WO2007074989A9

WO2007074989A9 - Thick steel plate for welded structure having excellent strength and toughness in central region of thickness and small variation of properties through thickness and method of producing the same

Info

Publication number: WO2007074989A9
Application number: PCT/KR2006/005548
Authority: WO
Inventors: Sang-Ho Kim; Jae-Gi Lee; In-Shik Suh; Choong-Jae Park
Original assignee: Posco
Priority date: 2005-12-26
Filing date: 2006-12-19
Publication date: 2010-06-17
Also published as: JP2009521601A; CN101346483B; DE112006003553B4; KR100660229B1; CN101346483A; JP5701483B2; DE112006003553T5; DE112006003553B9; WO2007074989A1

Abstract

Disclosed herein is a thick steel plate for welded structure having excellent strength and toughness in a central region and exhibiting small variation in properties through thickness. The steel plate comprises 0.05 ~ 0.10 % of C, 0.10 ~ 0.5 % of Si, 1.3 ~ 1.7 % of Mn, 0.0005 ~ 0.0025 % of B; 0.005 ~ 0.03 % of Ti, 0.010 % or less of N, 0.005 ~ 0.03 % of Nb, 0.005 ~ 0.055 % of Sol. Al, and the balance of Fe and other unavoidable impurities, in terms of weight%, wherein a content ratio of Ti/N is 2.0 or more, and a CP represented by Expression 1 is 40 ~ 50. CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x %Ni + 12.5 x %Cr + 14.4 x %Mo... (l)

Description

THICK STEEL PLATE FOR WELDED STRUCTURE HAVING EXCELLENT STRENGTH AND TOUGHNESS IN CENTRAL

REGION OF THICKNESS AND SMALL VARIATION OF PROPERTIES THROUGH THICKNESS AND METHOD OF

PRODUCING THE SAME Technical Field

[1] The present invention relates to a thick steel plate for welded structure having excellent strength and toughness in a central region of its thickness and exhibiting small variation in properties through the thickness, and a method of producing the same. More particularly, the present invention relates to a method of producing a high strength thick steel plate, which has excellent strength and toughness in a central region of thickness, and exhibits small variation in properties through the thickness along with weldability ensured by minimizing an addition of alloy elements.

[2]

Background Art

[3] Conventionally, a high strength steel plate has been produced in such a way to enhance so-called hardenability of steel by adding a great amount of alloy elements. In this case, a great amount of low temperature structures such as martensite or bainite are formed in the steel via cooling treatments such as quenching and tempering, so that the strength of the steel is enhanced.

[4] When producing steel plates for ships, sea structures, buildings, etc., a welding process is inevitably performed. In this regard, if the steel plate for the welded structure contains a great amount of alloy elements, the welding process can cause a welded part to be significantly deteriorated in low temperature toughness.

[5] In order to solve the problem described above, Japanese Patent Laid-open Publication No. (Sho) 62-0170459 discloses a technique, which ensures the strength of steel by quenching the steel after rolling the steel while preventing coarsening of structure in a heat affect zone (HAZ) through restriction in a carbon equivalent of the steel along with use of TiN inclusions.

[6] Likewise, Japanese Patent Laid-open Publication No. (Hei) 7-0268540 discloses a technique, which enhances the toughness of steel by preventing coarsening of structure upon welding through restriction in an amount of elements such as C, Si, Mn, and the like while controlling contents of Ti, Al, and the like to form a great amount of Ti-Al based non-metallic inclusions in the steel. [7] Both techniques described above have a common feature in that the weldability of the steel is enhanced at first by suppressing the contents of alloy elements in the steel as much as possible, and distributing a great amount of non-metallic inclusions, which become transformation precipitation nuclei of the structure while imparting pinning effect of preventing coarsening of the structure, and then, the strength of the steel is enhanced by forming a great amount of low temperature transformation structures through quenching of the steel. In other words, according to the techniques described above, it is possible to enhance both strength and weldability of the steel in order of 1) ensuring the conditions to enhance the weldability of the steel by restricting the amount of the alloy elements while distributing the fine inclusions, and 2) ensuring the conditions to enhance the strength of the steel by increasing the cooling rate.

[8] However, it is difficult to apply these techniques to a thick steel plate having a thickness of 50 mm or more. This is attributed to the fact that, as the thickness of the steel plate is increased, difference in cooling rate between the surface and the interior of the steel plate is significantly increased, and this causes low temperature transformation soft structures such as polygonal ferrite or pearlite to be formed mainly in the interior of the steel, particularly, in the center region of thickness of the steel plate, so that, even though a great amount of low temperature transformation structures are formed on the surface of the steel, the whole strength of the steel is reduced in comparison to a thin steel plate.

[9] In order to solve the problem as described above, Korean Patent No. 10-0266378 discloses a method which produces a bainite steel plate through hot rolling of a steel slab comprising 0.001 ~ 0.010 wt% of C in a region of ultra low carbon content, 0.60 wt% or less of Si, 0.20 ~ 3.00 wt% of Mn, 0.005 ~ 0.20 wt% of Ti, 0.01 ~ 0.20 wt% of Nb, 0.0003 ~ 0.0050 wt% of B, and 0.100 wt% of Al in such a way of heating the steel slab to a temperature of 1,100 ~ 1,350 ⁰C, isothermally maintaining the steel slab for 5 to 300 seconds or cooling the steel plate at a cooling rate of 1 °C/sec between rolling passes at a temperature of 1,100 ~ 900 ⁰C, and finishing the rolling at a temperature of 800 ⁰C or more, followed by cooling the steel plate.

[10] In the above method, the steel slab is the ultra low carbon steel which contains 0.010 wt% or less of carbon, and has a bainite structure, which is different from that of a typical bainite structure, and is typically known as ultra low carbon bainite (ULCB).

[11] The ULCB structure is observed in high strength and high toughness steel, which has good matrix toughness and low variation in properties resulting from low variation of hardness through the thickness. However, as can be appreciated from an embodiment disclosed in the publication, such an ULCB structure has a yield strength of 400 MPa level at point t/4. With this yield strength, the yield strength in the center region of the thickness of the steel can be inferred to be about 350 MPa, which is still less than 390 MPa, a target yield strength of the present invention in the center region of the thickness of the steel. In addition, since the ULCB-based steel is degraded in toughness at a welded part, a securing temperature thereof is merely 0 ⁰C.

