WO2010038470A1 - 母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板およびその製造方法 - Google Patents
母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板およびその製造方法 Download PDFInfo
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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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
- C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
Definitions
- the present invention relates to a thick steel plate having excellent low-temperature toughness of a base material and a weld heat-affected zone and small strength anisotropy, and a method for producing the same.
- Steel sheets produced by this method can be used for all types of welded structures such as shipbuilding, bridges, buildings, marine structures, pressure vessels, tanks, line pipes, etc., but particularly require fracture toughness tests at around -70 ° C. It is effective for use in the low temperature field.
- This application claims priority based on Japanese Patent Application No. 2008-256122 filed in Japan on October 1, 2008 and Japanese Patent Application No. 2009-000202 filed on January 5, 2009 in Japan. , The contents of which are incorporated herein.
- Patent Document 1 Patent Document 2, and Patent Document 3 describe a so-called 9% Ni steel (Ni is 8.5 to 9.5 in mass%) as a steel type used for an inner tank of a liquefied natural gas (LNG) storage tank. %, And has a structure mainly composed of tempered martensite, and in particular, a low temperature toughness, for example, a steel material excellent in Charpy impact absorption energy at ⁇ 196 ° C.).
- LNG liquefied natural gas
- Patent Document 4 and Patent Document 5 contain Ni of about 4.0% as a steel type used for ships, have a structure mainly composed of tempered martensite, and have low temperature toughness, for example, A steel material excellent in Charpy impact absorption energy at -70 ° C is disclosed.
- Patent Document 6 discloses a method of performing a preheat treatment for reducing segregation before heating and rolling a cast slab.
- Patent Document 7 discloses a method of reducing defects at the center of the plate thickness by using two rolling processes.
- the effect of reducing segregation is small, and band-like Ni segregation remains and the toughness of the weld heat affected zone becomes low.
- the rolling ratio from the casting slab to the final sheet thickness (the rolling ratio is a value obtained by dividing the sheet thickness before rolling by the sheet thickness after rolling) is small, and the first hot rolling is performed. Since the reduction ratio, temperature, etc. are not controlled, the toughness of the base metal and the weld heat-affected zone decreases due to coarsening of the structure and residual segregation.
- Patent Document 8 discloses a method by a process called TMCP (Thermal Mechanical Controlled Processing) in which water cooling is performed immediately after rolling as a method of manufacturing a steel material having excellent weld heat affected zone toughness.
- TMCP Thermal Mechanical Controlled Processing
- the anisotropy of strength increases and a problem arises in safety. That is, it is difficult to manufacture a steel material containing Ni and excellent in toughness of the base material and the weld heat-affected zone and having small strength anisotropy with the existing technology.
- the requirements for toughness on the user side are minimum strength anisotropy, base metal toughness of 150J or higher even at low temperatures of -70 ° C, and securing of weld heat affected zone toughness of 100J or higher even at low temperatures of -70 ° C It is desired.
- the problem to be solved by the present invention is to provide a steel sheet that is excellent in toughness of the base material and the weld heat affected zone and has a small anisotropy.
- the present invention provides a steel sheet that is excellent in toughness of a base material and a weld heat-affected zone and has low anisotropy, and the gist thereof is as follows.
- the steel is in mass%, C: 0.04% or more and 0.10% or less, Si: 0.02% or more and 0.40% or less, Mn: 0.5 %: 1.0% to 1.0%, P: 0.0010% to 0.0100%, S: 0.0001% to 0.0050%, Ni: 2.0% to 4.5%, Cr: 0 1% to 1.0%, Mo: 0.1% to 0.6%, V: 0.005% to 0.1%, Al: 0.01% to 0.08%, N : A steel composition containing 0.0001% or more and 0.0070% or less, with the balance being Fe and inevitable impurities, and Ni in a portion of the steel sheet having a thickness of 1 ⁇ 4 from the steel sheet surface in the thickness direction.
- the segregation ratio is 1.3 or less
- the flatness of prior austenite is 1.05 or more and 3.0 or less
- the effective crystal grain size is 10 ⁇ m or less.
- the wherein the Vickers hardness is less than 310HV than 265HV, a small steel sheet excellent and strength anisotropy in the low temperature toughness of the base metal and weld heat affected zone.
- the steel sheet having excellent low-temperature toughness of the base material and the weld heat-affected zone described in (1) above and having low strength anisotropy is further in mass%, Nb: 0.005% or more and 0.03% or less, Ti: 0.005% to 0.03%, Cu: 0.01% to 0.7%, B: 0.0002% to 0.05%, Ca: 0.0002% to 0.0040%
- the steel composition may contain one or two or more of REM: 0.0002% or more and 0.0040% or less, with the balance being Fe and inevitable impurities.
- the second embodiment of the present invention is mass%, C: 0.04% or more and 0.10% or less, Si: 0.02% or more and 0.40% or less, Mn: 0.5% or more and 1 0.0% or less, P: 0.0010% to 0.0100%, S: 0.0001% to 0.0050%, Ni: 2.0% to 4.5%, Cr: 0.1% 1.0% or less, Mo: 0.1% or more and 0.6% or less, V: 0.005% or more and 0.1% or less, Al: 0.01% or more and 0.08% or less, N: 0.00.
- a cast slab containing 0001% or more and 0.0070% or less, the balance being Fe and inevitable impurities, and having a thickness of 5.5 to 50 times the final plate thickness is 1250 ° C.
- the method for producing a steel sheet having excellent low-temperature toughness of the base material and the weld heat-affected zone and having low strength anisotropy as described in (3) above is further mass%, and Nb: 0.005% or more and 0.03 %: Ti: 0.005% to 0.03%, Cu: 0.01% to 0.7%, B: 0.0002% to 0.05%, Ca: 0.0002% to 0 .0040% or less, REM: 0.0002% or more and 0.0040% or less of one or two or more steel compositions, the balance being Fe and inevitable impurities may be used.
- the present invention it is possible to use a steel plate having excellent base material and weld heat-affected zone toughness and low strength anisotropy.
- the weldability is improved through increased welding heat input, etc., and the degree of freedom in design increases because it is less likely to restrict the direction in which the steel plate is used. I can say that.
- Ni segregation ratio has a relationship between Ni segregation ratio and weld heat affected zone toughness. It is a graph which shows the influence of the heating temperature and holding time of the 1st hot rolling which influence on Ni segregation ratio. It is a graph which shows the relationship between Ni segregation ratio and the reduction rate in the 1st hot rolling. It is a graph which shows the relationship between Ni segregation ratio and the temperature before the last 1 pass in the first hot rolling. It is a graph which shows the relationship between an effective crystal grain size and base material toughness. It is a graph which shows the relationship between a prior-austenite grain flatness and a 0.2% yield strength difference. It is a graph which shows the relationship between an effective crystal grain diameter and the heating temperature of the 2nd hot rolling.
