KR20140113975A - High strength steel plate having excellent brittle crack arrestability and method for manufacturing same - Google Patents

Abstract

A high strength post-steel sheet excellent in brittle crack propagation stopping property and a method for producing the same are provided.
(C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5) of not less than 0.34% and not more than 0.49%, and optionally Nb, Ti (211) plane X-ray intensity ratio of not less than 1.0 in a plane parallel to the steel sheet surface, at least one of Cu, Ni, Cr, Mo, V, Ca, B and REM Is present at least 1/3 of the plate thickness including the center of the plate thickness, the bainite fraction in the central portion of the plate thickness is 80% or more, and the Charpy wavefront transition temperature Is -40 ° C or less. Rolling at a temperature of the central portion of the plate thickness in hot rolling at a temperature of (Ar ₃ point + 60) ° C or lower and a cumulative rolling reduction of 50% or higher in a temperature range of Ar ₃ point or higher and then cooling at a cooling rate of 4.0 ° C / Cool.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high strength steel sheet having excellent brittle crack propagation stopping properties,

The present invention relates to a brittle crack arrestability suitable as a thick steel plate having a plate thickness exceeding 50 mm, which is used for large structures such as ships, offshore structures, low-temperature storage tanks, High strength steel plate and a method of manufacturing the same.

In large structures such as ships, offshore structures, low-temperature storage tanks, and construction / civil structures, the impact of brittle fracture accidents on economics and the environment is so great that safety improvement is always required. Toughness at the operating temperature and brittle crack propagation stopping properties are required for the steel material to be used.

Container ships and bulk carrier vessels use high strength heavy materials for the outer plate of ship's hull. However, recently, with the increase in size of the hull, In general, the brittle crack propagation stopping property of a steel sheet tends to deteriorate as a high strength or a shrunk material, so that the demand for brittle crack propagation stopping properties is further enhanced.

As a means for improving the brittle crack propagation stopping property of steel, there has been known a method of increasing the Ni content in the past. In a low tank of liquefied natural gas (LNG), 9% Ni steel ) Are used on a commercial scale.

However, since the increase in the amount of Ni causes a considerable increase in cost, it is difficult to apply to applications other than the LNG tank.

On the other hand, a relatively thin steel material having a plate thickness of less than 50 mm, which is used for ships and line pipes that do not reach cryogenic temperatures such as LNG, is subjected to grain refinement by the TMCP method, The low-temperature toughness can be improved, and excellent brittle crack propagation stopping properties can be imparted.

Further, in order to improve the brittle crack propagation stopping property without increasing the alloy cost, a steel material having ultrafine-grained structure of the surface layer portion is proposed in Patent Document 1. [

A steel material excellent in brittle crack propagation stopping property described in Patent Document 1 is a steel material having shear lips (shear-lips) generated in the surface layer of a steel material when the brittle crack propagates is effective for improving the brittle crack propagation stopping property The crystal grains in the shear rib portion are made finer to absorb the propagation energy of the propagating brittle crack.

As the manufacturing method, the step of cooling the surface layer portion to the Ar ₃ transformation point (Ar ₃ temperature) or lower by controlled cooling after hot rolling, and thereafter repeating the step of stopping the control cooling and reheating the surface layer portion to the transformation point or more at least once , It is described that the ferrite structure or the bainite structure is formed in the surface layer portion by repeating transformation or processing recrystallization by applying a pressure to the steel material during the process.

In Patent Document 2, in order to improve the brittle crack propagation stopping property in a steel material having a microstructure mainly composed of ferrite-pearlite, both surface portions of the steel have average grain diameter equivalent (equivalent grain diameter equivalent it is important to suppress the unevenness of ferrite grain size by constituting a layer having a ferrite structure having a ferrite grains of 2 or more and 50% or more of aspect ratios of the grains: , And as a method of suppressing unevenness, a rolling reduction ratio per pass in finish rolling is set to 12% or less, thereby suppressing the local recrystallization phenomenon.

However, the steel material excellent in the brittle crack propagation stoppage described in Patent Documents 1 and 2 has a structure in which only a surface layer portion of steel is once cooled and then heat is recovered, and a certain structure is obtained by performing processing in a double heat. It is not easy to use, and in particular, in the case of a lumber material having a plate thickness exceeding 50 mm, the load on the rolling and cooling equipment is large.

