JP2015206113A - High strength steel material excellent in fatigue crack propagation property and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel material having excellent balance of properties such as rolling and weldability, and roughness and fatigue crack propagation property and a manufacturing method therefor.SOLUTION: There is provided a high strength steel sheet excellent in strength, rolling property, roughness, weldable fatigue crack propagation property having an aggregate structure consisting of a martensite single phase or bainite and martensite and having X ray diffraction intensity ratio of 110and 110, and 110and 100and 111at a sheet thickness center position satisfying predetermined values by conducting heating at 900 to 1300°C, hot rolling with 50% or more of a cumulative draft in a temperature range of Arpoint or more and accelerate cooling from (Arpoint-80°C) or more to 600°C or less at a cooling speed of 3°C/s or more on a crude steel raw material containing, by mass%, C:0.02 to 0.25%, Si:0.01 to 0.60%, Mn:0.5 to 3.0% and Al:0.10% or less and further one or more kinds of Cu, Ni, Cr, Mo, Nb, V, Ti and B while satisfying Ceq of 0.45% or less and Pcm of 0.28% or less.

Description

  The present invention relates to a steel material suitable for various welded structures such as ships, offshore structures, bridges, construction machines, buildings, tanks, and particularly suitable for members subjected to repeated loads, fatigue crack propagation resistance. The present invention relates to a high-strength steel material excellent in and a manufacturing method thereof. The “steel material” here includes thick steel plates, steel pipes, shaped steels, thin steel plates and the like.

  In recent years, when building welded structures such as ships, offshore structures, bridges, construction machines, buildings, tanks, etc., to streamline the design, reduce the weight of steel used, and save labor in welding work by reducing the thickness. In many cases, high-strength steel materials are applied. For this reason, in addition to having excellent ductility and excellent low-temperature toughness, the applied high-strength steel materials have excellent fatigue resistance to ensure excellent weldability and structural safety. It is required to have

  In welded structures, there are many cases where fatigue cracks occur from the weld toes and propagate through the steel material of the welded structure to cause fracture (fatigue failure). This is attributed to the fact that the weld toe portion tends to be a stress concentration portion due to its shape, and that a tensile residual stress is generated after welding.

  For this reason, in order to suppress the occurrence of cracks from the weld toe, additional residual welding is performed to improve the shape of the weld toe. The technology to introduce is widely known.

  However, applying such a technology to a large number of welding toes on an industrial scale requires a great deal of labor and a lot of time, and is realistic from the viewpoint of productivity and cost. Is hard to say. Thus, even if a fatigue crack occurs, if the subsequent crack propagation rate in the steel material can be reduced, the fatigue life of the welded structure can be extended. For these reasons, it is strongly desired to improve the fatigue crack propagation characteristics of steel materials.

  In response to such a request, for example, in Patent Document 1, C: 0.02-0.2%, Si: 0.01-0.8%, Mn: 0.3-2.5%, P: 0.0001-0.04%, S: 0.0001- The α (111) plane inside the steel sheet has a composition containing 0.02%, Al: 0.005 to 0.1%, N: 0.001 to 0.01%, the ferrite phase occupies 70% or more, and is measured on a measuring plane parallel to the steel sheet surface A steel sheet with good fatigue crack propagation characteristics is described in which the ratio of the strength ratio to the α (100) plane strength ratio is 1.25 to 2.0. In the technique described in Patent Document 1, at least the austenite non-recrystallized rolling reduction is 10% or more in rough rolling, and further, in finish rolling, the cumulative rolling reduction is in the α-γ two-phase region or the α-phase temperature region. It is said that rolling is performed to 75% or more.

Patent Document 2 includes, in mass%, C: 0.06 to 0.20%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.006% or less, Al: 0.10% or less. The ferrite phase having a hardness of less than 150 HV on average contains 60% or more by volume, and the second phase has a hardness of less than 240 HV on average, at the plate thickness center position and the plate thickness 1/4 position ( 200) plane X-ray diffraction intensity ratio is 2.0 or more, or (110) plane X-ray diffraction intensity ratio is 2.5 or more, and {100} plane, {110} plane, {111} plane, {211} plane The thickness in the thickness direction of ferrite grain colonies in which any one of the above is aligned within 5 ° with respect to the rolling surface is 5 μm or less on average at the thickness center position and the thickness 1/4 position. A thick steel plate for welded structure having excellent fatigue crack propagation characteristics in the thickness direction is described. In the technique described in Patent Document 2, the cumulative reduction ratio in a ferrite single-phase region or a two-phase region at least 500 ° C. is 50% or more and the rolling speed S is 2.0 × 10 ⁻² / s or less. It is supposed to perform rolling including rolling.

In addition, Patent Document 3 includes C: 0.015-0.20%, Si: 0.05-2.0%, Mn: 0.1-2.0%, P: 0.05% or less, and S: 0.02% or less in weight percent. Describes a steel plate having a low fatigue crack propagation rate in the plate thickness direction in which the (200) diffraction intensity ratio is 2.0 to 15.0 and the area ratio of recovered or recrystallized ferrite grains is 15 to 40%. In the technique described in Patent Document 3, rolling is performed at a cumulative reduction rate of 20 to 90% in the recrystallization temperature range, and Ar to a cumulative reduction rate of 10 to 80% in the non-recrystallization temperature range above the Ar ₃ transformation point. ₃ subjected to finish rolling at a cumulative reduction rate of 40% to 90% by transformation point 600 ° C. or higher, after the completion of rolling, cooling at 30~300s between the atmosphere, at room temperature at a cooling rate of thereafter to 5 to 100 ° C. / s Control cooling to ~ 600 ° C.

