JP2008248314A - Method for producing thick steel plate excellent in total elongation and fatigue crack propagation resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a thick steel plate of thin sheet having ≤20 mm thickness excellent in total elongation and fatigue-cracking propagation resistance, and suitable to a welded structural material which strongly requires the structural safety, such as a vessel. <P>SOLUTION: The steel composed by mass% of 0.03 to <0.10% C, 0.05-0.50% Si, 0.5-2.0% Mn, ≤0.05% P, ≤0.02% S and if necessary, one or more kinds of Cu, Ni, Cr, Mo, Nb, V, Ti, B, Ca and 0.3-0.4 Ceq(=äC}+äMn}/6+äCu+Ni}/15+äCr+Mo+V}/5, wherein, each element shows content by mass.%), and the balance substantially Fe, is heated to 1,000-1,250°C and after rolling at Ar<SB>3</SB>point or higher and in ≥50% accumulated rolling-reduction ratio and at Ar<SB>3</SB>point or higher of rolling-finish temperature, the accelerated cooling is performed from the temperature range of Ar<SB>3</SB>-10°C to Ar<SB>3</SB>-80°C to 600-300°C at ≥10°C/s. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a thick steel plate excellent in fatigue crack propagation resistance, which is suitable for welded structures such as ships, offshore structures, bridges, buildings, tanks, and the like where structural safety is strongly required. The present invention relates to an on-line manufacturing method of a thin steel plate having a thickness of 20 mm or less, which causes a problem of deterioration.

  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, and in order to prevent ultimate failure such as member breakage, it is effective to improve the resistance to propagation of fatigue cracks in 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 position 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.
Japanese Patent No. 2785643 Japanese Patent No. 2962134 Japanese Patent No. 3489243 Japanese Patent No. 3434434

  By the way, all the steel plates according to the inventions described in Patent Documents 1 to 4 have introduced bainite or martensite as a hard phase in order to improve fatigue crack propagation resistance. It causes deterioration of 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 this application, in order to obtain such a hard phase, the cooling stop temperature can be lowered. Necessary and for thin objects of 20 mm or less, there is a problem that the total elongation is significantly reduced due to a decrease in cooling stop temperature.

  However, in steel materials used for structures such as ships, marine structures, bridges, buildings, and tanks, the total elongation value is often specified in the standard, and even when improving fatigue crack propagation resistance, Although it is premised on that the total elongation satisfies the standard value, there has not been a production guideline that can control both the fatigue crack propagation resistance and the total elongation.

  Accordingly, an object of the present invention is to provide a method for producing a thin steel plate having a thickness of 20 mm or less that is excellent in total elongation and excellent in fatigue crack propagation resistance.

  The present inventor has studied in detail the influence of chemical composition and manufacturing conditions on total elongation and fatigue crack propagation for thick steel plates manufactured by rolling using a reversible rolling mill and an on-line process by accelerated cooling. In the method of manufacturing a thick steel plate having a thickness of less than 20 mm, it has been found that Ceq is controlled to a specific range by lowering the C addition amount, and both elongation and fatigue crack propagation resistance are achieved by lowering the cooling stop temperature. .

The present invention has been made by further studying the obtained knowledge. 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 being substantially Fe, 1000 ° C. or higher, then heated to 1250 ° C. or less, the cumulative rolling reduction of 50% or more at Ar ₃ point or more, after rolling over the rolling end temperature Ar _{3 point,} Ar 3 -10 Production of a thick steel plate excellent in total elongation and fatigue crack propagation resistance, characterized by accelerated cooling at a rate of 10 ° C / s or higher from a temperature range of from ₃ ° C to Ar 3-80 ° C to 600 ° C or lower and 300 ° C or higher Method.
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. Total elongation according to 1, characterized by containing one or more of V: 0.10% or less, Ti: 0.03% or less, B: 0.003% or less, Ca: 0.005% or less And a method for producing thick steel plates with excellent fatigue crack propagation resistance.

  According to the present invention, a method for producing a thick steel sheet having a thickness of less than 20 mm and excellent in total elongation and having high fatigue crack propagation resistance can be obtained. For example, a fatigue crack can be obtained from a stress-concentrated part or a welded part. Even if it occurs over time, it is possible to delay the subsequent propagation and increase the safety of the steel structure, which is extremely useful in the industry.

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 online process based on rolling / accelerated cooling control, the total elongation is deteriorated and weldability is deteriorated. Therefore, 0.03% to 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%, preferably 0.10 to 0.40%.

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 impurity and degrades 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 impurity and degrades toughness, the 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, where each element is a content (% by mass)), which is the subject of the present application, depends on rolling and accelerated cooling control When producing a thick steel plate having excellent fatigue crack propagation resistance by an online process, it is specified in order to ensure a strength of 500 MPa class. When Ceq is less than 0.3, it becomes difficult to secure a strength of 500 MPa, while when it exceeds 0.4, an unnecessary increase in strength and deterioration of the total elongation are caused.

