist where it is 590 MPa or more and less than 780 MPa.

Here, the KP value is represented by the following formula (1), and the KV value is represented by the following formula (2).
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo] (1)
KV = [V] + [Nb] (2)
<< In formula, [] means content (mass%) of each element. >>

  In the present invention, a high-tensile steel sheet in which the average volume fraction of the mixed structure (MA structure) composed of martensite and austenite in the steel structure is 3% or less is preferable because the base material toughness is further improved. At this time, the average equivalent circle diameter of the MA structure is preferably 1 μm or less, and the average aspect ratio of the MA structure is preferably 2.5 or less.

  The high-tensile steel plate of the present invention preferably has a Mo content of 0.05 to 1.5% and a Nb content of 0.005 to 0.04%.

  In the present invention, when a high-tensile steel plate further containing Ni: 5% or less and / or Cu: 1.2% or less, or when the Mn content is 1.25 to 1.8%, Cu: 1 .High tensile steel sheets containing more than 2% and containing 2.0% or less, and high tensile steel sheets satisfying these Mn and Cu contents, and further containing Ni: 5% or less, have improved weldability. preferable. Further, high-tensile steel sheets containing Ca: 0.005% or less, Si: 1% or less, P: 0.020% or less, S: 0.010% or less, Al: 0.2% or less, respectively. The suppressed high-tensile steel sheet is a preferable embodiment because the weldability is further improved.

  Such a high-tensile steel plate of the present invention has good characteristics even when the thickness (plate thickness) is 80 mm or more.

The above-described high-tensile steel sheet can be produced by rolling 50% or more of the total rolling reduction in the partially recrystallized region when hot rolling is performed at a point of Ac _{3 to} 1300 ° C. Here, the partial recrystallization region refers to a steel plate test piece having a γ grain size (austenite grain size): 100 ± 10 μm in the temperature range under the conditions of strain rate: 10 sec ⁻¹ and equivalent strain: 0.2. When the structure is frozen after 10 seconds of reduction, the temperature range is 20 to 80% by volume of recrystallized grains.

It is also recommended to cool to 200 ° C. after the hot rolling and then perform tempering at a temperature below the A _c1 point. The cooling at this time is preferably performed by water cooling.

  In addition, after the hot rolling, it is also a preferable aspect that water cooling is performed to a temperature below the Bs point.

  The chemical composition of the high-strength steel sheet according to the present invention typically consists of the balance of the above elements and the remainder Fe and unavoidable impurities, but other chemical components are within a range that does not impair the effects of the present invention. It may be contained.

  According to the present invention, in addition to excellent base material properties and weldability, a high strength steel sheet having excellent low acoustic anisotropy is provided by controlling old γ grains in a steel structure to a specific form. it can. Thereby, it is possible to accurately investigate a welding defect by an ultrasonic flaw detection test, and for example, it can be applied as a highly reliable material in the field of large structures such as building structures and bridges.

  Furthermore, the high-strength steel sheet of the present invention can be manufactured by a method using a phenomenon called partial recrystallization, and the above-described excellent characteristics can be secured even as a non-tempered steel sheet.

  The present inventors have secured characteristics such as low acoustic anisotropy, base material strength / toughness, and high heat input HAZ toughness, which were difficult to achieve with the above-described prior art, without reheating and quenching and further tempering treatment. As a result of intensive studies to develop a steel plate that can be used, it has been found that such an object can be achieved to a high degree if the morphology of the old γ grains is appropriately controlled in addition to a specific chemical composition. Further, the inventors have found that such a structure control of the steel sheet can be implemented by strictly controlling the hot rolling conditions and utilizing the phenomenon of partial recrystallization, and thus completed the present invention.

  The “old γ grains” as used in the present invention means old austenite grains as described above. Generally, when the structure is cooled from the austenite state, a structural transformation occurs and another structure such as ferrite or martensite. However, the term that refers to the austenite grains before transformation from the standpoint of the steel material (steel plate, etc.) after transformation is “old γ grains”. Hereinafter, the present invention will be described in detail.

  In the steel sheet of the present invention, 90% by volume or more of the steel structure is bainite, and the old γ grains have an average aspect ratio of 1.8 or less and an average equivalent circle diameter of 100 μm or less.

First, from the viewpoint of reducing the acoustic anisotropy of the steel sheet and improving the toughness of the base metal, investigations were made focusing on the form of the old γ grains. As for acoustic anisotropy, the transverse wave sound velocity ratio C _SL / C _SC defined in JIS Z 3060 [the transverse wave sound velocity obtained with the vibration direction as the L direction (main rolling direction) and the C direction (direction perpendicular to the L direction)] Value C _SL (m / sec) to C _SC (m / sec)] is set to a low value of, for example, 1.02 or less, that is, low acoustic anisotropy. The relationship was investigated.