On the other hand, Patent Document 3 discloses a technique on the extension of TMCP, which not only makes the ferrite grains finer but also focuses on the sub-grains formed in the ferrite grains and improves the brittle crack propagation stopping characteristics.

Concretely, the rolling conditions for obtaining a fine ferrite crystal grain (a), a rolling condition for securing a fine ferrite crystal grain (b) (C) dislocations introduced by machining (rolling) while developing a texture in the fine ferrite are rearranged by thermal energy to form a sub-grain (D) the brittle crack propagation stopping property is improved by a cooling condition for suppressing coarsening of the formed fine ferrite grains and fine sub-grain grains.

It is also known to improve the brittle crack propagation stopping property by developing aggregate structure by applying a pressure drop to the transformed ferrite in the control rolling. Separation is generated on the fracture surface of the steel in a direction parallel to the surface of the steel sheet, thereby relieving the stress at the brittle crack tip, thereby increasing the resistance to brittle fracture.

For example, Patent Document 4 discloses a technique in which the (110) plane X-ray intensity ratio according to controlled rolling is set to 2 or more and the average grain diameter equivalent to a circle) of 20 mu m or more is 10% or less, it is described that the anti-brittle fracture characteristics are improved.

Patent Document 5 discloses a steel sheet for welding structure steel which is excellent in brittle crack propagation stopping performance of a joint portion and has an X-ray surface strength ratio of a (100) face of 1.5 or more on a rolled surface inside a plate thickness, And the brittle crack propagation stopping property is excellent due to the displacement of the stress load direction due to the development of the texture and the angle in the crack propagation direction. In Patent Documents 6 to 9, the average rolling reduction in the control rolling is specified so that the brittle crack propagation stopping portion that develops the aggregate structure in each portion in the plate thickness direction (1/4 sheet thickness, A method of manufacturing a steel for welded structure excellent in performance is disclosed.

Japanese Patent Publication No. 7-100814 Japanese Patent Application Laid-Open No. 2002-256375 Japanese Patent Publication No. 3467767 Japanese Patent Publication No. 3548349 Japanese Patent Publication No. 2659661 Japanese Patent Application Laid-Open No. 2008-214652 Japanese Laid-Open Patent Publication No. 2010-047805 Japanese Laid-Open Patent Publication No. 2009-221585 Japanese Laid-Open Patent Publication No. 2010-202931

 Inoue et al.: Longitudinal brittle crack propagation behavior in thick shipbuilding steels, Japan Society of Naval Architects and Ocean Engineers lecture No.3, 2006, pp359-362.

However, in the case of a large container ship exceeding 6,000TEU (Twenty-foot Equivalent Unit) in recent years, a steel plate having a plate thickness exceeding 50 mm is used. However, Inoue et al., A brittle brittle crack propagation behavior in thick steel for shipbuilding, In the lecture on the brittle crack propagation stopping performance of the steel sheet with a plate thickness of 65 mm, the results of brittle cracking did not stop in the large brittle crack propagation stop test of the base material .

In addition, the ESSO test (ESSO test compliant with the guideline for brittle crack arrest design (2009, CLASS NK)) shows that the value of Kca at the operating temperature of -10 ° C does not satisfy 3000 N / mm ^3/2 In the case of a hull structure in which a steel plate having a thickness exceeding 50 mm is applied, it has been suggested that safety is a problem.

The steel sheet excellent in brittle crack propagation stopping property described in the above-mentioned Patent Documents 1 to 5 has a thickness of about 50 mm as the main object from the manufacturing conditions and the disclosed experimental data. In the case of applying the steel sheet with a thickness exceeding 50 mm, And the characteristics of crack propagation in the plate thickness direction required in the hull structure are not verified at all.

In Patent Documents 6 to 9, it is necessary to set a high reduction ratio per pass at the time of rolling in order to develop the texture of the central portion of the plate thickness. Therefore, various constraints are generated in terms of manufacturing conditions, And improvement thereof has been demanded.

Therefore, the present invention is also applicable to a brittle crack propagation stop which can be stably produced in an industrially very simple process of controlling the aggregate structure in the plate thickness direction, Strength steel sheet excellent in characteristics and a method for producing the same.

