KR20170117235A - High strength thick steel plate with superior brittle crack arrestability and high heat input welding performance and method for manufacturing same - Google Patents

Abstract

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 of producing the same. The degree of integration of the RD // (110) plane at the plate thickness portion is 1.3 or more, the degree of integration of the RD // (110) plane at the center of the plate thickness, Of 1.8 or more and having a Charpy wavefront transition temperature vTrs of -50 캜 or less at a plate thickness portion and a plate thickness central portion, 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 having excellent brittle crack propagation stopping characteristics and high heat input welding characteristics,

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

In large structures such as ships, accidents involving brittle fracture 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 the steel material to be used.

Container ships, bulk carriers and other vessels use high-strength large thickness materials for their outer plates of ship's hull. In recent years, with the increase in the size of the hull, the high strength thickening progresses further. In general, the brittle crack propagation stopping property of the steel sheet tends to deteriorate as the high strength or the shrunk material. Therefore, the demand for brittle crack propagation stopping property is further enhanced .

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

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

On the other hand, a comparatively thin steel 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 subjected to a decreasing process by a thermo-mechanical control process a grain size) is improved to improve the low-temperature toughness, whereby 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 (ultrafine grained steel) in which the structure of the surface layer of the plate thickness is made ultra fine is proposed in Patent Document 1.

In Patent Document 1, attention is paid to the fact that a shear-lips (plastic deformation region) generated in a surface layer portion of a steel sheet is effective in improving brittle crack propagation stopping characteristics when a brittle crack propagates, Which is characterized in that crystal grains are refined to absorb the propagation energy of propagating brittle cracks, and a steel material excellent in brittle crack propagation stopping property is disclosed.

Patent Document 1 discloses a method of cooling a portion of the surface layer of the plate thickness below the transformation point of Ar ₃ by controlled cooling after hot rolling and then stopping the controlled cooling, The ferrite structure or the bainite structure is formed in the surface layer portion by repeatedly transforming or recrystallizing the steel material by repeatedly performing the recuperate process one or more times, thereby producing a bainite structure.

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

However, since the steel material having excellent brittle crack propagation stoppage characteristics disclosed in Patent Documents 1 and 2 is obtained by cooling only the surface layer portion of the steel sheet once and then heating the same, The control 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 processing (rolling) while developing an aggregate texture in the fine ferrite is rearranged by thermal energy to produce a fine ferrite structure And (d) the brittle crack propagation stopping property is improved by cooling conditions for suppressing coarsening of fine fine ferrite grains and fine sub-grain grains formed.

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 relieving 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, (grain size of 20 탆 or more in diameter equivalent to a circle in the crystal grains) of not more than 10% improves the brittle fracture characteristics.

Patent Document 5 discloses a welded structure steel excellent in brittle crack propagation stopping performance of a welded joint and characterized in that the X-ray plane intensity ratio of the (100) face on the rolled surface inside the plate thickness is 1.5 or more And excellent brittle crack propagation stopping characteristics can be obtained by a displacement 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

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. In Non-Patent Document 1, the brittle crack propagation stopping performance of a steel sheet having a thickness of 65 mm was evaluated, and a result of the brittle crack not stopping in the large brittle crack propagation stop test of the base material was reported.

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 is less than 3000 N / mm ^3/2 The results show that, in the case of a hull structure to which a steel sheet having a thickness exceeding 50 mm is applied, securing of 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 as the heat input of the welding becomes larger, the structure of the heat affected zone (HAZ) becomes coarse, so that the toughness of the weld heat affected zone is lowered. In order to solve the problem of reduction in toughness due to such large heat welding, a steel material for large heat input welding has already been developed and put to practical use. For example, Patent Document 6 discloses a technique for preventing coarsening of a weld heat affected part structure by controlling TiN precipitated in a steel, and also preventing grain coarsening in grains by dispersion of ferrite generating nuclei, Discloses a technique for increasing the toughness of a weld heat affected zone by promoting ferrite transformation. However, although the toughness of the weld heat affected zone of the large heat weld zone is excellent, the brittle crack propagation stop characteristics are not taken into consideration and satisfactory properties are not obtained.

