KR20150126697A - High strength thick steel plate for high heat input welding with excellent brittle crack arrestability and manufacturing method therefor - Google Patents

Abstract

Provided is a high strength steel sheet for high heat input welding excellent in brittle crack propagation stopping property suitable for use in ships and having a plate thickness of 50 mm or more, and a method for producing the same. (110) surface in the central portion of the plate thickness has an aggregate texture of 1.5 to 4.0, and the Charpy wavefront transition at the surface layer portion and the plate thickness central portion has a specific component composition, the metal structure is bainite- A steel sheet having a temperature vTrs of -40 캜 or lower and a method for producing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet for high heat input welding which is excellent in brittle crack propagation stopping properties and a method for manufacturing the steel sheet. 2. Description of the Related Art HIGH STRENGTH THICK STEEL PLATE FOR HIGH HEAT INPUT WELDING WITH EXCELLENT BRITTLE CRACK ARRESTABILITY AND MANUFACTURING METHOD THEREFOR

The present invention relates to a high-strength thick steel plate for high heat input welding having excellent brittle crack arrestability and a method for manufacturing the same. More particularly, the present invention relates to a high- A plate thickness of 50 mm or more suitable for the following.

In large structures such as ships, brittle fracture accidents have a large impact on the economy and the environment. For this reason, safety is always required to be improved, and toughness and brittle crack propagation stopping characteristics at the operating temperature are required for steel materials to be used.

A vessel such as a container ship or a bulk carrier uses a steel plate having a heavy thickness for the outer plate of the ship's hull. In recent years, with the increase in the size of the hull, further high-strength thickening progresses, and in general, the brittle crack propagation stopping property of the steel sheet tends to deteriorate as the strength or shore resistance, It is further improving.

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 a storage tank of liquefied natural gas (LNG), 9% Ni steel is used on a commercial scale have.

However, the increase in the amount of Ni is inevitable, and therefore it is difficult to apply to applications other than the LNG storage tank.

On the other hand, a comparatively thin steel plate having a plate 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 manufactured by a thermo-mechanical control process By providing a decreasing a crystal grain size and improving the low-temperature toughness, excellent brittle crack propagation stopping properties can be imparted.

Further, in order to improve the brittle crack propagation stopping property without raising the alloy cost, a steel material having an ultra fine grained microstructure of the surface layer portion is proposed in Patent Document 1. [

In Patent Document 1, attention is paid to the fact that a shear lip (shear-lips) generated in a surface layer of steel material is effective in improving brittle crack propagation stopping characteristics when a brittle crack propagates, Which is characterized by being made fine and absorbing the propagation energy possessed by the propagating brittle cracks, which is excellent in brittle crack propagation stopping property.

In Patent Document 1, the surface layer portion is cooled to an A _r3 transformation point or lower by controlled cooling after hot rolling, and then controlled cooling is stopped to recuperate the surface layer portion beyond the transformation point, And the steel material is subjected to repetitive transformation or machining recrystallization by applying a pressure drop between the steel material and the ferrite structure or bainite structure at the surface layer portion .

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 form a layer having a ferrite structure having a ferrite structure having a ferrite grain size of 5 μm or less and an aspect ratio of the grains of 2 or more to suppress unevenness of the ferrite grain size. As a method of suppressing unevenness, it is described that the maximum rolling reduction per pass in finish rolling is set to 12% or less and local recrystallization is suppressed.

However, since a steel material excellent in brittle crack propagation stopping property described in Patent Documents 1 and 2 is obtained by cooling only the surface layer portion of steel once and then heating the steel surface portion, and further processing is performed in the double heat, a specific structure is obtained. It is not easy. Particularly, 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.

Specifically, in a steel sheet having a thickness of 30 to 40 mm, it is not necessary to control the surface temperature of the steel sheet such that cooling and repetition of the surface layer are complicated, and (a) rolling conditions for securing fine ferrite grains, (b) (C) a dislocation introduced by machining (rolling) while developing a texture in the fine ferrite is rearranged by thermal energy, and (b) And (d) cooling conditions for suppressing the formation of fine ferrite grains and fine grain grains of fine grains, thereby improving brittle crack propagation stopping characteristics.

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. This method generates separation on the fracture surface of the steel 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 set to 2 or more by controlled rolling, (diameter equivalent to a circle in the crystal grains) of 20 mu m or more is 10% or less.

