JP5825224B2 - High tensile steel sheet with excellent surface arrestability and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile steel plate having excellent surface arrestability and a method for producing the same.

  In each field of structural steel sheets, the trend toward thickening and high strength is remarkable with the increase in size of structures. Recently, guarantee of fracture safety at a high level has been demanded at the same time, and there is an increasing demand for thick steel plates that have both strength and fracture safety. In particular, for materials for transporting liquefied gas, application of high strength steel having a tensile strength (TS) of 720 to 950 MPa has begun, for example.

  Moreover, the steel plate used for a liquefied gas carrier ship needs to be equipped with the high brittle cracking resistance on the use. That is, from the viewpoint of suppressing the occurrence of brittle cracks, it is necessary to increase the toughness of the weld heat affected zone (hereinafter abbreviated as “HAZ”) and to improve the base metal arrest characteristics from the viewpoint of stopping the propagation of brittle cracks. . The drop weight test is the most common and simple method used for welding construction tests by the classification society as an indicator of the base metal arrest characteristics, but as a high-strength steel manufacturing method, Therefore, it is difficult to ensure sufficient drop weight test characteristics.

  Among the liquefied gas carriers used for transporting liquefied gas, the LPG ship with the most construction results is used not only for transporting LPG but also for transporting various liquefied gases such as ammonia and vinyl chloride. When an LPG ship is used as a multi-purpose ship and is also loaded with anhydrous ammonia, it is necessary to pay attention to sulfide stress corrosion cracking (hereinafter abbreviated as “SSC”). SSC is considered to be a type of hydrogen embrittlement cracking that occurs because a large amount of hydrogen generated by a corrosion reaction penetrates into steel due to the presence of hydrogen sulfide. The ease of occurrence of SSC in steel (hereinafter abbreviated as “SSC sensitivity”) is affected by its chemical composition, microstructure, and the like. It is well known that it is effective to contain Ni in the steel to improve the low temperature toughness of the steel. However, as described in Non-Patent Document 1, when Ni is contained, active path cracking occurs. (Active Pass Corrosion: APC) mechanism promotes cracking and SSC resistance is degraded. For this reason, when using a steel plate in an environment where SSC should be considered, Ni cannot be contained for improving the low temperature toughness of the steel.

  In order to solve this problem, Patent Document 1 discloses that steel is alloyed and B is contained. Here, it is said that the strength of steel (hereinafter abbreviated as “base material strength”) can be secured, the increase in hardness of HAZ can be suppressed, and the SSC sensitivity of steel can be suppressed.

  In addition, Patent Document 2 discloses that the hardenability is reduced by reducing C without containing B to prevent the hardening of HAZ, while Nb is contained and the base material is utilized by utilizing its precipitation hardening action. An invention that compensates for the lack of strength is disclosed.

  Patent Document 3 discloses an invention of a high-tensile steel plate having high arrest characteristics that can be used as a large structural member by adding Nb without containing Ti. Patent Document 4 discloses a method for producing a steel sheet having sufficient drop weight test characteristics such that the NDT temperature (nil ductility transition temperature) is −70 ° C. or less, paying attention to the arrest characteristics of the surface layer. . Patent Document 5 discloses a method for producing a high-tensile steel plate having excellent HAZ toughness and SSC resistance.

JP 55-76044 A Japanese Patent Laid-Open No. 2-8322 JP 2000-45021 A JP-A-9-104948 JP 2012-92377 A

Yasuyoshi Yamane et al., Stress Corrosion Cracking Behavior of Low Alloy Steels in Sulfide Environment, Kawasaki Steel Technical Report, Vol. 17 (1985) No. 2, pp. 178-184

  For steel plates used in LPG carriers, in order to meet demands for capacity expansion and safety improvements, the use of high strength steel with a tensile strength (TS) of about 720 to 950 MPa and a plate thickness of 50 mm or more is considered. Has been.

  However, the steels described in Patent Documents 1 and 2 both have a tensile strength (TS) of about 580 to 700 MPa and cannot meet the above requirements.

  Further, materials such as LPG carriers and pressure vessels are required to improve the arrest characteristics of the base material from the viewpoint of stopping the propagation of brittle cracks. NRL (Naval Research Laboratory) drop weight test is the most common and simple method for estimating the arrestability of the base metal, and is an alternative to the Charpy impact test for welding construction tests by the classification society. Sometimes it is done.

