JP6969601B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel sheet having excellent brittle crack propagation stopping characteristics and a method for manufacturing the same, which is used for large structures such as ships, marine structures, low temperature storage tanks, line pipes and construction / civil engineering structures. In particular, the present invention relates to a steel sheet and a method for manufacturing the same, which can exhibit excellent brittle crack propagation stopping characteristics when and after the structure is unexpectedly deformed due to a collision, a large earthquake, or the like. Here, the brittle crack propagation stop characteristic means the ability to stop the propagation of brittle cracks.

In large structures such as ships, marine structures, low temperature storage tanks, line pipes and construction / civil engineering structures, accidents associated with brittle fracture have a great impact on the economy and the environment. A high degree of safety is required for such accidents. For this reason, low temperature toughness is often required for the steel materials used in these structures. Recently, in particular, brittle crack propagation stop characteristics at low temperatures, so-called arrest characteristics, are required from the viewpoint of preventing fracture even when cracks occur in the structure due to an unexpected accident or the like. became. Against this background, in the field of shipbuilding, the Nippon Kaiji Kyokai issued the "Brittle Rhagades Arest Design Guideline" in 2009, and in 2013, the International Association of Classification Societies issued a high arrest in the unified rules regarding the application of extra-thick steel sheets. Interest is increasing more and more, such as showing the way of thinking about steel.

Normally, structures are often subjected to large plastic deformation, so assuming an unexpected accident that actually occurs, the steel material may have a predetermined low temperature toughness even when it is damaged by plastic deformation and thereafter. It is considered necessary. Especially in recent years, steel materials for ships have high brittle crack propagation stop characteristics even after undergoing plastic deformation, for example, the fracture toughness value at -10 ° C, that is, Kca (-10 ° C) after undergoing 10% plastic deformation. ) Is 6000 N / mm ^1.5 or more, and a high fracture toughness value is increasingly required.

In addition, from the experience of brittle and destructive damage accidents of large-scale building structures in the Great Hanshin-Awaji Earthquake, even if the structure receives a large force and undergoes large plastic deformation, the after-seismic stress of repeated earthquakes or the subsequent structure From the viewpoint of ensuring safety for continuous use of materials, steel materials for building structures are required to have sufficient low-temperature toughness after undergoing plastic deformation.

Such a high brittle crack propagation stop property cannot be easily realized even in a rolled state that is not subjected to plastic deformation. Further, it is general that the toughness of a steel material decreases when it is work-hardened by undergoing plastic deformation, and it can be said that it is more difficult to realize excellent brittle crack propagation stop characteristics in the state after plastic deformation.

As a means for improving the low temperature toughness of steel materials, especially the brittle crack propagation stopping property, a method of increasing the Ni content has been conventionally known. In the storage tank of liquefied natural gas (LNG), 9% Ni steel is commercially available. Used on a scale. However, since an increase in Ni content inevitably leads to a significant increase in cost, it is difficult to apply it to applications other than LNG storage tanks.

On the other hand, in the case of so-called cold region specifications such as LNG, which does not reach extremely low temperatures, or winter specifications in Japan, the so-called TMCP method (Thermo-Mechanical Control Process), which is a processing heat treatment method that combines controlled rolling and controlled cooling, has been conventionally performed. Has been widely used. In this method, (1) austenite is repeatedly recrystallized to make austenite finer, and (2) the cumulative rolling reduction of austenite in the low-temperature unrecrystallized region is increased, and the austenite grains are expanded. By increasing the number of deformation zones and introducing a large number of deformation zones, in the subsequent ferrite transformation, the number of ferrite nucleation sites is increased to make the ferrite finer, and (3) γ in controlled cooling after rolling. It is characterized by finely granulating ferrite and introducing a fine bainite structure by adjusting the / α conversion ratio.

The TMCP method can impart extremely excellent brittle rhagades propagation stopping characteristics to steel materials with relatively thin steel plates used for ships and line pipes. However, when the plate thickness of the steel material increases, for example, when the plate thickness exceeds 40 mm, there is room for improvement in the brittle crack propagation stop characteristic.

Further, in controlled rolling, a TMCP method is also known in which the brittle crack propagation stop characteristic is improved by rolling down the transformed ferrite to develop an texture. This is a method of increasing the resistance to brittle fracture by generating a separation on the fracture surface of the steel material in a direction parallel to the plate thickness direction and relaxing the stress at the tip of the brittle crack. However, when the plate thickness of the steel material becomes thick, it becomes difficult to fully exert the effect of such a TMCP method. Further, if excessive processing is applied below the transformation point, there is a problem that the toughness of the steel material in the plate thickness direction deteriorates.

On the other hand, in recent years, a technique for ultra-miniaturizing the structure of the surface layer portion of a steel material without increasing the alloy cost has been proposed as a means for improving the brittle crack propagation stop property. For example, in Patent Document 1, it is noted that the shear lip (plastic deformation region) generated in the surface layer portion of the steel material is effective in improving the brittle crack propagation stop property when the brittle crack propagates, and the crystal grains in the shear lip portion are described. A method of making the particles finer to absorb the propagation energy of the propagating brittle crack is disclosed.

_{This is performed by repeating the process of cooling the surface layer portion below the Ar 3} transformation point by controlled cooling after hot rolling, then stopping the controlled cooling to reheat the surface layer portion above the transformation point, and during this period. In addition, by applying a rolling force to the steel material, repeated transformation or processing recrystallization is generated, and an ultrafine ferrite structure or bainite structure is formed on the surface layer portion.

Further, in Patent Document 2, the structure of the surface layer portion of the steel material that improves the brittle crack propagation stop characteristic is a fine ferrite crystal grain or a bainite structure having an average circle equivalent diameter of 3 μm or less, and the fine crystal grain is an aggregate structure. It is disclosed that it is effective to form a grain and flatten the crystal grains.

