JP4433844B2 - Method for producing high strength steel with excellent fire resistance and toughness of heat affected zone - Google Patents

Description

  The present invention relates to a high-strength steel having a yield strength of 414 MPa or more, which is used for offshore structures, line pipes, pressure vessels, etc., and is particularly excellent in fire resistance and weld joint toughness. This is a manufacturing method.

  Structural steel is usually designed and manufactured to have sufficient strength at room temperature. However, the strength of steel generally decreases with an increase in temperature. In particular, it is known that a conventional structural steel material has a large decrease in strength at a high temperature of 500 ° C. or higher. For this reason, in structures that are likely to become hot due to fire, etc., especially in buildings where humans live, steel materials have a certain level of fire resistance (high temperature strength) so that the structure will not collapse even in high temperatures. It is necessary to make it. As means for imparting fire resistance, for example, Patent Document 1 discloses a technique for increasing high-temperature strength by precipitation strengthening by adding Mo, V, or the like.

  Moreover, steel used for offshore structures and the like is finished into a desired shape structure by welding. Therefore, these steels are required to have excellent toughness of the welded joint (welded metal and heat affected zone) of the welded joint as well as the toughness of the base metal itself from the viewpoint of the safety of the structure.

  By the way, welding of steel with a large plate thickness is usually performed by multi-layer welding, but in such welding, the heat-affected zone is subjected to a complicated thermal history, so that a local embrittlement zone is likely to occur, Degradation of the toughness of the weld zone (boundary between the weld metal and base metal) and the two-phase reheat zone (coarse grain in the first cycle and heated to the two-phase zone of α and γ in the second cycle) It becomes. This is because the bond portion is exposed to a high temperature just below the melting point, so that the austenite grains are most coarsened and are easily transformed into a fragile upper bainite structure by subsequent cooling. Further, since a brittle structure such as a woodman stetten structure or island martensite is also generated in the bond portion, the toughness tends to be further deteriorated.

  As a countermeasure against this problem, for example, a technique of finely dispersing TiN in steel to suppress coarsening of austenite or using it as a core of ferrite transformation has been put into practical use. Further, in Patent Document 2 and Patent Document 3, a rare earth element (REM) is added in combination with Ti to disperse fine particles in the steel, thereby preventing austenite grain growth and improving the toughness of the weld. Technology is disclosed. In addition, Ti oxide dispersion technology, BN ferrite nucleation ability combined with oxide dispersion technology, and addition of Ca and REM control the form of sulfides to obtain high toughness Technology has also been proposed.

On the other hand, the two-phase region reheat zone, that is, the region that is exposed to a high temperature just below the melting point during the first welding and becomes a two-phase region of ferrite and austenite by reheating during the subsequent lap welding becomes more brittle than the bond portion. Tend. This is because carbon is concentrated in the austenite region by reheating after the second pass, and this generates a fragile bainite structure including island-like martensite during cooling, and deteriorates toughness. As a countermeasure, Patent Document 4 discloses a technique for suppressing generation of island martensite by reducing C and Si and further increasing the strength of the base material by adding Cu.
JP-A-5-311324 Japanese Patent Publication No. 03-053367 Japanese Patent Laid-Open No. 60-184663 JP 05-186823 A

  In general, it is known that precipitation strengthening effective in improving fire resistance significantly reduces the toughness of steel. Therefore, for steels that require severe low temperature toughness for the base metal and welds, such as steel used for offshore structures, etc., when precipitation strengthening is used to improve fire resistance, Toughness will be greatly degraded. Therefore, conventionally, there has been no steel having both fire resistance and low temperature toughness of the base material and the welded portion.

  The purpose of the present invention is to solve the above-mentioned problems of the prior art, and is excellent in the strength and toughness of the base metal and the toughness of the heat affected zone of the base metal, and also in the fire resistance not considered in the conventional steel It is in proposing the method of manufacturing advantageously.