  Patent Document 4 includes, in mass%, C: 0.03 to 0.15%, Si: 0.60% or less, Mn: 0.80 to 1.80%, Ti: 0.005 to 0.050%, and Nb: 0.001 to 0.1%. X of the (110) plane parallel to the plate surface in the range from 2mm position in the plate thickness direction to 3/10 position in the plate thickness direction from the front and back surfaces with the composition including one or two selected A thick steel sheet excellent in fatigue crack propagation characteristics in the thickness direction with a line strength ratio of 2.0 or more is described. In the technique described in Patent Document 4, the hot rolling is performed in the first rolling in which the cumulative rolling reduction is 10% or more in a temperature range higher than the austenite partial recrystallization temperature, and in the plate thickness direction from the surface to a thickness of 2 mm. One pass in the temperature range where the range corresponding to the 3/10 position in the thickness direction and / or the range corresponding to the 2/10 position in the thickness direction from the surface to the 7/10 position in the thickness direction is a two-phase structure. The second rolling is such that the rolling reduction ratio is less than 5% and the cumulative reduction ratio is 50% or more, and the end temperature of the second rolling is 600 ° C. or more.

  Further, in Patent Document 5, in mass%, C: 0.01 to 0.10%, Si: 0.19 to 0.60%, Mn: 0.5 to 2.0%, sol.Al: more than 0.005% and 0.10% or less, N: 0.0005 to 0.03% Nb: 0.005 to 0.08%, Ti: 0.005 to 0.03%, V: 0.005 to 0.080%, and a composition satisfying the condition represented by the specific formula, and the composition ratio of ferrite and bainite is 90% or more in total. A steel material having excellent fatigue crack growth characteristics in which the area ratio of pearlite is 2 to 10% and the half width of the X-ray diffraction intensity from the (110) plane is 0.13 to 0.3 ° is described. In the technique described in Patent Document 5, hot rolling to a finishing temperature of 720 to 800 ° C. is performed on a cast slab heated to 1000 to 1250 ° C., and an average cooling rate between 650 to 400 ° C. is 5 to 25 ° C. / The accelerated cooling set to s is stopped at a temperature of 400 ° C. or lower, and then the cooling is finished so that the recuperated temperature width becomes 70 ° C. or lower.

  Patent Document 6 describes a steel plate excellent in fatigue crack growth resistance. The steel sheet described in Patent Document 6 has a composition including C: 0.030 to 0.30%, Si: 0.50% or less, Mn: 0.8 to 2.0%, Al: 0.01 to 0.10%, N: 0.010% or less, Number ratio of acicular ferrite having an aspect ratio of 2 or more, acicular ferrite grown in the γ grain direction in an area fraction of 1 to 60%, and a major axis in the range of 5 to 100 μm at 1/4 thickness position Is a steel sheet having a structure of 80% or more.

  Patent Document 7 describes a steel sheet having high absorbed energy in the Charpy impact test and excellent fatigue crack growth resistance. The steel sheet described in Patent Document 7 has a composition containing C: 0.01 to 0.1%, Si: 0.03 to 0.6%, Mn: 0.3 to 2%, sol. Al: 0.001 to 0.1%, N: 0.0005 to 0.008%. , A bainite having an area ratio of 60 to 85%, a total of 0 to 5% martensite and pearlite, and a steel sheet having a structure in which the balance is ferrite.

  Patent Document 8 describes a thick steel plate having excellent base material toughness and fatigue crack growth characteristics. The thick steel plate described in Patent Document 8 is a composition containing C: 0.030 to 0.300%, Si: 0.50% or less, Mn: 0.80 to 2.00%, Al: 0.01 to 0.10%, N: 0.0100% or less in mass%. And a soft part composed of recrystallized ferrite and a multiphase structure mainly composed of a hard part composed of at least one of martensite and bainite, and the area fraction of the hard part is 15 to 85%, The average equivalent circle diameter is 10 μm or more, the average hardness is Hv 200 to 700, the difference in average hardness between the hard part and the soft part is Hv 100 or more, and the average equivalent circle diameter of the recrystallized ferrite grains is 20 μm or less. This is a thick steel plate with an average lath length of sight and bainite of 5 μm or less.

Japanese Unexamined Patent Publication No. 2000-17379 Japanese Patent No. 4876972 Japanese Patent No. 3434378 JP 2010-242211 A Japanese Patent No. 4706477 Japanese Patent No. 4976749 Japanese Patent No. 4466196 Japanese Patent No.4721956

  However, in the techniques described in Patent Documents 1 to 4, strong texture is applied in the α-γ two-phase region or the ferrite single-phase region to develop a texture having a specific orientation. However, such rolling has a heavy load on the rolling mill and is liable to cause troubles such as breakdowns, and the rolling efficiency is greatly reduced, resulting in a reduction in productivity. Furthermore, there is a problem that work-hardened ferrite has a high risk of significantly reducing the base material toughness.

  Furthermore, according to the techniques described in Patent Documents 1 to 4, the fatigue crack propagation rate in the plate thickness direction can be reduced. However, in Patent Documents 1 to 4, in the directions other than the plate thickness direction, There is no mention of reducing the fatigue crack propagation rate. In an actual welded structure, a fatigue crack propagates not only in the plate thickness direction but also in the plate width direction and plate length direction, and in many cases leads to failure. In the techniques described in Patent Documents 1 to 4, as a compensation for extremely reducing the fatigue crack propagation rate in one direction, for example, the fatigue crack in the plate length direction is accelerated and progresses, and the ultimate failure The possibility that the safety of the entire structure is lowered is included.

  In the technique described in Patent Document 5, a structure having a large half width (that is, a high dislocation density) is introduced in order to improve fatigue crack propagation characteristics. In order to obtain such a structure, it is necessary to contain a large amount of alloy elements and to reduce the accelerated cooling stop temperature. However, a large amount of alloy elements causes a decrease in weldability, and a decrease in accelerated cooling stop temperature has a problem that it is difficult to ensure ductility.