On the other hand, from the viewpoint of safety, the toughness of the weld heat-affected zone when the steel material is welded is also required for the steel material. At that time, the most important issue is the toughness of the bond portion of the weld heat-affected zone. This is because the bond portion is exposed to a high temperature just below the melting point at the time of high heat input welding, so that the austenite crystal grains are most likely to be coarsened, and are easily transformed into a fragile upper bainite structure by subsequent cooling. Further, in the bond portion, embrittled structures such as Woodmanstedten structure and island-like martensite are likely to be generated, which also causes a decrease in toughness. In the present invention, the bond portion is a joint surface of a steel material, which is a place near the melting point (about 1500 ° C.) at the time of high heat input welding.

Here, as a measure for improving the toughness of the bond portion, techniques for suppressing coarsening of austenite by fine dispersion of TiN and using it as a ferrite transformation nucleus have been put into practical use.
Further, in Patent Document 3 and Patent Document 4 aiming at improving toughness in a welded bond portion having a heat input amount of 230 kJ / cm, fine particles are dispersed in steel by compound-adding a rare earth element (REM) with Ti. A method of preventing the grain growth of austenite and improving the toughness of the bonded portion is shown.
Further, a technique for dispersing Ti oxide and a technique for improving the toughness of the bonded portion by combining the ferrite nucleation ability of BN have been developed. In addition, it is known that the morphology of sulfide can be controlled by adding Ca or REM, and a bond portion having higher toughness can be obtained.

Japanese Unexamined Patent Publication No. 4-141517 Japanese Unexamined Patent Publication No. 5-271863 Special Fair 03-53367 Gazette Japanese Patent Application Laid-Open No. 60-184663 Japanese Unexamined Patent Publication No. 8-225836 Japanese Unexamined Patent Publication No. 8-253812 Japanese Unexamined Patent Publication No. 11-256228

Proceedings of the Japan Welding Society, Vol. 15, No. 1, pp. 148-154 (1997)

However, the techniques disclosed in Patent Documents 1 and 2 described above provide a structure effective for brittle rhagades propagation stop characteristics by temporarily cooling only the surface layer of the steel material, reheating it, and processing it during the reheating. It is gained, and it is considered that it is not easy to control it on an actual production scale. Further, in the above invention, a fine structure is obtained by utilizing the processing and recrystallization of ferrite, but the processed and recrystallized ferrite tends to grow and lacks structural stability. Therefore, there is a problem that non-uniformity of the structure and the material is likely to occur due to a slight change in the heat history.

On the other hand, in Patent Document 5, a method of improving the descaling device attached to the rolling mill to control the timing of cooling and rolling is adopted. By adopting such a method, Patent Document 3 proposes to stabilize an unstable structure such as the above-mentioned fine ferrite crystal grains.
However, this method expands the descaling device, which is arranged in a limited space, to an application different from the original purpose. Therefore, there are engineering restrictions such as the need for remodeling aimed at significantly increasing capacity. Furthermore, it is considered that the method has not been able to fundamentally solve the instability of the ferrite structure, which is easily affected by the thermal history.

Here, as described in the prior art, even if an ultrafine structure having excellent brittle crack propagation stopping characteristics is stably obtained, such an ultrafine structure is inevitable as shown in Non-Patent Document 1. Increases the hardness of the surface layer. That is, the hardness of the surface layer portion is significantly higher than the hardness of the central portion of the plate thickness, such that the Vickers hardness of the surface layer portion exceeds about 200 with respect to the Vickers hardness of the central portion of the plate thickness of 160 to 170. Therefore, there is a concern that the ultrafine structure of the surface layer may offset even if good brittle crack propagation stopping characteristics are obtained.

Therefore, in Patent Document 6, V-added steel is used for the purpose of making the hardness distribution in the plate thickness direction uniform. Ferrite is generated in the steel structure by directly cooling the heated slab of the steel to give a temperature difference, and then the steel is rolled and reheated again in the temperature range near the transformation point during or after rolling. A method to make it is proposed. In this method, the precipitation hardening of V is applied only to the central portion of the plate thickness to achieve uniform hardness distribution in the plate thickness direction and to improve the brittle crack propagation stop characteristic.
However, proper precipitation of the V compound requires further complication of the process and is therefore not considered to necessarily lead to the resolution of the above-mentioned tissue instability.

As described above, the above-mentioned prior art is a technique intended to form an ultrafine structure on the surface of a steel material to improve the brittle crack propagation stop property, but all of them are on an industrial scale. It is considered difficult to obtain the tissue in a stable manner. Further, in these prior arts, the disclosed steel material itself has been shown to have excellent brittle crack propagation stopping properties, but does not mention the fracture toughness value after undergoing plastic deformation. Therefore, it was not clear whether the brittle crack propagation stop property was sufficient after undergoing plastic deformation.

Here, Patent Document 7 states that not only the ferrite crystal grains are refined, but also the subgrains formed in the ferrite crystal grains play a major role in the brittle crack propagation stop property, and these ferrites. A technique for solving the above problems by clarifying the relationship between the crystal grains and their subgrain structure and the conditions of the TMCP method, which is a production condition, is described.

With this technique, a steel sheet having the structure can be stably obtained on an industrial scale, and the fracture toughness value at 0 ° C., that is, Kca (0 ° C.) after undergoing plastic deformation has been successfully improved. doing. However, in recent years, it has become required to exhibit a high fracture toughness value in a lower temperature range after undergoing plastic deformation. For example, the technique described in Patent Document 5 still has room for improvement in that it is desired to increase the fracture toughness value at −10 ° C., that is, Kca (-10 ° C.).

