KR20140097463A - High-strength thick steel plate for construction having excellent characteristics for preventing diffusion of brittle cracks, and production method therefor - Google Patents

Abstract

The present invention provides a structural high strength steel sheet excellent in brittle crack propagation stopping property and having a plate thickness of 50 mm or more suitable for use in ships, and a method of producing the same. Wherein the metal structure is a bainite-based body and the integrated degree I of the RD // (110) face at the central portion of the plate thickness is 1.5 or more, and the Charpy wavefront transition temperature vTrs at the surface layer portion and the plate thickness central portion is -40 DEG C Or less, and more preferably, the Charpy wave front transition temperature at the central portion of the plate thickness and the degree of integration I of the RD / (110) plane satisfy the following formula (1) .
vTrs _{(1 / 2t)} -12 x I _{RD // (110) [1 / 2t]} ? -70 ... (One),
vTrs _{(1 / 2t)} : wavefront transition temperature (° C) at the center of plate thickness,
I _{RD // (110) [1 / 2t]} : RD // (110) density at the center of the plate thickness

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 high-strength thick steel plate for structural use having excellent brittle crack arrestability and a method of manufacturing the same. More particularly, the present invention relates to a plate suitable for use in ships Thickness of 50 mm or more.

In large structures such as ships, brittle fracture has a great impact on the economy and the environment. Therefore, safety is always required to be improved. For the steel to be used, toughness at the operating temperature toughness, and brittle crack propagation stopping characteristics.

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, Progress is being made. In general, the brittle crack propagation stopping property of a steel sheet tends to deteriorate as a high strength or a shrunk material, and the demand for brittle crack propagation stopping properties is further enhanced.

As a means for improving the brittle crack propagation stopping property of steel, a method of increasing the Ni content has been conventionally known. 9% Ni steel is used on commercial scale in low-tanks of Liquefied Natural Gas.

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 sheet having a thickness of less than 50 mm, which is used for a ship or a line pipe which does not reach an ultra low temperature such as LNG, is subjected to grain refinement by the TMCP (Thermo-Mechanical Control Process) Fine grain formation) is promoted, the low temperature toughness is improved, and excellent brittle crack propagation stopping properties can be imparted.

In addition, a steel material having ultra fine crystallization of the surface layer to improve the brittle crack propagation stopping property without increasing the alloy cost has been 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 a manufacturing method, a process of cooling the surface layer portion to below the transformation point A _r3 by controlled cooling after hot rolling, and then stopping controlled cooling to recuperate the surface layer portion to the transformation point or more It is described that the ferrite structure or the bainite structure is formed in the surface layer portion by repeating at least one time and by repeatedly transforming or recrystallizing the steel by pushing down the steel material have.

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 circle-equivalent average grain size: It is important to suppress the non-uniformity of the ferrite grain size by constituting the ferrite structure having a ferrite structure of not less than 5 mu m and an aspect ratio of the grains of not less than 2 as 50% or more, As a method, it is described that the maximum rolling reduction per pass in finish rolling is set to 12% or less to suppress 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 on the fracture surface of the steel is generated in a direction parallel to the plane of the steel, thereby relaxing 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 intensity ratio in the (110) plane showing a texture developing degree is controlled to be 2 or more by controlled rolling, (brittle fracture property) by setting the coarseness of 20 mu m or more to 10% or less in diameter equivalent to a circle in the crystal grains.

Patent Document 5 discloses a steel for weld structure which is excellent in brittle crack propagation stopping property 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.

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

 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 large container ships exceeding the recent 6,000TEU (Twenty-foot Equivalent Unit), a steel plate with a plate thickness exceeding 50 mm is used. Non-Patent Document 1 reports the results of evaluating the brittle crack propagation stopping performance of a steel sheet having a thickness of 65 mm and failing to stop the brittle cracks in the large brittle crack propagation stop test of the base metal.