FIG. 1 shows C: 0.03%, Si: 0.1%, Mn: 1.5%, Ni: 0.4%, Cr: 0.6%, Mo: 0.3%, Nb: 0.0. Steel sheet containing 01%, Ti: 0.014%, B: 0.010%, KP value: 3.0, KV value: 0.01, sheet thickness: 80 mm, old γ grain: 60 μm (no tempering) ) Is a graph showing the relationship between the average aspect ratio (horizontal axis) of the prior γ grains and the transverse wave sound velocity ratio C _SL / C _SC (vertical axis). Thus, when the average aspect ratio (ratio of “major axis / minor axis”) of the old γ grains is 1.8 or less, for example, low acoustic anisotropy such as a shear wave velocity ratio of 1.02 or less is achieved. I found. The average aspect ratio of the prior γ grains is more preferably less than 1.5, and still more preferably less than 1.3.

As for the base material toughness, JIS No. 4 specimens were collected from a 1/4 thickness part of the steel sheet, and subjected to a Charpy impact test at -5 ° C. The measured absorbed energy (vE _-5 ) and old γ grains The relationship with form was investigated.

FIG. 2 shows C: 0.03%, Si: 0.1%, Mn: 1.5%, Ni: 0.4%, Cr: 0.6%, Mo: 0.3%, Nb: 0.0. In the steel sheet (without tempering) containing 01%, Ti: 0.014%, B: 0.010%, KP value: 3.0, KV value: 0.01, and plate thickness: 80 mm Is a graph showing the relationship between the average equivalent circle diameter (horizontal axis) and the base material toughness (vE _-5 , vertical axis). Thus, it has been found that when the average equivalent circle diameter of the old γ grains is 100 μm or less, excellent base material toughness such as vE ₋₅ of 200 J or more can be secured. More preferably, the average equivalent circle diameter of the prior γ grains is 60 μm or less, and more preferably 40 μm or less. On the other hand, the lower limit of the average equivalent circle diameter of the old γ grains is preferably 3 μm. If the average equivalent circle diameter of the former γ grains is less than 3 μm, the hardenability is extremely lowered, and a large amount of alloying elements are required to obtain a bainite structure. Become. More preferably, it is 10 micrometers or more, More preferably, it is 20 micrometers or more.

  The average aspect ratio and the average equivalent circle diameter of the old γ grains are measured as follows. A test piece obtained by mirror-polishing a ¼ portion of the plate thickness is etched using an AGS solution manufactured by Yamamoto Scientific Tool Research Co., Ltd. or a 2% nitric acid-ethanol solution (2% nital solution). It is recommended that the etching conditions be 5 to 10 minutes at room temperature for the AGS solution and 5 to 30 seconds at room temperature for the 2% nital solution. The test piece after the etching treatment is observed at a magnification of 400 times using an optical microscope, and photographed. The obtained micrograph (observation visual field: 10 visual fields) is subjected to image analysis using “Image-Pro Plus” manufactured by Media Cybernetics, and the major diameter, minor diameter, and equivalent circle diameter of the former γ grains are measured.

  The average aspect ratio can be obtained by determining the aspect ratio (major axis / minor axis) of each old γ grain observed in the observation field and determining the average value of the aspect ratios. In addition, the equivalent circle diameter of the old γ grain is measured by image analysis of the area of each old γ grain, and the equivalent circle diameter of each old γ grain (the observed old γ grain is considered to be a perfect circle). (Diameter when done). The average equivalent circle diameter of the old γ grains is obtained by averaging the equivalent circle diameters of all the old γ grains observed in the observation field.

  Furthermore, in the steel plate of the present invention, 90% by volume or more of the steel structure is bainite in order to suppress a decrease in the base material strength (for example, the yield strength at 0.2% elongation) associated with the shape control of the old γ grains. That is, the refinement of the old γ grains in terms of securing the base material toughness results in a decrease in the strength of the steel sheet due to a decrease in hardenability, but this is avoided by using the steel structure as the main component.

  FIG. 3 shows C: 0.03%, Si: 0.1%, Mn: 1.5%, Ni: 0.4%, Cr: 0.6%, Mo: 0.3%, Nb: 0.0. In steel sheet (with no tempering) including 01%, Ti: 0.014%, B: 0.010%, KP value: 3.0, KV value: 0.01, plate thickness: 80 mm, It is a graph showing the relationship between a bainite fraction (horizontal axis) and the yield strength at the time of 0.2% elongation (vertical axis). Thus, by using 90% by volume or more of the steel structure as bainite, an excellent proof stress such as 430 MPa or more can be ensured, and further, the tensile strength can be set to 590 MPa or more and less than 790 MPa. In addition, the yield strength and tensile strength at the time of 0.2% elongation are values obtained by taking a JIS No. 4 test piece from a 1/4 thickness portion of the steel sheet and conducting a tensile test. A more preferable bainite fraction in the steel structure is 95% by volume or more, and more preferably 97% by volume or more.

  The bainite fraction in the steel structure was determined by conducting image analysis on the optical micrograph obtained when measuring the average aspect ratio and average equivalent circle diameter of the old γ grains as described above to obtain ferrite, pseudo All the lath-like structures other than polygonal ferrite and MA (Martensite Austenite Constituent) are regarded as bainite, and the area ratio of bainite is measured. Calculate as a value.

