JP4972451B2 - Low yield ratio high strength steel sheet with excellent low temperature toughness of weld heat affected zone and base metal and method for producing the same - Google Patents

Low yield ratio high strength steel sheet with excellent low temperature toughness of weld heat affected zone and base metal and method for producing the same Download PDF

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JP4972451B2
JP4972451B2 JP2007112061A JP2007112061A JP4972451B2 JP 4972451 B2 JP4972451 B2 JP 4972451B2 JP 2007112061 A JP2007112061 A JP 2007112061A JP 2007112061 A JP2007112061 A JP 2007112061A JP 4972451 B2 JP4972451 B2 JP 4972451B2
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祐二 ▲高▼橋
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株式会社神戸製鋼所
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/14Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE BY DECARBURISATION, TEMPERING OR OTHER TREATMENTS
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE BY DECARBURISATION, TEMPERING OR OTHER TREATMENTS
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/002Bainite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE BY DECARBURISATION, TEMPERING OR OTHER TREATMENTS
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/005Ferrite

Description

  The present invention relates to a low-yield-ratio high-tensile steel sheet excellent in low-temperature toughness of a weld heat-affected zone and a base material. When used in applications exposed to low temperatures, for example, liquefied ammonia and liquefied propane gas are mixed. The present invention relates to a low-yield ratio high-tensile steel sheet that is excellent in low-temperature toughness of a weld heat-affected zone and a base material, which can be applied to a multipurpose tank. The present invention is not limited to the above-described high-strength steel plate welding method, and can be applied to submerged arc welding, electrogas arc welding, etc., but in the following, it is said that securing the toughness of the weld heat affected zone is particularly difficult. The case where the single-sided submerged arc welding with large heat input is applied will be described as an example.

  In recent years, single-sided submerged arc welding with large heat input, for example, with a heat input of 50 to 200 kJ / cm, has been widely used to produce marine structures and low-temperature tanks for storing liquefied gas such as LPG in a short period of time. It has been adopted. However, while the welding can realize high efficiency of construction, it is difficult to stably secure the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) formed by welding, and it is due to low heat input. In many cases, it is necessary to apply multilayer welding. Therefore, the high heat input welding method capable of high-efficiency construction is adopted for the production of the low temperature tank and the like, and the HAZ toughness (low temperature toughness) is excellent even at a low temperature of about −60 ° C. There is a need for steel sheets.

  On the other hand, the steel sheet used for the liquefied ammonia tank has a low yield strength YS of 440 MPa or less to prevent stress corrosion cracking (SCC) and a tensile strength TS of 510 MPa or less to reduce the total weight of the steel material. It is required that In the case of a tank in which liquefied ammonium and liquefied propane gas are mixed, it is required that the steel sheet (base material) used has excellent low-temperature toughness. It is known that liquefied ammonia causes stress corrosion cracking (SCC) of steel materials, and it is specified as a characteristic of a steel sheet that the yield strength YS is suppressed to 440 MPa or less (Non-Patent Document 1).

  However, in the multi-purpose use where the above liquefied ammonium and liquefied propane gas are mixed, it is necessary to satisfy the characteristics required for both, and it is installed in ships as the marine structures such as ships increase in size. As the capacity of the tank is increased, it is also required to increase the tensile strength of the steel sheet, and simultaneously achieving a lower yield ratio (yield ratio YR = YS / TS) due to the upper limit of the yield strength YS is a major issue. It has become.

  Until now, various methods have been proposed to improve the low temperature toughness of the HAZ. For example, Patent Literature 1 and Patent Literature 2 propose a method for improving HAZ toughness by suppressing the austenite grain coarsening by pinning particles such as TiN and Al oxide. Patent Document 3 and Patent Document 4 disclose a technique for miniaturizing crystal grains by making many ferrite transformation nuclei exist in austenite grains. Specifically, the use of TiN, MnS, BN, Ti oxide or the like as ferrite transformation nuclei achieves refinement of crystal grains and improves the low temperature toughness of HAZ.