[12] In order to enhance the strength of the ULCB-based steel, it is necessary to perform complicated processes in such a way of adding any one of Cu, Ni, Cr and Mo or combinations thereof at a great amount or adding a great amount of Cu, performing heat treatment, and the like. In this case, due to the great amount of alloy elements added thereto, not only manufacturing costs can be possibly increased, but also the toughness of the welded part can be significantly deteriorated.

[13]

Disclosure of Invention Technical Problem

[14] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a thick steel plate having a large thickness of 50 mm or more for low-alloy welded structure, which exhibits a tensile strength of 530 MPa or more, a yield strength 390 MPa or more in a central region of thickness of the steel plate, a ductility-brittleness transition temperature of -50 ⁰C or less, and a hardness variation of 50 Hv or less through the thickness.

[15]

[16] *

Technical Solution

[17] In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a thick steel plate, comprising: 0.05 ~ 0.10 % of C; 0.10 ~ 0.5 % of Si; 1.3 ~ 1.7 % of Mn; 0.0005 ~ 0.0025 % of B; 0.005 ~ 0.03 % of Ti; 0.010 % or less of N; 0.005 ~ 0.03 % of Nb; 0.005 ~ 0.055 % of Sol. Al; and the balance of Fe and other unavoidable impurities, in terms of weight%, wherein a content ratio of Ti/N is 2.0 or more, and a composition parameter (CP) represented by Expression 1 is in the range of 40 ~ 50;

[18] CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x %Ni

+ 12.5 x %Cr + 14.4 x %Mo ... (1)

[19] Preferably, the thick steel plate further comprises at least one component selected from the group consisting of 0.5 % or less of Cu; 0.5 % or less of Ni; 0.15 % or less of Cr; and 0.15 % or less of Mo in terms of weight%.

[20] Preferably, among the unavoidable impurities, contents of P and S are controlled to

0.012 % or less, and 0.005 % or less, respectively, in terms of weight%.

[21] Preferably, in order to further decrease negative influence caused by the impurities, the contents of P and S are controlled to 0.010 % or less, and 0.003 % or less, re- spectively, in terms of weight%.

[22] Preferably, a fraction of polygonal ferrite is 10 % or less in the central region

(ranging from t/4 to 3t/4, when t indicates a total thickness of the steel plate) of the thickness, and a fraction of martensite is 10 % or less in a surface region of the steel plate (ranging from a depth of 1 mm below the surface to t/4, the opposite side being the same).

[23] Preferably, the steel plate has a hardness variation of Hv 50 or less through the thickness.

[24] In addition, the present invention is effective for the steel plate which has a thickness of 50 - 100 mm.

[25] In accordance with another aspect of the present invention, there is provided a method for producing a steel plate, comprising the steps of: finish rolling a steel slab at a reduction rate of 30 % or more at a temperature of Ar3 ~ an austenite recrystal- lization temperature, after reheating the steel slab to a temperature of 1,000 ~ 1,250 ⁰C, the steel slab comprising 0.05 ~ 0.10 % of C, 0.10 ~ 0.5 % of Si, 1.3 ~ 1.7 % of Mn, 0.0005 ~ 0.0025 % of B, 0.005 ~ 0.03 % of Ti, 0.010 % or less of N, 0.005 ~ 0.03 % of Nb, 0.005 ~ 0.055 % of Sol. Al, and the balance of Fe and other unavoidable impurities in terms of weight% wherein a content ratio of Ti/N is 2.0 or more, and a composition parameter (CP) represented by Expression 2 is in the range of 40 ~ 50; and cooling the hot rolled steel plate in such a way of beginning cooling at a rate of 1.5 °C/sec or more in view of a central region of the steel plate at a temperature of Ar3 or more and finishing cooling at a temperature of 350 ~ 550 ⁰C.

[26] CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x %Ni

+ 12.5 x %Cr + 14.4 x %Mo ... (2)

[27] Preferably, the steel slab further comprises at least one component selected from the group consisting of 0.5 % or less of Cu; 0.5 % or less of Ni; 0.15 % or less of Cr; and 0.15 % or less of Mo in terms of weight%.

[28] Preferably, the steel slab comprises 0.012 % of P, and 0.005 % or less of S as the impurities.

[29] Preferably, the steel slab comprises 0.010 % of P, and 0.003 % or less of S.

[30] Preferably, the present invention is effective for the steel plate which has a thickness of 50 ~ 100 mm.

[31]

Advantageous Effects

[32] As apparent from the above description, the method according to the present invention produces a thick steel plate having a thickness of 50 mm or more for welded structure, which exhibits excellent strength and toughness in a center region of thickness, and small variation in properties through the thickness, along with ensuring weldability of the steel plate by minimizing an addition of alloy elements. [33]

Brief Description of Drawings

[34] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[35] Fig. 1 is a graphical representation depicting a fraction of martensite in a surface region and a fraction of polygonal ferrite in a central region of thickness in relation to CP; and

[36] Fig. 2 is a graphical representation depicting a distribution of hardness variation through thickness in relation to CP.

[37]

Best Mode for Carrying out the Invention

[38] The present invention will be described as follows.

[39]

[40] Microstructure of steel plate

[41] The present invention is preferably applied to a steel plate which has bainite structures or acicular ferrite structures, which contain substantially no polygonal ferrite or martensite. In particular, in order to obtain target strength and toughness of the steel plate according to the present invention, it is necessary to suppress a fraction of polygonal ferrite to 10 % or less in a central region of thickness (when t indicates a total thickness of the steel plate, the central region of thickness refers to the range of t/ 4 ~ 3t/4, that is, the thickness center (t/2) ± XlA), and in order to obtain a hardness variation through the thickness according to the present invention, it is necessary to suppress a fraction of martensite to 10 % or less in a surface region of the steel plate (ranging from a depth of 1 mm below the surface to XlA of the thickness, the opposite side being the same).

[42] In this case, if the steel plate satisfies composition of the present invention, the steel plate has the structures comprising acicular ferrite as a main structure, and bainite as a secondary structure in the whole region of the steel plate in a thickness direction excluding the depth of 1 mm below the surface and a segregation part in the central region of thickness.