Patent Document 5 discloses a welded structure steel excellent in brittle crack propagation stopping performance of a joint part and having an X-ray surface strength ratio of a (100) face of 1.5 or more on a rolled surface inside a plate thickness, Discloses that excellent brittle crack propagation stopping property can be obtained by a deviation between 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 Japanese Patent Publication No. 3546308

 Inoue et al.: Longitudinal brittle crack propagation behavior in thick steel for shipbuilding, Journal of the Japan Society of Naval Architects and Ocean Engineers Third lecture, 2006, pp359 ~ 362  "Design Guidelines for Brittle Cracked Arrests" September 2009, Japan Society of Naval Architects

However, in 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 evaluates the brittle crack propagation stopping performance of a steel sheet having a thickness of 65 mm, and reports the result that the brittle crack does not stop in the large brittle crack propagation stop test of the base material.

Also, in the standard ESSO test (ESSO test compliant with WES 3003) of the test specimen, the value of Kca (hereinafter also referred to as Kca (-10 ° C)) at the operating temperature of -10 ° C is less than 3000 N / mm ^3/2 In the case of a hull structure in which a steel sheet having a thickness exceeding 50 mm is applied, it has been suggested that securing safety is a problem.

It is considered that the steel sheet having excellent brittle crack propagation stopping properties described in the above-mentioned Patent Documents 1 to 5 is mainly a steel sheet having a thickness of about 50 mm from the manufacturing conditions and the disclosed experimental data. It is unclear whether a predetermined characteristic can be obtained when the technique described in Patent Documents 1 to 5 is applied to a skirting material exceeding 50 mm and the characteristics of crack propagation in the plate thickness direction required in the hull structure are not verified at all.

On the other hand, with the thickening of the steel sheet, the welding operation is performed with a high efficiency (high efficiency) such as submerged arc welding, electrogas arc welding, electroslag welding, Thermal welding is applied. Generally, it is known that when the heat input amount of the welding becomes large, the structure of the heat affected zone (HAZ) becomes coarse, and the toughness of the weld heat affected zone is lowered. In order to solve the problem of reduction in toughness caused by such large heat welding, a steel material for large heat input welding has already been developed and put to practical use. For example, in Patent Document 6, coarsening of the weld heat affected zone structure is prevented by controlling TiN precipitated in the steel, and the in-crystal grain ferrite transformation Thereby increasing the toughness of the welding heat affected zone. However, although the toughness of the weld heat affected zone of the large heat weld zone is excellent, brittle crack propagation stop characteristics are not considered, and satisfactory characteristics are not obtained.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide a heat treatment method and a heat treatment method capable of stably producing a steel sheet with excellent brittle crack propagation stopping properties, Strength steel sheet and a method of manufacturing the same.

DISCLOSURE OF THE INVENTION The present inventors have repeatedly made intensive studies for attaining the above-described object, and have obtained the following perceptions for a high strength steel sheet having excellent brittle crack propagation stopping properties even in case of a steel sheet.

1. Standard steel ESSO tests were conducted on steel plates with plate thicknesses exceeding 50 mm. 1 (a) and 1 (b) show an example in which the crack 3 protruding from the notch 2 of the standard ESSO test piece 1 stops propagating to the tip shape 4 in the base material 5 And it is confirmed that when the branch 3a of short cracks as shown schematically in Fig. 1 (a) is confirmed, a high degree of arrestability can be obtained. It is presumed that the stress is relaxed by the branch 3a of cracks.

2. In order to obtain the above-mentioned wave-front form, it is necessary to make a structure in which the cracks are branched. Here, it is advantageous to use 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. In addition, the (100) plane, which is a cleavage plane, It is effective to integrate them obliquely with respect to the rolling direction or the plate width direction.

3. On the other hand, if the degree of integration of the (100) plane is excessively increased, a branch of a large crack occurs from a very short crack branch. In the standard ESSO test, it is necessary to suppress the branching of the brittle crack as described in Non-Patent Document 2 showing the brittle crack array design guide of the hull structure. Therefore, in order to prevent the branching of the brittle crack, It is necessary to define the upper limit of

4. As a result of detailed observation and analysis of the wavefront of the standard ESSO test, it is effective to control the material of the central part of the plate thickness, which is the tip of the crack, to improve the performance of the arrester. Especially, It is effective to satisfy the following expression (2).