Further, considering the case where welding is performed on a thick material having a plate thickness of about 50 mm, large heat input welding with a heat input of more than 300 kJ / cm may be applied to the welding of the thick material. However, in the above-mentioned conventional technique, when such large heat input welding is applied, there is a problem that sufficient toughness cannot be obtained particularly at the bond portion.
That is, in the technology mainly using TiN, since the bond portion is heated to the temperature range where TiN melts, not only the suppression of austenite coarsening and the action as a ferrite transformation nucleus disappear, but also the solid solution Ti and N There was a problem that the formation caused embrittlement of other tissues and a significant decrease in toughness was observed. Further, even in the techniques disclosed in the above-mentioned Patent Documents 3 and 4 in which Ca and REM are added, the same phenomenon as the technique mainly using TiN occurs, and the high toughness of the bond portion in the large heat input welding is improved. There was a problem that it was difficult to secure. Further, the technique using the oxide of Ti has a problem that the fine dispersion of the oxide cannot be sufficiently homogeneous.

The present invention has high toughness in the bonded portion even when high heat input welding is applied, and has high brittle crack propagation stopping characteristics at a low temperature of -10 ° C even after undergoing plastic deformation. By clarifying the structure required for the above, it is an object of the present invention to provide a steel plate having excellent low temperature brittle crack propagation stopping characteristics. Another object of the present invention is to provide a manufacturing method capable of stably obtaining such a structure without using an expensive alloying element for a steel sheet.

The inventors aimed to develop a steel material having excellent brittle crack propagation stop characteristics not only in the state where the steel material was manufactured but also after being subjected to plastic deformation, and the relationship between the structure and the brittle crack propagation stop characteristics. investigated. In particular, we found a stable structure that is effective for brittle crack propagation stop characteristics, and examined the relationship with the conditions of the TMCP method (hereinafter simply referred to as TMCP conditions). In this study, structural steel having a tensile strength of 400 MPa class to 510 MPa class, which is often used especially for large structures, was used. The test steel was made into steel pieces by continuous casting, and then steel materials were manufactured under various TMCP conditions.
As a result of diligent studies, as described below, a steel sheet having excellent brittle crack propagation stopping characteristics even after being subjected to plastic deformation and a manufacturing method capable of stably obtaining the steel sheet have been completed. I arrived.

That is, in the present invention, in order to improve the brittle crack propagation stop property, it is important to improve the toughness of the steel sheet and utilize the texture by rolling.
First, as a method for improving toughness, it was confirmed that fine graining of ferrite and pearlite crystal grains is an effective means for steel materials mainly composed of a ferrite phase and a pearlite phase. The term "ferrite phase and pearlite phase" as used herein means that the ferrite phase and the pearlite phase occupy 50% or more of the area ratio. Further, the rest of the structure of the steel sheet of the present invention is not particularly limited as long as it has a structure recognized by a normal steel sheet. Examples of the remaining tissue include bainite and / and martensite.

On the other hand, for the utilization of the texture, a method of relaxing the stress at the tip of the crack due to the generation of separation is often used. However, under the rolling conditions that generate separation, which is necessary to achieve a high Kca (-10 ° C) after plastic deformation, for example, 6000 N / mm ^{1.5 or more, the toughness of the obtained steel material may decrease.} high.

Therefore, in the present invention, it is decided to use a method of accumulating cleavage planes in a direction that resists the propagation direction of cracks, instead of improving by a method of generating separation. Specifically, it was decided to utilize an aggregate structure that accumulates tissues in the {100} <011> orientation 5.5 times or more as compared with the so-called random case in which the aggregate structure is not developed.

Further, in the present invention, in order to improve the brittle crack propagation stopping property and to improve the toughness of the bonded portion, a diligent study was conducted.
As a result, it was found that the toughness of the bond portion is affected by the embrittled structure, and that this embrittled structure can be greatly improved by controlling the method of adding Ca, which plays a role in controlling the morphology of sulfide.

That is, in the present invention, as a method for adding Ca, CaS is crystallized at the solidification stage when melting steel. Furthermore, it was found that if the amount of Ca and S added and the amount of dissolved oxygen in the molten steel at the time of addition are controlled to secure the amount of solid solution S after the crystallization of CaS, MnS is deposited on the surface of CaS. rice field.
It is known that such MnS has a ferrite nucleation ability, and it has been found that formation of a dilute band of Mn around the MnS promotes ferrite transformation. It was also found that the precipitation of ferrite-generated nuclei such as TiN, BN, and AlN on MnS further promotes ferrite transformation.
Based on these findings, the inventors succeeded in finely dispersing ferrite metamorphosis-generated nuclei that do not dissolve even at high temperatures, and made it possible to make the structure of the bond portion finer and tougher.

The present invention has been obtained based on the above findings, and the gist structure of the present invention is as follows.
1. 1. By mass%, C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 1.0 to 2.0%, Al: 0.005 to 0.08%, P: 0.015 % Or less, S: 0.010% or less, Nb: 0.003 to 0.017%, Ti: 0.005 to 0.02%, N: 0.0035 to 0.0075%, Ca: 0.0005 to A component composition containing 0.0030% and B: 0.0005 to 0.0020%, Ca, O and S satisfying the following formula (1), and the balance consisting of Fe and unavoidable impurities, and ferrite. And a structure mainly composed of pearlite, the structure has a {100} <011> orientation strength of 5.5 or more at 1/2 part of the plate thickness, and a ferrite phase and pearlite at 1/2 part of the plate thickness. 10% or more of the crystal grains of the phase have an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less, and the Kca (-10 ° C.) of the steel plate after applying a strain of 10% is 6000 N / mm ^1.5 or more and a bond. A steel plate having a charpy toughness value of vTrs ≤ −30 ° C.
0 <([Ca%]-(0.18 + 130 x [Ca%]) x [O%]) / 1.25 / [S%] <1 ... (1)
However, [Ca%], [O%], and [S%] represent the content (mass%) of each component (Ca, O, and S) in the steel.

Here, the 1/2 portion of the plate thickness means a position at a depth of 1/2 of the plate thickness from the surface (rolled surface) of the steel plate.