[43]

[44] Composition of steel plate

[45] According to the present invention, the steel plate comprises 0.05 ~ 0.10 % of C;

0.10 ~ 0.5 % of Si; 1.3 ~ 1.7 % of Mn; 0.012% or less of P; 0.005% or less of S; 0.0005 ~ 0.0025 % of B; 0.005 ~ 0.03 % of Ti; 0.005 ~ 0.03 % of Nb; 0.005 ~ 0.055 % of Sol. Al; 0.01 % or less of N; and the balance of Fe and other unavoidable impurities, in terms of weight%. In addition, a content ratio of Ti/N is 2.0 or more, and a composition parameter (CP) represented by the following Expression 1 is in the range of 40 - 50.

[46] CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x %Ni

+ 12.5 x %Cr + 14.4 x %Mo ... (1)

[47]

[48] Composition of the steel plate according to the invention will now be described in detail.

[49]

[50] Carbon (C): 0.05 - 0.10 wt%

[51] C is an element, which is effective to increase the strength of the steel plate by allowing solid solution strengthening while enhancing the hardenability of the steel plate. In order to ensure a desired tensile strength in the central region of the thickness of the steel plate, it is necessary to contain carbon in an amount of 0.05 wt% or more. Furthermore, in order to ensure low temperature toughness at a welded part, it is necessary to contain carbon in an amount of 0.05 wt% or more such that soft structures such as acicular ferrite can be formed by allowing a boron carbide to be formed after welding. However, an excessive content of carbon causes the hardness to increase in the surface region, thereby increasing a hardness variation in the thickness direction. Furthermore, the excessive content of carbon also causes a degradation in toughness of matrix, and an increase in fraction of martensite-austenite constituent (MA, martensite island) at the welded part, thereby significantly deteriorating the toughness of the welded part. Thus, an upper limit of the carbon content is set to 0.1 wt%.

[52]

[53] Silicon (Si): 0.10 - 0.5 wt%

[54] Si is an element which assists aluminum in deoxidization of molten steel. Thus, it is necessary to contain 0.10 wt% or more of Si. However, if the silicon content becomes excessive, the martensite island formed in a HAZ is not decomposed, thereby significantly increasing possibility of brittle fracture while deteriorating the toughness of the matrix. Thus, it is disadvantageous to contain Si above 0.5 wt%.

[55]

[56] Manganese (Mn): 1.3-1.7 wt%

[57] Mn is an element which serves to increase the strength of the steel while lowering a yield ratio thereof. In particular, Mn suppresses a fraction of polygonal ferrite, thereby increasing the hardenability of the steel plate. Thus, it is necessary to contain 1.3 wt% of Mn. However, if the manganese content becomes excessive, the steel plate is increased in strength, but deteriorated in toughness, particularly, in toughness at the heat affected zone (HAZ). Thus, it is necessary to suppress the content of Mn to 1.7 wt% or less.

[58]

[59] Boron (B): 0.0005 ~ 0.0025 wt%

[60] B is an essential element of the present invention, and enables the hardenability of steel to increase with a small addition thereof. In order to obtain a target strength in the central region of the thickness according to the present invention while allowing the central region of the thickness to have the acicular ferrite structure which substantially does not contain the polygonal ferrite, it is necessary to contain 0.0005 wt% or more of B. However, if boron is excessively contained at an amount above 0.0025 wt%, the steel plate is degraded in hardenability in the central region, deteriorating the strength of the steel plate while enlarging the hardness variation in the thickness direction. Thus, it is necessary to suppress the content of B to 0.0025 wt% or less.

[61]

[62] Titanium (Ti): 0.005 ~ 0.03 wt%

[63] According to the present invention, Ti is another essential element along with B. In order to obtain hardenability enhancing effect of B, it is necessary for boron to exist in an atomic state after rolling operation. In this regard, if N exhibiting a great affinity to B exists as solid solution N in steel during a reheating or rolling process, N and B form BN compounds, thereby eliminating the hardenability enhancing effect of B. Accordingly, it is necessary to suppress formation of the BN compounds in such a way of forming TiN before forming BN by adding Ti which exhibits a stronger affinity than that of B with respect to N. To this end, it is necessary to contain at least 0.005 wt% of Ti. However, if the Ti content exceeds 0.03 wt%, the effect obtained by addition of Ti is saturated. Furthermore, if the Ti content becomes excessive, there may occur nozzle clogging or lots of coarsened inclusions during continuous casting, thereby deteriorating the toughness of steel. Thus, it is necessary to suppress the content of Ti to 0.03 wt% or less.

[64]

[65] Nitrogen (N): 0.010 wt% or less

[66] Although N is an unavoidable element in a steel manufacturing process, it reacts with

Ti and/or Al and forms nitrides, thereby serving to form fine structures. Meanwhile, in order to add 0.010 wt% or more of N to the steel, it is necessary to perform a particular process, such as inputting an excessive amount of manganese nitride or cyanide- containing compounds during the steel manufacturing process, and in this case, N exists in a solid solution state in the steel, thereby deteriorating the hardenability enhancing effect of B. Thus, it is necessary to suppress the content of N to 0.010 wt% or less.

[67] More preferably, the contents of N and Ti are controlled in terms of a content ratio therebetween. Specifically, since it is possible to effectively suppress solid solution N with Ti by controlling a weight ratio of Ti and N (a ratio of Ti/N) to become 2.0 or more, the Ti content is controlled to have a Ti/N weight ratio of 2.0 or more.

[68]

[69] Niobium (Nb): 0.005 ~ 0.03 wt%

[70] Nb is yet another essential element along with B and Ti in the present invention. In order to sufficiently apply the hardenability enhancing effect of B, it is necessary to add Nb simultaneously with B and Ti. Nb serves to cause austenite structures to have a fine grain size, and to enlarge a non-recrystallization region while contributing to refinement and strength enhancement of final structures. To this end, it is necessary to contain 0.005 wt% or more of Nb. However, since Nb is an expensive element and the Nb content exceeding 0.03 wt% does not ensure significant increase in effect thereof while deteriorating the toughness at the welded part, an upper limit of Nb is set to 0.03 wt%.

