JP2010248599A - 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 thick steel plate with a low yield ratio and high toughness contains: <0.10% C (excluding 0%), 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%), 0.05 to 0.50% Al, 0.008 to 0.030% Ti, 0.0020 to 0.010% N and 0.0035% or less Ca (excluding 0) and also contains one or more kinds selected from a group comprising 2% or less Cu (excluding 0), 2% or less Ni (excluding ) and 2% or less Cr (excluding 0). The steel plate has a microstructure in a position of t/4 (t: plate thickness) which is formed of a mixed structure of ferrite and bainite, wherein the retained austenite are dispersed in the bainite, the ferrites have an average particle size of 10 to 50 μm, and the retained austenite existing in the bainite occupies ≥2.0% by area fraction with respect to the whole area. <P>COPYRIGHT: (C)2011,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, in addition to high strength and high toughness, from the viewpoint of earthquake resistance, when applying elastoplastic design, the yield ratio represented by the ratio of yield stress YS and tensile strength TS It is also required to lower YR (YR = YS / TS). 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 has been proposed.

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. Furthermore, Patent Document 4 has succeeded in combining the low yield ratio and the base material toughness by containing retained austenite, but it is unclear whether the HAZ toughness is satisfactory. is there.

JP-A-55-97425 JP 55-41927 A JP 2005-23423 A JP 2000-345283 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 less than C: 0.10% (meaning “mass%”, the same applies to chemical components hereinafter) (not including 0%) ), 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 (Excluding 0%), Al: 0.05 to 0.50%, Ti: 0.008 to 0.030%, N: 0.0020 to 0.010% and Ca: 0.0035% or less (0 In addition to Cu: 2% or less (not including 0%), Ni: 2% or less (not including 0%) and Cr: 2% or less (not including 0%) Containing one or more selected from the group consisting of ferrite and bainite in a microstructure at a position of t / 4 (t: plate thickness) Consisting of a mixed structure, residual austenite is dispersed in bainite, and the average grain size of ferrite is 10 to 50 μm, and the residual austenite present in bainite is present in an area fraction of 2.0% or more. It has a gist in that it is a thing.

  In the thick steel plate of the present invention, (a) Mo: 0.5% or less (not including 0%), Nb: 0.05% or less (not including 0%) as other elements as necessary. , B: 0.003% or less (excluding 0%) and V: 0.1% or less (not including 0%), (b) Mg: 0.01% or less (Not including 0%) and / or rare earth elements: 0.01% or less (not including 0%), (c) Zr: 0.1% or less (not including 0%), etc. It is useful and the properties of the steel sheet are improved depending on the components 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, the retained austenite is dispersed in bainite, and the average particle diameter of ferrite and the bainite By adjusting the fraction of retained austenite present in the steel 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 for the purpose of achieving both the low yield ratio and the contradictory properties of the base material toughness. As a result, it was 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 deteriorates 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 = mixed structure of martensite and austenite). Also, if the MA exists randomly in the steel structure as in the prior art, a good yield ratio-toughness balance cannot be realized, and the presence of MA dispersed in the bainite provides a better yield ratio-toughness balance. I found it excellent. Further, it has also been found that stable characteristics can be ensured by containing a certain amount or more of retained austenite (hereinafter sometimes referred to as “residual γ”) in the fine second phase present as MA. From the viewpoint of the formation of residual γ and HAZ toughness, an appropriate component system (a large amount of Al was added to minimize C and Ti and N were added in an appropriate amount) was found, and the present invention was completed.

  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 sheet of the present invention, as a feature on the microstructure, residual γ is present dispersed in bainite, but the area fraction of residual γ present in bainite is 2.0% of the total area. % Or more is also necessary. If the area fraction of residual γ is less than 2.0%, the yield stress YS becomes too high.

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

[C: less than 0.10% (excluding 0%)]
C is an element necessary for securing the strength of the steel sheet, and is an element necessary for securing residual γ. However, when C is contained excessively, the toughness is reduced instead. For these reasons, the C content needs to be less than 0.10%. In addition, the preferable minimum of C content is 0.03% (more preferably 0.04%), and a preferable upper limit is 0.08% (more preferably 0.05%).

[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 generating residual γ. 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 the 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 0.010%. In addition, S is an impurity inevitably contained in steel, and it is difficult to make the amount 0% in industrial production.

[Al: 0.05 to 0.50%]
Al is an element effective as a deoxidizer and an element that promotes the formation of residual γ. In order to exert such effects, the Al content needs to be 0.05% or more. However, if Al is excessively contained, the base material toughness is deteriorated. For these reasons, the upper limit was made 0.50%. In addition, the minimum with preferable Al content is 0.10%, and a preferable upper limit is 0.40%.

[Ti: 0.008 to 0.030%]
Ti has an effect of dispersing TiN in steel and preventing the coarsening of residual 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.

[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 0.0020%, More preferably, it is 0.0015%.

[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. 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%).

  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.

