JP5470904B2 - TS of 570 MPa or more, total elongation of 25% or more, and fatigue crack propagation rate at ΔK = 15 MPa√m of 8.75 × 10 −9 m / cycle or less, excellent in total elongation and fatigue crack propagation resistance, plate thickness of 20 mm or less Of manufacturing thick steel plate - Google Patents

Description

  The present invention is mainly applied to a welded structure such as a ship, an offshore structure, a bridge, a building, a tank, and the like, which has a strong demand for structural safety, and a method for producing a thick steel plate having excellent fatigue crack propagation resistance. In particular, the present invention relates to a method suitable as an on-line manufacturing method for a thin steel plate having a thickness of 20 mm or less, in which a decrease in elongation is a problem.

  Steel materials used in structures such as ships, offshore structures, bridges, buildings, tanks, etc. have excellent mechanical properties such as strength and toughness and weldability, as well as repeated loads and winds during normal operation. In addition, the structural safety of the structure must be ensured against repeated vibration caused by earthquakes.

  It is required to have excellent fatigue characteristics for repeated loads. In order to prevent ultimate failure such as fracture of members, it is effective to improve the propagation resistance of fatigue cracks possessed by steel materials. I think.

  In the case of general welded structures, the weld toe tends to be a stress concentration part, and the tensile residual stress due to welding also acts and often becomes a source of fatigue cracks. It is known to introduce a compressive residual stress by tanning welding or shot peening.

  However, since there are a large number of weld toes in the welded structure and the burden is high in cost, these methods are unsuitable for implementation on an industrial scale, and the fatigue resistance characteristics of the welded structure are It is often achieved by improving the fatigue crack propagation characteristics of the steel material used.

  In these steel structures, welding is often performed in various directions with respect to the steel sheet, for example, in various directions with respect to the rolling direction, and therefore the direction of fatigue crack initiation and propagation is also various. In addition, it is desirable that the fatigue crack propagation resistance performance of the steel plate is high regardless of the direction in the steel.

  Patent Document 1 relates to a steel plate for a tanker, and the structure thereof is composed of a mixed structure of a first phase of ferrite and a second phase of bainite and / or pearlite. It is described that it has excellent fatigue crack growth characteristics.

  Patent Document 2 discloses a steel sheet having a fatigue crack growth-suppressing effect characterized in that the structure is composed of a base of a hard part and a soft part dispersed in the base, and the hardness difference between the two is 150 or more in terms of Vickers hardness. Is described.

In Patent Document 3, the steel structure of the cross section is ferrite and bainite, the ferrite phase has an area ratio of 38% or more and 52% or less, the hardness of the ferrite phase portion is 80HV0.02-150HV0.02, and The tensile strength with excellent fatigue crack growth resistance is 55 kgf, characterized in that the boundary between the ferrite phase and the bainite phase exists at a density of 50 to 300 locations / mm on a straight line drawn at an arbitrary location in the cross section. Ferritic bainite duplex steel with a / mm ² or more is described.

  Patent Document 4 discloses a ferrite in which the distance from the interface between the second phases in the fatigue crack propagation direction to the interface to the next second phase is 25 μm or less, and the cross-sectional structure in the plate thickness direction is 60 to 90% in area ratio. It consists of a parent phase and a second phase, the hardness of the second phase: Hv (SP) and the hardness of the ferrite: Hv (F) satisfy the value represented by a specific formula, and the aspect ratio of the second phase: A steel material having excellent fatigue crack propagation characteristics is described, wherein 1 (major axis length) / d (minor axis length) is 1 / d> 3.42.

  Patent Document 5 relates to a method for producing a steel material having excellent ductility and fatigue crack propagation characteristics, by stopping temporary cooling in the middle of accelerated cooling after rolling, and holding accelerated cooling again after holding for a predetermined time, thereby increasing the thickness of the steel sheet. It is described that the microstructure is mainly composed of fine ferrite homogeneous in the direction and microstructure pearlite dispersed in the microstructure.

Patent Document 6 relates to a method of manufacturing a steel material having excellent fatigue crack propagation characteristics with a small strength difference in the thickness direction, and from Patent Document 5, the amount of C is 0.10 to 0 so that a high-strength thick material can be manufactured. It is described that the accelerated cooling is started at a high temperature after air cooling for a certain time after completion of hot rolling at ₃ or more points of Ar.