Therefore, the present invention is to provide a high-strength, high-strength, high-strength, high-strength, high-strength, high-strength, high- Steel sheet and a method for producing the same.

The present inventors repeatedly conducted extensive research aimed at achieving the above-described problems, and obtained the following perceptions for a high strength steel plate having excellent brittle crack propagation stopping properties even in case of a steel sheet under a low temperature.

1. Detailed examination of the wave front of the standard ESSO test for steel sheets with a plate thickness exceeding 50 mm resulted in a decrease in the width of the brittle cracks in the case of a wavefront shape as shown in Fig.1 (b) As a result, the stress intensity factor of the crack tip becomes smaller, and as a result, the arrestability of the steel sheet increases. 1 (a) and 1 (b) schematically show an example in which 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 wavefront shape as described above, it is necessary to improve the performance of the edge of the plate thickness portion and the center portion of the plate thickness. It is effective to improve the toughness of the plate thickness portion and the plate thickness central portion as a method for improving the performance of the plate thickness portion and the center portion of the plate thickness. However, there is a limitation in improving the toughness of the central portion of the plate thickness because there is a limitation in the cooling rate and the reduction rate in the steel sheet having a plate thickness exceeding 50 mm.

3. As a method for improving the performance of the alloy in addition to the improvement in toughness, it is effective to control the texture of the central portion of the plate thickness. In particular, integrating (110) planes parallel to the rolling direction, It is effective to perform the texture control so that the advancing cracks are deviated obliquely from the rolling direction or the plate width direction, respectively.

4. Further, by making the cumulative reduction ratio at 20% or more and the average reduction ratio per one pass at 5.0% or less in the state where the plate thickness central portion is in the austenite recrystallization temperature range, the structure of the plate thickness portion can be miniaturized do. Thereafter, when the cumulative reduction ratio in the state where the center of the plate thickness is in the austenite non-recrystallization temperature range is 40% or more and the average reduction rate per pass is 7.0% or more, the toughness and aggregate structure It is possible to develop the above-described organization.

5. As a method for improving the toughness of the heat-welded portion, it is possible to finely divide the complex sulfide of TiN, CaS and MnS, to inhibit the growing of grains when exposed to a high temperature by welding, It is effective to accelerate the transformation of the grain during the cooling process to refine the structure of the heat affected zone 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, 0.50% or less of Si, 1.0 to 2.0% of Mn, 0.030% or less of P, 0.0005 to 0.0040% of S, 0.005 to 0.10% 0.004 to 0.030% of Ti, 0.0036 to 0.0075% of N, 0.0005 to 0.0030% of Ca and 0.0040% or less of O, and each content of Ca, O and S satisfies the following formula (1) (110) surface in the plate thickness portion is 1.3 or more, and the RD // (110) surface in the plate thickness center portion is a ferrite- And the Charpy fracture transition temperature vTrs at the center of the plate thickness portion and the plate thickness portion is not higher than -50 캜. The brittle crack propagation stopping property is excellent in the substitution heat welding High strength steel plate.

0 < (Ca- (0.18 + 130 x Ca) x 0) /1.25/S? (One)

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

(2) satisfying the following expression (2), characterized in that the Charpy wavefront transition temperature at the plate thickness part and the plate thickness center and the degree of integration of the RD // High strength steel for welding.

vTrs _{(surface layer)} + 1.9 x vTrs _{(1 / 2t)} -6 x I _{RD // (110) [surface layer]} -84 x I _{RD // (110) [1 / 2t]} (2)

However, in the formula (2)

vTrs _{(surface layer)} : Charpy wavefront transition temperature (° C)

vTrs _{(1 / 2t)} : Charpy wavefront transition temperature (° C) of plate thickness central portion (t / 2)

I _{RD // (110) [surface layer]} : density of the RD // (110) surface of the plate thickness portion

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

Also, t is the plate thickness (mm).

3. The steel composition according to claim 1, further comprising, by mass%, 0.003 to 0.050% Nb, 0.5% or less Cu, 0.5% or less Ni, 0.5% or less of Cr, V: not more than 0.2%, and B: not more than 0.0003 to not more than 0.0030%, wherein the brittle crack propagation stopping property is excellent.

