WO2015022729A1 - 鋼板 - Google Patents
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- WO2015022729A1 WO2015022729A1 PCT/JP2013/071864 JP2013071864W WO2015022729A1 WO 2015022729 A1 WO2015022729 A1 WO 2015022729A1 JP 2013071864 W JP2013071864 W JP 2013071864W WO 2015022729 A1 WO2015022729 A1 WO 2015022729A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
Definitions
- the present invention has a yield strength of 460 N / mm 2 to 580 N / mm 2 , a tensile strength that is suitable for application to welded structures such as buildings, construction machines, offshore structures, large cranes for ships, and civil engineering structures. is has a strength of 550N / mm 2 ⁇ 670N / mm 2, further comprising a uniform properties in the thickness direction, excellent weldability and base material toughness and HAZ toughness, than the plate thickness 80mm
- the present invention relates to a suitable thick high strength steel sheet.
- thick high-tensile steel sheets have been applied to welded structures such as buildings, construction machines, offshore structures, large-scale cranes for ships, and civil engineering structures as the size of the structures increases.
- thick high-tensile steel plates are applied to large structures, the difference in strength and toughness in the plate thickness direction is highly advanced by predicting their deformation behavior and fracture behavior when constructing and designing complex welded structures. It is not preferable when building reasonable safety. Therefore, a thick high-tensile steel sheet having uniform characteristics in the sheet thickness direction is required. Thick high-tensile steel plates are often used in parts requiring high safety in large offshore structures and large cranes.
- HAZ toughness high weld heat affected zone toughness
- thick high-tensile steel sheets with a plate thickness of 80 mm or more are usually alloys such as C, Mn, Cr, Mo, and V that improve the hardenability for the purpose of imparting a predetermined strength to the center of the plate thickness.
- Appropriate amounts of elements are added and manufactured by quenching and tempering. During quenching, it is well known that strength and toughness change depending on the depth in the thickness direction from the surface layer to the thickness center due to the difference in cooling rate in the thickness direction. Further, as the plate thickness increases, not only the difference in cooling rate during the quenching process, but also the difference in heating rate between the surface layer and the plate thickness center portion during heating during the quenching process increases.
- the time for holding at a high temperature is longer than that in the center portion of the plate thickness, and the crystal grains tend to be coarser than in the center portion of the plate thickness. If there is a difference in crystal grains between the vicinity of the surface layer and the central portion of the plate thickness, there may be a difference in materials such as strength.
- the position of 1/4 of the plate thickness along the plate thickness direction from the surface of the steel plate in other words, 1 / of the plate thickness from the surface of the steel plate to the center of the plate thickness in the plate thickness direction.
- the characteristics of the position advanced by 4 (hereinafter referred to as 1/4 t portion) are defined.
- the thickness of the plate is 1 ⁇ 2 of the plate thickness from the surface of the steel plate toward the center of the plate thickness. Even at the position (hereinafter referred to as 1 / 2t portion), stable and high characteristics are required.
- a plate unique to thick high-tensile steel plates It is important to eliminate the non-uniformity in the thickness direction. It is clear that the weldability is determined by the alloy composition from many studies, and can be evaluated by an index such as a Pcm value. In many cases, good weldability that does not require preheating can be achieved by limiting the content of alloy elements having high hardenability such as Cr and Mo and setting the Pcm value to, for example, 0.25% or less.
- Patent Document 1 and Patent Document 2 disclose inventions relating to a method for producing a high-tensile steel sheet containing 0.6% to 1.5% and 0.5% to 2.0% of Cu, respectively. These inventions are premised on the application of thermal processing control that performs controlled rolling during hot rolling and, as a rule, includes accelerated cooling after rolling. For this reason, the manufacturing methods disclosed in Patent Documents 1 and 2 are not suitable for manufacturing thick high-tensile steel sheets targeting 80 mm or more. In addition, when these manufacturing methods are used, the microstructure in the vicinity of the surface thickness part of the sheet thickness and the center part is greatly increased due to the effect of controlled rolling, etc. There is a concern.
- Patent Document 3 discloses a method for producing a high-toughness high-strength steel (high-strength steel sheet) containing 0.5% to 4.0% Cu and having excellent elongation characteristics and a tensile strength of 686 MPa or more. ing.
- the object of Patent Document 3 is a high strength steel having a tensile strength of 686 MPa or more that exceeds the assumption of the present invention, and has high hardenability in which addition of alloy elements such as Cr, Mo, and V is allowed. High strength steel. Therefore, the manufacturing method described in Patent Document 3 cannot be adopted as a means for solving the problem intended by the present invention because of concern about material uniformity in the thickness direction.
- Patent Document 4 discloses a high-tensile steel sheet containing 0.8% to 1.5% Cu and having excellent weld toughness. Although this high-tensile steel plate is added with Cu and Ni, the assumption of the plate thickness is 77 mm as can be seen from the examples in Reference 4, and the intention is different from the present invention suitable for a plate thickness of 80 mm or more. . In Patent Document 4, it is specified that in the production of a high-strength steel sheet, rolling is performed while restricting the total amount of rolling at 900 ° C. or less, and direct water cooling is performed after rolling. Therefore, there is a great concern for the material uniformity in the thickness direction.
- the N / Al ratio is defined to be in the range of 0.3 to 3.0, the Al content is 0.013% or less as disclosed in the examples.
- deoxidation with normal Al cannot be performed, and there is a concern that the conventional general production method is slightly deviated, lacks in stability, and costs increase.
- Patent Document 5 Patent Document 6 and Patent Document 7 all disclose a method for producing a steel for high heat input welding containing 0.2% to 2.0% Cu and excellent in low temperature toughness.
- a feature of these steel sheets is that the S content is controlled to be 0.003% to 0.008%.
- fine MnS is precipitated in the steel, and excellent HAZ toughness is obtained for high heat input welding.
- the target plate thickness is a thin material of about 32 mm, which is greatly different from the intention of the present invention.
- the addition of S promotes the generation of MnS inclusions that are highly likely to adversely affect toughness, particularly in thick high-tensile steel sheets. For this reason, the techniques disclosed in Patent Documents 5 to 7 are not excellent methods on the premise of producing thick high-tensile steel sheets.
- Patent Document 8 discloses a high-strength thick steel plate excellent in CTOD characteristics and containing 0.70% to 1.75% Cu.
- the strength level of these steel plates is 780 MPa class (tensile strength of 780 MPa or more), which is significantly different from the strength intended by the present invention.
- these steel sheets contain 0.005% to 0.0015% of B, the increase in hardness in the vicinity of the plate thickness surface layer portion becomes extremely large. Therefore, it is estimated that the strength difference in the thickness direction is large in the steel sheet disclosed in Patent Document 8. Further, these steel sheets have an Al content of 0.01% or less, and cannot be deoxidized by ordinary Al. Therefore, it is not suitable for solving the problems of the present invention, such as a slight deviation from the conventional general manufacturing method and a high cost for stability.
- Cu addition is a technique that has been applied to many inventions.
- the present invention is, in the conventional invention could not be achieved, the yield strength 460N / mm 2 ⁇ 580N / mm 2, with a tensile strength of 550N / mm 2 ⁇ 670N / mm 2, for example, there a thick high tensile steel plate or 80mm
- a thick high-tensile steel sheet having uniform characteristics in the thickness direction and excellent in weldability, base metal toughness and HAZ toughness is provided.
- the inventors of the present invention repeated many experiments on a method for producing a thick high-tensile steel plate.
- the Pcm value is controlled to be in the range of 0.25% or less and substantially contains Cr, Mo, V and B with high hardenability. It was found that it is important not to let them.
- high weldability shows that a weld crack does not generate
- the content of Cu and Ni is limited to a specific high concentration range, and then TMCP which has been the mainstream of conventional Cu-added steels. It has been found effective to apply a quenching and tempering treatment rather than a treatment (Thermo Mechanical Control Process).
- FIG. 1 shows a cross-sectional hardness distribution in the thickness direction after quenching and tempering treatment in two types of steel plates having a thickness of 110 mm containing 1.15% Cu and 1.81% or 3.22% Ni.
- FIG. 1 shows a cross-sectional hardness distribution in the thickness direction after quenching and tempering treatment in two types of steel plates having a thickness of 110 mm containing 1.15% Cu and 1.81% or 3.22% Ni.
- FIG. 1 shows a cross-sectional hardness distribution in the thickness direction after quenching and tempering treatment in two types of steel plates having a thickness of 110 mm containing 1.15% Cu and 1.81% or 3.22% Ni.
- FIG. 1 shows a cross-sectional hardness distribution in the thickness direction after quenching and tempering treatment in two types of steel plates having a thickness of 110 mm containing 1.15% Cu and 1.81% or 3.22% Ni.
- FIG. 1 shows a cross-sectional hardness distribution in the thickness direction after quenching and tempering treatment in
- the range of high hardness has spread, and the Vickers hardness at 1 / 8th of the plate thickness (hereinafter referred to as 1 / 8t part) and the Vickers hardness at 1 / 2t part along the plate thickness direction from the surface of the steel plate.
- the difference ( ⁇ Hv) is 38.
- the ⁇ Hv of the 3.22% Ni-containing steel is significantly higher than that of the 1.81% Ni steel.
- the surface of the steel sheet does not mean a specific surface during rolling, but simply means one surface of the steel sheet.
- FIG. 2 shows the results of experimental determination of the relationship between ⁇ Hv and the content of alloy elements.
- FIG. 2 shows ⁇ Hv, which is the difference between the hardness at 1/8 t portion and the hardness at 1/2 t portion of a steel plate having a thickness of 100 mm with varying contents of Cu and Ni. .
- the number in the circle in the figure is ⁇ Hv.
