JP2010059472A - Thick steel plate with low yield ratio and high toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate which has a low yield ratio, and besides, shows high toughness characteristics. <P>SOLUTION: The steel plate includes: 0.03 to 0.15% C, 1.0% or less Si (excluding 0%), 1.0 to 2.0% Mn, 0.015% or less P (excluding 0%), 0.010% or less S (excluding 0%), 0.005 to 0.060% Al, 0.008 to 0.030% Ti, 0.0020 to 0.010% N and 0.010% or less O (excluding 0%). Also, the steel plate has a microstructure formed of a mixed structure of ferrite and bainite in which island-shaped martensites are dispersed, in a position of t/4 (t: plate thickness). The ferrites have an average particle size of 10 to 50 μm, and the island-shaped martensites existing in the bainite occupy 1 to 20 area% by fraction with respect to the whole area. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a low yield ratio high toughness steel plate suitable for use in the fields of architecture, offshore structures, line pipes, shipbuilding, civil engineering, construction machinery and the like.

  In recent years, in various welded structural steel materials, the yield ratio YR (YR = YS / TS) represented by the ratio of the yield stress YS and the tensile strength TS is lowered from the viewpoint of earthquake resistance in addition to high strength and high toughness. It is also required to do. In general, it is known that the yield ratio of steel can be reduced by making the microstructure of steel a structure in which a hard phase such as bainite or martensite is appropriately dispersed in a soft phase such as ferrite. It has been.

  As a production method for obtaining a structure in which a hard phase is appropriately dispersed in the soft phase as described above, for example, Patent Document 1 discloses a two-phase ferrite and austenite between quenching (Q) and tempering (T). A heat treatment method for performing quenching (Q ′) from the zone is known.

Patent Document 2 discloses a method never manufacturing process is increased, after completion of the rolling at Ar ₃ transformation point temperature or higher, accelerated cooling to a temperature of the steel is below Ar ₃ transformation point temperature to produce the ferrite A method of delaying the onset of is disclosed.

On the other hand, Patent Literature 3 discloses a three-phase structure of ferrite, bainite, and island martensite as a technique for achieving a low yield ratio without performing a complicated heat treatment as disclosed in Patent Literature 1 and Patent Literature 2. A method has been proposed.
JP-A-55-97425 JP 55-41927 A JP 2005-23423 A

  However, with the technologies proposed so far, in the processing of circular steel pipes with a thickness of 40 mm or more that require a high workability (D / t <10, D: steel pipe diameter mm, t: plate thickness), etc., extremely low It has been difficult to make the yield ratio (YR <70%) and high toughness (vTrs <−20 ° C.) compatible.

  This invention is made | formed in view of such a situation, The objective is to provide the thick steel plate which exhibits a high toughness characteristic with a low yield ratio.

  The low yield ratio high toughness thick steel plate according to the present invention that has solved the above-mentioned problems is C: 0.03 to 0.15% (meaning of “mass%”, the same applies to chemical components hereinafter), Si: 1.0% or less (not including 0%), Mn: 1.0 to 2.0%, P: 0.015% or less (not including 0%), S: 0.010% or less (0% Not included), Al: 0.005 to 0.060%, Ti: 0.008 to 0.030%, N: 0.0020 to 0.010%, and O: 0.010% or less (not including 0%) ), Each of which has a microstructure of t / 4 (t: plate thickness) and has a mixed structure of ferrite and bainite, and island-like martensite is dispersed in the bainite. The average particle size of 10 to 50 μm and present in bainite Fraction of that island martensite and has a gist in that 1-20 area% relative to the total area.

