Steel plate and its manufacturing method

Abstract

To provide a steel plate that can be used in an unpainted state even under outdoor atmospheric corrosion environments such as bridges, especially in harsh corrosion environments such as at sea or near the coast where the air carries a lot of salt, wherein the steel plate offers superior weather resistance, along with enhanced toughness, total elongation, and fatigue crack propagation resistance.SOLUTION: A steel plate has a predetermined composition. The hard structure of the steel plate comprises at least one selected from perlite, bainite and martensite. The hard structure satisfies the following formula (1) and formula (2). L(L) / L(Z)≤5.0 (1) and L(L) / L(C)≤5.0 (2), where L(L): average length of hard structure in rolling direction (L direction), L(Z): average length of hard structure in thickness direction (Z direction), and L(C): average length of hard structure in width direction (C direction).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet having excellent weather resistance, total elongation and fatigue crack propagation resistance, and a method for producing the same. The steel plate of the present invention can be suitably used for welded structures that are used in outdoor atmospheric corrosive environments and for which structural safety is highly required, such as ships, marine structures, bridges, buildings, tanks, etc. In particular, the steel plate of the present invention can be suitably used for structures such as bridges that are used in severe corrosive environments such as at sea or near coasts where there is a high amount of airborne salt. [Background technology]

[0002] Steel structures used outdoors, such as bridges, are usually subjected to some kind of corrosion prevention treatment before use. For example, weathering steel is often used in environments with low levels of airborne salt. When used in atmospheric exposure environments, weathering steel is covered on the surface with a highly protective rust layer containing concentrated alloying elements such as Cu, P, Cr, and Ni, which significantly reduces the corrosion rate. Bridges made of such weathering steel are known to be able to withstand decades of service without painting in environments with low levels of airborne salt.

[0003] On the other hand, it is known that in high-salinity environments, it is difficult for weathering steel to form a protective rust layer, making it difficult to achieve practical weathering resistance. For this reason, in environments with high levels of airborne salt, such as at sea or near the coast, ordinary steel that has been treated with anti-corrosion coatings, such as paint, is generally used.

[0004] However, painted steel requires periodic repainting and other repairs due to deterioration of the paint film over time, the formation of rust, and swelling of the paint film. Repainting work often requires work at high altitudes, making the work itself difficult and increasing labor costs. Therefore, when painted steel is used, repainting work increases the maintenance costs of the structure, which in turn increases the life cycle cost. Therefore, there is a demand for steel that can be used unpainted, even in environments with high levels of airborne salt, such as near the coast.

[0005] In response to these demands, steel materials containing various alloying elements, particularly Ni and Cu, have been developed that can be used without painting in environments with high levels of airborne salt, such as near the coast.

[0006] As an example of a steel material with excellent corrosion resistance, Patent Document 1 discloses a highly coastal weathering steel material with excellent earthquake resistance that contains 0.0003 to 0.0050% B and further contains one or more of 0.1 to 1.5% Cu, 0.1 to 6.0% Ni, and 0.005 to 0.500% Mo.

[0007] Patent Document 2 discloses a highly coastal weathering steel material with excellent earthquake resistance, which contains 0.0003 to 0.0050% B and one or more of 0.1 to 2.0% Cu, 0.1 to 6.0% Ni, and 0.005 to 1.000% Mo.

[0008] These steels with improved weather resistance are widely used in ships, marine structures, bridges, buildings, tanks, and other structures. Furthermore, the steel material is required to have excellent fatigue properties in addition to excellent mechanical properties such as strength and toughness, and excellent weldability. That is, when the above-mentioned structures are used, the structures are subjected to repeated loads such as those caused by wind, waves, and vibrations due to earthquakes. Therefore, the steel plate is required to have fatigue properties that can ensure the safety of the structure even when such repeated loads are applied. In particular, in order to prevent ultimate destruction such as breakage of components, it is required to improve the fatigue crack propagation resistance of the steel plate.

[0009] That is, various studies have been conducted to improve the fatigue crack propagation resistance of steel plates. For example, Patent Document 3 proposes a steel plate for tankers that has excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment. The steel plate has a mixed structure consisting of ferrite and one or more of bainite and pearlite. The average grain size of the ferrite in the steel plate is 20 μm or less.

[0010] Patent Document 4 proposes a steel plate with excellent fatigue crack propagation resistance. The steel plate has a microstructure consisting of hard and soft parts, and is characterized in that the difference in hardness between the hard and soft parts is 150 or more in Vickers hardness.

[0011] Patent Document 5 proposes a dual-phase steel having a microstructure consisting of bainite and ferrite with an area fraction of 38 to 52%. The technology proposed in Patent Document 5 improves fatigue crack propagation resistance by controlling the Vickers hardness of the ferrite phase and the number of boundaries between the ferrite and bainite phases per unit length. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-355731 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-355732 [Patent Document 3] Japanese Patent Application Publication No. 06-322477 [Patent Document 4] Japanese Patent Application Publication No. 07-242992 [Patent Document 5] Japanese Patent Application Publication No. 08-225882 Summary of the Invention [Problem to be solved by the invention]

[0013] However, while Patent Documents 1 and 2 are both prior art that refer to weather resistance, they do not at all consider the need to simultaneously achieve weather resistance and toughness and fatigue crack propagation resistance, which are important properties for steel plates used in structures. In addition, the microstructure of the steel plate is important for improving toughness and fatigue crack propagation resistance, and neither of these documents considered a manufacturing method for controlling the microstructure.

[0014] Furthermore, the total elongation value is generally specified in the standards for steel materials used in structures such as ships, marine structures, bridges, buildings, and tanks. Therefore, even steel plates with excellent fatigue crack propagation resistance are required to satisfy the standard value for total elongation.