  However, in any of the above methods, when performing single-sided submerged arc welding with high heat input, precipitates such as TiN are considerably dissolved, and it is difficult to suppress subsequent grain coarsening, Further improvement is required to ensure excellent HAZ toughness (hereinafter sometimes referred to as “HAZ toughness” or simply “HAZ toughness”) at a low temperature of about −60 ° C.

  In addition, the HAZ toughness improvement technologies proposed so far have not been considered to have a low yield ratio (for example, 75% or less) required as a multipurpose tank in which liquefied ammonium and liquefied propane gas are mixed. Is the actual situation.

  By the way, in order to realize a low yield ratio of a steel sheet, it is generally necessary to make the steel structure two-phase (dual phase), that is, to secure a soft phase (usually ferrite) that governs the yield strength and tensile strength. It is known that mixed organization of hard phases (perlite, bainite, martensite, etc.) is effective. As such a technique, for example, Patent Document 5 discloses a technique for realizing a low yield ratio by controlling rolling to a two-phase temperature of ferrite and austenite to form a mixed structure of ferrite + bainite or ferrite + bainite + martensite. It is shown.

  In this technology, as the chemical component increases, the hard phase hardens as the C content increases, and it is easy to reduce the yield ratio. On the other hand, there is a problem that it is disadvantageous for weldability and low temperature toughness. It is a characteristic that is incompatible with the low yield ratio, and it is extremely difficult to achieve both. As with steels used in conventional building applications, high C steel, which has a relatively low toughness level and is advantageous for low yield ratios, has not been a problem. It cannot be applied to steel materials that require low temperature toughness.

Several techniques have been proposed so far to achieve both low temperature toughness and low yield ratio of steel sheets (base materials). For example, Patent Document 6 discloses that low-temperature toughness is transitioned to the fracture surface by actively generating island-shaped martensite (MA), which is a hard phase, and defining the form (aspect ratio) and size. It has been proposed to improve the temperature (vTrs) to around -80 ° C. However, since the formation of hard MA causes the starting point and propagation of fracture, there is a limit to improving the low temperature toughness while maintaining the necessary properties as a steel material.
IGC CODE 17.13 (International Code for the Construction and Equipment of Shipping Carrying Liquidated Gases in Bulk) 2002 Edition Japanese Patent Publication No. 55-026164 Japanese Patent No. 2950076 Japanese Patent Publication No. 07-068577 Japanese Patent Publication No. 05-017300 Japanese Patent No. 3371744 JP 2002-3983 A

  The present invention has been made in view of such circumstances, and the object thereof is excellent in low temperature toughness of HAZ even when welding is performed with high heat input, and also in low temperature toughness of a base material (steel plate). The object is to provide an excellent low yield ratio high strength steel sheet.

  The high-tensile steel sheet of the present invention that can achieve the above-mentioned object is C: 0.05 to 0.09% (meaning “mass%”, the same applies to chemical components), Si: 0.05 to 0.00. 25%, Mn: 1.2 to 1.6%, P: 0.01% or less (excluding 0%), S: 0.003% or less (excluding 0%), Al: 0.02 0.04%, B: 0.0006 to 0.0020%, N: 0.0030 to 0.0080%, Ti: 0.005 to 0.025%, respectively, the balance being iron and inevitable impurities And, in the microstructure at the position of t / 4 (t: plate thickness), the ferrite fraction occupies 60 to 85 area% of the entire structure, and the island-like martensite fraction is 5% or less (including 0%), The balance consists of a mixed structure of bainite structure, and the average crystal grain size of the ferrite is 14 μm or less. The average Vickers hardness Hv of the second phase is intended to include the features in that it is 265-400.

In the high-tensile steel plate of the present invention, the chemical component composition preferably satisfies the following formula (1).
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0}

  In the high-tensile steel plate of the present invention, if necessary, (a) Cu: 0.4% or less (not including 0%), Ni: 0.4% or less (not including 0%), Cr: 0 1 type selected from the group consisting of 0.4% or less (excluding 0%), Mo: 0.4% or less (not including 0%), and V: 0.02% or less (not including 0%) As described above, it is also preferable to contain (b) Nb: 0.005 to 0.025%, (c) Ca: 0.003% or less (not including 0%), and the like. Is improved.