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

In the above formula (2)

vTrs _{(1 / 2t)} : Wavefront transition temperature (° C) at the plate thickness center part (= 1 / 2t)

I _{RD // (110) [1 / 2t]} : the degree of integration of the RD // (110) plane of the central portion of the plate thickness (= 1 / 2t)

t is the plate thickness (mm).

5. Further, in the state where the temperature is in the austenite recrystallization temperature range, the rolling is carried out at a cumulative rolling reduction of 20% or more to achieve grain refinement of the structure, and thereafter, a state in the austenite non- , Rolling is performed at a cumulative reduction ratio of 40 to 70% and a difference between the rolling temperature of the first pass and the rolling temperature of the last pass is within 40 DEG C, thereby controlling the texture of the central portion of the plate thickness , The above-described organization can be realized.

6. As a method for improving the toughness of the heat-welded portion of the primary heat source, the composite sulfide of TiN, CaS and MnS is finely divided to inhibit grain growth when exposed to high temperatures of welding, It is effective to miniaturize the structure of the heat-affected portion at room temperature.

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

1. A steel composition comprising, by mass%, 0.03 to 0.15% of C, 0.01 to 0.5% of Si, 1.40 to 2.50% of Mn, 0.005 to 0.08% of Al, 0.03% or less of P and 0.0005 to 0.0030% , N: 0.0036 to 0.0070%, Ti: 0.004 to 0.030%, Ca: 0.0005 to 0.0030%, and each content of Ca, S and O satisfies the following formula (1) (110) surface in the central portion of the plate thickness is 1.5 to 4.0, and the Charpy in the surface layer portion and the central portion of the plate thickness are inevitable impurities, and the metal structure is mainly composed of bainite, And has a wave front transition temperature (vTrs) of -40 占 폚 or less, and has excellent brittle crack propagation stopping properties.

0.30? (Ca- (0.18 + 130 x Ca) x O) /1.25/S? 0.80 (1)

In the formula (1), Ca, O and S are defined as the content (% by mass).

2. The steel composition according to claim 1, further comprising, by mass%, Nb: not more than 0.05%, Cu: not more than 1.0%, Ni: not more than 1.0%, Cr: not more than 0.5%, Mo: not more than 0.5%, V: : Not more than 0.003%, and REM: not more than 0.01%. The high strength steel sheet for high-strength heat welding with excellent brittle crack propagation stopping properties as set forth in 1 above.

(3) The brittle crack propagation stop characteristics described in (1) or (2), wherein the Charpy wavefront transition temperature at the central portion of the plate thickness and the degree of integration of the RD // High strength steel plate.

However, in the formula (2)

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

vTrs _{(1 / 2t)} : Wavefront transition temperature (° C) of plate thickness center part (1 / 2t)

I _{RD // (110) [1 / 2t]} : The degree of integration of the RD // (110) surface of the plate thickness center part (1 / 2t).

Also, t is the plate thickness (mm).

4. The steel material having the composition described in 1 or 2 is heated to a temperature of 1000 to 1200 캜 to perform rolling in which the cumulative rolling reduction ratio in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more At this time, when the central portion of the plate thickness is in the austenite recrystallization temperature range, the rolling is performed at a cumulative rolling reduction of 20% or more. Then, in a state where the plate thickness central portion is in the austenite non- Is 40 to 70%, 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 40 Deg.] C, and thereafter cooled to 450 DEG C or less at a cooling rate of 4.0 DEG C / s or more. A method of manufacturing a high strength steel plate for welding.

5. The method of claim 4, further comprising the step of tempering at a temperature of not more than A _c1 after accelerated cooling to 450 캜 or less. ≪ / RTI >

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain a high strength low-strength steel sheet excellent in brittle crack propagation stopping property and high heat resistance welded joint toughness, and a method for producing the same. It is preferable to apply the present invention 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, more preferably a plate thickness of 60 mm or more, It is effective because it exerts a remarkable superiority. For example, in the shipbuilding industry, by applying the present invention to hatch side coaming or deck members in the strength deck structure of a large container line and a bulk carrier, Very useful.

Fig. 1 schematically shows 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. 1 (b) Fig.