  In the thick steel plate of the present invention, as required, other elements include (a) Cu: 2% or less (not including 0%), Ni: 2% or less (not including 0%), and Cr: 2 % Or less (0% not included), (b) Mo: 0.5% or less (0% not included), (c) Nb: 0.050% or less (0% 1) or more selected from the group consisting of B: 0.0030% or less (not including 0%) and V: 0.1% or less (not including 0%), (d) Mg: 0 0.005% or less (excluding 0%), (e) Ca: 0.0035% or less (excluding 0%), (f) Zr: 0.1% or less (excluding 0%) and / or Hf: 0.05% or less (not including 0%), (g) Co: 2.5% or less (not including 0%) and / or W: 2.5% or less ( % The contained no), (h) a rare earth element: not more than 0.010% (excluding 0), and the like are useful also be included, properties of the steel sheet is improved in accordance with the ingredients contained.

  According to the present invention, the chemical composition of the steel material is appropriately adjusted, the microstructure is a mixed structure of ferrite and bainite, island-like martensite is dispersed in bainite, and the average particle diameter of ferrite and By adjusting the fraction of island-like martensite present in bainite to an appropriate range, a thick steel plate having a low yield ratio and exhibiting high toughness characteristics could be realized. Such steel plates are extremely useful as materials for welded structures such as buildings, marine structures, line pipes, shipbuilding, civil engineering, and construction machinery.

  The inventors of the present invention have made extensive studies on the optimum structure with the aim of achieving both the conflicting characteristics of low yield ratio and base material toughness. As a result, we found that a low yield ratio requires a hard phase and a soft phase, and securing the base material toughness can be achieved by appropriately controlling the size of the soft phase and the hardness and size of the hard phase. .

  In conventional ferritic martensite steel, hard martensite degrades toughness. On the other hand, in ferrite bainite steel, the hardness of bainite is insufficient and a low yield ratio cannot be obtained. The optimum structure form is a three-phase structure of ferrite, bainite and island martensite (MA). In addition, if the MA exists randomly in the steel structure as in the prior art, a good yield ratio-toughness balance can be realized. If the MA is dispersed in the bainite, the yield ratio-toughness balance is more favorable. I found it excellent. The inventors of the present invention have intensively studied components and production methods for allowing MA to be present in bainite, and have reached the present invention.

  The steel sheet of the present invention is a mixed structure of ferrite and bainite in a microstructure at a position of t / 4 (t: plate thickness), and the ferrite fraction of this structure is preferably about 10 to 90 area%. If the ferrite fraction is less than 10 area%, the yield stress YS becomes too high and the yield ratio becomes large. If it exceeds 90 area%, the tensile strength TS: 490 MPa or more cannot be secured.

  In the steel sheet of the present invention, it is necessary that the average particle diameter of the ferrite is 10 to 50 μm. If the average grain size of this ferrite is less than 10 μm, the yield stress YS becomes too high, and if it exceeds 50 μm, the base metal toughness deteriorates.

  In the thick steel plate of the present invention, MA is present in a dispersed manner in the bainite, but the fraction of MA present in the bainite is 1 to 20 area% with respect to the total area. It is also necessary to be. When the MA fraction is less than 1 area%, the yield stress YS becomes too high, and when it exceeds 20 area%, the base metal toughness deteriorates.

  In the thick steel plate of the present invention, the chemical component composition also needs to be adjusted appropriately. The reasons for limiting the range of each component are as follows.

[C: 0.03-0.15%]
C is an element necessary for securing the strength of the steel sheet, and in order to ensure MA, it is necessary to contain 0.03% or more. However, when C is contained excessively, the toughness is reduced instead. For these reasons, the upper limit needs to be 0.15%. The preferable lower limit of the C content is 0.04% (more preferably 0.05%), and the preferable upper limit is 0.09% (more preferably 0.08%).

[Si: 1.0% or less (excluding 0%)]
Si is an effective element for securing the strength of the steel sheet, and is an element necessary for MA generation. However, if Si is contained excessively, the toughness is reduced instead. For these reasons, the upper limit was made 1.0%. In addition, the minimum with preferable Si content is 0.1%, and a preferable upper limit is 0.7% (more preferably 0.5%).