2. 2. Further, the component composition is, in mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.5%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0. The steel sheet according to 1 above, which contains one or more selected from 5.5%, V: 0.001 to 0.15% and REM: 0.0100% or less.

3. 3. The step (a) of heating the steel material having the component composition according to 1 or 2 to at least Ac ₃ points or more and 1000 ° C. or less, and then (Ar ₃ points −5 ° C.) or less, (Ar _3). Step (b) of rolling with an average rolling reduction rate of 4% or more and a cumulative rolling reduction rate of 50% or more per pass in a temperature range of −150 ° C.) or higher, followed by a cooling rate of 5 ° C./s. A method for manufacturing a steel sheet, which comprises the step (c) of controlling and cooling to a temperature range of 600 ° C. or lower.

4. The method for manufacturing a steel sheet according to 3 above, wherein after the step (c) _{, tempering is performed in a temperature range of 1 Ac or less.}

According to the present invention, even after undergoing plastic deformation, it has excellent brittle crack propagation stopping characteristics even in a low temperature region. Therefore, when it is used for a large structure, even if the steel material is greatly deformed due to an unexpected accident or the like, it is possible to prevent the structure from collapsing on a large scale.

Hereinafter, the present invention will be specifically described. In the steel sheet according to the present invention, the composition, the texture, and the structure inside the steel sheet are defined as follows.

[Ingredient composition]
First, the reason for limiting the composition of steel in the present invention will be described. The "%" indication regarding the component composition shall mean "mass%" unless otherwise specified.

C: 0.03 to 0.20%
C is an element that improves the strength of steel, and in the present invention, the content of C is required to be 0.03% or more in order to secure the desired strength. On the other hand, if the C content exceeds 0.20%, not only the weldability is deteriorated but also the toughness is adversely affected. Therefore, the C content is set to 0.03 to 0.20%. The C content is preferably 0.04 to 0.15%.

Si: 1.0% or less Si is effective as a deoxidizing element and as a reinforcing element for steel, but if the content exceeds 1.0%, not only the surface texture of steel is impaired, but also the toughness is extremely high. Deteriorates to. Therefore, the Si content is 1.0% or less. The Si content is preferably 0.03 to 0.70%.

Mn: 1.0 to 2.0%
Mn is contained as a reinforcing element that enhances the strength of the ferrite structure. If the Mn content is less than 1.0%, the effect is not sufficient. On the other hand, when the Mn content is more than 2.0%, the weldability is deteriorated and the steel material cost is also increased. Therefore, the Mn content in the present invention is 1.0 to 2.0%.

P: 0.015% or less P is an unavoidable impurity in steel, and the toughness deteriorates as the content increases. Therefore, in the present invention, the P content is set to 0.015% or less in order to maintain good toughness even in a steel sheet having a plate thickness of more than 50 mm. The P content is preferably 0.010% or less, more preferably 0.006% or less. On the other hand, the lower limit may be 0%, but the cost increases each time it is reduced. Therefore, it is preferable to set the lower limit to 0.005% of the concentration that can be reduced within a range that does not result in high cost.

S: 0.010% or less S is an unavoidable impurity in steel, and the toughness deteriorates as the content increases. Therefore, in order to maintain good toughness, the S content is suppressed to 0.010% or less. The S content is preferably 0.005% or less, more preferably 0.003% or less. On the other hand, the lower limit may be 0% as in P, but it is preferable that the lower limit is 0.0005%, which is a concentration that can be reduced within a range that does not result in high cost.

Al: 0.005-0.08%
Al is an element added as a deoxidizing material, and it is necessary to add 0.005% or more in order to obtain its effect. On the other hand, when the Al content exceeds 0.08%, the toughness is lowered and the toughness of the weld metal portion is lowered when welded. Therefore, the Al content is set to 0.005 to 0.08%. The Al content is preferably 0.020 to 0.06%.

Nb: 0.003 to 0.017%
Nb precipitates as NbC in the steel structure during ferrite transformation or reheating, and contributes to increasing the strength of the steel. In addition, Nb has the effect of expanding the unrecrystallized region in rolling in the austenite region and contributes to the refinement of ferrite, so that it is also effective in improving the toughness of steel. The effect is exhibited by the content of 0.003% or more. On the other hand, if it is contained in an amount of more than 0.017%, coarse NbC is precipitated, which causes a decrease in toughness. Therefore, the Nb content is set to 0.003 to 0.017%.

Ti: 0.005-0.02%
Ti has the effect of forming nitrides, carbides, or carbonitrides by containing a small amount of Ti, and refining the crystal grains to improve the toughness of the base metal. Therefore, Ti is contained in an amount of 0.005% or more. On the other hand, when the Ti content exceeds 0.02%, the toughness of the steel sheet and the heat-affected zone of the weld is lowered. Therefore, the upper limit of the Ti content is 0.02%. The Ti content is preferably 0.005 to 0.017%.

N: 0.0035 to 0.0075%
N is an element necessary for securing the required amount of TiN, and if it is less than 0.0035%, a sufficient amount of TiN cannot be obtained. On the other hand, when N exceeds 0.0075%, the amount of solid solution N increases in the region where TiN is melted by the welding heat cycle, and the toughness of the steel sheet is significantly lowered. Therefore, the N content is set to 0.0035 to 0.0075%.

Ca: 0.0005 to 0.0030%
Ca is an element having an effect of refining the structure of the weld heat affected zone and improving the toughness of the steel sheet, and the Ca content is set to 0.0005% or more in order to sufficiently obtain such an effect. On the other hand, if Ca is excessively contained, coarse inclusions are formed and the toughness of the base metal is deteriorated. Therefore, the Ca content is set to 0.0030% or less.

B: 0.0005 to 0.0020%
B is an element having an effect of enhancing the hardenability of steel in a small amount, and the B content is set to 0.0005% or more from the viewpoint of obtaining good hardenability. On the other hand, when the B content exceeds 0.0020%, the toughness of the welded portion decreases. Therefore, the B content is set to 0.0005 to 0.0020%.