The inventors of the present invention have conducted intensive studies on a high strength steel sheet having excellent brittle crack propagation stopping properties and a manufacturing method for stably obtaining the steel sheet even in a low steel sheet having a plate thickness of more than 50 mm, The area having an aggregate structure in which the (211) plane X-ray intensity ratio on the plane parallel to the surface of the steel sheet was 1.0 or more was found to have a total thickness including the center of the plate thickness, Of the brittle crack propagation stopping property, it is recognized that excellent brittle crack propagation stopping property is obtained. It has also been found that it is desirable to produce such a steel sheet in combination with a chemical composition of a specific range and a manufacturing condition of a specific range, particularly rolling and cooling conditions at the center of the plate thickness.

The present invention is accomplished by further studying the obtained recognition, that is,

1. A steel sheet having an aggregate structure having a (211) plane X-ray intensity ratio of 1.0 or more on a plane parallel to the steel sheet surface in an area of 1/3 or more of the total plate thickness including the center of the plate thickness, Characterized in that the bainite fraction in the central portion is 80% or more and the fracture transition temperature at 1/4 of the plate thickness is -40 占 폚 or less. The brittle crack propagation stopping property is excellent in high strength After the steel plate.

2. The steel according to claim 1, wherein the chemical composition of the steel is 0.03 to 0.20% of C, 0.03 to 0.50% of Si, 0.50 to 2.20% of Mn, 0.030% or less of P, 0.010% or less of S and 0.005 to 0.08% , N: 0.0045% or less, and the carbon equivalent (Ceq) represented by the following formula (1) is 0.34% or more and 0.49% or less and the balance of Fe and inevitable impurities. High strength steel sheet with excellent characteristics.

Ceq = C + Mn / 6 + Cu + Ni / 15 + V + Mo +

Note that each symbol represents the content (mass%) of each component.

3. The steel composition according to any one of claims 1 to 3, wherein the steel contains 0.005 to 0.030% Ti, 0.005 to 0.050% Nb, 0.01 to 0.50% Cu, 0.01 to 1.00% Ni, 0.01 to 0.50% The brittle crack described in 2, characterized in that it contains at least one selected from 0.01 to 0.50% of V, 0.001 to 0.10% of B, 0.0030% or less of B, 0.0050% or less of Ca and 0.010% or less of REM Strength after high strength steel plate excellent in stopping characteristics.

4.2 or heating the steel material having a chemical composition as described in 3, at a temperature of 900~1200 ℃, and the temperature of the plate thickness center at the hot rolling (Ar ₃ point + 100), the cumulative rolling reduction in the temperature range above ℃ (Ar ₃ point + 60) ° C or lower, a cumulative reduction ratio of 50% or higher in a temperature range of Ar ₃ point or higher, an average value of a reduction rate per pass of 6.0% or higher, Wherein the rolled steel sheet is rolled to a rolling reduction range of 5.0 to 20.0% and then cooled to 450 DEG C or less at a cooling rate of 4.0 DEG C / s or more.

The post-steel sheet obtained by the present invention is excellent in brittle crack propagation stopping property because aggregate structure is appropriately controlled according to angular positions in the plate thickness direction even when the plate thickness is 50 mm or more. The present invention is applied to a steel sheet having a plate thickness of 50 mm or more, preferably a plate thickness of more than 50 mm, more preferably a plate thickness of 55 mm or more, and more preferably a plate thickness of 60 mm or more, And therefore, it is effective. In particular, the present invention is applied to a deck member joined to a hatch side coaming in a strength deck structure such as a container ship or a bulk carrier as a structural member for a ship, which contributes to safety improvement of the ship. , Is very useful in industry.

(Mode for carrying out the invention)

In the present invention, 1. the texture of the steel sheet, 2. the microstructure at the center of the plate thickness, and 3. the toughness of the base material.

1. Aggregate structure inside steel plate

In the present invention, in order to improve the crack propagation stopping property against the crack propagating in the direction parallel to the sheet surface in the rolling direction or in the direction perpendicular to the sheet surface, . When the (211) plane is developed on the plane parallel to the surface of the steel sheet at the center of the plate thickness, microscopic cracks occur before the crack progresses, which is resistance to crack propagation.

(211) plane X-ray intensity ratio in a plane parallel to the steel sheet surface is not less than 1.0 in a region of not less than 1/3 of the entire plate thickness including the center of the plate thickness in order to generate microscopic cracks prior to crack propagation It is assumed that there is an aggregate structure. As described above, since the microscopic cracks are generated before crack propagation and the cracks become resistant to the cracks, the effect obtained when the region having the texture is included in the region including the central portion of the plate thickness is not less than 1/3 of the total plate thickness, Are not specified. However, even if the area is increased by more than 3/4 of the total thickness of the plate, the increase of the action effect becomes saturated, so that the texture of the aggregate is reduced It is not necessary to increase the area having a thickness greater than 3/4 of the total thickness of the plate. However, needless to say, even if the entire thickness of the plate thickness is the aggregate structure, the above-mentioned action and effect is exerted.