4. The steel composition according to any one of claims 1 to 3, wherein the steel composition further contains at least one of Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020%, and REM: 0.001 to 0.020% And the brittle crack propagation stopping property described in any one of < 1 >

5. A steel material having a composition according to any one of claims 1, 3 or 4, which is heated to a temperature of 900 to 1150 캜 and a total of cumulative rolling reduction at a temperature range of austenite recrystallization temperature and austenite non- , The rolling reduction is performed so that the cumulative rolling reduction is 20% or more and the average rolling reduction per one pass is 5.0% or less in the state where the center of the plate thickness is in the austenite recrystallization temperature range. Subsequently, Rolling was carried out at a temperature in the kneading temperature range of not lower than 40 占 폚 and at an average rolling reduction per pass of not less than 7.0% By weight based on the total weight of the high-strength steel sheet and the brittle crack propagation stopping property.

6. Preparation of a mixture was cooled to below 600 ℃, adding, A _C1 point below the tempering temperature (tempering) for the high-strength steel sheet excellent in heat 5 brittle crack propagation stop characteristics according to, it characterized in that a step of welding is substituted by a Way.

According to the present invention, there is obtained a high strength steel sheet for large heat input welding, which has an aggregate structure appropriately controlled in the plate thickness direction and excellent in brittle crack propagation stopping property, and a method for producing the same. The application of the present invention to a steel sheet having a plate thickness of 50 mm or more, preferably more than 50 mm, more preferably 55 mm or more, and even more preferably 60 mm or more, And therefore, it is effective. 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.

1 is a schematic view showing a wavefront form of a standard steel ESSO test of a steel sheet having a plate thickness of more than 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 thickness, toughness and aggregate structure at the center portion, 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 expressed in 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. Therefore, C is set to 0.03 to 0.15%. Preferably 0.05 to 0.15%.

Si: 0.50% or less

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 may not be obtained. On the other hand, if it exceeds 0.50%, not only the surface quality of the steel is deteriorated but also the toughness is deteriorated. Therefore, the addition amount should be 0.50% or less. Preferably, it is in the range of 0.01 to 0.40%.

Mn: 1.0 to 2.0%

Mn is added as a strengthening element. If it is less than 1.0%, the effect is not sufficient. On the other hand, if it exceeds 2.0%, the toughness of the welded portion is deteriorated. Therefore, Mn is set to 1.0 to 2.0%. Preferably, it is in the range of 1.1 to 1.8%.

P: not more than 0.030%

P is an inevitably incorporated impurity, and if it exceeds 0.030%, the toughness of the base material and the welded portion is lowered. Therefore, the upper limit is set to 0.030%.

S: 0.0005 to 0.0040%

S is required in excess of 0.0005% to produce the required CaS or MnS. On the other hand, if it exceeds 0.0040%, the toughness of the base material deteriorates. Therefore, S is set to 0.0005 to 0.0040%.

Al: 0.005 to 0.10%

Al acts as a deoxidizing agent, which requires a content of not less than 0.005%. However, if the content exceeds 0.10%, the toughness is lowered and, in the case of welding, the toughness of the weld metal portion is lowered. Therefore, Al is defined in the range of 0.005 to 0.10%. Preferably 0.005 to 0.08%, and more preferably 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 0.0075%

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. When the Ti content exceeds 0.0075%, TiN re-dissolves when it is subjected to a welding heat cycle, so that solid solute N is excessively generated, and the toughness is remarkably deteriorated. Therefore, N is set to 0.0036 to 0.0075%. Preferably, it is in the range of 0.0037 to 0.0068%.

Ca: 0.0005 to 0.0030%

Ca is an element having an effect of improving toughness by fixing S. In order to exhibit such effects, it is necessary to contain Ca in an amount of 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%.