Patent Document 7 relates to a high-strength steel material excellent in ductile crack initiation characteristics and fatigue crack propagation characteristics and a method for producing the same, and has a microstructure containing fine ferrite in an area fraction of 10 to 40%, bainite, martensite or It is described that a thick material having a tensile strength of 550 MPa or more is produced by direct quenching after hot rolling is completed at Ar ₃ points or more and then air-cooled for a certain time with the mixed structure of both.

Japanese Patent No. 2785643 Japanese Patent No. 2962134 Japanese Patent No. 3489243 Japanese Patent No. 3434434 JP 2005-314811 A JP 2005-314812 A JP 2007-197777 A

  By the way, all of the steel sheets according to the inventions described in Patent Documents 1 to 4, 6, and 7 introduce bainite or martensite as a hard phase in order to improve fatigue crack propagation resistance. The use of will lead to degradation of total elongation.

  In particular, when producing a thick steel plate with excellent fatigue crack propagation resistance by an on-line process by rolling and accelerated cooling control, which is the subject of the present invention, in order to obtain such a hard phase, the cooling stop temperature is lowered. In a thin material with a plate thickness of 20 mm or less, there was a problem that the total elongation was remarkably lowered due to a decrease in the cooling stop temperature. That is, when the plate thickness was 20 mm or less, fatigue crack propagation resistance and total elongation were reduced. It was difficult to achieve both. When the plate thickness exceeds 20 mm (18 mm), this coexistence is relatively easy.

  In steel materials used for structures such as ships, offshore structures, bridges, buildings, tanks, etc., the total elongation value is often specified in the standard, and even when improving fatigue crack propagation resistance, the total elongation is However, there is no production guideline that can control both fatigue crack propagation resistance and total elongation. The inventions described in Patent Documents 5 to 7 are also applicable to a sheet thickness of 20 mm or more, require air cooling for a certain period of time from rolling to the start of cooling, and are said to be optimal as a method for manufacturing a sheet material having a sheet thickness of 20 mm or less. hard. As described above, there is a demand for a technique for appropriately controlling both fatigue crack propagation resistance and total elongation in a plate thickness of 20 mm or less.

  Accordingly, an object of the present invention is to provide a manufacturing method suitable for manufacturing a thin steel plate having a thickness of 20 mm or less, particularly less than 18 mm, which is excellent in total elongation and excellent in fatigue crack propagation resistance. To do.

  The present inventor is directed to a thick steel plate manufactured by an on-line process in which an accelerated cooling is performed by an accelerating cooling device provided on a conveying line and rolling by a reversible rolling mill, and chemical components affecting the total elongation and fatigue crack propagation In the case of producing a thick steel plate having a thickness of 20 mm or less by examining the influence of production conditions in detail, the C addition amount is lowered to control Ceq within a specific range, and the cooling stop temperature is lowered to reduce the temperature after rolling. It was found that both excellent elongation and fatigue crack propagation resistance can be achieved without requiring air cooling for a certain period of time before the start of accelerated cooling.

The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. C: 0.03% or more, less than 0.10%, Si: 0.05 to 0.50%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.02% or less, Ceq (= {C} + {Mn} / 6 + {Cu + Ni} / 15 + {Cr + Mo + V} / 5, where each element is contained (mass%)): 0.3 to 0.4 the steel balance consisting of Fe and unavoidable impurities, 1000 ° C. or higher, then heated to 1250 ° C. or less, as the cumulative rolling reduction at Ar ₃ point or more is 50% or more, Ar ₃ point or more rolling end temperature after rolling, subsequently online, up to 600 ° C. or less 300 ° C. or higher than the temperature range above ₃ points Ar, 3 0, characterized in that accelerated cooling at ° C. / s or more, TS more than 570 MPa, a total elongation of 25% or more , ΔK = fatigue in 15MPa√m crack propagation velocity 8.75x10 ^-9 m / cyc The following, total elongation and manufacturing method of steel plate excellent thickness 20mm below the fatigue crack propagation resistance e.
2. Further, as a steel component, in mass%, Cu: 0.4% or less, Ni: 0.8% or less, Cr: 0.4% or less, Mo: 0.4% or less, Nb: 0.05% or less, 1. TS of 1 characterized by containing 1 type or 2 types or more of V: 0.05% or less, Ti: 0.03% or less, B: 0.003% or less, Ca: 0.005% or less. 570MPa or more, the total elongation of 25% or more, [Delta] K = fatigue crack propagation speed 8.75x10 ^-9 m / cycle follows in 15MPa√m, total elongation and excellent thickness 20mm below the fatigue crack propagation resistance Manufacturing method of thick steel plate.