  The high-strength steel disclosed in Patent Document 3 has sufficient properties in strength, toughness, and arrestability, but since an off-line quenching / tempering method is selected, it is an important surface layer in the NRL drop weight test. It is difficult to ensure the arrestability. Moreover, in the invention disclosed in Patent Document 4, TS is as low as about 490 MPa, and the plate thickness is 16 mm to 40 mm. Furthermore, since the invention disclosed in Patent Document 5 is not an invention that focuses on the arrestability of the surface layer, it cannot be said that a method of manufacturing a steel sheet that is excellent in the arrestability of the surface layer is sufficiently disclosed.

  In view of such circumstances, the present invention has a high strength of TS of 720 MPa or more and high toughness without using Ni which is harmful to sulfide stress corrosion cracking. An object is to provide a high-tensile steel sheet having excellent properties at low cost.

  In order to solve the above problems, the present inventors have made various studies, and as a result, have obtained the findings shown in the following (a) to (c).

  (a) In order to improve the HAZ toughness of the high-tensile steel sheet, the hardenability is optimized to make the HAZ crystal structure a mixed structure composed of martensite and bainite, and the C content is reduced to reduce the island-shaped martens. It is important to suppress the generation of sites. However, when the C content is reduced, it is necessary to add a large amount of alloy elements to ensure the strength, which is not preferable from the viewpoint of manufacturing cost. In the present invention, the C content is increased in order to minimize the Mo content, which is particularly expensive.

  The HAZ toughness that deteriorates as the C content increases can be improved by reducing the Si content. If the Si content is reduced, C is easily dissolved in the base material, and as a result, formation of island martensite can be suppressed and the low temperature toughness of the HAZ can be improved.

  (b) Brittle fracture occurs in the steel sheet when the local stress at the tip of the crack is equal to or greater than the critical microscopic fracture stress, which is a characteristic of the steel sheet. Therefore, in order to improve the fracture resistance of the steel plate, the critical fracture stress of the steel plate may be improved or the local stress at the crack tip may be reduced.

  First, in order to improve the critical fracture stress of a steel sheet, it is effective to refine the metal structure of the steel sheet, and the critical fracture stress of the steel sheet increases as the crystal grains are refined. For this purpose, there is a method of increasing the hardenability of the steel sheet and reducing the grain size on the surface of the steel sheet. However, in order to improve the hardenability, it is necessary to add many alloy elements, resulting in an increase in cost.

  Next, as a method of reducing the local stress at the crack tip, it is generally known to use separation. Separation is a vertical crack that occurs in the thickness direction of a steel sheet in which a texture has developed. When this separation occurs, the crack tip restraint is released, and as a result, the local stress is reduced, so that brittle fracture is less likely to occur.

  (c) In other words, it is necessary to improve the brittle fracture characteristics by sufficiently developing the texture while refining the structure of the steel sheet. As a result of repeated studies, the inventors can obtain a steel sheet with a sufficiently developed texture if rolling is performed so that the average ferrite crystal grain size in the thickness direction within 1 mm of the steel sheet surface is 10 μm or less. It was found that the NDT temperature of the steel sheet surface layer can achieve less than -50 ° C.

At this time, if the temperature at the time of rolling is significantly lower than the Ar ₃ point, the ferrite phase starts to be excessively precipitated on the surface layer of the steel sheet, and the critical fracture stress of the steel sheet is lowered, so that the drop weight test characteristics are deteriorated. Therefore, the area within 1 mm from the surface layer parallel to the rolling direction of the steel sheet may have a mixed structure in which the area ratio of the bainite phase is 85 to 99% and the area ratio of the ferrite phase is 1% to 15%. Such a mixed structure can be obtained, for example, by completing rolling at a finishing temperature of (Ar ₃ points + 30 ° C.) or lower and water cooling from a temperature of (Ar ₃ points−30 ° C.) or higher.