[0015] Here, since fatigue crack propagation resistance and total elongation are contradictory properties, conventional technologies such as those described in Patent Documents 1 to 5 were unable to achieve both excellent fatigue crack propagation resistance and total elongation.

[0016] Furthermore, from the viewpoint of ensuring the safety of structures, steel plates are required to have excellent fatigue crack propagation resistance not only in one direction but also in all directions: the thickness direction, the rolling direction, and the width direction.

[0017] In general structures, steel plates are welded freely from various directions, resulting in a variety of directions in which fatigue cracks can initiate and propagate. Furthermore, in welded areas with narrow corners, fatigue cracks are inevitable due to their structural characteristics, and once they do occur, they tend to propagate primarily in the plate thickness direction. Therefore, to prevent structural collapse due to fatigue cracks, it is important to suppress the propagation of fatigue cracks in the plate width and rolling directions, even after they penetrate the steel plate thickness direction.

[0018] However, the directional dependency of fatigue crack propagation resistance was not taken into consideration in the conventional techniques described in Patent Documents 1 to 5. In addition, the steel materials proposed in Patent Documents 3 to 5 do not have sufficient weather resistance in environments with high levels of airborne salt, such as near the coast.

[0019] The present invention has been made in view of the above circumstances, and has an object to provide a steel plate that can be used without painting even when used in outdoor atmospheric corrosive environments such as bridges, particularly in severe corrosive environments such as at sea or near the coast where there is a large amount of airborne salt, and that has excellent weather resistance, toughness, total elongation, and fatigue crack propagation resistance. [Means for solving the problem]

[0020] That is, the gist of the present invention is as follows. 1. The composition contains, in mass%, C: 0.01% or more and 0.20% or less, Si: 0.05% or more and 1.00% or less, Mn: 0.10% or more and 2.00% or less, P: 0.003% or more and 0.035% or less, S: 0.0001% or more and 0.0350% or less, Al: 0.001% or more and 0.100% or less, and Ni: 0.80% or more and 6.00% or less, and further contains Cu: 1.00% or less and Mo: 1. A steel plate containing one or two elements selected from the group consisting of ferrite, bainite, and martensite, with the remainder being Fe and unavoidable impurities, and having a microstructure consisting of ferrite with an area fraction of 55% or more and hard structure harder than ferrite with an area fraction of 45% or less, wherein the hard structure contains one or more elements selected from pearlite, bainite, and martensite, and wherein the hard structure satisfies the following formulas (1) and (2). L(L) / L(Z)≦5.0 (1) L(L) / L(C)≦5.0 (2) L (L): Average length of hard structure in the rolling direction (L direction) L(Z): Average length of hard structure in the thickness direction (Z direction) L(C): Average length of hard structure in the plate width direction (C direction)

[0021] 2. The steel sheet according to 1 above, wherein the chemical composition further contains, in mass%, one or more selected from Cr: 1.000% or less, W: 1.00% or less, Co: 1.000% or less, Sn: 0.300% or less, Sb: 0.300% or less, Nb: 0.100% or less, V: 0.150% or less, Ti: 0.100% or less, B: 0.0050% or less, Zr: 0.1000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less.

[0022] 3. A method for producing the steel sheet described in 1 or 2 above, which comprises heating a slab having the chemical composition described in 1 or 2 above to a temperature range of 1000°C or higher and 1300°C or lower, then hot rolling the slab at a cumulative reduction rate of 50% or higher in a temperature range below the slab heating temperature and equal to or higher than the Ar3 transformation point to form a hot-rolled sheet, and cooling the hot-rolled sheet at an average cooling rate of 0.10°C / s or higher.

[0023] 4. The method for producing a steel sheet according to 3 above, wherein after the cooling, the steel sheet is further heated to a reheating temperature of not less than the Ac1 transformation point but less than the Ac3 transformation point, and after such heating, is cooled to a cooling stop temperature between 350 and 600°C at an average cooling rate in the range of 2.00 to 7.00°C / s, and further quenched. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a steel plate that can be used without painting even when used in welded structures used at low temperatures in cold regions, particularly in outdoor atmospheric corrosive environments such as bridges, and even when used in severe corrosive environments such as at sea or near the coast where there is a high amount of airborne salt or where antifreezing agents are sprayed. Furthermore, according to the present invention, the maintenance costs of the steel structure of the structure, and therefore the life cycle costs, can be reduced. Furthermore, according to the present invention, the steel sheet has both excellent fatigue crack propagation resistance and total elongation, and has excellent fatigue crack propagation resistance in all directions, including the plate thickness direction, rolling direction, and width direction, making it possible to ensure the safety of steel structures. DETAILED DESCRIPTION OF THE INVENTION

[0025] The chemical composition and microstructure of the steel sheet, the properties of the steel sheet, and the manufacturing method thereof will be described below in this order. Note that the present invention is not limited to the following embodiments. [Component composition] First, the chemical composition of the steel sheet of the present invention will be described. In the description of the chemical composition, % indicating the content of each component means % by mass. C: 0.01% or more, 0.20% or less C is an element that increases the strength of steel, and must be contained in an amount of 0.01% or more to ensure the required strength for structural steel. On the other hand, if the C content exceeds 0.20%, weldability, total elongation, and toughness deteriorate. Therefore, the C content is set to 0.20% or less. The C content is preferably set to 0.10% or less, and more preferably set to 0.08% or less.

[0026] Si: 0.05% or more, 1.00% or less The Si content must be 0.05% or more to ensure deoxidation and the strength of the steel sheet. Preferably, it is 0.10% or more. On the other hand, if the Si content exceeds 1.00%, toughness and weldability deteriorate significantly. Therefore, the Si content is set to 1.00% or less. Preferably, the Si content is 0.80% or less.