Here, the (211) plane and the number representing the X-ray intensity Sembilan target material 211 density (X-ray diffraction intensity ratio of texture) of the crystal plane, the target material 211, X-ray diffraction intensity (I ₍₂₁₁ of _reflection) (I _{211 (211)} / I _{0 (211)} ) of the X-ray diffraction intensity (I _{0 (211)} ) of the (211) reflection of a random standard sample having no texture.

2. Microstructure in the middle of plate thickness

The bainite fraction in the central portion of the plate thickness parallel to the rolling direction is set to at least 80% in order to obtain a suitable aggregate structure in the central portion of the plate thickness described above. The bainite fraction is expressed as an area fraction.

The (211) plane on the plane parallel to the steel sheet surface is developed when the austenite structure processed at the time of rolling is transformed into a ferrite or bainite structure. In the case of the ferrite-cementite structure, there is the influence of recovery and the like, and this texture does not develop in a wide range in the plate thickness direction. It is possible to maintain the highest (211) plane X-ray intensity ratio in a wide range by transforming the transformed structure into a bainite structure. In the present invention, the microstructure at the central portion of the plate thickness means microstructure of at least one-third of the plate thickness including the central portion of the plate thickness. The present invention includes a steel sheet whose overall cross section in the direction of the thickness direction is the microstructure.

3. Base material toughness

Since the base material toughness has good properties to suppress the progress of the brittle cracks, the steel sheet according to the present invention is characterized in that, as a representative position of the material of the steel sheet, the Charpy of the Charpy test piece taken from the 1/4 position of the plate thickness Defines the Charpy wavefront transition temperature in the impact test.

In order to obtain a brittle crack propagation stop performance of Kca (-10 ° C) ≥7000 N / mm ^3/2 , which is considered as a target in securing structural safety with a finishing material having a plate thickness of 50 mm or more, 1/4 The Charpy wavefront transition temperature in the Charpy impact test with the test piece taken from the position is specified to be -40 DEG C or less.

The chemical composition and the manufacturing conditions of the steel suitable for the steel sheet having the above-described texture and base material toughness are as follows. In the description of the chemical components, "%" means mass%.

C: 0.03 to 0.20%

C is an element for improving the strength of steel. In the present invention, it is required to contain 0.03% or more in order to secure a desired strength, but when it exceeds 0.20%, weldability is deteriorated and toughness is adversely affected. Therefore, it is preferable that C is specified in the range of 0.03 to 0.20%. More preferably, it is 0.05 to 0.15%.

Si: 0.03 to 0.50%

Si is effective as a deoxidizing element and also as a strengthening element of steel, but it has no effect when the content is less than 0.03%. On the other hand, if it exceeds 0.50%, not only the surface properties of steel are deteriorated but also toughness is extremely deteriorated. Therefore, the content thereof is preferably 0.03% or more and 0.50% or less. Preferably 0.05 to 0.45%.

Mn: 0.50 to 2.20%

Mn can be contained as a strengthening element. If it is less than 0.50%, the effect is not sufficient. If it exceeds 2.20%, the toughness and weldability of the base material deteriorate and the cost of the steel material also increases, so that it is preferably 0.50% or more and 2.20% or less. Preferably, it is 0.60 to 2.15%.

P, S

P and S are inevitable impurities in the steel, but P is more than 0.030% and S is more than 0.010%, the toughness deteriorates. Therefore, it is preferably not more than 0.030% and not more than 0.010% , And still more preferably 0.005% or less.

Al: 0.005 to 0.08%

Al acts as a deoxidizer, and is preferably contained in an amount of 0.005% or more. However, if it exceeds 0.08%, the toughness is lowered and, in the case of welding, the toughness of the weld metal portion is lowered. Therefore, it is preferable that Al is specified in the range of 0.005 to 0.08%. More preferably, it is 0.02 to 0.04%.