(Mode for carrying out the invention)

In the present invention, 1. steel composition, 2. plate toughness, toughness at the center portion and texture at the center of the plate thickness, 3. metal structure, and 4. manufacturing conditions.

1. Steel composition

Hereinafter, preferred chemical components in the present invention will be described. In the description,% is mass%.

C: 0.03 to 0.15%

C is an element which improves the strength of steel. In the present invention, it is required to contain 0.03% or more in order to secure a desired strength. On the other hand, if it exceeds 0.15%, not only the weldability is deteriorated but also the toughness is adversely affected. For this reason, C is specified in the range of 0.03 to 0.15%. Preferably, it is 0.05 to 0.15%.

Si: 0.01 to 0.5%

Si is effective as a deoxidizing element and also as a strengthening element of steel. However, when the content is less than 0.01%, the effect is not obtained. On the other hand, if it exceeds 0.5%, not only the surface quality of the steel is deteriorated but also the toughness is extremely deteriorated. Therefore, the addition amount is set at 0.01 to 0.5%. Preferably, it is in the range of 0.02 to 0.45%.

Mn: 1.40 to 2.50%

Mn is added as a strengthening element. If it is less than 1.40%, the effect is not sufficient. On the other hand, if it exceeds 2.50%, the weldability deteriorates and the cost of steel increases. Therefore, Mn is set to 1.40 to 2.50%. Preferably, it ranges from 1.42 to 2.40%.

P: not more than 0.03%

If P is more than 0.03%, the toughness of the welded portion is remarkably deteriorated. Therefore, the upper limit is set to 0.03%. It is preferably 0.02% or less.

S: 0.0005 to 0.0030%

S is required in excess of 0.0005% to produce the required CaS and MnS. On the other hand, if it exceeds 0.0030%, the toughness of the base material deteriorates. Therefore, S is set to 0.0005 to 0.0030%. And preferably in the range of 0.0006 to 0.0025%.

Al: 0.005 to 0.08%

Al acts as a deoxidizer, and it requires a content 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. For this reason, Al is specified in the range of 0.005 to 0.08%. Preferably, it is 0.02 to 0.06%.

Ti: 0.004 to 0.030%

By adding a trace amount of Ti, a nitride, a carbide or a carbonitride is formed, the coarsening of austenite in the weld heat affected zone is suppressed, and / or the ferrite transformation is promoted as ferrite transformation nuclei, And has an effect of improving the toughness of the base material. The effect is obtained by adding 0.004% or more. However, the content exceeding 0.030% lowers the toughness of the base material and the weld heat affected zone by coarsening of the TiN particles. For this reason, Ti is set in the range of 0.004 to 0.030%. Preferably, it is in the range of 0.006 to 0.028%.

N: 0.0036 to 0.0070%

N is an element necessary for securing a necessary amount of TiN. If it is less than 0.0036%, a sufficient amount of TiN can not be obtained and the toughness of the welded portion deteriorates. If it exceeds 0.0070%, TiN is re-dissolved when a welding heat cycle is received, and solid solute N is excessively generated, and the toughness is remarkably deteriorated. Therefore, N is set to 0.0036 to 0.0070%. Preferably, it is in the range of 0.0038 to 0.0065%.

Ca: 0.0005 to 0.0030%

Ca is an element having an effect of improving toughness by fixing S. In order to exhibit such an effect, it is necessary to contain at least 0.0005% or more. However, if the content exceeds 0.0030%, the effect is saturated. Therefore, in the present invention, Ca is limited to a range of 0.0005 to 0.0030%. And preferably in the range of 0.0007 to 0.0028%.

In the present invention, it is necessary to satisfy the following expression (1).

0.30? (Ca- (0.18 + 130 x Ca) x O) /1.25/S? 0.80 (1)

In the formula (1), Ca, O and S are defined as the content (% by mass).

It is necessary to contain Ca and S so as to satisfy the relationship of the formula (1). In this case, a form of complex sulfide in which MnS is precipitated on CaS is formed. Since this complex sulfide functions as the nucleus of the ferrite transformation, the structure of the weld heat affected zone becomes finer and the toughness of the weld heat affected zone is improved. (Ca- (0.18 + 130 x Ca) x O) /1.25/S is less than 0.30, CaS is not crystallized, so S precipitates in the form of MnS alone. This MnS is elongated by rolling at the time of steel sheet production, causing deterioration of the toughness of the base material, and since MnS is melted at the weld heat affected zone, which is the main point of the present invention, fine dispersion is not achieved. On the other hand, if the value of (Ca- (0.18 + 130 x Ca) x O) /1.25/S exceeds 0.80, S is almost fixed by Ca and MnS serving as a ferrite generating nucleus is not precipitated on CaS , Sufficient toughness improvement is not achieved. The preferable range of the value of (Ca- (0.18 + 130 x Ca) x O) /1.25/S is 0.32 to 0.78%.