  The inventors diligently studied a method capable of improving the high temperature strength of the base material while improving the toughness of the weld heat affected zone of the high strength steel. As a result, in addition to the pinning effect of TiN, the amount of Ca added to control the morphology of the sulfide is made within an appropriate range, so that the structure of the heat affected zone is refined and the fire resistance is improved. It has been found that the adverse effects of elements such as Mo added to the effect can be mitigated, and that both the fire resistance of the base metal and the toughness of the weld heat affected zone can be achieved. Furthermore, when the influence of the rolling conditions on the strength toughness of the base metal was examined, it was found that a high-tensile steel excellent in the strength toughness of the base material can be produced by controlling the rolling temperature, cooling rate and cooling stop temperature.

The present invention developed based on the above knowledge is C: 0.03-0.10 mass%, Si: 0.05-0.30 mass%, Mn: 0.6-1.7 mass%, P: 0.015 mass%. Hereinafter, S: 0.0005-0.0050 mass% , Al: 0.005-0.06 mass% , Cr: 0.05-1.0 mass% , Mo: 0.2-0.5 mass% , Nb: 0.005 to 0.05 mass%, Ti: 0.005 to 0.02 mass%, N: 0.0030 to 0.0065 mass%, Ca: 0.0005 to 0.0030 mass%, and Ca, O and S content is the following relational expression;
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
The meet and containing, after heating the steel material and the balance being Fe and unavoidable impurities in 1,050 to 1250 ° _C., and _(Ar 3 + 100 ℃) over a cumulative rolling reduction in the temperature range 50% or more, a finishing temperature Fire rolling and weld joint toughness characterized by cooling to 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec after hot rolling with (Ar ₃ −100 ° C.) to (Ar ₃ + 100 ° C.) It is a manufacturing method of high-tensile steel excellent in.

  In addition to the above component composition, the steel material of the present invention further includes one or more selected from V: 0.01 to 0.06 mass%, Cu: 0.1 to 1.0 mass%, and Ni: 0.02 to 1.5 mass%. It is characterized by containing.

  Further, the present invention is characterized in that the high-tensile steel after cooling is further tempered at a temperature of 700 ° C. or lower.

  According to the present invention, the base metal has good strength at normal temperature and high temperature (600 ° C.) and toughness, and also has excellent toughness in the heat affected zone, yield strength is 414 MPa or more, and tensile strength is 517 MPa or more (API Standard 2W-60) high strength steel can be manufactured at low cost. As a result, it greatly contributes to increasing the size, safety and construction efficiency of buildings and offshore structures.

Next, the reason why the composition range of each component is limited in the present invention will be described.
C: 0.03 ~ 0.10mass%
C is required to be contained in an amount of 0.03 mass% or more in order to obtain the necessary strength as structural steel. On the other hand, if the amount is too large, the toughness of the welded portion is reduced, so the upper limit needs to be 0.10 mass%. Preferably, it is 0.04-0.09 mass%.

Si: 0.05-0.30mass%
Si needs to be added in an amount of 0.05 mass% or more as a deoxidizing component. On the other hand, if it exceeds 0.30 mass%, the toughness of the multi-layer welded portion is deteriorated, so it is necessary to limit it to 0.30 mass% or less.

Mn: 0.6-1.7mass%
Mn needs to be added by 0.6 mass% or more in order to ensure the strength of the base material. On the other hand, if it exceeds 1.7 mass%, the toughness of the welded portion is remarkably deteriorated, so it is necessary to make it 1.7 mass% or less. Preferably, it is 0.8-1.6 mass%.

P: 0.015 mass% or less When P exceeds 0.015 mass%, the toughness of the welded portion is deteriorated, so that it is limited to 0.015 mass% or less. Preferably, it is 0.012 mass% or less.

S: 0.0005-0.0050 mass%
If S is contained in excess of 0.0050 mass%, the toughness of the base metal and the welded portion is deteriorated, so it is 0.0050 mass% or less. Preferably, it is 0.0040 mass% or less. On the other hand, if S is less than 0.0005 mass%, the amount of sulfide effective for refining the HAZ structure cannot be secured, so S is set to 0.0005 mass% or more.

