WO2009119274A1 - 耐応力除去焼鈍特性と低温靭性に優れた高強度鋼板 - Google Patents
耐応力除去焼鈍特性と低温靭性に優れた高強度鋼板 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium 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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/30—Stress-relieving
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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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- 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
- C21D8/0273—Final recrystallisation annealing
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- 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
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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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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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
- 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
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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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
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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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
- C21D2211/004—Dispersions; Precipitations
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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
- C21D2211/005—Ferrite
Definitions
- the present invention relates to a high-strength steel sheet having excellent low-temperature toughness (Heat Affected Zone: hereinafter sometimes referred to as “HAZ”).
- Cr—Mo steel has been generally applied as a steel material in which the strength reduction due to SR treatment as described above is reduced as much as possible.
- strength reduction after SR treatment is suppressed by adding a high concentration of Cr, and high temperature strength is improved by adding Mo.
- Patent Document 1 proposes “tough steel for pressure vessels” that basically contains 0.26 to 0.75% Cr and 0.45 to 0.60% Mo. As described above, this technique is based on the above basic idea in that the addition of Cr suppresses the coarsening of the carbide after the SR treatment and suppresses the strength reduction after the SR treatment. Therefore, even in such a steel material, since the Cr content is large, the problem that the low-temperature toughness (particularly the HAZ toughness) is lowered remains unresolved.
- Patent Document 2 proposes a “high-strength tough steel for pressure vessels” that basically contains 0.10 to 1.00% Cr and 0.45 to 0.60% Mo.
- this technique the reaction of Fe 3 C with coarse M 23 C 6 due to the SR treatment for a long time is suppressed by addition of Cr.
- Cr is contained in a relatively wide range, but actually only Cr content of 0.29% or more is shown, and low temperature toughness (especially HAZ toughness). Is expected to decrease.
- Patent Document 3 proposes a steel sheet having improved SR resistance and improved HAZ toughness.
- this technique is also based on containing a large amount of Cr and Mo.
- fracture surface transition temperature vTrs some ductile-brittle fracture surface transition temperatures vTrs after normal SR treatment (hereinafter simply referred to as “fracture surface transition temperature vTrs”) have obtained relatively good values. It is expected that the toughness will decrease after the required high temperature and long time severe SR treatment.
- the present invention has been made in view of the above circumstances, and its purpose is that even when stress-relieving annealing is performed for a long time after welding, there is little decrease in strength (that is, stress-relieving annealing characteristics are good). ) Moreover, the object is to provide a high-strength steel sheet excellent in low-temperature toughness in the base material after HA treatment and HAZ.
- the high-strength steel sheet according to the present invention that has solved the above problems is C: 0.05 to 0.18% (meaning “mass%”; the same applies hereinafter), Si: 0.10 to 0.50% , Mn: 1.2 to 2.0%, Al: 0.01 to 0.10%, Cr: 0.05 to 0.30%, Ti: 0.008 to 0.025% and V: 0.01 -0.05% each containing iron and inevitable impurities, P in the inevitable impurities is suppressed to 0.008% or less, and the following formulas (1) to (3) are satisfied It has a gist in terms.
- the average particle diameter of cementite in the structure is preferably 0.165 ⁇ m or less in terms of the equivalent circle diameter.
- the “equivalent circle diameter” refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of cementite.
- the high-strength steel sheet of the present invention in addition to the basic elements, if necessary, (a) at least one of Cu: 0.05 to 0.8% and Ni: 0.05 to 1%, b) Mo: 0.01 to 0.3%, (c) B: 0.0004% or less, (d) Ca: 0.0005 to 0.005%, etc. are also useful and contained.
- the properties of the steel sheet are further improved depending on the types of components to be added.
- a high-strength steel sheet having a fine cementite particle size can be obtained.
- the high-strength steel sheet thus obtained is extremely useful as a tank material because it can suppress a decrease in strength after SR treatment and is excellent in low-temperature toughness of the base material and HAZ after SR treatment.
- the present inventor has examined components from various angles that can maintain good weldability without causing a decrease in strength even after prolonged SR treatment.