N: 0.0045% or less

N combines with Al in the steel to adjust the grain diameter at the time of rolling and strengthen the steel. However, if it exceeds 0.0045%, the toughness deteriorates, and therefore, it is preferably 0.0045% or less. More preferably, it is 0.0040% or less.

Carbon equivalent (Ceq): not less than 0.34%, not more than 0.49%

Carbon equivalent is an important indicator for predicting tissue strength, transformation behavior, and so on. When the carbon equivalent is less than 0.34%, it is difficult to obtain the above-mentioned bainite fraction at the center of the plate thickness. In addition, if it exceeds 0.49%, the toughness deteriorates. Therefore, it is preferable that the tear strength is 0.34% or more and 0.49% or less. More preferably, it is 0.35 to 0.48%.

The carbon equivalent (Ceq) is obtained by the following formula.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5

The symbol of each element is the content (% by mass).

The balance of Fe and inevitable impurities is the preferred basic composition in the present invention. As an inevitable impurity, for example, O is allowed if it is 0.0050% or less.

In order to further improve the characteristics, it is possible to contain at least one selected from Ti, Nb, Cu, Ni, Cr, Mo, V, B, Ca and REM.

Ti: 0.005 to 0.030%

Ti has an effect of forming a nitride, a carbide, or a carbonitride by a minute amount to improve the toughness of the base material by making the grain finer. The effect is obtained by the content of 0.005% or more, but the content exceeding 0.030% lowers the toughness of the base material and the weld heat affected zone. Therefore, when containing Ti, the content is preferably 0.005 to 0.030% . More preferably, it is 0.008 to 0.028%.

Nb: 0.005 to 0.050%

Nb is precipitated as NbC at the time of ferrite transformation or reheating, thereby contributing to enhancement of strength. Further, it has an effect of expanding the non-recrystallized region in the rolling of the austenite region and contributes to the grain refinement of the ferrite, which is also effective in improving the toughness. The effect is obtained by the content of 0.005% or less, but if the content is more than 0.050%, coarse NbC precipitates and conversely causes a decrease in toughness. Therefore, when Nb is contained, the upper limit is set to 0.050% desirable. More preferably, it is 0.008 to 0.040%.

Cu, Ni, Cr, Mo

Cu, Ni, Cr, and Mo all increase the quenching properties of the steel. But it can be added for improvement of the function such as toughness, high temperature strength or weather resistance, but the excessive content deteriorates the toughness and the weldability. Therefore, when it is contained, 0.50%, Ni of 1.00%, Cr of 0.50%, and Mo of 0.50%. It is more preferable to set the upper limit to 0.45% of Cu, 0.95% of Ni, 0.45% of Cr, and 0.45% of Mo, respectively. On the other hand, when the content of each element is less than 0.01%, the effect is not exhibited. Therefore, when the content is contained, the content is preferably 0.01% or more in each element.

V: 0.001 to 0.10%

V is an element which improves the strength of steel by precipitation strengthening as V (CN), and may be contained in an amount of 0.001% or more in order to exhibit this effect. However, if it exceeds 0.10%, the toughness is lowered. Therefore, when V is contained, it is preferably contained in the range of 0.001 to 0.10%. More preferably, it is 0.008 to 0.095%.

B: not more than 0.0030%

B is an element which increases the quenching property of the steel in a minute amount, and the effect is exhibited by the content of 0.0006% or more. However, when the content exceeds 0.0030%, the toughness of the welded portion is lowered. Therefore, when B is contained, the content is preferably 0.0030% or less. More preferably, it is 0.0028% or less.

Ca: 0.005% or less, REM: 0.01% or less

Ca and REM may improve the toughness by making the structure of the weld heat affected zone finer, and if contained, the effect of the present invention is not impaired. However, if it is contained excessively, coarse inclusions are formed to deteriorate toughness of the base material. Therefore, when contained, the upper limit of the amount is preferably 0.005% and 0.01%, respectively.

Hereinafter, preferable production conditions in the present invention will be described.

It is preferable that the manufacturing conditions specify slab heating conditions, hot rolling conditions, and cooling conditions after hot rolling.

[Heating slab]

It is preferable that the molten steel of the above composition is melted in a converter or the like and made into a steel material (slab) by continuous casting or the like and heated to 900 to 1200 占 폚 and then hot-rolled.