  In addition, it is important that the steel material used in the actual structure is a steel material in which various properties such as strength, ductility, toughness, weldability and the like are improved in a well-balanced manner as well as fatigue crack propagation characteristics. The technique described in Patent Document 6 is a technique for effectively utilizing acicular ferrite and improving fatigue crack growth resistance. However, Patent Document 6 does not mention characteristics such as ductility and toughness, It remains unclear whether the steel sheet manufactured by the technique described in Patent Document 6 has the necessary properties in a well-balanced manner other than fatigue crack growth resistance as a structural steel sheet.

  Moreover, in the technique described in Patent Document 7, when a fatigue crack encounters bainite, the crack propagates while stopping at the boundary or bending so as to avoid bainite. It is said that the resistance to fatigue crack growth will be improved. However, in Patent Document 7, there is a description of fatigue crack growth resistance and toughness, but there is no description of ductility, weldability, etc., which is important as a structural steel sheet, and the technology described in Patent Document 7 It remains unclear whether or not the steel plate manufactured in (1) has the properties necessary for a structural steel plate in a well-balanced manner.

  Further, in the technique described in Patent Document 8, fatigue crack propagation characteristics are obtained by forming a multiphase structure in which a sufficiently refined ferrite and a low-temperature transformation phase having a short lath length transformed from processed γ are combined. Both characteristics of base metal toughness can be made compatible. However, Patent Document 8 remains unclear as to whether or not it has a well-balanced property such as ductility and weldability necessary for a steel sheet for actual structures other than fatigue crack growth rate and toughness.

  An object of the present invention is to solve the problems of the prior art and to provide a high-strength steel material excellent in fatigue crack resistance and a method for producing the same.

Here, “high strength” refers to a case where the tensile strength TS is 490 MPa or more. The term “excellent in fatigue crack propagation resistance” as used herein means that the fatigue crack propagation rate da / dN is at least 1.75 × 10 ⁻⁸ (ΔK: 15 MPa√m, regardless of the crack propagation direction. m / cycle) or less, ΔK: 25 MPa√m, 8.5 × 10 ⁻⁸ (m / cycle) or less.

  The upper limit of the crack propagation rate in each stress intensity factor range described above is the stress expansion for the NK class KA steel described in “Metal Fatigue Crack Propagation Resistance Data” vol. The upper limit of the data band of the relationship between the coefficient range and the fatigue crack propagation rate was used as a reference value, and the case where the fatigue crack propagation rate was 1/2 or less of the standard within the same stress intensity factor range was determined.

  In addition, the high-strength steel material that is the object of the present invention is a steel material that is excellent in fatigue crack propagation resistance as described above, and further excellent in ductility, low-temperature toughness, and weldability. Here, “excellent ductility” refers to a case where the elongation El value obtained by the tensile test is 20% or more. The term “excellent in low temperature toughness” as used herein refers to a case where the absorbed energy at −40 ° C. is 53 J or more in the Charpy impact test conducted in accordance with the provisions of JIS Z 2242-2005. And “Excellent weldability” means MAG welding (heat input 14kJ) in a welding atmosphere with a preheating temperature of 25 ° C, an air temperature of 20 ° C, and a humidity of 60% in accordance with the rules of JIS Z 3158. / y) y-type weld cracking test is conducted and no cracking occurs.

In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting the fatigue crack propagation rate. As a result, in order to reduce the fatigue crack propagation rate regardless of the crack propagation direction, first, the X-ray diffraction intensity ratio (110) of the (110) plane parallel to the plate surface at the 1/4 position of the plate thickness. It was found that the X-ray diffraction intensity ratio (110) _M of the (110) plane parallel to the plate surface at the center position of _Q and the plate thickness must be 2.0 or less. This is because the X-ray diffraction intensity ratio (110) _Q of the (110) plane parallel to the plate surface at the 1/4 position of the plate thickness and the X-ray diffraction of the (110) plane parallel to the plate surface at the center position of the plate thickness. Strength ratio (110) When _M exceeds 2.0, only the fatigue crack propagation rate in the plate thickness direction is significantly reduced, and the fatigue crack propagation rate in the other direction (plate width direction and plate longitudinal direction) is reversed. The direction dependency of the fatigue crack propagation rate becomes too large. For this reason, it was found that both (110) _Q and (110) _M must be 2.0 or less in order to reduce the fatigue crack propagation rate regardless of the crack propagation direction.

Furthermore, the inventors of the present invention have the X-ray diffraction intensity ratio (110) _{M of} the (110) plane parallel to the plate surface and the X-ray diffraction intensity ratio of the (100) plane parallel to the plate surface at the center position of the plate thickness ( 100) _M , X-ray diffraction intensity ratio (111) _M of (111) plane parallel to the plate surface is expressed by the following equations (3) to (5)
2.0 × (110) _M ≦ (100) _M ≦ 6.0 × (110) _M ...... (3)
2.5 × (110) _M ≦ (111) _M ≦ 7.0 × (110) _M ...... (4)
(100) _M ≤ (111) _M ...... (5)
It was found that by adjusting the texture so as to satisfy all of the above, the fatigue crack propagation rate is remarkably reduced regardless of the crack propagation direction.

At the center of the plate thickness, the X-ray diffraction intensity ratio (110) _{M of} the (110) plane parallel to the plate surface, the X-ray diffraction intensity ratio (100) _{M of the} (100) plane parallel to the plate surface, and parallel to the plate surface If the X-ray diffraction intensity ratio (111) _M of the (111) plane is not satisfied with any one of the equations (3) to (5), the thickness direction, the plate width direction, and the plate length direction In one or more directions, a reduction in fatigue crack propagation rate is not observed. Therefore, in the present invention, it has been found that the texture needs to be adjusted so that (110) _M , (100) _M, and (111) _M satisfy all the expressions (3) to (5).

  In addition, the inventors adjusted the texture as described above, and further adjusted the composition to an appropriate range, resulting in not only fatigue crack propagation resistance, but also strength, ductility, low temperature toughness, Found that weldability was improved in a well-balanced manner.