- Such locally hard areas are extremely small areas with respect to the overall thickness of the thick high-tensile steel sheet, and are considered to have little effect on the strength of the steel. . Therefore, when measuring the hardness distribution of the cross section of a steel plate, it is desirable to exclude the data of the above-mentioned local hardening part.
- the lower limit value of the A value is not particularly limited. However, from the viewpoint of securing HAZ toughness and strength, which will be described later, the lower limit is 1.2% and 0.7% for the Ni content and the Cu content, respectively. Therefore, the lower limit value of the A value is preferably 1.9%, which is the sum of the lower limit values of the Cu content and the Ni content.
- the present inventors simulated a weld heat affected zone at ⁇ 40 ° C. in order to investigate the effects of Cu content and Ni content on HAZ toughness (vE (HAZ)), which is a major element of the present invention.
- HAZ toughness HAZ toughness
- An impact test was also conducted. The result is shown in FIG.
- Charpy absorbed energy at ⁇ 40 ° C. is 42 J or more, the occurrence of brittle fracture can be prevented. Therefore, whether or not the Charpy absorbed energy at ⁇ 40 ° C. is 42 J or more was determined as a pass / fail criterion.
- the numerical value in the circle in FIG. 3 is the Charpy absorbed energy at ⁇ 40 ° C. As can be seen from FIG.
- the HAZ toughness is strongly influenced by the alloy composition (content of alloy components).
- the toughness of the base material needs to be examined in consideration of the microstructure, specifically the crystal grain size, in addition to the alloy composition.
- the tensile strength of 550N / mm 2 ⁇ 670N / mm 2 steel as contemplated by the present invention in general, the microstructure becomes ferrite and bainite mixed tissue. Therefore, it is not easy to evaluate the crystal grain size from observation of a microstructure using an optical microscope that has been conventionally performed.
- an EBSD method electron beam backscatter diffraction pattern analysis method
- a region surrounded by a grain boundary having an angle of 30 ° or more is defined as a crystal grain.
- the circle equivalent grain size of the crystal grain was defined as the crystal grain size.
- the frequency distribution of the measured crystal grain size was calculated, and the crystal grain size at which the cumulative frequency from the fine grain side was 70% was defined as the average crystal grain size.
- An example of actual measurement is shown in FIG. FIG.
- the crystal grain in each plate board thickness position of the surface layer part (namely, surface part or outermost layer), 1 / 8t part, 2 / 8t part (1 / 4t part), and 3 / 8t part of the steel plate after quenching and tempering
- the diameter was obtained, and the cumulative frequency (%) with respect to the crystal grain size was obtained.
- the crystal grain size corresponding to a cumulative frequency of 70% is the average crystal grain size.
- the average crystal grain size at each plate thickness position varies depending on the sampling position in the plate thickness direction of the steel plate, and is generally 20 ⁇ m or more at the outermost layer and 1 / 8t portion. On the other hand, it was 15 ⁇ m or less in the 2 / 8t part and the 3 / 8t part.
- FIG. 5 shows 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni shown above.
- the crystal grain size and test temperature change at 20 ° C intervals in a quenched and tempered steel with a plate thickness of 140 mm containing -0.012% Ti-0.012% Nb-0.035% Al-0.0039% N
- the relationship with the toughness obtained by the Charpy test carried out is shown.
- the fracture surface transition temperature (vTrs) obtained by the Charpy test was used as an index of toughness.
- vTrs identifies the ductile fracture surface and the brittle fracture surface from the fracture surface characteristics of the test piece, measures the area ratio of the brittle fracture surface to the total fracture surface area, and determines the area ratio of the brittle fracture surface. Is the temperature at which the area ratio of the brittle fracture surface is 50%. vTrs indicates that the smaller the value, the better the toughness.
- the sampling position of the Charpy test piece is the same position as the site where the crystal grain size was measured, and the sampling direction is a direction perpendicular to the rolling direction. In FIG.
- the vertical axis represents vTrs (toughness), and the horizontal axis d ⁇ 1/2 represents the reciprocal of the square root of the average crystal grain size.
- vTrs and d ⁇ 1/2 there is a substantially linear correlation between vTrs and d ⁇ 1/2 . This corresponds to a relationship conventionally called a Hall-Petch relationship.
- the vTrs on the vertical axis is also affected by the component system, and it is known that the toughness improves particularly when the Ni content increases.
- HAZ toughness is important as a steel material.
- high toughness is required not only for HAZ toughness but also for the base material (portion not affected by welding heat).
- brittle fracture is assumed to occur due to welding defects, etc., but most of these defects are not defects that exist on the surface that are easy to find, but are brittle fractures that occur inside the steel sheet. It has the greatest impact. This is because it is assumed that defects inside the steel sheet are likely to be the most severe stress state with respect to the crack growth, although the possibility that defects in the steel plate are found is low, and depending on the acting stress state.
- the toughness of the base material in the vicinity of the defect must be high in order to prevent it from occurring. It is assumed that such a severe stress state is mainly in the region of 1 / 8t to 7 / 8t, which is the inner side of the steel sheet. Therefore, the toughness required for the base metal should be defined on the inner side closer to the center of the steel plate than the 1/8 t portion rather than the vicinity of the surface layer of the plate thickness.
- the average crystal grain size is 35 ⁇ m or less, vTrs ⁇ ⁇ 10 ° C. can be satisfied.
- Each point in FIG. 5 is taken from the plate thickness position indicated by the display in ().
- the steel sheet surface layer portion does not significantly affect the destruction of the actual structure. Therefore, in the present invention, the average crystal grain at the position excluding the region from the outermost layer portion to 1/8 t portion. Define the diameter. Since the thick steel plate is held in the heat treatment furnace for a long time, the crystal grain size tends to be coarser on the steel plate surface layer side than on the plate thickness central portion.
- the average crystal grain size at 1/8 t part of the plate thickness is 35 ⁇ m or less. Furthermore, by setting the average crystal grain size of the 3 / 8t part of the plate thickness to 35 ⁇ m or less, the average crystal grain size of both the 1 / 8t part and the 3 / 8t part of the plate thickness may be set to 35 ⁇ m or less. As described above, the finer the average crystal grain size, the better the toughness, but it is not easy to make it fine. Therefore, the lower limit value of the average crystal grain size may be 5 ⁇ m, 10 ⁇ m, or 15 ⁇ m. In order to improve the safety of steel structures, there is a concept that requires higher toughness for the base material in consideration of strain aging and the like.
- the cooling rate at the time of quenching is higher in the vicinity of the surface layer of the steel plate than in the steel plate, there is a tendency that a sufficient quenching structure can be obtained, but the strength is increased. Therefore, the toughness in the vicinity of the surface layer is not necessarily high compared to the inside of the steel plate (for example, 1/4 t portion).
- the inside of the plate thickness inside of 1 / 8t
- the internal toughness is considered from 1 / 8t, it is considered sufficient for ensuring the safety of the structure, and the internal average crystal grain size is specified from 1 / 8t. It was decided.
- the steel plate manufactured based on the above techniques exhibits excellent weldability, base metal toughness, and weld heat affected zone toughness while ensuring uniformity in the thickness direction. In particular, the effect is large in a steel plate having a plate thickness of 80 mm or more. However, in a steel sheet having a plate thickness exceeding 200 mm, the cooling rate at the central portion of the plate thickness is remarkably lowered, leading to the coarsening of the microstructure, so that there is a high possibility that the predetermined strength and toughness cannot be satisfied.
- the thickness of the steel plate manufactured according to the present invention may be 200 mm or less. If necessary, the upper limit of the plate thickness may be 175 mm, 150 mm, or 125 mm. The lower limit of the plate thickness may be 90 mm or 100 mm.
- the present invention is substantially free of these elements, for example, with respect to a thick high-tensile steel plate of 80 mm or more, which contains a large amount of alloy elements such as Cr and Mo, and the contents of Cu and Ni. This is based on the fact that the conditions under which steel having excellent weldability, base metal toughness and HAZ toughness can be manufactured are specified by appropriately controlling the thickness.
- the steel sheet according to one embodiment of the present invention has a chemical composition of mass%, C: 0.03% to 0.12%, Si: 0.05% to 0.30%, Mn: 1 20% to 1.65%, Cu: 0.7% to 2.5%, Ni: 1.2% to 3.0%, Nb: 0.005% to 0.030%, Ti: 0.005 % To 0.030%, Al: 0.015% to 0.065%, N: 0.0020% to 0.0060%, Mo: 0% to 0.04%, Cr: 0% to 0.08% , V: 0% to 0.01%, B: 0% to 0.0005%, P: 0.010% or less, S: 0.002% or less, Ca: 0% to 0.0030%, Mg: 0 % To 0.0030%, REM: 0% to 0.0030%, balance: Fe and impurities; A value represented by the following formula (a) is 4.5% or less; Indicated Pcm value be 0.25%; yield strength 460N / mm 2 ⁇ 580N / mm
- A Cu + Ni (a)
- Pcm C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 ⁇ B (b)
- C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are the contents of each element, and the unit is mass%.
- the average crystal grain size in a 3 / 8t portion that is a position of 3/8 of the plate thickness along the plate thickness direction from the surface of the steel plate. It may be 35 ⁇ m or less.
- the average crystal grain size in the 1 / 8t portion may be 25 ⁇ m or less.
- the average crystal grain size in a 3 / 8t portion that is a position of 3/8 of the plate thickness along the plate thickness direction from the surface of the steel plate. It may be 25 ⁇ m or less.
- the plate thickness of the steel plate may be 80 mm or more.
- the present invention it is possible to provide a thick high-tensile steel plate that is excellent in the uniformity of the base material in the plate thickness direction and excellent in weldability, base material toughness, and HAZ toughness.