[71]

[72] Soluble aluminum (Sol. Al): 0.005 ~ 0.055 wt%

[73] Al is generally used as a deoxidizing agent for the steel. Thus, it is necessary to contain 0.005 wt% or more of Sol. Al as an effective component thereof. However, if the Al content exceeds 0.055 wt%, the deoxidizing effect is saturated, and thus, an upper limit of Sol. Al is set to 0.055 wt%.

[74]

[75] The composition of the steel plate described above is advantageous to impart excellent properties such as high strength and high toughness to the steel plate while reducing a variation of the properties through the thickness. In addition to this composition, it is preferable that the steel plate further comprises at least one component selected from the group consisting of Cu, Ni, Cr, and Mo in order to obtain more advantageous effect.

[76]

[77] Copper (Cu): 0.5 wt% or less and Nickel (Ni): 0.5 wt% or less

[78] Cu and Ni are elements, which serve to enhance the hardenability of steel without significantly reducing the toughness at the welded part, thereby suppressing the polygonal ferrite from being formed in the steel, and serve to enhance the strength of steel through solid solution strengthening. However, since Cu and Ni are expensive elements and excessive addition thereof cause saturation of their effects, both upper limits of Cu and Ni are set to 0.5 wt%.

[79] [80] Chrome (Cr): 0.15 wt% or less

[81] Cr is an element which can remarkably enhance the hardenability of steel. Thus, as the content of Cr increases in the steel, the polygonal ferrite is suppressed from being formed in the steel, thereby enhancing the strength of steel. However, if the Cr content is excessive in the steel, not only the steel is deteriorated in weldability, but also the martensite can be formed therein. In addition, Cr is a very expensive material. Thus, it is desirable that the Cr content be 0.15 wt% or less.

[82]

[83] Molybdenum (Mo): 0.15 wt% or less

[84] Mo provides the same effect as that of Cr. Thus, although Mo is effective in view of polygonal ferrite suppression and strength increase, if the Mo content is excessive in the steel, not only the steel is deteriorated in weldability, but also the martensite can be formed therein. In addition, Mo is a very expensive material. Thus, it is desirable that the Mo content be 0.15 wt% or less.

[85]

[86] Additionally, the steel plate of the present invention may comprise P, S, and the like as unavoidable elements during the steel manufacturing process. More preferably, the content of these elements are restricted so as to satisfy the following conditions in order to further enhance the properties of the steel.

[87]

[88] Phosphorus (P): 0.012 wt% or less (Preferably, 0.010 wt% or less)

[89] P is an element, which causes grain boundary segregation resulting in embrittlement of steel. Thus, it is necessary for the steel comprising the acicular ferrite and/or bainite as the main structure to minimize the content of P in order to enhance the toughness of the steel. However, since minimization of P content to an ultimately low level is accompanied with a severe load in the manufacturing process, and 0.012 wt% or less of P in the steel does not significantly suffer from the above problems, an upper limit of P is set to 0.012 wt%. More preferably, the P content is set to 0.010 wt% or less in order to prevent negative influence of P as described above.

[90]

[91] Sulfur (S): 0.005 wt% or less (Preferably, 0.003 wt% or less)

[92] S is an element which causes hot shortness of the steel. Like the P content, an upper limit of S is set to 0.005 wt% or less, and preferably, 0.003 wt% or less in consideration of the load during the steel manufacturing process.

[93] In addition to the composition of the steel plate, the steel plate of the present invention has a composition parameter (CP) in the range of 40 ~ 50 as is represented by the following Expression 2. With the composition parameter, it is possible to simultaneously determine how much the polygonal ferrite is suppressed from being formed in the central region of the thick steel plate and how much the martensite is suppressed from being formed in the surface region thereof when cooling the thick steel plate with water.

[94] CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x %Ni

+ 12.5 x %Cr + 14.4 x %Mo ... (2)

[95]

[96] The reason of suggesting the CP will be described hereinafter.

[97] With results of investigation, inventors of the present invention reported that, when using a conventional cooling method to cool a thick steel plate having a thickness of 50 mm - 100 mm as a target steel plate of the present invention, although it could be varied according to the thickness of the steel plate and a cooling manner, a maximum cooling rate in the central region of thickness of the thick steel plate was about 3 - 6 °C/sec. Meanwhile, in order to obtain the maximum cooling rate in the central region described above, a cooling rate in a surface region corresponding to a depth of lmm directly below the surface of the steel plate must be 20 - 40 °C/sec. Thus, there is a large gap in cooling rate between the central region of the thickness and the surface region.

[98] Accordingly, for a typical steel plate, the surface region tends to be formed with martensite due to its rapid cooling rate, whereas the central region tends to be formed with polygonal ferrite due to its slow cooling rate. If such a tendency is not suppressed, the martensite increases in its fraction in the surface region of the steel plate, and the polygonal ferrite increases in its fraction in the central region thereof. The martensite is a representative hard structure, which enhances the strength of steel while deteriorating the toughness thereof. On the contrary, the polygonal ferrite is a representative soft structure, which is effective to ensure the toughness but is inappropriate to enhance the strength of steel. Accordingly, if the steel has such a structural variation described above, the steel exhibits a severe variation of properties, which results in deteriorated toughness of the surface region, and lowered strength of the central region. In addition, with the structural variation described above, a hardness difference between the central region and the surface region becomes significant, making it difficult to obtain effect of reducing the hardness variation which is one of the objects of the present invention.

[99] The CP is a parameter that the inventors of the present invention derived through investigation over a long period of time to solve the problems described above. If the CP is maintained in a predetermined range, the tendency of forming the martensite in the surface region and forming the polygonal ferrite in the central region can be suppressed, thereby minimizing the variation of properties through the steel plate. In order to obtain the object of the present invention, it is necessary to suppress both fractions of martensite and polygonal ferrite to 10 % or less in the whole region of the steel plate excluding abnormal regions. The abnormal regions of the steel plate refer to a central segregation region formed in the central region of the steel plate, and a region from the surface of the steel plate to the depth of 1 mm directly under the surface. The central segregation region refers to a region of the steel plate where an abnormally large amount of solid solution elements are segregated, thereby making it difficult to ensure the typical properties of steel, and is formed since the region from the surface of the steel plate to the depth of 1 mm directly under the surface is strongly influenced by the cooling rate.