- the chemical composition is appropriately controlled and the content of Cr, Mn and V is controlled so as to satisfy the relational expression (1), cementite can be refined and strength reduction can be suppressed.
- the present applicant has already filed for this invention (Japanese Patent Application No. 2006-338933).
- Japanese Patent Application No. 2006-338933 Japanese Patent Application No.
- a strengthening method of improving strength by dispersing a large amount of fine precipitates in the matrix and hindering the movement of dislocations by the pinning effect of dislocations by the precipitates is known as precipitation strengthening. According to this way of thinking, it can be expected that the extent of decrease in strength increases as cementite coarsens.
- FIG. 2 is a graph showing the relationship between the equivalent-circle diameter of cementite and the amount of decrease in strength ( ⁇ TS) before and after SR treatment. According to this graph, it can be seen that reducing the cementite particle size is important in reducing the strength reduction amount ⁇ TS.
- FIG. 3 is a graph showing the relationship between the P value and the cementite equivalent circle diameter. It is recognized that the cementite coarsening suppression effect tends to increase as the P value increases. Moreover, when P value became 7.2 or more, it became clear that cementite could be disperse
- the present inventor further studied to improve the low temperature toughness of the steel sheet even after the above invention was completed. As a result, by satisfying the following formulas (2) and (3) at the same time, it was found that excellent low temperature toughness could be secured even after severe high temperature and long time SR treatment, and the present invention was completed.
- Di value The value on the left side of the above equation (2) (hereinafter, this value is referred to as “Di value”) is an index for organizing the intensity, and the value itself is known (for example, Japanese Patent Laid-Open 9-202936).
- this Di value has not been used as an indicator of toughness. This is because, among the components defined by the formula (2), elements and impurities necessary for determining the austenite grain size, which is the dominant factor of toughness, were not defined.
- the present inventor has found that the steel sheet can be made excellent in low-temperature toughness by simultaneously satisfying the expression (3) incorporating such elements and impurities.
- the value on the left side of the equation (3) (hereinafter referred to as “Pt value”) determines the fracture unit which is the controlling factor of the low temperature toughness.
- the components directly involved in achieving the stress-relieving annealing characteristics and low temperature toughness of the steel sheet as the subject of the present invention are C, Si, Mn, Al, Cr , Ti, V and P as an inevitable impurity.
- Cu, Ni, Mo, and B are components that are included according to requirements different from the subject of the present invention, but also affect low-temperature toughness. For this reason, those contents determined according to other requirements also need to be included in the calculation of the Di value and the Pt value. For this reason, the above formulas (2) and (3) also define the contents of these elements. Therefore, when these elements are not contained, the amount of these elements may be calculated as 0 from the above formulas (2) and (3).
- the steel sheet can be made excellent in both SR resistance and low temperature toughness.
- C 0.05 to 0.18%
- C is an indispensable element for securing the strength of the steel sheet.
- a production method by reheating quenching and tempering is adopted, if the C content is less than 0.05%, the necessary strength is ensured.
- the C content is excessive, the toughness and weldability are remarkably impaired, so it is necessary to set the content to 0.18% or less.
- the preferable lower limit of the C content is 0.06%, and the preferable upper limit is 0.16%.
- Si 0.10 to 0.50%
- Si is an element essential for improving the strength and deoxidation of the steel sheet. In order to exhibit such an effect effectively, it is necessary to contain 0.10% or more. However, if the Si content is excessive, the toughness of the steel sheet is lowered, so it is necessary to make it 0.50% or less.
- the minimum with preferable Si content is 0.15%, and a preferable upper limit is 0.40%.
- Mn is an element indispensable for improving the hardenability of the steel sheet and improving the strength.
- the solid solubility in cementite is the second highest after that of Cr, and it is an element effective in suppressing agglomeration and coarsening of cementite by dissolving in cementite as described above.
- it is necessary to contain 1.2% or more of Mn.
- the minimum with preferable Mn content is 1.30%, and a preferable upper limit is 1.80%.