  In the high-tensile steel sheet of the present invention, the average volume fraction of the mixed structure (hereinafter sometimes referred to as “MA structure”) composed of martensite and austenite in the steel structure is preferably 3% or less. It is. As will be apparent from Examples described later, as a result of various studies conducted by the present inventors, it is clear that the toughness of the base material is further improved by suppressing the average volume fraction of the MA structure to 3% or less. Because it became. From the viewpoint of securing the toughness of the base material, the proportion of the MA structure can be tolerated up to 3%, but the proportion of the MA structure should be as small as possible, more preferably 2% or less. The most preferable ratio of the MA structure is 0%, but it is very difficult to make the amount of the MA structure generated to 0% in actual operation, and it is generated not a little.

  In order to suppress the formation of the MA structure to 3% or less, as will be described later, it is only necessary to quench with water cooling after hot rolling, and to suppress the formation of the MA structure by increasing the temperature-decreasing rate during cooling. .

  Thus, if the average volume fraction of the MA structure is suppressed to 3% or less, the toughness of the base material is remarkably improved, so that the amount of addition of Mo, Nb, etc. can be reduced (details will be described later). .

  It is preferable that the structure other than the MA structure is substantially a bainite structure. “Substantially” means that it is inevitably mixed with other structures that are inevitably generated, and basically represents an MA structure and a bainite structure.

  The average volume fraction of the MA structure in the steel structure is determined by observing the structure at a position where the depth from the steel sheet surface is t / 4 with an optical microscope, where the thickness of the steel sheet is t. Since the structure of the steel sheet changes greatly depending on the heating conditions and the cooling conditions, there is a slight variation in the proportion of the structure generated at the surface and the center of the steel sheet. Therefore, when the amount of MA structure generated in the surface portion of the steel sheet is compared with the amount of MA structure generated in the center portion, the amount of MA structure generated is relatively smaller in the surface portion than in the center portion. Therefore, in the present invention, the average volume fraction of the MA structure is determined by observing the structure at a position where the depth from the steel sheet surface is t / 4 and using this as a representative value. In addition, the test piece (specimen) used for evaluating the characteristics of the steel sheet is generally cut out from a position where the depth from the steel sheet surface is t / 4. It is appropriate that the structure composition at the position where the depth from the surface is t / 4 is the structure composition of the entire steel sheet.

  In the high-tensile steel plate of the present invention, it is preferable that the average equivalent circle diameter of the MA structure is 1 μm or less. This is because the toughness of the base material is further increased by making the MA structure finer. The average equivalent circle diameter of the MA structure is more preferably 0.7 μm or less.

  In order to control the average equivalent circle diameter of the MA structure to 1 μm or less, as described later, water cooling may be performed after hot rolling. This is because when the cooling rate is increased by water cooling and the temperature is lowered rapidly, the concentration of C into untransformed austenite during bainite transformation is relaxed, and the average equivalent circle diameter of the MA structure becomes relatively small.

  In the high-tensile steel sheet of the present invention, it is preferable that the average aspect ratio of the MA structure is 2.5 or less. This is because the shape of the MA structure affects the toughness of the base material, and the toughness of the base material is improved by making the MA structure spherical. A more preferable average aspect ratio of the MA structure is 2.2 or less.

  In order to control the average aspect ratio of the MA structure to 2.5 or less, it is only necessary to make the old γ grain size as small as possible. By reducing the old γ grain size, crystal growth of the MA structure is inhibited and the aspect ratio is reduced. Get smaller. Specific means for controlling the prior γ particle size will be described later.

  The average volume fraction, average equivalent circle diameter, and average aspect ratio of the MA structure are measured as follows. First, a test piece in which a part having a depth of t / 4 from the steel sheet surface is mirror-polished is etched using a corrosive solution, and the treated test piece is observed at a magnification of 1000 times using an optical microscope. And take a photo. For the etching treatment for observing the MA structure, solution A obtained by mixing ethanol (96% by mass) and picric acid (4% by mass), distilled water (99% by mass) and sodium metabisulfite (1 A corrosive liquid obtained by mixing 50 parts by mass of 60 parts by mass (A liquid: B liquid) of B liquid obtained by mixing (mass%) is used.

  Next, the obtained microphotograph (observation visual field: 10 visual fields) was subjected to image analysis using “Image-Pro Plus” manufactured by Media Cybernetics, etc., and the average area ratio, average equivalent circle diameter and average aspect ratio of MA tissue were analyzed. And the average area ratio is defined as the average volume ratio.

  Regarding the average equivalent circle diameter of the MA tissue, the area of each MA tissue is measured by image analysis, and the equivalent circle diameter of each MA tissue is obtained from the area. Next, the average equivalent circle diameter is obtained by averaging the equivalent circle diameters of all MA structures observed in the observation field. The equivalent circle diameter is a diameter when the observed MA structure is regarded as a perfect circle.

  The average aspect ratio of the MA structure can be obtained by calculating the aspect ratio (major axis / minor axis) of each MA structure observed in the observation visual field and determining the average value of the aspect ratios.