In producing the low-yield ratio high-tensile steel sheet as described above, after performing hot rolling, the steel sheet is heated at an average cooling rate of 10 ° C./second or more from a temperature exceeding (Ar 3 transformation point−40 ° C.) ( (Ar 3 transformation point −40 ° C.) or lower, the cooling is temporarily interrupted at that temperature, and air cooling is performed for 30 to 150 seconds. Subsequently, the temperature at the t / 4 (t: plate thickness) position is (Ar 3 350 ° C. greater than the temperature range of transformation temperature -80 ℃) ~ (Ar 3 transformation point -190 ℃), 550 ℃ may be to cool at a temperature range up to 10 ° C. / sec or more average cooling rate.

  According to the present invention, the yield strength YS of the steel plate (base material) is 440 MPa or less, the tensile strength TS is 510 MPa or more, and the low temperature toughness of the steel plate (base material) is excellent. Even when welding is performed, HAZ exhibits excellent toughness at −60 ° C., which contributes to an increase in the size of a welded structure such as a multi-purpose tank in which liquefied ammonia and liquefied propane gas are mixed, and for example, high heat input. The single-sided submerged arc welding method can be employed, and the welded structure can be manufactured in a shorter period of time.

  This inventor examined from various angles in order to implement | achieve the high-tensile steel plate excellent in the low temperature toughness of HAZ and a base material. As a result, the chemical component composition was adjusted while setting C to 0.09% or less and Si to 0.25% or less, and in the microstructure at the t / 4 (t: plate thickness) position, The mixed structure (ferrite + second phase) with a properly defined fraction is used, the average crystal grain size of ferrite is 14 μm or less, and the average Vickers hardness Hv of the second phase is 265-400. As a result, the inventors have found that the above object can be achieved brilliantly and completed the present invention.

  In the high-tensile steel plate of the present invention, the ferrite fraction needs to be adjusted to 60 to 85 area% in the microstructure at the t / 4 (t: plate thickness) position. If the ferrite fraction is less than 60 area%, the yield strength YS: 440 MPa or less cannot be achieved, and if it exceeds 85 area%, the tensile strength TS: 510 MPa or more cannot be ensured. A preferable range of this ferrite fraction is 65 area% or more and 80 area% or less.

  In order to ensure the toughness of the base material (that is, to improve the propagation of fracture), it is also an important requirement to control the average crystal grain size of ferrite to be 14 μm or less. FIG. 1 is a graph showing the relationship between the average crystal grain size of ferrite (ferrite grain size) and the toughness of the base material (evaluated by the fracture surface transition temperature vTrs). is there. As is clear from this result, it is understood that good low temperature toughness (fracture surface transition temperature vTrs of −100 ° C. or less) of the base material can be secured by setting the ferrite grain size to 14 μm or less. A preferable upper limit of the ferrite particle diameter is 12 μm.

  The high-tensile steel sheet of the present invention has a microstructure mainly composed of ferrite and includes bainite or (bainite + island martensite) as a second phase (hard phase). It is necessary to adjust the hardness of the second phase appropriately. That is, by appropriately defining these requirements, it is possible to prevent the starting point of fracture and to secure the strength of the base material without reducing the low temperature toughness of the base material.

  From such a viewpoint, the fraction of the island martensite MA needs to be 5 area% or less (including 0 area%). That is, when the MA fraction exceeds 5 area%, the strength of the base material is improved, but the low temperature toughness is lowered.

  The hardness of the second phase is also influenced by the ferrite fraction (as a result, the fraction of the second phase), but basically the ratio depends on the ratio of bainite and MA in the second phase. The value is determined. Also, the hardness varies depending on the chemical composition. By setting the hardness of the second phase within the range of 265 to 400 in terms of Vickers hardness Hv, the strength of the base material can be secured without reducing the low temperature toughness of the base material. FIG. 2 is a graph showing the influence of the MA fraction and the second phase hardness on the properties of the base material, and is a summary of data of examples described later. In FIG. 2, “◯” indicates the strength and low-temperature toughness of the base metal (tensile strength TS: 510 MPa or more, fracture surface transition temperature: −100 ° C. or less) steel plate (example of the present invention), “Δ” indicates the base The steel plate (comparative example) which did not satisfy at least one of the properties of the material strength and low temperature toughness is shown. As is apparent from this result, the MA fraction is controlled to 5 area% or less, and the hardness of the second phase is controlled within the range of 265 to 400 by Hv. , Strength) can be secured.