  According to the present invention, a method for producing a thin steel plate having a total thickness of 20 mm or less, and optimally less than 18 mm, having excellent fatigue crack propagation resistance, is obtained. Even if fatigue cracks occur over time from welds, welds, etc., it is possible to delay the subsequent propagation and increase the safety of the steel structure, which is extremely useful industrially.

The figure which shows the shape of the one side notch simple tension type fatigue test piece (plate thickness of 12 mm).

The composition of the present invention and the definition of production conditions will be described in detail.
[Ingredient composition] In the description, “%” means “mass%”.
C
C is added in an amount of 0.03% or more to ensure strength. When 0.10% or more is added, in the on-line process by rolling / accelerated cooling control, along with deterioration of the total elongation, weldability deteriorates, so 0.03% or more and less than 0.10% is added.

Si
Si is added in an amount of 0.05% or more to ensure deoxidation and strength. If added over 0.50%, weldability and toughness deteriorate, so 0.05 to 0.50% (0.05% or more and 0.50% or less), preferably 0.10 to 0.40%. And

Mn
Mn is added in an amount of 0.5% or more in order to ensure strength and toughness by increasing hardenability. If it exceeds 2.0%, the total elongation is lowered and the weldability is deteriorated, so 0.5 to 2.0%, preferably 0.8 to 1.6% is added.

P
P is an unavoidable impurity and deteriorates toughness. Therefore, its content is preferably as small as possible, and is 0.05% or less, preferably 0.03% or less in terms of manufacturing cost.

S
Since S is an inevitable impurity and deteriorates toughness, its content is preferably as small as possible, and is 0.02% or less, preferably 0.01% or less in terms of manufacturing cost.

Ceq (= {C} + {Mn} / 6 + {Cu + Ni} / 15 + {Cr + Mo + V} / 5, where each element is contained (mass%)): 0.3 to 0.4
Ceq (= {C} + {Mn} / 6 + {Cu + Ni} / 15 + {Cr + Mo + V} / 5) is a fatigue crack propagation resistance by an on-line process that performs rolling and accelerated cooling control targeted by the present invention. When manufacturing a thick steel plate having excellent strength, it is specified to ensure a strength of 570 MPa. If Ceq is less than 0.3, it is difficult to ensure a strength of 570 MPa class. On the other hand, if it exceeds 0.4, an unnecessary increase in strength and deterioration of the total elongation are caused. 4

  The above is the basic component composition of the steel according to the present invention, but when further improving or imparting strength, toughness, weldability and weather resistance, Cu, Ni, Cr, Mo, Nb, V, Ti, B, One or more of Ca are added.

Cu
Cu increases the strength by solid solution and improves the weather resistance, so it is added according to the desired properties. When added, if it exceeds 0.4%, the weldability is impaired, and flaws are likely to occur during the manufacture of the steel material. Therefore, when added, the content is made 0.4% or less, preferably 0.3% or less.

Ni
Ni improves low-temperature toughness and weather resistance, and improves hot brittleness when Cu is added, so it is added according to desired characteristics. If it exceeds 0.8%, the weldability is impaired and the steel material cost increases, so when added, the content is made 0.8% or less, preferably 0.6% or less.

Cr
Cr increases the strength and improves the weather resistance, so it is added according to the desired properties. If over 0.4%, weldability and toughness are impaired, so when added, the content is made 0.4% or less, preferably 0.3% or less.

Mo
Since Mo increases strength, it is added according to desired characteristics. When added, if it exceeds 0.4%, weldability and toughness are impaired, so when added, the content is made 0.4% or less, preferably 0.2% or less.

Nb
Nb suppresses austenite recrystallization during rolling to achieve finer grains, and at the same time, precipitates during air cooling after accelerated cooling and increases strength. Therefore, Nb is added according to desired characteristics. When added, if it exceeds 0.05%, the toughness is impaired, so 0.05% or less, preferably 0.03% or less.

V
V precipitates during air cooling after accelerated cooling and increases the strength, so it is added according to the desired characteristics. When added, if it exceeds 0.05%, weldability and toughness are impaired, so 0.05% or less, preferably 0.03% or less.

Ti
Ti increases strength and improves weld toughness, so it is added according to desired properties. When adding, if it exceeds 0.03%, the steel material cost increases, so 0.03% or less, preferably 0.02% or less.