- Ni content is 1.81% or 3.22%, all other components are It is the figure which showed the result of having measured the hardness distribution in the plate
- the Cu content is 0.3% to 3.6%, and the Ni content is 0.57% to 3%.
- the critical CTOD value ( ⁇ c) obtained in the CTOD test performed three times at a test temperature of ⁇ 10 ° C. using a full-thickness CTOD test piece located 3 mm from the fusion line (FL) with the material (FL + 3 mm) It is the figure which showed the relationship between the average value of Cr and the content of Cr.
- C 0.03% to 0.12% C is an element that improves the strength of the base material.
- the C content needs to be 0.03% or more.
- the lower limit of the C content may be 0.04%, 0.05%, 0.06%, or 0.07%.
- the upper limit of C content is 0.12%.
- the upper limit of the C content may be 0.11%, 0.10%, 0.09%, or 0.08%.
- Si 0.05% to 0.30% Si is an element effective for deoxidation and an element for improving strength. In order to acquire the effect, it is necessary to make Si content 0.05% or more. In order to improve the strength, the lower limit of the Si content may be 0.06%, 0.08%, 0.10%, or 0.13%. On the other hand, if the Si content exceeds 0.30%, the HAZ toughness decreases, so the upper limit of the Si content is set to 0.30%. In order to improve HAZ toughness, the upper limit of the Si content may be 0.25%, 0.22%, 0.20%, or 0.18%.
- Mn 1.20% to 1.65%
- Mn is an element effective for deoxidation and an element for improving strength. In order to acquire the effect, it is necessary to make Mn content 1.20% or more. In order to improve the strength, the lower limit of the Mn content may be 1.25%, 1.28%, 1.30%, 1.33%, 1.35%, or 1.37%. On the other hand, if the Mn content exceeds 1.65%, the material uniformity in the plate thickness direction is impaired due to the increase in hardenability, and segregation in the slab becomes prominent, thereby reducing the HAZ toughness. Therefore, the upper limit of the Mn content is 1.65%. In order to improve HAZ toughness, the upper limit of the Mn content may be 1.60%, 1.58%, 1.55%, 1.52%, 1.50%, or 1.47%.
- Cu 0.7% to 2.5%
- Cu is a main alloy element for the steel sheet according to the present embodiment, and is a few elements that improve the strength of the base material without impairing the weldability and the HAZ toughness.
- the minimum of Cu content is made into 0.7%.
- the lower limit of the Cu content is 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05% or 1.1. % May be used.
- the upper limit of the Cu content is set to 2.5%.
- the upper limit of Cu content may be 2.3%, 2.1%, 1.9%, 1.7%, 1.6%, 1.5%, or 1.4%. .
- Ni 1.2% to 3.0%
- Ni is also a major alloying element for the steel sheet according to the present embodiment, and is an effective element for improving the base material strength and toughness and for improving the HAZ toughness.
- the Ni content needs to be 1.2% or more as shown in FIG.
- the lower limit of the Ni content is 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, 1.5%, 1.55% or 1 It may be 6%.
- the upper limit of Ni content is limited to 3.0%.
- the upper limit of the Ni content is 2.8%, 2.6%, 2.4%, 2.2%, 2.0%, 1.9% or 1. It may be 8%.
- Nb 0.005% to 0.030%
- Nb is an element that improves the strength and is effective in making the base material crystal grains fine. In order to acquire the effect, it is necessary to make Nb content 0.005% or more.
- the lower limit of the Nb content may be 0.007%, 0.010%, 0.012%, 0.013%, or 0.015% in order to improve the strength and refine the crystal grains.
- the upper limit of the Nb content is 0.030%.
- the upper limit of the Nb content may be 0.027%, 0.025%, 0.022%, or 0.020%.
- Ti 0.005% to 0.030%
- Ti is an element that forms a nitride and contributes to the refinement of crystal grains in the weld heat affected zone. In order to acquire the effect, it is necessary to make Ti content 0.005% or more. In order to improve HAZ toughness, the lower limit of the Ti content may be 0.007%, 0.010%, or 0.012%. On the other hand, if the Ti content exceeds 0.030%, the nitride is coarsened, and there is a concern that the HAZ toughness is reduced. Therefore, the upper limit of Ti content is 0.030%. In order to prevent reduction in HAZ toughness, the upper limit of Ti content may be 0.025%, 0.020%, or 0.018%.
- Al 0.015% to 0.065% or less
- Al is an element that is effective for deoxidation, and at the same time, forms nitrides and is effective for refining the base material and HAZ crystal grains.
- the lower limit of the Al content may be set to 0.020%, 0.025%, 0.028%, 0.031%, or 0.035% for refining the base material and the HAZ crystal grains.
- the upper limit of the Al content is 0.065%.
- the upper limit of the Al content may be 0.060%, 0.055%, 0.052%, 0.050%, or 0.048%.
- N 0.0020% to 0.0060%
- N is an element that combines with elements such as Ti and Al to form nitrides. From the viewpoint of forming nitrides, the N content needs to be 0.0020% or more. In order to form nitride more reliably, the lower limit of the N content may be 0.0024% or 0.0028%. On the other hand, if the N content exceeds 0.0060%, the HAZ toughness decreases, so the upper limit of the N content is set to 0.0060%. In order to prevent a decrease in HAZ toughness, the upper limit of the N content may be 0.055%, 0.050%, or 0.045%.
- Mo and V are elements that increase the hardenability and increase the difference in hardness between the surface layer portion and the thickness center portion in the thick high-tensile steel plate. Moreover, when Cr, Mo and V are contained, there is a concern that the HAZ toughness is lowered. Therefore, it is necessary to reduce these elements in the steel plate according to the present embodiment. As described above, in order to evaluate the HAZ toughness, a Charpy test is often used. Recently, a CTOD test for obtaining a CTOD value that can be reflected in a design considering more fracture mechanics is also performed.
- the CTOD value is the crack opening displacement, and is the opening amount at the crack tip when a brittle fracture occurs from the fatigue crack tip.
- a method for obtaining this CTOD value experimentally is the CTOD test.
- the CTOD test is usually performed at a design temperature at which the structure is actually operated.
- the CTOD value is affected by the microstructure of the steel plate at the tip of the fatigue crack, that is, hardness, crystal grain size, carbide state, presence of brittle structure, etc., so it is more sensitive to these metallurgical factors than the Charpy test. It is said that it is. In many cases, if the CTOD value is 0.1 mm or more, it is determined that the steel sheet has sufficient resistance to brittle fracture.
- FIG. 6 is a diagram showing the results of conducting a CTOD test on actual welded joints of a plurality of steel plates with varying amounts of Mo and evaluating the influence of the amount of Mo.
- 0.06% C-0.18% Si-1.35% Mn-1.05% Cu-1.25% Ni-0.013% Ti was used as a basic component system, and it contained Mo.
- Steel whose amount was changed from no addition (content contained as impurities) to 0.12% was melted, and a steel plate having a thickness of 100 mm was manufactured by hot rolling. Thereafter, the steel plate was quenched at 900 ° C.
- the vertical axis represents the average value of three critical CTOD values ⁇ c (may be described as ⁇ c-10 ° C.) at ⁇ 10 ° C.
- the horizontal axis represents the Mo content.
- Mo reduces the CTOD characteristics of the welded joint, and in particular, the CTOD characteristics at the FL + 3 mm position and the FL position.
- ⁇ c ⁇ 0.1 mm is used as a criterion for acceptance, it is understood that the Mo content needs to be 0.04% or less.
- the Mo content it is desirable that the content is small. However, it is not desirable to prevent the Mo content from being completely contained because the cost increases.
- the upper limit of the Mo content is set to 0.04%. A more preferable upper limit of the content is 0.03%, 0.02% or 0.01%.
- FIG. When the content of both Cr and V increases, ⁇ c falls below 0.1 mm at a certain content.
- the upper limit values of the contents of both of which ⁇ c is not less than 0.1 mm are determined from FIGS. 7 and 8, the upper limit of the Cr content is 0.08% and the upper limit of the V content is 0.01%.
- the upper limit of Cr content is 0.08% regardless of impurities or intentional inclusion.
- the upper limit of the Cr content may be 0.06%, 0.05%, 0.04%, or 0.03%.
- the upper limit of the V content is 0.01% regardless of impurities or intentional inclusion.
- the upper limit of the V content may be 0.008%, 0.005%, 0.003%, or 0.001%.
- Cr, Mo, and V may be mixed as impurities from scrap or the like during the production of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.
- B is also an element effective for improving the hardenability by increasing the hardness after quenching treatment with a small amount of content like Cr, Mo and V.
- the difference in the quenching hardness between the surface layer portion and the plate thickness center portion increases due to the inclusion of B. Therefore, B is not preferable from the viewpoint of uniformity in the thickness direction.
- the upper limit of the B content is set to 0.0005%. Even when it is intentionally contained, the upper limit is 0.0005%.
- the upper limit of the B content may be 0.0004%, 0.0003%, 0.0002%, or 0.0001% for further uniformity in the thickness direction.
- the lower limit is not particularly limited, and the lower limit is 0%.
- the upper limit of P is 0.010% or less, preferably 0.007%, 0.005% or less or 0.003%, and the upper limit of S is limited to 0.002% or less.
- the upper limit of S may be limited to 0.001% or 0.0008%.
- Ca has the effect of reducing the influence of MnS, which is harmful to toughness, by spheroidizing the sulfide of the steel sheet. In order to acquire this effect, you may contain 0.0001% or more. However, since the weldability is impaired when the Ca content is excessive, the Ca content is limited to 0.0050% or less. In order to improve weldability, the upper limit of the Ca content may be 0.0040%, 0.0035%, or 0.0030%. Ca may be mixed as an impurity from scrap, refractory, or the like during manufacturing of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.