Al: 0.005-0.06mass%
In order to deoxidize molten steel, Al must be contained in an amount of 0.005 mass% or more. On the other hand, if it exceeds 0.06 mass%, the toughness of the base metal is reduced and the weld metal part is diluted by dilution during welding. In order to deteriorate toughness by mixing, it is necessary to limit to 0.06 mass% or less.

Cr: 0.05-1.0mass%
Cr effectively works to increase the strength of steel at room temperature and high temperature. To obtain this effect, it is necessary to add 0.05 mass% or more. On the other hand, addition exceeding 1.0 mass% deteriorates toughness. Therefore, Cr is added in the range of 0.05 to 1.0 mass%.

Mo: 0.2-0.5mass%
Mo is an effective element for improving the hardenability and increasing the strength of the steel by precipitation strengthening and the like, and is particularly effective for increasing the medium / high temperature strength. On the other hand, addition of a large amount increases the cost and causes deterioration of weldability, so is limited to a range of 0.2 to 0.5 mass%.

Nb: 0.005-0.05mass%
Nb is effective in improving the normal temperature and high temperature strength of steel, but 0.005 mass% or more needs to be added to obtain the effect. On the other hand, addition exceeding 0.05 mass% deteriorates the toughness of the welded portion. Therefore, Nb is added in the range of 0.005 to 0.05 mass%.

Ti: 0.005-0.02mass%
Ti precipitates as TiN during solidification and suppresses austenite coarsening in the weld zone, and also serves as a ferrite transformation nucleus to refine the ferrite grains and contribute to higher toughness. If it is less than 0.005 mass%, the effect is small. On the other hand, if it exceeds 0.02 mass%, the expected effect cannot be obtained due to the coarsening of TiN particles.

N: 0.0030-0.0065mass%
N is necessary for securing a necessary amount of TiN, and in order to obtain a sufficient amount of TiN, it is necessary to contain 0.0030 mass% or more. On the other hand, if it exceeds 0.0065 mass%, the amount of solid solution N in the temperature region where TiN dissolves by heating during welding increases, and the toughness is significantly reduced.

Ca: 0.0005 to 0.0030 mass%
Ca is an element that fixes S and improves toughness. In order to exhibit such an effect, it is necessary to contain at least 0.0005 mass%. However, since the effect is saturated even if it contains exceeding 0.0030 mass%, the addition amount of Ca shall be the range of 0.0005-0.0030 mass%.

0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
In order to finely disperse the ferrite transformation nuclei that do not dissolve even at high temperatures, Ca, O, and S have the following formula:
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
Here, Ca, O, S: content of each element (mass%)
It is necessary to contain so as to satisfy the relationship. In the above formula, (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) is a value indicating the ratio of the atomic concentration between Ca concentration and S effective for sulfide form control, The form of sulfide can be estimated (Hirida et al., “Iron and Steel”, Japan Iron and Steel Institute, 66 (1980), No. 3, p. 354-362). When this equation is satisfied, the sulfide precipitate is in the form of a composite sulfide in which MnS is deposited on CaS. On the other hand, when (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) ≦ 0, since CaS does not crystallize, S precipitates in the form of MnS alone. This MnS is elongated at the time of rolling the steel sheet to cause a decrease in the toughness of the base material, and the fine dispersion of α transformation generation nuclei in the weld heat affected zone, which is the main point of the present invention, is not achieved. On the contrary, when 1 ≦ (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S), S is completely fixed by Ca, and MnS acting as ferrite nuclei is formed on CaS. Since it does not precipitate, the composite sulfide does not exhibit a sufficient function as an α-forming nucleus. Therefore, (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) is in the range of more than 0 and less than 1. Preferably it is the range of 0.2-0.8.