O: 0.0040% or less

O is an inevitable impurity contained in the steel, becomes an oxide at the time of solidification, and precipitates, thereby lowering the cleanliness of the steel. Therefore, in the present invention, it is preferable to reduce O as much as possible. Particularly, when the content of O exceeds 0.0040%, the CaO inclusions are coarsened and the toughness of the base material is lowered. Therefore, O is set to 0.0040% or less.

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

0 < (Ca- (0.18 + 130 x Ca) x O) /1.25/S<0.8. (One)

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

Ca, O and S must be contained 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, CaS does not crystallize, so S precipitates in the form of MnS alone. This MnS is elongated by the rolling at the time of steel sheet production and causes deterioration of the toughness of the base material, and the MnS is melted in the weld heat affected zone, which is the main point of the present invention. On the other hand, when the value of (Ca- (0.18 + 130 x Ca) x O) /1.25/S exceeds 0.8, 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 0.10 to 0.8%.

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

Nb: 0.003 to 0.050%

Nb is precipitated as NbC upon ferrite transformation or reheating, and contributes to enhancement of strength. In addition, it has an effect of expanding the non-recrystallized temperature region in the rolling of the austenite region and contributes to grain refinement of ferrite and bainite. The effect is exhibited by containing Nb in an amount of 0.003% or more, and when it is contained, it is preferably 0.003% or more. However, when it is added in an amount exceeding 0.050%, coarse NbC precipitates and conversely toughness is lowered. Therefore, when it is contained, the upper limit is preferably 0.050%. More preferably, it is in the range of 0.005 to 0.040%.

Cu, Ni, Cr, Mo, W

Cu, Ni, Cr, Mo, and W are all elements that increase the hardenability of the steel. It can be directly added to improve the strength after rolling, and can be added for improving functions such as toughness, high temperature strength, weather resistance, and the like. These effects are exhibited by the content of not less than 0.01%, and when it is contained, the content is preferably not less than 0.01%. However, if it is contained excessively, the toughness and weldability are deteriorated. Therefore, it is preferable that the upper limit is 0.5% for Cu, 1.0% for Ni, 0.5% for Cr, 0.5% for Mo and 0.4% for W Do.

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: 0.0003 to 0.0030%

B is an element that enhances the quenching of the steel in a trace amount. In order to exhibit this effect, 0.0003% or more may be contained. 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 in the range of 0.0003 to 0.0026%.

In the present invention, in order to improve the characteristics, it is possible to contain at least one of Mg, Zr, and REM in addition to the above-mentioned component composition.

Mg: 0.0005 to 0.0050%

Since Mg is an element having an effect of improving toughness by dispersion of oxides, Mg may be added as needed. Since this effect is exhibited by containing 0.0005% or more, it is preferable that the effect is 0.0005% or more. However, since the effect is saturated even when it is contained in an amount exceeding 0.0050%, in the case of adding Mg, the addition amount is preferably 0.0005 to 0.0050%.

Zr: 0.001 to 0.020%

Zr has an effect of forming oxides in the steel and dispersing the oxides thereof, thereby improving the toughness of the welded heat affected zone, thereby improving the toughness. Since this effect is exhibited by containing 0.001% or more, it is preferable that the effect is 0.001% or more. However, if it is added excessively, the effect becomes saturated, and coarse inclusions are formed to deteriorate the toughness of the base material. Therefore, when added, the upper limit of the addition amount is preferably 0.020%.

REM: 0.001 to 0.020%

The REM has the effect of forming oxides in the steel and dispersing the oxides, thereby improving the toughness of the weld heat affected zone by improving the toughness. The REM does not impair the effect of the present invention. Since this effect is exhibited by containing 0.001% or more, it is preferable that the effect is 0.001% or more. However, if it is added excessively, the effect becomes saturated, and coarse inclusions are formed to deteriorate toughness of the base material. Therefore, when added, the upper limit of the addition amount is preferably 0.020%.

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, a method of strengthening degassing or adding a deoxidizing agent can be employed.

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

2. Plate thickness Toughness and center texture

In the present invention, in order to obtain the wave form of FIG. 1 capable of improving the crack propagation stopping property against the 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, And the degree of integration of the RD // (110) plane.