  In the high-tensile steel sheet of the present invention, in order to satisfy the basic characteristics as the steel sheet, it is necessary to appropriately adjust the chemical composition while reducing the C and Si contents. The reasons for limiting the ranges of the basic components (C, Si, Mn, P, S, Al, B, N, Ti) are as follows.

[C: 0.05-0.09%]
In order to suppress the formation of MA, which is a hard phase, and to ensure the HAZ toughness at −60 ° C., it is necessary to suppress the C content to 0.09% or less. On the other hand, C is an element essential for ensuring the strength of the steel sheet, so 0.05% or more is contained. The C content is preferably 0.06% or more and 0.08% or less.

[Si: 0.05 to 0.25%]
By reducing Si to 0.25% or less, the formation of MA can be sufficiently suppressed, and the low temperature toughness of HAZ can be easily ensured. On the other hand, since Si is an element that is used for deoxidation of molten steel and effectively works to improve strength, it is necessary to contain 0.05% or more.

  In addition, as described above, in order to reliably increase the HAZ toughness and to provide other characteristics such as strength and toughness of the steel plate (base material), it is necessary to set the content of components other than the above within the following ranges.

[Mn: 1.2 to 1.6%]
Mn is an element useful for capturing S as MnS and suppressing degradation of HAZ toughness due to S. It is also an element that contributes to increasing the strength of the steel sheet by increasing the hardenability. In order to exhibit such an action effectively, it is necessary to contain 1.2% or more of Mn. Preferably it is 1.3% or more. However, if the amount of Mn becomes excessive, the HAZ toughness deteriorates on the contrary, so it is suppressed to 1.6% or less. Preferably, it is 1.55% or less.

[P: 0.01% or less (excluding 0%)]
P is an element that deteriorates the HAZ toughness, so it is necessary to reduce it as much as possible. In the present invention, P is suppressed to 0.01% or less.

[S: 0.003% or less (excluding 0%)]
S is an element that generates coarse sulfides and degrades the HAZ toughness. Therefore, it is necessary to reduce as much as possible, and in the present invention, it is suppressed to 0.003% or less.

[Al: 0.02-0.04%]
Al is an element that is used as a deoxidizer and that also generates AlN-based precipitates to improve the HAZ toughness during high heat input welding. In the present invention, Al is contained in an amount of 0.02% or more. However, when the Al content is excessive, oxide inclusions such as alumina increase and MA formation is promoted to deteriorate the HAZ toughness. Therefore, the Al content is suppressed to 0.04% or less.

[B: 0.0006 to 0.0020%]
B has the effect of fixing solute N which is harmful to the HAZ toughness by generating BN and promoting the formation of intragranular ferrite. Solid solution B also has the effect of suppressing the coarsening of grain boundary ferrite and the formation of ferrite side plates, and making the crystal grains in the austenite grains finer. In order to fully exhibit this effect, it is necessary to contain B 0.0006% or more. On the other hand, when there is too much B, a crystal | crystallization is formed in a fixed direction by the effect | action of excess solute B, and HAZ toughness deteriorates on the contrary. Therefore, the B content is limited to 0.0020% or less. In addition, the minimum with preferable B content is 0.0008%, and a preferable upper limit is 0.0018%.

[N: 0.0030 to 0.0080%]
N is an element that improves the HAZ toughness by forming a nitride with an element such as Ti or Al, and therefore needs to be contained in an amount of 0.0030% or more (preferably 0.0040% or more). In addition, the solid solution N causes the HAZ toughness to deteriorate. By increasing the total nitrogen amount, the above-mentioned nitrides increase, but solute N also becomes excessive. Therefore, in the present invention, the N content is suppressed to 0.0080% or less (preferably 0.0070% or less).