Further, in the component composition of the present invention, it is important that Ca, O and S satisfy the following formula (1).
This is because if the equation (1) is not satisfied, the effect of promoting the ferrite transformation cannot be obtained.
0 <([Ca%]-(0.18 + 130 x [Ca%]) x [O%]) / 1.25 / [S%] <1 ... (1)
However, [Ca%], [O%], and [S%] represent the content (mass%) of each component (Ca, O, and S) in the steel.

The composition of the steel sheet according to the present invention is basically composed of the above elements, the balance Fe and unavoidable impurities.

Further, in the present invention, in order to further improve the characteristics, one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, V, and REM are further added to the basic composition. Can be contained as described in.

Cu: 0.01-0.5%
Cu is an element that enhances the hardenability of steel, and contributes to the improvement of steel functions such as toughness, high temperature strength, and weather resistance, in addition to the improvement of strength after rolling. These effects are exhibited by the content of 0.01% or more, but the excessive content deteriorates the toughness and weldability. Therefore, when Cu is contained, the range is 0.01 to 0.5%.

Ni: 0.01-1.5%
Ni is an element that enhances the hardenability of steel, and in addition to improving the strength after rolling, it contributes to improving the functions of steel such as toughness, high temperature strength, and weather resistance. These effects are exhibited by the content of 0.01% or more. On the other hand, excessive content deteriorates toughness and weldability, and also increases the cost of the alloy. Therefore, when Ni is contained, the range is 0.01 to 1.5%.

Cr: 0.01-0.5%
Like Cu, Cr is an element that enhances the hardenability of steel, and contributes to the improvement of steel functions such as toughness, high temperature strength, and weather resistance, in addition to the improvement of strength after rolling. These effects are exhibited by the content of 0.01% or more, but the excessive content deteriorates the toughness and weldability. Therefore, when Cr is contained, the range is set to 0.01 to 0.5%.

Mo: 0.01-0.5%
Like Cu and Cr, Mo is an element that enhances the hardenability of steel, and contributes to the improvement of steel functions such as toughness, high temperature strength, and weather resistance, in addition to the improvement of strength after rolling. These effects are exhibited by the content of 0.01% or more, but the excessive content deteriorates the toughness and weldability. Therefore, when Mo is contained, the range is 0.01 to 0.5%.

V: 0.001 to 0.15%
V is an element that improves the strength of steel by precipitation strengthening that precipitates as V (CN). This effect is exhibited by containing 0.001% or more of V. On the other hand, if V is contained in an amount of more than 0.15%, the toughness is rather lowered. Therefore, when V is contained, the range is set to 0.001 to 0.15%.

REM: 0.0100% or less REM (rare earth metal) is an element that has the effect of refining the structure of the weld heat-affected zone and improving toughness, similar to Ca, and the effect of the present invention if contained in an appropriate amount. Contribute to. Therefore, REM can be arbitrarily contained. However, if the REM is excessively contained, coarse inclusions are formed and the toughness of the base metal is deteriorated. Therefore, when REM is contained, the content thereof is set to 0.0100% or less. On the other hand, the lower limit of the REM content is not particularly limited, but the content is preferably 0.0005% or more in order to sufficiently obtain the content effect.

[Aggregate organization]
In the present invention, in order to improve the brittle crack propagation stop characteristic for cracks propagating in a direction parallel to the plate surface such as the rolling direction or the direction perpendicular to the rolling direction, {100} <011> in the plate thickness 1/2 portion of the steel plate. The azimuth structure is accumulated 5.5 times or more as compared with the so-called random structure in which the aggregated structure is not developed (in the present invention, {100} <011> azimuth intensity is 5.5 or more. It is important to make it an aggregate organization. That is, when the {100} <011> azimuth grain is developed in the plate thickness 1/2 portion, the cleavage planes are effectively aligned diagonally with respect to the direction in which the crack grows, which becomes a resistance to the crack growth.

That is, in the present invention, the texture is an aggregate structure in which the {100} <011> directional strength at 1/2 portion of the plate thickness is 5.5 or more. By controlling the texture so as to satisfy such conditions, it is possible to obtain an excellent brittle crack propagation stop characteristic of ^{Kca (-10 ° C.) ≧ 6000 N / mm 1.5 even after undergoing plastic deformation.} Here, Kca (-10 ° C.) means the brittle crack propagation stop toughness value when the temperature of the steel sheet is −10 ° C., and can be measured by the temperature gradient type standard ESSO test of Examples described later. can. When better crack propagation stop performance is required, it is preferable that the {100} <011> directional strength at 1/2 portion of the plate thickness is 6.3 or more. This is because when the {100} <011> directional strength is 6.3 or more, the crack propagation tends to be zigzag. Therefore, the stress intensity factor of the steel sheet becomes small, and the cracks generated in the steel sheet are likely to stop. On the other hand, the upper limit of the {100} <011> directional strength in 1/2 portion of the plate thickness is not particularly limited, and the higher the plate thickness, the better.

In the present invention, the {100} <011> directional intensity can be obtained as a random intensity ratio by the X-ray pole projection method, and specifically, it can be measured by the method described in the examples described below. .. In the measurement of {100} <011> directional strength in the present invention, a test piece collected from 1/2 part of the plate thickness is used for measurement.

[Structure of steel plate]
The structure of the steel sheet in the present invention is mainly ferrite and pearlite. When ferrite and pearlite are the main constituents, it is preferable that the total of the ferrite phase and the pearlite phase occupies 50% or more, preferably 70% or more in area ratio, and the remaining structure is a bainite or / and martensite phase. Of course, the ferrite phase and the pearlite phase may be 100%. The ratio of the ferrite phase and the pearlite phase is not particularly limited and varies depending on the rolling and cooling conditions, but the total ratio of the two may be 70% or more. The structure is defined in this way because it is a structure suitable for ensuring a tensile strength of 400 MPa class to 510 MPa class, which is often used for large structures. The area ratio of the structure other than the ferrite phase and the pearlite phase shall be less than about 30%.