- Mg and REM are elements that improve the HAZ toughness by forming an oxide in the steel sheet. In order to acquire this effect, you may contain 0.0001% or more. However, if the contents of Mg and REM are excessive, coarse oxides are generated, leading to a decrease in toughness. Therefore, the Mg content and the REM content are limited to 0.0030% or less, respectively. If necessary, the upper limit of these contents may be 0.0025% or 0.0020%. Mg and REM may be mixed as impurities from scraps, refractories, etc. during the production of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.
- REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. Note that the content of REM means the total content of these elements.
- the steel sheet according to the present embodiment contains the above components, with the balance being iron and impurities.
- the steel plate according to the present embodiment in addition to the above components, for the purpose of further improving the strength, toughness, etc. of the steel material itself, or as impurities from auxiliary materials such as scrap, Sb, As, Sn, Pb, Zr, Zn, W, and Co may be contained.
- the upper limit of the content is preferably as follows.
- the upper limit of the Sb content may be 0.02%. In order to improve HAZ toughness, the upper limit of the Sb content may be 0.01%, 0.005%, or 0.002%. Since As and Sn impair the toughness of HAZ, the upper limit of the content of As and Sn may be 0.02%. As needed, it is good also considering the upper limit of content of As and Sn as 0.01%, 0.005%, or 0.002%. In order to improve strength and toughness, the contents of Pb, Zr, Zn, and W may be 0.1% or less, 0.01%, or 0.005% or less, respectively. There is no particular need to determine these lower limits, and it is 0%. Co may be contained as an impurity in Ni. Since Co impairs the HAZ toughness, the upper limit of the Co content may be 0.3%, 0.1%, or 0.05%. There is no particular need to determine the lower limit, and the lower limit is 0%.
- a value ( Cu + Ni): 4.5% or less
- ⁇ Hv is an index mainly showing the uniformity of strength
- Cu + Ni that is, the total of Cu content and Ni content
- ⁇ Hv which is the difference between the Vickers hardness and the Vickers hardness at the 1/2 t portion, exceeds 20, and the characteristics in the thickness direction are not uniform. From this result, in addition to the limitation of the range of each element described above, the upper limit of the A value is set to 4.5%.
- the upper limit of the A value is set to 4.2%, 4.0%, 3.8%, 3.5%, 3.3% or 3 as necessary. It may be 0%.
- the lower limit of the A value is not particularly limited, but 1.9% of the total of the lower limits of the Cu content and the Ni content is a substantial lower limit.
- A Cu + Ni (1)
- Cu and Ni in the above formula (1) are the contents of each element, and the unit is mass%.
- the chemical composition is such that the Pcm value obtained by the following formula (2) is 0.25% or less.
- the Pcm value is often applied as an index representing the weld cracking sensitivity similarly to the carbon equivalent (Ceq), and is calculated from the content of the alloy contained in the steel.
- Formula (2) includes elements such as Cr, Mo, V, and B that are not substantially contained in the present invention. However, since these elements may be mixed as impurities from various alloy raw materials in the process of industrial production, the inclusion of alloy elements including such impurities is necessary when evaluating weldability. The amount needs to be evaluated.
- each alloy element is not contained (not detected), the calculation may be performed with the term set to 0.
- Pcm C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 ⁇ B
- C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are the contents of each element, and the unit is mass%.
- the upper limit of the Pcm value is set to 0.25%.
- the lower limit of the Pcm value does not need to be specified, but the lower limit may be 0.15% or 0.18%.
- the steel plate according to the present embodiment can be manufactured by the following manufacturing method.
- a molten steel having a steel component (chemical composition) adjusted to the above-described range is made into a slab by continuous casting or an ingot-making method (casting step: S1).
- the obtained slab is heated (heating step: S2).
- the lower limit of the target heating temperature in the heating step is preferably 950 ° C. for the purpose of sufficiently reducing the thickness of the thick high-tensile steel plate to the center of the plate thickness.
- the heating temperature exceeds 1250 ° C., the scale of the steel sheet cannot be peeled off and the steel sheet surface flaws may occur, so the upper limit is desirably set to 1250 ° C.
- the heated slab is hot rolled to form a steel plate (hot rolling step: S3).
- the steel sheet is cooled as it is to 350 ° C. or less (cooling process: S4).
- cooling process: S4 In order to reheat beyond the Ac3 transformation point after the cooling step, accelerated cooling may be performed as necessary if there is a restriction on the cooling location. If the cooling stop temperature in the cooling process exceeds 350 ° C., it is not desirable because embrittlement may occur due to coarse precipitates such as aluminum nitride.
- the Ac1 transformation point mentioned here refers to a temperature at which austenite starts to occur locally when the steel is heated from the ferrite phase at room temperature.
- the two-phase state of ferrite and austenite becomes an austenite single phase.
- the temperature at which this austenite single phase is obtained is called the Ac3 transformation point.
- These transformation points can usually be obtained experimentally by utilizing the difference in thermal expansion coefficient between ferrite and austenite. That is, an expansion-temperature curve obtained by heating steel at a constant heating rate (for example, 2.5 ° C./min) can be measured and experimentally obtained from the change point of thermal expansion.
- a quenching process in which the temperature is higher than the Ac3 transformation point and water-cooled and a tempering process in which the temperature is lower than the Ac1 transformation point and air-cooled are performed (quenching and tempering process: S5).
- the heating temperature at the time of quenching is less than the Ac3 transformation point, a sufficient quenched structure cannot be obtained, so that the strength or toughness is lowered.
- the heating temperature at the time of quenching is low from the viewpoint of preventing coarsening of crystal grains. For this reason, it is good also considering the upper limit of heating temperature as 930 degreeC, 910 degreeC, or 890 degreeC. Further, if the heating temperature at the time of tempering exceeds the Ac1 transformation point, the strength or toughness may be remarkably lowered.
- the sheet thickness surface side becomes a flat fine-grained austenite structure by hot rolling, and the central side is hardly affected by rolling, and isotropic and slightly coarse-grained austenite structure generated by recrystallization.
- the region from the surface layer affected by rolling to the vicinity of 1 / 8t part is a microstructure mainly composed of fine ferrite and bainite structures transformed from the processed austenite.
- a coarse ferrite and bainite structure is formed on the inner side of the 2 / 8t portion.
- the average crystal grain size of 3 / 8t part becomes 35 ⁇ m or more.
- the crystal grains of the steel sheet (directly quenched steel) that has been directly quenched in this way are finer on the 1/8 t side than the surface layer than the center of the sheet thickness, and are completely opposite to the steel sheet according to the present embodiment. It has a microstructure. That is, in the directly quenched steel, even if the crystal grain size of the base material in the 1 / 8t part is defined, the crystal grain size inside the steel plate is coarser than the crystal grain size in the 1 / 8t part.
- the toughness of the base material cannot be defined due to restrictions within the scope of the present invention.
- the direct quenching + tempering method is not suitable as a means for securing material uniformity in the thickness direction and imparting excellent toughness in the thick high-tensile steel plate.
- the temperature of the steel sheet is 550 ° C. or higher and the Ac1 transformation is performed for the purpose of uniformizing the crystal grain size in the thickness direction during quenching between the hot rolling process and the quenching and tempering process. It is preferable to further include a step (preliminary heat treatment step: S6) of performing a preheat treatment so that the holding time in this temperature range is 5 hours or more and 500 hours or less. By performing this preliminary heat treatment step, the difference in crystal grain size in the plate thickness direction as shown in FIG. 4 can be reduced.
- this pre-heat treatment prevents the coarsening of crystal grains that occur when the heating time of the surface layer to 1/8 t part takes a long time in the heating process during the quenching treatment of thick high-tensile steel sheets as described above. Therefore, it is a process performed prior to quenching.
- the metallurgical meaning of this pre-heat treatment is to cause Ti and Nb carbonitrides or aluminum nitride precipitates finely precipitated after hot rolling to act as pinning particles during quenching by Ostwald growth. The purpose is to coarsen to an appropriate size.
- the toughness was further improved by reducing the average crystal grain size. Furthermore, the grain refinement effect by the pre-heat treatment described above is more effective for the crystal grains on the surface layer side, so the difference in toughness with the center of the plate thickness becomes smaller and the toughness in the plate thickness direction is also improved. It tends to be uniform. However, when the holding time in the pre-heat treatment is 500 hours or more, the coarsening of the precipitated particles proceeds remarkably during the pre-heat treatment, and the number density of the particles is reduced accordingly, so that the pinning effect is reduced. Therefore, it is desirable that the upper limit of the holding time be 500 hours. Note that when the preliminary heat treatment temperature exceeds the Ac1 transformation point, austenite transformation partially occurs in the steel sheet.
- the heating temperature (holding temperature) at the time of the preliminary heat treatment be equal to or lower than the Ac1 transformation point.
- the steel sheet is cooled to 350 ° C. or lower and then quenched.
- the quenching process is a process of cooling the steel sheet heated to a temperature exceeding the Ac3 transformation point. From the viewpoint of preventing the coarsening of crystal grains, it is preferable that the heating temperature during quenching is low. For this reason, it is good also considering the upper limit of heating temperature as 930 degreeC, 910 degreeC, or 890 degreeC.
- a tempering process is carried out following the quenching process.
- the tempering process is an important process for the purpose of controlling the strength and toughness within a predetermined range.
- the tempering process is performed at a temperature equal to or lower than the Ac1 transformation point for the purpose of ensuring the uniformity of the material in the thickness direction.
- the temperature range is preferably 500 ° C. to 650 ° C., more preferably 550 ° C. to 610 ° C.
- it is effective to perform tempering treatment at the above temperature. It is.
- Steel strips obtained by melting steels A1 to A10 and B1 to B29 having the composition shown in Tables 1 and 2 were made into steel plates having a thickness of 80 to 200 mm according to the manufacturing conditions shown in Tables 3 and 4. It was.