[Ti: 0.005 to 0.025%]
Ti is an element that generates TiN-based precipitates and promotes the formation of intragranular ferrite, and is also effective in suppressing austenite grain coarsening. It is also an element contributing to high strength. In order to exhibit such an action effectively, it is necessary to contain 0.005% or more of Ti, and preferably 0.010% or more. However, if Ti is excessively contained, the HAZ toughness is lowered instead, so it is necessary to make it 0.025% or less, preferably 0.024% or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. In addition, by optimizing the balance of B, N and Ti within the above component range and strictly optimizing the amount of dissolved B, the crystal grains in the austenite grains can be refined. As a result, the HAZ It is effective in remarkably increasing the low temperature toughness.

FIG. 3 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.010% Nb is a basic component, and B, N, and Ti are within the specified ranges described later. (B-NT / 1.3) {B is B content (mass ppm), NT is N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) Relationship
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
When (N-Ti / 3.4) <0, NT = 0 is indicated.
The same applies to equation (1) below}
A heat cycle test was performed using steel sheets having various values of and the low temperature toughness (vE- 60 ) of HAZ was measured as described in the examples described later, and these results were organized. In the thermal cycle test, welding heat input: 60 kJ / cm (plate thickness 12 mm) was assumed, and after heating and holding at 1400 ° C. × 5 seconds, cooling from 800 ° C. to 500 ° C. was performed in 150 seconds.

As shown in FIG. 3, the value of (B-NT / 1.3) is −20 ppm or more and 10 ppm or less as shown in the following formula (1) in order to achieve vE −60 : 100 J or more as the low temperature toughness of HAZ. It can be seen that it is effective to be within the range.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)

  As shown in the above formula (1), by optimizing the balance of B, N and Ti, the grain boundary ferrite (coarse ferrite from the austenite grain boundary) due to the solid solution B existing at the grain boundary in the austenite grain It is considered that the effect of suppressing the generation of ferrite side plates from the grain boundaries and the effect of BN as ferrite transformation nuclei could be maximized.

  The chemical component composition defined in the high-strength steel sheet of the present invention and the requirements for optimizing the balance of B, N and Ti are as described above. However, if necessary, (a) Cu: 0.4% or less (Not including 0%), Ni: 0.4% or less (not including 0%), Cr: 0.4% or less (not including 0%), Mo: 0.4% or less (including 0%) And V: 0.02% or less (excluding 0%) and one or more selected from the group consisting of (b) Nb: 0.005 to 0.025%, (c) Ca: 0.003 % Or less (not including 0%) is also preferable, and the characteristics are improved depending on the components to be included. The reason for setting the range when these elements are contained is as follows.

[Cu: 0.4% or less (not including 0%), Ni: 0.4% or less (not including 0%), Cr: 0.4% or less (not including 0%), Mo: 0. 1 or more selected from the group consisting of 4% or less (excluding 0%) and V: 0.02% or less (not including 0%)]
Cu, Ni, Cr, Mo and V are all useful elements for securing the strength. Cu is an element effective for increasing the strength (tensile strength TS) by solid solution strengthening and precipitation strengthening. However, if excessively contained, the hot workability is inhibited, so the content is suppressed to 0.4% or less. Ni is an element that simultaneously improves the strength and toughness of the base material. In order to exhibit such an action effectively, it is preferable to contain 0.2% or more. However, excessive addition increases the cost and may induce SCC in liquefied ammonia, so it is suppressed to 0.4% or less.

  V is an element useful for increasing the hardenability and ensuring high strength, and for increasing the temper softening resistance. However, if it is excessively contained, the HAZ toughness deteriorates, so the content is suppressed to 0.02% or less.

  Cr and Mo are effective elements for increasing the strength of the base material. However, excessive addition degrades toughness, so it is better to keep them all at 0.4% or less.

[Nb: 0.005 to 0.025%]
Nb is an element useful for sufficiently suppressing the formation of coarse grain boundary ferrite and achieving crystal grain refinement in the austenite grains. In order to sufficiently exhibit such an effect, the amount when Nb is contained is preferably 0.005% or more. However, if excessively contained, island-like martensite (MA), which is a hard phase, is easily generated, and crystals are formed in a certain direction, leading to deterioration of HAZ toughness.