In the standard ESSO test (ESSO test compliant with WES 3003), the value of Kca (hereinafter also referred to as Kca (-10 ° C)) at the operating temperature of -10 ° C does not satisfy 3000 N / mm ^3/2 , And in the case of a hull structure to which a steel plate having a thickness exceeding 50 mm is applied, it is suggested that safety is a problem.

The steel sheet excellent in brittle crack propagation stoppage described in the above-mentioned Patent Documents 1 to 5 is a main object up to a plate thickness of about 50 mm from the production conditions and the disclosed experimental data, and when applied to a slip material exceeding 50 mm, It is unclear whether a predetermined characteristic will be obtained and the characteristics of the crack propagation in the plate thickness direction necessary for the hull structure are not verified at all.

Therefore, the present invention provides a high strength steel sheet excellent in brittle crack propagation stopping property which can be stably produced by an industrially very simple process of controlling the texture in the plate thickness direction by optimizing the rolling conditions, and a method of manufacturing the same The purpose is to provide.

The inventors of the present invention have repeatedly carried out intensive studies for achieving the above-mentioned problems, and have obtained the following perceptions for high strength steel plates having excellent crack propagation stopping properties even in the case of low-strength steel plates.

1. A steel plate having a plate thickness exceeding 50 mm is subjected to a standard ESSO test to obtain a steel plate having a short crack branch 3a as shown schematically in Fig. Confirmed. It is presumed that the stress is relaxed by the branch 3a of the crack. 1 (a) and 1 (b) schematically show that the cracks 3 protruding from the notch 2 of the standard ESSO test piece 1 stop propagating to the tip shape 4 in the base material 5 .

2. In order to obtain the above-mentioned wave-front form, it is necessary to make a structure in which the cracks are branched. It is more advantageous to have a steel structure mainly composed of bainite in which a packet or the like is present, rather than a steel structure mainly composed of ferrite. It is also effective to integrate the (100) plane, which is a cleavage plane, obliquely with respect to the rolling direction or the plate width direction, which is the advancing direction of the crack.

3. As a result of observing and analyzing the wavefront of the standard ESSO test in detail, it is effective to control the material of the center of the plate thickness, which is the tip of the crack, to improve the performance of the arc. It is particularly effective to satisfy the following expression (1) as an index relating to toughness and texture of the plate thickness center portion.

vTrs _{(1 / 2t)} -12 x I _{RD // (110) [1 / 2t]} ? -70 ... (One)

vTrs _{(1 / 2t)} : Wavefront transition temperature at the center of plate thickness (캜)

I _{RD // (110) [1 / 2t]} : density of RD // (110) plane at the center of the plate thickness

t: plate thickness (mm)

4. Further, in the state where the austenite is in the recrystallization temperature range, the rolling is carried out at a cumulative rolling reduction of 20% or more to achieve grain refinement. Thereafter, the accumulated reduction ratio is set to 40% or more in a state of being in the austenite non-recrystallization temperature region. In addition, by rolling the difference between the rolling temperature of the first pass and the rolling temperature of the last pass within 40 DEG C, the texture of the central portion of the plate thickness can be controlled to realize the above-described structure.

The present invention is further based on the obtained recognition, and is further reviewed. That is,

1. The metal structure is a bainite-based material and has an aggregate I of an RD // (110) plane (Rolling Direction parallel to (110) plane) at the center of the plate thickness of 1.5 or more, Wherein the Charpy wavefront transition temperature of the steel sheet is in the range of vTrs ≤ -40 DEG C, and the brittle crack propagation stopping property is excellent.