[Mn: 1.0 to 2.0%]
Mn is an element effective in improving the hardenability and ensuring the strength of the steel sheet. In order to exert such effects, it is necessary to contain Mn in an amount of 1.0% or more. However, if Mn is excessively contained, the base material toughness deteriorates, so the upper limit is made 2.0%. In addition, the minimum with preferable Mn content is 1.3%, and a preferable upper limit is 1.8%.

[P: 0.015% or less (excluding 0%)]
P is an impurity that is inevitably mixed in, and adversely affects the toughness of the base material and HAZ. From such a viewpoint, P is preferably suppressed to 0.015% or less. The upper limit with preferable P content is 0.01%.

[S: 0.010% or less (excluding 0%)]
S is an impurity that combines with alloy elements in the steel sheet to form various inclusions and adversely affects the ductility and toughness of the steel sheet, so it is preferably as small as possible. In consideration of the degree of cleanliness of practical steel, the upper limit is preferably suppressed to 0.01%. In addition, S is an impurity inevitably contained in steel, and it is difficult to make the amount 0% in industrial production.

[Al: 0.005 to 0.060%]
Al is an element effective as a deoxidizing agent, and also exhibits the effect of improving the toughness of the base material by refining the microstructure of the steel sheet. In order to exert such effects, the Al content needs to be 0.005% or more. However, if Al is excessively contained, the base material toughness is deteriorated. For these reasons, the upper limit was made 0.060%. In addition, the minimum with preferable Al content is 0.01% (more preferably 0.02% or more), and a preferable upper limit is 0.04%.

[Ti: 0.008 to 0.030%]
Ti has the effect of dispersing TiN in the steel and preventing coarsening of austenite grains (γ grains) during heating before rolling. In order to exhibit such an effect, it is necessary to contain 0.008% or more of Ti. However, when the Ti content is excessive, the toughness of the base material and the HAZ deteriorates, so the content is made 0.030% or less.

[N: 0.0020 to 0.010%]
N contained as an impurity combines with Al, Ti, Nb, B, and the like, and has the effect of forming a nitride to refine the base metal structure, and also γ during heating and welding before base metal rolling. Contributes to grain refinement. In order to exert such an effect, N needs to be contained by 0.0020% or more. However, the solute N causes the base material toughness to deteriorate. As the total nitrogen amount increases, the aforementioned nitride increases, but the solid solution N also becomes excessive and harmful, so it is necessary to make it 0.010% or less.

[O: 0.010% or less (excluding 0)]
O is contained as an unavoidable impurity, but exists as an oxide in steel. However, if the content exceeds 0.010%, a coarse oxide is generated, and the base metal toughness and the HAZ toughness deteriorate. For these reasons, the upper limit of the O content is set to 0.010%. The upper limit with preferable O content is 0.005% (more preferably 0.003%).

  In the steel sheet of the present invention, in addition to the above components, the steel plate is composed of iron and inevitable impurities (for example, Sb, Se, Te, etc.), but may contain trace components (allowable components) to the extent that the properties are not impaired. Such a steel sheet is also included in the scope of the present invention. It is also effective to contain the following elements as necessary. The reasons for limiting the range when these components are contained are as follows.

[One or more selected from the group consisting of Cu: 2% or less (excluding 0%), Ni: 2% or less (not including 0%), and Cr: 2% or less (not including 0%)]
Cu, Ni and Cr are all effective elements for improving the hardenability and improving the strength of the steel sheet, and are contained as necessary. However, if the content of these elements is excessive, the base material toughness is lowered, so that it is preferable that both be 2% or less (more preferably 1% or less). A preferable lower limit for exhibiting the above effect is 0.20% (more preferably 0.40%).

[Mo: 0.5% or less (excluding 0%)]
Mo is an element that improves the hardenability and is effective in securing the strength of the steel sheet, and is appropriately used to prevent temper embrittlement. Such an effect increases as the content thereof increases. However, if the Mo content becomes excessive, the toughness of the base material and the HAZ deteriorates, so that the content is preferably 0.5% or less. More preferably, it should be 0.30% or less.