- Al 0.01 to 0.10%
- Al is added as a deoxidizer, but if it is less than 0.01%, sufficient effects are not exhibited. Further, if Al is contained excessively exceeding 0.10%, the toughness of the steel sheet is deteriorated and the crystal grains are coarsened, so the upper limit is made 0.10%.
- the minimum with preferable Al content is 0.02%, and a preferable upper limit is 0.08%.
- Cr 0.05-0.30%
- Mn is an element effective for improving the strength by increasing the hardenability of the steel sheet by adding a small amount. Further, like Mn, it is an element effective for solid solution in cementite to suppress the cementite coarsening. In order to exert such an effect effectively, Cr needs to be contained in an amount of 0.05% or more, but if it is contained excessively, weldability deteriorates, so it should be made 0.30% or less.
- the minimum with preferable Cr content is 0.10%, and a preferable upper limit is 0.25%.
- Ti hardly dissolves in the base material, and forms carbides and nitrides to contribute to strength improvement and austenite grain size refinement during heating.
- the inclusion of Ti can form a nitride to suppress austenite coarsening and obtain a ferrite structure necessary for securing low temperature toughness. Such an effect is effectively exhibited when the Ti content is 0.008% or more, but the effect is saturated even if the Ti content exceeds 0.025%.
- V 0.01 to 0.05%
- V is an element that has a high solid solubility in cementite and is effective in exhibiting a cementite grain coarsening suppression effect, as in Mn and Cr.
- V is an essential element for forming fine carbonitrides and improving the toughness of the steel sheet. In order to exhibit these effects, it is necessary to contain V 0.01% or more. However, if the content exceeds 0.05%, the HAZ toughness is lowered. The minimum with preferable V content is 0.02%, and a preferable upper limit is 0.04%.
- the basic components in the high-strength steel sheet of the present invention are as described above, and the balance is iron and inevitable impurities.
- Inevitable impurities include steel raw materials or P, S, N, O, etc. that can be mixed in the manufacturing process.
- P especially P, if the amount is excessive, the effect of grain boundary segregation due to prolonged SR treatment becomes significant, and the low temperature toughness deteriorates, so 0.008% It is preferable to suppress to the following.
- Cu and Ni are effective elements for enhancing the hardenability of the steel sheet. In order to exhibit such an effect effectively, it is preferable to contain 0.05% or more of all. However, since the above effect is saturated even if contained excessively, Cu is preferably 0.8% or less and Ni is preferably 1% or less. More preferably, Cu is 0.5% or less, and Ni is 0.8% or less. Cu and Ni may contain either one or both.
- Mo effectively acts to ensure the strength of the steel sheet after annealing. Such an effect is effectively exhibited when the Mo content is 0.01% or more. However, even if the Mo content is excessive, the above effect is saturated. More preferably, it is 0.2% or less.
- B is an element effective for improving the hardenability of the steel sheet by adding a very small amount. However, if excessively contained, the severe SR treatment will adversely affect the low temperature toughness, so the upper limit is 0.0004% or less. It is preferable to do.
- Ca 0.0005 to 0.005%
- Ca is an element effective for improving the toughness of the steel sheet by controlling inclusions. Such an effect is effectively exhibited when the content is 0.0005% or more. However, if the content is excessive, the above effect is saturated, so 0.005% or less is preferable.
- the high-strength steel sheet of the present invention can control the average grain size of cementite to 0.165 ⁇ m or less as long as the chemical composition and the relationship of the above formula (1) are satisfied, thereby reducing the strength after SR treatment. Can be suppressed.
- the slab is cast with a continuous casting machine, heated to a heating temperature of about 1000-1200 ° C, and after rolling in the temperature range of 800-1000 ° C, it is allowed to cool. Subsequently, it is reheated to the Ac3 transformation point or higher for quenching, and then tempered at a temperature of 600 to 700 ° C.
- the austenite crystal grains become fine and the structure becomes difficult to be baked, and when it exceeds 1200 ° C., abnormal grain growth (exagrated grain growth) may occur.