2. The structural steel for high strength after breaking excellent in brittle crack propagation stopping property as set forth in 1, wherein the Charpy toughness value at the central portion of the plate thickness and the degree of integration I of the RD // (110) face satisfy the following expression (1).

vTrs _{(1 / 2t)} -12 x I _{RD // (110) [1 / 2t]} ? -70 ... (One)

vTrs _{(1 / 2t)} : Wavefront transition temperature at the center of plate thickness (캜)

I _{RD // (110) [1 / 2t]} : density of RD // (110) plane at the center of the plate thickness

t: plate thickness (mm)

3. A steel composition, comprising, by mass%, 0.03 to 0.20% of C, 0.03 to 0.5% of Si, 0.5 to 2.5% of Mn, 0.005 to 0.08% of Al, 0.005% or less of N, 0.005 to 0.03% of N, and the balance of Fe and inevitable impurities. The steel sheet according to any one of 1 to 2, wherein the steel has excellent brittle crack propagation stopping properties.

4. The steel composition according to claim 1, further comprising, by mass%, 0.005 to 0.05% of Nb, 0.01 to 0.5% of Cu, 0.01 to 1.0% of Ni, 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.010% or less.

5. A slab having a composition according to any one of claims 3 or 4 is heated to a temperature of 1000 to 1200 占 폚 and the sum of the cumulative rolling reduction in the austenite recrystallization temperature range and the austenite non- Rolling at least 65% is carried out. At this time, the cumulative rolling reduction ratio is 20% or more when the center portion of the plate thickness is in the austenite recrystallization temperature range. Then, when the central portion of the plate thickness is in the austenite non-recrystallization temperature range, the cumulative rolling reduction is 40% or more, and during the rolling in the state where the plate thickness central portion is in the austenite non- The difference between the rolling temperature of the first pass and the rolling temperature of the last pass falls within 40 占 폚. And then cooled to 450 DEG C or less at a cooling rate of 4 DEG C / s or more. The method for manufacturing a structural high strength steel sheet excellent in brittle crack propagation stopping characteristics.

6. A method of manufacturing a high strength steel plate excellent in brittle crack propagation stopping property as set forth in 5, wherein the steel sheet has a step of accelerating and cooling the steel sheet to 450 DEG C or less and further tempering the steel sheet at a temperature not higher than the _Ac1 point.

According to the present invention, it is possible to obtain a high strength steel sheet having a plate thickness of 50 mm or more and a manufacturing method thereof, wherein the aggregate structure is appropriately controlled in the plate thickness direction and the brittle crack propagation stopping property is excellent, More preferably, to a steel sheet having a thickness of 55 mm or more. In the field of shipbuilding, it is very useful in industry such as contributing to the safety of a ship by applying it to hatch side coaming or deck member in the strength deck structure of large container lines and bulk carriers.

Fig. 1 is a view schematically showing a wavefront form of a standard ESSO test of a steel sheet with a plate thickness exceeding 50 mm. Fig. 1 (a) is a view of the test piece observed from the plane side, Fig.

(Mode for carrying out the invention)

In the present invention, 1. the toughness and texture of the central portion of the plate thickness, and 2. the metal texture are defined.

1. Toughness and texture

In the present invention, in order to improve the crack propagation stopping property with respect to cracks advancing in the horizontal direction (in-plane direction of the steel sheet) in the rolling direction or in the direction perpendicular to the rolling direction, toughness at the center of the plate thickness and RD / The degree of integration I is appropriately defined according to the desired brittle crack propagation stopping characteristics.

First, the good toughness of the base material is a premise for suppressing crack propagation. Therefore, in the steel sheet according to the present invention, the Charpy wavefront transition temperature at the surface layer portion and the central portion of the plate thickness is specified to be -40 占 폚 or less. It is also preferable that the Charpy wavefront transition temperature at the central portion of the plate thickness is -50 캜 or less.

By developing the degree of integration I of the RD // (100) plane, brittle crack propagation stopping performance is improved by the effect of the stress relaxation at the brittle crack tip by causing the cleavage plane to be obliquely integrated with respect to the crack main direction, .

Brittle crack propagation stopping performance, which is considered to be a goal in securing structural safety, with a plate thickness exceeding 50 mm, which has been used for the outer shell of a ship such as a container line or a bulk carrier in recent years, is Kca (-10 ° C) ≥ In the case of obtaining 6000 N / mm ^3/2 , it is necessary to set the degree of integration I of the RD // (110) face at the central portion of the plate thickness to 1.5 or more, preferably 1.7 or more.