- the heating temperature was 950 ° C. to 1250 ° C., followed by hot rolling, followed by air cooling or water cooling. Thereafter, for test numbers 5, 10, 15 and 26, a preliminary heat treatment was performed before the quenching treatment. Quenching and tempering treatments were performed on the steel plates with test numbers 1 to 51 except for test number 18. Test No. 18 was water-cooled to 100 ° C. immediately after rolling, and only tempered without quenching. Thereafter, in order to evaluate the strength characteristics of the base material, a No. 14 tensile test piece specified in JIS Z 2201 was collected, and a tensile test specified in JIS Z 2241 was performed.
- the yield strength 460N / mm 2 ⁇ 580N / mm 2 ⁇ 580N / mm 2 was judged to be acceptable.
- the impact test piece was extract
- the average value of the three absorbed energy at ⁇ 40 ° C. was described as vE-40 (base material), and 42 J or more was accepted.
- the tensile test piece it extract
- the impact test specimens were collected from three places of 1 / 8t part, 1 / 4t part, and 1 / 2t part. Tables 3 and 4 show 1 / 2t part (thickness center part) with the lowest toughness. Only the test results of) are listed.
- the sampling direction was a direction perpendicular to the rolling direction.
- a cylindrical test piece having a diameter of 3 mm ⁇ and a length of 10 mm was sampled by machining from a 1/4 t part of the plate thickness, and a thermocouple was attached to the end of the test piece. It was read from the change in the amount of thermal expansion in the longitudinal direction of the test piece when heated from room temperature to 950 ° C. at a heating rate of 2.5 ° C./min by high frequency induction heating.
- the crystal orientation is an angle of 30 ° or more
- a region surrounded by a grain boundary having a grain size was defined as a crystal grain, and the equivalent circle diameter of the crystal grain was defined as a crystal grain size.
- the frequency distribution with respect to the crystal grain size of each sample was measured, and the crystal grain size at which the cumulative frequency calculated from the fine grain side was 70% was defined as the average crystal grain size.
- the Vickers hardness distribution (load 98N) in the cross section in the thickness direction was measured, and the difference in hardness between the 1/8 t portion and the 1/2 t portion of the thickness was described as ⁇ Hv as an index of material uniformity. Moreover, the case where ⁇ Hv was 20 or less was regarded as acceptable.
- the 1 / 8t part of the plate thickness is present in two locations in the steel sheet (that is, the position that becomes the 1 / 8t part and the 7 / 8t part when viewed from one surface). Or the larger of the difference in hardness between the 1 / 8t part and the 1 / 2t part.
- a butt joint having a groove shape of K type with a heat input of 3.5 kJ / mm to 4.5 kJ / mm was prepared by submerged arc welding. From this butt joint, three impact test pieces based on JIS Z 3128 were taken with the notch position as a fusion line, and an impact test was conducted at a test temperature of ⁇ 40 ° C. The average value of the three test pieces is shown in Tables 3 and 4 as vE-40 (HAZ).
- the test grain numbers 5, 10 and 15 in which the preliminary heat treatment within the scope of the present invention was performed the average crystal grain size was 1/8 t part and 3/8 t of the plate thickness as compared with the others.
- the average grain size of all the parts is 25 ⁇ m or less.
- the test numbers 5, 10 and 15 have better toughness of the base material than other steels.
- test numbers 18 to 22 in Table 4 the components are within the scope of the present invention, but the production conditions are not desirable, and the base material characteristics and / or uniformity in the thickness direction satisfy the target values.
- Test numbers 23 to 51 are steel plates manufactured using steel whose chemical composition deviates from the scope of the present invention. Test numbers 23 to 51 do not satisfy the target values for at least one of the base material strength and toughness, ⁇ Hv, crack stop temperature, vE-40 (HAZ) and ⁇ c-10 ° C., as shown in Table 4. As a result.
- Test No. 18 was obtained by performing only a tempering process on a steel sheet that had been subjected to a water cooling process (direct quenching) immediately after rolling, and was manufactured without the quenching process.
- the base metal toughness is as low as 29 J and ⁇ Hv is as high as 29.
- Test number 19 is an example in which the tensile properties of the base material do not satisfy the target value as a result of the quenching temperature being a two-phase quenching process.
- Test No. 20 is an example in which the tempering temperature is 705 ° C., the yield strength is low as a result of exceeding the Ac1 transformation point, and ⁇ Hv does not satisfy the target value.
- the cooling stop temperature after rolling is as high as 395 ° C., and heating for quenching is started therefrom.
- the cooling stop temperature is high, the coarsening of the precipitate occurs in the heating stage of the next quenching process, and the toughness of the base material is lowered.
- the test number 22 is an example in which the quenching temperature is 950 ° C. and deviates from a desirable range.
- the crystal grain size is coarse and the base material toughness is low.
- Test Nos. 23, 25 and 27 are examples in which the contents of C, Si and Mn deviate from the scope of the present invention.
- test number 24 is an example in which C is 0.14%, which is out of the range of the present invention, and the Pcm value is out of 0.27%.
- the base metal toughness is low
- ⁇ Hv is inferior to 32 in the thickness direction uniformity
- the crack stop temperature is as high as 25 ° C.
- the absorbed energy and ⁇ c of the weld are low.
- test number 28 is 1.89% for Mn, 0.11% for Cr for test number 46, and 0.0006% for B for test number 49, both deviating from the scope of the present invention.
- test numbers 28, 46, and 49 all have values of ⁇ Hv exceeding 20, and the yield strength and base material toughness do not satisfy the scope of the present invention.
- test number 26 is 0.37% for Si
- test number 29 is 0.012% for P
- test number 30 is 0.004% for S
- test number 40 is 0.038% for Nb
- test number 42 is Ti is 0.036%
- test number 44 is 0.077% Al
- test number 45 is N 0.0075%
- test number 47 is Mo 0.05%
- test number 48 is V 0.012% Both deviate from the scope of the present invention.
- the HAZ toughness decreases.
- Test No. 32, 35 and 36 are examples in which Cu deviates from the range of the present invention and the Pcm value exceeds 0.25%. As a result, the crack stop temperature of all of these is 25 ° C., which does not satisfy the target, and the HAZ toughness is also low.
- Ni is 1.05%, which deviates from the scope of the present invention, and vE-40 (HAZ) and ⁇ c-10 ° C. are low.
- Test number 36 is an example where Ni deviates from the scope of the present invention, so Cu + Ni is 6.00%, which is outside the scope of the invention 4.5%. As a result, ⁇ Hv is 59. Not satisfied with the goal. Test No. 33 and Test No.
- Test number 34 are examples in which Cu is contained within the scope of the present invention, but Ni deviates from the scope of the present invention. That is, in test number 33, Ni is 0.92%, which is out of the range of the present invention, and as a result, the toughness of the base material and the welded part does not satisfy the target.
- Test number 34 is an example in which Ni is 3.15%, which is out of the range of the present invention.
- Cu + Ni is 4.63%, which deviates from 4.5% of the present invention.
- ⁇ Hv is as high as 45.
- Test No. 39 is an example in which Nb deviates slightly, and the base material has low yield strength and tensile strength. Test No.
- Test No. 41 is an example in which Ti is shifted to a low level of 0.003%, and vE-40 (HAZ) is low.
- Test No. 43 is an example in which Al is shifted to a low level of 0.014%, the crystal grains of the base material are not sufficiently refined, and the toughness of the base material is low.
- Test Nos. 50 and 51 are examples in which the individual component ranges are within the scope of the present invention, but the A value or the Pcm value is independently shifted.
- Test number 50 is an example in which the A value is 4.60% and deviates from 4.5%, which is the scope of the present invention. In this case, ⁇ Hv is 31, which does not satisfy the scope of the present invention.
- Test No. 51 has a Pcm value of 0.27%, which is outside the scope of the present invention. As a result, the crack stop temperature is as high as 25 ° C. and does not satisfy the target value.
- the present invention it is possible to provide a thick high-tensile steel plate that is excellent in the uniformity of the base material in the thickness direction and excellent in the toughness, weldability and HAZ toughness of the base material.