- the reason for setting the rolling end temperature in the temperature range of 800 to 1000 ° C. is to improve the productivity as much as possible.
- the steel is slowly cooled and then reheated to the Ac3 transformation point or higher to perform a quenching process.
- the austenite-transformed structure is rapidly cooled to obtain a quenched structure such as martensite, thereby improving the strength. That is, if the heating temperature in this step is less than the Ac3 transformation point, the steel sheet using transformation strengthening cannot be strengthened.
- tempering is performed to optimize the strength. In this process, if the tempering temperature is less than 600 ° C., the strength of the steel plate becomes too high, and if it exceeds 700 ° C., the strength of the steel plate becomes too low.
- the high-strength steel sheet of the present invention obtained in this way has a finely dispersed cementite. For this reason, the steel sheet of the present invention is reduced in strength reduction after SR treatment as much as possible, and has excellent low-temperature toughness. Therefore, the steel plate of the present invention is extremely useful as a material for large steel containers.
- the steel sheet of the present invention by setting the P value defined by the above formula (1) to 7.2% or more, the SR resistance after severe SR treatment and the low temperature toughness of HAZ are good. It becomes.
- the “severe SR process” is not limited to the time, but needs to be discussed in consideration of the relationship with temperature.
- the condition that the TP value defined by the following formula (4) is 18.5 or more is defined as “severe SR processing”. . That is, the steel sheet of the present invention has good SR resistance even when it is SR-treated under such a condition that the TP value defined by the following formula (4) is 18.5 or more.
- TP value T (20 + logt 0 ) ⁇ 10 3 (4)
- T SR processing heating temperature (K)
- t 0 SR processing heating time (hour)
- the slab was cast with a continuous casting machine and hot rolled (slab heating temperature: 1000 to 1200 ° C., rolling end temperature: 800 to 1000 ° C. ) And heat treatment (heated to 900 to 930 ° C. and quenched, then tempered at 600 to 680 ° C.) to obtain various steel plates (plate thickness t: 70 to 72 mm).
- the heating temperature at this time is t (t: plate thickness) in the temperature distribution from the front surface to the back surface of the steel slab calculated based on the atmospheric temperature in the furnace from the start of heating to extraction by the process computer and the in-furnace time. This is the temperature of a / 4 portion (a point at a depth of 1/4 of the plate thickness from the surface of the steel plate).
- each steel plate obtained as mentioned above the equivalent-circle diameter of cementite was measured by the following method.
- each steel plate was subjected to SR treatment corresponding to the above TP value of 18 to 18.5, and the tensile strength before and after SR treatment was measured by the following method (tensile test), and the strength was lowered before and after SR treatment.
- the toughness of the base material base material toughness after SR treatment vE ⁇ 46
- SR treatment conditions are the same as above
- HAZ toughness fracture surface transition temperature vTrs
- HAZ toughness HAZ toughness after SR
- SR treatment conditions are the same as above
- specimens of ASTM A370-05 were collected in the same manner as described above, at -46 ° C in accordance with ASTM A370-05.
- a Charpy impact test was performed and the absorbed energy (vE ⁇ 46 ) was measured. Then, those having a value (average value) of vE ⁇ 46 of 50 J or more were evaluated as having excellent HAZ toughness.
- No. No. 7 uses a steel type that does not contain Ti, which is an essential element in the present invention, so that TiN, which is a nitride of Ti, is not precipitated, and austenite crystal grains are not heated.
- TiN which is a nitride of Ti
- the organization is large and easy to burn.
- this steel it becomes an upper bainite structure similarly to the above, and both the base material and HAZ have deteriorated toughness.
- FIG. 2 shows the relationship between the cementite equivalent circle diameter and the strength reduction amount ( ⁇ TS), and FIG. 3 shows the relationship between the P value and the cementite equivalent circle diameter. It is.
- FIG. 4 shows the relationship between the Pt value and the base material toughness (vE ⁇ 46 ).