[Nb: selected from the group consisting of 0.050% or less (not including 0%), B: 0.0030% or less (not including 0%), and V: 0.1% or less (not including 0%) One or more
Nb, B, and V are added as necessary in order to exhibit the effect of improving the hardenability and improving the strength of the base material. V also has the effect of increasing the temper softening resistance. However, if Nb is contained in a large amount, the generation of carbides increases and the toughness of the base material and the HAZ deteriorates, so 0.050% or less (more preferably 0.04% or less, more preferably 0.03% or less). ) Is good. Further, when B is contained in a large amount, the toughness of the base material deteriorates, so it is good to be 0.0030% or less (more preferably 0.020% or less, 0.015% or less). V is preferably contained in an amount of 0.01% or more in order to effectively exhibit the above effects, but if contained in a large amount, the toughness of the base material and the HAZ deteriorates, so 0.1% or less (more preferably 0). .05% or less).

[Mg: 0.005% or less (excluding 0%)]
Since Mg has the effect of improving the HAZ toughness by forming MgO and suppressing the coarsening of austenite grains in the HAZ, it is contained as necessary. However, if the Mg content is excessive, the inclusions become coarse and the HAZ toughness deteriorates. Therefore, the content is preferably 0.005% or less (more preferably 0.0035% or less).

[Ca: 0.0035% or less (excluding 0%)]
Ca is an element that contributes to the improvement of HAZ toughness by controlling the form of sulfide. However, even if it exceeds 0.0035% and contains excessively, HAZ toughness will deteriorate on the contrary. In addition, the upper limit with preferable Ca content is 0.020%, More preferably, it is 0.0015%.

[Zr: 0.1% or less (excluding 0%) and / or Hf: 0.05% or less (0%
Is not included)]
Zr and Hf are elements effective for improving the HAZ toughness by forming a nitride with N as in the case of Ti, refining the austenite grains of the HAZ during welding. However, if excessively contained, the HAZ toughness is reduced. Therefore, when these elements are contained, Zr is set to 0.1% or less and Hf is set to 0.05% or less.

[Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%)]
Co and W are contained as necessary because they have the effect of improving the hardenability and increasing the strength of the base material. However, since the HAZ toughness deteriorates if contained excessively, the upper limit is set to 2.5%.

[Rare earth element (REM): 0.010% or less (not including 0)]
REM is an element that contributes to improving the toughness of HAZ by making the shape of inclusions (oxides, sulfides, etc.) inevitably mixed in steel materials finer and spheroidized, and is contained if necessary. The Such an effect increases as the content thereof increases. However, if the content of REM becomes excessive, inclusions become coarse and the HAZ toughness deteriorates. Therefore, the content is preferably suppressed to 0.010% or less. In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium).

In producing the thick steel plate of the present invention, steel satisfying the above-mentioned chemical composition amount is melted by a normal melting method, and after this molten steel is cooled to form a slab, it is heated to, for example, a range of 950 to 1300 ° C. After performing hot rolling, the rolling reduction in the temperature range of Ar ₃ transformation point + 100 ° C. to Ar ₃ transformation point + 150 ° C. is set to 10% or more, the finish rolling temperature is set to 800 to 700 ° C., and the cooling start is finished. Accelerated cooling is started within −50 ° C. from the rolling temperature, water cooled to 400 ° C. to 150 ° C. at an average cooling rate of 5 to 50 ° C./second, and then air cooled. The reason for setting the range of each condition in this method is as follows.

[Heating temperature: 950-1300 ° C]
Although it is necessary to set it as 950 degreeC or more from a viewpoint of once making the structure of a steel plate all austenite, when heating temperature exceeds 1300 degreeC, austenite will coarsen and it will become difficult to obtain a predetermined structure | tissue in a subsequent process.