B
B increases the hardenability and increases the strength, so it is added according to the desired properties. When adding, if it exceeds 0.003%, the weldability deteriorates, so 0.003% or less, preferably 0.002% or less.

Ca
Since Ca improves the toughness of the steel sheet by controlling the sulfide form, it is added according to the desired characteristics. When adding, if the content exceeds 0.005%, the effect is saturated, so the upper limit is made 0.005%.

[Production conditions]
The steel material according to the present invention heats steel having the above-described components to 1000 ° C. or more and 1250 ° C. or less, and after rolling with an Ar ₃ point or more and a cumulative reduction ratio of 50% or more, 600 ° C. from a temperature range of Ar ₃ or more point. It is obtained by accelerated cooling to 10 ° C./s or higher up to 300 ° C. or lower.

  The steel material according to the present invention is as thin as 20 mm or less, and the temperature of the steel sheet immediately decreases after rolling. Therefore, it is preferable to manufacture the steel material using an online process in which a rolling device and an accelerated cooling device are provided on the transport line.

1. Heating temperature The heating temperature is set to 1000 ° C. or higher in order to secure the rolling temperature. If the temperature exceeds 1250 ° C, the crystal grains of the steel become coarse, so the upper limit is set to 1250 ° C.

2. If rolling conditions rolling end temperature is lower than ₃ points Ar, becomes a two-phase region rolling, since the total elongation is deteriorated, and Ar ₃ point or more. In addition, when the cumulative rolling reduction at ₃ or more points of Ar is less than 50%, the refinement of the ferrite grains and the refinement of the structure through the refinement of the austenite grains become insufficient, and the toughness that is the basic performance of the steel sheet deteriorates. 50% or more.

3. Accelerated cooling condition The accelerated cooling start temperature has a lower limit of Ar _{3 in} order to avoid a decrease in strength due to excessive ferrite precipitation.

  The accelerated cooling stop temperature is set to 600 ° C. or lower and 300 ° C. or higher in order to transform untransformed austenite into a hard phase. When the accelerated cooling stop temperature exceeds 600 ° C., fatigue crack propagation resistance deteriorates, and sufficient strength, specifically, strength of 570 MPa or more may not be obtained.

  On the other hand, when the accelerated cooling stop temperature is lower than 300 ° C., the above-described component steel sheet cannot obtain a sufficient total elongation, specifically, 25% elongation in the full thickness tensile test (test piece shape JIS No. 5).

  The cooling rate is set to 10 ° C./s or more in order to prevent coarsening of ferrite that deteriorates fatigue crack propagation characteristics during cooling and coarsening of the structure through this.

  The preferable range of the cooling rate is 30 ° C./s or more for obtaining a sufficient coarsening suppression effect, and 60 ° C./s or less for reducing the residual stress of the steel sheet.

In the above rules, the temperature is the surface temperature of the steel material, and the cooling rate is the average cooling rate in the thickness direction of the steel material. The Ar ₃ point can be determined by Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (where the element symbol is the content in mass% of each element in the steel). .

The combination of the above-described component composition and manufacturing conditions enables the use of a steel plate having a thickness of 20 mm or less, particularly a thickness of less than 18 mm, which is excellent in total elongation and has high fatigue crack propagation resistance. Acceleration cooling excellent in fatigue crack propagation resistance, with an elongation of 570 MPa or more, a total elongation of 25% or more (JIS No. 5), and a fatigue crack propagation rate at ΔK = 15 MPa√m of 8.75 × 10 ⁻⁹ m / cycle or less A die-thick steel plate is obtained.

  Steel sheets having a thickness of 12 mm to 17 mm were prepared from the steel pieces having the composition shown in Table 1 under the manufacturing conditions shown in Table 2, and the mechanical properties of the obtained steel sheets were investigated.

  With regard to fatigue crack propagation characteristics, the fatigue crack propagation behavior when a crack propagates in the plate thickness direction was investigated using a single-side notched simple tensile fatigue test piece shown in FIG. The test conditions were based on ASTM E647, and the stress ratio was 0.1, the frequency was 15 Hz, and the room temperature atmosphere was used.

The tensile strength was obtained by a tensile test using a full thickness test piece of JISZ22015 collected in the plate width direction. As for toughness, an absorbed energy at 0 ° C .: vE ₀ (J) was determined by a Charpy impact test. A Charpy impact test piece (JISZ2202) was taken in parallel to the rolling direction from the center of the plate thickness.

