Classifications

• • 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
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • 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
• • 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
• • 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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
• • 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
• • 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
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
• • 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
• • 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
• • 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

Definitions

• the present invention relates to a Ni-containing thick steel plate having excellent low-temperature toughness, and particularly to a steel plate suitable as a member for a storage tank for liquefied natural gas.
• high Ni-containing steel plates having excellent mechanical properties at low temperatures have been used in many cases for LNG storage tanks for liquefied natural gas (hereinafter referred to as LNG).
• LNG liquefied natural gas
• steel plates made of high Ni-containing steel containing 9% by mass of Ni hereinafter referred to as 9% Ni steel are often used, and have many applications.
• Non-Patent Document 1 shows that low temperature toughness is improved by reducing impurity elements such as P and S. It is described to do.
• Non-Patent Document 2 describes that low temperature toughness is improved by stabilizing retained austenite.
• Ni is an expensive metal, and it is desired to further reduce the Ni content.
• Patent Documents 1 to 3 disclose a technique for obtaining a thick steel plate having a Ni content lower than that of 9% Ni steel and having good low temperature toughness.
• the amount of austenite contained, the aspect ratio, and the average equivalent-circle particle diameter are specified and the mechanical properties are improved by manufacturing the method by satisfying them, having a predetermined chemical component. It is said.
• the toughness of the weld heat affected zone is improved if it has a predetermined chemical component and the Fe content extracted by the extraction residue method is equal to or greater than a predetermined amount after the reproducible thermal cycle test.
• patent document 3 it is supposed that the brittle crack propagation stop characteristic is improved by using a steel having a predetermined chemical component and having a specific texture.
• the present invention has been made in view of such circumstances, and an object thereof is to provide a Ni-containing thick steel plate that is inexpensive and has excellent low-temperature toughness.
• the inventors have made C, Si, Mn, P, S, Al, Ni essential elements, and further cooled to liquid nitrogen temperature.
• the residual austenite after sub-zero treatment is less than 1.7%, and the average grain size of the grains surrounded by the high-angle grain boundaries with an orientation difference of 15 ° or more is 5 ⁇ m or less in terms of the equivalent circle diameter.
• excellent low temperature toughness can be ensured even when the Ni content is reduced as compared with the conventional 9% Ni steel.
• the retained austenite is unstable at -165 ° C where LNG tanks are used even though it is stable at room temperature.
• the presence of residual austenite at ⁇ 165 ° C. is considered to deteriorate toughness because the retained austenite transforms into a martensite structure due to processing-induced transformation at the crack tip of the steel material when the LNG tank breaks. Therefore, by reducing the retained austenite after sub-zero treatment corresponding to -165 ° C where LNG tanks are used and making the structure finer in this way, the Ni content is reduced compared to the conventional 9% Ni steel. Even if it makes it, it is estimated that low-temperature toughness is improved.
• the present invention has been made based on the above findings, and provides the following (1) to (4).
• (1) By mass%, C: 0.01 to 0.15%, Si: 0.02 to 0.20%, Mn: 0.45 to 2.00%, P: 0.020% or less, S: 0.005% or less, Al: 0.005 to 0.100%, Ni: 5.0 Containing 8.0%, with the balance being Fe and inevitable impurities,
• the amount of retained austenite when cooled to the liquid nitrogen temperature is less than 1.7% by volume, and the average grain size of the grains surrounded by the large tilt grain boundaries with an orientation difference of 15 ° or more is equivalent to a circle equivalent diameter of 5 ⁇ m or less.
• a Ni-containing thick steel plate characterized by being.
• Ni-containing thick steel plate according to (1) further containing one or two of Cr: 1.00% or less and Mo: 1.000% or less in mass%.
• the Ni-containing thick steel plate according to the present invention will be described in detail according to the component composition, structure and manufacturing method.
• the% display in a component is the mass%.
• C 0.01-0.15%
• C is an important element for solid solution strengthening of steel. If the C content is less than 0.01%, sufficient strength cannot be obtained. On the other hand, if C exceeds 0.15%, weldability and workability deteriorate. Therefore, the C content is in the range of 0.01 to 0.15%. Preferably, it is 0.03 to 0.10% of range.
• Si 0.02 to 0.20%
• Si is an effective element as a deoxidizer in molten steel, and is also an effective element for solid solution strengthening.
• the Si content is less than 0.02%, a sufficient deoxidation effect cannot be obtained.
• Si is added in excess of 0.20%, there arises a problem that ductility is lowered and inclusions are increased. Therefore, the Si content is in the range of 0.02 to 0.20%. Preferably, it is 0.03 to 0.10% of range.