  Said steel structure is securable by manufacturing a steel plate by the method of a postscript using the steel of the chemical composition mentioned later.

  Furthermore, in the steel plate of this invention, the chemical composition is specified also from a viewpoint of ensuring the outstanding weldability (especially high heat input HAZ toughness and weld crack resistance). Specifically, after limiting C to extremely low C, Mn, Cr and Mo which are hardenability improving elements are positively added, and the KP value determined by the content of these quenching improving elements is appropriately controlled. At the same time, B is further added, and the addition of V and Nb, which are large heat input HAZ toughness reducing elements, is appropriately controlled as the KV value. When appropriate amounts of these components are added, the continuous cooling curve of bainite (see the CCT diagram in FIG. 4) moves to the short time side and the low temperature side, and the ferrite CCT wire moves to the long time side (from the solid line). Move to dashed line).

  For this reason, conventionally, in order to produce martensite at a high cooling rate and ferrite or pseudopolygonal ferrite at a low cooling rate, the hardness depends greatly on the cooling rate, and the hardness of the HAZ part is reduced during small heat input welding. (Improvement of weld crack resistance) and base material strength were not compatible, and preheating-free was difficult to achieve. According to the present invention, low-temperature transformation bainite was produced at both high and low cooling rates. As a result, the dependence of the hardness on the cooling rate was reduced, and both the reduction of the hardness of the HAZ part during welding (improvement of weld crack resistance) and the securing of the base metal strength were achieved.

  On the other hand, in the case of high heat input welding, since the cooling rate of HAZ is slow, conventionally ferrite or pseudopolygonal ferrite is generated, and a coarse and massive island-like martensite structure is formed accordingly, resulting in deterioration of HAZ toughness. However, in the present invention, low-temperature transformation bainite is generated even when the cooling rate is slow, so that it becomes a film-like martensite structure instead of a lump, and at the same time, the generated martensite structure becomes fine because it is extremely low C, HAZ toughness can be secured.

  In addition, as described above, the addition of the element having a very low C and a hardenability improving element is important in that the steel structure is mainly composed of bainite. Hereinafter, the chemical composition of the steel sheet of the present invention will be described.

C: 0.010 to 0.06%
C is an important element for achieving both the weld crack resistance of the HAZ part during welding and the strength of the base material and improving the high heat input HAZ toughness. If C exceeds 0.06%, martensite is generated instead of low-temperature transformation bainite on the high cooling rate side, and the weld crack resistance and high heat input HAZ toughness are not improved. Preferably it is 0.055% or less. In addition, if it is less than 0.010%, the necessary minimum base material strength cannot be obtained. Preferably it is 0.020% or more, More preferably, it is 0.030% or more.

Mn: 1.25 to 2.5%, Cr: 0.1 to 2.0%, Mo: 0.005 to 1.5%
These elements have the effect of improving the hardenability, make it easy to produce low-temperature transformation bainite at a high cooling rate to a low cooling rate, and as described above, make it extremely low C and simultaneously add a predetermined amount of B. Therefore, it is an important element for achieving both the weld cracking resistance of the HAZ part and securing the base metal strength at the time of small heat input welding and increasing the high heat input HAZ toughness.

  The contents of Mn, Cr, and Mo are required to be 1.25% or more, 0.1% or more, and 0.005% or more, respectively. If these contents are not satisfied, the desired hardenability improving effect is not exhibited and the base material strength is insufficient. Preferably, Mn: 1.3% or more, Cr: 0.3% or more, Mo: 0.05% or more. More preferably, Cr is more than 0.5%, and Mo is 0.1% or more. However, if the contents of Mn, Cr, and Mo exceed 2.5%, 2.0%, and 1.5%, respectively, the toughness of the base material decreases. Preferably, Mn is 2.2% or less, Cr is 1.5% or less, and Mo is 1.3% or less.

  Further, the KP value determined by the content of these elements needs to be 2.4 or more and 4.5 or less. If the KP value is less than 2.4, the above-described effect cannot be exhibited effectively, and pseudopolygonal ferrite and ferrite are easily generated, and a base material strength of 590 MPa or more cannot be obtained. Preferably it is 2.7 or more. However, if the KP value exceeds 4.5, the high heat input HAZ toughness decreases. Preferably it is 4.3 or less.

V: 0.04% or less (including 0%), Nb: 0.001 to 0.04% or less V has the effect of increasing hardenability and temper softening resistance with a small amount of addition. However, if added over 0.04%, the high heat input HAZ toughness decreases. V is preferably 0.03% or less. Nb is also added in a small amount to improve hardenability and contribute to improvement of the strength of the base material. Therefore, the amount of Nb added is 0.001% or more. Preferably it is 0.005% or more, More preferably, it is 0.006% or more. However, if the amount of Nb added exceeds 0.04%, the high heat input HAZ toughness decreases. Preferably it is Nb: 0.03% or less.

  Furthermore, the KV value determined by these elements needs to be 0.040 or less. This is because, as described above, when both of these elements are added in an excessive amount, the high heat input HAZ toughness is lowered. Preferably it is 0.035 or less.