[0027] Mn: 0.10% or more, 2.00% or less Mn is an element that increases the strength of steel by improving its hardenability. That is, to ensure the required strength for structural steel, Mn must be contained in an amount of 0.10% or more. Preferably, it is 0.20% or more. On the other hand, if the Mn content exceeds 2.00%, the total elongation, toughness, and weldability deteriorate. Therefore, the Mn content is set to 2.00% or less. Preferably, the Mn content is 1.80% or less.

[0028] P: 0.001% or more, 0.035% or less P is an element that contributes to improving the weather resistance of steel. To achieve this effect, the P content must be 0.001% or more. On the other hand, if the P content exceeds 0.035%, the weldability and toughness deteriorate. Therefore, the P content is set to 0.035% or less.

[0029] S: 0.0001% or more, 0.0350% or less S is an element that deteriorates weldability and toughness. For this reason, the S content must be 0.0350% or less. On the other hand, if the S content is reduced to less than 0.0001%, production costs increase. Therefore, the S content must be 0.0001% or more.

[0030] Al: 0.001% or more, 0.100% or less Al is an element necessary for deoxidation during steelmaking. To achieve this effect, the Al content must be 0.001% or more. The Al content is preferably 0.005% or more, more preferably 0.010% or more. On the other hand, an Al content exceeding 0.100% adversely affects total elongation and weldability. Therefore, the Al content is set to 0.100% or less. The Al content is preferably less than 0.080%, more preferably less than 0.060%.

[0031] Ni: 0.80% or more, 6.00% or less Ni refines the rust grains in the rust layer, forming a dense rust layer and suppressing the penetration of oxygen and chloride ions, which are corrosion-accelerating factors, into the base steel. This effect is achieved when the Ni content is 0.80% or more. Therefore, the Ni content is set to 0.80% or more. The Ni content is preferably 0.90% or more, and more preferably 1.00% or more. On the other hand, if the Ni content exceeds 6.00%, weldability is impaired and the alloy cost increases excessively. Therefore, the Ni content is set to 6.00% or less. The Ni content is preferably 5.00% or less, and more preferably 4.00% or less.

[0032] In addition to the above elements, the steel sheet of the present invention must further contain elements selected from Cu: 1.00% or less and Mo: 1.00% or less, either singly or in combination.

[0033] Cu: 1.00% or less Cu refines the rust grains in the rust layer, forming a dense rust layer and suppressing the penetration of oxygen and chloride ions, which are corrosion-accelerating factors, into the base steel. To achieve this effect, when Cu is contained alone, it is desirable to contain 0.05% or more. The Cu content is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, weldability is impaired and defects are more likely to occur during the production of the steel sheet. Therefore, when Cu is contained, its content is set to 1.00% or less. The Cu content is preferably 0.80% or less, more preferably 0.60% or less.

[0034] Mo: 1.00% or less Mo dissolves during the anodic reaction of steel and forms MoO4 in the rust layer. 2- The distribution of Mo prevents chloride ions, which are corrosion-accelerating factors, from penetrating the rust layer and reaching the base steel. Furthermore, the precipitation of Mo-containing compounds on the steel surface inhibits the anodic reaction of the steel. To achieve this effect, if Mo is contained alone, it is desirable to contain 0.05% or more. On the other hand, if the Mo content exceeds 1.00%, weldability is impaired and alloy costs increase. Therefore, if Mo is contained, its content should be 1.00% or less. The Mo content is preferably 0.80% or less, more preferably 0.50% or less.

[0035] When both Cu and Mo are contained, the total content of Cu and Mo is preferably 0.05% or more, while the upper limit of the total content of both Cu and Mo is allowed to be up to 1.00% for each.

[0036] A steel sheet according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Furthermore, the chemical composition of the steel sheet according to another embodiment of the present invention may further contain at least one of the following elements. By containing these optional elements, the properties of the steel sheet, such as strength, toughness, weldability, and weather resistance, can be further improved.

[0037] Cr:1.000% or less Cr is an element that has the effect of further improving the strength of the steel sheet. Cr also has the effect of forming a dense rust layer to further improve weather resistance. In order to obtain these effects, when Cr is contained, the Cr content is preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.000%, the weldability and toughness are impaired and the weather resistance is also adversely affected. Therefore, when Cr is contained, the Cr content is set to 1.000% or less. The Cr content is preferably 0.700% or less, more preferably 0.500% or less.

[0038] W: 1.00% or less W is an element that improves the weather resistance of steel. W dissolves during the anodic reaction and forms WO4 in the rust layer. 2- By distributing W as a tungsten complex, chloride ions, a corrosion-accelerating factor, are electrostatically prevented from penetrating the rust layer and reaching the steel substrate. Furthermore, the precipitation of compounds containing W on the steel surface inhibits the anodic reaction of the steel. In addition, the formation of fine rust densifies the rust layer, preventing chloride ions, a corrosion factor, from penetrating the rust layer and reaching the steel substrate. To fully achieve these effects, it is preferable to include 0.01% or more of W, and more preferably 0.03% or more. On the other hand, a W content exceeding 1.00% significantly increases the alloy cost. Therefore, if W is included, the W content should be 1.00% or less. The W content is preferably 0.70% or less, and more preferably 0.50% or less.

[0039] Co: 1.000% or less Co is distributed throughout the rust layer, forming a dense rust layer, thereby improving weather resistance. To achieve this effect, the Co content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.010% or more. On the other hand, even if the Co content is higher than 1.000%, the effect saturates and the alloy cost increases. Therefore, when Co is contained, the Co content is set to 1.000% or less. The Co content is preferably 0.500% or less.