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Abstract
Description
厚手高張力鋼板を大型構造物に適用する場合、板厚方向における強度差及び靱性差は、複雑な溶接構造物を構築、設計する上で、それらの変形挙動や破壊挙動を予測して高度かつ合理的な安全性を構築する場合に好ましいものではない。そのため、板厚方向に均一な特性を有する厚手高張力鋼板が求められている。
厚手高張力鋼板は、大型海洋構造物や大型クレーンなどにおいて、高度な安全性が要求される部位に使用される場合が多い。構造物の破壊で最も懸念されるのは、溶接欠陥などの溶接継手部から、脆性破壊が発生することである。したがって、溶接部においては欠陥の発生を防止するために優れた溶接性が求められるとともに、脆性破壊に対しては高い溶接熱影響部靭性(以下、HAZ靱性とする)を求められる場合が多い。
一般に、多くの鋼材規格では鋼板の表面から板厚方向に沿って板厚の1/4の位置、言い換えれば、鋼板の表面から板厚方向に板厚の中心部へ向かって板厚の1/4進んだ位置(以下、1/4t部)の特性が規定されている。しかしながら、海洋構造物などにおいて板厚が厚くなり、かつ、破壊に対して高度な安全性が要求されるようになると、鋼板の表面から板厚中心部方向へ向かって板厚の1/2の位置(以下、1/2t部)においても安定して高い特性が必要となる。
さらに、応力除去焼鈍後の特性やHAZ靭性を確保するためには、高い濃度のCuと同時に高い濃度のNiを含有することが有効であることを知見した。さらに、板厚方向の材質均一性を有する厚手高張力鋼板とするためには、Cu、Niの含有量を特定の高い濃度範囲に制限した上で、従来のCu添加鋼の主流であったTMCP処理(Thermo Mechanical Control Process)ではなく、焼入れおよび焼戻し処理を適用することが有効であることを見出した。
図5において、縦軸がvTrs(靭性)、横軸のd-1/2が平均結晶粒径の平方根の逆数である。この図においては、横軸のd-1/2×100の値が大きいほど結晶粒径が細かいことを表している。
図5から明らかなように、vTrsとd-1/2とにはほぼ直線の相関関係が認められる。これは、従来からホール・ペッチの関係と呼ばれる関係に相当する。なお、縦軸のvTrsは、成分系にも影響され、特にNi含有量が増加すると靱性が向上することが知られている。図5はNi含有量が1.23%の場合であり、このNi量はHAZ靭性の向上を図るために必要なNi量の下限値である1.2%に近い。そのため、この図5を用いると靭性に与える影響が最も大きい合金成分であるNiの含有量が本発明範囲の下限値に近い場合に、どの程度の結晶粒径が必要なのかを予測することが可能となる。以下、詳細に説明する。
溶接部の欠陥からの破壊を想定した場合、万が一、脆性き裂が発生したとしても、母材でそれを阻止するためには、欠陥近傍の母材の靭性が高くなければならない。このような厳しい応力状態となるのは、主に鋼板の内部側である1/8t部~7/8t部の領域であると想定される。それゆえ、母材に対して必要な靭性は、板厚の表層近傍よりもむしろ1/8t部より鋼板の中心に近い内部側で規定されるべきである。
以上の理由から、一般に要求される-40℃におけるシャルピーの吸収エネルギー(vE-40)として要求される42J以上のエネルギー値は、鋼板表面から1/8t部より鋼板の内側において必要とされる。そこで、本発明においては、1/8t部より内部側の結晶粒径を規定する。
さて、従来の鋼板で得られた遷移曲線から考えると、-40℃での吸収エネルギー42Jを満足するためにはvTrsが-10℃以下であることが必要である。
図5から、vTrsが-10℃(図中の破線)に相当する平均結晶粒径は、35μmである。従って、平均結晶粒径が35μm以下であれば、vTrs≦-10℃を満足できることがわかった。図5中の各点は、( )内の表示で示された板厚位置から採取されたものである。先に述べたように、鋼板表層部は実際の構造物の破壊にあまり影響しないと考えられるので、本発明では、最表層部から1/8t部までの領域を除いた位置での平均結晶粒径を規定する。厚手鋼板は長時間にわたって熱処理炉内で保持されるため、鋼板表層部側の方が板厚中心部に比べて結晶粒径が粗大になる傾向がある。そのため、特に板厚の1/8t部の平均結晶粒径を35μm以下とすることが重要である。さらに、板厚の3/8t部の平均結晶粒径を35μm以下とすることで、板厚の1/8t部及び3/8t部の両方の平均結晶粒径を35μm以下としても差し支えない。
なお、上記のように平均結晶粒径は細粒であるほど靱性が向上するが、細粒にすることは容易でない。そのため、平均結晶粒径の下限値を5μm、10μm又は15μmとしてもよい。
鋼構造物の安全性を向上させるために、歪時効などを考慮し、母材に対してより高い靭性を必要とする考え方がある。特に歪時効の場合、発明者らの検討によれば、5%程度の歪を冷間で付与し、その後、250℃(2時間保持)で時効処理を実施した場合、シャルピー遷移温度が、-15℃程度上昇することが判明している。そこで、歪時効を考慮してさらに高い靭性が要求される場合は、vTrsは、さらに15℃低い、-25℃以下であることが望ましい。このためには、同じく図5より、板厚の1/8t部の平均結晶粒径を25μm以下とすればよいことが判った。すなわち、前記と同様な理由により、板厚の3/8t部の平均結晶粒径も25μm以下としてもよい。
なお、鋼板の表層付近は鋼板内部より焼入れ時の冷却速度が高くなるため、十分な焼入れ組織が得られやすい半面、強度が高くなる傾向がある。そのため、表層付近の靭性は鋼板内部(例えば、1/4t部)と比べ、必ずしも高いとは言えない。しかしながら、先に述べた様に、構造物としての脆性破壊に対する安全性を考えた場合、極端な曲げ変形が生じない条件下では、溶接欠陥などの潜在的なき裂の発見が容易であり、かつ、拘束力が低い表層近傍より板厚内部(1/8tより内部)の方がより脆性き裂の発生に対してはより厳しくなる傾向にある。このため、本発明においては、1/8tより内部の靭性を考慮すれば、構造物の安全性の確保に対して、十分であると考え、1/8tより内部の平均結晶粒径を規定することとした。
以上のような技術に基づいて製造された鋼板は、その板厚方向の均一性を確保しながら、優れた溶接性、母材靱性および溶接熱影響部靭性を示す。特に、板厚が80mm以上の鋼板においてその効果は大きい。しかしながら、板厚が200mmを超える鋼板では、板厚中心部の冷却速度が著しく低下し、ミクロ組織の粗大化を招くことにより、所定の強度および靱性を満足できなくなる可能性が高い。したがって、本発明により製造する鋼板の板厚は200mm以下としてもよい。必要に応じて、板厚の上限を175mm、150mm又は125mmとしてもよい。板厚の下限を90mm又は100mmとしてもよい。
A=Cu+Ni…(a)
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B…(b)
ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V及びBは、各元素の含有量で、その単位は質量%である。
まず、本実施形態に係る鋼板の化学組成の限定理由を述べる。
Cは、母材の強度を向上させる元素である。その効果を得るためには、C含有量を、0.03%以上とする必要がある。強度の向上のため、C含有量の下限を0.04%、0.05%、0.06%又は0.07%としてもよい。一方、C含有量が0.12%を超えると焼入れ性の増加により板厚方向の材質均一性が損なわれる。また、溶接部の硬さが上昇すると同時にHAZ靭性が低下する。そのため、C含有量の上限を0.12%とする。HAZ靭性の改善のため、C含有量の上限を0.11%、0.10%、0.09%又は0.08%としてもよい。
Siは、脱酸に有効な元素であるとともに、強度を向上させる元素である。その効果を得るためには、Si含有量を0.05%以上とする必要がある。強度の向上のため、Si含有量の下限を0.06%、0.08%、0.10%又は0.13%としてもよい。一方、Si含有量が0.30%を超えると、HAZ靭性が低下するので、Si含有量の上限を0.30%とする。HAZ靭性の向上のため、Si含有量の上限を0.25%、0.22%、0.20%又は0.18%としてもよい。
Mnは、脱酸に有効な元素であるとともに、強度を向上させる元素である。その効果を得るためには、Mn含有量を1.20%以上とする必要がある。強度の向上のため、Mn含有量の下限を1.25%、1.28%、1.30%、1.33%、1.35%又は1.37%としてもよい。一方、Mn含有量が1.65%を超えると、焼入れ性の増加により板厚方向の材質均一性が損なわれるとともに、鋳片での偏析が顕著になってHAZ靭性を低下させる。そのため、Mn含有量の上限を1.65%とする。HAZ靭性の向上のため、Mn含有量の上限を1.60%、1.58%、1.55%、1.52%、1.50%又は1.47%としてもよい。
Cuは、本実施形態に係る鋼板にとって主要な合金元素であり、溶接性およびHAZ靭性を損なわずに母材の強度を向上させる数少ない元素である。Cu含有量を0.7%以上とすることで強度の上昇に著しい効果がある。このため、Cu含有量の下限は0.7%とする。強度の向上のため、Cu含有量の下限を0.75%、0.8%、0.85%、0.9%、0.95%、1.0%、1.05%又は1.1%としてもよい。一方、Cu含有量が2.5%を超えた場合、焼入れ性の上昇を招き、図3に示されたようにHAZ靭性が低下することが懸念される。そのため、Cu含有量の上限を2.5%とする。HAZ靭性の向上のため、Cu含有量の上限を2.3%、2.1%、1.9%、1.7%、1.6%、1.5%又は1.4%としてもよい。
Niも、本実施形態に係る鋼板にとって主要な合金元素であり、母材強度および靭性の改善、ならびにHAZ靭性改善に有効な元素である。Ni含有量は、HAZ靭性の観点から、図3に示されたように、1.2%以上が必要である。上記の特性の改善のため、Ni含有量の下限を1.25%、1.3%、1.35%、1.4%、1.45%、1.5%、1.55%又は1.6%としてもよい。一方、Ni含有量が3.0%を超えると、図2に示されたように、板厚方向の材質差を生じる。そのため、Ni含有量の上限を3.0%に制限する。板厚方向の材質差をより小さくするため、Ni含有量の上限を2.8%、2.6%、2.4%、2.2%、2.0%、1.9%又は1.8%としてもよい。