  Furthermore, the present inventors have found that the above-described texture can be easily adjusted by applying a strong pressure in the austenite non-recrystallization temperature range.

  The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.

(1) By mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.60%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.02% or less, Al: 0.10% or less, and Cu : 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less 1 type or 2 types or more of the following formula (1)
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
(Here, C, Mn, Cr, Mo, V, Ni, Cu: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.45% or less, and the following formula (2)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
The weld cracking susceptibility composition Pcm defined by the above is adjusted so as to satisfy 0.28% or less, and the composition composed of the balance Fe and unavoidable impurities and the structure at the 1/4 position of the plate thickness are composed of martensite alone. Phase, or a mixed phase of martensite phase and bainite phase, and the X-ray diffraction intensity ratio (110) _Q of (110) plane parallel to the plate surface at 1/4 position of the plate thickness is 2.0 or less The X-ray diffraction intensity ratio (110) of the (110) plane parallel to the plate surface at the plate thickness central position (110) _M is 2.0 or less and the (110) plane X-ray parallel to the plate surface at the plate thickness central position Diffraction intensity ratio (110) _M , (100) plane X-ray diffraction intensity ratio (100) _M parallel to the plate surface at the plate thickness center position, and (111) plane X-ray diffraction intensity ratio (100) plane at the plate thickness center position ( 111) _M represents the following formulas (3) to (5)
2.0 x (110) _M ≤ (100) _M ≤ 6.0 x (110) _M ...... (3)
2.5 x (110) _M ≤ (111) _M ≤ 7.0 x (110) _M ...... (4)
(100) _M ≤ (111) _M ...... (5)
A high-strength steel material excellent in fatigue crack resistance, characterized by having a structure satisfying

  (2) In (1), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.010% or less and REM: 0.010% or less by mass%. High strength steel material.

(3) When the steel material is subjected to a hot rolling process and an accelerated cooling process to obtain a high-strength steel material, the steel material is, in mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.60%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.02% or less, Al: 0.10% or less, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less selected from the following formula (1)
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
(Here, C, Mn, Cr, Mo, V, Ni, Cu: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.45% or less, and the following formula (2)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
The weld cracking sensitive composition Pcm defined by is adjusted so as to satisfy 0.28% or less, and a steel material having a composition comprising the balance Fe and unavoidable impurities, the hot rolling step, the steel material The heating temperature is a process of heating to a temperature in the range of 900 to 1300 ° C., followed by hot rolling where the cumulative reduction ratio is 50% or more at the Ar ₃ transformation point or higher, and the accelerated cooling step is (Ar ₃ transformation) Point −80 ° C) or higher temperature range, average cooling rate: 3 ° C / s or higher, cooling to 600 ° C or lower, without cooling to room temperature following the accelerated cooling step Or, after cooling to room temperature, a tempering process is performed in which tempering is performed at a temperature of 400 ° C or higher and lower than the Ac ₁ transformation point. Method.

  (4) In (3), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.010% or less and REM: 0.010% or less by mass%. A manufacturing method of high strength steel.

  According to the present invention, a large amount of alloying elements, no special process is required, and in addition to excellent fatigue crack propagation characteristics regardless of the crack propagation direction, strength, ductility, toughness, Furthermore, a high-strength steel material whose weldability is improved in a well-balanced manner can be manufactured easily, stably and inexpensively, and has a remarkable industrial effect. Further, according to the present invention, there is an effect that it is possible to increase the safety margin of fatigue fracture of the welded structure.

It is explanatory drawing which shows the shape of a SENT test piece typically.

  First, the reasons for limiting the composition of the high-strength steel material of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.02-0.25%
C is an element contributing to an increase in strength through solid solution strengthening and further improvement in hardenability. In order to secure a desired high strength by such an effect, the content of 0.02% or more is required. On the other hand, when it contains exceeding 0.25%, weldability will be inhibited. For this reason, C was limited to the range of 0.02 to 0.25%. In addition, Preferably it is 0.05 to 0.20%.

Si: 0.01-0.60%
Si is an element that acts as a deoxidizer and contributes to an increase in strength through solid solution strengthening. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if it exceeds 0.60%, weldability and toughness are lowered. For this reason, Si was limited to the range of 0.01 to 0.60%. In addition, Preferably it is 0.05 to 0.40%.

Mn: 0.5-3.0%
Mn is an element that contributes to the increase in strength of the steel material at low cost and improves the toughness through the improvement of hardenability. In order to acquire such an effect, 0.5% or more of content is required. On the other hand, when it contains exceeding 3.0%, weldability will fall. For this reason, Mn was limited to the range of 0.5 to 3.0%. In addition, Preferably it is 0.5 to 1.8%.

P: 0.05% or less
P is an element that degrades the toughness of steel, and it is desirable to reduce it as much as possible, but it is acceptable up to 0.05%. Therefore, P is limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

S: 0.02% or less
S exists as sulfide inclusions in steel, and lowers the ductility and toughness of the steel. For this reason, it is desirable to reduce S as much as possible, but it is acceptable up to 0.02%. For these reasons, S is limited to 0.02% or less. In addition, Preferably it is 0.01% or less.

Al: 0.10% or less
Al is an element that acts as a deoxidizer and contributes to refinement of crystal grains. In order to acquire such an effect, it is desirable to contain 0.01% or more, but when it contains more than 0.10%, an oxide inclusion will increase and toughness and ductility will fall. For this reason, Al was limited to the range of 0.10% or less. In addition, Preferably it is 0.06% or less.

Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less One or more selected
Cu, Ni, Cr, Mo, Nb, V, Ti, and B are all elements that contribute to an increase in the strength of the steel material. In order to form a desired structure, one or more kinds are selected and contained. It was decided.