- the present invention will be described in detail.
- the inventors diligently studied the conditions under which the Ni-added steel used at about ⁇ 70 ° C. is excellent in base metal toughness and weld heat-affected zone toughness and has low strength anisotropy.
- it is necessary to use hot rolling twice in the manufacturing process it is necessary to use a cast slab having a thickness that can ensure a sufficient reduction ratio as a whole, and heating conditions in each hot rolling, It was clarified that it is necessary to strictly control the reduction ratio and temperature.
- Each of the two hot rollings has a role.
- the primary role of the first hot rolling is to reduce the band-like Ni segregation unique to the Ni-containing hot-rolled steel sheet, and the main role of the second hot rolling is to form a quenched structure, It is miniaturization and suppression of tissue flattening.
- the present invention it is most important to use a cast slab having a thickness that can be sufficiently reduced by two hot rollings.
- the present inventors conducted tests for evaluating the base material toughness and the weld heat affected zone toughness on various steel plates manufactured by one or two hot rolling. As a result, as shown in Table 1, two properties are excellent only when the hot rolling is performed twice and the total reduction ratio obtained by dividing the cast slab thickness by the product plate thickness is 5.5 or more. I found out. On the other hand, when the total reduction ratio exceeds 50, the productivity is remarkably lowered. Therefore, the total reduction ratio in the present invention is defined as 5.5 or more and 50 or less.
- the total rolling reduction is 7.5 or more, the base material and the weld heat affected zone toughness are further improved, so the total rolling reduction is desirably 7.5 or more and 50 or less. Furthermore, when the total rolling reduction is 10 or more, the base material and the weld heat affected zone toughness are further improved, so that the total rolling reduction is more preferably 10 or more and 50 or less.
- Table 1 the evaluation of the base material toughness was OK when it was 150 J or more, and NG when it was less than 150.
- the case of 100 J or more was determined as OK, and the case of less than 100 J was determined as NG. Comprehensive judgment makes OK when both evaluations are OK, and makes NG when one or both evaluations are NG.
- the details of the first hot rolling will be described.
- the main purpose of the first hot rolling is to improve the weld heat-affected zone toughness by reducing the band-like Ni segregation characteristic of Ni-added hot-rolled steel sheets.
- the inventors diligently investigated the cause of the low temperature toughness of the Ni-added steel used at about -70 ° C., particularly the weld heat affected zone toughness when high efficiency welding is performed. As a result, it has been found that the band-like Ni segregation is one factor in reducing the toughness of the weld heat affected zone.
- the band-like Ni segregation is a Ni-segregated at the time of solidification formed into a band-like form parallel to the rolling direction by hot rolling. As the band-like Ni segregation develops, a region having a low Ni concentration is formed locally, so that the weld heat affected zone toughness is lowered.
- the present inventors investigated the relationship between Ni segregation ratio and weld heat affected zone toughness.
- a Charpy specimen having a thickness of 32 mm was taken from a welded joint prepared under a heat input of 29 to 30 kJ / mm by SMAW (Shield Metal Arc Weld), and Charpy impact absorption energy was evaluated at -70 ° C.
- the notch part of the Charpy test piece was made to correspond to the bond part.
- FIG. 1 when the Ni segregation ratio of a portion (hereinafter referred to as a 1/4 t portion) having a thickness of 1/4 from the steel sheet surface in the thickness direction of the steel sheet is 1.3 or less.
- the 1/4 se portion Ni segregation ratio in the present invention is defined as 1.3 or less.
- the segregation ratio of the 1/4 t part is 1.2 or less, the weld heat-affected zone toughness is more excellent. Therefore, the Ni segregation ratio is desirably 1.2 or less.
- the segregation ratio of the 1/4 t part is 1.1 or less, the weld heat-affected zone toughness is further excellent. Therefore, the Ni segregation ratio is more preferably 1.1 or less.
- the segregation ratio of the 1/4 t part can be measured by EPMA (Electron Probe Micro Analyzer).
- the Ni amount data of 400 points is obtained at intervals of 5 ⁇ m over a length of 2 mm in the plate thickness direction, centering on a portion that enters inside by 1 ⁇ 4 from the surface in the plate thickness direction from the surface of the steel plate.
- the average of the remaining 390 points is used as an average value, and 10 points in order from the largest of the 390 points.
- the average value is the maximum value.
- divided the maximum value by the average value is made into the segregation ratio of 1 / 4t part.
- the lower limit of the segregation ratio is not particularly defined because of the need for weld heat affected zone toughness, but is 1.0 in the calculation.
- the toughness of the weld heat affected zone is excellent in the toughness of the weld heat affected zone at ⁇ 70 ° C. as described above, and the absorbed energy is 100 J or more in the weld heat affected zone Charpy test at ⁇ 70 ° C. It means 100J or more.
- the heating temperature refers to the surface temperature of the slab before passing through the first pass.
- the holding time refers to the time from when the slab surface reaches the heating temperature until 3 hours have elapsed until the heating furnace is extracted.
- the higher the temperature and the longer the holding time the smaller the Ni segregation ratio due to diffusion.
- the inventors investigated the influence of the combination of the heating temperature and holding time of the first hot rolling on the segregation ratio. Specifically, the first hot rolling was performed under conditions of a reduction ratio of 2.0 and a temperature of 1020 ° C. before the last one pass. As a result, as shown in FIG.
- the heating temperature of the first hot rolling is defined as 1250 ° C. or more and the holding time is defined as 8 hours or more. Note that when the heating temperature is 1380 ° C. or more and the holding time is 50 hours, the productivity is greatly reduced. Therefore, the upper limit of the heating temperature is 1380 ° C. and the holding time is 50 hours or less. If the heating temperature is 1300 ° C. or more and the holding time is 20 hours or more, the Ni segregation ratio is further reduced. Therefore, the heating temperature is preferably 1300 ° C. or more and the holding time is 20 hours or more.
- the segregation reduction effect can be expected even during the first hot rolling biting and air cooling after rolling. This is because, when recrystallization occurs, the effect of reducing segregation through grain boundary movement is present, and when recrystallization does not occur, there is an effect of reducing segregation through diffusion under a high dislocation density. For this reason, the band-like Ni segregation ratio decreases as the rolling reduction ratio of the first hot rolling increases.
- the inventors investigated the influence of the reduction ratio of the first hot rolling on the segregation ratio. Specifically, the first hot rolling was performed under the conditions of a heating temperature of 1280 ° C., a holding time of 10 hours, and a temperature before the final one pass of 1020 ° C. As a result, as shown in FIG.