Nbは、強度を向上させるとともに、母材結晶粒の細粒化に有効な元素である。その効果を得るためには、Nb含有量を0.005%以上とする必要がある。強度向上と結晶粒の微細化とのため、Nb含有量の下限を0.007%、0.010%、0.012%、0.013%又は0.015%としてもよい。一方、Nb含有量が、0.030%を超えるとHAZ靭性が低下するので、Nb含有量の上限を0.030%とする。HAZ靭性の向上のため、Nb含有量の上限を0.027%、0.025%、0.022%又は0.020%としてもよい。
Tiは窒化物を形成し溶接熱影響部における結晶粒の細粒化に寄与する元素である。その効果を得るためには、Ti含有量を0.005%以上とする必要がある。HAZ靭性の向上のため、Ti含有量の下限を0.007%、0.010%、又は0.012%としてもよい。一方、Ti含有量が0.030%を超えると、窒化物が粗大化してしまいかえってHAZ靭性の低下を招くことが懸念される。そのため、Ti含有量の上限を0.030%とする。HAZ靭性の低下防止のため、Ti含有量の上限を0.025%、0.020%、又は0.018%としてもよい。
Alは脱酸に有効であると同時に、窒化物を形成し母材およびHAZ結晶粒の細粒化に有効な元素である。その効果を得るためには、Al含有量を0.015%以上とする必要がある。母材およびHAZ結晶粒の細粒化のため、Al含有量の下限を0.020%、0.025%、0.028%、0.031%又は0.035%としてもよい。一方、Al含有量が0.065%を超えると粗大な窒化物が形成され靱性が低下する傾向がある。そのため、Al含有量の上限を0.065%とする。靭性低下防止のため、Alの含有量の上限を0.060%、0.055%、0.052%、0.050%又は0.048%としてもよい。
NはTi、Al等の元素と結合し、窒化物を形成する元素である。窒化物の形成の観点からはN含有量を0.0020%以上とすることが必要である。より確実に窒化物を形成させるため、Nの含有量の下限を0.0024%又は0.0028%としてもよい。一方、N含有量が0.0060%を越えると、HAZ靭性が低下するので、N含有量の上限を0.0060%とする。HAZ靭性の低下防止のため、Nの含有量の上限を0.055%、0.050%又は0.045%としてもよい。
Mo:0%~0.04%
V:0%~0.01%
Cr、MoおよびVは、焼入れ性を増加させ、厚手高張力鋼板においては、表層部と板厚中心部との硬さの差を大きくする元素である。またCr、Mo及びVを含有すると、HAZ靭性が低下することが懸念される。そのため、本実施形態に係る鋼板においてはこれらの元素を低減する必要がある。
前述したように、HAZ靱性の評価は、多くの場合シャルピー試験が用いられるが、最近はより破壊力学を考慮した設計に反映できるCTOD値を求めるCTOD試験も実施される。CTOD値とはき裂開口変位のことであり、疲労き裂先端からの脆性破壊が発生した時のき裂先端の開口量である。このCTOD値を実験的に求める方法がCTOD試験である。CTOD試験は、通常、構造物が実際に運用される設計温度で実施される。CTOD値は、疲労き裂先端の鋼板のミクロ組織、すなわち、硬さや結晶粒径、炭化物の状態、脆化組織の有無などに影響されるので、シャルピー試験よりこれらの冶金学的な要因に敏感であるといわれている。なお、多くの場合、CTOD値が0.1mm以上であれば、その鋼板は、脆性破壊に対する十分な抵抗性を有すると判断される。
図6は、縦軸が-10℃での限界CTOD値δc(δc-10℃と記載する場合がある)の3本の平均値であり、横軸がMo含有量である。図6から、Moは、溶接継手のCTOD特性、その中でもFL+3mmの位置及びFL位置でのCTOD特性を低下させることが分かる。また、δc≧0.1mmを合格の目安とすると、Mo含有量を0.04%以下にする必要があることが分かる。
Mo含有量については、少ない方が望ましいが、完全に含有しないようにすることはコストの上昇を招くため望ましくない。また、不純物として又は意図的に含有される場合を考慮し、Moの含有量の上限を0.04%とする。より好ましい含有量の上限は0.03%、0.02%又は0.01%である。
なお、Cr、Mo、Vは溶鋼製造時にスクラップ等から不純物として混入する場合があるが、その下限を特に制限する必要はなく、その下限は0%である。
ここで、REMとは、ランタノイドの15元素にYおよびScを合わせた17元素の総称であり、これらの元素のうちの1種または2種以上を含有させることができる。なお、REMの含有量はこれらの元素の合計含有量を意味する。
本実施形態に係る鋼板は、上記成分を含有し、残部が鉄および不純物である。しかしながら、本実施形態に係る鋼板には、上記成分の他に、鋼材自体の強度、靭性等を一段と改善する目的で、あるいはスクラップ等の副原料からの不純物として、さらに、Sb、As、Sn、Pb,Zr、Zn、W、Coを含有してもよい。しかしながら、その含有量の上限は、以下の通りとすることが望ましい。
SbはHAZの靭性を損なうため、Sb含有量の上限を0.02%としてもよい。HAZ靭性を向上させるため、Sb含有量の上限を、0.01%、0.005%又は0.002%としてもよい。
AsおよびSnはHAZの靭性を損なうため、AsおよびSnの含有量の上限を0.02%としてもよい。必要に応じて、AsおよびSnの含有量の上限を、0.01%、0.005%又は0.002%としてもよい。
また、強度及び靭性の向上のため、Pb、Zr、Zn及びWの含有量を、それぞれ0.1%以下、0.01%又は0.005%以下としてもよい。これらの下限を特に決める必要はなく、0%である。
Coは、Niの中に不純物として含まれる場合がある。CoはHAZ靭性を損なうため、Co含有量の上限を0.3%、0.1%又は0.05%としてもよい。その下限を特に決める必要はなく、その下限は0%である。
本実施形態では母材の板厚方向について、主として強度の均一性を示す指標であるΔHvを制御する必要がある。図2から分かるように、Cu+Ni、すなわち、Cu含有量とNi含有量との合計であり、下記式(1)で表されるA値が、4.5%を超えると1/8t部でのビッカース硬さと1/2t部でのビッカース硬さとの差であるΔHvが20を超え、板厚方向における特性が不均一となる。この結果から、上記の個々の元素の範囲の限定に加えて、A値の上限を4.5%とする。板厚方向の硬さの差をより低減するため、必要に応じて、A値の上限を4.2%、4.0%、3.8%、3.5%、3.3%又は3.0%としてもよい。A値の下限は特に限定する必要がないが、Cu含有量及びNi含有量のそれぞれの下限の合計の1.9%が実質的な下限となる。
A=Cu+Ni…(1)
ここで、上記式(1)中のCuおよびNiは各元素の含有量で、その単位は質量%である。
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B…(2)
ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V及びBは各元素の含有量で、その単位は質量%である。
本実施形態に係る鋼板においては、Pcm値が0.25%を超えると0℃で溶接した時の低温割れが発生しやすくなることから、Pcm値の上限を0.25%としている。Pcm値の下限を特に規定する必要はないが、その下限を0.15%又は0.18%としてもよい。
まず、上述した範囲に調整した鋼成分(化学組成)を有する溶鋼を、連続鋳造または造塊分塊法によってスラブとする(鋳造工程:S1)。その後、得られたスラブを加熱する(加熱工程:S2)。なお、加熱工程において目標とする加熱温度は、厚手高張力鋼板を圧延する際に板厚中心部まで十分に圧下の効果を得る目的で、その下限を950℃とすることが望ましい。一方で、加熱温度が1250℃を超えると、鋼板のスケールが剥離できず、鋼板表面疵が発生してしまう場合があるので、その上限を1250℃とすることが望ましい。
なお、ここで言うAc1変態点とは、鋼を室温のフェライト相から昇温した場合、オーステナイトが局部的に生じ始める温度を言う。また、さらに昇温した場合には、フェライトとオーステナイトとの二相状態からオーステナイト単相になる。このオーステナイト単相になる時の温度をAc3変態点と言う。これら変態点は、通常、フェライトとオーステナイトとの熱膨張率の差を利用して実験的に求めることができる。すなわち、鋼を一定の加熱速度(例えば2.5℃/minなど)で加熱して得られる膨張-温度曲線を測定し、熱膨張の変化点から実験的に求めることができる。
冷却工程後、Ac3変態点以上に加熱し水冷する焼入れ処理、およびAc1変態点以下の温度に加熱し空冷する焼戻し処理を行なう(焼入れ焼戻し工程:S5)。
焼入れ時の加熱温度がAc3変態点未満であると、十分な焼入れ組織が得られないため、強度又は靭性が低下する。一方で、結晶粒の粗大化を防止する点から、焼入れ時の加熱温度は低い方が好ましい。このため、加熱温度の上限を930℃、910℃又は890℃としてもよい。また、焼戻し時の加熱温度がAc1変態点超であると、強度又は靭性が著しく低下する場合がある。
近年、本実施形態のように圧延後に冷却された鋼板を再加熱し焼入れ処理および焼戻し処理をする方法ではなく、圧延後に直接冷却を施し、それを焼き戻す方法(直接焼入れ+焼戻し処理)が、高張力鋼板の製造においても適用される例がある。しかし、この方法は、本実施形態に係る鋼板には適さない。その理由は、以下のとおりである。
圧延後に直接焼入れ処理をされた鋼板の結晶粒径は、加熱および圧延温度に依存する。結晶粒径の細粒化を図るために、低温加熱を実施したり、低温で圧延を実施したりすると、冷却されやすい鋼板表面側において圧延温度が低くなる。その結果、圧延直後には、板厚表面側が熱間圧延により扁平な細粒なオーステナイト組織となり、中心部側が圧延の影響を受けにくい、再結晶によって生成した等方的でやや粗粒なオーステナイト組織となる場合が多い。これらのオーステナイト組織を有する鋼板に直接焼入れを実施すると、圧延の影響を受ける表層から1/8t部付近までの領域は、加工されたオーステナイトから変態した細粒フェライトおよびベイナイト組織を主体するするミクロ組織となり、逆に2/8t部より内側では、粗粒なフェライトおよびベイナイト組織となる。この結果、3/8t部の平均結晶粒径が35μm以上となる。