[Ca: 0.003% or less (excluding 0%)]
Ca is an element effective for fixing S, which adversely affects HAZ toughness, as CaS, and for improving the toughness by controlling the form of nonmetallic inclusions in a granular form. In order to exert such effects sufficiently, it is preferable to contain 0.0010% or more of Ca, but even if Ca is contained excessively, these effects are saturated and the HAZ toughness deteriorates. Therefore, the Ca content is preferably 0.003% or less.

  In order to produce the steel material of the present invention having the above-described structure, a low yield ratio high-tensile steel plate excellent in low temperature toughness of HAZ can be obtained by, for example, the following method.

After heating the steel material satisfying the above-described component composition to a predetermined temperature (for example, a temperature of 1000 ° C. or higher), hot rolling to a predetermined plate thickness and finishing the hot rolling [however, the rolling end temperature is (Ar 3 transformation point -40 ° C.) than the temperature, the temperature from 10 ° C. / sec or more average cooling rate (Ar 3 transformation point -40 ° C.) is cooled to a temperature below (first cooling), the The cooling is temporarily interrupted at the temperature and air cooling is performed for 30 to 150 seconds, and the temperature at the t / 4 (t: plate thickness) position is (Ar 3 transformation point −80 ° C.) to (Ar 3 transformation point −190 ° C.). The temperature from 350 ° C. to over 550 ° C. is cooled at an average cooling rate of 10 ° C./second or more (second cooling). The reason for setting the range of each condition in this method is as follows.

[First cooling]
(A) Predetermined ferrite fraction can be ensured by precipitating fine ferrite from overcooled austenite by the first cooling and then air cooling. When the cooling start temperature after the hot rolling is less than (Ar 3 transformation point −40 ° C.) or less, coarse ferrite precipitates before cooling and a fine ferrite structure cannot be obtained. On the other hand, when the average cooling rate is less than 10 ° C./second, a sufficient degree of supercooling cannot be obtained, the nucleation sites are lowered, and it becomes difficult to obtain ferrite having an average crystal grain size of 14 μm or less. In the present invention, the “Ar 3 transformation point” is a value determined by the following formula (2).
Ar 3 transformation point = 930-230. [C] +25. [Si] -74. [Mn] -56. [Cu] -16. [Ni] -9. [Cr] -5. [Mo] -1620. [Nb] (2)
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.

(B) The above cooling needs to be performed to a temperature of (Ar 3 transformation point −40 ° C.) or lower (however, the second cooling start temperature or higher). In order to precipitate fine ferrite from austenite by the first cooling, it is necessary to make it supercooled by this cooling. For this purpose, the temperature at which the cooling is stopped is (Ar 3 transformation point −40 ° C.) or less. The temperature must be When the temperature becomes higher than this, the effect of supercooling is lost, and it is produced as coarse ferrite during the subsequent air cooling, and the same structure as in the case of not cooling is obtained.

(C) It is necessary to temporarily stop cooling at the first cooling stop temperature and perform air cooling for 30 to 150 seconds. However, if the time (air cooling time) at this time is less than 30 seconds, the ferrite fraction is insufficient. In other words, if it exceeds 150 seconds, the ferrite fraction becomes too high, and the ferrite particle size also increases, making it difficult to ensure the toughness and strength of the base material. Preferably it is less than 100 seconds. In addition, the air cooling at this time is the temperature (second cooling) at a temperature of (Ar 3 transformation point −80 ° C.) to (Ar 3 transformation point −190 ° C.) after the air cooling. However, if this temperature is less than (Ar 3 transformation point-190 ° C.) or exceeds (Ar 3 transformation point−80 ° C.), the ferrite fraction is reduced. It will be insufficient. In the present invention, “air cooling” means a state in which the cooling rate is less than 1.0 ° C./second by stopping cooling and allowing to cool.