  The above is the basic component composition of the steel according to the present invention. In the case of further improving or imparting strength, toughness, weldability and weather resistance, Cu, Ni, Cr, Mo, Nb, V, Ti, B Add one or two or more.

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%, weldability deteriorates, so 0.003% or less, preferably 0.002% or less.

Ca
Ca is added for the purpose of improving the toughness of the steel plate through the sulfide form control. When adding, if the content exceeds 0.005%, the effect is saturated, so the upper limit is made 0.005%.

[Production conditions]
Steel components according to the steel according to the present invention described above, 1000 ° C. or higher, then heated to 1250 ° C. or less to complete the rolling at a cumulative reduction of 50% or more of the rolling performed Ar ₃ point or more by Ar ₃ or more points Thereafter, it is obtained by accelerated cooling at a rate of 10 ° C./s or higher from Ar ₃ −10 ° C. to a temperature range of Ar ₃ −80 ° C. to 600 ° C. or lower and 300 ° C. or higher.

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 or less.

2. Rolling conditions When the rolling end temperature is lower than the Ar ₃ point, the rolling becomes two-phase region and the total elongation deteriorates. Moreover, when the cumulative rolling reduction of ₃ or more points of Ar is less than 50%, the refinement | miniaturization of a ferrite grain and refinement | miniaturization of a microstructure through refinement | miniaturization of an austenite grain become inadequate, and toughness which is the basic performance of a steel plate deteriorates.

3. Accelerated cooling condition The accelerated cooling start temperature is set to Ar ₃ -10 ° C. or lower in order to precipitate pro-eutectoid ferrite and improve fatigue crack propagation resistance. Moreover, in order to avoid strength reduction due to excessive ferrite precipitation, the lower limit of the accelerated cooling start temperature is set to Ar ₃ point-80 ° C.

  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 is deteriorated, and sufficient strength, specifically, a strength of 490 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 plate cannot achieve a sufficient total elongation, specifically, a 15% elongation cannot be obtained in a full thickness tensile test (test piece shape JIS1A). 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 recommended 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.

The above 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 is determined by Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (where the element symbol represents the content in mass% of each element in the steel). be able to.

  By the combination of the above-described component composition and manufacturing conditions, a thick steel plate having a thickness of 20 mm or less having excellent total elongation and high fatigue crack propagation resistance can be obtained.

  Using steel slabs having the composition shown in Table 1, steel sheets having a thickness of 12 mm were prepared under the production 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 determined by a tensile test using a full thickness test piece of JISZ2201 1A sampled in the plate width direction. As for toughness, the absorbed energy at 0 ° C .: vE0 (J) was determined by 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. The plate number No. in which the components and the production method are within the specified range of the present invention. 3-No. 6, no. The steel sheets of 15, 17, and 18 all have a fatigue crack propagation rate of 8.75 × 10 ⁻⁹ m / cycle or less at ΔK = 15 MPa√m, and are excellent in resistance to fatigue crack propagation.

  In addition, TS is 490 MPa or more, elongation is 15% or more, vE0 (J) is 200 J or more as a basic characteristic of structural steel sheet, excellent overall elongation and resistance to fatigue crack propagation, and for structural use 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 50 kg class 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 9 has the same tendency as the steel plate of Example 8.

  In Examples Nos. 10 to 14, 16 and 19, steel type E was 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 ratio of ₃ points or more is less than 50% of the present invention specified value. 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 No. 13, the pro-eutectoid ferrite does not precipitate, and the fatigue crack propagation resistance is inferior.

  The cooling rate deviates from the scope of the present invention. In the steel plate No. 14, the strength decreases, the toughness deteriorates due to the coarsening of the structure, and the fatigue crack propagation resistance also decreases. No. in which the stop temperature during accelerated cooling exceeds the range of the present invention. Steel plate No. 16 exhibits excellent total elongation, but is inferior in fatigue crack propagation resistance.

  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.

The figure which shows the shape of a one-side notch simple tension type fatigue test piece.

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 being substantially Fe, 1000 ° C. or higher, then heated to 1250 ° C. or less, the cumulative rolling reduction of 50% or more at Ar ₃ point or more, after rolling over the rolling end temperature Ar _{3 point,} Ar 3 -10 Production of a thick steel plate excellent in total elongation and fatigue crack propagation resistance, characterized by accelerated cooling at a rate of 10 ° C / s or higher from a temperature range of from ₃ ° C to Ar 3-80 ° C to 600 ° C or lower and 300 ° C or higher Method.   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: 0.10% or less, Ti: 0.03% or less, B: 0.003% or less, Ca: 0.005% or less. A method for producing thick steel plates with excellent total elongation and fatigue crack propagation resistance.

Images