• Mn 0.45-2.00% Mn is an effective element from the viewpoint of ensuring hardenability and improving strength. If the Mn content is less than 0.45%, the effect cannot be obtained sufficiently. On the other hand, if the Mn content exceeds 2.00%, the weldability deteriorates. Therefore, the Mn content is in the range of 0.45 to 2.00%. Preferably, it is in the range of 0.55 to 1.00%.
• P 0.020% or less
• the low temperature toughness is deteriorated, but if the content is 0.020% or less, it is acceptable. For this reason, the upper limit of the P content is 0.020%.
• Al 0.005-0.100%
• Al is an effective element as a deoxidizer in molten steel, and is also an effective element for improving low temperature toughness. If the Al content is less than 0.005%, these effects cannot be obtained sufficiently, while if the Al content exceeds 0.100%, the weldability is lowered. Therefore, the Al content is set in the range of 0.005 to 0.100%. Preferably, it is 0.020 to 0.050%.
• Ni 5.0-8.0%
• Ni is an important element in the present invention, and is an element that enhances the hardenability and improves the toughness of the ferrite ground. If the Ni content is less than 5.0%, this effect cannot be exhibited sufficiently. On the other hand, if the Ni content exceeds 8.0%, the cost increases. Therefore, the Ni content is in the range of 5.0 to 8.0%. Further, from the viewpoint of further reducing the cost, it is desirable that the Ni content is in the range of 5.0 to 7.5%.
• one or two of Cr and Mo can be contained in the following ranges as a selected component of the first group as necessary.
• Cr 1.00% or less Cr has an effect of improving hardenability and improving low temperature toughness by refining the martensite structure. However, if its content exceeds 1.00%, the weldability deteriorates and the manufacturing cost increases. For this reason, when Cr is contained, the content is made 1.00% or less. In order to effectively exhibit the above effects, the Cr content is preferably 0.05% or more. More preferably, it is in the range of 0.10 to 0.75%.
• Mo 1.000% or less Mo has an effect of improving hardenability and improving low-temperature toughness by refining the martensite structure. However, if its content exceeds 1.000%, the weldability deteriorates and the manufacturing cost increases. For this reason, when it contains Mo, the content shall be 1.000% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.005% or more. More preferably, it is in the range of 0.010 to 0.500%.
• one or more selected from Cu, V, Nb, Ti and B can be contained in the following range as the second group of selected components as required.
• Cu 1.00% or less Cu is an element that enhances hardenability. However, if its content exceeds 1.00%, the hot workability is lowered and the cost is also increased. For this reason, when it contains Cu, the content shall be 1.00% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.05% or more.
• V 0.100% or less
• V is an element that precipitates as carbonitride and has the effect of refining the structure and helps to improve toughness. However, if its content exceeds 0.100%, weldability deteriorates. For this reason, when it contains V, the content shall be 0.100% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.005% or more.
• Nb 0.100% or less
• Nb is an element that precipitates as carbonitride and has the effect of refining the structure and helps to improve toughness. However, if its content exceeds 0.100%, weldability deteriorates. For this reason, when it contains Nb, the content shall be 0.100% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.005% or more.
• Ti 0.100% or less Ti has the effect of improving toughness by fixing solute N harmful to toughness as TiN. However, if its content exceeds 0.100%, coarse carbonitride precipitates and toughness deteriorates. For this reason, when Ti is contained, the content is made 0.100% or less. In order to effectively exhibit the above effects, the content is preferably 0.005% or more. More preferably, it is 0.010 to 0.050% or less.
• B 0.0030% or less
• B is an element that enhances hardenability by adding a small amount. However, if its content exceeds 0.0030%, toughness deteriorates. For this reason, when it contains B, the content shall be 0.0030% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.0003% or more.
• one or two of Ca and REM can be contained in the following ranges as a third group selection component as necessary.
• Ca 0.0050% or less Ca is an element that fixes S and suppresses the formation of MnS that causes a decrease in toughness. However, if its content exceeds 0.0050%, the amount of inclusions in the steel increases, which leads to deterioration of toughness. For this reason, when it contains Ca, the content shall be 0.0050% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.0005% or more.
• REM 0.0050% REM (rare earth metal) is an element that fixes S and suppresses the formation of MnS, which causes a decrease in toughness. However, if its content exceeds 0.0050%, the amount of inclusions in the steel increases, which leads to deterioration of toughness. For this reason, when it contains REM, when it adds, the content shall be 0.0050% or less of range. In order to effectively exhibit the above effects, the content is preferably 0.0005% or more.