- the reduction ratio in order to achieve a Ni segregation ratio of 1.3 or less, it was discovered that the reduction ratio must be 1.2 or more. On the other hand, when the rolling ratio exceeds 10, the productivity is significantly reduced. Therefore, the reduction ratio in the first hot rolling is defined as 1.2 or more and 10 or less. In addition, since the segregation ratio becomes smaller when the reduction ratio is 2.0 or more, the reduction ratio is desirably 2.0 or more and 10 or less.
- the temperature before the last one pass in the first hot rolling It is also very important to control the temperature before the last one pass in the first hot rolling to an appropriate temperature. This is because if the temperature before the final one pass is too low, diffusion does not progress during air cooling after the rolling is completed, and the segregation ratio becomes low. Conversely, if the temperature before the final one pass is too high, the dislocation density decreases rapidly due to recrystallization. This is because the diffusion effect under high dislocation density at the time of air cooling after the end of rolling is lowered, and the segregation ratio is lowered. In the first hot rolling, there is a temperature range in which dislocations remain moderately and diffusion is likely to proceed. The present inventors investigated the relationship between the temperature before the last one pass and the segregation ratio in the first hot rolling.
- the first hot rolling was performed under the conditions of a heating temperature of 1290 ° C. for the first hot rolling, a holding time of 10 hours, and a temperature before the final one pass of 1020 ° C.
- the temperature before the last one pass in the first hot rolling is defined as 800 ° C. or more and 1250 ° C. or less.
- the temperature before the last one pass in the first hot rolling is desirably 950 ° C.
- the steel slab surface temperature at the time of transition is set to 300 ° C. or lower.
- the heating temperature refers to the temperature of the slab surface.
- the holding time refers to the time from when the slab surface reaches the heating temperature until 3 hours have elapsed until the heating furnace is extracted.
- the reduction ratio is a value obtained by dividing the plate thickness before rolling by the plate thickness after rolling.
- the temperature before the last pass is the temperature of the slab surface measured immediately before the final pass of rolling, and can be measured with a radiation thermometer or the like. With air cooling, the surface temperature of the steel sheet is between 800 ° C. and 500 ° C., and the cooling rate is 5 ° C./s or less.
- the main purpose of the second hot rolling is to secure strength by generating a quenched structure, to improve base material toughness by making the structure finer, and to reduce strength anisotropy by reducing the structure flatness.
- the Vickers hardness is less than 265 HV, a steel plate having a large thickness is required, resulting in a decrease in fuel consumption and an increase in welding construction cost due to an increase in the weight of the structure.
- the Vickers hardness exceeds 310 HV, the weld heat-affected zone toughness decreases, making it impossible to apply highly efficient welding.
- the Vickers hardness is defined as 265HV or more and 310HV or less.
- the Vickers hardness is an average of five points measured at a load of 10 kgf at a portion of the sample cut out in a plane parallel to the rolling direction and the plate thickness direction of the steel plate and entering from the steel plate surface by 1 ⁇ 4 of the plate thickness. Value.
- the main component of the structure is martensite, and the effective grain size corresponds to the region surrounded by the large-angle grain boundary, that is, the effective crystal grain size, and with the refinement of the effective crystal grain size, Base material toughness is improved.
- the present inventors obtained the relationship shown in FIG.
- the effective crystal grain size exceeds 10 ⁇ m, the base material toughness decreases, so the effective crystal grain size is defined as 10 ⁇ m or less.
- the smaller the effective crystal grain size the better.
- the lower limit value of the effective crystal grain size is set to 1 ⁇ m.
- the effective crystal grain size is desirably 1 ⁇ m or more and 6 ⁇ m or less.
- the base material toughness is further improved when the effective crystal grain size is 3 ⁇ m or less, the effective crystal grain size is more preferably 1 ⁇ m or more and 3 ⁇ m or less.
- the effective crystal grain size can be estimated by observing the vicinity of the brittle fracture starting point of the fracture surface after the Charpy test, quantifying the area of a number of cleavage cleavage surfaces, and calculating the average equivalent circle diameter.
- excellent base material toughness means that the absorbed energy at ⁇ 70 ° C. in the Charpy test of the weld heat affected zone is 150 J or more.
- the strength anisotropy is evaluated by the difference in 0.2% proof stress between the test piece taken perpendicular to the rolling direction and the test piece taken parallel to the rolling direction, and the strength anisotropy is small. It means that the 0.2% yield strength difference is 50 MPa or less.
- the flatness of the prior austenite is defined as 3.0 or less.
- the productivity is greatly reduced. Therefore, the lower limit of the prior austenite flatness is defined as 1.05.
- the flatness of the prior austenite is desirably 1.05 or more and 1.6 or less.
- the flatness of prior austenite is desirably 1.05 or more and 1.2 or less.
- the flatness of the prior austenite grains is calculated as follows. That is, a sample cut in a plane parallel to the rolling direction and the plate thickness direction of the steel plate enters the interior of the steel plate by a quarter of the plate thickness using an eyepiece with a mesh in an optical microscope. Observe the structure and calculate the ratio of the number of old austenite grain boundaries crossing the line in the rolling longitudinal direction and the number of old austenite grain boundaries crossing the line in the thickness direction perpendicular to the rolling direction at the same distance. The flatness of the prior austenite grains can be obtained.
- the heating temperature in the second hot rolling is defined as 900 ° C. or more and 1270 ° C. or less.
- the heating temperature in the second hot rolling is desirably 900 ° C. or higher and 1120 ° C. or lower.
- the holding time during heating in the second hot rolling is not particularly defined, it is preferably 2 hours or more and 10 hours or less from the viewpoint of uniform heating and ensuring productivity.
- the reduction ratio in the second hot rolling is also important. As the reduction ratio increases, the effective crystal grain size decreases through recrystallization or increase in dislocation density.
- the present inventors investigated the relationship between the effective crystal grain size and the reduction ratio. As a result, as shown in FIG. 8, it was discovered that the rolling ratio needs to be 2.0 or more in order to make the effective crystal grain size 10 ⁇ m or less. In addition, when the rolling ratio exceeds 40, the productivity is significantly reduced. Therefore, the reduction ratio in the second hot rolling is defined as 2.0 or more and 40 or less. In addition, when the reduction ratio in the second hot rolling is 10 or more, the effective crystal grain size is further refined, and thus the reduction ratio is desirably 10 or more and 40 or less.
- the temperature before the final first pass of the second hot rolling is also important.
- the temperature before the final pass decreases, the flatness of the prior austenite increases, and when it increases, the effective crystal grain size increases.
- the present inventors examined the temperature before the last pass for setting the flatness of prior austenite to 3.0 or less and the effective crystal grain size to 10 ⁇ m or less.