The above is the basic composition of the present invention. It is possible to contain at least one of Nb, Cu, Ni, Cr, Mo, V, B and REM in order to further improve the characteristics.

Nb: not more than 0.05%

Nb is precipitated as NbC upon ferrite transformation or reheating, and contributes 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 for improving toughness. Since the effect is exhibited by containing 0.005% or more, it is preferable that the effect is 0.005% or more. However, when it is added in an amount exceeding 0.05%, coarse NbC precipitates and conversely toughness is lowered. Therefore, when it is contained, the upper limit is preferably 0.05%. More preferably, it is in the range of 0.007 to 0.045%.

Cu, Ni, Cr, Mo

Cu, Ni, Cr, and Mo are all elements that increase the hardenability of the steel. Can be added for the purpose of improving the toughness, high-temperature strength, weather resistance, and the like, while contributing directly to increase in strength after rolling. When these effects are exhibited by containing 0.01% or more, % Or more. However, if it is contained excessively, toughness and weldability are deteriorated, and therefore, when it is contained, it is preferable that the upper limit is 1.0% for Cu, 1.0% for Ni, 0.5% for Cr and 0.5% for Mo. More preferably, it is in a range of 0.02 to 0.95% of Cu, 0.02 to 0.95% of Ni, 0.02 to 0.46% of Cr and 0.02 to 0.46% of Mo.

V: not more than 0.2%

V is V (C, N), an element which improves the strength of steel by precipitation strengthening, and may be contained in an amount of 0.001% or more so as to exhibit this effect. However, if it exceeds 0.2%, the toughness is lowered. Therefore, when V is contained, the content is preferably 0.2% or less, and more preferably 0.001 to 0.10%.

B: not more than 0.003%

B is an element which increases the quenching property of the steel in a minute amount, and may be contained in an amount of 0.0005% or more in order to exhibit this effect. However, if it is contained in an amount exceeding 0.003%, the toughness of the welded part is lowered. Therefore, when B is contained, the content is preferably 0.003% or less. More preferably, it is in the range of 0.0006 to 0.0025%.

REM: Not more than 0.01%

The REM may be added as necessary since the effect of the present invention is not impaired even if the REM improves the texture of the weld heat affected zone and improves toughness. Since this effect is exhibited by containing 0.0010% or more, it is preferable that the effect is 0.0010% or more when contained. However, if it is added excessively, a coarse inclusion is formed and the toughness of the base material is deteriorated. Therefore, when the additive is added, the upper limit of the addition amount is preferably 0.01%.

In addition, O is contained in the steel as an inevitable impurity, thereby lowering the cleanliness. Therefore, in the present invention, it is preferable to reduce O as much as possible. In particular, when the O content exceeds 0.0050%, the CaO inclusions are coarsened and the toughness of the base material is lowered. Therefore, it is preferably 0.0050% or less.

In the present invention, in order to crystallize Ca as CaS, it is necessary to reduce the amount of O having strong bonding force with Ca before Ca addition, and it is preferable that the amount of remaining oxygen before Ca addition is 0.0050% or less. As a method for reducing the residual oxygen amount, it is possible to employ methods such as strengthening degassing, or introducing a deoxidizing agent.

The remainder other than the above-mentioned components are Fe and inevitable impurities.

2. Plate thickness The toughness of the surface and center parts and the texture of the central part of the plate thickness

In the present invention, in order to improve the brittle crack propagation stopping property against a crack progressing in the horizontal direction (in-plane direction of the steel sheet) in the rolling direction or in the direction perpendicular to the rolling direction, the toughness in the surface layer portion and the central portion, The degree of integration of the RD // (100) plane in the brittle crack propagation stopping property is appropriately defined in accordance with the desired brittle crack propagation stopping property.