• the Ni-containing thick steel sheet of the present invention has the above component composition and is surrounded by large-angle grain boundaries having a residual austenite of less than 1.7% when cooled to a liquid nitrogen temperature and an orientation difference of 15 ° or more.
• the average grain size of the crystal grains has a structure with an equivalent circle diameter of 5 ⁇ m or less.
• the structure at ⁇ 165 ° C. in which the LNG tank is used is important. For this reason, a sub-zero treatment for maintaining the liquid nitrogen temperature was performed. Define the later organization. If the retained austenite after the sub-zero treatment is 1.7% or more by volume, sufficient low temperature toughness cannot be obtained. Although there is a report that retained austenite improves low-temperature toughness, the Ni-containing thick steel sheet of the present invention adversely affects toughness. This is because the Ni-containing thick steel sheet of the present invention has a lower Ni content than the conventional 9% Ni steel, so even if residual austenite is present at -165 ° C, it is unstable and the steel at the crack tip is unstable.
• the residual austenite after cooling to liquid nitrogen temperature shall be less than 1.7% by volume ratio. Preferably, it is 1.0% or less, more preferably 0.5% or less.
• the average crystal grain size of the crystal grains surrounded by the large tilt grain boundaries with an orientation difference of 15 ° or more is set to 5 ⁇ m or less, preferably 3 ⁇ m or less, in terms of equivalent circle diameter.
• a slab or steel slab having the above composition is heated at 900 to 1100 ° C. for 10 hours or less, and then at a temperature range of 870 ° C. or less, the cumulative rolling reduction is 40% or more and 70% or less.
• the cumulative rolling reduction is 40% or more and 70% or less.
• a direct quenching process in which the obtained hot-rolled steel sheet is immediately quenched to 200 ° C. or less at a cooling rate of 5 ° C./s or more. It is preferable to heat at a temperature rise rate of 0.05 to 1.0 ° C./s to a temperature range of 500 to 650 ° C., hold at that temperature range for 10 minutes to 60 minutes, and temper.
• Heating temperature 900 to 1100 ° C
• heating time 10 hours or less
• the heating temperature is less than 900 ° C
• coarse AlN deposited in the casting stage of steel slabs does not dissolve and toughness decreases.
• the rolling conditions shown below cannot be substantially satisfied.
• austenite becomes coarse grains and the toughness decreases.
• the heating time exceeds 10 hours, the austenite grains become coarse and the toughness decreases. For this reason, the heating temperature is 900 to 1100 ° C., and the heating time is 10 hours or less.
• Rolling ratio Cumulative rolling reduction of 40% or more and 70% or lower at 870 ° C or lower If the cumulative rolling reduction is less than 40% in the austenite non-recrystallized region of 870 ° C or lower, the martensite structure is not sufficiently refined and toughness Decreases. On the other hand, when the cumulative rolling reduction exceeds 70%, it is difficult to substantially roll at the finishing temperature shown below. For this reason, the rolling reduction is set to 40% or more and 70% or less at 870 ° C. or less.
• Finishing temperature 700 ⁇ 820 °C
• the finishing temperature is less than 700 ° C.
• ⁇ - ⁇ two-phase region rolling occurs, and a bainite phase is generated, so that the desired strength cannot be satisfied.
• the finishing temperature exceeds 820 ° C.
• sufficient reduction in the austenite non-recrystallized region becomes substantially difficult, a fine structure cannot be obtained, and the toughness decreases. Therefore, the finishing temperature is 700 to 820 ° C.
• Cooling starts immediately after the end of rolling. If it does not start immediately, a bainite phase is formed, and the desired strength cannot be satisfied. For this reason, cooling is started immediately after the end of rolling.
• “immediately” means within about 120 seconds after the end of rolling.
• Cooling rate 5 ° C./s or more
• a cooling rate shall be 5 degrees C / s or more.
• it is 10 ° C./s or more.
• Cooling stop temperature 200 ° C. or less
• the cooling stop temperature exceeds 200 ° C., transformation into a uniform martensite structure does not occur in the steel sheet, and desired strength and toughness cannot be obtained. For this reason, the cooling stop temperature is set to 200 ° C. or lower.
• Tempering heating rate 0.05 to 1.0 ° C / s
• the tempering temperature rising rate is less than 0.05 ° C./s, the precipitated carbide is coarsened and the toughness is lowered.
• the tempering temperature rising rate is set to 0.05 to 1.0 ° C./s.
• Tempering temperature 500-650 ° C
• the tempering temperature is less than 500 ° C.