[0040] Sn: 0.300% or less Sn is an element that improves the weather resistance of steel. Furthermore, Sn is present in the rust layer near the surface of the base steel, where it refines the rust particles, preventing chloride ions, a corrosion-accelerating factor, from penetrating the rust layer and reaching the base steel. Furthermore, Sn suppresses anodic reactions on the steel surface. To fully achieve this effect, the Sn content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, an Sn content exceeding 0.300% leads to deterioration of the ductility and toughness of the steel. Therefore, when Sn is contained, the Sn content is set to 0.300% or less. The Sn content is preferably 0.100% or less, and more preferably 0.050% or less.

[0041] Sb: 0.300% or less Sb is present in the rust layer near the surface of the base steel, and by refining the rust particles, it prevents chloride ions, a corrosion-accelerating factor, from penetrating the rust layer and reaching the base steel. Sb also suppresses anodic reactions on the steel surface. To fully achieve this effect, the Sb content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, an Sb content exceeding 0.300% leads to deterioration of the ductility and toughness of the steel. Therefore, if Sb is contained, the Sb content is set to 0.300% or less. The Sb content is preferably 0.150% or less, and more preferably 0.100% or less.

[0042] Nb: 0.100% or less Nb is an element that has the effect of suppressing the recrystallization of austenite during hot rolling and refining the final crystal grains. Nb also precipitates during air cooling, further improving strength. To achieve this effect, when Nb is contained, the Nb content is preferably 0.005% or more. On the other hand, when the Nb content exceeds 0.100%, the hardenability becomes excessive, martensite is formed, and the desired structure cannot be obtained, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.050% or less.

[0043] V:0.150% or less V precipitates during air cooling, further improving strength. Also, VO4 3- The presence of V as a V component prevents chloride ions, which are corrosion-accelerating factors, from penetrating the rust layer and reaching the base steel. To fully obtain this effect, it is preferable to include 0.005% or more of V. On the other hand, if the V content exceeds 0.150%, the effect saturates. Therefore, if V is contained, the V content should be 0.150% or less.

[0044] Ti:0.100% or less Ti is an element that increases strength. To fully obtain this effect, it is preferable to contain 0.005% or more of Ti. On the other hand, if the Ti content exceeds 0.100%, it will cause a deterioration in toughness. Therefore, if Ti is contained, the Ti content should be 0.100% or less.

[0045] B: 0.0050% or less B is an element that has the effect of improving hardenability, and as a result, further improving strength. To obtain this effect, when B is contained, the B content is preferably 0.0001% or more. On the other hand, when the B content exceeds 0.0050%, the hardenability becomes excessive, martensite is formed, and the desired structure cannot be obtained, and weldability is also reduced. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0030% or less.

[0046] Zr: 0.1000% or less Zr is an element that increases strength. To fully obtain this effect, it is preferable to contain 0.0050% or more of Zr. On the other hand, if the Zr content exceeds 0.1000%, the strength-improving effect saturates. Therefore, if Zr is contained, the Zr content is set to 0.1000% or less.

[0047] Ca:0.0100% or less Ca is an element that fixes S in steel and improves the toughness of the weld heat-affected zone. To fully obtain this effect, it is preferable to contain 0.0001% or more of Ca. On the other hand, if the Ca content exceeds 0.0100%, the amount of inclusions in the steel increases, which actually leads to a deterioration in toughness. Therefore, if Ca is contained, the Ca content is set to 0.0100% or less.

[0048] Mg: 0.0100% or less Mg is an element that fixes S in steel and improves the toughness of the weld heat-affected zone. To fully obtain this effect, it is preferable to contain 0.0001% or more of Mg. On the other hand, if the Mg content exceeds 0.0100%, the amount of inclusions in the steel increases, which actually leads to a deterioration in toughness. Therefore, if Mg is contained, the Mg content is set to 0.0100% or less.

[0049] REM: 0.0200% or less REM (rare earth metals) are elements that fix S in steel and improve the toughness of the weld heat-affected zone. To fully obtain this effect, it is preferable to contain 0.0001% or more of REM. On the other hand, if the REM content exceeds 0.0200%, the amount of inclusions in the steel increases, which actually leads to a deterioration in toughness. Therefore, if REM is contained, the REM content should be 0.0200% or less.

[0050] [Microstructure] Next, the microstructure of the steel sheet of the present invention will be described. The steel sheet according to one embodiment of the present invention is composed of ferrite in an area fraction of 55% or more and hard structure in an area fraction of 45% or less. In the present invention, the term "hard structure" refers to a structure that contains one or more selected from pearlite, bainite, and martensite and is harder than ferrite. Furthermore, the hard structure must satisfy the following formulas (1) and (2). These formulas reflect the blockiness of the hard structure. L(L) / L(Z)≦5.0 (1) L(L) / L(C)≦5.0 (2) L (L): Average length of hard structure in the rolling direction (L direction) L(Z): Average length of hard structure in the thickness direction (Z direction) L(C): Average length of hard structure in the plate width direction (C direction)

[0051] The microstructure in the present invention refers to the microstructure at the 1 / 4 position of the plate thickness t of the steel plate (1 / 4t position), because the effects of the present invention can be obtained by specifying the microstructure at such a position. The area fraction of each structure and the length in each direction of the hard structure can be measured by taking a test piece at a depth of 1 / 4 t from the surface of the steel plate, etching each cross section with nital, and observing it. More specifically, the area fraction and the length in each direction of the hard structure can be determined by the method described in the Examples.