B: 0.0006 to 0.005%
B is an element for improving hardenability and facilitates formation of low-temperature transformation bainite at a low cooling rate, and is extremely low C as described above, and at the same time, by adding an appropriate amount of Mn, Cr, Mo, at the time of small heat input welding. It exhibits the effect of increasing the weld crack resistance and the base metal strength of the HAZ part. However, if the amount of B is less than 0.0006%, the effect of improving hardenability is insufficient, and a satisfactory base material strength cannot be obtained. Preferably it is 0.0007% or more, More preferably, it is 0.0010% or more. However, if the amount of B exceeds 0.005%, the hardenability deteriorates and the base material strength is insufficient. Preferably it is 0.003% or less.

Ti: 0.005 to 0.03%
Ti is useful in that it forms nitrides with N to refine γ grains in the HAZ part during high heat input welding and contributes to improvement in HAZ toughness. However, if Ti exceeds 0.03%, the HAZ toughness decreases. Preferably it is 0.02% or less. If it is less than 0.005%, the effect of improving the high heat input HAZ toughness is not sufficient. Preferably it is 0.007% or more.

N: 0.0020 to 0.010%
As described above, N is useful in that it forms Ti and nitrides and contributes to the improvement of HAZ toughness during high heat input welding. However, N combines with B to reduce the solid solution B, inhibits the hardenability improvement effect of B, and also has the effect of lowering the toughness of the base metal and the high heat input HAZ toughness. When it exceeds 0.010%, the effect becomes remarkable. Preferably it is 0.008% or less. If it is less than 0.0020%, the effect of improving the high heat input HAZ toughness by forming a nitride with Ti is not sufficient. Preferably it is 0.0030% or more.

  Furthermore, in the present invention, it is recommended to positively add the following elements or to suppress the content thereof with the aim of further improving the weldability.

Ni: 5% or less Ni is an element useful for improving the toughness of the base metal, but if added over 5%, scale wrinkles are likely to occur, so the upper limit is preferably made 5%. More preferably, it is 4% or less.

Cu: 1.2% or less Cu is an element that improves the strength of the base metal by solid solution strengthening and precipitation strengthening and also has an effect of improving hardenability. However, if added over 1.2%, the high heat input HAZ toughness decreases, so it is preferable to keep it to 1.2% or less. More preferably, it is 1.0% or less.

  However, when the amount of Mn is in the range of 1.25 to 1.8%, it is possible to compensate for the decrease in the high heat input HAZ toughness due to Cu. Thermal HAZ toughness can be ensured. However, even in this case, if the Cu content exceeds 2.0%, the high heat input HAZ toughness decreases, so the upper limit is preferably made 2.0%. More preferably, it is 1.5% or less.

Ca: 0.005% or less Ca is an element having an effect of spheroidizing MnS and reducing the anisotropy of inclusions. In order to exert such an effect, it is preferable to add 0.0005% or more. More preferably, it is 0.0010% or more. However, if added in excess of 0.005%, the toughness of the base material decreases, so it is preferable to keep it to 0.005% or less. More preferably, it is 0.004% or less.

Si: 1% or less Si is an element useful as a deoxidizer, but if it exceeds 1%, weldability and base metal toughness are lowered, so it is preferable to keep the content to 1% or less. More preferably, it is 0.6% or less.

P: 0.020% or less, S: 0.010% or less P and S are harmful impurity elements that adversely affect physical properties such as toughness, P: 0.020% or less, S: 0.010% or less It is preferable that each is suppressed.

Al: with 0.2% or less Al is a deoxidizing element, is an element to improve the hardenability improving effect based on B by increasing the solid solution B by fixing the N, exceeds 0.2% Since the toughness of the base material decreases, the upper limit is preferably made 0.2%. More preferably, it is 0.1% or less.

  The chemical composition of the high-strength steel sheet of the present invention is as described above. However, when 90 volume% or more of the steel structure is bainite and the average volume fraction of the MA structure in the steel structure exceeds 3%, Mo The content and the Nb content are preferably Mo: 0.05 to 1.5% and Nb: 0.005 to 0.04%, respectively. This is because by adding a relatively large amount of Mo and Nb, an extreme decrease in YR is prevented and 0.2% proof stress is secured.

  On the other hand, when 90 volume% or more of the steel structure is bainite and the average volume fraction of the MA structure in the steel structure is 3% or less, the Mo content and the Nb content are respectively set to Mo: 0.005 to 1.5. % And Nb: 0.001 to 0.04% are preferable. This is because even if the addition amounts of Mo and Nb are reduced, 0.2% proof stress can be secured by reducing the MA structure.

Next, the manufacturing method of the steel plate which concerns on this invention is demonstrated. In the production method of the present invention, in addition to using steel satisfying the above chemical composition, it is necessary to strictly manage the hot rolling conditions, particularly when controlling the morphology of the old γ grains as described above. Specifically, when hot rolling is performed by heating from point A _{3 to} 1300 ° C., 50% or more of the total reduction amount, preferably 70% or more of the total reduction amount, is rolled in the partial recrystallization region. By utilizing the phenomenon of partial recrystallization in such an operation, the old γ grains in the steel sheet can be suppressed to the above-described form (average aspect ratio and average equivalent circle diameter).