[Ar ₃ transformation point + 100 ° C. to Ar ₃ transformation point + 150 ° C. temperature reduction: 10% or more]
By setting the rolling reduction in this temperature range to 10% or more, the particle size of the ferrite can be refined. If the temperature is out of this range or the rolling reduction is less than 10%, the ferrite grain size becomes coarse. In the present invention, the “Ar ₃ transformation point” is a value determined by the following equation (1).
Ar ₃ = 910−230 × [C] + 25 × [Si] −74 × [Mn] −56 × [Cu]
-16x [Ni] -9x [Cr] -5x [Mo] -1620x [Nb] (1)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [Nb] are respectively C, Si, Mn, Cu, Ni, Cr, Mo and The Nb content (% by mass) is shown.

[Finish rolling temperature: 800-700 ° C]
When the finish rolling temperature exceeds 800 ° C., the ferrite particle size becomes coarse, and when it becomes less than 700 ° C., the ferrite particle size becomes less than 10 μm, and the yield stress YS becomes too high.

[Cooling start temperature: within -50 ° C from finish rolling temperature]
When the cooling start temperature after rolling becomes lower than −50 ° C. than the finish rolling temperature, the ferrite becomes coarse.

[Accelerated cooling from 400 ° C to 150 ° C at an average cooling rate of 5 to 50 ° C / second]
If the average cooling rate during accelerated cooling is less than 5 ° C / second, ferrite grains become coarse, and if it exceeds 50 ° C / second, the amount of ferrite is insufficient. The reason why the cooling stop temperature is set to 400 to 150 ° C. is to generate a predetermined amount of MA.

  The temperature shown above is controlled by the temperature at the position of the steel sheet surface. The steel plate of the present invention is assumed to be a thick steel plate, but the plate thickness at this time is about 40 mm or more, and the upper limit is not particularly limited, but is usually 100 mm or less.

  In the present invention, by specifying the chemical component composition and the structure in the specific region as described above, a low yield ratio thick steel plate excellent in toughness (base material toughness) can be realized. The toughness of the heat affected zone (hereinafter referred to as “HAZ”) is also basically good. That is, the steel plate of the present invention is applied as a welded structure in the fields of architecture, offshore structures, line pipes, shipbuilding, civil engineering, construction machinery, etc., and has good HAZ toughness when welded. Although it is required to be present, such HAZ toughness is also good.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

Experimental example 1
Various molten steels having chemical composition shown in Tables 1 to 3 below are melted by a normal melting method, and the molten steel is cooled to form a slab, followed by hot rolling and cooling under the conditions shown in Tables 4 and 5 below. And various steel plates (thickness: 50 mm) were obtained. In Tables 1 and 3 below, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Tables 1 to 3 below, “-” indicates that no element was added.

  About each obtained steel plate, while measuring a base material structure (ferrite particle size, MA fraction) and mechanical properties (base material tensile properties, base material impact properties) by the following methods, it also evaluates HAZ toughness. did. The measurement results are shown in Tables 6 and 7 below.

[Measurement of ferrite fraction and ferrite particle size]
A 2 cm square specimen taken from the position of t / 4 part (t: thickness) of each steel plate is mirror-polished, etched with a nital etchant (2% nitric acid-ethanol solution), and the structure is observed with an optical microscope. (Magnification 100 times: n = 10), the ferrite particle size (average value) was measured based on the method of the comparative method defined in JIS G 0552.

[Measurement of MA fraction]
For the position of t / 4 part (t: thickness) of each steel plate, the mirror-polished test piece is repeller-corroded, the structure is observed with an optical microscope, and a region of 1000 × magnification and 50 μm square is photographed at n = 10. The MA present in the bainite structure was identified by an image analysis apparatus (Media Cybernetics: Image-Pro Plus), and the area ratio (average value) was measured.

[Evaluation of tensile properties of base metal]
Yield stress YS (yield point YP) and tensile strength TS were measured by taking a JIS No. 4 test piece from the position of t / 4 part (t: thickness) of each steel plate and conducting a tensile test according to JIS Z2241. The yield ratio YR was calculated.