First, good parent material toughness is a prerequisite for suppressing crack propagation. In the steel sheet according to the present invention, the Charpy wavefront transition temperature vTrs in the plate thickness portion and the plate thickness central portion is specified to be -50 占 폚 or less as toughness in the plate thickness portion and the center portion.

In addition, by developing the texture of the RD // (110) plane, the cleavage planes are integrated obliquely with respect to the crack main direction, and the brittle crack tip The brittle crack propagation stopping performance is improved by the effect of stress relaxation. Recent brittleness of the container ship or a bulk carrier, such as in yukjae after exceeding the thickness 50㎜ be used in the hull, the Kca (-10 ℃) considered the goals when securing the structure stability ≥6000N / ㎜ ^3/2 The degree of integration of the RD // (110) face in the plate thickness portion is 1.3 or more, preferably 1.6 or more, and the degree of integration of the RD // (110) face in the center of the plate thickness is 1.8 Or more, and preferably 2.0 or more. Therefore, in the present invention, the degree of integration of the RD // (110) face in the plate thickness portion is 1.3 or more, preferably 1.6 or more, and the degree of integration of the RD // (110) face in the plate thickness central portion is 1.8 or more, Preferably 2.0 or more.

On the other hand, if the degree of integration of the RD // (110) plane in the plate thickness portion or the plate thickness central portion exceeds 4.0, the aggregate structure is excessively developed, so that not only fine crack branching occurs but also brittle cracks are clearly branched The brittle crack propagation stopping performance due to the stress relaxation effect of the brittle crack tip becomes difficult to be exerted. Therefore, it is preferable that the degree of integration of the RD // (110) plane in the plate thickness portion and the degree of integration of the RD // (110) plane in the center portion of the plate thickness are all 4.0 or less.

Here, the degree of integration of the RD // (110) surface in the plate thickness portion or the plate thickness center portion means the following. First, a sample having a thickness of 1 mm is taken from the surface layer portion or the center portion of the plate thickness, and a surface parallel to the surface of the plate is subjected to mechanical polishing and electrolytic polishing to prepare a test piece for X-ray diffraction . In the case of the sheet thickness surface layer portion, the surface closer to the outermost surface is to be polished. X-ray diffraction measurement was carried out using this test piece by using an X-ray source and an X-ray diffractometer to measure the (200), (110) and (211) ), And the orientation distribution functions of the three-dimensional crystal orientation are obtained from the obtained positive electrode viscosity 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 degrees from? ₂ = 0 to 90 degrees in Bunge notation Dimensional integrating value of the RD / (110) plane is obtained by integrating the value of the three-dimensional crystal orientation density function of the RD / (110) plane, and dividing the integrated value by the number of the accumulated orientations.

It is preferable that the Charpy wavefront transition temperature and the RD // (110) surface density of the plate thickness portion and the plate thickness central portion satisfy the following expression (2) in addition to the above-described base material toughness and texture definition.

vTrs _{(surface layer)} + 1.9 x vTrs _{(1 / 2t)} -6 x I _{RD // (110) [surface layer]} -84 x I _{RD // (110) [1 / 2t]} (2)

However, in the formula (2)

vTrs _{(surface layer)} : Charpy wavefront transition temperature (° C)

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

I _{RD // (110) [surface layer]} : density of the RD // (110) surface of the plate thickness portion

I _{RD // (110) [1 / 2t]} : This is the degree of integration of the RD // (110) plane at the center of the plate thickness. T is the plate thickness (mm).

By satisfying the above expression (2), more excellent brittle crack propagation stopping performance can be obtained.

3. Metal structure

In the present invention, it is assumed that the metal structure is a ferrite core. Here, in the present invention, it is assumed that the area fraction of the ferrite phase is 60% or more of the entire metal structure. The remainder is allowed if bainite, martensite (including island-shaped martensite), pearlite, etc. are 40% or less in total area fraction.