In the steel plate of the present invention, crystal grains of the ferrite phase and the pearlite phase (in the present invention, when simply referred to as crystal grains, crystals of the ferrite phase or the pearlite phase) at 1/2 portion of the plate thickness of the above ferrite phase and pearlite phase main structure. Of the grains), the toughness can be effectively improved by setting the area ratio of the crystal grains having an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less to be 10% or more, preferably 26% or more.
Here, the aspect ratio is the ratio of the major axis to the minor axis of the crystal grain. The major axis is the longest diameter of the crystal grain, and the minor axis is the largest width perpendicular to the major axis. The abundance ratio of crystal grains satisfying the above regulations can be obtained by the method described in the examples described below.

Of the ferrite grains and pearlite grains, setting the area ratio of crystal grains having an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less to 10% or more means increasing the number of fine structures having a minor axis diameter of 5 μm or less. Therefore, the area ratio of the crystal grains was set to 10% or more because the arrest property could be improved. Therefore, the upper limit does not have to be particularly limited and may be 100%. In addition, the aspect ratio is defined with the maximum length in the rolling direction as the major axis, which confirms the crystal grains that are reliably processed (rolled) and contribute to the formation of the texture among the crystal grains. Because.
The aspect ratio, the minor axis diameter, and the area ratio of the ferrite phase can be obtained by the methods described in Examples described later.

[Toughness of steel plate]
According to the present invention, by controlling the composition and structure as described above, a high-strength steel sheet having excellent toughness can be obtained. It is important that the steel sheet has excellent toughness in order to suppress the growth of cracks. Specifically, it is preferable that the Charpy absorption energy at -40 ° C: vE (-40 ° C) at 1/2 part of the plate thickness, which generally shows the lowest toughness among steel sheets, is 250 J or more, and 280 J or more. It is more preferable to set it to 300 J or more, and it is further preferable to set it to 300 J or more. On the other hand, the upper limit of vE (−40 ° C.) is not particularly limited, but may be 420 J or less, or 400 J or less, from the viewpoint of the upper shelf of absorbed energy.

[Brittle crack propagation stop characteristics]
As described above, in the steel sheet of the present invention, Kca (-10 ° C.) is 6000 N / mm ^1.5 or more even after undergoing plastic deformation by controlling the structure, that is, improving the toughness by refining the crystal grains. , Excellent brittle crack propagation stop characteristics can be realized.

[Charpy toughness value of bond part]
In the present invention, it is important that the Charpy toughness value of the bonded portion is vTrs ≦ −30 ° C. This is because the bond portion is most likely to become brittle.

[Tensile strength]
The tensile strength (TS) of the steel sheet of the present invention is preferably 400 MPa class to 510 MPa class. This is because the tensile strength at which the effect of the present invention is most obtained.

[Plate thickness]
The thickness of the steel plate of the present invention is preferably in the range of 10 to 45 mm. This is because the plate thickness is the most effective for the present invention.

[Production method]
Next, an embodiment of the method for manufacturing a steel sheet in the present invention will be illustrated and described.
The steel sheet of the present invention can be produced by hot rolling a steel material having the above-mentioned composition under specific conditions. Specifically, it is important to carry out the following steps (a) to (c) in order.
(A) A step of heating a steel material to Ac ₃ points or more and 1000 ° C. or less.
(B) Next, in _{the temperature range of (Ar 3} points -5 ° C) or lower and (Ar ₃ points -150 ° C) or higher, the average value of the rolling reduction rate per pass is 4% or more and the cumulative rolling reduction rate is 50% or more. The process of hot rolling.
(C) Further, a step of cooling to a temperature range of 600 ° C. or lower at a cooling rate of 5 ° C./s or higher.

Further, after the step (c), the next step (d) can be arbitrarily performed.
(D) Tempering is performed in a temperature range of _{1 point or less.} Specifically, a step of heating a steel sheet cooled to a temperature 100 ° C. or more lower than an arbitrary tempering temperature of _{1 point or less of Ac and tempering it at a temperature of 1} point or less of Ac. The lower limit of the tempering temperature is not particularly limited, but it is preferably 200 ° C. from the viewpoint of obtaining the effect of the tempering treatment.

Further, the next step (e) can be optionally performed after the step (a) and before the step (b).
(E) A step of applying a reduction of a cumulative reduction rate of 30% or more and 98% or less in the austenite region (specifically, Ar _{3 points over −5 ° C. to the heating temperature or less in the step (a) above).}

Hereinafter, the reasons for limiting the conditions in each of the above steps (a) to (e) will be described. In the present invention, the manufacturing conditions of the steel sheet other than the conditions in each of the above steps (a) to (e) may be according to a conventional method.

[The heating step of (a) above]
Heating temperature: Ac ₃ points or more, 1000 ° C. or less Prior to hot rolling, the steel material having the above composition is heated. At that time, if the heating temperature is _{less than 3} points of Ac, a ferrite-austenite two-phase structure is formed, the entire plate becomes a non-uniform structure, and the desired effect cannot be sufficiently obtained in the subsequent rolling process. On the other hand, when the heating temperature exceeds 1000 ° C., the austenite grains become coarse and the crystal grain shape required by the present invention cannot be realized. Therefore, the heating temperature of the steel material shall be Ac ₃ points or more and 1000 ° C or less. From the viewpoint of improving the toughness of the steel sheet, the heating temperature is preferably (Ac ₃ points + 10) ° C. or higher, 980 ° C. or lower, and more preferably 950 ° C. or lower.

The steel material used in the heating step is not particularly limited and can be produced by any conventionally known method. For example, a molten steel having the above-mentioned composition can be melted in a converter or the like, and a steel piece (slab) obtained by continuous casting can be used as the above-mentioned steel material.