  Cu is an element that contributes to increase in strength by solid solution and also contributes to improvement in weather resistance. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, a large content exceeding 1.0% lowers weldability and hot workability, and tends to generate flaws during the production of steel. For this reason, when contained, Cu is limited to 1.0% or less. In addition, Preferably it is 0.5% or less.

  Ni is an element that dissolves and contributes to an increase in strength and improves low-temperature toughness. Ni also contributes effectively to improving weather resistance and improving hot brittleness that occurs when Cu is added. In order to acquire such an effect, it is desirable to contain 0.2% or more. On the other hand, if the content exceeds 2.0%, the weldability is lowered and the material cost is increased. For these reasons, when Ni is contained, Ni is limited to 2.0% or less. In addition, Preferably it is 1.5% or less.

  Cr is an element that contributes to an increase in strength and an improvement in weather resistance. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, if it contains more than 1.0%, weldability and toughness are lowered. For this reason, when contained, Cr is limited to 1.0% or less. In addition, Preferably it is 0.5% or less.

  Mo is an element that contributes to an increase in strength. In order to acquire such an effect, it is necessary to contain 0.005% or more. On the other hand, when it contains exceeding 1.0%, the weldability and toughness will be reduced. For this reason, when contained, Mo is limited to 1.0% or less. In addition, Preferably it is 0.5% or less.

  Nb contributes to increasing the strength of the steel material through solid solution strengthening and precipitation strengthening, and suppresses austenite grain recrystallization during hot rolling and refines the structure. In order to acquire such an effect, it is necessary to contain 0.008% or more. On the other hand, if it exceeds 0.1%, toughness is reduced. For this reason, when it contained, Nb was limited to 0.1% or less. In addition, Preferably it is 0.05% or less.

  V, like Nb, is an element that contributes to an increase in strength by precipitation strengthening. In order to acquire such an effect, it is necessary to contain 0.003% or more. On the other hand, if the content exceeds 0.1%, the toughness and weldability are lowered. For this reason, when contained, V is limited to 0.1% or less. In addition, Preferably it is 0.07% or less.

  Ti contributes to an increase in strength through precipitation strengthening and also contributes to an improvement in weld toughness. In order to acquire such an effect, it is necessary to contain 0.008% or more. On the other hand, if the content exceeds 0.1%, the material cost increases. For these reasons, when Ti is contained, Ti is preferably limited to 0.1% or less. In addition, More preferably, it is 0.05% or less.

  B is an element that improves hardenability and contributes to an increase in the strength of the steel by containing a small amount. In order to acquire such an effect, it is desirable to contain 0.0005% or more. On the other hand, when it contains exceeding 0.005%, weldability will fall. For this reason, when contained, B is limited to 0.005% or less. In addition, Preferably it is 0.003% or less.

  The above components are basic components, but in the present invention, in addition to the above basic composition, as a selective element, for the purpose of improving toughness and ductility, Ca: 0.010% or less, REM: 0.010% or less One or two selected from among them can be selected and contained as necessary.

One or two selected from Ca: 0.010% or less, REM: 0.010% or less
Both Ca and REM are elements that contribute to the improvement of the ductility and toughness of the steel material through the form control of inclusions, and can be selected and contained as necessary. In order to acquire such an effect, it is desirable to contain Ca: 0.0005% or more and REM: 0.0005% or more. On the other hand, if the content exceeds Ca: 0.010% and REM: 0.010%, the amount of oxide inclusions increases, and the toughness decreases. For this reason, when it contains, it is preferable to limit to Ca: 0.010% or less and REM: 0.010% or less, respectively. Preferably, Ca is 0.005% or less, and REM is 0.005% or less.

  The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, O: 0.01% or less and N: 0.01% or less are acceptable.

The steel material of the present invention has the above-described composition, and as a weldability index, the following formula (1)
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
(Here, C, Mn, Cr, Mo, V, Ni, Cu: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.45 or less, and the following formula (2)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
The weld cracking susceptibility composition Pcm defined by is adjusted to 0.28 or less to ensure excellent weldability. When the carbon equivalent Ceq defined by the above equation (1) does not satisfy 0.45 or less, and when the weld crack susceptibility composition Pcm defined by the above equation (2) does not satisfy 0.28 or less, the weldability is descend. In addition, when calculating the formulas (1) and (2), when the element indicated in the formula is not included, the content of the element is calculated as zero.

  The steel structure generally changes in the plate thickness direction depending on the composition and manufacturing history, but in the present invention, the structure at a 1/4 position of the plate thickness that exhibits a standard structure is limited. The steel material of the present invention has the above-described composition, and the structure at a 1/4 position of the plate thickness exhibiting a standard structure is a martensite single phase or a structure composed of a martensite phase and a bainite phase. Have. By setting it as such a structure | tissue, desired high intensity | strength can be achieved. If a ferrite phase or pearlite is included, the desired high strength cannot be secured. Thereby, desired high strength, high toughness, and high ductility can be secured even as a low alloy component system. The “martensite phase” here refers to a tempered martensite phase.

  For this reason, in the steel material of the present invention, the structure at 1/4 position of the plate thickness is limited to a structure composed of a single martensite phase or a mixed phase of martensite phase and bainite phase.

In the steel material of the present invention, in addition to the above-described organization, the X-ray diffraction intensity ratio (110) _Q of the (110) plane parallel to the plate surface at the 1/4 position of the plate thickness and the plate at the center position of the plate thickness The X-ray diffraction intensity ratio (110) _M of the (110) plane parallel to the plane has a texture where both are 2.0 or less.

When the X-ray diffraction intensity ratio of the (110) plane parallel to the plate surface increases, the fatigue crack propagation rate decreases only in the plate thickness direction, and not only in the other direction, but the decrease in crack propagation rate is not observed. Conversely, the crack propagation speed may be accelerated. In welded structures, the occurrence of fatigue cracks is often unspecified, and reducing the fatigue crack propagation rate only in the plate thickness direction may not necessarily extend the fatigue life of the entire structure. Is expensive. In the present invention, in order to avoid such an adverse effect, (110) _Q and (110) _M are limited to 2.0 or less.