• the effect of improving toughness due to precipitation of fine carbides such as cementite cannot be obtained sufficiently.
• the tempering temperature exceeds 650 ° C.
• coarse carbides are precipitated and the toughness is lowered. Therefore, the tempering temperature is 500 to 650 ° C.
• Tempering holding time 10 minutes or more and 60 minutes or less
• the tempering holding time is less than 10 minutes, the effect of improving toughness due to precipitation of fine carbides such as cementite cannot be obtained sufficiently.
• the tempering holding time exceeds 60 minutes, the toughness decreases due to precipitation of coarse carbides.
• the manufacturing cost increases.
• the tempering holding time is 10 minutes or more and 60 minutes or less. Cooling after tempering may be either water cooling or air cooling, but if the cooling rate is too high, the temperature difference between the surface and the inside of the steel sheet will increase, causing distortion inside the steel sheet and reducing the low temperature toughness. It is preferable to set it as s or less.
• Two-phase heat treatment heating rate 0.1 to 1.5 ° C / s
• Two-phase heat treatment temperature 650-800 ° C
• the two-phase region heat treatment temperature is less than 650 ° C.
• sufficient austenite reverse transformation does not occur, and the effect of refinement of the structure cannot be obtained, so that the effect of improving toughness cannot be obtained.
• the austenite reverse transformation amount is small, C is easily concentrated in the austenite, and the retained austenite increases.
• the heat treatment temperature in the two-phase region exceeds 800 ° C.
• the reverse transformed austenite becomes coarse and the toughness decreases.
• tissue after cooling also coarsens, toughness falls.
• the manufacturing cost increases. For this reason, the heat treatment temperature in the two-phase region is set to 650 to 800 ° C.
• the two-phase region heat treatment temperature is preferably 720 to 780 ° C.
• Two-phase region heat treatment holding time 10 minutes or more and 60 minutes or less
• the two-phase region heat treatment holding time is less than 10 minutes, sufficient austenite reverse transformation does not occur, and the effect of improving toughness due to refinement of the structure cannot be obtained.
• the two-phase region heat treatment holding time exceeds 60 minutes, austenite grains become coarse and toughness decreases.
• generated after cooling also coarsens, toughness falls.
• C concentrates in austenite retained austenite increases. In addition, the manufacturing cost increases. For this reason, the two-phase region heat treatment holding time is 10 minutes or more and 60 minutes or less.
• Cooling rate after heat treatment in two-phase region 5 ° C./s or more
• austenite does not transform into a martensite structure, and desired strength and toughness cannot be obtained.
• the cooling rate is slow, the amount of C dissolved in ferrite decreases with decreasing temperature, so C moves from ferrite around the austenite that has undergone reverse transformation to austenite, and C concentrates and remains in the austenite. It becomes easy to become austenite.
• a cooling rate shall be 5 degrees C / s or more. Preferably it is 10 degrees C / s or more.
• Cooling stop temperature after two-phase region heat treatment 200 ° C. or less
• the cooling stop temperature exceeds 200 ° C.
• transformation into a uniform martensite structure does not occur in the steel sheet, and desired strength and toughness cannot be obtained.
• C is concentrated in austenite and tends to be retained austenite. For this reason, the cooling stop temperature is set to 200 ° C. or lower.
• tempering is performed in the same manner as described above. That is, it is heated to a temperature range of 500 ° C. to 650 ° C. at a rate of temperature increase of 0.05 ° C./s to 1.0 ° C./s, and is tempered by holding for 10 minutes to 60 minutes in the same temperature range.
• Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace to obtain a small steel ingot (150 kg). After heating these steels under the conditions shown in Table 2, they were hot-rolled to a thickness of 7 to 50 mm, quenched immediately after rolling, and some of the steel plates were subsequently tempered. The remaining steel sheet was subjected to a two-phase region heat treatment after quenching and then a tempering treatment. The obtained steel sheet was subjected to a tensile test, a Charpy impact test, an austenite volume fraction measurement, and a grain size measurement of a crystal grain surrounded by a large-angle grain boundary having an orientation difference of 15 ° or more in the following manner.
• Tensile test pieces having a parallel part length of 30 mm, a GL of 24 mm, and a parallel part diameter of 6 ⁇ were taken from the rolling direction at a thickness of 1/2 of each steel plate, and a tensile test was performed at room temperature.
• Tensile strength (TS) and yield strength (YS) were calculated from the obtained stress-strain curve. TS with 690MPa or higher and YS with 590MPa or higher were considered superior to TS and YS.

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• 2012
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