First, good base material toughness is a prerequisite for suppressing crack propagation. In the steel sheet according to the present invention, as the toughness at the plate thickness portion and the center portion, the Charpy fracture surface transition temperature at the plate thickness portion and the central portion 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.

Further, by developing the texture of the RD // (100) plane, by integrating the cleavage plane obliquely with respect to the main direction of cracking, by the effect of stress relaxation at the tip of the brittle crack due to generation of fine cracks The brittle crack propagation stopping performance is improved. 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) ≥ When obtaining 6000 N / mm ^3/2 , it is necessary to set the degree of integration of the RD // (110) face at the center of the plate thickness to 1.5 or more, preferably 1.7 or more. Therefore, in the present invention, the degree of integration of the RD // (110) face at the central portion of the plate thickness is 1.5 or more, preferably 1.7 or more.

On the other hand, if the degree of integration of the RD // (110) face in the central portion of the plate thickness is more than 4.0, the aggregate structure is excessively developed, so that not a fine crack branching occurs but brittle cracks are clearly branched, The brittle crack propagation stopping performance due to the stress relaxation effect at the crack tip becomes difficult to be exerted. Therefore, the degree of integration of the RD // (110) plane is set in the range of 1.5 to 4.0.

Here, the degree of integration 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 mechanically polished and electrolytically polished to prepare a test piece for X-ray diffraction. Using this specimen, the X-ray diffraction apparatus was used with an Mo source, and X-ray diffraction measurement was carried out to obtain the pole figure of the (200), (110) and (211) Dimensional crystal orientation density function is calculated by Bunge's method. Next, from the obtained three-dimensional crystal orientation density function, in a cross-sectional view of 19 pieces in total at intervals of 5 degrees from ψ ₂ = 0 ° to 90 ° in the Bunge notation, the orientation of the (110) plane parallel to the rolling direction The value obtained by integrating the values of the three-dimensional crystal orientation density function to obtain the integrated value and dividing the integrated value by the number of the accumulated orientations is referred to as the integration degree of the RD // (110) plane.

It is preferable that the Charpy wavefront transition temperature at the center of the plate thickness and the degree of integration of the RD // (110) plane satisfy the following formula (2), in addition to the above-mentioned base material toughness and texture definition. By satisfying the following expression (2), more excellent brittle crack propagation stopping performance can be obtained.

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

However, in the formula (2)

vTrs _{(1 / 2t)} : Charpy 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.

Also, t is the plate thickness (mm).

3. Metal structure

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 desired toughness is 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 of the RD // (110) plane is not more than 1.5, preferably not more than 1.7. On the other hand, in the case where the structure rolled at the austenite non-recrystallization temperature region is transformed into bainite, the transformation time is not sufficient, so that the texture of the specific orientation is preferentially formed, The integration degree of the RD // (110) plane is 1.5 or more, preferably 1.7 or more. Therefore, the metal structure obtained after rolling and cooling is mainly made of bainite. In the present invention, the metal structure is bainite-based, which means that the area fraction of the bainite phase is 80% or more of the total. The remainder is allowed if the total area fraction of ferrite, martensite (including island-shaped martensite), pearlite and the like is 20% or less.

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, 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 to define the cumulative rolling reduction ratio and the temperature condition of rolling in a state where the central portion of the plate thickness is in the austenite non-recrystallized region. By defining these, the Charpy wavefront transition temperature vTrs and the RD // (110) density at the central portion of the plate thickness can be set to a desired value at the surface layer portion and the plate thickness center portion of the steel sheet.

Molten steel having the above composition is firstly melted in a converter or the like and made into a steel material (slab) by continuous casting or the like.

Next, it is preferable to heat the steel material to a temperature of 1000 to 1200 占 폚 and then perform 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. On the other hand, when the temperature exceeds 1200 ° C, the austenite grains are coarsened, resulting in deterioration of the toughness as well as remarkable oxidation loss and lower yield. Therefore, the heating temperature is preferably 1000 to 1200 占 폚. 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 transformation time in this case is not sufficient, so that the texture of the specific orientation is preferentially formed, the degree of integration of the RD // (110) plane can be made to be 1.5 or more, preferably 1.7 or more, by performing variant selection.