[0052] Ferrite area fraction: 55% or more, hard structure area fraction: 45% or less The microstructure of the steel sheet in the present invention is a composite structure in which hard structures are dispersed in a ferrite phase. The presence of hard structures at the tip of a fatigue crack causes bending or branching of the fatigue crack, which leads to fracture surface roughness-induced crack closure and stress shielding effects, reducing the driving force for fatigue crack propagation and improving fatigue crack propagation resistance. Furthermore, since ferrite is effective in improving total elongation, if the area fraction of ferrite is less than 55% (i.e., the area fraction of hard structure exceeds 45%), the desired total elongation cannot be obtained. Therefore, the area fraction of ferrite is set to 55% or more (i.e., the area fraction of hard structure is 45% or less). It is preferable that the area fraction of ferrite is 60% or more (i.e., the area fraction of hard structure is 40% or less). On the other hand, although there is no particular upper limit for the area fraction of ferrite, it is preferable that it is 97% or less (i.e., the area fraction of hard structure is 3% or more).

[0053] The organization in the present invention is as follows. Ferrite includes polygonal ferrite, pearlite includes pearlite and pseudo-pearlite, bainite includes upper bainite, acicular ferrite, and granular bainite, and martensite includes island martensite, lath martensite, and lenticular martensite.

[0054] L(L) / L(Z)≦5.0 (1) L(L) / L(C)≦5.0 (2) In the present invention, the relationships between the average length (L(L)) of the hard structure in the rolling direction (L direction), the average length (L(Z)) of the hard structure in the plate thickness direction (Z direction), and the average length (L(C)) of the hard structure in the plate width direction (C direction) are defined by the above formulas (1) and (2). The above formulas (1) and (2) reflect the blockiness of the hard tissue. If formulas (1) and (2) are not satisfied, the hard tissue will become band-shaped, resulting in a direction in which the frequency of contact between the tip of the fatigue crack and the hard tissue will decrease. Therefore, it is not possible to obtain the desired fatigue crack propagation resistance in all directions. In other words, in the present invention, by satisfying both the above formulas (1) and (2), the desired fatigue crack propagation resistance can be obtained in all directions of the steel plate.

[0055] In the present invention, the total elongation, tensile strength, toughness, and fatigue crack propagation resistance properties are as described below. [Total elongation] The steel sheet of the present invention has the above-described chemical composition and microstructure, and as a result, has excellent total elongation (EL). The EL value is not particularly limited and can be determined in accordance with standards such as JIS G 3106, but is preferably 15% or more, more preferably 16% or more, even more preferably 17% or more, and most preferably 20% or more. On the other hand, the upper limit of EL is not particularly limited, but is preferably, for example, 40% or less. EL can be measured by the method described in the Examples below.

[0056] [Tensile strength] The steel sheet of the present invention has the above-mentioned chemical composition and microstructure, and as a result, can have excellent tensile strength (TS). The TS value is not particularly limited, but it is preferably 400 MPa or more. On the other hand, the upper limit of TS is also not limited, but for example, the steel sheet may be subjected to a tensile strength of 400 MPa (50 kgf / mm) according to JIS. 2 In order to achieve the JIS 490 MPa (60 kgf / mm) grade, the TS must be 510 MPa or less. 2 ) grade, the upper and lower limits of TS should be 490 MPa and 610 MPa, respectively.

[0057] [Toughness] The steel plate of the present invention has excellent toughness as a result of having the aforementioned chemical composition and microstructure. The toughness of the steel plate of the present invention is not particularly limited, but the Charpy absorbed energy vE0 at 0°C, which is one of the toughness indicators, is preferably 100 J or more, more preferably 150 J or more, and even more preferably 200 J or more. Meanwhile, the upper limit of vE0 is not particularly limited, as the higher the better. Note that vE0 can be measured by the method described in the Examples below.

[0058] [Fatigue crack propagation resistance] The steel sheet of the present invention has the above-mentioned component composition and microstructure, and as a result, can have excellent fatigue crack propagation resistance in all directions, including the thickness direction, rolling direction, and width direction. In the present invention, the fatigue crack propagation rate (da / dN) can be used as an index of fatigue crack propagation resistance, but the value of the fatigue crack propagation rate is not particularly limited. The fatigue crack propagation rate in the thickness direction (Z direction) is within the stress intensity factor range ΔK: 25 MPa m 1 / 2 The fatigue crack propagation rate under these conditions was 4.25 × 10, which confirms that the fatigue durability is sufficiently better than that of conventional steel. -8 (m / cycle) or less. In addition, the stress intensity factor range ΔK is 25 MPa m for both the fatigue crack propagation rate in the rolling direction (L direction) and the fatigue crack propagation rate in the cross direction (C direction). 1 / 2 The fatigue crack propagation rate under these conditions was 8.50 × 10, which confirms that the fatigue durability of the steel is sufficiently good compared to conventional steel. -8 (m / cycle) or less is preferable.

[0059] Plate Thickness In the present invention, the thickness of the steel plate is not particularly limited and can be any value. In the present invention, the thickness is preferably 6 mm or more, which is the thickness normally used for structural members. As mentioned above, the temperature deviation between the leading and trailing ends of the steel sheet is likely to be large, and the effect of the present invention is particularly remarkable in steel sheets that are required to have excellent elongation characteristics across the entire thickness. Therefore, the thickness of the steel sheet is preferably 50 mm or less, and more preferably 25 mm or less.

[0060] [Manufacturing method] Next, a method for manufacturing a steel sheet according to the present invention will be described. The steel sheet according to one embodiment of the present invention can be obtained by subjecting a steel material having the above-described chemical composition to the following steps (1), (2), and (3). Furthermore, by optionally subjecting a steel material to steps (4), (5), and (6), a steel sheet having even better fatigue crack propagation resistance and total elongation can be manufactured. (1)Heating (2) Hot rolling (3) Cooling (4) Reheating (5) Cooling (6) Quenching

[0061] The conditions for each step are explained below. In the present invention, unless otherwise specified, the temperature refers to the surface temperature of the workpiece (steel material or hot-rolled steel sheet). The cooling rate refers to the cooling rate of the surface temperature. The surface temperature can be measured, for example, with a radiation thermometer.