すなわち、このように直接焼入れ処理が行われた鋼板(直接焼入れ鋼)の結晶粒は、表層から1/8t側が板厚中心部側より細粒となり、本実施形態に係る鋼板とは全く逆のミクロ組織の構成となる。すなわち、直接焼入れ鋼において、1/8t部における母材の結晶粒径を規定したとしても、それより鋼板内部での結晶粒径が1/8t部での結晶粒径よりも粗大であるので、本発明範囲における制約で母材の靭性を規定することはできない。さらに、表層側の結晶粒が細粒であることから、板厚方向の硬さ分布においても表層側が硬化する傾向が高く、ΔHv≦20を満足できない。
以上のように、厚手高張力鋼板において、板厚方向の材質均一性を確保し、かつ優れた靱性を付与する手段としては、直接焼入れ+焼戻し法は適さない。板厚方向の材質均一性を確保し、1/8t部の平均結晶粒径を35μm以下とするためには、一旦冷却後に焼入れ処理および焼戻し処理を行う必要がある。
図9から分かるように、予備熱処理の温度が450℃の場合は、保持時間が長時間となると徐々に平均結晶粒径が小さくなる傾向があるものの、平均結晶粒径を25μm以下とするには、100時間以上と非常に長い保持時間が必要である。一方、予備熱処理の温度が550℃の場合は、保持時間が5時間以上で、平均結晶粒径が25μm以下となり、明らかな細粒化が認められた。以上のことから、粗大化しやすい板厚の1/8t部近傍の平均結晶粒径の微細化を図るためには、予備熱処理として、550℃以上で5時間以上の保持を行なうことが望ましいことが分かった。平均結晶粒径が細粒化されることで、より靭性が向上した。さらに、上述の予備熱処理による結晶粒微細化効果は、表層側の結晶粒に対しての方がより効果がより大きいので、板厚中央部との靭性の差異が小さくなり板厚方向も靭性が均一化する傾向にある。しかしながら、予備熱処理における保持時間が500時間以上となると、予備熱処理中に析出粒子の粗大化が著しく進行するとともに、それに伴い粒子の個数密度が減少することによってピン止め効果はかえって小さくなる。したがって、その保持時間の上限を500時間とすることが望ましい。なお、予備熱処理温度がAc1変態点を越えると鋼板内でオーステナイト変態が部分的に生じる。この場合、フェライトとオーステナイトとで析出物の成長速度が異なることから、鋼板内で均一な析出物の成長が期待できない。そのため、予備熱処理の際の加熱温度(保持温度)は、Ac1変態点以下とすることが好ましい。
また、同じ突合せ継手から、ノッチ位置をCGHAZ(Coarse grain HAZ)とよばれるフージョンラインとして、BS7448に準拠した全厚CTOD試験片(B×Bタイプ)を採取し、試験温度-10℃で、API(American Petroleum Institue)規格RP 2Z及びBS(British Standards)規格7448に準拠したCTOD試験をそれぞれ3本行った。これらの最低値をδc-10℃として表3、表4に記載した。なお、衝撃試験においては42J以上を、CTOD試験(δc)では0.1mm以上を合格として評価した。
なお、衝撃試験の結果とCTOD試験の結果とには、大まかな相関があるとされているが、一方が良好であってももう一方が低い場合もある。そのため、破壊に対して要求が厳しい構造物では、HAZ靭性として両者を満足する必要がある。
さらに、試験番号22は、焼入れ温度が950℃と望ましい範囲を逸脱して実施された例である。試験番号22は、結晶粒径が粗大であり、母材靭性が低くなっている。
試験番号23、25および27は、それぞれC、SiおよびMnの含有量が本発明範囲を低めにはずれた例である。これらは母材の引張強さが目標値を満足しておらず、さらに試験番号23、25では降伏強度も低い。
逆に、試験番号24は、Cが0.14%と本発明範囲を高めにはずれかつ、Pcm値も0.27%とはずれた例である。その結果、母材靱性が低く、ΔHvが32と板厚方向の均一性に劣り、割れ停止温度も25℃と高く、溶接部の吸収エネルギーもδcも低い。同様に、試験番号28は、Mnが1.89%、試験番号46はCrが0.11%および試験番号49はBが0.0006%といずれも本発明範囲を高めに逸脱している。これらの元素はすべて母材の焼入れ性を上昇させる元素なので、試験番号28、46、49は、いずれもΔHvが20を超える値であり、降伏強度および母材靱性が本発明範囲を満足しないものもある。
一方、試験番号26はSiが0.37%、試験番号29はPが0.012%、試験番号30はSが0.004%、試験番号40はNbが0.038%、試験番号42はTiが0.036%、試験番号44はAlが0.077%、試験番号45はNが0.0075%、試験番号47はMoが0.05%、試験番号48はVが0.012%、といずれも本発明範囲を高めに逸脱している。これらの元素が、本発明範囲を超えて含有されると、HAZ靭性が低下する。従って、vE-40(HAZ)もしくはδc-10℃の値いずれかまたは両者が目標値を満足していない。
次に、本発明鋼にとって、主要な元素であるCu、Niの効果について述べる。試験番号31、試験番号37および試験番号38は、Cuがいずれも本発明範囲を低めにはずれている。そのため、試験番号31では、引張強さが低く、試験番号37、38では、引張強度及び降伏強度が低い。さらに、試験番号37は、Niが3.29%と本発明範囲を高めにはずれていることから、ΔHvも55と高く、試験番号38は、逆にNiが1.11と低めにはずれており、溶接部の靭性がいずれも低い。
さらに、試験番号32、35および36は、Cuが本発明範囲を高めにはずれ、さらにPcm値も0.25%を超えた例である。その結果、これらのすべてで割れ停止温度が25℃となっており目標を満足しておらず、HAZ靱性も低い。その中でも試験番号35は、Niも1.05%と本発明範囲を低めに逸脱しており、vE-40(HAZ)およびδc-10℃が低い。また、試験番号36は、逆にNiが本発明範囲を高めにはずれた例であるので、Cu+Niが6.00%と発明範囲である4.5%を外れており、その結果ΔHvが59と目標を満足していない。
試験番号33および試験番号34は、Cuが本発明範囲内で含有されたものであるがNiが本発明範囲を逸脱した例である。すなわち、試験番号33は、Niが0.92%と本発明範囲を低めにはずれており、その結果、母材及び溶接部の靭性が目標を満足していない。一方、試験番号34は、逆にNiが3.15%と本発明範囲を高めにはずれた例で、同時にCu+Niも4.63%と本発明範囲である4.5%を逸脱しているので、ΔHvが45と高くなっている。
試験番号39は、Nbが低めにはずれた例であり、母材の降伏強度および引張強さが低い。試験番号41はTiが0.003%と低めにはずれた例でありvE-40(HAZ)が低い。試験番号43は、Alが0.014%と低めにはずれた例であり、母材の結晶粒の細粒化が不十分であり、母材の靭性が低い。試験番号50および51は、個々の成分範囲については本発明範囲であるが、A値またはPcm値がそれぞれ単独ではずれた例である。試験番号50は、A値が4.60%と本発明範囲である4.5%を逸脱した例であるこの場合、ΔHvが31となり本発明範囲を満足しない。試験番号51はPcm値が0.27%と本発明範囲を逸脱しており、その結果、割れ停止温度が25℃と高く目標値を満足してない。
Claims (5)
- 化学組成が、質量%で、
C:0.03%~0.12%、
Si:0.05%~0.30%、
Mn:1.20%~1.65%、
Cu:0.7%~2.5%、
Ni:1.2%~3.0%、
Nb:0.005%~0.030%、
Ti:0.005%~0.030%、
Al:0.015%~0.065%、
N:0.0020%~0.0060%、
Mo:0%~0.04%、
Cr:0%~0.08%、
V:0%~0.01%、
B:0%~0.0005%、
P:0.010%以下、
S:0.002%以下、
Ca:0%~0.0030%、
Mg:0%~0.0030%、
REM:0%~0.0030%、
残部:Fe及び不純物であり;
下記(1)式で示されるA値が4.5%以下であり;
下記(2)式で示されるPcm値が0.25%以下であり;
降伏強度が460N/mm2~580N/mm2、かつ、引張強さが550N/mm2~670N/mm2であり;
表面から板厚方向に沿って板厚の1/8の位置である1/8t部の硬さと、前記表面から前記板厚方向に沿って前記板厚の1/2の位置である1/2t部の硬さとの差が、ビッカース硬度で20以下であり;
電子ビーム後方散乱回析パターン解析法を用いた結晶方位解析を行い、結晶方位差が30°以上の粒界で囲まれる領域を結晶粒と定義し、前記結晶粒の円相当粒径を結晶粒径と定義し、前記結晶粒径の頻度分布を算出した場合の累積頻度が細粒側から70%となる前記結晶粒径を、平均結晶粒径と定義したとき、前記1/8t部における前記平均結晶粒径が35μm以下である;
ことを特徴とする鋼板。
A=Cu+Ni…(1)
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B…(2)
ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V及びBは、各元素の含有量で、その単位は質量%である。 - さらに、前記鋼板の前記表面から前記板厚方向に沿って前記板厚の3/8の位置である3/8t部における前記平均結晶粒径が35μm以下であることを特徴とする請求項1に記載の鋼板。
- さらに、前記1/8t部における前記平均結晶粒径が25μm以下であることを特徴とする請求項1に記載の鋼板。
- さらに、前記鋼板の前記表面から前記板厚方向に沿って前記板厚の3/8の位置である3/8t部における前記平均結晶粒径が25μm以下であることを特徴とする請求項3に記載の鋼板。
- 前記鋼板の前記板厚が、80mm以上であることを特徴とする請求項1~4のいずれか1項に記載の鋼板。
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CN104520463A (zh) | 2015-04-15 |
JP5510620B1 (ja) | 2014-06-04 |
EP2860276B1 (en) | 2018-05-02 |
KR20150029615A (ko) | 2015-03-18 |
KR101542709B1 (ko) | 2015-08-12 |
EP2860276A4 (en) | 2015-12-30 |
JPWO2015022729A1 (ja) | 2017-03-02 |
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CN104520463B (zh) | 2016-03-30 |
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