[Second cooling]
(A) The second cooling is for making the second phase hardness within a predetermined range (265 to 400 in Vickers hardness Hv) while controlling the fraction of MA. If the average cooling rate at this time is less than 10 ° C./second, a pearlite structure or a large amount of coarse MA is generated, and the toughness and strength of the base material cannot be secured. When the cooling stop temperature is 350 ° C. or lower, martensite MA is generated from the second phase and the hardness is increased. On the other hand, when the cooling stop temperature exceeds 350 ° C., the MA is decomposed and the fraction is reduced and finely dispersed. However, if the cooling stop temperature exceeds 550 ° C., the second phase becomes a pearlite structure, and the toughness and strength of the base material cannot be ensured.

  After cooling to above 350 ° C. and below 550 ° C. as described above, it is preferable to perform air cooling (AC). Moreover, tempering can also be performed at 500-600 degreeC, and intensity | strength adjustment is attained by adding such a process.

  In addition, the temperature shown above was managed by the temperature of the position of t / 4 part (t: board thickness) as a position which exhibits the average performance of a steel plate. The steel material of the present invention can be advantageously applied to so-called thick steel plates. The plate thickness at this time is about 7 mm or more, and the upper limit is not particularly limited, but is usually about 40 mm or less.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  The steel pieces having the chemical composition shown in Table 1 below are heated to 1100 ° C., hot-rolled to a predetermined plate thickness (12 mm or 30 mm), and after the hot rolling is finished, It cooled to the temperature range (1st cooling), the cooling was stopped in the middle, and it cooled by air. Then, it cooled to 550 degrees C or less (cooling stop temperature) with the cooling rate of 10 degrees C / sec or more from the said temperature range (2nd cooling). The manufacturing conditions at this time are shown in Tables 2 and 3.

For each steel plate obtained as described above, the matrix structure (ferrite fraction, ferrite grain size, MA fraction, second phase hardness), matrix characteristics [plate thickness, yield strength YS, tensile strength TS, toughness (fracture surface transition temperature vTrs)], and HAZ toughness (vE- 60 ) were evaluated in the following manner.

[Measurement of ferrite fraction and ferrite particle size]
For the fraction of ferrite (polygonal ferrite), t / 4 parts (t: plate thickness) of each steel plate were observed with an optical microscope at a magnification of 200 times and a field of view: 300 μm × 300 μm. The average value of 5 fields of view was obtained. Further, the average crystal grain size of ferrite was measured by a comparison method defined in JIS G 0551 by observing 10 visual fields at 400 times at a position of t / 4 part (t: plate thickness) of each steel plate.

[MA fraction]
At the position of t / 4 part (t: thickness) of each steel plate, after repeller corrosion, an optical microscope is used to observe an area of 1 field of view: 50 μm × 50 μm at a magnification of 1000 times, and using image analysis software Measurements were made and the average of 10 fields of view was determined.

[Second phase hardness]
Measurement was made using a 10 kgf micro Vickers hardness meter at a position of t / 4 part (t: plate thickness) of each steel plate, and an average value of 10 fields of view was obtained.

[Evaluation of base material properties]
JIS Z 2201 No. 1B test piece was taken from the total thickness of each steel plate in the direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z 2241, yield strength YS (if there is a yield point, Yield point YP, 0.2% yield strength (σ 0.2 ) when not present, and tensile strength (TS) were measured. And the thing whose yield strength: 440MPa or less, tensile strength: 510MPa, and yield ratio (YS / TS) was 75% or less was evaluated as a low yield ratio high-tensile steel plate.

  Further, a V-notch test piece of JIS Z 2202 was taken in the rolling direction from a portion of each steel plate cut by 1 mm from the surface side, and a Charpy impact test was performed in the manner of JIS Z 2242 to measure the fracture surface transition temperature vTrs. And it evaluated that the fracture surface transition temperature vTrs had the outstanding base material toughness with -100 degrees C or less.