  Further, as described above, a mixed structure of ferrite and bainite is effective in the surface layer, but a mixed structure of bainite and martensite is preferably used in the steel sheet to ensure both strength and toughness. That is, in order to achieve the target strength and toughness inside the steel sheet, the region between the (1/4) t position and the (1/2) t position of the sheet thickness t of the steel sheet is a mixture of bainite and martensite. Further, the ratio Pr / Pt of the crystal grain at the (1/4) t position of the thickness t of the steel sheet to the grain size Pt in the thickness direction of the rolling direction is 4 or more. Good.

  The high-tensile steel plate according to the present invention has been completed based on such knowledge, and the following high-strength steel plate shown in (1) to (3) and the method for producing a high-tensile steel plate shown in (4) The gist. Hereinafter, they are referred to as “present invention (1)” to “present invention (4)”, respectively. The present invention (1) to the present invention (4) may be collectively referred to as “the present invention”.

(1) By mass%, C: 0.10 to 0.20%, Si: 0.02 to 0.25%, Mn: 0.7 to 1.6%, P: 0.02% or less, S: 0.008% or less, Cr: 0.5-1.2%, Mo: 0.1% or more and less than 0.3%, Ti: 0.004-0.025%, B: 0.0005-0. A steel sheet containing 003%, Al: 0.01 to 0.05% and N: 0.01% or less, the balance having a chemical composition consisting of Fe and impurities,
The region within 1 mm from the surface layer parallel to the rolling direction of the steel sheet is composed of a mixed structure in which the area ratio of the bainite phase is 85 to 99% and the area ratio of the ferrite phase is 1% to 15%, and is parallel to the rolling direction of the steel sheet. The average ferrite crystal grain size in the thickness direction in the region within 1 mm from the surface layer is 10 μm or less, and
The region between the (1/4) t position and the (1/2) t position of the sheet thickness t of the steel sheet is composed of a mixed structure of bainite and martensite, and further, (1/4) of the sheet thickness t of the steel sheet. A high-tensile steel sheet, wherein a ratio Pr / Pt of a grain size Pr in a rolling direction to a grain size Pt in a plate thickness direction is 4 or more.

  (2) The high-strength steel sheet according to (1) above, further containing one or both of V: 0.1% or less and Nb: 0.02% or less by mass%.

  (3) The high-strength steel sheet according to (1) or (2) above, further containing Sn: 0.50% or less by mass%.

(4) The method for producing a high-tensile steel plate according to any one of (1) to (3) above, comprising the following steps (i) to (iii):
(i) a step of heating the slab having the chemical composition of any one of (1) to (3) to a temperature of 1000 to 1200 ° C. and soaking in the temperature range;
(ii) After descaling and rolling, rolling is completed at a finishing temperature of (Ar ₃ points + 30 ° C.) or less (excluding (Ar ₃ points + 30 ° C.)) , and (Ar ₃ points −30 ° C.) Water cooling from the above temperature to 350 ° C. or less at a cooling rate of 5 ° C./sec or more, and
(iii) A step of reheating and cooling to a temperature of 500 ° C. or more and Ac ₁ point or less.

  According to the present invention, TS has a high strength of not less than 720 MPa and high toughness without using Ni harmful to sulfide stress corrosion cracking, and has excellent low temperature toughness in HAZ and excellent arrestability of the base material surface layer. Tensile steel sheets can be provided at low cost.

  Below, each requirement of this invention is demonstrated in detail. Here, “%” representing the chemical composition means “mass%” unless otherwise specified.

(A) About high-tensile chemical composition C: 0.10 to 0.20%
C has the effect of increasing the strength of the steel and improving the hardenability of the steel, so it is necessary to contain 0.10% or more. On the other hand, if the C content is excessive, the hardness of the HAZ becomes excessively high and the amount of island martensite generated increases. For this reason, low temperature joint toughness is impaired. In order to avoid this, the C content needs to be 0.20% or less. Therefore, the C content is 0.10 to 0.20%. The lower limit of the desirable C content is 0.11%, and the upper limit of the desirable C content is 0.15%.

Si: 0.02-0.25%
Si is useful as a deoxidizing element and also has an effect of increasing the strength of steel, so it is necessary to contain 0.02% or more. On the other hand, since it is necessary to avoid Si solid solution in the base material in order to suppress the formation of island martensite, the Si content needs to be limited to 0.25% or less. Therefore, the Si content is 0.02 to 0.25%. The preferable lower limit of the Si content is 0.03%, and the preferable upper limit is 0.20%.