Here, the partially recrystallized region is a steel plate test piece having a γ grain size of 100 ± 10 μm in the temperature region and is subjected to a reduction of 10 sec under the conditions of strain rate: 10 sec ⁻¹ and equivalent strain: 0.2. When the structure is later frozen (for example, water-cooled), the temperature range is 20 to 80% by volume of recrystallized grains. Since this partial recrystallization region varies depending on the chemical composition of the steel plate, it is only necessary to confirm by performing the above operation on a steel plate test piece having the same chemical composition as each steel plate before hot rolling.

  In order to produce the high-tensile steel sheet of the present invention, the total reduction ratio in the partial recrystallization region is important, and the cooling means and cooling conditions after hot rolling are not particularly limited, and may be air-cooled as usual.

  However, 90% by volume or more of the steel structure is a bainite structure, and in order to suppress the average volume ratio of the MA structure in the steel structure to 3% or less, the temperature is below the Bs point after the hot rolling. It is preferable to cool with water. This is because when the steel sheet after hot rolling is rapidly cooled by water cooling, the formation of the MA structure is suppressed, the average equivalent circle diameter of the structure is reduced, and the toughness of the base material is improved.

  Although the said water cooling conditions are not specifically limited, The said water cooling employ | adopted by this invention points out the cooling whose temperature-fall rate is 3 degrees C / sec or more. More preferably, the cooling rate during water cooling is 5 ° C./sec or more, and more preferably 10 ° C./sec or more.

Moreover, after the said hot rolling, you may cool to 200 degrees C or less, and you may then temper at the temperature below _Ac1 point as needed. For example, when higher base metal toughness is required, it is effective because the MA structure, which is a toughness inhibiting factor, can be decomposed into ferrite and cementite by the tempering.

  The cooling means for cooling to 200 ° C. is not particularly limited, and may be air-cooled as usual, but it is of course possible to suppress MA crystal growth by water-cooling instead of air-cooling.

  The other steps and conditions for producing the steel plate of the present invention are not particularly limited, and the usually used production steps and conditions (temperature, time, etc.) of the high-tensile steel plate may be adopted as appropriate. In the present invention, various properties such as low acoustic anisotropy, base material strength / toughness, and weldability can be secured with a non-tempered steel sheet not subjected to so-called tempering treatment. Therefore, the manufacturing process can be omitted and the production cost can be reduced.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

  Steels having chemical compositions shown in Tables 1 and 2 were melted by an ordinary melting method to form slabs, and then hot-rolled under the conditions shown in Tables 3 and 4 to produce evaluation steel plates having a predetermined thickness. Some steel plates were tempered at the temperatures shown in Tables 3 and 4 after being cooled to 200 ° C. after the hot rolling.

  The following measurements were performed on the steel sheet for evaluation thus obtained. The results are shown in Tables 5 and 6.

[Form of former γ grains and bainite fraction]
A test piece obtained by mirror-polishing a 1/4 thickness part of each steel plate was etched with a 2% nital solution, and the spot was observed at 400 times using an optical microscope and photographed. Image analysis was performed using “Image-Pro Plus” manufactured by Media Cybernetics, and the morphology (average aspect ratio and average equivalent circle diameter) and bainite fraction of the old γ grains in the steel structure were measured for the 10 visual fields. did. At this time, a lath-like structure other than ferrite, pseudo-polygonal ferrite and MA was regarded as bainite.

[Tensile strength (TS), yield strength at 0.2% elongation (0.2% yield strength), and yield ratio (YR)]
A JIS No. 4 test piece was taken from a 1/4 thickness portion of each steel plate and subjected to a tensile test to measure TS, 0.2% proof stress, and YR. Among these, regarding TS and 0.2% yield strength, 590 MPa ≦ TS <780 MPa and 0.2% yield strength ≧ 430 MPa were regarded as acceptable.

[Base material toughness]
JIS No. 4 test specimens were collected from a 1/4 thickness portion of each steel plate, and the absorbed energy (vE _-5 ) was measured by conducting a Charpy impact test at -5 ° C. vE _-5 ≧ 200 J was accepted.

[Acoustic anisotropy (ratio of sound speed of shear wave)]
The shear wave sound velocity ratio C _SL / C _SC was measured in accordance with JIS Z 3060. C _SL / C _SC ≦ 1.02 was accepted.

[HAZ toughness]
A HAZ reproduction test was conducted under the conditions of a maximum heating temperature of 1350 ° C. and Tc (cooling time of 800 to 500 ° C.) = 40 sec, or a maximum heating temperature of 1400 ° C. and Tc = 100 sec, and a JIS No. 4 specimen was collected after the test. Then, a Charpy impact test was conducted at −5 ° C. to determine the absorbed energy (vE ₋₅ ). Tc = 40 sec corresponds to a heat input of 5 kJ / mm, and Tc = 100 sec corresponds to a heat input of 15 kJ / mm. Also in that the HAZ reproduction test in any of the conditions were judged as acceptable vE _-5 ≧ 80 J.