[100] Of course, as described above, the present invention strictly sets the upper and lower limits of the respective components so as to ensure the strength, toughness and weldability of the steel, which are desired to be obtained in the present invention, and it is possible to achieve the object of the present invention if the steel plates satisfies the upper and lower limits of the respective components. However, according to tests, even if the steel plate satisfied the composition according to the present invention, the object of the present invention was not achieved in some cases. Accordingly, the inventors of the present invention compared the case where the object of the present invention was achieved with the case where the object of the present invention was not achieved when the steel plate satisfied the composition according to the present invention, and concluded that the reason of failure in achieving the object of the present invention could be explained using the CP described above, and that the object of the present invention could be achieved by controlling the CP to a predetermined range in the condition that the steel plate satisfies the composition according to the present invention.

[101] A result of experiments is shown in Fig. 1, by which fractions of polygonal ferrite and martensite in the whole region of the steel plate excluding the abnormal regions according to CP as listed in Table 1 are calculated by a point counting method. In Fig. 1, a fraction of polygonal ferrite was measured at a cooling rate of 1.5 °C/sec, that is, 50 % of the maximum cooling rate of 3 °C/sec which can be typically obtained in the central region of the steel plate having a thickness of 50 ~ 100 mm if such a steel plate is subjected to accelerated cooling. In addition, a fraction of martensite was measured at a cooling rate of 40 °C/sec which can be typically obtained at a depth of 1 mm directly under the surface of the steel plate having the thickness of 50 ~ 100 mm if the steel plate is subjected to accelerated cooling.

[102] As can be appreciated from Fig. 1, when the CP represented by Expression 1 is 40 or more, the fraction of polygonal ferrite is 10 % or less, which means that, if the CP is 40 or more, the fraction of polygonal ferrite can be maintained to 10 % or less even with the cooling rate of 1.5 °C/sec (below 3 °C/sec, which is the typical cooling rate in the central region of the steel plate having the thickness of 50 ~ 100 mm). When the CP is 50 or less, the fraction of martensite is 10 % or less, which means that, if the CP is 50 or more, the fraction of martensite can be maintained at 10 % or less even with the cooling rate of 40 °C/sec, a typical cooling rate at the depth of 1 mm directly under the surface of the steel plate having the thickness of 50 ~ 100 mm.

[103] Fig. 2 shows a difference between a maximum value and a minimum value of

Vickers hardness measured at 2 mm intervals in the thickness direction for steel plates, each having a thickness of 100 mm and a different CP from others. As can be appreciated from Fig. 2, when the CP is in the range of 40 ~ 50 according to the present invention as described above, the hardness difference between the maximum value and the minimum value can be controlled to 50 Hv or less. As described above, this is caused by restricting the fraction of martensite and the fraction of polygonal ferrite to 10 % or less in the surface region and in the central region of the thickness, respectively.

[104] In addition, the thick steel plate satisfying all conditions of the present invention described above has the thickness of 50 ~ 100 mm, and a hardness variation of Hv 50 or less in the thickness direction.

[105] Preferably, such a thick steel plate satisfying the conditions as described above is manufactured by manufacturing conditions as follows.

[106]

[107] (Rolling and cooling conditions)

[108] According to the present invention, effect of the present invention can be substantially achieved by controlling the composition and structure of the steel plate as described above even with controlled rolling and accelerated cooling which are known in the art. However, in order to further enhance the effect of the present invention, it is necessary to control rolling and cooling conditions more accurately than the conventional method.

[109]

[110] Reheating temperature: 1 ,000 ~ 1 ,250 ⁰C

[111] When hot rolling a steel slab comprising components described above, it is necessary to heat the steel slab to a predetermined temperature. In order to achieve the object of the present invention, it is necessary to allow B to exist in an atomic state in a steel plate after hot rolling. To this end, it is necessary to prevent BN from being precipitated in the steel plate during cooling after the rolling in such a way of reducing a content of solid solution N in the steel plate by maintaining B in a solid solution state while allowing N to be precipitated as TiN therein. In order to ensure this effect, the steel slab is heated to 1,000 ⁰C or more, thereby causing BN formed during solidification of molten steel to be dissolved and exist as solid solution in the steel. Meanwhile, if the steel slab is heated to a temperature of 1,250 ⁰C or more, TiN pre- cipitates are dissolved in the steel, thereby allowing a great amount of solid solution N to be contained in the steel. [112] Thus, it is necessary to control the reheating temperature of the slab to 1,000 < T reheating ≤ 1,250 C

[113]

[114] Finish rolling temperature: Ar₃ ~ austenite recrystallization temperature

[115] A temperature for finish rolling is one of essential components to achieve the object of the present invention. If finish rolling is performed at a ferrite transformation temperature of Ar₃ or less, polygonal ferrite is formed, thereby making it difficult to maintain a fraction of polygonal ferrite to 10 % or less, which satisfies the condition of the present invention. Thus, it is necessary to perform the finish rolling at a temperature of Ar₃ or more. Meanwhile, if the finish rolling is performed at a significantly high temperature above the austenite recrystallization temperature, a fraction of martensite may increase to 10 % or more, and recrystallization grains may be coarsened, thereby not only deteriorating the toughness of the steel plate but also significantly increasing the hardenability in the surface region of the steel plate. Thus, it is preferable to set an upper limit of the finish rolling temperature to the austenite recrystallization temperature or less.

[116] As such, the finish rolling temperature is preferably in the range of Ar₃ < T_{Fimsh ro}ii_mg ≤ austenite recrystallization temperature.

[117]

[118] Reduction rate for finish rolling: 30 % or more

[119] In order to sufficiently achieve austenite grain refinement effect upon the finish rolling, the reduction rate for the finish rolling is preferably 30 % or more, and more preferably, 45 % or more. If the reduction rate is less than 30 % during the finish rolling, the effect of austenite grain refinement is not satisfactory, reducing the toughness of the steel plate while insufficiently enhancing the strength of the steel plate.