When a metal structure mainly composed of ferrite is obtained by the rolling conditions in the ordinary austenite back-rolling in a structure mainly composed of ferrite, although desired toughness can not be obtained, a transformation from austenite to ferrite after rolling (110) surface is 1.3 or more, and preferably 1.6 or more in the intended sheet thickness surface layer portion, and the degree of integration of RD / (110) , The degree of integration of the RD // (110) plane can not be achieved to be 1.8 or more, preferably 2.0 or more. Thus, by devising the rolling conditions as will be described later, the degree of integration of the RD // (110) face is 1.3 or more, preferably 1.6 or more, in the plate thickness portion of the ferrite- 110) plane is 1.8 or more, preferably 2.0 or more.

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 case of hot rolling, in addition to the cumulative rolling reduction ratio in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range, in addition to the case where the plate thickness central portion is in the austenite recrystallization temperature region, It is desirable to specify the cumulative reduction ratio and the average reduction ratio per pass. By defining these, desirable characteristics can be obtained for the degree of integration of the Charpy wavefront transition temperature vTrs and RD // (110) plane in the plate thickness portion and the plate thickness central 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.

Then, it is preferable to heat the steel material to a temperature of 900 to 1150 占 폚 and then perform hot rolling. In order to obtain good toughness, it is effective to lower the heating temperature and reduce the crystal grain size before rolling. However, when the heating temperature is lower than 900 占 폚, it is not possible to sufficiently secure the time for rolling in the austenite recrystallization temperature range. In addition, at a temperature higher than 1150 DEG C, the austenite grains are coarse and not only deteriorate toughness but also oxidative loss becomes remarkable and the yield is lowered. Therefore, the heating temperature is preferably 900 to 1150 캜. A more preferable heating temperature range from the viewpoint of toughness is 1000 to 1100 deg.

Generally, when a metal structure mainly composed of ferrite is obtained by performing normal austenite back-rolling, desired toughness can not be obtained. However, when transformation from austenite to ferrite occurs after rolling, As a result, the resulting texture becomes random. Therefore, in normal austenite reverse rolling, the degree of integration of the RD // (110) face in the sheet thickness surface layer portion of the present invention is 1.3 or more, preferably 1.6 or more, and the RD / / (110) plane can not attain 1.8 or more, preferably 2.0 or more. Therefore, in the present invention, it is preferable to specify the hot rolling condition as described below. Accordingly, even in the structure of the ferrite main body, the degree of integration of the RD // (110) Preferably 1.6 or more, and the degree of integration of the RD // (110) face in the central portion of the plate thickness is 1.8 or more, preferably 2.0 or more.

In the hot rolling, it is preferable that rolling is first performed so that the cumulative rolling reduction is not less than 20% and the average rolling reduction per one pass is 5.0% or less 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 is made fine and the finally obtained metal structure is also refined to improve toughness. On the other hand, by setting the average reduction ratio per pass in this temperature range to 5.0% or less, deformation can be introduced particularly near the surface layer of the steel material. As a result, the degree of integration of the RD // (110) surface in the surface layer portion of the sheet thickness can be made to be 1.3 or more, preferably 1.6 or more, and further the surface layer portion of the plate thickness is made fine and the toughness improving effect of the plate thickness portion is obtained .

Next, it is preferable to perform rolling with a cumulative rolling reduction of 40% or more and an average rolling reduction per pass of 7.0% 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 this temperature range to 40% or more, the texture of the central portion of the plate thickness is sufficiently developed and the average reduction ratio per pass is 7.0% or more, May be 1.8 or more, and preferably 2.0 or more.

Further, it is preferable that the cumulative rolling reduction ratio as a whole of the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more. 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 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 steel sheet after completion of rolling is preferably cooled to 600 占 폚 or less at a cooling rate of 4.0 占 폚 / s or more. By setting the cooling rate to 4.0 DEG C / s or more, the fine structure can be obtained without coarsening of the structure, and desired excellent toughness can be obtained. If the cooling rate is less than 4.0 DEG C / s, the structure becomes coarse and the target toughness can not be obtained. By keeping the cooling stop temperature at 600 占 폚 or less, progress of recrystallization can be avoided and the desired aggregate structure obtained by hot rolling and subsequent cooling can be maintained. If the cooling-stop temperature is higher than 600 ° C, recrystallization proceeds even after stopping the cooling, and desired aggregate structure 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, surface temperature, and cooling conditions. For example, the temperature at the center of the plate thickness of the steel sheet is obtained by calculating the temperature distribution in the plate thickness direction by using a difference method.