[0062] Any steel material can be used as long as it has the above-mentioned composition. The composition of the final steel sheet is the same as that of the steel material used. For example, a steel slab can be used as the steel material. (1)Heating process Heating temperature: 1000℃ or higher, 1300℃ or lower First, the steel material is heated to a temperature range of 1000°C or higher and 1300°C or lower. If the heating temperature is lower than 1000°C, the deformation resistance of the steel material in the subsequent hot rolling process will be high, increasing the load on the hot rolling mill and making hot rolling difficult. On the other hand, if the heating temperature exceeds 1300°C, the grain size of the steel sheet structure will become too large, resulting in a deterioration in toughness. If the steel is held in the heating process, the holding time is preferably 1 hour or longer.

[0063] (2) Hot rolling The heated steel material is then hot-rolled to produce a hot-rolled sheet. In this process, to ensure toughness, which is a basic performance of the product steel sheet, the cumulative reduction is set to 50% or more in the temperature range below the slab heating temperature and above the Ar3 transformation point. If the cumulative reduction is less than 50%, the ferrite grains within the sheet thickness become coarse, resulting in localized regions of low ductility, which make brittle cracks more likely to occur and deteriorate toughness. Other conditions related to the hot-rolling process are not particularly limited and can be those known in the art. In addition, the rolling reduction in the recrystallization temperature range is preferably 30% or more, more preferably 45% or more, to effectively achieve grain refinement through recrystallization. Also, the rolling reduction in the non-recrystallization temperature range is preferably 60% or less, more preferably less than 30%, to effectively prevent grain elongation. The recrystallization temperature is determined by determining the softening curve of the steel material in a two-stage compression test, and the temperature at which the softening degree reaches 50% is defined as the recrystallization temperature. The Ar3 transformation point can be calculated, for example, by the following formula (3). Ar3(℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo+0.35…(3) Here, the element symbols in the above formula (3) represent the content (mass%) of each element in the steel, and are set to zero when the element is not contained.

[0064] (3) Cooling Next, cooling is performed, that is, the hot-rolled sheet is cooled after completion of hot rolling (first cooling step). This cooling can be performed by any method, for example, air cooling or accelerated cooling. The average cooling rate is 0.10°C / s or more. If the average cooling rate is less than 0.10°C / s, the shape of the hard structure becomes band-like, and the desired fatigue crack propagation resistance cannot be obtained. Other cooling conditions are not particularly limited, and known cooling conditions can be used. Furthermore, there is no problem with the dispersion of hard phases in the microstructure of the steel material produced by the above-mentioned production steps (1), (2), and (3).

[0065] Next, by carrying out the following steps (4), (5), and (6), it is possible to further improve the fatigue crack propagation resistance and total elongation. (4) Reheating The steel sheet cooled as described in (3) above is then heated to a temperature (reheating temperature) equal to or higher than the Ac1 transformation point but lower than the Ac3 transformation point. This temperature is referred to as "reheating treatment" below. By heating the steel sheet to a reheating temperature that results in a two-phase region of ferrite and austenite, the microstructural variations resulting from cooling deviations can be eliminated without damaging the pre-heating structure. As a result, the fatigue crack propagation resistance can be further improved in all directions: the rolling direction, the sheet width direction, and the sheet thickness direction. If the reheating temperature is equal to or higher than the Ac3 transformation point, the decarburization reaction specific to the temperature range from the Ac1 transformation point to below the Ac3 transformation point does not proceed, and fatigue crack propagation resistance cannot be further improved.On the other hand, if the reheating temperature is lower than the Ac1 transformation point, the variation in microstructure caused by cooling deviation cannot be eliminated, and fatigue crack propagation resistance cannot be further improved.

[0066] The Ac1 transformation point can be determined, for example, by the following formula (4). Ac1(℃)=723+29.1×Si-10.7×Mn-16.9×Ni+16.9×Cr…(4) The Ac3 transformation point can be calculated, for example, by the following formula (5). Ac3(℃)=961.6-311.9×C+49.5×Si-36.4×Mn+438.1×P-2818×S+12.7×Al-51×C u-29×Ni-8.7×Cr+13.5×Mo+308.1×Nb-140×V+318.9×Ti+611.2×B-969×N…(5) Here, the element symbols in the above formulas (4) and (5) represent the content (mass%) of each element in the steel, and are set to zero when the element is not contained.

[0067] In the reheating treatment, it is preferable to heat the steel sheet to a reheating temperature and then hold the temperature. If the holding time is less than 10 minutes, reverse transformation to the austenite phase may not start over the entire length of the steel sheet, resulting in a significant decrease in hardenability in some regions. Therefore, it is preferable to hold the steel sheet for 10 minutes or more.

[0068] (5) Cooling The steel sheet heated in the reheating treatment step is cooled to a cooling stop temperature arbitrarily set in the range of 350 to 600°C (second cooling step). The average cooling rate is set to 2 to 7°C / s. A lower average cooling rate is preferable in terms of improving toughness because it promotes pearlite transformation. However, if the average cooling rate is less than 2°C / s, pearlite tends to form in a band-like shape, making crack propagation along the band structure more likely. As a result, it is not possible to further improve fatigue crack propagation resistance. On the other hand, if the average cooling rate exceeds 7°C / s, pearlite transformation does not proceed sufficiently in the microstructure inside the steel sheet, and bainite transformation or martensitic transformation tends to proceed. In this case, the amount of hard structure increases, resulting in a deterioration in total elongation. For this reason, the average cooling rate is set to 2 to 7°C / s. The average cooling rate is more preferably 5°C / s or less.