[120]

[121] Initial cooling temperature: Ar₃ or more

[122] Even though the finish rolling is completed at a temperature of Ar₃ or more, if water cooling is not begun at the temperature of Ar₃ or more, coarse polygonal ferrite is formed in the steel plate during air cooling. In this case, the structure of the steel plate desired to be formed in the present invention is not obtained, and the strength and toughness of the steel plate are also deteriorated. Accordingly, in order to achieve the object of the present invention, it is necessary to begin the cooling process before the temperature of the steel plate reaches a temperature of forming the ferrite, that is, Ar₃.

[123] [124] Cooling rate: 1.5 °C/sec or more

[125] When accelerated cooling is performed according to a typical method, the object of the present invention can be achieved if the steel plate has the composition of the present invention described above. However, if the cooling rate of the steel plate is very slow, for example, if the steel plate is cooled in air after the rolling, a great amount of polygonal ferrite is formed within the whole region of the steel plate, thereby failing to achieve the object of the present invention. Thus, in order to effectively achieve the object of the present invention, it is necessary to perform cooling of the steel plate such that the polygonal ferrite is suppressed from being formed in the central region of the thickness of the steel plate.

[126] To this end, the cooling rate in the central region of the steel plate must be 1.5 °C/sec or more.

[127]

[128] Final cooling temperature: 350 ~ 550 ⁰C

[129] If cooling of the steel plate is finished at a temperature of 550 ⁰C or more, the polygonal ferrite is likely to be formed in the central region of the thickness, and interferes with formation of acicular ferrite desired to be formed in the present invention. In addition, if cooling of the steel plate is stopped at a temperature less than 350 ⁰C, a fraction of low temperature structures such as bainite or martensite increases. The low temperature structures such as the bainite or the martensite cause so-called continuous yielding in which a yielding point is not exhibited in the stress-strain curve of a material, so that, as the fraction of the low temperature structures increases to a predetermined level, the yield strength decreases.

[130] Accordingly, in the case of a thick steel plate in which it is difficult to have low temperature structure over the whole thickness of the steel plate as in the present invention, the yield strength may be effectively enhanced by preventing the fraction of the low temperature structures from increasing. In this regard, the final cooling temperature is preferably in the range of 350 ~ 550 ⁰C.

[131]

Mode for the Invention

[132] Example 1

[133] In order to confirm properties of steel plates produced according to the present invention, after each of steel slabs having compositions listed in the following Table 1 was subjected to temper rolling, hot rolling was performed at an accumulated reduction rate of 40 % or more at a temperature between non-recrystallization temperature and Ar3 in relation to the respective compositions, where the steel plates had a thickness of 50 mm or 100 mm. After the rolling, cooling of the steel plate was begun at a tern- perature of Ar₃ + 10 ⁰C or more while controlling a cooling rate to be 3 °C/sec in a central region of each steel plate. [134] [135] Table 1

[Table 1] [Table ]

[136] IS: Inventive steel, CS: Comparative steel [137] [138] In Table 1, contents of the respective elements are listed in terms of weight %, and, although not shown in Table 1, Sol. Al is added to the steel slabs at an amount to satisfy the content (0.005 ~ 0.055 wt%) according to the present invention.

[139] With the conditions as described above, the steel plates having a thickness of 50 mm or 100 mm were produced. Fine structures and hardness differences were measured only for the steel plates having the thickness of 100 mm. Mechanical properties were measured for the steel plates having the thickness of 100 mm or 50 mm using samples cut from central regions of the respective steel plates (for each sample, the center of the sample was coaxial with the center of the steel plate in the thickness direction). In addition, impact toughness was measured at a fusion line of a welded part where heat input of 300 kJ/cm was performed during welding. Results of these tests are shown in Table 2.

[140] In Table 2, VF means a fraction of polygonal ferrite, which was measured by the point counting method for 0.01 mπf of a central region of each steel plate in the thickness direction (that is, when t indicates a total thickness of the steel plate, the central region refers to the range of t/4 ~ 3t/4 from the surface of the steel plate), and VM means a fraction of martensite, which was measured in the range from a depth of 2 mm below the surface to t/4 of the thickness by the same method as that of VF. A hardness difference is a difference between the maximum hardness and the minimum hardness for Vickers hardness which were measured in the whole region, excluding the depth of 2 mm below the surface of the steel plate and a segregation part in the central region of the thickness. DBTT of a matrix and the welded part is a ductility-brittleness transition temperature, which was measured at 20 ⁰C intervals from room temperature (20 ⁰C) to -140 ⁰C by Charpy V-notch impact test, and exhibited a value of 200 J.

[141]

[142] Table 2

[Table 2] [Table ]

[143] IS: Inventive steel, CS: Comparative steel [144]

[145] In Table 2, the steel plates produced according to the present invention have the polygonal ferrite, the content of which is suppressed to 10 % or less in the central region of each steel plate having the thickness of 100 mm, and the martensite, the content of which is suppressed to 10 % or less at the surface region, so that the hardness difference in the thickness direction is controlled to 50 Hv or less. In addition, for the steel plates having the thickness of 50 mm or 100 mm, the central region of each steel plate has a yield strength of 399 MPa or more, a tensile strength of 536 MPa or more, and a DBTT of -52 ⁰C or less. From the results shown in Table 2, it can be appreciated that the object of the present invention is achieved. Meanwhile, with a result of measuring the toughness of the welded part, phases of the steel plates produced by the present invention exhibit an excellent impact toughness of 132 J or more at -20 ⁰C, and an excellent DBTT of -35 ⁰C or less.

[146] On the contrary, for Comparative Steel 1 which comprises a composition according to an invention disclosed in Korean Patent No. 10-0266378 Bl, fractions of structures and a hardness variation in the thickness direction are similar to those of the present invention. However, when it is applied to a steel plate having a thickness of 100 mm, the strength of the central region and the weldability of the welded part are also significantly lowered due to its excessively low content of C.

[147] For Comparative Steel 2 which has an excessively high content of C, the strength is significantly high, but the hardness difference is 50 Hv or more due to an increased fraction of martensite and increased solid solution strengthening effect of C in the surface region. In particular, the DBTT of the matrix is above -50 ⁰C, and the toughness of the welded part does not satisfy the object of the present invention.