Here, the degree of integration I of the RD // (110) face at the center of the plate thickness means the following. First, a sample having a thickness of 1 mm is taken from the central portion of the plate thickness, and a surface parallel to the plate surface is subjected to mechanical polishing / electrolytic polishing to prepare a test piece for X-ray diffraction. X-ray diffraction measurement was carried out using this specimen and an Mo source to obtain pole figures of (200), (110) and (211) And the three dimensional orientation distribution function is obtained from Bunge's method. Next, from the obtained three-dimensional crystal orientation density function, the orientation in which the (110) plane becomes parallel to the rolling direction in a cross section of 19 pieces in total at intervals of 5 degrees from? ₂ = 0 to 90 degrees in Bunge notation And the integrated value of the three-dimensional crystal orientation density function is obtained. The value obtained by dividing the integrated value by the number of the accumulated orientations is called the integration degree I of the RD // (110) plane.

It is preferable that the Charpy toughness value at the central portion of the plate thickness and the degree of integration I of the RD // (110) surface satisfy the following expression (1), in addition to the above-described base material toughness and texture definition. By satisfying the following expression (1), more excellent brittle crack propagation stopping performance can be obtained.

vTrs _{(1 / 2t)} -12 x I _{RD // (110) [1 / 2t]} ? -70 ... (One)

vTrs _{(1 / 2t)} : Wavefront transition temperature at the center of plate thickness (캜)

I _{RD // (110) [1 / 2t]} : RD // (110) density at the center of the plate thickness

t: plate thickness (mm)

2. Metal structure

In the present invention, the metal structure is mainly made of bainite. When the metal structure is bainite-based, it is assumed that the area fraction of the bainite phase is 80% or more of the total. The remainder is ferrite, martensite (including island-shaped martensite), pearlite and the like in an area fraction of the total of 20% or less.

In order to obtain the above toughness and texture, it is effective to perform the controlled rolling at the austenite non-recrystallization temperature region and then transform into bainite. In the case of transformation from austenite to ferrite after rolling, although the intended toughness can not be obtained, since the transformation time is sufficiently present at the time of transformation from austenite to ferrite, the resulting structure becomes random, The degree of integration I of the RD // (110) plane can not attain 1.5 or more, preferably 1.7 or more. On the other hand, when a structure rolled at the austenite non-recrystallization temperature region is transformed into bainite, a so-called variant is selected in which the transformation time is not sufficient and the texture of the specific orientation is preferentially formed , And the degree of integration I of the RD // (110) plane is 1.5 or more, preferably 1.7 or more. Therefore, the metal structure obtained after rolling and cooling becomes bainite-based.

3. Chemical composition

Hereinafter, preferred chemical components in the present invention will be described. In the description,% is 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. When it exceeds 0.20%, weldability is deteriorated and toughness is also 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 0.5%

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 0.5%, not only the surface properties of the steel are deteriorated but also the toughness is extremely deteriorated. Therefore, the addition amount is preferably 0.03% or more and 0.5% or less.

Mn: 0.5 to 2.5%

Mn is added as a strengthening element. If it is less than 0.5%, the effect is not sufficient. If it exceeds 2.5%, the weldability deteriorates and the cost of the steel material also increases, so that it is preferably 0.5% or more and 2.5 or less.

Al: 0.005 to 0.08%

Al serves as a deoxidizing agent, which requires a content of 0.005% or more, but 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, the content of Al is preferably set in the range of 0.005 to 0.08%, more preferably 0.02 to 0.04%.

N: 0.0050% or less

N bonds with Al in the steel to form AIN, thereby adjusting the crystal grain diameter at the time of rolling and strengthening the steel. However, when N exceeds 0.0050%, toughness deteriorates, so that N is preferably 0.0050% or less.