Mn: 0.7 to 1.6%
Mn is an element indispensable for enhancing the hardenability and ensuring the strength and toughness of the steel, and it is necessary to contain 0.7% or more. However, when the Mn content exceeds 1.6%, the deterioration of the low temperature joint toughness becomes remarkable. Therefore, the Mn content is set to 0.7 to 1.6%. The minimum with preferable Mn content is 0.9%, and a preferable upper limit is 1.4%.

P: 0.02% or less, S: 0.008% or less Each of P and S is an element mixed in the steel sheet as an impurity, and it is desirable that it is as low as possible from the viewpoint of toughness. When considering the balance between securing toughness and economy, the P content is acceptable up to 0.02%, and the S content is acceptable up to 0.008%.

Cr: 0.5 to 1.2%
Cr has the effect of improving the hardenability of steel and greatly improving the strength and toughness, so it is necessary to contain 0.5% or more. However, if its content exceeds 1.2%, the toughness of the joint, particularly the low temperature toughness, deteriorates. Therefore, the Cr content is 0.5 to 1.2%. The minimum with preferable Cr content is 0.6%, and a preferable upper limit is 1.1%.

Mo: 0.1% or more and less than 0.3% Mo, like Mn and Cr, has the effect of improving strength and toughness, so it is contained in an amount of 0.1% or more. However, if the Mo content is 0.3% or more, the HAZ toughness is deteriorated, and the cost increase becomes remarkable. Therefore, the Mo content is 0.1% or more and less than 0.3%. The minimum with preferable Mo content is 0.15%, and a preferable upper limit is 0.28%.

Ti: 0.004 to 0.025%
Ti can suppress the coarsening of the austenite grain size at the time of slab heating and can improve the base material toughness, so it is necessary to contain 0.004% or more. However, if it exceeds 0.025%, not only does the effect reach its peak, but carbides are produced and the HAZ toughness deteriorates. Therefore, the Ti content is 0.004 to 0.025% or less. The preferable lower limit of the Ti content is 0.01%, and the preferable upper limit is 0.02%.

B: 0.0005 to 0.003%
Since B can improve the hardenability by adding a small amount, it is necessary to contain B in an amount of 0.0005% or more. However, when the content exceeds 0.003%, the weldability is remarkably deteriorated. Therefore, the B content is set to 0.0005 to 0.003%. The minimum with preferable B content is 0.0010%, and a preferable upper limit is 0.0020%.

Al: 0.01 to 0.05%
Al is effective for deoxidation and also effective for forming nitrides and making crystal grains finer. Therefore, Al is contained in an amount of 0.01% or more. However, if it exceeds 0.05%, the effect is saturated. Therefore, the Al content is set to 0.01 to 0.05%. The minimum with preferable Al content is 0.015%, and a preferable upper limit is 0.04%. In addition, Al content means content of acid-soluble Al (sol.Al).

N: 0.01% or less N is an element mixed in the steel sheet as an impurity, and reacts with B to generate BN. Therefore, although the content of N is preferably low, it is acceptable up to 0.01%.

  The high-tensile steel plate according to the present invention has the above-described chemical composition, and the balance is made of Fe and impurities. Here, an impurity means the component mixed by various factors of a manufacturing process including raw materials, such as an ore and a scrap, when manufacturing a steel plate industrially.

  The high-tensile steel plate according to the present invention may contain at least one of V, Nb and Sn in addition to the above elements as follows. The purpose of inclusion of each element and the range of the preferred content are as follows.

V: 0.1% or less V has an effect of suppressing a reduction in strength when the steel sheet is tempered, and may be contained as necessary. However, if the content exceeds 0.1%, weldability is deteriorated. Therefore, when V is contained, the content is made 0.1% or less. When it is desired to obtain the above effect, it is preferable to contain V by 0.01% or more.

Nb: 0.02% or less Nb, like V, has the effect of suppressing strength reduction during tempering, and is necessary because it has the effect of improving the toughness by refining crystals during rolling. You may make it contain according to. However, when the content exceeds 0.02%, deterioration of weldability and HAZ toughness is caused. Therefore, when Nb is contained, the content is set to 0.02% or less. In order to obtain the above effect, it is preferable to contain Nb in an amount of 0.005% or more.