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 + 233Nb-39.7V-5.7Ti-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 1

(Steel symbols A to R) having the compositions shown in Table 1 were hot rolled to a thickness of 50 to 80 mm as a steel material (slab 250 mm in thickness) by a solvent in a converter and subjected to continuous casting, Nos. 1 to 30 of the disclosed steel were obtained. For some, tempering was also performed after cooling. Table 2 shows hot rolling, cooling and tempering conditions.

The obtained steel sheet was subjected to a tensile test to determine the yield strength (YS) and the tensile strength (YS) of the test piece of JIS 14A having a diameter of 14 from the ¼ portion of the thickness of the steel sheet so that the longitudinal direction of the test piece was perpendicular to the rolling direction TS; tensile strength) were measured.

After the obtained steel sheet was mirror-polished, the sheet metal cross-section parallel to the longitudinal direction of the steel sheet was mirror-polished, and the metal structure exposed and exposed by etching was observed by an optical microscope.

Further, in order to evaluate the toughness value, the JIS No. 4 impact test specimen was cut from the surface layer portion and the plate thickness central portion (hereinafter, the plate thickness central portion was sometimes referred to as 1/2 t portion) in the direction of the longitudinal axis of the test piece parallel to the rolling direction And a Charpy impact test was conducted to obtain the wave front transition temperature (vTrs). Here, the impact test piece of the surface layer portion of the plate thickness is assumed to have a depth of 1 mm from the surface of the steel sheet, the surface nearest to the surface.

Next, a standard ESSO test (temperature gradient type ESSO test) was conducted to evaluate the brittle crack propagation stopping characteristic, and the Kca value (Kca (-10 DEG C)) at -10 DEG C was obtained.

In order to evaluate the welding characteristics of the large heat input, the disclosed steel sheet was subjected to grooving (improvement angle: 20 °), and a commercially available low-temperature steel electrogas arc welding wire was used, Joints were made by electrosurgical welding (EGW) with large-volume heat welding (300 to 750 kJ / cm). Thereafter, using a Charpy impact test piece in which a notch was placed in a weld bond with respect to the surface of the plate thickness direction and the position of 1 mm of the back surface, the HAZ toughness was measured at a test temperature of -20 ° C Energy: vE -20 캜 (three average values) were obtained.

In addition, the degree of integration of the RD // (110) plane in the plate thickness portion and the plate thickness center portion was obtained as follows. First, a sample having a thickness of 1 mm is taken from the surface layer portion or the center portion of the plate thickness, and a surface parallel to the surface is mechanically polished and electrolytically polished to prepare a test piece for X-ray diffraction. In the case of the sheet thickness surface layer portion, the surface nearest to the outermost surface is polished. Using this test piece, 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 bearing density function is 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 degrees from? ₂ = 0 to 90 degrees in Bunge notation Dimensional crystal orientation density function of the RD / (110) plane is calculated to obtain the integrated value, and the value obtained by dividing the integrated value by the number of the accumulated orientations is defined as the degree of integration of the RD // (110) plane.

Table 3 shows the results of these tests. (10 ° C) of 6,000 N / mm in the case of the steel sheet having the toughness value (wave-surface transition temperature vTrs) and the aggregate structure within the range of the present invention (Manufacture Nos. ^3/2 or more, indicating excellent brittle crack propagation stopping performance. Also, the absorbed energy of the weld heat affected zone: vE -20 DEG C was 125 J or more, which was excellent. In addition, in the case of the published steel sheets (Manufacturing Numbers 1 to 13) 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 plate (production No. 14) which does not satisfy the above-mentioned requirements. In addition, the metal structures of these disclosed steel sheets (Manufacturing Nos. 1-14) were ferrite-based.

On the other hand, in the case of the disclosed steel plates (Production Nos. 15 to 30) in which the composition or the manufacturing conditions are outside the scope of the present invention and the toughness or texture of the steel sheet does not satisfy the requirements of the present invention, the value of Kca / Mm < ^3/2 >, and / or the absorption energy of the weld heat affected zone: vE -20 [deg.] C is less than 100J.