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 center 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 the rolling at a cumulative rolling reduction of 40 to 70% or more in a state where the temperature of the center of the plate thickness is in the austenite non-recrystallization temperature range. By making the cumulative reduction ratio at this temperature range equal to or more than 40%, the texture of the central portion of the plate thickness is sufficiently developed, and the degree of integration of the RD // (110) face at the center of the plate thickness is set to 1.5 or more, preferably 1.7 or more .

If the cumulative rolling reduction at this temperature range exceeds 70%, the aggregate structure is excessively developed and the degree of integration of the RD // (110) surface exceeds 4.0. Therefore, the range of the cumulative reduction ratio is set to 40 to 70%.

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 region is within 40 캜. 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 can be 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.

The cumulative rolling reduction ratio of the total of the austenite recrystallization temperature zone and the austenite non-recrystallization temperature zone is preferably 65% or more. If the overall reduction rate is small, the reduction of the structure is not sufficient, and the toughness and the strength can not achieve the desired value. By setting the cumulative rolling reduction ratio to 65% or more as a whole, it is possible to secure a sufficient reduction in the amount of tearing on the structure, and the toughness and the degree of integration 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. From the viewpoint of the rolling efficiency, it is preferable to terminate in the austenite non-recrystallization temperature range.

The rolled steel sheet is preferably cooled to 450 DEG C or less at a cooling rate of 4.0 DEG C / s or more. By setting the cooling rate to 4.0 DEG C / s or higher, the structure is not coarsened, and the ferrite transformation is suppressed, whereby the fine bainite structure can be obtained and the desired excellent toughness and the degree of integration can be obtained. When the cooling rate is less than 4.0 占 폚 / s, since the coarsening of the structure and the ferrite transformation proceed at each plate thickness position, desired structure is not obtained and the strength of the steel sheet is also lowered.

By setting the cooling stop temperature to 450 占 폚 or less, the bainite transformation can be sufficiently advanced, and desired toughness and densities 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 or pearlite is also produced, and the bainite-based structure of 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 center of the plate thickness can be obtained by simulation calculation or the like from the plate thickness, the surface temperature, and the cooling conditions. 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 steel sheet after cooling has been completed. By performing tempering, the toughness of the steel sheet can be further improved. The tempering temperature is set to the A _C1 point or less as the steel sheet average temperature, and the tempering treatment can be performed so as not to damage the desired structure obtained by rolling and cooling. 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 +

233 Nb-39.7 V-5.7 Ti-895B

In the above formula, the symbol of each element is the content (mass%) in the steel and is set to 0 when 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

(Steel symbols A to Q) having the compositions shown in Table 1 were melted by a converter and made into a steel material (slab 250 mm in thickness or 300 mm in thickness) by continuous casting, hot rolled to a thickness of 55 to 100 mm, Cooling was carried out. 1 to 27 were obtained. For some, tempering was also performed after cooling. Table 2 shows the hot rolling and cooling conditions.

The obtained steel sheet was subjected to a tensile test to obtain a yield strength (YS) and a tensile strength (TS) of JIS 14A test specimens having a diameter of 14 mm from 1/4 of the plate thickness so that the longitudinal direction of the test pieces was perpendicular to the rolling direction ) Were measured.

Further, in order to evaluate the toughness value, the JIS No. 4 impact test specimen was measured from the surface layer portion of the thickness and the center portion of the plate thickness (hereinafter, the center portion of the plate thickness was sometimes referred to as 1/2 t portion) And the Charpy impact test was carried out to obtain wave front transition temperatures (vTrs). 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.

After the obtained steel sheet was subjected to mirror-surface polishing of the plate thickness cross section parallel to the rolling length direction of the steel sheet, the metal structure developed by etching was observed by an optical microscope.

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

Also, the degree of integration 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. Using this specimen, X-ray diffraction measurement was carried out using an Mo source and an X-ray diffractometer to obtain the positive electrode viscosity of (200), (110) and (211) The crystal orientation distribution density function is calculated by Bunge's method. From the obtained three-dimensional crystal orientation distribution density function, ψ ₂ = 0 to 90 °, Bunge's notation shows a total of 19 cross sections in a total of 19 sections, The value obtained by integrating the values of the dimensional crystal orientation distribution density function and obtaining the integrated value and dividing the integrated value by the number of the accumulated orientations is defined as the degree of integration of the RD // (110) plane.