Sn: 0.50% or less Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly to reduce the Fe ^3+, by having an effect of reducing Fe ³⁺ concentration as oxidizing agent, since inhibit corrosion promoting effect of Fe ^3+, thereby improving the weather resistance in high airborne salt environments. For this reason, you may make it contain as needed. However, when the content exceeds 0.50%, these effects are saturated. Therefore, when it contains Sn, the content shall be 0.50% or less. When it is desired to obtain the above effects, it is preferable to contain 0.03% or more of Sn.

(B) About structure of high-tensile steel sheet (B-1) About structure of region within 1 mm from surface layer parallel to rolling direction of steel sheet NDT temperature of steel sheet surface layer can achieve less than −50 ° C. and drop weight test characteristics In order to improve the brittle fracture characteristics, it is necessary to sufficiently develop the texture while reducing the structure of the steel sheet in order to obtain a high-tensile steel sheet with excellent arrestability on the surface layer of the base material . Specifically, in a region within 1 mm from the surface layer parallel to the rolling direction of the steel sheet, the average ferrite crystal grain size in the sheet thickness direction is 10 μm or less, and the area ratio of the bainite phase is 85 to 99%. It is necessary to make a mixed structure having an area ratio of 1% to 15%.

In order to obtain such a structure, the rolling step in the manufacturing process is important. If the rolling temperature is too high, the texture does not develop sufficiently, and the surface layer structure is not sufficiently refined, so that desired drop weight test characteristics cannot be obtained. On the other hand, if the water cooling start temperature is too low, the ferrite phase begins to be excessively precipitated on the surface layer of the steel sheet, and the critical fracture stress of the steel sheet is lowered. Specifically, such a structure can be obtained by completing rolling at a finishing temperature of (Ar ₃ points + 30 ° C.) or lower and subsequently cooling with water from a temperature of (Ar ₃ points−30 ° C.) or higher.

  The average ferrite crystal grain size in the plate thickness direction in the region within 1 mm from the surface layer parallel to the rolling direction of the steel plate is obtained by cutting and grinding the steel plate so that the surface parallel to the rolling direction and the plate thickness direction of the steel plate is exposed. , Nittal (2-5% (volume fraction) of nitric acid ethanol solution) or an appropriate corrosive solution is used to reveal the ferrite grain boundary and take a microscopic observation photograph of the area within 1 mm from the surface layer. It can be determined by measuring the average line segment length per crystal grain of a crystal grain intersecting with an arbitrarily drawn straight line.

(B-2) About the structure of the steel plate in the thickness direction The structure inside the steel plate greatly contributes to the strength and toughness of the steel plate. Therefore, in order to ensure strength and toughness, a bainite structure having high toughness and a martensite structure having high strength characteristics are combined inside the steel plate. Specifically, a mixed structure of a bainite structure and a martensite structure is used in a region between the (1/4) t position and the (1/2) t position of the sheet thickness t. Other structures may also be present in the steel, but should be kept to 5% or less. In terms of area ratio, it is preferable to have a mixed structure comprising a bainite structure of 70 to 90% and a martensite structure of 10 to 30%.

  Furthermore, in order to ensure toughness, in the present invention, the ratio Pr / Pt of the crystal grain at the (1/4) t position of the sheet thickness t of the steel sheet to the grain size Pt in the sheet thickness direction of the grain size Pr in the rolling direction. Needs to be 4 or more.

  Here, the ratio Pr / Pt of the grain at the (1/4) t position of the plate thickness of the steel plate to the particle size Pt in the plate thickness direction of the particle size Pr in the rolling direction is determined in the rolling direction and the plate thickness direction of the steel plate. Cut out the steel sheet so that the parallel planes are exposed, take a microstructural observation photograph in the vicinity of (1/4) t of the thickness t of the steel sheet, select 10 arbitrary crystal grains in the field of view, and select each crystal grain The ratio may be calculated from and averaged.

(C) About the manufacturing method of a high strength steel plate The above high strength steel plates can be manufactured through the following processes, for example. Except for the steps in which conditions are specified, normal conditions can be adopted.