In the present invention, in addition to the essential components described above, one or more selected from V, Cu and Ni can be contained in order to further increase the strength and toughness.
V: 0.01-0.06mass%
V is effective for increasing the high temperature strength even if added in a small amount, but in order to obtain the effect, addition of 0.01 mass% or more is preferable. On the other hand, since excessive addition deteriorates the toughness of the welded portion, it is preferably made 0.06 mass% or less.

Cu: 0.1-1.0mass%
Cu is effective for increasing the strength of the base material, but 0.1 mass% or more is preferably added in order to obtain the effect. On the other hand, if it exceeds 1.0 mass%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. Therefore, the upper limit is preferably set to 1.0 mass%.

Ni: 0.02 to 1.5 mass%
Ni is effective in improving the normal temperature strength and the low temperature toughness, but it is preferable to add 0.02 mass% or more in order to obtain the effect. On the other hand, excessive addition causes an increase in cost, so the upper limit is preferably 1.5 mass%.

Next, the manufacturing conditions of the present invention will be clarified.
Molten steel having the above composition is melted by a usual method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material such as a slab by a usual method such as a continuous casting method or an ingot-bundling rolling method. This material is used to produce high-strength steel by the following process.
First, the steel material adjusted to the above-described component composition is heated to a temperature range of 1050 to 1250 ° C. The reason for heating to 1050 ° C. or higher is to crimp the casting defect. However, when heated to a temperature exceeding 1250 ° C., TiN becomes coarse and causes the toughness of the weld to deteriorate, so the heating temperature must be regulated to 1250 ° C. or lower.

After the steel material is heated to the above temperature, it is necessary to perform hot rolling in which the cumulative rolling reduction in a temperature range of (Ar ₃ + 100 ° C.) or higher is 50% or higher. By performing the above rolling in this temperature range, it is possible to press the casting defects generated in the center of the material, promote the recrystallization of austenite grains, refine the structure, and thereby achieve the refinement of the ferrite structure. Toughness can be ensured. However, if the cumulative rolling reduction is less than 50%, casting defects will not be crimped or abnormal coarse grains will remain during heating, which adversely affects the toughness of the base metal.

Subsequent to hot rolling in the temperature range above (Ar ₃ + 100 ° C.), after performing hot rolling with the finish rolling temperature in the range of (Ar ₃ −100 ° C.) to (Ar ₃ + 100 ° C.), the cooling rate Cool to 2 ° C./sec to 600 ° C. or lower, then air cool.
The reason why the finishing rolling temperature is in the range of (Ar ₃ −100 ° C.) to (Ar ₃ + 100 ° C.) is that when rolling is finished at a temperature exceeding (Ar ₃ + 100 ° C.), coarse austenite grains remain. As a result, the toughness at the center of the plate thickness deteriorates. On the other hand, when the rolling is finished at a temperature lower than (Ar ₃ -100 ° C.), the ferrite grains on the steel sheet surface are hardened due to processing strain and the surface toughness is deteriorated. Moreover, the reason for setting the cooling rate after hot rolling to 2 to 20 ° C./sec is that the strength of the base material is greatly reduced because the ferrite fraction increases at less than 2 ° C./sec. This is because when the temperature exceeds ° C./sec, the surface of the steel sheet is remarkably hardened and the toughness is lowered. Furthermore, the reason why the cooling stop temperature is set to 600 ° C. or less is that when the cooling is stopped at a temperature exceeding 600 ° C., the ferrite fraction increases and the strength of the base material is greatly reduced.

  In the present invention, for the purpose of reducing the residual stress of the steel sheet, it is preferable to perform a tempering treatment at a temperature of 700 ° C. or lower after the above cooling. In order to obtain this effect, the tempering temperature is preferably 500 ° C. or higher. On the other hand, when it exceeds 700 ° C., various carbides are precipitated and coarsened, and it is difficult to ensure the high temperature strength thereafter. A more preferable tempering temperature is 500 to 650 ° C.