[Evaluation of impact properties (toughness) of base metal]
For the impact properties (toughness) of the base material, a V-notch Charpy test was performed, and vTrs (brittle fracture surface transition temperature) was obtained from a transition curve. A JIS No. 4 test piece was collected from t / 4 parts (t plate thickness), and the test was carried out in accordance with JIS Z2242. At this time, for each temperature measurement (at least 4 temperatures), the test was conducted at n = 3, and a brittle fracture surface transition curve was drawn so as to pass through the point with the highest brittle fracture surface ratio among the three points. A temperature of 50% was calculated as the brittle fracture surface transition temperature vTrs (a line was drawn so that vTrs was on the highest temperature side).

[HAZ toughness test]
As a HAZ toughness evaluation method simulating a thermal cycle when submerged arc welding (2 kJ / mm) is performed, a heating temperature is maintained at 1400 ° C. for 5 seconds, and then a cooling time is 800 to 500 ° C. (Tc): 25 seconds. After heat-treating each test steel plate in the thermal cycle, Charpy absorbed energy (V notch) at a temperature of −40 ° C. was measured. In addition, as a test piece, it is a rod shape of size 10 mm × 10 mm × 55 mm taken from the position of the plate thickness t / 4 part (t: plate thickness) and has a V notch having a depth of 2 mm on one side of the central part. used. At this time, the V Charpy impact value (vE- ₄₀ ) was determined to be 50 J or more.

  As is clear from these results, Experiment No. 1-22 is an example which satisfies the requirements prescribed | regulated by this invention, and the low yield specific thickness steel plate with favorable toughness is obtained for both a base material and HAZ. In contrast, Experiment No. Nos. 23 to 56 are examples that do not satisfy any of the requirements defined in the present invention, and it is understood that any of the characteristics is not obtained. Experiment No. The microstructure of the steel plate obtained in 2 is shown in FIG. 1 (drawing substitute micrograph) (the part that appears white indicates MA).

Experiment No. 2 is a drawing-substituting micrograph showing the microstructure of the steel sheet obtained in 2.

Claims (9)

  C: 0.03 to 0.15% (meaning “mass%”, chemical components are the same hereinafter), Si: 1.0% or less (not including 0%), Mn: 1.0 to 2.0 %, P: 0.015% or less (not including 0%), S: 0.010% or less (not including 0%), Al: 0.005 to 0.060%, Ti: 0.008 to 0 0.030%, N: 0.0020 to 0.010%, and O: 0.010% or less (not including 0%), each of which is a microplate at a position of t / 4 (t: thickness) In the structure, it is composed of a mixed structure of ferrite and bainite, island-like martensite is dispersed in bainite, and the average particle diameter of ferrite is 10 to 50 μm, and the island-like martensite existing in bainite The fraction is 1 to 20 area% with respect to the total area. Low yield ratio high toughness steel plate characterized.   Furthermore, at least one selected from the group consisting of Cu: 2% or less (not including 0%), Ni: 2% or less (not including 0%) and Cr: 2% or less (not including 0%) The low yield ratio high toughness thick steel plate according to claim 1, which is contained.   The low yield ratio high toughness thick steel plate according to claim 1 or 2, further comprising Mo: 0.5% or less (not including 0%).   Further, Nb: 0.050% or less (excluding 0%), B: 0.0030% or less (not including 0%), and V: 0.1% or less (not including 0%) The low yield ratio high toughness thick steel plate according to any one of claims 1 to 3, which contains one or more selected.   Furthermore, Mg: 0.005% or less (excluding 0%) is contained, The low yield ratio high toughness thick steel plate in any one of Claims 1-4.   The low yield ratio high toughness thick steel plate according to any one of claims 1 to 5, further comprising Ca: 0.0035% or less (not including 0%).   Furthermore, it contains Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%). Low yield ratio high tough steel plate.   Furthermore, Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%) are contained. Low yield ratio high tough steel plate.   The low yield ratio high toughness thick steel plate according to any one of claims 1 to 8, further comprising rare earth element: 0.010% or less (not including 0).