P, S

P and S are inevitable impurities in the steel. When P is more than 0.03%, S is more than 0.01%, toughness is deteriorated. Therefore, it is preferably not more than 0.03% and not more than 0.01% , And still more preferably 0.005% or less.

Ti: 0.005 to 0.03%

Ti has an effect of forming a nitride, a carbide, or a carbonitride by the addition of a minute amount and improving the toughness of the base material by refining the crystal grains. The effect is obtained by the content of 0.005% or more, but the content exceeding 0.03% lowers the toughness of the base material and the weld heat affected zone, so that the Ti content is preferably in the range of 0.005 to 0.03%.

However, in order to further improve the characteristics, it is possible to contain at least one of Nb, Cu, Ni, Cr, Mo, V, B, Ca and REM.

Nb: 0.005 to 0.05%

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 temperature region in the rolling of the austenite region and contributes to grain refinement of the bainite packet, which is also effective in improving toughness. The effect is exhibited by the addition of 0.005% or more, but when it is added in an amount exceeding 0.05%, coarse NbC precipitates and conversely the toughness is lowered. Therefore, the upper limit is preferably 0.05%.

Cu, Ni, Cr, Mo

Cu, Ni, Cr, and Mo all increase the quenching properties of the steel. And can be added for the purpose of improving functions such as toughness, high-temperature strength, weather resistance, and the like. These effects can be exhibited by containing not less than 0.01% Is preferably 0.01% or more. However, if it is contained excessively, the toughness and weldability are deteriorated. Therefore, when it is contained, the upper limit is preferably 0.5% for Cu, 1.0% for Ni, 0.5% for Cr and 0.5% for Mo.

V: 0.001 to 0.10%

V is an element that improves the strength of steel by precipitation strengthening as V (C, N). In order to exhibit this effect, the content may be 0.001% or more, but if it exceeds 0.10%, the toughness is lowered. Therefore, when V is contained, it is preferable that the content is in the range of 0.001 to 0.10%.

B: not more than 0.0030%

B may be added as an element to increase the quenching property of the steel in a trace amount. 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.

Ca: not more than 0.0050%, REM: not more than 0.010%

Since Ca and REM improve the toughness of the weld heat affected zone to improve the toughness and the effect of the present invention is not impaired, it may be added as needed. However, if it is contained excessively, coarse inclusions are formed to deteriorate the toughness of the base material. Therefore, in case of incorporation, it is preferable that Ca is 0.0050% and REM is 0.010%.

4. Manufacturing conditions

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

As the manufacturing conditions, it is preferable to specify the heating temperature of the steel material (slab), the hot rolling condition, the cooling condition, and the like. Particularly, in the hot rolling, in addition to the cumulative rolling reduction in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range, the plate thickness central portion is in the austenite recrystallization temperature region and the austenite non- It is preferable that the cumulative rolling reduction ratio is defined for each of the cases and the temperature condition of rolling in a state where the center of the plate thickness is in the austenite non-recrystallized region. By defining these, the toughness at the surface layer portion and the plate thickness central portion of the post-steel sheet, the RD // (110) density I at the central portion of the plate thickness, and the strength at 1/4 of the plate thickness, .

Molten steel of the above composition is first melted in a converter furnace or the like, and made into a steel material by continuous casting or the like. Subsequently, the steel material is heated to a temperature of 1000 to 1200 占 폚, followed by hot rolling.

When the heating temperature is less than 1000 占 폚, it is not possible to sufficiently secure the time for rolling in the austenite recrystallization temperature range. When the temperature exceeds 1200 ° C, the austenite grains are coarse to cause deterioration of toughness, oxidation loss becomes remarkable, and the yield decreases. Therefore, the heating temperature is preferably 1000 to 1200 ° C. A more preferable heating temperature range from the viewpoint of toughness is 1000 to 1150 占 폚.