[Hot rolling step of (b) above]
Next, the hot rolling step of (b) is performed. In this hot rolling process, the average value of the reduction rate per pass is 4% or more and the cumulative reduction rate is 50 in the temperature range _{from (Ar 3} points -5 ° C) or less to (Ar _{3 points -150 ° C) or more.} % Or more rolling.

The reason for limiting to the above temperature range is that the target texture cannot be obtained when the temperature is higher than _{(Ar 3 points -5 ° C).} On the other hand, if the _{temperature is lower than (Ar 3} points −150 ° C.), the processing conditions become too strict and cause a decrease in the toughness of the steel sheet.
The reason why the cumulative reduction rate is 50% or more is that when it is less than 50%, 10% or more of the crystal grains in the L cross section of the plate thickness 1/2 part have an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less. This is because it is not possible to achieve any of the target values of. Further, in order to make the {100} <011> directional strength at 1/2 part of the plate thickness 5.5 or more, the cumulative reduction rate needs to be 50% or more. The upper limit of the cumulative reduction rate is not particularly limited, but is preferably 80% or less from the viewpoint of avoiding a decrease in toughness.

Furthermore, the reason for defining the average reduction rate per pass is to control the shear strain distribution in the plate thickness direction to further refine the {100} <011> directional strength in the 1/2 portion of the plate thickness. Is. That is, by setting the average reduction rate per pass to 4% or more in addition to the above-mentioned cumulative reduction rate: 50% or more, it is possible to secure {100} <011> directional strength: 5.5 or more. Will be.

In the present invention, it is possible to further refine the structure by applying a reduction with a cumulative reduction rate of 30% or more in the austenite region after the step (a) and before the step (b). It can be done (step (e) above). As a result, the toughness value of 1/2 portion of the plate thickness can be as high as 300 J or more.

[Cooling step of (c) above]
Cooling rate: 5 ° C / s or higher Cooling stop temperature: 600 ° C or lower For steel sheets that have been hot-rolled, the cooling rate is 600 ° C / s or higher from the viewpoint of retaining the texture obtained during hot rolling. Cool to a cooling stop temperature below ° C. The upper limit of the cooling rate is not particularly limited, but is preferably 50 ° C./s or less from the viewpoint of manufacturing cost and the like. The lower limit of the cooling stop temperature is not particularly limited, but is preferably 0 ° C. or higher from the viewpoint of manufacturing cost and the like.

[Tempering step of (d) above]
Tempering temperature: Ac ₁ point or less In the present invention, after the cooling step, the tempering process can be performed at a tempering _{temperature: Ac 1 point or less.} In the tempering treatment, it is preferable that the steel sheet cooled to a temperature 100 ° C. or higher lower than the tempering temperature is heated again _{and tempered at a temperature of 1 Ac or less.} This is because if the tempering temperature is _{higher than Ac 1} point, the texture developed during rolling may be lost. The lower limit of the tempering temperature is not particularly limited, but it is preferably 350 ° C. or higher in order to obtain the effect of such tempering.

As described above, the heating temperature, _{the cumulative reduction rate in the temperature range of (Ar 3} points -5 ° C) or lower (Ar ₃ points -150 ° C) or higher, the average reduction rate per pass, and the cooling rate after rolling are appropriately controlled. By doing so, the required structure and aggregated structure can be secured, and Kca (-10 ° C.) ≧ 6000 N / mm ^3/2 can be stably achieved.
The temperature of the steel sheet in the above description is the surface temperature of the steel sheet measured by a radiation thermometer.
Further, Ac ₃ points, Ar ₃ points and Ac ₁ points can be obtained by the following empirical simple equations (Equation 1, Equation 2 and Equation 3). In the formula, [element symbol] is the component composition amount (%) of the element of the element symbol in steel. Further, t in the formula 2 is the plate thickness (mm) of the steel plate.

Examples of steel sheets manufactured based on the present invention, which have good brittle crack propagation stopping characteristics even after undergoing plastic deformation (including comparative examples outside the scope of the invention) are shown below.
Table 1 shows the chemical composition of the test steel. The steel slab having these chemical components was hot-rolled into a steel plate having a plate thickness of 10 to 45 mm, and the characteristics of the obtained steel plate were evaluated. Table 2 shows the manufacturing conditions of the test steel sheet. This table also shows _{the ac 3} point and Ar ₃ point temperatures obtained by calculation.

For each of the obtained steel sheets, toughness, tensile strength, texture, proportion of crystal grains having an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less (area ratio) and brittle crack propagation stop characteristics were obtained by the following methods. Was evaluated. The evaluation results are shown in Table 3.

[Toughness]
In order to evaluate the toughness of the obtained steel sheet, a Charpy impact test was performed to measure the Charpy absorption energy vE (-40 ° C.) at −40 ° C. of 1/2 part of the plate thickness of each steel sheet. For the Charpy impact test, a No. 4 impact test piece (length 55 mm, width 10 mm, thickness 10 mm) specified in JIS (Japanese Industrial Standards) is used, and the test piece is a steel plate in the longitudinal direction of the test piece. The test piece was sampled so as to be parallel to the rolling direction of the sheet and so that the position of 1/2 of the thickness of the test piece was 1/2 of the thickness of the steel sheet from which the test piece was collected. For a steel sheet having a thickness of 10 mm, the scale (black skin) on the surface was removed and used as a test piece as it was.

[Tensile strength]
JIS 14B test pieces were collected from any part of the obtained steel sheet so that the longitudinal direction of the test piece was perpendicular to the rolling direction and the center of the test piece was 1/2 part of the thickness of the steel sheet. Using the test piece, a tensile test was performed in accordance with JIS Z2241 to determine the tensile strength (TS).