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Abstract
Description
6.7[Cr]+4.5[Mn]+3.5[V]≧7.2(質量%) …(1)
但し、[Cr],[Mn]および[V]は、夫々Cr,MnおよびVの含有量(質量%)を示す。
1.16×([C]/10)1/2×(0.75×[Si]+1)×(5.1×([Mn]-1.2)+5)×(0.35×[Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+1)×(1.75×[V]+1)×(200×[B]+1)≦2.08 …(2)
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo],[V]および[B]は、夫々C,Si,Mn,Cu,Ni,Cr,Mo,VおよびBの含有量(質量%)を示す。
-{Di-900×[Ti]+50×([P]-0.008)+3500×([B]-0.0004)}≧9.62 …(3)
但し、[Ti],[P]および[B]は、夫々Ti,PおよびBの含有量(質量%)を示し、Diは上記(2)式の左辺の値を意味する。
6.7[Cr]+4.5[Mn]+3.5[V]≧7.2(質量%)…(1)
但し、[Cr],[Mn]および[V]は、夫々Cr,MnおよびVの含有量(質量%)を示す。
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo],[V]および[B]は、夫々C,Si,Mn,Cu,Ni,Cr,Mo,VおよびBの含有量(質量%)を示す。
但し、[Ti],[P]および[B]は、夫々Ti,PおよびBの含有量(質量%)を示し、Diは上記(2)式の左辺の値を意味する。
Cは、鋼板の強度を確保する上で不可欠の元素であり、再加熱焼入れ・焼戻しによる製造方法を採用する場合、C含有量を0.05%未満にすると、必要な強度を確保するためには、他の合金元素を多量に含有させる必要があり、コストアップになってしまう。また、C含有量が過剰になると、靭性と溶接性を著しく損ねることから、0.18%以下とする必要がある。C含有量の好ましい下限は0.06%であり、好ましい上限は0.16%である。
Siは、鋼板の強度向上と脱酸に不可欠な元素である。こうした効果を有効に発揮させるには0.10%以上含有させる必要がある。しかしながら、Si含有量が過剰になると鋼板の靭性が低下するので、0.50%以下とする必要がある。Si含有量の好ましい下限は0.15%であり、好ましい上限は0.40%である。
Mnは、鋼板の焼入れ性を高めて強度の向上に必要不可欠な元素である。また、セメンタイトへの固溶度(solid solubility)がCrに次いで高く、上記の通りセメンタイトに固溶することによって、セメンタイトの凝集粗大化を抑制する上で有効な元素である。こうした効果を有効に発揮させるためには、Mnは1.2%以上含有させる必要がある。しかしながら、Mn含有量が過剰になると、溶接部の靭性が低下するので、2.0%を上限とする。Mn含有量の好ましい下限は1.30%であり、好ましい上限は1.80%である。
Alは、脱酸剤として添加されるが、0.01%未満では十分な効果が発揮されない。また、Alは、0.10%を超えて過剰に含有させると鋼板における靭性の悪化や結晶粒の粗大化を招くので0.10%を上限とする。Al含有量の好ましい下限は0.02%であり、好ましい上限は0.08%である。
Crは、Mnと同様に少量の添加で鋼板の焼入れ性を高めて強度の向上に有効な元素である。また、Mnと同様にセメンタイトへ固溶してセメンタイトの凝集粗大化を抑制する上で有効な元素である。こうした効果を有効に発揮させるためには、Crは0.05%以上含有させる必要があるが、過剰に含有されると溶接性が悪くなるので、0.30%以下にすべきである。Cr含有量の好ましい下限は0.10%であり、好ましい上限は0.25%である。
Tiは、母材には殆ど固溶せず、炭化物や窒化物を形成して強度向上や加熱時のオーステナイト粒径微細化に寄与する。本発明の成分系では、Tiの含有によって、窒化物を形成してオーステナイトの粗大化を抑制し、低温靭性確保に必要なフェライト(ferrite)組織を得ることができる。こうした効果は、Ti含有量が0.008%以上で有効に発揮されるが、0.025%を超えて過剰に含有させてもその効果は飽和する。