- the prior austenite flatness increases when the temperature before the last pass is less than 680 ° C.
- the effective crystal grain size increases when the temperature exceeds 1000 ° C. as shown in FIG. I found it.
- the temperature before the last pass in the second hot rolling step is defined as 680 ° C. or more and 1000 ° C. or less.
- the temperature before the last pass is 800 ° C. or more and 920 ° C. or less, the prior austenite flatness and the effective crystal grain size become smaller. Therefore, the temperature before the last pass is desirably 800 ° C. or more and 920 ° C. or less.
- Si is an element essential for ensuring strength
- its addition amount is set to 0.02% or more.
- an increase in the amount of Si causes a decrease in weldability, so the upper limit is made 0.40%.
- Mn is an element effective for increasing the strength, and it is necessary to add 0.5% or more at a minimum. Conversely, if it exceeds 1.0%, temper embrittlement susceptibility becomes high and brittle fracture resistance Decreases. Therefore, the addition amount of Mn is defined as 0.5% or more and 1.0% or less.
- the addition amount of P is defined as 0.0010 or more and 0.0100% or less.
- the addition amount of S is defined as 0.0001% or more and 0.0050% or less.
- Ni is an element effective for improving the brittle fracture resistance. If it is less than 2.0%, the improvement in brittle fracture resistance is small, and if it exceeds 4.5%, the production cost increases. Therefore, the addition amount of Ni is defined as 2.0% or more and 4.5% or less. Note that when the Ni content is 3.6% or less, the alloy cost is further reduced. Therefore, the Ni addition amount is desirably 2.0% or more and 3.6% or less.
- Cr is an element effective for increasing the strength and needs to be added at least 0.1%, but conversely if added over 1.0%, the weld heat-affected zone toughness decreases. Therefore, the addition amount of Cr is defined as 0.1% or more and 1.0% or less.
- Mo Mo is an element effective for increasing the strength without increasing the susceptibility to temper embrittlement. If the addition amount is less than 0.1%, the effect of increasing the strength is small, and if it exceeds 0.6%, the manufacturing cost increases and the weld heat affected zone toughness decreases. Therefore, the addition amount of Mo is defined as 0.1% or more and 0.6% or less. Since the manufacturing cost can be further reduced when the Mo amount is 0.3% or less, the Mo amount is desirably 0.1% or more and 0.3% or less.
- V is an element effective for securing strength. If the addition is less than 0.005%, the effect is small, and if it exceeds 0.1%, the weld heat-affected zone toughness is lowered. Therefore, the addition amount of V is defined as 0.005% or more and 0.1% or less.
- Al is an element effective as a deoxidizing material, and if it is added less than 0.01%, deoxidation is insufficient and the base metal toughness is reduced, and if it exceeds 0.08%, the toughness of the heat affected zone of the weld is affected. Incurs a decline. Therefore, the addition amount of Al is defined as 0.01% or more and 0.08% or less.
- N is less than 0.0001%, productivity decreases due to an increase in the refining load, and when it exceeds 0.007%, the base material toughness and the weld heat affected zone toughness decrease. Therefore, the addition amount of N is defined as 0.0001% or more and 0.007% or less.
- Nb is an element effective for securing strength. If the addition is less than 0.005%, the effect is small, and if it exceeds 0.03%, the weld heat-affected zone toughness is lowered. Therefore, the amount of Nb added is specified to be 0.005% or more and 0.03% or less.
- Ti is an element effective for improving toughness. If the addition is less than 0.005%, the effect is small, and if it exceeds 0.03%, the weld heat-affected zone toughness is lowered. Therefore, the addition amount of Ti is defined as 0.005% or more and 0.03% or less.
- Cu is an element effective for securing strength. If the addition is less than 0.01%, the effect is small, and if it exceeds 0.7%, the weld heat affected zone toughness is lowered. Therefore, the addition amount of Cu is defined as 0.01% or more and 0.7% or less.
- B is an element effective for securing strength. If the addition is less than 0.0002%, the effect is small, and if it exceeds 0.05%, the toughness of the base material is lowered. Therefore, the addition amount of B is defined as 0.0002% or more and 0.05% or less.
- Ca is an element effective for preventing nozzle clogging. If the addition is less than 0.0002%, the effect is small, and if the addition exceeds 0.0040%, the toughness is reduced. Therefore, the addition amount of Ca is defined as 0.0002% or more and 0.0040% or less.
- REM is an element effective for improving the toughness of the weld heat affected zone. If the addition is less than 0.0002%, the effect is small, and if the addition exceeds 0.0040%, the toughness is reduced. Therefore, the amount of REM added is defined as 0.0002% or more and 0.0040% or less.
- Zn, Sn, Sb, Zr, Mg, etc. that may be mixed as raw materials including additive alloys or inevitable impurities eluted from furnace materials during melting when the steel of the present invention is melted If the amount is less than 0.002%, the effect of the present invention is not impaired.
- Table 2 shows the plate thickness, chemical composition, manufacturing method, Ni segregation ratio, Vickers hardness, effective crystal grain size, and prior austenite grain flatness of Examples 1 to 13 and Comparative Examples 1 to 13.
- Table 3 shows the plate thickness, chemical composition, production method, Ni segregation ratio, Vickers hardness, effective crystal grain size, and prior austenite grain flatness of Examples 14 to 26 and Comparative Examples 14 to 26.
- Table 4 shows the evaluation results of the characteristics.
- the tempering was performed in the range of 630 ° C to 680 ° C.
- Yield stress and tensile strength were measured by a metal material tensile test method described in JIS Z 2241.
- the test piece is a metal material tensile test piece described in JIS Z 2201, No. 5 test piece from a steel plate having a thickness of 20 mm or less, and No. 10 test taken from a 1/4 t portion from the steel plate surface from a steel plate having a thickness of 40 mm or more. A piece was used. The specimens were collected so that the longitudinal direction was parallel and perpendicular to the rolling direction. They are called the L direction and the C direction, respectively. The yield stress was 0.2% proof stress calculated by the offset method. Two tests were performed at room temperature, and an average value was adopted.
- the anisotropy of strength was evaluated by the difference between the yield stress in the C direction and the yield stress in the L direction. The case where the pressure was 50 MPa or less was determined to be OK, and the case where it exceeded 50 MPa was determined to be NG.
- Charpy impact absorption energy was measured by a metal material impact test method described in JIS Z 2242.
- the test piece was a metal material impact test piece described in JIS Z 2202, and a test piece having a width of 10 mm was taken from t / 4 part.
- a test piece having a width of 5 mm was taken from a steel plate having a thickness of 6 mm.