In the present invention, it is preferable to specify the hot rolling condition and the subsequent cooling conditions as described below. As a result, since the structure rolled at the austenite non-recrystallization temperature is transformed into bainite, the selection of the so-called variant in which the aggregate structure of the specific orientation is preferentially formed because the transformation time in this case is not sufficient The integration degree I of the RD // (110) plane can be set to 1.5 or more, preferably 1.7 or more.

In the hot rolling, it is preferable to perform the rolling at a cumulative rolling reduction of 20% or more in a state where the central portion of the plate thickness is in the austenite recrystallization temperature range. By setting the cumulative reduction ratio to 20% or more, austenite becomes fine and the finally obtained metal structure is also refined to improve toughness. If the cumulative rolling reduction is less than 20%, the austenite is not sufficiently refined and toughness is not improved in the finally obtained structure.

Next, it is preferable to perform rolling at a cumulative rolling reduction of 40% or more in a state where the temperature of the center of the plate thickness is in the austenite non-recrystallization temperature range. By setting the cumulative reduction ratio at the temperature range to 40% or more, the texture of the central portion of the plate thickness is sufficiently developed, and the degree of integration I of the RD // (110) plane at the center of the plate thickness is set to 1.5 or more, preferably 1.7 or more can do.

Further, if the temperature in the central portion of the plate thickness is in the austenite non-recrystallization temperature range and the rolling takes too much time, the structure becomes coarse and the toughness is lowered. Therefore, it is preferable that the difference between the rolling temperature of the first pass and the rolling temperature of the last pass during the rolling in the state where the center of the plate thickness is in the austenite non-recrystallized zone is within 40 占 폚. Here, the rolling temperature refers to the temperature of the central portion of the steel sheet immediately before rolling. The temperature at the central portion of the plate thickness is obtained by simulation calculation or the like from the plate thickness, the surface temperature, and the heat history. For example, the temperature distribution in the plate thickness direction is calculated using the difference method, whereby the temperature at the center of the plate thickness of the steel sheet is obtained.

It is preferable that the cumulative rolling reduction ratio of the total of the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more. If the overall reduction rate is small, the reduction of the structure is not sufficient, and the toughness and strength can not attain the desired value. By setting the cumulative rolling reduction ratio to 65% or more as a whole, it is possible to secure a sufficient amount of rolling reduction for the structure, and toughness and strength can attain desired values.

The austenite recrystallization temperature zone and the austenite non-recrystallization zone temperature can be grasped by performing a preliminary experiment to give a heat with a changed condition to a steel having the composition of the composition.

The finish temperature of the hot rolling is not particularly limited. However, from the viewpoint of the rolling efficiency, it is preferable to terminate the heat treatment in the austenite non-recrystallization temperature range.

The steel sheet after completion of rolling is preferably cooled to 450 DEG C or less at a cooling rate of 4 DEG C / s or more. By setting the cooling rate to 4 DEG C / s or higher, a fine bainite structure can be obtained by suppressing the ferrite transformation without coarsening of the structure, thereby achieving the desired excellent toughness, texture and strength . When the cooling rate is less than 4 DEG C / s, since the coarsening of the structure and the ferrite transformation progress at each plate thickness position, not only a desired structure is obtained but also the strength of the steel sheet is lowered. By setting the cooling stop temperature to 450 DEG C or lower, the bainite transformation can be sufficiently advanced, and a metal structure having desired toughness and texture can be obtained. When the cooling stop temperature is higher than 450 DEG C, the bainite transformation does not proceed sufficiently and a structure such as ferrite and pearlite is also generated, and the bainite-based structure targeted by the present invention can not be obtained. The cooling rate and the cooling stop temperature are set at the central portion of the thickness of the steel sheet. The temperature at the central portion of the plate thickness is obtained by simulation calculation or the like from the plate thickness, the surface temperature, and the cooling condition. For example, the temperature distribution in the plate thickness direction is calculated using the difference method, whereby the temperature at the center of the plate thickness of the steel sheet is obtained.