Vは、前述の如く、MnやCrと同様に、セメンタイトへの固溶度が高く、セメンタイト粒粗大化抑制効果を発揮するのに有効な元素である。またVは、微細な炭窒化物を形成させて鋼板の靭性を向上させるのに必要不可欠な元素である。これらの効果を発揮させるためには、Vは0.01%以上含有させる必要がある。しかしながら、0.05%を超えて過剰に含有させると、HAZ靭性を低下させることになる。V含有量の好ましい下限は0.02%であり、好ましい上限は0.04%である。
CuおよびNiは、鋼板の焼入れ性を高めるのに有効な元素である。こうした効果を有効に発揮させるためには、いずれも0.05%以上含有させることが好ましい。しかしながら、過剰に含有させても上記効果が飽和してしまうので、Cuで0.8%以下、Niで1%以下とすることが好ましい。より好ましくはCuで0.5%以下、Niで0.8%以下である。CuとNiは、いずれか一方を含有させても、両方を含有させてもよい。
Moは、焼鈍後の鋼板の強度を確保するのに有効に作用する。こうした効果は、Mo含有量が0.01%以上で有効に発揮されるが、過剰に含有させても上記効果が飽和してしまうので、0.3%以下とすることが好ましい。より好ましくは0.2%以下である。
Bは極少量の添加で鋼板の焼入れ性を向上させるのに有効な元素であるが、過剰に含有させると過酷なSR処理によって低温靭性に悪影響を及ぼすため、その上限を0.0004%以下とすることが好ましい。
Caは、介在物の制御により鋼板の靭性を向上させるのに有効な元素である。こうした効果は含有量が0.0005%以上で有効に発揮されるが、過剰に含有されると、上記効果が飽和するので0.005%以下とするのがよい。
但し、T:SR処理加熱温度(K)、t0:SR処理加熱時間(時間)
Ar3=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]+0.35(t-8) …(5)
各鋼板のt(t:板厚)/4の部位を透過型電子顕微鏡(TEM)により倍率:7500倍で約200μmの視野を10視野観察した。それによって得られた画像データを画像解析して、面積分率と個数からセメンタイトの1個当りの面積を算出し、セメンタイトの切断面を円と仮定したときの直径を円相当径として導出した。このとき、面積が0.0005μm2以下の粒はノイズと判断して除外した。
SR処理前・後の各鋼板のt(t:板厚)/4部位から、圧延方向に対して直角の方向にJIS Z 2201の4号試験片を採取して、JIS Z 2241の要領で引張試験を行なって引張強度(TS)を測定した。そして、SR処理前・後の引張強度TSの差によって強度低下量ΔTSを測定し、このΔTS(平均値)が35MPa未満のものを耐SR特性が良好と判定した。
SR処理後の各鋼板のt(板厚)/4部位から、圧延方向に対して直角の方向にASTM A370-05(0.500-in.Round Spacimen)試験片を採取し、ASTM A370-05に準拠して、-46℃でシャルピー衝撃試験を行い、吸収エネルギー(vE-46)を測定した。そして、vE-46の値(平均値)が200J以上のものを母材靭性に優れると評価した。
下記の条件で溶接した各鋼板について、SR処理(条件は上記と同じ)を行い、上記と同様にしてASTM A370-05試験片を採取し、ASTM A370-05に準拠して、-46℃でシャルピー衝撃試験を行い、吸収エネルギー(vE-46)を測定した。そして、vE-46の値(平均値)が50J以上のものをHAZ靭性に優れると評価した。
溶接方法:被覆アーク溶接
最高入熱量:50kJ/cm
溶接材料:LB-62L
電流:170A
電圧:26V
溶接速度:6.0cm/min
予熱パス間温度:75℃以上
パス数:バック側14パス、ファイナル側17パス
開先形状:X開先
Claims (7)
- C:0.05~0.18%(「質量%」の意味。以下同じ)、Si:0.10~0.50%、Mn:1.2~2.0%、Al:0.01~0.10%、Cr:0.05~0.30%、Ti:0.008~0.025%およびV:0.01~0.05%を夫々含有し、残部が鉄および不可避的不純物からなり、該不可避的不純物中のPを0.008%以下に抑制し、且つ下記(1)~(3)式を満足することを特徴とする高強度鋼板。
6.7[Cr]+4.5[Mn]+3.5[V]≧7.2(質量%) …(1)
但し、[Cr],[Mn]および[V]は、夫々Cr,MnおよびVの含有量(質量%)を示す。