In order to evaluate the welding heat characteristic of the substitution heat, the disclosed steel sheet was subjected to a forming a groove (improvement angle: 20 °) and welded by electro-gas welding using commercially available low- / Cm, and the toughness of the bond portion was evaluated as a HAZ toughness by a 2 mmV notch Charpy test. The test was carried out with vE- ₂₀ (average of three values) of the Charpy absorbed energy at -20 ° C.

Table 3 shows the results of these tests. (No. 1 to 11) exhibited excellent brittle crack propagation stopping performance of Kca (-10 DEG C) of 6000 N / mm < ³ > / ² or more in the range of the present invention. Also, the absorbed energy at the bond portion of the heat input weld joint was: vE-20 ≥ 88J, which was excellent. (2) to (11) in which the Charpy toughness value (wave-front transition temperature) and the RD // (110) density of the surface layer portion and the plate thickness center satisfy the expression (2) (-10 [deg.] C) as compared with a known steel sheet (manufactured No. 1) which does not satisfy the above requirement. In addition, the metal structures of these disclosed steel plates (Manufacturing Nos. 1 to 11) were mainly bainite-based.

On the other hand, although the steel sheet components are within the preferred range of the present invention, the steel plates (Production Nos. 20 to 27) in which the heating and rolling conditions in the steel sheet production conditions are outside the preferred range of the present invention, The value of 6000 N / mm ^3/2 did not reach the value. For the steel plates (Manufacturing Nos. 12 to 19) in which the components of the steel sheet did not meet the conditions of the present invention, the absorbed energy of the heat input welded joint: vE- ₂₀ was 22 J or less, which was inferior to that of the present invention .

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

Claims (5)

A steel composition comprising, by mass%, 0.03 to 0.15% of C, 0.01 to 0.5% of Si, 1.40 to 2.50% of Mn, 0.005 to 0.08% of Al, 0.03% or less of P, 0.0005 to 0.0030% of S, : 0.0036 to 0.0070%, Ti: 0.004 to 0.030%, Ca: 0.0005 to 0.0030%, and each content of Ca, S and O satisfies the following formula (1) (110) surface in the central portion of the plate thickness is 1.5 to 4.0, and the Charpy wavefront transition in the surface layer portion and the plate thickness central portion Characterized in that the temperature vTrs is not higher than -40 占 폚, and the brittle crack propagation stopping property is excellent.
0.30? (Ca- (0.18 + 130 x Ca) x O) /1.25/S? 0.80 (1)
In the formula (1), Ca, O and S are defined as the content (% by mass). The method according to claim 1,
The steel composition further contains, by mass%, Nb: not more than 0.05%, Cu: not more than 1.0%, Ni: not more than 1.0%, Cr: not more than 0.5%, Mo: not more than 0.5%, V: not more than 0.2% % Or less, and REM: 0.01% or less in terms of brittle crack propagation stopping property. 3. The method according to claim 1 or 2,
Wherein the Charpy wavefront transition temperature at the central portion of the plate thickness and the degree of integration of the RD // (110) face satisfy the following formula (2).
vTrs _{(1 / 2t)} -12 x I _{RD // (110) [1 / 2t]} ? -70 (2)
However, in the formula (2)
vTrs _{(1 / 2t)} : Charpy wavefront transition temperature (° C) of plate thickness center part (1 / 2t)
I _{RD // (110) [1 / 2t]} : The degree of integration of the RD // (110) surface of the plate thickness center part (1 / 2t).
Also, t is the plate thickness (mm). A steel material having a composition according to any one of claims 1 to 6 is heated to a temperature of 1000 to 1200 占 폚 and the total of the cumulative rolling reduction in the austenite recrystallization temperature range and the austenite non- %. At this time, when the central portion of the plate thickness is in the austenite recrystallization temperature range, rolling at a cumulative rolling reduction of 20% or more is performed. Subsequently, the central portion of the plate thickness is in the austenite non- The rolling temperature is set so that the cumulative rolling reduction is in the range of 40 to 70% and the rolling temperature in 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- Is cooled to 450 占 폚 or less at a cooling rate of 4.0 占 폚 / s or more, and the brittle crack propagation stopping property A method for producing a substituted thermal welding high strength steel sheet after. 5. The method of claim 4,
Further _comprising the step of tempering the steel sheet at a temperature of not more than A _c1 after accelerated cooling to 450 DEG C or less.

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