Further, in addition to the limitations on the texture described above, the steel material of the present invention further has an X-ray diffraction intensity ratio (110) _M of (110) plane parallel to the plate surface at the plate thickness central position, and the plate surface at the plate thickness central position. The X-ray diffraction intensity ratio (100) _{M of} the (100) plane parallel to the plate and the X-ray diffraction intensity ratio (111) _M of the (111) plane parallel to the plate surface at the center of the plate thickness are as follows: (5) Formula
2.0 × (110) _M ≦ (100) _M ≦ 6.0 × (110) _M ...... (3)
2.5 × (110) _M ≦ (111) _M ≦ 7.0 × (110) _M ...... (4)
(100) _M ≤ (111) _M ...... (5)
The texture is adjusted to satisfy all of the above.

When (110) _M , (100) _M , and (111) _M satisfy all the above expressions (3) to (5), the fatigue crack propagation rate is reduced regardless of the propagation direction. If any one of the above formulas (3) to (5) is not satisfied, a reduction in fatigue crack propagation rate is recognized in one or more of the plate thickness direction, the plate width direction, and the plate length direction. Disappear. For this reason, in the present invention, (110) _M , (100) _M , and (111) _M are adjusted so as to satisfy all the above expressions (3) to (5).

  The steel material of the present invention having the composition and structure described above is a high-strength steel material with improved balance of strength, ductility, toughness, and weldability in addition to excellent fatigue crack propagation characteristics regardless of the crack propagation direction. It becomes.

  Below, the preferable manufacturing method of this invention steel material is demonstrated.

  First, the molten steel having the above composition is preferably melted by a conventional melting method such as a converter and used as a steel material such as a slab by using a conventional casting method such as a continuous casting method. In addition, it cannot be overemphasized that the manufacturing method of a steel raw material is not limited to an above-described method.

  The obtained steel material is subjected to a hot rolling process and an accelerated cooling process to obtain a hot rolled steel material.

In the hot rolling step, the steel material having the above composition is heated to a temperature in the range of 900 to 1300 ° C., and then subjected to hot rolling at an Ar ₃ transformation point or higher and a cumulative reduction ratio of 50% or higher. It is preferable that The temperature is the steel surface temperature.

Heating temperature: 900-1300 ℃
When the heating temperature of the steel material is less than 950 ° C., the deformation resistance increases, the load on the rolling mill increases, and the productivity decreases. On the other hand, if the temperature exceeds 1300 ° C., the crystal grains become coarse and it becomes difficult to ensure the desired toughness. For this reason, the heating temperature is preferably limited to a temperature in the range of 900 to 1300 ° C.

Cumulative rolling reduction above Ar ₃ transformation point: 50% or more In hot rolling, the austenite grains can be refined by setting the cumulative rolling reduction above Ar ₃ transformation point to 50% or more. The resulting structure is refined and the strength, toughness, and fatigue crack propagation characteristics can be improved. On the other hand, if the temperature range for hot rolling in the hot rolling is less than the Ar ₃ transformation point, the ferrite phase is mixed in and the strength decreases, and the work texture of ferrite develops too much, resulting in fatigue crack resistance. Anisotropy occurs. Also, if the cumulative rolling reduction is less than 50%, the austenite grains cannot be refined, and the structure obtained by the subsequent transformation becomes coarse, so that the desired strength, toughness, and fatigue crack propagation resistance cannot be ensured. The Ar ₃ transformation point is, for example, the following formula:
Ar ₃ transformation point (° C) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
(Here, C, Mn, Cu, Cr, Ni, Mo: content of each element (mass%))
Can be calculated based on the amount of elements contained in the steel. It should be noted that elements not contained are calculated as zero.

In addition, it is preferable that the rolling end temperature of hot rolling is not less than the Ar ₃ transformation point. If the rolling end temperature is less than the Ar ₃ transformation point, the ferrite phase is mixed in, the strength is lowered, and the work texture of the ferrite develops too much, resulting in anisotropy in fatigue crack resistance.

Next, the accelerated cooling step is a step of cooling from a temperature range of (Ar ₃ transformation point −80 ° C.) or higher to a temperature range of 600 ° C. or lower at an average cooling rate of 3 ° C./s or higher. The temperature is the steel surface temperature, and the cooling rate is the average value in the thickness direction of the steel.

Accelerated cooling start temperature: (Ar ₃ transformation point-80 ° C) or more If the accelerated cooling start temperature is less than (Ar ₃ transformation point-80 ° C), the structure becomes a structure mainly composed of ferrite + pearlite, and the desired high strength can be secured. Disappear.

Average cooling rate: 3 ° C / s or more When the cooling rate is less than 3 ° C / s on average, the structure becomes a structure mainly composed of ferrite and pearlite, and a desired high strength cannot be secured. In addition, Preferably it is 10 degrees C / s or more.

Cooling stop temperature: 600 ° C. or less In the temperature range where the accelerated cooling stop temperature exceeds 600 ° C., the structure becomes a structure mainly composed of ferrite and pearlite, and a desired high strength cannot be secured. The temperature range is preferably 400 ° C. or lower. Therefore, in the accelerated cooling process, the hot-rolled steel material is cooled from a temperature range of (Ar ₃ transformation point -80 ° C) or higher to an average cooling rate of 3 ° C / s or higher to a temperature range of 600 ° C or lower. It is preferable to set it as the process to give.

  In the present invention, the tempering step is performed after the accelerated cooling step.

  The tempering step may be carried out immediately after completion of the accelerated cooling step without being cooled to room temperature, or may be carried out by reheating in another line after cooling to room temperature.