[Mo: 0.5% or less (not including 0%), Nb: 0.05% or less (not including 0%), B: 0.003% or less (not including 0%), and V: 0.00. 1 or more selected from the group consisting of 1% or less (excluding 0%)]
Mo, 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 the Mo content is excessive, the toughness of the base material and the HAZ deteriorates, so it is preferable to set the content to 0.5% or less (preferably 0.30% or less). Further, when Nb is contained in a large amount, the generation of carbides increases and the toughness of the base material and HAZ deteriorates, so 0.05% or less (more preferably 0.04% or less, more preferably 0.03% or less). It is preferable that When B is contained in a large amount, the toughness of the base material deteriorates, so it is good to be 0.003% or less (more preferably 0.002% or less, 0.0015% 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: 0.01% or less (not including 0%) and / or rare earth element (REM): 0.01% or less (not including 0%)]
Mg and rare earth elements are elements that contribute to improving the toughness of HAZ by refining and spheroidizing inclusions (oxides, sulfides, etc.) that are inevitably mixed in steel. Contained by. Such an effect increases as the content increases. However, if the content is excessive, inclusions become coarse and the HAZ toughness deteriorates. In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium).

[Zr: 0.1% or less (excluding 0%)]
Zr, like Ti, is an element that forms N and nitride, refines the HAZ austenite grains during welding, and is effective in improving HAZ toughness. However, if excessively contained, the HAZ toughness is reduced. For this reason, when Zr is contained, Zr is made 0.1% or less.

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 500 ° C. to 300 ° C. at an average cooling rate of 5 to 50 ° C./second, and then 300 seconds into a temperature range from this cooling stop temperature to −100 ° C. What is necessary is just to make it stay above. 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. from the finish rolling temperature, ferrite becomes coarse.

[Accelerated cooling to 500 to 300 ° 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 500 to 300 ° C. is to generate a predetermined amount of residual γ.

[Residence for 300 seconds or more in the temperature range from the cooling stop temperature to -100 ° C]
When the residence time is less than 300 seconds, the amount of residual γ is insufficient. Although there is no particular upper limit, it is preferably less than 1000 seconds from the viewpoint of productivity. The “temperature range from the cooling stop temperature to −100 ° C.” means a temperature range in which the residual γ is stabilized.

  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.

  Various molten steels (steel types A to P and A1 to N1) having chemical composition compositions shown in Tables 1 and 2 below are melted by a normal melting method, and the molten steel is cooled to form a slab. The steel sheet (thickness: 50 mm) was obtained by hot rolling and cooling under the conditions shown in. In Table 1 below, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Tables 1 and 2 below, “-” indicates that no element was added.

  For each steel plate obtained, the base material structure (ferrite grain size, ferrite fraction, residual γ fraction), mechanical properties (base material tensile properties, base material impact properties) are measured by the following methods, The HAZ toughness was also evaluated. The measurement results are shown in Tables 5 and 6 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 residual γ area fraction]
A test piece is cut out from the surface of each steel plate at a depth of t / 4 part (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. The parallel cross-section was observed using an electrolytic emission scanning microscope “SUPRA35 (trade name)” (FE-SEM) manufactured by Carl Zeiss, observation magnification: 600 ×, observation field: 0.04 mm ² , observation location: 5 locations It calculated by observing on the conditions of this, and analyzing by the EBSD (Electron Back Scatter Diffraction Parameters) method.

[Evaluation of tensile properties of base metal]
A U14A test piece is taken from the position of t / 4 part (t: thickness) of each steel plate, and a tensile test is performed according to JIS Z2241, thereby measuring a yield stress YS (yield point YP) and a tensile strength TS, 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 U14A 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, a V Charpy impact value (vE- ₄₀ ) of 100 J or more was regarded as acceptable.

  As is clear from these results, Experiment No. Nos. 1 to 16 are examples that satisfy the requirements defined in the present invention, and a low yield specific thickness steel plate having good toughness is obtained for both the base material and HAZ. In contrast, Experiment No. Examples Nos. 17 to 35 are examples that deviate from any of the requirements defined in the present invention (Experiment Nos. 17 to 30 are those that deviate from the chemical component composition defined in the present invention, and Experiments Nos. 31 to 35 are out of manufacturing conditions. As a result, it is understood that one of the characteristics is not obtained.

Claims (4)

  C: Less than 0.10% (meaning “mass%”, the same applies to chemical components) (not including 0%), Si: not more than 1.0% (not including 0%), Mn: 1.0 -2.0%, P: 0.015% or less (excluding 0%), S: 0.010% or less (not including 0%), Al: 0.05-0.50%, Ti: 0 0.008 to 0.030%, N: 0.0020 to 0.010% and Ca: 0.0035% or less (not including 0%), respectively, Cu: 2% or less (not including 0%) ), Ni: 2% or less (not including 0%) and Cr: 2% or less (not including 0%), containing at least one selected from the group consisting of t / 4 (t: thickness) In this microstructure, it consists of a mixed structure of ferrite and bainite, and residual austenite is dispersed in bainite. And a high yield toughness steel plate having a low yield ratio, characterized in that the ferrite has an average grain size of 10 to 50 μm and the retained austenite in the bainite is 2.0% or more in area fraction. .   Mo: 0.5% or less (not including 0%), Nb: 0.05% or less (not including 0%), B: 0.003% or less (not including 0%), and V: 0.1 2. The low yield ratio high toughness thick steel plate according to claim 1, comprising at least one selected from the group consisting of 1% or less (not including 0%).   The low yield according to claim 1 or 2, further comprising Mg: 0.01% or less (not including 0%) and / or rare earth elements: 0.01% or less (not including 0%). Specific high toughness steel plate.   Furthermore, Zr: 0.1% or less (0% is not included) The low yield ratio high toughness thick steel plate in any one of Claims 1-3.