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

Claims (6)

A steel composition comprising, by mass%, 0.03 to 0.15% of C, 0.50% or less of Si, 1.0 to 2.0% of Mn, 0.030% or less of P, 0.0005 to 0.0040% of S, 0.005 to 0.10% 0.004 to 0.030% of N, 0.0036 to 0.0075% of N, 0.0005 to 0.0030% of Ca and 0.0040% or less of O, and each content of Ca, O and S satisfies the following formula (1) The degree of integration of the RD // (110) plane in the plate thickness portion is 1.3 or more, and the degree of integration of the RD // (110) plane in the plate thickness center portion is Fe / inevitable impurity, Of not less than 1.8 and having a Charpy wavefront transition temperature vTrs of not more than -50 캜 at a plate thickness portion and a center portion of the plate thickness, characterized by having a brittle crack propagation stopping property and a substitution heat welding property, .
0 < (Ca- (0.18 + 130 x Ca) x O) /1.25/S<0.8. (One)
In the formula (1), Ca, O and S are defined as the content (% by mass). The method according to claim 1,
Characterized in that the Charpy wavefront transition temperature at the plate thickness portion and the plate thickness center portion and the degree of integration of the RD // (110) face satisfy the following expression (2) High strength steel for welding.
vTrs _{(surface layer)} + 1.9 x vTrs _{(1 / 2t)} -6 x I _{RD // (110) [surface layer]} -84 x I _{RD // (110) [1 / 2t]} (2)
However, in the formula (2)
vTrs _{(surface layer)} : Charpy wavefront transition temperature (° C)
vTrs _{(1 / 2t)} : Charpy wavefront transition temperature (° C) of plate thickness central portion (t / 2)
I _{RD // (110) [surface layer]} : density of the RD // (110) surface of the plate thickness portion
I _{RD // (110) [1 / 2t]} : This is the degree of integration of the RD // (110) surface of the plate thickness center part (t / 2).
Also, t is the plate thickness (mm). The method according to claim 1,
Wherein the steel composition further contains at least one of the following groups (A) and (B): (1) the brittle crack propagation stopping property and the substitution heat welding property;
(A) in an amount of from 0.003 to 0.050% Nb, 0.5% or less Cu, 1.0% or less of Ni, 0.5% or less of Cr, 0.5% or less of Mo, 0.4% or less of W, B: 0.0003 to 0.0030%
(B) at least one of Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020% and REM: 0.001 to 0.020% 3. The method of claim 2,
Wherein the steel composition further contains at least one of the following groups (A) and (B): (1) the brittle crack propagation stopping property and the substitution heat welding property;
(A) in an amount of from 0.003 to 0.050% Nb, 0.5% or less Cu, 1.0% or less of Ni, 0.5% or less of Cr, 0.5% or less of Mo, 0.4% or less of W, B: 0.0003 to 0.0030%
(B) at least one of Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020% and REM: 0.001 to 0.020% A steel material having a composition as claimed in any one of claims 1 to 3 is heated to a temperature of 900 to 1150 占 폚 and a total of the austenite recrystallization temperature zone and the austenite non- , The rolling reduction is carried out so that the cumulative rolling reduction rate is not less than 20% and the average rolling reduction per one pass is not more than 5.0% in the state where the cumulative rolling reduction rate is not less than 65% and the central portion of the plate thickness is in the austenite recrystallization temperature range ,
Subsequently, the rolled steel sheet was rolled so that the cumulative rolling reduction was 40% or more and the average rolling reduction per pass was 7.0% or more in a state where the center of the plate thickness was in the austenite non-recrystallization temperature range. s or more at a cooling rate of at least 600 ° C. The method of manufacturing a high strength steel plate for high heat input of high heat input excellent in brittle crack propagation stopping property and large heat input welding characteristics. 6. The method of claim 5,
Further comprising a step of tempering the steel to a temperature of not more than the point A _C1 after cooling the steel sheet to 600 DEG C or less.

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