- Each of the shapes was a V-notch test piece, and the sample was taken so that the line formed by the notch bottom was parallel to the plate thickness direction and the longitudinal direction of the test piece was perpendicular to the rolling direction.
- the test temperature was ⁇ 70 ° C., and the average value of three tests was adopted.
- Evaluation of the weld heat affected zone toughness was performed by collecting Charpy test pieces from a welded joint prepared by SMAW.
- the SMAW conditions were heat input of 1.5 to 2.0 kJ / cm, preheating and interpass temperature of 100 ° C. or less.
- the notch part of the Charpy test piece was made to correspond to the bond part.
- the test temperature was ⁇ 70 ° C., and the average value of three tests was adopted.
- 100J or more was OK and less than 100J was NG.
- Example 1 a steel plate having a thickness of 12 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 1 in which a steel plate was produced with the same components and production method as in Example 1, the holding time and Ni segregation ratio in the first hot rolling were out of the scope of the present invention. was inferior in weld heat affected zone toughness.
- Example 2 a band-like segregation was controlled to produce a steel plate having a thickness of 25 mm. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 2 in which a steel plate was produced with the same components and production method as in Example 2, the heating temperature and segregation ratio in the first hot rolling were out of the scope of the present invention.
- the weld heat-affected zone toughness was poor.
- steel sheets having a thickness of 50 mm were manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 3 in which a steel plate was produced with the same components and production method as in Example 3, the reduction ratio and segregation ratio in the first hot rolling were out of the scope of the present invention.
- the weld heat-affected zone toughness was poor.
- Example 4 a steel plate having a thickness of 12 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 4 in which a steel sheet was produced with the same components and production method as in Example 4, the Si amount and the P amount were outside the scope of the present invention. The toughness was inferior.
- Example 5 a steel plate having a thickness of 25 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 5 in which a steel sheet was produced with the same components and production method as in Example 5, the amount of Ni deviated from the scope of the present invention. Therefore, the steel sheet of Comparative Example 5 was in base metal toughness and weld heat affected zone toughness. It was inferior.
- Example 6 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped NI segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 6 in which the steel sheet was produced with the same components and production method as in Example 6, the temperature before the last one pass and the prior austenite grain flatness in the second hot rolling were out of the scope of the present invention.
- the steel plate of Comparative Example 6 had a large strength anisotropy.
- a steel plate having a thickness of 12 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 7 in which the steel sheet was manufactured with the same components and manufacturing method as in Example 7, the temperature before the last one pass and the effective crystal grain size in the second hot rolling were out of the scope of the present invention.
- the steel plate of Example 7 was inferior in the base material toughness.
- a band-like segregation was controlled to produce a steel plate with a plate thickness of 25 mm.
- This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 8 in which a steel sheet was produced with the same components and production method as in Example 8, the C amount and Vickers hardness were outside the scope of the present invention. It was inferior to the affected area toughness.
- Example 9 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 9 in which a steel plate was produced with the same components and production method as in Example 9, the Mn content was outside the scope of the present invention, so the steel plate of Comparative Example 9 was inferior in the base material toughness.
- Example 10 a steel plate having a thickness of 25 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 10 in which the steel sheet was manufactured with the same components and manufacturing method as in Example 10, the temperature and the segregation ratio before the last one pass of the first hot rolling were out of the scope of the present invention. The steel plate was inferior in weld heat affected zone toughness.
- Example 11 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 11 in which the steel sheet was produced with the same components and production method as in Example 11, the heating temperature and effective crystal grain size in the second hot rolling were out of the scope of the present invention. The steel sheet was inferior in the base metal toughness.
- Example 12 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 12 in which the steel sheet was produced with the same components and production method as in Example 12, the reduction ratio and effective crystal grain size in the second hot rolling were out of the scope of the present invention.
- the steel sheet was inferior in the base metal toughness.
- Example 13 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 13 in which a steel plate was produced with the same components and production method as in Example 13, the reduction ratio and Ni segregation ratio in the first hot rolling were out of the scope of the present invention. Was inferior in weld heat affected zone toughness.
- Example 14 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 14 in which a steel sheet was produced with the same components and production method as in Example 14, the total rolling reduction ratio was outside the scope of the present invention, so the steel sheet of Comparative Example 14 was inferior in the base material toughness.
- Example 15 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 15 in which the steel sheet was produced with the same components and production method as in Example 15, the total reduction ratio, the reduction ratio in the second hot rolling, and the effective crystal grain size were out of the scope of the present invention.
- the steel plate of Comparative Example 15 was remarkably inferior in the base material toughness.
- Example 16 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Example 16 in which the steel sheet was produced with the same components and production method as in Example 16, the total reduction ratio, the reduction ratio in the first hot rolling, and the Ni segregation ratio were out of the scope of the present invention.
- the steel plate of Example 16 was inferior in base metal toughness and weld heat affected zone toughness.
- Example 17 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 17 in which the steel sheet was produced with the same components and production method as in Example 17, the total reduction ratio, the reduction ratio in the first hot rolling, the reduction ratio in the second hot rolling, the Ni segregation ratio, Since the effective crystal grain size was out of the range of the present invention, the steel plate of Comparative Example 17 was inferior in the base metal toughness and the weld heat affected zone toughness.
- Example 18 a steel plate having a thickness of 12 mm was manufactured by controlling band-shaped segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 18 in which a steel sheet was manufactured using the same components and manufacturing method as in Example 18, the amount of Mo was outside the scope of the present invention, so the steel sheet of Comparative Example 18 was inferior in weld heat affected zone toughness.
- Example 19 a steel plate having a thickness of 25 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 19 in which the steel sheet was manufactured with the same components and manufacturing method as in Example 19, the temperature before the last one pass of the first hot rolling and the Ni segregation ratio were out of the scope of the present invention.
- Example 20 a steel plate having a thickness of 25 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 20 in which the steel sheet was manufactured with the same components and manufacturing method as in Example 20, the S content and the Cr content were out of the scope of the present invention. The toughness was inferior.
- Example 21 a steel plate having a thickness of 50 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 21 in which a steel sheet was manufactured with the same components and manufacturing method as in Example 21, the V content and Al content were outside the scope of the present invention.
- the toughness was inferior.
- Example 22 a steel plate having a thickness of 25 mm was manufactured by controlling band-shaped segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 22 in which a steel sheet was produced with the same components and production method as in Example 22, the reduction ratio and effective crystal grain size in the second hot rolling were out of the scope of the present invention.
- the steel sheet was inferior in the base metal toughness.
- Example 23 a steel plate having a thickness of 25 mm was manufactured by controlling band-shaped Ni segregation. This steel plate was excellent in base material toughness and weld heat-affected zone toughness and small in strength anisotropy.