If the heating temperature is less than 900 ° C, the time for performing the rolling in the austenite recrystallization temperature can not be sufficiently secured. If the heating temperature is higher than 1200 ° C, the austenite grains are coarsened, The loss is remarkable and the yield is lowered. Therefore, the heating temperature is 900 to 1200 占 폚. A preferable range of the heating temperature in terms of toughness is 1000 to 1150 deg. C, more preferably 1000 to 1050 deg.

[Hot Rolling]

The temperature of the plate thickness center at the hot rolling (temperature at which the half of the sheet thickness position, hereinafter the same hereinafter) is (Ar ₃ point + 100), the cumulative reduction rate, (Ar ₃ point + 60 at least ℃ ) ℃ or less, Ar ₃ point or more cumulative rolling reduction, (Ar ₃ point +60) ℃ or less, Ar mean value and, the rolling reduction per one pass of the at least _three points (Ar ₃ point +60) ℃ or less, Ar ₃ point or higher It is desirable to define a range of the reduction rate per one pass.

In the hot rolling, the rolling reduction at a cumulative rolling reduction of not less than 30% at the center of the plate thickness center (Ar ₃ point + 100) ° C or more is performed, and the austenite is made fine, . It is more preferable that the cumulative rolling reduction in this temperature range is 35% or more. In the present invention, Ar ₃ point (° C) is obtained by the following formula.

Ar ₃ point = 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu

In the formula, the symbol of each element is the content (mass%) in the steel, and is set to 0 when it is not contained.

Next, the rolling is performed at a cumulative rolling reduction of 50% or more and an average rolling reduction per pass of 6.0% or more at a temperature range of the center of the plate thickness (Ar ₃ point + 60) ° C or lower and a temperature range of Ar ₃ or higher. If the cumulative rolling reduction in this temperature range is less than 50%, the toughness of the steel sheet deteriorates. In addition, the, the non-recrystallized austenite inverse of (Ar ₃ point +60) ℃ or less, a cumulative rolling reduction of 50% or more in the temperature range more than Ar ₃ point to a plane X-ray intensity ratio (211) by 1.0 or more. The cumulative reduction ratio at this temperature range is more preferably 55% or more.

In the finishing rolling of the hearth material, the region where the (211) plane X-ray intensity ratio at the surface parallel to the steel sheet surface is 1.0 or more tends to be narrowed in that it is subjected to small- Therefore, in the present invention, the average value of the reduction rate per pass at the temperature range of Ar ₃ point or more and the temperature at the center of the plate thickness is (Ar ₃ point + 60) ° C or more, 6.0% or more, Is defined as 5.0 to 20.0%. This makes it possible to set the region where the (211) plane X-ray intensity ratio is 1.0 or more to be a region including 1/3 or more of the total plate thickness including the center of the plate thickness. When the average value of the reduction rate per pass is less than 6.0% or when the minimum value of each pass reduction ratio is less than 5.0%, the region where the toughness is lowered and the (211) plane X- The area including the center can not be a region of 1/3 or more of the entire thickness of the plate. On the other hand, if the maximum value of the reduction rate of each pass exceeds 20.0%, the toughness deteriorates due to the influence of the work deformation. The average value of the reduction rate per pass at this temperature range is more preferably 6.5% or more, and the reduction ratio of each pass is more preferably 5.5 to 18.0%. In hot rolling, rolling may be performed outside the prescribed temperature range. It is sufficient if the rolling including the cumulative rolling reduction of the above-mentioned prescribed condition is performed at the prescribed temperature range.

[Cooling after hot rolling]

The steel sheet after completion of rolling is cooled to 450 DEG C or less at a cooling rate of 4.0 DEG C / s or more. When the cooling rate is less than 4.0 占 폚, transformation into bainite does not proceed sufficiently. Therefore, the region where the (211) plane X-ray intensity ratio becomes 1.0 or more is divided into 1/3 And a desired microstructure, that is, a structure having a bainite fraction of 80% or more in the central portion of the plate thickness is not obtained. If the cooling-stop temperature exceeds 450 DEG C, the transformation into bainite does not proceed sufficiently, and thus a desired microstructure can not be obtained. As the cooling method, a method such as water cooling or gas cooling can be used.

Not only the desired texture is obtained by the above-described manufacturing conditions, but also the fracture facet size in the Charpy impact test is made finer, and the Charpy wavefront transition temperature at the plate thickness 1/4 position is -40 DEG C Or less.

In the above description, the temperature at the central portion of the plate thickness is obtained from the plate surface temperature measured by the radiation thermometer by heat transfer calculation. The temperature condition for cooling after hot rolling is also the temperature at the center of the plate thickness.