(C-1) Preparation of Slab First, a slab (steel ingot) having the above-described chemical composition is prepared. The manufacturing method of slab is not ask | required. For example, it may be manufactured by a continuous casting method.

(C-2) Heating process It is good to heat a slab at 1000-1200 degreeC, and to soak in the temperature range. When the heating temperature is 1000 ° C. or higher, the alloy element is sufficiently dissolved, so that the strength can be ensured. And since a crystal grain becomes fine when heating temperature is 1200 degrees C or less, toughness is securable. The soaking is preferably performed until the temperature difference between the steel sheet surface temperature and the steel sheet inside is 40 ° C. or less. The soaking time should be sufficient to satisfy the above heating conditions according to the equipment specifications.

(C-3) Rolling process Before rolling, the slab is preferably descaled. Since the slab satisfying the chemical composition defined in the present invention has a low Si content of 0.25% or less, the growth of the scale is fast, and the steel sheet surface flaw due to the scale tends to occur during rolling. Therefore, the slab extracted from the heating furnace is preferably descaled before rolling. Descaling is preferably performed for each rolling pass.

After rolling, quenching is performed directly by accelerated cooling equipment. If the rolling finishing temperature is too high, the rolled structure on the surface of the steel sheet is insufficient, so that good surface layer arrest characteristics cannot be obtained. Therefore, it is important to complete the rolling at a finishing temperature of (Ar ₃ points + 30 ° C.) or less. The rolling completion temperature is (Ar ₃ points−30 ° C.) or higher because of the relationship with the subsequent water cooling. A preferable rolling completion temperature is Ar ₃ or more.

The water cooling start temperature is set to (Ar ₃ points-30 ° C.) or higher. When the water cooling start temperature is set to (Ar ₃ points-30 ° C.) or higher, a uniform quenched structure is easily obtained, and ferrite does not excessively precipitate on the steel sheet surface layer, so that the surface layer arrest characteristic can be ensured. From the relationship with the rolling finish temperature in the previous step, the water cooling start temperature does not exceed (Ar ₃ points + 30 ° C.), but if the water cooling start temperature exceeds (Ar ₃ points + 30 ° C.), the crystal There is a risk of grain coarsening.

  At this time, the cooling is performed to 350 ° C. or lower at a cooling rate of 5 ° C./sec or higher. In particular, in order to obtain a uniform quenched structure, it is preferable to cool to a temperature of 200 ° C. or lower at a cooling rate of 5 ° C./sec or higher.

(C-4) Tempering step After quenching, tempering is performed as the final step. Specifically, it is reheated to a temperature of 500 ° C. or higher and Ac ₁ point or lower and cooled. By setting the tempering temperature to 500 ° C. or higher, a sufficient tempering effect can be obtained and low temperature toughness can be ensured. However, if the tempering temperature exceeds ₁ Ac, the steel sheet having the chemical composition of the present invention may not be able to ensure the strength of HT720 grade. The steel plate after tempering may be air-cooled. Although further high toughness can be obtained by performing water cooling after tempering, the steel of the present invention can ensure sufficient toughness even with air cooling.

In addition, Ar ₃ point (° C.), Ac ₃ point (° C.) and Ac ₁ point (° C.) are calculated from the chemical composition and thickness t (mm) of the steel sheet according to the following equations, respectively.
Ar ₃ points = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo + 0.35 (t-8)
Ac ₃ points = 910.7-295.7C + 61.2Si-30.3Mn + 333.1P-27.1Cu-2.6Cr-27.5Ni-1.8Mo + 70.9V
Ac ₁ point = 712 + 20.1Si-17.8Mn-19.8Mo + 11.9Cr-19.1Ni
(Here, the element symbol in each formula means the content (mass%) of each element.)

  Steel having the chemical composition shown in Table 1 was melted in a laboratory vacuum melting furnace, the obtained 180 kg slab was rolled with a small rolling mill, and the obtained steel plate was heat-treated to give a test material (steel No. 0.1-33).

  Table 2 shows the manufacturing conditions for each test material.

  In any specimen, after the slab was heated, soaking was performed for 60 minutes at the slab heating temperature shown in Table 2. While performing descaling in each rolling pass, the sheet was rolled to a thickness of 50 to 60 mm, then directly quenched, tempered and allowed to cool to the atmosphere.