- Comparative Example 23 in which the steel sheet was produced with the same components and production method as in Example 23, the amount of N, Vickers hardness and the time from the end of rolling in the second hot rolling to the start of water cooling fall within the scope of the present invention. Since it came off, the steel plate of Comparative Example 23 was inferior in the base metal toughness.
- band-shaped segregation was controlled to produce a steel plate having a thickness of 40 mm. This steel plate was excellent in base metal toughness and weld heat affected zone toughness.
- Comparative Example 24 in which a steel sheet was produced with the same components and production method as in Example 24, the total rolling reduction ratio was outside the scope of the present invention, so the steel sheet of Comparative Example 24 was inferior in the base material toughness.
- Example 25 band-shaped segregation was controlled to produce a steel plate having a thickness of 40 mm. This steel plate was excellent in base metal toughness and weld heat affected zone toughness.
- Comparative Example 25 in which the steel sheet was produced with the same components and production method as in Example 25, the time from the end of the second hot rolling to the start of water cooling and the Vickers hardness deviated from the scope of the present invention.
- the steel plate of Comparative Example 25 was inferior in the base material toughness.
- Example 26 band-shaped segregation was controlled to produce a steel plate having a thickness of 40 mm. This steel plate was excellent in base metal toughness and weld heat affected zone toughness.
- Comparative Example 26 in which a steel plate was produced with the same components and production method as in Example 26, the water cooling end temperature and Vickers hardness were outside the scope of the present invention, so the steel plate of Comparative Example 26 was inferior in the base metal toughness. It was. From the above examples, it is clear that the steel plates of Examples 1 to 26, which are thick steel plates manufactured according to the present invention, are steel plates having excellent weld heat affected zone toughness and small strength anisotropy.
- the present invention it is possible to use a steel plate having excellent base material and weld heat-affected zone toughness and low strength anisotropy.
- the welding workability is improved through an increase in welding heat input and the like, and the degree of freedom in design is increased because the direction in which the steel plate is used is less likely to occur.
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Abstract
Description
本願は、2008年10月1日に、日本に出願された特願2008-256122号及び、2009年1月5日に、日本に出願された特願2009-000202号に基づき優先権を主張し、その内容をここに援用する。
つまり、Niを含有する鋼材で、母材および溶接熱影響部の靱性に優れ、かつ強度異方性が小さい鋼材を製造することは、既存技術では困難である。
(1)本発明の第一実施態様は、鋼が、質量%で、C:0.04%以上0.10%以下、Si:0.02%以上0.40%以下、Mn:0.5%以上1.0%以下、P:0.0010%以上0.0100%以下、S:0.0001%以上0.0050%以下、Ni:2.0%以上4.5%以下、Cr:0.1%以上1.0%以下、Mo:0.1%以上0.6%以下、V:0.005%以上0.1%以下、Al:0.01%以上0.08%以下、N:0.0001%以上0.0070%以下を含有し、残部がFe及び不可避的不純物からなる鋼組成であり、鋼板表面から鋼板厚さ方向に鋼板厚さの1/4だけ入った部位のNi偏析比が1.3以下であり、旧オーステナイトの扁平度が1.05以上3.0以下であり、有効結晶粒径が10μm以下であり、ビッカース硬さが265HV以上310HV以下であることを特徴とする、母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板である。
本発明者らは、-70℃程度で使用されるNi添加鋼が母材靱性と溶接熱影響部靱性に優れ、かつ強度異方性が小さくなる条件を鋭意検討した。その結果、製造工程に2回の熱間圧延が必要であること、全体で十分な圧下比を確保できる厚さの鋳造スラブを使用する必要があること、さらにそれぞれの熱間圧延において加熱条件、圧下比や温度などを厳格に制御する必要があることを明らかにした。2回の熱間圧延にはそれぞれ役割がある。即ち、1回目の熱間圧延の主要な役割はNi含有熱延鋼板特有のバンド状Ni偏析を低減することであり、2回目の熱間圧延の主要な役割は焼き入れ組織の生成、組織の微細化、組織の扁平化抑制である。
なお、加熱温度とは、スラブ表面の温度を指す。また、保持時間とは、スラブ表面が加熱温度に到達後3時間経過してから、加熱炉抽出までの時間を指す。圧下比とは、圧延前の板厚を圧延後の板厚で除した値である。また、最終1パス前温度とは、圧延の最終パスの噛込直前に測定されたスラブ表面の温度であり、放射温度計などにより測定が可能である。空冷とは、鋼板の表面温度が800℃から500℃の間で冷却速度が5℃/s以下であるものとする。
溶接構造物用の材料として使用するために、焼き入れ組織の生成による強度確保が必要である。ビッカース硬さが265HVを下回ると板厚の大きい鋼板が必要となって構造物の重量増大による燃費低下、溶接施工コストの増大が生じる。一方ビッカース硬さが310HVを超えると溶接熱影響部靱性が低下して高能率な溶接の適用が不可能になる。よって、ビッカース硬さを265HV以上310HV以下と規定する。なお、ビッカース硬さは鋼板の圧延方向と板厚方向に平行な面で切り出したサンプルの、鋼板表面から板厚の1/4だけ内部に入った部位で、荷重10kgfで5点測定された平均値をいう。