1.16×([C]/10)1/2×(0.75×[Si]+1)×(5.1×([Mn]-1.2)+5)×(0.35×[Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+1)×(1.75×[V]+1)×(200×[B]+1)≦2.08 …(2)
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo],[V]および[B]は、夫々C,Si,Mn,Cu,Ni,Cr,Mo,VおよびBの含有量(質量%)を示す。
-{Di-900×[Ti]+50×([P]-0.008)+3500×([B]-0.0004)}≧9.62 …(3)
但し、[Ti],[P]および[B]は、夫々Ti,PおよびBの含有量(質量%)を示し、Diは上記(2)式の左辺の値を意味する。 - 組織中のセメンタイトの平均粒径が円相当径で0.165μm以下である請求項1に記載の高強度鋼板。
- 更に他の元素として、Cu:0.05~0.8%およびNi:0.05~1%の少なくともいずれかを含有するものである請求項1に記載の高強度鋼板。
- 更に他の元素として、Mo:0.01~0.3%を含有するものである請求項1に記載の高強度鋼板。
- 更に他の元素として、B:0.0004%以下を含有するものである請求項1に記載の高強度鋼板。
- 更に他の元素として、Ca:0.0005~0.005%を含有するものである請求項1に記載の高強度鋼板。
- C:0.05~0.18%(「質量%」の意味。以下同じ)、Si:0.10~0.50%、Mn:1.2~2.0%、Al:0.01~0.10%、Cr:0.05~0.30%、Ti:0.008~0.025%およびV:0.01~0.05%、Cu:0.8%以下、Ni:1%以下、Mo:0.3%以下、B:0.0004以下およびCa0.005%以下を夫々含有し、残部が鉄および不可避的不純物からなり、該不可避的不純物中のPを0.008%以下に抑制し、且つ下記(1)~(3)式を満足することを特徴とする高強度鋼板。
6.7[Cr]+4.5[Mn]+3.5[V]≧7.2(質量%) …(1)
但し、[Cr],[Mn]および[V]は、夫々Cr,MnおよびVの含有量(質量%)を示す。
1.16×([C]/10)1/2×(0.75×[Si]+1)×(5.1×([Mn]-1.2)+5)×(0.35×[Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+1)×(1.75×[V]+1)×(200×[B]+1)≦2.08 …(2)
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo],[V]および[B]は、夫々C,Si,Mn,Cu,Ni,Cr,Mo,VおよびBの含有量(質量%)を示す。
-{Di-900×[Ti]+50×([P]-0.008)+3500×([B]-0.0004)}≧9.62 …(3)
但し、[Ti],[P]および[B]は、夫々Ti,PおよびBの含有量(質量%)を示し、Diは上記(2)式の左辺の値を意味する。
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JP2006045672A (ja) * | 2004-07-07 | 2006-02-16 | Jfe Steel Kk | 高張力鋼板の製造方法および高張力鋼板 |
JP2008150656A (ja) * | 2006-12-15 | 2008-07-03 | Kobe Steel Ltd | 耐応力除去焼鈍特性と溶接性に優れた高強度鋼板 |
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JPS6237342A (ja) * | 1985-08-09 | 1987-02-18 | Nippon Kokan Kk <Nkk> | 高温強度と耐sr割れ性に優れた高靭性、高温高圧容器用鋼 |
JP2006045672A (ja) * | 2004-07-07 | 2006-02-16 | Jfe Steel Kk | 高張力鋼板の製造方法および高張力鋼板 |
JP2008150656A (ja) * | 2006-12-15 | 2008-07-03 | Kobe Steel Ltd | 耐応力除去焼鈍特性と溶接性に優れた高強度鋼板 |
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