  In Tables 3-6, steel plate No. The numbers after A or B are steel type Nos. In Tables 1 and 2. Means. In addition, “Tc = 40 sec” in the HAZ characteristics of Tables 5 and 6 is a result of measuring a test piece subjected to a HAZ reproduction test under the conditions of a maximum heating temperature of 1350 ° C. and Tc = 40 sec, and “Tc = 100 sec” It means the result of measurement on a test piece that was subjected to a HAZ reproduction test under the conditions of a maximum heating temperature of 1400 ° C. and Tc = 100 sec. In Table 6, “-” in the old γ grains means that the old γ grains were substantially unobservable.

From Tables 5 and 6, it can be considered as follows. Each steel plate shown in Table 5 has the composition, steel structure, and TS satisfying the requirements of the present invention, low acoustic anisotropy, and other various base material properties (0.2% proof stress, vE _-5 ). And weldability (HAZ characteristics) were both good.

  On the other hand, each steel plate shown in Table 6 is a comparative example lacking any of the requirements defined in the present invention, and has the following problems.

  Each steel sheet of B2-1, B2-2, B13-1, and B13-2 is manufactured under hot rolling conditions in which the amount of reduction in the partial recrystallization region falls below the range of the present invention, and B2-1 and B13 The steel sheet of -1 is an example in which the average aspect ratio of old γ grains is large, and the steel sheets of B2-2 and B13-2 are large in average circular equivalent diameter of old γ grains. The steel plate of B2-1 and B13-1 has a large shear wave sound velocity ratio and the low acoustic anisotropy is not reduced, and the steel plate of B2-2 and B13-2 has poor base material toughness.

  The steel plate of B20 is an example with a high C content, and the base metal toughness is inferior. The steel plate of B21 is an example having a low C content, and TS and 0.2% proof stress are inferior.

  The steel sheet of B22 has a low Mo content, the steel sheet of B23 has a low Mo content and Nb content, and the steel sheet of B28 has a low Cr content and a KP value, both of which have a low bainite fraction and are substantially free of old γ grains. Not observed. These steel plates are inferior in TS and 0.2% yield strength.

  The steel plate of B24 is an example in which the KV value is high, and the steel plate of B25 is a high V amount and KV value, and the base metal toughness is inferior.

  The steel plate of B26 is an example in which the Mn amount and the KP value are low, and TS and 0.2% proof stress are inferior.

  The B27 steel plate has a high KP value, the B29 steel plate has a low Mn amount, and has a high Mo amount and KP value. In these steel plates, TS is very high and the base metal toughness is inferior.

  The steel sheet of B30 is an example in which the Mn content, Cr content and KP value are low and the Cu content is high, and the base metal toughness is inferior.

  The steel sheet of B31 has a low Ti content, and the steel sheet of B32 has a low N content, and the HAZ toughness (in the case of Tc = 40 sec) is inferior.

  Steels having the chemical compositions shown in Tables 7 and 8 were melted by a normal melting method to form slabs, then hot-rolled under the conditions shown in Tables 9 and 10, and then the same as the cooling stop temperatures shown in Tables 9 and 10. The steel sheet for evaluation which consists of predetermined | prescribed board thickness was manufactured by cooling with the cooling rate shown to a table | surface.

  Some of the steel plates were cooled to 200 ° C. at the cooling rates shown in Tables 9 and 10 after the hot rolling, and then tempered at the temperatures shown in Tables 9 and 10.

  Moreover, about some steel plates, after the said hot rolling, it water-cools to the cooling stop temperature shown to Table 9 and 10 at the cooling rate shown to the same table, and then air-cools to 200 degrees C or less, Then, it shows to Table 9 and 10 Tempering was performed at temperature.

  Thus, about the obtained steel plate for evaluation, each measurement was performed similarly to the said Example 1. FIG. The results are shown in Tables 11 and 12.

  The average volume fraction, the average equivalent circle diameter and the average aspect ratio of the MA structure were measured by the following methods.

[Average volume fraction of MA texture, average equivalent circle diameter and average aspect ratio]
When the thickness of the steel sheet is t, a test piece obtained by mirror-polishing a portion having a depth of t / 4 from the steel sheet surface is etched using a corrosive solution, and the processed test piece is subjected to an optical microscope. Using magnification: observing at 1000 times and taking a picture. Etching is performed by mixing A solution obtained by mixing ethanol (96% by mass) and picric acid (4% by mass), distilled water (99% by mass) and sodium metabisulfite (1% by mass). Then, the corrosive liquid obtained by mixing the B liquid obtained by mixing at 50 parts by mass (60 parts by mass (A liquid: B liquid)) was used.

  Next, with respect to the obtained micrograph (observation visual field 10 visual fields), image analysis was performed using “Image-Pro Plus” manufactured by Media Cybernetics, etc., and the average area ratio, average equivalent circle diameter, average aspect ratio of MA tissue were analyzed. And the average area ratio was defined as the average volume ratio.