[0069] Furthermore, if the cooling stop temperature is less than 350°C, pearlite tends to form in a band-like structure, making it easier for cracks to propagate along the band structure, preventing further improvement in fatigue crack propagation resistance. On the other hand, if the cooling stop temperature exceeds 600°C, the steel will be quenched with a large amount of untransformed austenite remaining, resulting in the excessive formation of hard bainite and martensite. As a result, it is impossible to further improve total elongation.

[0070] (6) Quenching In the present invention, the steel plate cooled to the cooling stop temperature can be quenched. The quenching temperature is preferably in the range of 350 to 600°C. The other conditions for quenching are not particularly limited and can be any known conditions, but it is preferable to water-cool the steel plate to a temperature below the Ms point, preferably below 200°C. The Ms point can be calculated, for example, by the following equation (6). Ms(℃)=517-300×C-11×Si-33×Mn-17×Ni-22×Cr-11×Mo…(6) Here, the element symbols in the above formula (6) represent the content (mass%) of each element in the steel, and are set to zero when the element is not contained.

[0071] In the production method according to the present invention, any item not described in this specification can be carried out in a conventional manner. [Example]

[0072] The effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0073] The steel plates were manufactured in the following manner. First, a steel slab (steel material) having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was produced by a converter-continuous casting method. Next, the steel slabs were heated to the heating temperatures shown in column (1) of Table 2, and then hot-rolled to obtain hot-rolled steel sheets at the cumulative reduction, the reduction in the recrystallized region, and the reduction in the non-recrystallized region shown in column (2) of Table 2. The thicknesses (final thicknesses) of the hot-rolled steel sheets thus obtained are also shown in Table 2. The hot-rolled steel sheets were then cooled under the conditions shown in column (3) of Table 2 to obtain steel sheets. The thickness of the steel sheets was the same as the final thickness described above. Some of the steel sheets were also subjected to the processes shown in columns (4), (5), and (6) of Table 2.

[0074] [Table 1]

[0075] [Table 2]

[0076] The microstructure, mechanical properties, weather resistance, and fatigue crack propagation resistance of each of the steel plates were evaluated using the following evaluation methods. The average length of the hard structure in the rolling direction (L direction) is (L(L)), the average length of the hard structure in the plate thickness direction (Z direction) is (L(Z)), and the average length of the hard structure in the plate width direction (C direction) is (L(C)).

[0077] (microstructure) First, samples for microstructure observation were taken from the 1 / 4t position in the thickness direction of the steel plate so that the rolling direction (L direction) cross section, plate width direction (C direction) cross section, and plate thickness direction (Z direction) cross section would be the observation surfaces. Note that the rolling direction (L direction) cross section refers to a cross section perpendicular to the plate width direction, the plate width direction (C direction) cross section refers to a cross section perpendicular to the plate thickness direction, and the plate thickness direction (Z direction) cross section refers to a cross section perpendicular to the rolling direction. Next, the surface of the sample was etched with nital, and the structure was photographed using an optical microscope at 400x magnification and a scanning electron microscope (SEM) at 2000x magnification. The photographed images were used to identify the existing structure. Furthermore, the optical microscope images were analyzed using image analysis software (Photoshop) to binarize the ferrite and hard structure regions, thereby measuring the area fraction of ferrite and the area fraction of hard structure, as well as the length of the hard structure in each direction. These values ​​were calculated by averaging five fields of view observed per sample.

[0078] (mechanical properties) A full-thickness tensile test specimen was taken from the steel plate in the plate width direction (C direction). Using the full-thickness tensile test specimen, a tensile test was performed in accordance with JIS Z 2241 to measure the tensile strength (TS) and total elongation (EL). In addition, a Charpy impact test specimen was taken from the center of the plate thickness of the steel plate parallel to the rolling direction (L direction), and a Charpy impact test was performed at 0°C in accordance with JIS Z 2202 to measure the absorbed energy vE0.

[0079] (weather resistance) Test pieces measuring 50 mm × 50 mm × 4 mm were taken from each of the steel plates, and the end faces and back faces of the test pieces were sealed with tape. The front faces were also sealed with tape so that the area of ​​the exposed surface was 40 mm × 40 mm. The weather resistance of the test pieces thus obtained was evaluated. To evaluate weather resistance, a corrosion test was conducted to simulate the environment inside a girder that is not exposed to rain, which is considered to be the most severe environment for an actual bridge or other structure. This corrosion test was conducted by repeatedly cycling temperature and humidity with salt attached to the surface of the sample. The temperature and humidity cycle consisted of an 11-hour drying process at a temperature of 40°C and a relative humidity of 40% RH, followed by a 1-hour transition period, and then an 11-hour moistening process at a temperature of 25°C and a relative humidity of 95% RH, followed by a 1-hour transition period, for a total of 24 hours per cycle, simulating a temperature and humidity cycle in a real environment. Before the temperature and humidity cycle started and after every 7 cycles, the salt content on the test specimen surface was 1.4 mg / dm 2Before the drying process, artificial seawater was dropped onto the surface of the test piece so that the Under these conditions, a test was conducted over 26 weeks, covering 182 temperature and humidity cycles.