The tempering step is a step of tempering at a temperature of 400 ° C. or higher and lower than the Ac ₁ transformation point. Thereby, ductility and toughness can be adjusted. When the tempering temperature is less than 400 ° C., the ductility and toughness hardly change even when tempering is performed. On the other hand, at the Ac ₁ transformation point or higher, toughness is reduced, for example, part of the material transforms to austenite, and island martensite is generated during subsequent cooling. For this reason, the heating temperature of the tempering treatment is limited to a temperature not lower than 400 ° C. and lower than the Ac ₁ transformation point. In addition, Preferably it is 400-650 degreeC.

The Ac ₁ transformation point is based on the content of each alloy element, for example,
Ac ₁ transformation point (℃) = 723-14Mn + 22Si-14.4Ni + 23.3Cr
(However, Mn, Si, Ni, Cr: content of each element (mass%)) can be used for calculation.

  Molten steel having the composition shown in Table 1 was melted in a converter and cast into a slab (steel material) (wall thickness: 250 to 400 mm) by a continuous casting method. The obtained steel material is subjected to a hot rolling process, an accelerated cooling process, further cooling to room temperature and reheating under the conditions shown in Table 2, or immediately following on-line cooling without cooling to room temperature. The tempering process was given on the conditions shown in 2, and the steel plate with a plate thickness of 12-65 mm was manufactured.

  A test piece for X-ray diffraction test and a test piece for structure observation were collected from the obtained steel plate.

  Test specimens for X-ray diffraction test (size: thickness 1.5 mm × width 25 mm × length 30 mm) were collected in parallel to the plate surface from the plate thickness 1/4 position and the plate thickness center position of each steel plate. After mechanically and chemically polishing the measurement surface (25 x 30 mm surface) to remove the processed layer, the X-ray diffraction intensity of the (110), (100), and (111) surfaces is obtained by X-ray diffraction. It was. In addition, the random test piece was prepared and the X-ray diffraction intensity was calculated | required similarly.

The ratio of the obtained X-ray diffraction intensity and the X-ray diffraction intensity of the random test piece was determined, respectively, and the (110) plane X-ray diffraction intensity ratio (110) _Q parallel to the plate surface at the plate thickness 1/4 position, X-ray diffraction intensity ratio (110) _{M of} (110) plane parallel to the plate surface at the center position of the plate thickness, X-ray diffraction intensity ratio (100) _M of (100) plane parallel to the plate surface at the center position of the plate thickness, The X-ray diffraction intensity ratio (111) _M of the (111) plane parallel to the plate surface at the center of the plate thickness was used.

  The obtained results are shown in Table 3.

  Further, from the obtained steel plate, a structure observation test piece was collected so that the observation surface was at a 1/4 thickness position. The obtained test piece was polished and corroded with 3% nital etchant to reveal the structure. The tissue was observed and imaged using an optical microscope (magnification: 500 times). While identifying a structure | tissue about the obtained structure | tissue photograph, the structure fraction (area%) of each phase was measured using commercially available image analysis software. It should be noted that the observation of the structure was 5 visual fields or more, the structural fraction in each visual field was obtained, and the arithmetic average was obtained as the structural fraction of the steel sheet.

  The obtained results are also shown in Table 3.

In addition, specimens were collected from the obtained steel plates and subjected to fatigue crack propagation tests, tensile tests, impact tests, and weldability tests. The test method was as follows.
(1) Fatigue Crack Propagation Test Fatigue crack propagation characteristics were investigated according to ASTM E647 using CT specimens. The CT specimen is full thickness for steel sheets with a thickness of 25 mm or less, and for steel sheets with a thickness of more than 25 mm and less than 50 mm, the thickness is reduced at both sides, centering around 1/2 position of the plate thickness. The thickness was reduced to 25 mm by reducing both sides around the 1/4 position of the thickness. In addition, from each steel sheet, the CT specimen is divided into two types in which the direction in which the fatigue crack propagates is perpendicular to the rolling direction (sheet width direction) and the direction in which the fatigue crack propagates is in the rolling direction (sheet length direction). Test specimens were collected. The test using the CT test piece was performed according to ASTM E647 under the conditions of a stress ratio R of 0.1 and a frequency of 20 Hz in a room temperature atmosphere.

  A fatigue crack propagation test in which the fatigue crack propagates in the thickness direction was also conducted. SENT (Single edge notch tension) type test pieces shown in FIG. 1 were collected from each steel plate. In the SENT type test piece, a fatigue precrack 3 was introduced at the tip of the mechanical notch 2, and a crack gauge 4 was stuck on both sides.

  The fatigue crack propagation test using the SENT type test piece was performed in a room temperature atmosphere under the conditions of a stress ratio R: 0.1 and a frequency: 20 Hz. Then, the relationship between the stress intensity factor range and the fatigue crack propagation rate from the point when the fatigue precrack 3 was introduced 1 mm or more from the tip of the mechanical notch 2 was obtained.

  During the test, each crack length was detected by a crack gauge 4 affixed on both sides, and the average length on both sides was taken as the crack length a.

  The stress intensity factor range ΔK was calculated by the Brown formula shown below.