  Tensile test pieces were sampled in the direction perpendicular to the rolling direction from the (1/4) t position of the thickness t of each of the obtained test materials (steel plates No. 1 to 33) and subjected to a tensile test. The results are shown in Table 3 as “base material YS (MPa) and base material TS (MPa)”.

  Further, from the (1/4) t position of the plate thickness t of each obtained test material, a JIS No. 4 test piece was taken in a direction parallel to the rolling direction, and the ductility-brittle transition temperature (vTrs) was measured. The results are shown in Table 3 as “base material vTrs (° C.)”.

  Moreover, the NRL drop weight test piece was extract | collected from the surface vicinity of each test material, and NDT temperature was calculated | required. The results are shown in Table 3 as “NDT temperature (° C.)”.

In addition, a welded joint using each specimen as a base material was produced by submerged arc welding with a heat input of 36 kJ / cm, and the notch position was set to F.D. L. A Charpy test was conducted. The results are shown in Table 3 as “HAZ vE- ₅₀ (J)”.

  In addition, a test piece was collected from the vicinity of the surface of each specimen, and after polishing the surface parallel to the rolling direction and the plate thickness direction, it was corroded with nital (2-5% (volume fraction) nitric acid ethanol solution), Tissue observation was performed. The exposed ferrite grain boundaries were observed with an optical microscope, and the average line segment length of the crystal grains intersecting the straight line drawn in the thickness direction was measured. The results are shown in Table 3 as “average particle diameter (μm) within 1 mm of steel sheet surface layer”.

In addition, from the test piece corrosion-treated with the above-mentioned nital, the microstructure within the surface layer of 1 mm was observed, and the area ratio of bainite and ferrite was measured. The area ratio was determined by the following calculation formula.
Area ratio of bainite (%) = (area of bainite phase) / (area of entire field of view) × 100
Area ratio of ferrite (%) = (area of ferrite phase) / (area of entire field of view) × 100

  In addition, specimens were also taken from the (1/4) t position and (1/2) t position of the thickness t of each specimen, and similarly corroded using nital, and the thickness (1 / 4) It was confirmed that the region between the t position and the (1/2) t position was a mixed structure of bainite (B) and martensite (M). The results are shown in Table 3 as “structure in the region between the (1/4) t position and the (1/2) t position” of the thickness t of the steel sheet.

  Furthermore, using the microstructural observation photograph at the (1/4) t position of the thickness t of each test material, the crystal grains at the (1/4) t position of the thickness t of the steel sheet, and the grains in the rolling direction The ratio Pr / Pt of the diameter Pr to the particle diameter Pt in the plate thickness direction was measured. At this time, ten arbitrary crystal grains in the field of view were selected as the grain size of the crystal grains, and the average value of the ratios of the respective grain sizes was calculated. The results are shown in Table 3 as “ratio Pr / Pt of the grain size Pr in the rolling direction to the grain size Pt in the thickness direction of the crystal grains at the (1/4) t position of the thickness t of the steel plate”.

The target values of the base material strength are the yield point (YS) of 620 MPa or more, the tensile strength (TS) of 720 MPa or more, the base material toughness target value of vTrs of −50 ° C. or less, and the NDT temperature target of The target value of less than −50 ° C. and HAZ toughness is the absorbed energy vE ₋₅₀ at −50 ° C. is 50 J or more.

  From Table 3, steel plate No. satisfying the conditions defined in the present invention. In all of Nos. 1 to 20, the strength and toughness of the base material, the NDT temperature, and the HAZ toughness all satisfied the target values.

  On the other hand, steel plate No. In No. 21, the C content was excessive, and the toughness of the base metal and HAZ did not meet the target. Steel plate No. In No. 22, the Si content was excessive and the HAZ toughness was insufficient. Steel plate No. In No. 23, the Mn content was excessive, and the toughness of the base metal and HAZ did not meet the target. Steel plate No. In No. 24, the Cr content was excessive and the HAZ toughness was insufficient. Steel plate No. In No. 25, the Mo content was excessive and the HAZ toughness was insufficient. Steel plate No. In No. 26, the Ti content was excessive and the base metal toughness was insufficient. Steel plate No. In No. 27, the B content was excessive and the HAZ toughness was insufficient. Steel plate No. In No. 28, the Al content was excessive and the base metal toughness was insufficient.