[148] For Comparative Steel 3 which has a content of Si above the upper limit of the present invention, the strength and the hardness difference satisfy the object of the present invention, but the toughness of the welded part and the matrix is significantly low.

[149] For Comparative Steel 4 which has a content of Mn below the lower limit of the present invention, the fractions of structures and the hardness variation in the thickness direction for the steel plate having the thickness of 100 mm do not satisfy the object of the present invention due to low hardenability. On the other hand, for Comparative Steel 5 which has a content of Mn above the upper limit of the present invention, the impact toughness of the matrix and the welded part is significantly low due to low hardenability, failing to satisfy the object of the present invention.

[150] For Comparative Steel 6 which has a content of P above the upper limit of the present invention, the impact toughness of the matrix and the welded part is significantly low, and thus does not satisfy the object of the present invention.

[151] For Comparative Steel 7 which does not contain B, the fraction of polygonal ferrite in the central region of the thickness is 10 % or more which is a higher value than that of the present invention, and results in deterioration of the strength in the central region of the thickness for the steel plates having the thickness of 100 mm, thereby failing to satisfy the object of the present invention. On the other hand, for Comparative Steel 8 which excessively contains B, the effect of B is weakened due to precipitation of boron nitride and the like, causing a hardness difference of 50 Hv or more while deteriorating the strength and toughness of the central region of the thickness for the steel plates having the thickness of 100 mm to a degree not satisfying the object of the present invention.

[152] For Comparative Steel 9 which does not contain Ti, and Comparative Steel 10 which has a Ti/N ratio of 2 or less, since the content of solid solution N is not sufficiently lowered, BN is formed. Thus, the fraction of polygonal ferrite is 10 % or more, and the hardness difference is 50 Hv or more in the central region of the thickness. In addition, the tensile strength of the central region does not satisfy the object of the present invention.

[153] For Comparative Steel 11 which does not contain Nb, the hardenability enhancing effect of B is not realized, thereby causing the fraction of polygonal ferrite in the central region of the thickness to exceed 10 % while exhibiting the hardness difference of 50 Hv or more. In addition, the strength of the central region of the thickness for the steel plates having the thickness of 100 mm does not satisfy the object of the present invention.

[154] Comparative Steels 12 and 13 have the compositions of the present invention, and different CP from that of the present invention. Specifically, for Comparative Steel 12 which has a CP below the lower limit of the present invention, the fraction of polygonal ferrite in the central region of the thickness exceeds 10 %, and the hardness difference therein exceeds 50 Hv. In addition, the strength of the central region of the thickness for the steel plates having the thicknesses of 100 mm and 50 mm does not satisfy the object of the present invention. For Comparative Steel 13 which has a CP above the upper limit of the present invention, the fraction of martensite in the surface region exceeds 10 %, and the hardness difference therein also exceeds 50 Hv. In addition, for Comparative Steel 13, the impact toughness of the matrix and the welded part is significantly low due to excessive increase in strength, failing to satisfy the object of the present invention.

[155] With the results as described above, it is possible to confirm that the steel plate according to the present invention has advantageous effects.

[156]

[157] Example 2

[158] After temper rolling steel slabs having compositions of Inventive Steels 1 to 11 listed in Table 1, rolling and cooling were performed with conditions listed in Table 3 as follows, thereby forming thick steel plates, each having a thickness of 100 mm. [159]

[160] Table 3

[Table 3] [Table ]

[161] IM: Inventive material, CM: Comparative material, IS: Inventive steel [162] [163] In Table 3, Tnr indicates an austenite recrystallization temperature, and Ar3 indicates an initial transformation temperature from austenite to ferrite. In addition, T4 and T5 indicate an initial temperature of finish rolling, and a final temperature of finish rolling, respectively.

[164] After obtaining samples from the steel plates produced under the conditions of Table 3 with the same manner as that of Example 1, mechanical properties of the samples were measured, results of which are shown in Table 4.

[165] [166] Table 4

[Table 4] [Table ]

[167] IM: Inventive material, CM: Comparative material [168] [169] In Table 4, DBTT means a ductility-brittleness transition temperature. [170] For Inventive materials 1 to 25 which were produced according to rolling and cooling conditions of the present invention as described above, a fraction of polygonal ferrite in a central region of thickness is 10 % or less, a fraction of martensite in a region excluding regions corresponding to a depth of 2 mm below the surface of the steel plate is 10 % or less, thereby providing a hardness difference of 50 Hv or less, which satisfies the object of the present invention. In addition, the central region of each steel plate has a yield strength of 395 MPa or more, a tensile strength of 532 MPa or more, and a DBTT of -52 ⁰C or less, which results in excellent low temperature toughness.

[171] For Comparative Material 1, reheating was performed at a temperature not in the range of the present invention. In this case, a great amount of solid solution N exists in the material, and causes insufficient hardenability enhancing effect of B, so that an excessive fraction of polygonal ferrite is formed in the central region of the thickness, and results in the hardness difference above 50 Hv. In addition, both yield strength and tensile strength in the central region of the thickness are significantly lower than target values.

[172] For Comparative Material 2, the reheating was performed at a temperature excessively higher than that of the present invention. In this case, although the hardness difference and the strength thereof satisfy the object of the present invention, austenite grains are excessively coarsened, thereby providing an excessively high DBTT in the central region of the thickness, which does not satisfy the object of the present invention.

[173] For Comparative Material 3, finish rolling was performed at a temperature of austenite recrystallization temperature or more. In this case, a great amount of martensite is formed in the surface region, and causes the hardenability of the surface region to be excessively increased, thereby providing a large hardness difference. In addition, due to substantially non-grain refinement caused by the rolling, the DBTT is - 32 ⁰C, which is significantly lower than that of the inventive materials.

[174] For Comparative Material 5, finish rolling was performed at a temperature of Ar3 or less causing the ferrite transformation. As a result, a great amount of polygonal ferrite is formed in the surface region and the central region, thereby providing a hardness difference exceeding 50 Hv, a tensile strength of 486 MPa in the central region of the thickness, and a DBTT of -39 ⁰C, which are lower than those of the inventive materials.