ΔK = Δσ (πa) ^1/2 F (a / w)
(Where F (a / w) = 1.12−0.231 (a / w) +10.55 (a / w) ² -21.72 (a / w) ³ +30.39 (a / w) ⁴ , a: crack length Mm, w: specimen width mm (plate thickness))
When the fatigue crack propagates, the crack propagation rate in the stress intensity factor range ΔK = 15 MPa√m is 1.75 × 10 ⁻⁸ m / cycle or less, and the crack propagation rate in ΔK = 25 MPa√m is 8.5 × 10 ⁻⁸ m The case where it was less than / cycle was evaluated as ○. Otherwise, it was set as x.
(2) Tensile test Full thickness tensile specimens for steel sheets with a thickness of 40 mm or less so that the tensile direction is the direction T perpendicular to the rolling direction with reference to the Japan Maritime Association Steel Ship Rules. (U1 tensile test piece), and for steel plates with a thickness of more than 40 mm, a tensile test piece (U14 tensile test piece) was collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Yield strength (0.2% yield strength) YS, tensile strength TS, elongation El) was determined. The case where the tensile strength TS was 490 MPa or more and the elongation El was 16% or more was evaluated as ◯, and the others were evaluated as x.
(3) Impact test From the obtained steel plate, in accordance with the Japan Maritime Association steel ship regulations, the test piece end face is a steel plate with a plate thickness of 40 mm or less so that the length direction of the test piece is parallel to the rolling direction L. For steel sheets with a surface thickness of 2mm below the surface, and for steel sheets with a thickness of more than 40mm, take an impact test piece (U4 2mmV notch) from 1/4 position of the thickness, and conduct an impact test at -40 ℃. The absorption energy (J) was calculated. Three of each steel plate was tested. When all three samples showed 53J or more, the evaluation was ○, and the others were evaluated as ×.
(4) Weldability test In accordance with the provisions of JIS Z 3158, a y-type weld crack test piece is collected from the obtained steel sheet, the preheating temperature is 25 ° C, the temperature is 20 ° C, and the humidity is 60%. A y-type weld cracking test for MAG welding (heat input 14 kJ / cm) was carried out in an atmosphere to investigate whether cracks occurred. The case where no crack was generated was evaluated as ◯, and the other cases were evaluated as ×.

  Table 4 shows the obtained results.

In all of the inventive examples, high strength satisfying a desired strength (TS: 490 MPa or more), high ductility satisfying a desired ductility (El: 16% or more), and a desired low-temperature toughness (vE- ₄₀ : 53 J High toughness that satisfies the above requirements) and excellent weldability with no weld cracking. Furthermore, the fatigue crack propagation rate is reduced regardless of the crack propagation direction, and excellent fatigue crack propagation. It is a steel plate with characteristics. On the other hand, in the comparative example outside the scope of the present invention, any of strength, ductility, toughness, weldability, and fatigue crack propagation resistance does not satisfy the desired characteristics.

1 SENT type test piece 2 Mechanical notch 3 Pre-fatigue crack 4 Crack gauge

Claims (4)

% By mass
C: 0.02 to 0.25%, Si: 0.01 to 0.60%,
Mn: 0.5 to 3.0%, P: 0.05% or less,
S: 0.02% or less, Al: 0.10% or less, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less , Ti: 0.1% or less, B: 0.005% or less selected from one or more, carbon equivalent Ceq defined by the following formula (1) is 0.45% or less, and the following formula (2) The weld crack susceptibility composition Pcm defined is adjusted to satisfy 0.28% or less, the composition consisting of the balance Fe and inevitable impurities, and the structure at 1/4 position of the plate thickness is a martensite single phase. Or a mixed phase of martensite phase and bainite phase, and the X-ray diffraction intensity ratio (110) _Q of (110) plane parallel to the plate surface at 1/4 position of the plate thickness is 2.0 or less, the thickness parallel to the plate surface at the center position (110) plane X-ray diffraction intensity ratio of (110) _M is 2.0 or less, and, parallel to the plate surface in the sheet thickness center position (110) plane X-ray diffraction intensity ratio of (110) _M, parallel to the plate surface in the sheet thickness center position (100) plane X-ray diffraction intensity ratio of (100) _M, and in the sheet thickness center position (111) plane A structure in which the X-ray diffraction intensity ratio of (111) _M satisfies the following formulas (3) to (5):
A high-strength steel material with excellent fatigue crack propagation characteristics characterized by having
Record
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
Here, C, Mn, Cr, Mo, V, Ni, Cu: Content of each element (mass%)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)
2.0 x (110) _M ≤ (100) _M ≤ 6.0 x (110) _M ...... (3)
2.5 x (110) _M ≤ (111) _M ≤ 7.0 x (110) _M ...... (4)
(100) _M ≤ (111) _M ... (5)   2. The composition according to claim 1, wherein, in addition to the composition, the composition further contains one or two kinds selected from Ca: 0.010% or less and REM: 0.010% or less by mass%. High strength steel. When steel material is subjected to hot rolling process and accelerated cooling process to make high strength steel material,
The steel material in mass%,
C: 0.02 to 0.25%, Si: 0.01 to 0.60%,
Mn: 0.5 to 3.0%, P: 0.05% or less,
S: 0.02% or less, Al: 0.10% or less, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less , Ti: 0.1% or less, B: 0.005% or less selected from one or more, carbon equivalent Ceq defined by the following formula (1) is 0.45% or less, and the following formula (2) The weld crack sensitivity composition Pcm defined is 0.28% or less, adjusted to satisfy the content, and the steel material of the composition consisting of the balance Fe and inevitable impurities,
The hot rolling step is a step in which the steel material is heated to a temperature in the range of 900 to 1300 ° C., and then subjected to hot rolling at an Ar ₃ transformation point or higher and a cumulative reduction ratio of 50% or higher. ,
The accelerated cooling step is a step of cooling from a temperature range of (Ar ₃ transformation point −80 ° C.) or higher to an average cooling rate: 3 ° C./s or higher and a temperature range of 600 ° C. or lower,
Following the accelerated cooling step, a tempering step of performing a tempering treatment at a temperature of 400 ° C. or higher and lower than the Ac ₁ transformation point after cooling to room temperature or after cooling to room temperature is characterized in that A method for producing high-strength steel materials with excellent fatigue crack propagation resistance.
Record
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
Here, C, Mn, Cr, Mo, V, Ni, Cu: Content of each element (mass%)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)   4. The composition according to claim 3, wherein the composition further includes one or two selected from Ca: 0.010% or less and REM: 0.010% or less by mass% in addition to the composition. Manufacturing method of high strength steel.

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