2回目の熱間圧延では、母材の靱性を高めるため、組織の微細化が必要である。本発明の強度範囲では、組織の主体はマルテンサイトであり、実効的な粒径は大角粒界で囲まれた領域、すなわち有効結晶粒径に対応し、有効結晶粒径の微細化に伴って母材靱性が向上する。本発明者らは、有効結晶粒径と母材靱性の関係を調査した結果、図5に示すような関係を得た。有効結晶粒径が10μm超では、母材靱性が低下することから、有効結晶粒径を10μm以下と規定する。有効結晶粒径は小さいほど望ましいが、有効結晶粒径が1μm未満では生産性が著しく低下することから、有効結晶粒径の下限値は1μmとする。なお、有効結晶粒径が6μm以下では母材靱性がより向上することから、望ましくは有効結晶粒径を1μm以上6μm以下とする。さらに、有効結晶粒径が3μm以下では母材靱性がさらに向上することから、さらに望ましくは有効結晶粒径を1μm以上3μm以下とする。なお、有効結晶粒径は、シャルピー試験後の破面の脆性破壊起点近傍を観察し、多数のへき開破面の面積を定量化して、平均の円相当径を算出することで推定が可能である。本発明で母材靱性に優れるとは、溶接熱影響部のシャルピー試験の-70℃での吸収エネルギーが150J以上であることとする。
また、2回目の熱間圧延の最終1パス前温度も重要である。最終1パス前温度が低くなると旧オーステナイトの扁平度が大きくなり、高くなると有効結晶粒径が大きくなる。本発明者らは、旧オーステナイトの扁平度を3.0以下として、かつ有効結晶粒径を10μm以下とするための最終1パス前温度を検討した。その結果、図9に示すように、最終1パス前温度を680℃未満にすると旧オーステナイト扁平度が大きくなり、図10に示すように、1000℃超とすると有効結晶粒径が増大することを見いだした。よって、2回目の熱間圧延工程における最終1パス前温度を680℃以上1000℃以下と規定する。なお、最終1パス前温度を800℃以上920℃以下とすると、旧オーステナイト扁平度と有効結晶粒径がより小さくなることから、望ましくは最終1パス前温度を800℃以上920℃以下とする。
Cは、強度確保に必須の元素であるため、その添加量を0.04%以上とする。しかし、一方でC量の増大は粗大析出物の生成による母材靱性の低下や溶接性の低下を招くためその上限を0.10%とする。
Nbは強度確保に有効な元素である。0.005%未満の添加では効果が小さく、0.03%超の添加では溶接熱影響部靱性の低下を招く。よって、Nbの添加量を0.005%以上0.03%以下と規定する。
母材靭性は、JIS Z 2242に記載の金属材料衝撃試験方法によりシャルピー衝撃吸収エネルギーを測定した。試験片は、JIS Z 2202に記載の金属材料衝撃試験片とし、t/4部から幅10mmの試験片を採取した。板厚6mmの鋼板は幅5mmの試験片を採取した。形状はいずれもVノッチ試験片とし、ノッチ底のなす線が板厚方向と平行になるように、また試験片の長手方向が圧延方向と垂直になるように採取した。試験温度は-70℃とし、3本の試験を行った平均値を採用した。シャルピー衝撃吸収エネルギーの必要値は、船舶において多く使用される条件の150J以上を採用し、150J以上をOK、150J未満をNGとした。
溶接熱影響部靭性の評価は、SMAWで作成した溶接継手からシャルピー試験片を採取して行った。SMAWの条件は、入熱1.5~2.0kJ/cm、予熱およびパス間温度100℃以下とした。シャルピー試験片のノッチ部はボンド部に対応させた。試験温度は-70℃とし、3本の試験を行った平均値を採用した。溶接継手のシャルピー試験の場合、100J以上をOK、100J未満をNGとした。
実施例2では、バンド状偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例2と類似の成分および製造方法で鋼板を製造した比較例2では、1回目の熱間圧延における加熱温度と偏析比が本発明の範囲を外れたため、この比較例2の鋼板は溶接熱影響部靭性に劣っていた。
実施例3では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例3と類似の成分および製造方法で鋼板を製造した比較例3では、1回目の熱間圧延における圧下比と偏析比が本発明の範囲を外れたため、この比較例3の鋼板は溶接熱影響部靭性に劣っていた。
実施例4では、バンド状Ni偏析を制御して板厚12mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例4と類似の成分および製造方法で鋼板を製造した比較例4では、Si量とP量が本発明の範囲を外れたため、この比較例4の鋼板は母材靱性と溶接熱影響部靱性に劣っていた。
実施例5では、バンド状Ni偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例5と類似の成分および製造方法で鋼板を製造した比較例5では、Ni量が本発明の範囲を外れたため、この比較例5の鋼板は母材靱性と溶接熱影響部靭性に劣っていた。
実施例6では、バンド状NI偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例6と類似の成分および製造方法で鋼板を製造した比較例6では、2回目の熱間圧延における最終1パス前温度と旧オーステナイト粒扁平度が本発明の範囲を外れたため、この比較例6の鋼板は強度異方性が大きかった。
実施例7では、バンド状Ni偏析を制御して板厚12mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例7と類似の成分および製造方法で鋼板を製造した比較例7では、2回目の熱間圧延における最終1パス前温度と有効結晶粒径が本発明の範囲を外れたため、この比較例7の鋼板は母材靱性に劣っていた。
実施例8では、バンド状偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例8と類似の成分および製造方法で鋼板を製造した比較例8では、C量、ビッカース硬さが本発明の範囲を外れたため、この比較例8の鋼板は母材靱性と溶接熱影響部靱性に劣っていた。
実施例9では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例9と類似の成分および製造方法で鋼板を製造した比較例9では、Mn量が本発明の範囲を外れたため、この比較例9の鋼板は母材靱性に劣っていた。
実施例10では、バンド状Ni偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例10と類似の成分および製造方法で鋼板を製造した比較例10では、第一の熱間圧延の最終1パス前温度と偏析比が本発明の範囲を外れたため、この比較例10の鋼板は溶接熱影響部靱性に劣っていた。
実施例11では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例11と類似の成分および製造方法で鋼板を製造した比較例11では、2回目の熱間圧延における加熱温度と有効結晶粒径が本発明の範囲を外れたため、この比較例11の鋼板は母材靱性に劣っていた。
実施例12では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例12と類似の成分および製造方法で鋼板を製造した比較例12では、2回目の熱間圧延における圧下比、有効結晶粒径が本発明の範囲を外れたため、この比較例12の鋼板は母材靱性に劣っていた。
実施例13では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例13と類似の成分および製造方法で鋼板を製造した比較例13では、1回目の熱間圧延における圧下比、Ni偏析比が本発明の範囲を外れたため、この比較例13の鋼板は溶接熱影響部靱性に劣っていた。
実施例14では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例14と類似の成分および製造方法で鋼板を製造した比較例14では、全圧下比が本発明の範囲を外れたため、この比較例14の鋼板は母材靱性に劣っていた。
実施例15では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例15と類似の成分および製造方法で鋼板を製造した比較例15では、全圧下比および2回目の熱間圧延における圧下比、有効結晶粒径が本発明の範囲を外れたため、この比較例15の鋼板は母材靱性が著しく劣っていた。
実施例16では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例16と類似の成分および製造方法で鋼板を製造した比較例16では、全圧下比および1回目の熱間圧延における圧下比、Ni偏析比が本発明の範囲を外れたため、この比較例16の鋼板は母材靱性と溶接熱影響部靱性に劣っていた。