It is also possible to perform the tempering treatment on the cooled steel sheet. By performing the tempering, the toughness of the steel sheet can be further improved. The tempering temperature is set to be equal to or less than the A _c1 point of the steel sheet average temperature, so that the desired structure obtained by rolling and cooling can be prevented from being damaged. In the present invention, the point A _C1 (° C) is obtained by the following formula.

A _C1 point = 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B

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

The average temperature of the steel sheet is obtained by simulation calculation or the like from the plate thickness, the surface temperature and the cooling condition, etc., in the same manner as the temperature at the center of the plate thickness.

[Example]

The steel materials (steel symbols A to O) of each composition shown in Table 1 were melted in a converter and made into a steel material (slab thickness: 250 mm) by continuous casting, hot rolled to a plate thickness of 50 to 90 mm, Nos. 1 to 30 of the disclosed steel were obtained. Table 2 shows the hot rolling and cooling conditions.

The resulting steel sheet was subjected to a tensile test to determine the Yield Strength and Tensile Strength of JIS 14A test specimens having a diameter of 14 mm from 1/4 sheet thickness in such a manner that the longitudinal direction of the test pieces was perpendicular to the rolling direction, Strength) was measured.

An impact test piece of JIS No. 4 was taken from 1/2 of the plate thickness so that the direction of the shrinkage of the test piece was parallel to the rolling direction, and the Charpy impact test was conducted to obtain the wavefront transition temperature. Here, the impression test piece of the surface layer portion is assumed to have a depth of 1 mm from the surface of the steel sheet nearest to the surface.

Next, in order to evaluate the brittle crack propagation stopping characteristics, a standard ESSO test was performed to obtain a Kca value (Kca (-10 DEG C)) at -10 DEG C.

Further, the degree of integration I of the RD // (110) plane at the center of the plate thickness was obtained as follows. First, a sample having a thickness of 1 mm was taken from the central portion of the plate thickness, and a surface parallel to the plate surface was subjected to mechanical polishing and electrolytic polishing to prepare a test piece for X-ray diffraction. X-ray diffraction measurement was carried out using this test piece and an Mo source, and the (200), (110) and (211) positive electrode viscosities were obtained. From the obtained positive electrode viscosity, the three-dimensional crystal orientation density function was calculated by Bunge's method. Next, from the obtained three-dimensional crystal orientation density function, the orientation in which the (110) plane becomes parallel to the rolling direction in a cross section of 19 pieces in total at intervals of 5 deg. From? ₂ = 0 to 90 deg. Dimensional crystal orientation density function of the three-dimensional crystal orientation density function was integrated to obtain an integrated value. The value obtained by dividing the integrated value by the number of the accumulated orientations 19 was defined as the degree of integration I of the RD // (110) plane.

Table 3 shows the results of these tests. (10 ° C) of 6,000 N / mm ^3/2 or more in the case of a steel sheet having a toughness value in the central portion of the plate thickness and the aggregate structure within the range of the present invention (Production Nos. 1 to 13 and 27 to 30) Crack propagation stopping performance. Further, in the case of the published steel sheets (manufactured numbers 1 to 13) in which the Charpy toughness value and the RD // (110) density I of the surface layer portion and the plate thickness center satisfy the expression (1) A value of Kca (-10 DEG C) higher than that of the known steel sheets (manufactured numbers 27 to 30) was obtained.

On the other hand, although the composition of the steel sheet is within the preferred range of the present invention, the steel sheet (Production Nos. 21 to 26) in which the heating and rolling conditions in the steel sheet production conditions are outside the preferred range of the present invention, The value did not reach 6000 N / mm ^3/2 . In the steel sheets (Manufacturing Numbers 22, 23, and 26), the texture of the steel sheet does not meet the requirements of the present invention. For the disclosed components of the steel composition was outside the acceptable range of the invention steel sheet (Preparation No. 14-20), the toughness of the steel sheet does not satisfy the regulations of the present invention the value of Kca (-10 ℃) is 6000N / ㎜ ^{3 / 2} did not reach.