Table 2 shows the results of tensile, Charpy, and fatigue crack propagation tests together with the manufacturing conditions. The plate number No. in which the component composition and the production method are within the scope of the present invention. 3-No. 6, no. 15, 17, 18, no. 20-No. Each of the 23 steel plates has a fatigue crack propagation rate of 8.75 × 10 ⁻⁹ m / cycle or less at ΔK = 15 MPa√m, and is excellent in fatigue crack propagation resistance.

Further, TS is 570 MPa or more, elongation is 25% or more, and vE ₀ (J) is 200 J or more as a basic characteristic of the structural steel sheet, and exhibits excellent total elongation and fatigue crack propagation resistance, It was confirmed that the steel had adequate strength and toughness.

  On the other hand, Example No. using steel types A and B in which the addition amount of C exceeds the range of the present invention. The steel sheets 1 and 2 have excellent fatigue crack propagation speeds of the order of the present invention, but the total elongation is low.

  Example No. using steel grade G with Ceq less than the scope of the present invention. The steel plate No. 7 does not have the required strength as a 570 MPa grade steel. Example No. using steel grade H with Ceq exceeding the range of the present invention. The steel plate No. 8 has too high strength and deteriorates in total elongation.

  Example No. using steel type I in which the amount of Mn added exceeds the range of the present invention. The steel plate of No. 9 is too strong and deteriorates in total elongation.

  In Examples Nos. 10 to 14, the steel type E is manufactured under various manufacturing conditions. In Example 10, the slab heating temperature exceeded the range of the present invention, and the austenite grain size was coarsened, so that the toughness of the steel sheet after rolling was deteriorated.

Ar No. ₃ Rolling rate of ₃ points or more is less than the specified value of the present invention: 50%. Steel No. 11 also has poor toughness because austenite is not sufficiently recrystallized and has a rough structure after cooling.

Rolling finish temperature is below Ar ₃ point (two-phase rolling) No. 12 steel plate has deteriorated in total elongation. The accelerated cooling start temperature is higher than the Ar ₃ point. In Steel No. 13, proeutectoid ferrite was precipitated before the start of water cooling, and the ferrite was excessive. Therefore, the strength required for 570 MPa grade steel was not obtained.

  The cooling rate deviates from the scope of the present invention. The steel plate No. 14 has a reduced strength, a deteriorated toughness due to coarsening of the structure, and a fatigue crack propagation resistance. No. in which the stop temperature during accelerated cooling exceeds the range of the present invention. Although steel plate No. 16 exhibits excellent total elongation, the strength required for 570 MPa grade steel has not been obtained, and fatigue crack propagation resistance is also inferior.

  No. in which the stop temperature during accelerated cooling is lower than the range of the present invention. No. 19 steel plate exhibits excellent fatigue crack propagation resistance but is inferior in total elongation.

Claims (2)

C: 0.03% or more, less than 0.10%, Si: 0.05 to 0.50%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.02% or less, Ceq (= {C} + {Mn} / 6 + {Cu + Ni} / 15 + {Cr + Mo + V} / 5, where each element is contained (mass%)): 0.3 to 0.4 the steel balance consisting of Fe and unavoidable impurities, 1000 ° C. or higher, then heated to 1250 ° C. or less, as the cumulative rolling reduction at Ar ₃ point or more is 50% or more, Ar ₃ point or more rolling end temperature After rolling, on-line, it is accelerated cooling at 30 ° C / s or more from 600 ° C or less to 300 ° C or more from a temperature range of ( Ar ₃ points + 11 ° C) or more, TS is 570 MPa or more, and total elongation is more than 25%, ΔK = fatigue in 15MPa√m crack propagation velocity 8.75x10 ^- m / cycle or less of the total elongation and a manufacturing method of steel plate excellent thickness 20mm below the fatigue crack propagation resistance. Further, as a steel component, in mass%, Cu: 0.4% or less, Ni: 0.8% or less, Cr: 0.4% or less, Mo: 0.4% or less, Nb: 0.05% or less, 2. V: 0.05% or less; Ti: 0.03% or less; B: 0.003% or less; Ca: 0.005% or less. TS of 570 MPa or more, total elongation of 25% or more, and a fatigue crack propagation rate of 8.75 × 10 ⁻⁹ m / cycle or less at ΔK = 15 MPa√m, a plate thickness of 20 mm excellent in total elongation and fatigue crack propagation resistance The manufacturing method of the following thick steel plates.

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