[0080] After the corrosion test, the test pieces were immersed in a rust removal solution made up of 500 mL of 37% hydrochloric acid, 3.5 g of hexamethylenetetramine, 3 mL of Hibilon (an inhibitor manufactured by Aiko Chemical Co., Ltd.), and distilled water to make 1 L (liter) for rust removal. The mass was measured in accordance with the method described in the 145th Corrosion and Protection Symposium document "Improving the Accuracy of Corrosion Depletion Evaluation Methods." Furthermore, the difference between the obtained mass and the initial mass was calculated and divided by the area of ​​the test surface of the test piece to calculate the average thickness reduction of one surface of the test piece. In this example, such average thickness reduction was used as an index of weather resistance. An airborne salt level of approximately 0.5 mdd corresponds to an environment with a high level of airborne salt, such as near the coast, but previous knowledge has shown that the amount of steel plate thickness reduction in this corrosion test (over 182 days) is equivalent to the amount of steel plate thickness reduction due to corrosion when exposed to an actual environment with an airborne salt level of approximately 0.5 mdd for 182 days.

[0081] When the amount of corrosion after 100 years is calculated by extrapolating the average thickness reduction, if the average thickness reduction obtained during this corrosion test is 22 μm or less, the average thickness reduction after 100 years is evaluated to be 0.5 mm or less, which means no delamination of rust will occur.

[0082] Generally, it is known that the standard for determining whether unpainted weathering steel can be used for bridges is a thickness loss of 0.5 mm or less after 100 years, so this corrosion test is conducted on various steel materials, and if the average thickness loss obtained is 22 μm or less, the unpainted weathering steel can be used for bridges. Therefore, in Table 3, weathering resistance is judged to be excellent when the average thickness loss is 22 μm or less.

[0083] (fatigue crack propagation resistance) As an index of fatigue crack propagation resistance, the fatigue crack propagation rate (da / dN) in the thickness direction (Z direction), rolling direction (L direction), and width direction (direction perpendicular to the rolling direction, C direction) was calculated using the stress intensity factor range ΔK: 25 MPa m 1 / 2 The measurements were carried out under the following conditions. Rolling direction and width direction The fatigue crack propagation rate in the rolling direction (L direction) was measured using test specimens cut from the steel plate so that the load direction was the rolling direction. Similarly, the fatigue crack propagation rate in the cross direction (C direction) was measured using test specimens cut from the steel plate so that the load direction was the cross direction. These test specimens were compact tension test specimens in accordance with ASTM E647. In addition, in the above measurements, fatigue crack propagation tests were conducted based on the crack gauge method, and the fatigue crack propagation rate was determined. ·Thickness direction On the other hand, to measure the fatigue crack propagation rate in the thickness direction (Z direction), a single-edge notched simple tension fatigue test specimen was used. Such a test specimen was taken from the steel plate, and the fatigue crack propagation rate when the crack propagated in the thickness direction was measured.

[0084] The results of each evaluation are shown in Table 3.

[0085] [Table 3]

[0086] As can be seen from the results shown in Table 3, the steel sheets satisfying the conditions of the present invention had extremely excellent properties, satisfying all of the following five conditions. In particular, they had both excellent fatigue crack propagation resistance and total elongation, and also excellent fatigue crack propagation resistance in all directions: the thickness direction, the rolling direction, and the width direction. In contrast, the steel sheets of the comparative examples that did not satisfy the conditions of the present invention did not satisfy at least one of the following five conditions. EL: 15% or more vE0:100J or higher ·Fatigue crack propagation rate in L and C directions: ΔK: 25 MPa·m 1 / 2Under the condition of 8.50×10 -8 (m / cycle) or less ·Fatigue crack propagation rate in Z direction: ΔK: 25 MPa·m 1 / 2 Under the condition of 4.25×10 -8 (m / cycle) or less ·Average plate thickness reduction: 22μm or less

Claims

1. The component composition is, in mass%, C: 0.01% or more, 0.20% or less, Si: 0.05% or more, 1.00% or less, Mn: 0.10% or more, 2.00% or less, P: 0.003% or more, 0.035% or less, S: 0.0001% or more, 0.0350% or less, Al: 0.001% or more and 0.100% or less; Ni: 0.80% or more and 6.00% or less; moreover, Cu: 1.00% or less and Mo: 1.00% or less, and the balance being Fe and unavoidable impurities, The microstructure is composed of 55% or more of ferrite and 45% or less of a hard structure harder than ferrite, in terms of area fraction; The hard structure contains one or two selected from pearlite and bainite, and the area fraction of martensite is 0%, The hard structure of the steel plate satisfies the following formulas (1) and (2), and the Charpy absorbed energy vE 0 at 0° C. is 100 J or more. L(L) / L(Z)≦5.0 (1) L(L) / L(C)≦5.0 (2) L (L): Average length of hard structure in the rolling direction (L direction) L (Z): Average length of hard structure in the thickness direction (Z direction) L (C): Average length of hard structure in the plate width direction (C direction)

2. The component composition further comprises, in mass%, Cr: 1.000% or less, W: 1.00% or less, Co: 1.000% or less, Sn: 0.300% or less, Sb: 0.300% or less, Nb: 0.100% or less, V: 0.150% or less, Ti: 0.100% or less, B: 0.0050% or less, Zr: 0.1000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, 2. The steel sheet according to claim 1, further comprising one or more selected from the group consisting of REM: 0.0200% or less.

3. A method for producing the steel sheet according to claim 1 or 2, A slab having the component composition according to claim 1 or 2 is heated to a temperature range of 1000°C or more and 1300°C or less, and then Below slab heating temperature Ar ３ A method for manufacturing a steel sheet, comprising: hot rolling a hot-rolled sheet at a cumulative reduction rate of 50% or more in a temperature range above the transformation point; and cooling the hot-rolled sheet at an average cooling rate of 0.10°C / s or more.

4. After the cooling, further Ac １ Transformation point or above Ac ３ Heat to a reheating temperature below the transformation point, After this heating, the material is cooled to a cooling stop temperature between 350 and 600°C at an average cooling rate in the range of 2.00 to 7.00°C / s, The method for producing a steel sheet according to claim 3, further comprising quenching.

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