Further, in the case of a known steel sheet (Production Nos. 14 to 26) in which at least one of the toughness value and the texture in the central portion of the plate thickness is outside the scope of the present invention, Kca (-10 캜) does not reach 6000 N / mm ^3/2 I did.

1: Standard ESSO test specimen
2: Notch
3: Crack
3a: Branch
4: tip shape
5: Base material

Claims (8)

Wherein the metal structure is a bainite-based body and the integrated degree I of the RD // (110) face at the center of the plate thickness is 1.5 or more, and the Charpy wavefront transition temperature vTrs in the surface layer portion and the plate thickness central portion is -40 DEG C By mass or less, and having excellent brittle crack propagation stopping property. The method according to claim 1,
Wherein the Charpy wavefront transition temperature at the central portion of the plate thickness and the degree of integration I of the RD // (110) face satisfy the following formula (1).
vTrs _{(1 / 2t)} -12 x I _{RD // (110) [1 / 2t]} ? -70 ... (One)
vTrs _{(1 / 2t)} : Wavefront transition temperature at the center of plate thickness (캜)
I _{RD // (110) [1 / 2t]} : density of RD // (110) plane at the center of the plate thickness
t: plate thickness (mm) 3. The method according to claim 1 or 2,
The steel according to any one of claims 1 to 3, wherein the steel composition contains 0.03 to 0.20% of C, 0.03 to 0.5% of Si, 0.5 to 2.5% of Mn, 0.005 to 0.08% of Al, 0.03% , N: 0.0050% or less, and Ti: 0.005 to 0.03%, the balance being Fe and inevitable impurities. The method of claim 3,
The steel composition further contains, by mass%, 0.005 to 0.05% of Nb, 0.01 to 0.5% of Cu, 0.01 to 1.0% of Ni, 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.010% or less. A steel material having the composition described in claim 3 is heated to a temperature of 1000 to 1200 占 폚 and rolled at a total of 65% or more of the austenite recrystallization temperature zone and the austenite non- At this time, when the central portion of the plate thickness is in the austenite recrystallization temperature range, the cumulative reduction ratio is 20% or more. Then, when the center portion of the plate thickness is in the austenite non- recrystallization temperature range, The difference between the rolling temperature of the first pass and the rolling temperature of the last pass during rolling in a state where the plate thickness central portion is in the austenite non-recrystallization temperature range is within 40 占 폚, Wherein the steel sheet is cooled to 450 DEG C or less at a cooling rate of 4 DEG C / s or more, and the brittle crack propagation stopping property is excellent. 6. The method of claim 5,
A step of accelerating and cooling the steel sheet at a temperature of 450 DEG C or less and further tempering the steel sheet at a temperature not higher than the _Ac1 point, wherein the steel sheet has excellent brittle crack propagation stopping properties. A steel material having the composition described in claim 4 is heated to a temperature of 1000 to 1200 占 폚 and rolled at a total cumulative reduction ratio at a temperature range of austenite recrystallization temperature and austenite non recrystallization temperature of 65% At this time, when the central portion of the plate thickness is in the austenite recrystallization temperature range, the cumulative rolling reduction is not less than 20%, and when the plate thickness central portion is in the austenite non-recrystallization temperature range, the cumulative rolling reduction is 40% , And the difference between the rolling temperature of the first pass and the rolling temperature of the last pass during the rolling in the state where the center of the plate thickness is in the austenite non-recrystallization temperature range is within 40 DEG C, wherein the steel sheet is cooled to 450 DEG C or less at a cooling rate of 50 DEG C or more. 8. The method of claim 7,
A step of accelerating and cooling the steel sheet at a temperature of 450 DEG C or less and further tempering the steel sheet at a temperature not higher than the _Ac1 point, wherein the steel sheet has excellent brittle crack propagation stopping properties.

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