実施例17では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例17と類似の成分および製造方法で鋼板を製造した比較例17では、全圧下比、1回目の熱間圧延における圧下比、2回目の熱間圧延における圧下比、Ni偏析比、有効結晶粒径が本発明の範囲を外れたため、この比較例17の鋼板は母材靱性と溶接熱影響部靱性に劣っていた。
実施例18では、バンド状偏析を制御して板厚12mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例18と類似の成分および製造方法で鋼板を製造した比較例18では、Mo量が本発明の範囲を外れたため、この比較例18の鋼板は溶接熱影響部靱性に劣っていた。
実施例19では、バンド状Ni偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例19と類似の成分および製造方法で鋼板を製造した比較例19では、1回目の熱間圧延の最終1パス前温度とNi偏析比が本発明の範囲を外れたため、この比較例19の鋼板は溶接熱影響部靱性に劣っていた。
実施例20では、バンド状Ni偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例20と類似の成分および製造方法で鋼板を製造した比較例20では、S量とCr量が本発明の範囲を外れたため、この比較例20の鋼板は母材靱性と溶接熱影響部靱性に劣っていた。
実施例21では、バンド状Ni偏析を制御して板厚50mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例21と類似の成分および製造方法で鋼板を製造した比較例21では、V量とAl量が本発明の範囲を外れたため、この比較例21の鋼板は母材靱性と溶接熱影響部靱性に劣っていた。
実施例22では、バンド状偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例22と類似の成分および製造方法で鋼板を製造した比較例22では、2回目の熱間圧延における圧下比と有効結晶粒径が本発明の範囲を外れたため、この比較例22の鋼板は母材靱性に劣っていた。
実施例23では、バンド状Ni偏析を制御して板厚25mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れ、強度異方性が小さかった。一方、実施例23と類似の成分および製造方法で鋼板を製造した比較例23では、N量、ビッカース硬さと2回目の熱間圧延での圧延終了から水冷開始までの時間が本発明の範囲を外れたため、この比較例23の鋼板は母材靱性に劣っていた。
実施例24では、バンド状偏析を制御して板厚40mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れていた。一方、実施例24と類似の成分および製造方法で鋼板を製造した比較例24では、全圧下比が本発明の範囲を外れたため、この比較例24の鋼板は母材靱性に劣っていた。
実施例25では、バンド状偏析を制御して板厚40mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れていた。一方、実施例25と類似の成分および製造方法で鋼板を製造した比較例25では、2回目の熱間圧延の終了から水冷開始までの時間とビッカース硬さが本発明の範囲を外れたため、この比較例25の鋼板は母材靱性に劣っていた。
実施例26では、バンド状偏析を制御して板厚40mmの鋼板を製造した。この鋼板は、母材靭性、溶接熱影響部靭性に優れていた。一方、実施例26と類似の成分および製造方法で鋼板を製造した比較例26では、水冷終了温度、ビッカース硬さが本発明の範囲を外れたため、この比較例26の鋼板は母材靱性に劣っていた。
以上の実施例から、本発明により製造された厚鋼板である実施例1~26の鋼板は、溶接熱影響部靱性に優れ、強度異方性が小さい鋼板であることは明白である。
Claims (4)
- 鋼が、質量%で、
C :0.04%以上0.10%以下、
Si:0.02%以上0.40%以下、
Mn:0.5%以上1.0%以下、
P:0.0010%以上0.0100%以下、
S:0.0001%以上0.0050%以下、
Ni:2.0%以上4.5%以下、
Cr:0.1%以上1.0%以下、
Mo:0.1%以上0.6%以下、
V:0.005%以上0.1%以下、
Al:0.01%以上0.08%以下、
N:0.0001%以上0.0070%以下を含有し、
残部がFe及び不可避的不純物からなる鋼組成であり、
鋼板表面から鋼板厚さ方向に鋼板厚さの1/4だけ入った部位のNi偏析比が1.3以下であり、
旧オーステナイトの扁平度が1.05以上3.0以下であり、有効結晶粒径が10μm以下であり、
ビッカース硬さが265HV以上310HV以下である
ことを特徴とする、母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板。 - さらに質量%で、
Nb:0.005%以上0.03%以下、
Ti:0.005%以上0.03%以下、
Cu:0.01%以上0.7%以下、
B:0.0002%以上0.05%以下、
Ca:0.0002%以上0.0040%以下、
REM:0.0002%以上0.0040%以下
の1種または2種以上を含有し、残部がFe及び不可避的不純物からなる鋼組成である
ことを特徴とする、請求項1に記載の母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板。 - 質量%で、
C :0.04%以上0.10%以下、
Si:0.02%以上0.40%以下、
Mn:0.5%以上1.0%以下、
P:0.0010%以上0.0100%以下、
S:0.0001%以上0.0050%以下、
Ni:2.0%以上4.5%以下、
Cr:0.1%以上1.0%以下、
Mo:0.1%以上0.6%以下、
V:0.005%以上0.1%以下、
Al:0.01%以上0.08%以下、
N:0.0001%以上0.0070%以下
を含有し、残部がFe及び不可避的不純物からなる鋼組成であり、最終板厚の5.5倍以上50倍以下の厚さを有する鋳造スラブを、1250℃以上1380℃以下に加熱し、8時間以上保持する工程と;
前記鋳造スラブに、圧下比1.2以上10.0以下、最終1パス前温度が800℃以上1250℃以下で1回目の熱間圧延を行うことで鋼片を得る工程と;
前記鋼片を300℃以下まで空冷し、その後に900℃以上1270℃以下に加熱を行う工程と;
前記鋼片に、圧下比が2.0以上40.0以下、最終1パス前温度が680℃以上1000℃以下で2回目の熱間圧延を行う工程と;
前記2回目の熱間圧延後、100秒以内に水冷を開始し、前記鋼片の表面温度が200℃以下になるまで冷却する工程と;
550℃以上720℃以下で前記鋼片の焼き戻しを行う工程と;
を有することを特徴とする、母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板の製造方法。 - さらに質量%で、
Nb:0.005%以上0.03%以下、
Ti:0.005%以上0.03%以下、
Cu:0.01%以上0.7%以下、
B:0.0002%以上0.05%以下、
Ca:0.0002%以上0.0040%以下、
REM:0.0002%以上0.0040%以下
の1種または2種以上を含有し、残部がFe及び不可避的不純物からなる鋼組成であることを特徴とする、請求項3に記載の母材および溶接熱影響部の低温靭性に優れかつ強度異方性の小さい鋼板の製造方法。
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JP2019173054A (ja) * | 2018-03-27 | 2019-10-10 | 株式会社神戸製鋼所 | 高強度高延性鋼板 |
JP7048379B2 (ja) | 2018-03-27 | 2022-04-05 | 株式会社神戸製鋼所 | 高強度高延性鋼板 |
JP7048378B2 (ja) | 2018-03-27 | 2022-04-05 | 株式会社神戸製鋼所 | 高強度高延性鋼板 |
JP2022505860A (ja) * | 2018-10-26 | 2022-01-14 | ポスコ | 極低温靭性及び延性に優れた圧力容器用鋼板及びその製造方法 |
JP7183410B2 (ja) | 2018-10-26 | 2022-12-05 | ポスコ | 極低温靭性及び延性に優れた圧力容器用鋼板及びその製造方法 |
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JP4538095B2 (ja) | 2010-09-08 |
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