Example

(Steel symbols A to T) having the compositions shown in Table 1 were hot rolled to a thickness of 50 to 75 mm as a steel material (slab 280 mm in thickness) by a solvent in a converter and subjected to a continuous casting method, Nos. 1 to 28 of the disclosed steel were obtained. Table 2 shows the hot rolling and cooling conditions. The Ar ₃ point (占 폚) was calculated by the following formula.

Ar ₃ point = 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu

However, the symbol of each element is the content (mass%) in the steel, and it is set to 0 when it does not contain.

The obtained steel sheet was subjected to tensile test to obtain a yield strength (YS) and a tensile strength (TS) by taking a JIS 14A test piece of? 14 having a longitudinal direction orthogonal to the rolling direction from 1/4 sheet thickness Respectively.

From the 1/4 part of the plate thickness, the JIS No. 4 impact test piece was sampled so that the direction of the shrinkage of the test piece was parallel to the rolling direction, and Charpy impact test was conducted to obtain the wavefront transition temperature (vTrs). And that the Charpy wavefront transition temperature in a plate thickness of 1/4 part is -40 캜 or lower is within the scope of the present invention.

With respect to the bainite fraction in the central portion of the plate thickness, the plate thickness cross-section parallel to the rolling longitudinal direction of the central portion of the plate thickness was mirror-polished, and then an optical microscope photograph of the metal structure revealed by etching was taken , And imaging analysis.

In order to evaluate the texture of the steel sheet, the (211) plane X-ray intensity ratio at a plane parallel to the steel sheet surface was measured every 1 mm from the surface to the back surface of the steel sheet, and the (211) plane X- The area where the ratio becomes 1.0 or more is obtained.

Next, in order to evaluate the brittle crack propagation stopping property, a temperature gradient type ESSO test was performed to obtain Kca (-10 DEG C) (N / mm3 / ² ).

Table 3 shows the results of these tests.

In all of Nos. 1 to 28, the (211) plane X-ray intensity ratio on the plane parallel to the steel sheet surface was 1.0 or more in the central portion of the plate thickness.

A region where the transition temperature of the Charpy impact test at 1/4 of the plate thickness, the bainite fraction at the center of the plate thickness, and the (211) plane X-ray intensity ratio in the plane parallel to the steel sheet surface is 1.0 or more, (-10 ° C) exhibited excellent brittle crack propagation stopping performance of 7000 N / mm ^3/2 or more in the case of the published steel plates (No.

Claims (4)

(211) plane X-ray intensity ratio of 1.0 or more on a plane parallel to the surface of the steel sheet in an area of 1/3 or more of the entire thickness of the plate thickness including the central portion of the plate thickness, And a Charpy wavefront transition temperature at the 1/4 position of the plate thickness is -40 占 폚 or lower. The method according to claim 1,
The steel according to claim 1, wherein the chemical composition of the steel is 0.03 to 0.20% of C, 0.03 to 0.50% of Si, 0.50 to 2.20% of Mn, 0.030% or less of P, 0.010% or less of S, 0.005 to 0.08% : 0.0045% or less, and a carbon equivalent (Ceq) expressed by the following formula (1) is 0.34% or more and 0.49% or less, with the balance being Fe and inevitable impurities.
Ceq = C + Mn / 6 + Cu + Ni / 15 + V + Mo +
Note that each symbol represents the content (mass%) of each component. 3. The method of claim 2,
0.005 to 0.050% of Nb, 0.005 to 0.050% of Nb, 0.01 to 0.50% of Cu, 0.01 to 1.00% of Ni, 0.01 to 0.50% of Cr, 0.01 to 0.50% of Cr, 0.50%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.010% or less. A steel material having the chemical composition according to claim 2 or 3 is heated to a temperature of 900 to 1200 占 폚 and the temperature at the central portion of the plate thickness in the hot rolling is accumulated at a temperature range of (Ar ₃ point +100) (Ar ₃ point + 60) ° C or lower, a cumulative reduction ratio of 50% or higher at a temperature range of Ar ₃ point or higher, and an average value of a reduction rate per pass of 6.0% or higher, And rolling is carried out so that the rolling reduction ranges of the respective passes become 5.0 to 20.0%, and then cooled to 450 DEG C or less at a cooling rate of 4.0 DEG C / s or more.