Moreover, about base material toughness, the absorbed energy (vE- ₁₀ ) was measured by extract | collecting a JIS4 test piece from the board thickness 1/4 site | part of each steel plate, and performing a Charpy impact test at _-10 degreeC. the vE _-10 ≧ 200J was passed.

  In Tables 9-12, steel plate No. The numbers after A or B are steel grade numbers in Tables 7 and 8. Means. In addition, “Tc = 40 sec” in the HAZ characteristics of Tables 11 and 12 is a result of measuring a test piece that was subjected to a HAZ reproduction test under the conditions of a maximum heating temperature of 1350 ° C. and Tc = 40 sec, and “Tc = 100 sec” It means the result of measurement on a test piece that was subjected to a HAZ reproduction test under the conditions of a maximum heating temperature of 1400 ° C. and Tc = 100 sec. “Unmeasureable” in the old γ grains in Table 12 means that the old γ grains could not be identified because ferrite or pseudopolygonal ferrite was generated.

From Tables 11 and 12, it can be considered as follows. Each steel plate shown in Table 11 has the composition, steel structure, and TS satisfying the requirements of the present invention, low acoustic anisotropy, other various base material properties (0.2% proof stress), and weldability. All of the (HAZ characteristics) were good. In particular, the base material toughness (vE _-10 ) is very excellent among the base material characteristics.

  On the other hand, each steel plate shown in Table 12 is a comparative example that does not satisfy the requirements of the present invention, and any one of the above specifications is inferior.

It is a graph which shows the relationship between the average aspect-ratio of the old gamma grain in a steel plate, and the transverse wave sound speed ratio of a steel plate. It is a graph which shows the relationship between the average equivalent circular diameter of the former gamma grain in a steel plate, and base material toughness. It is a graph which shows the relationship between the bainite fraction in a steel plate, and the yield strength at the time of 0.2% elongation of a steel plate. It is a typical CCT diagram for demonstrating the concept of the component design in this invention from the surface of weldability improvement.

Claims (14)

C: 0.010 to 0.06% (meaning mass%, the same shall apply hereinafter),
Mn: 1.25 to 2.5%,
Cr: 0.1 to 2.0%,
Mo: 0.005 to 1.5%,
V: 0.04% or less (including 0%),
Nb: 0.001 to 0.04%,
Ti: 0.005 to 0.03%,
B: 0.0006 to 0.005%,
N: 0.0020 to 0.010%
Made of steel that meets
2.4 ≦ KP ≦ 4.5
KV ≦ 0.040
As well as satisfying each
90% by volume or more of the steel structure is bainite,
The old γ grains have an average aspect ratio of 1.8 or less and an average equivalent circle diameter of 100 μm or less.
A high-tensile steel sheet having a tensile strength of 590 MPa or more and less than 780 MPa.
However,
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo]
KV = [V] + [Nb]
<< In formula, [] means content (mass%) of each element. >>   The high-tensile steel sheet according to claim 1, wherein an average volume fraction of a mixed structure (MA structure) composed of martensite and austenite in the steel structure is 3% or less.   The high-tensile steel sheet according to claim 2, wherein the average equivalent circle diameter of the MA structure is 1 µm or less.   The high-tensile steel sheet according to claim 2 or 3, wherein the average aspect ratio of the MA structure is 2.5 or less. Furthermore, Ni: 5% or less and / or Cu: 1.2% or less are contained, The high-tensile steel plate in any one of Claims 1-4. The high-tensile steel plate according to any one of claims 1 to 4,
When the Mn content is 1.25 to 1.8%, Cu: more than 1.2%, and a high-tensile steel sheet containing 2.0% or less. The high-tensile steel plate according to claim 6, further comprising Ni: 5% or less. The high-tensile steel plate according to any one of claims 1 to 7, further containing Ca: 0.005% or less. The high-tensile steel sheet according to any one of claims 1 to 8, which is suppressed to Si: 1% or less, P: 0.020% or less, S: 0.010% or less, and Al: 0.2% or less. The high-tensile steel plate according to any one of claims 1 to 9, wherein the plate thickness is 80 mm or more. A method for producing the high-tensile steel sheet according to any one of claims 1 to 10,
Upon heated to A _c3 point to 1300 ° C. performing hot rolling method for producing a high-tensile steel plate, characterized in that rolling at least 50 percent of the total rolling reduction at partial recrystallization region.
Here, the partially recrystallized region is a steel plate test piece having a γ grain size of 100 ± 10 μm in the temperature region, and is subjected to a reduction of 10 sec ^-1 and an equivalent strain of 0.2 for 10 sec. When the structure is frozen later, 20 to 80% by volume is a temperature range in which recrystallized grains are formed. The manufacturing method of Claim 11 which cools to 200 degreeC after the said hot rolling, and performs tempering at the temperature below _Ac1 point after that.   The manufacturing method of Claim 12 which performs the said cooling by water cooling.   The manufacturing method of Claim 11 which water-cools to the temperature below Bs point after the said hot rolling.

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