  Steels adjusted to various component compositions shown in Table 1 were melted into steel slabs, and then the steel slabs were hot-rolled under the conditions shown in Table 2 to produce steel plates having a thickness of 40 to 75 mm. Thereafter, a tempering treatment was performed. Each thick steel plate thus obtained was subjected to a tensile test and a Charpy test. Tensile specimens were collected from ¼ position of each steel plate so that the tensile direction was parallel to the direction perpendicular to the rolling direction. The yield strength YS and the tensile strength TS were determined. In addition, Charpy impact test specimens were obtained from JIS No. 4 impact test specimens from the center of the thickness of each steel plate so that the length direction was parallel to the direction perpendicular to the rolling direction, and the absorbed energy at −40 ° C. was measured. Asked. In addition, in order to evaluate the toughness of each steel sheet after the welding heat cycle, a test piece having a width of 80 mm, a length of 80 mm, and a thickness of 15 mm was collected, and after heating the test piece to 1400 ° C., a cooling time of 800 to 500 ° C. After applying a heat cycle of 40 seconds (equivalent to about 40 kJ / cm heat input in submerged arc welding), a JIS No. 4 impact test piece was collected from this test piece in the same manner as described above, and a Charpy impact test was performed. The ductile brittle fracture surface transition temperature vTrs was measured.

The results of the above measurements are shown together in Table 2. Nos. 1 to 6 satisfying the conditions of the present invention all have a yield stress at room temperature of 414 MPa or more, a tensile strength of 517 MPa or more (API standard 2W-60), and an impact absorption energy vE of _{−40 ° C. at −40 ° C.} It has a characteristic of 200J or more, and it can be seen that the base material has excellent room temperature strength and toughness. Moreover, the ratio of the yield stress at 600 ° C. to the yield stress at normal temperature is 53 (56)% or more, which is excellent in high temperature strength. Furthermore, the toughness of the HAZ part by the reproducible thermal cycle is a steel material having excellent weld heat affected zone toughness with a ductile brittle fracture surface transition temperature (vTrs) of -20 ° C or lower. On the other hand, the comparative examples (Nos. 7 to 30) outside the scope of the present invention are the yield stress and tensile strength of the base material and the yield stress at vE- _{40 ° C.} and 600 ° C., or the reproduction HAZ vTrs. Any one or more are inferior to the examples of the present invention, and the steel is not excellent in the strength, toughness, high temperature strength of the base metal part, and toughness of the weld heat affected zone.

  According to the present invention, it is possible to produce a high-strength steel for a structure excellent in fire resistance and weld toughness. Therefore, in conventional steels with insufficient fire resistance, extra costs such as fireproof coating were necessary to prevent the collapse of the structure in the event of a fire. it can.

Claims (3)

C: 0.03-0.10 mass%,
Si: 0.05-0.30 mass%,
Mn: 0.6 to 1.7 mass%,
P: 0.015 mass% or less,
S: 0.0005 to 0.0050 mass%,
Al: 0.005 to 0.06 mass%,
Cr: 0.05-1.0 mass% ,
Mo: 0.2-0.5 mass%
Nb: 0.005 to 0.05 mass%,
Ti: 0.005-0.02 mass%,
N: 0.0030 to 0.0065 mass%,
Ca: 0.0005 to 0.0030 mass%, and the contents of Ca, O, and S are the following relational expressions:
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
The meet and containing, after heating the steel material and the balance being Fe and unavoidable impurities in 1,050 to 1250 ° _C., and _(Ar 3 + 100 ℃) over a cumulative rolling reduction in the temperature range 50% or more, a finishing temperature Fire rolling and weld joint toughness characterized by cooling to 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec after hot rolling with (Ar ₃ −100 ° C.) to (Ar ₃ + 100 ° C.) For producing high-strength steel with excellent resistance. In addition to the above component composition, V: 0.01 to 0.06 mass%, Cu: 0.1 to 1.0 mass%, and Ni: 0.02 to 1.5 mass%, one or two selected The method for producing high-tensile steel according to claim 1, comprising the above. The method for producing high-strength steel according to claim 1 or 2, wherein the high-tensile steel after cooling is further tempered at a temperature of 700 ° C or lower.