[Aggregate organization]
In order to evaluate the texture of the obtained steel sheet, the {100} <011> directional strength at 1/2 part of the sheet thickness was measured by the following method. First, a sample having a thickness of 1 mm including 1/2 part of the plate thickness was taken. Next, mechanical polishing and electrolytic polishing were performed in parallel with the plate surface of the collected sample, and 1/2 portion of the plate thickness was used as the polished surface to prepare a test piece for X-ray diffraction.
For each of the obtained test pieces, X-ray diffraction measurement was performed using an X-ray diffractometer using a Mo radiation source, and (200), (110), and (211) positive electrode point diagrams were obtained. By calculating the three-dimensional crystal orientation density function from the obtained positive point diagram, the ratio of the {100} <011> orientation intensity to the random intensity was calculated.

[Structure of steel plate]
A sample containing 1/2 part of the plate thickness was collected on the plane parallel to the plate thickness direction and the rolling direction. Then, the surface of the sample was mirror-polished to make 1/2 of the plate thickness a polished surface, and then the metal structure of the polished surface was exposed by etching. Then, an optical micrograph of the metal structure was taken, and the area ratios of the ferrite phase and the pearlite phase were determined according to the quadrature method. Further, the number of crystal grains in the metal structure and the aspect ratio and minor axis diameter of the crystal grains were obtained, and the ratio of crystal grains having an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less was calculated. The aspect ratio and minor axis diameter of the crystal grains were measured in a region of 500 × 500 μm centered on 1/2 portion of the plate thickness in the optical micrograph of the sample, and each crystal grain in the region was measured by image analysis. The lengths of the minor axis and the major axis of the above were obtained, and the abundance ratio of the crystal grains in the above range was determined.

[Brittle crack propagation stop characteristics]
In order to evaluate the brittle rhagades propagation stop property, a temperature gradient type standard ESSO test was performed on the rolled steel material and the steel material subjected to 10% prestrain, and the Kca values of the steel sheet at 0 ° C. and −10 ° C. were obtained. In the temperature gradient type ESSO test, the total thickness was used as it was.

[Welded joint toughness]
A joint test plate collected from a steel plate was subjected to V groove processing to prepare a large heat input welded joint having a welding heat input of 50 to 500 kJ / cm. From the obtained welded joint, a JIS No. 4 impact test piece having a notch position as a bond was collected from 1/2 part of the plate thickness, and a Charpy impact test was carried out to determine the ductility-brittle fracture surface transition temperature (vTrs). For a steel sheet having a thickness of 10 mm, the scale (black skin) on the surface was removed to obtain a test piece.

As shown in Table 3, Nos. 1 to 15 according to the present invention have Kca (0 ° C.) and Kca (-10 ° C.) of 6000 N / mm ^1.5 or more even after applying 10% plastic deformation. It showed excellent brittle crack propagation stop characteristics. On the other hand, Nos. 16 to 34 of Comparative Examples, which deviate from the present invention, have insufficient brittle crack propagation stopping characteristics, particularly Kca (-10 ° C.), after undergoing plastic deformation.

Although the above-mentioned embodiment is an example of applying the present invention to a steel plate manufactured by thick plate rolling, the present invention can also be applied to other steel material manufacturing processes.

Claims (4)

By mass%, C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 1.0 to 2.0%, Al: 0.005 to 0.08%, P: 0.015 % Or less, S: 0.010% or less, Nb: 0.003 to 0.017%, Ti: 0.005 to 0.02%, N: 0.0035 to 0.0075%, Ca: 0.0005 to A component composition containing 0.0030% and B: 0.0005 to 0.0020%, Ca, O and S satisfying the following formula (1), and the balance consisting of Fe and unavoidable impurities, and ferrite. And a structure mainly composed of pearlite, the structure has a {100} <011> orientation strength of 5.5 or more at 1/2 part of the plate thickness, and a ferrite phase and pearlite at 1/2 part of the plate thickness. 10% or more of the crystal grains of the phase have an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less, and the Kca (-10 ° C.) of the steel plate after applying a strain of 10% is 6000 N / mm ^1.5 or more, and welding is performed. A steel plate having a charpy toughness value of vTrs ≤ −30 ° C. when a large heat input welded joint having a heat input of 50 to 500 kJ / cm is manufactured.
0 <([Ca%]-(0.18 + 130 x [Ca%]) x [O%]) / 1.25 / [S%] <1 ... (1)
However, [Ca%], [O%], and [S%] represent the content (mass%) of each component (Ca, O, and S) in the steel. By mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.5%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: The steel sheet according to claim 1, which contains one or more selected from 0.001 to 0.15% and REM: 0.0100% or less. The step (a) of heating the steel material having the component composition according to claim 1 or 2 to at least ₃ points of Ac and 1000 ° C. or lower, and then (Ar ₃ points −5 ° C.) or less, (Ar). Step (b) of rolling with an average rolling reduction of 4% or more and a cumulative rolling ratio of 50% or more per pass in a temperature range of _{3 points -150 ° C) or higher, followed by a cooling rate of 5 ° C /} It has a structure mainly composed of ferrite and pearlite, which has a step (c) of controlled cooling to a temperature range of 600 ° C. or lower at s or more, and the structure is {100} <011 in 1/2 part of the plate thickness. > The orientation strength is 5.5 or more, and 10% or more of the ferrite phase and pearlite phase crystal grains in the 1/2 portion of the plate thickness have an aspect ratio of 2 or more and a minor axis diameter of 5 μm or less, and a strain of 10%. When a large heat input welded joint with a Kca (-10 ° C.) of 6000 N / mm ^1.5 or more and a welding heat input of 50 to 500 kJ / cm is manufactured, the charpy toughness value of the bond portion is vTrs ≦. A method for manufacturing a steel plate at -30 ° C. The method for manufacturing a steel sheet according to claim 3, wherein after the step (c) _{, tempering is performed in a temperature range of 1 point or less of Ac.}