  And steel plate No. No. 29 satisfies the range of the chemical composition specified in the present invention, but the water cooling start temperature is low, and the mixing ratio of the bainite phase and the ferrite phase within 1 mm of the steel surface layer and the ferrite average particle size are outside the range specified in the present invention. NDT temperature did not reach the target. Steel plate No. 30 satisfies the range of the chemical composition specified in the present invention, but the rolling completion temperature is high, and the mixing ratio of the bainite phase and the ferrite phase within 1 mm of the steel surface layer and the ferrite average particle size are outside the range specified in the present invention. NDT temperature did not reach the target. Steel plate No. No. 31 satisfied the range of the chemical composition specified in the present invention, but the water cooling start temperature was low, and the mixing ratio of the bainite phase and the ferrite phase within 1 mm of the steel surface layer and the ferrite average particle size were outside the range specified in the present invention. Therefore, the yield point was low and the NDT temperature did not reach the target. Steel plate No. 32 satisfies the range of the chemical composition defined in the present invention, but the cooling rate by water cooling is low, and the mixing ratio of the bainite phase and the ferrite phase within 1 mm of the steel surface layer is outside the range defined in the present invention. The goal was not reached. Steel plate No. 33 satisfies the range of the chemical composition defined in the present invention, but the slab heating temperature is low, and the structure in the region from the (1/4) t position to the (1/2) t position of the sheet thickness t of the steel sheet is Since it became a mixed structure of ferrite and bainite, the base metal strength (yield point and tensile strength) did not reach the target.

  According to the present invention, TS has a high strength of not less than 720 MPa and high toughness without using Ni harmful to sulfide stress corrosion cracking, and has excellent low temperature toughness in HAZ and excellent arrestability of the base material surface layer. Tensile steel sheets can be provided at low cost.

Claims (4)

In mass%, C: 0.10 to 0.20%, Si: 0.02 to 0.25%, Mn: 0.7 to 1.6%, P: 0.02% or less, S: 0.008 % Or less, Cr: 0.5 to 1.2%, Mo: 0.1% or more and less than 0.3%, Ti: 0.004 to 0.025%, B: 0.0005 to 0.003%, A steel plate containing Al: 0.01 to 0.05% and N: 0.01% or less, the balance being a chemical composition comprising Fe and impurities,
The area within 1 mm from the surface layer parallel to the rolling direction of the steel sheet is composed of a mixed structure in which the area ratio of the bainite phase is 85 to 99% and the area ratio of the ferrite phase is 1% to 15%.
And the average ferrite crystal grain size in the plate thickness direction in the region within 1 mm from the surface layer parallel to the rolling direction of the steel plate is 10 μm or less, and
The region between the (1/4) t position and the (1/2) t position of the sheet thickness t of the steel sheet is composed of a mixed structure of bainite and martensite.
A high-tensile steel plate characterized in that the ratio Pr / Pt of the grain size Pr in the rolling direction with respect to the grain size Pt in the plate thickness direction at the (1/4) t position of the steel plate thickness t is 4 or more. .   The high-tensile steel sheet according to claim 1, further comprising one or both of V: 0.1% or less and Nb: 0.02% or less in mass%.   The high-tensile steel sheet according to claim 1 or 2, further comprising Sn: 0.50% or less in mass%. The method for producing a high-tensile steel sheet according to any one of claims 1 to 3, further comprising the following steps (i) to (iii):
(i) a step of heating the slab having the chemical composition according to any one of claims 1 to 3 to a temperature of 1000 to 1200 ° C. and soaking in the temperature range;
(ii) After descaling and rolling, rolling is completed at a finishing temperature of (Ar ₃ points + 30 ° C.) or less (excluding (Ar ₃ points + 30 ° C.)) , and (Ar ₃ points −30 ° C.) Water cooling from the above temperature to 350 ° C. or less at a cooling rate of 5 ° C./sec or more, and
(iii) A step of reheating and cooling to a temperature of 500 ° C. or more and Ac ₁ point or less.