[175] For Comparative Material 6, finish rolling was performed at a temperature of Ar₃ or more, and cooling was performed at a temperature of Ar₃ or less. As a result, a fraction of polygonal ferrite exceeds 10 % in the whole range of the material like Comparative Material 5, and results in low strength. [176] For Comparative Material 4, finish rolling was performed at a reduction rate of 20 % being lower than that of the inventive materials, causing insufficient grain refinement effect while deteriorating the low temperature toughness. As a result, it has a DBTT of -33 ⁰C, and a yield strength of 387 MPa being lower than that of the inventive materials.

[177] For Comparative Material 7, rolling was performed with the conditions according to the present invention excluding that cooling was performed at a rate close to air cooling. The fraction of polygonal ferrite exceeds 10 % over the whole thickness range of the material as in Comparative Material 5 due to the slow cooling rate, thereby exhibiting insufficient strength and DBTT properties compared with the inventive material.

[178] For Comparative Material 8, although final cooling was performed at a temperature of Ar3 or more, cooling was performed at a temperature of 279 ⁰C being lower than the final cooling temperature of the present invention. As a result, an excessive amount of low temperature structure, for example, a fraction of martensite exceeding 10 %, is formed in the central region of the thickness, and causes continuous yielding to occur upon tensile testing, thereby degrading the yield strength.

[179] For Comparative Material 9, cooling was finished at a temperature of 626 ⁰C, which is higher than that of the present invention. In this case, transformation of polygonal ferrite excessively occurred in the material, thereby providing a hardness difference exceeding 50 Hv while deteriorating the yield strength in the central region of the thickness. From these result, it can appreciated that the mechanical properties of Comparative Material 9 do not satisfy the object of the present invention.

[180] With the examples described above, it could be confirmed that the object of the present invention was achieved by controlling the composition and the microstructure according to the present invention, and, essentially, through application of controlled rolling and cooling conditions of a typical thick steel plate. Meanwhile, it could also be confirmed that, when the conditions of the present invention, that is, manufacturing conditions of a steel plate determined by the compositions constituting the steel plate, were satisfied, the thick steel plate for welded structure had enhanced microstructures, and hardness difference through the thickness, as well as strength and toughness of the central region.

[181] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] A thick steel plate having excellent strength and toughness in a central region of its thickness and exhibiting small variation in properties through the thickness, comprising: 0.05 ~ 0.10 % of C; 0.10 ~ 0.5 % of Si; 1.3 ~ 1.7 % of Mn; 0.0005 ~ 0.0025 % of B; 0.005 ~ 0.03 % of Ti; 0.010 % or less of N; 0.005 ~ 0.03 % of Nb; 0.005 ~ 0.055 % of Sol. Al; and the balance of Fe and other unavoidable impurities, in terms of weight%, wherein a content ratio of Ti/N is 2.0 or more, and a composition parameter (CP) represented by Expression 1 is in the range of 40 ~ 50.

CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x %Ni + 12.5 x %Cr + 14.4 x %Mo ... (1)

[2] The thick steel plate according to claim 1, further comprising: at least one component selected from the group consisting of 0.5 % or less of Cu; 0.5 % or less of Ni; 0.15 % or less of Cr; and 0.15 % or less of Mo in terms of weight%.

[3] The thick steel plate according to claim 1, wherein among the unavoidable impurities, contents of P and S are controlled to 0.012 % or less, and 0.005 % or less, respectively, in terms of weight%.

[4] The thick steel plate according to claim 3, wherein the contents of P and S are controlled to 0.010 % or less, and 0.003 % or less, respectively, in terms of weight%.

[5] The thick steel plate according to any one of claims 1 to 4, wherein a fraction of polygonal ferrite is 10 % or less in the central region (ranging from t/4 to 3t/4, when t indicates a total thickness of the steel plate) of the thickness, and a fraction of martensite is 10 % or less in a surface region of the steel plate (ranging from a depth of 1 mm below the surface to t/4, the opposite side being the same).

[6] The thick steel plate according to any one of claims 1 to 4, wherein the steel plate has a hardness variation of Hv 50 or less through the thickness.

[7] The thick steel plate according to claim 6, wherein the steel plate has a thickness of 50 ~ 100 mm.

[8] A method for producing a steel plate having excellent strength and toughness in a central region of its thickness and exhibiting small variation in properties through the thickness, the method comprising the steps of: finish rolling a steel slab at a reduction rate of 30 % or more at a temperature of Ar₃ ~ an austenite recrystallization temperature after reheating the steel slab to a temperature of 1,000 ~ 1,250 ⁰C, the steel slab comprising 0.05 ~ 0.10 % of C,

0.10 ~ 0.5 % of Si, 1.3 ~ 1.7 % of Mn, 0.0005 ~ 0.0025 % of B, 0.005 ~ 0.03 % of Ti, 0.010 % or less of N, 0.005 ~ 0.03 % of Nb, 0.005 ~ 0.055 % of Sol. Al, and the balance of Fe and other unavoidable impurities in terms of weight% wherein a content ratio of Ti/N is 2.0 or more, and a composition parameter (CP) represented by Expression 2 is in the range of 40 ~ 50; and cooling the hot rolled steel plate in such a way of beginning cooling at a rate of

1.5 °C/sec or more in view of a central region of the steel plate at a temperature of Ar₃ or more and finishing cooling at a temperature of 350 ~ 550 ⁰C.

CP = 165 x %C + 6.8 x %Si + 10.2 x %Mn + 80.6 x %Nb + 9.5 x %Cu + 3.5 x

%Ni + 12.5 x %Cr + 14.4 x %Mo ... (2)

[9] The method according to claim 8, wherein the steel slab further comprises at least one component selected from the group consisting of 0.5 % or less of Cu; 0.5 % or less of Ni; 0.15 % or less of Cr; and 0.15 % or less of Mo in terms of weight%.

[10] The method according to claim 8, wherein among the unavoidable impurities, contents of P and S are controlled to 0.012 % or less, and 0.005 % or less, respectively, in terms of weight%.

[11] The method according to claim 10, wherein the contents of P and S are controlled to 0.010 % or less, and 0.003 % or less, respectively, in terms of weight%.

[12] The method according to any one of claims 8 to 11, wherein the steel plate has a thickness of 50 ~ 100 mm.