[Evaluation of HAZ toughness]
Single-sided submerged arc welding using the steel plate was performed by the FCB method. The FCB method is a method of laying a backing flux on a copper plate, pressing it against the back of the groove, and completing the welding while forming a back bead from one side of the surface. Yes. The groove shape is shown in FIG. 4 [(a) when the plate thickness is 12 mm, (b) when the plate thickness is 30 mm]. As the welding material, the following low-temperature steel welding material (manufactured by Kobe Steel) was used, and weld joints were produced under the welding conditions shown in FIG. 5 and Table 4.
[Welding material]
・ Wire; US-255
・ Front flux; PFI-50LT
・ Backing flux; MF-1R

Then, three V-notch test pieces of JIS Z 2202 each cut by 1 mm from the surface side and notched perpendicularly to the plate surface at the position of HAZ (bond part) were collected, and Charpy impact was performed according to the procedure of JIS Z 2242. A test was conducted. And the absorbed energy (vE- 60 ) in test temperature: -60 degreeC was measured. And the thing whose average value of this absorbed energy (vE- 60 ) is 100J or more was evaluated as excellent in the low temperature toughness of HAZ.

  These results are shown in Tables 5 and 6 below together with the actual welding conditions (construction method, heat input).

  From these results, it can be considered as follows (note that the following No. indicates the experiment No. in the table).

  No. satisfying the requirements defined in the present invention. Steel plates 1 to 14 are excellent in low-temperature toughness of HAZ and are excellent in base material properties (low-temperature toughness, yield strength YS: 440 MPa or less, tensile strength TS: 510 MPa or more, yield ratio YR: 75% or less). It is a high-strength steel plate that is welded by a high heat input single-sided submerged arc welding method and exhibits excellent characteristics when used for low-temperature applications.

  On the other hand, No. which does not satisfy the provisions of the present invention. Each of 15 to 29 has the following problems. That is, no. 15 to 18 are excellent in low temperature toughness of HAZ, but the ferrite fraction of the base material is low, and the desired base material characteristics (yield strength YS: 440 MPa or less, yield ratio YR: 75% or less, vTrs is −100 ℃ or less) is not obtained.

  No. 19 is excellent in low temperature toughness of HAZ, but the MA fraction of the base material is high, and the desired base material characteristics (tensile strength TS: 510 MPa or more, yield ratio YR: 75% or less, vTrs is − 100 ° C. or lower) is not obtained. No. No. 20 is excellent in low temperature toughness of HAZ, but the MA fraction of the base material is high and the second phase hardness is high, and the desired base material characteristics (yield ratio YR: 75% or less, vTrs is − 100 ° C. or lower) is not obtained. No. In 21-23, the ferrite particle size is also large and the base material toughness has deteriorated.

  No. No. 24 has a C content exceeding the upper limit. In No. 25, since the Si content and the Mn content exceed the upper limit, the MA fraction increases and both the HAZ toughness and the base metal toughness are inferior. No. No. 26 has an excessive Ti content. In No. 27, since the B content is excessive, (B-NT / 1.3) exceeds the upper limit of the formula (1) and both are inferior in HAZ toughness.

  No. No. 28 has a V content exceeding the preferred range, does not contain Ti or B, and (B-NT / 1.3) is below the lower limit of the formula (1), so HAZ toughness It is inferior to. No. In No. 29, the Ni content exceeds the preferable range, and B does not contain B. The strength decreases and the HAZ toughness is also inferior.

It is a graph which shows the relationship between a ferrite particle size and the fracture surface transition temperature vTrs of a base material. It is the graph which showed the influence which MA fraction and 2nd phase hardness have on the characteristic of a base material. It is a graph which shows the relationship between (B-NT / 1.3) and vE- 60 of HAZ. Sectional drawing of the groove shape in the welding in an Example is shown. The schematic diagram of the electrode arrangement | positioning at the time of FCB welding is shown.

Claims (4)

  1. C: 0.05 to 0.09% (meaning “mass%”, chemical components are the same hereinafter), Si: 0.05 to 0.25%, Mn: 1.2 to 1.6%, P: 0.01% or less (excluding 0%), S: 0.003% or less (not including 0%), Al: 0.02-0.04%, B: 0.0006-0.0020%, N: 0.0030 to 0.0080%, Ti: 0.005 to 0.025%, Nb: 0.005 to 0.025%, Ca: 0.003% or less (not including 0%) , respectively The balance is iron and inevitable impurities, and in the microstructure at the position of t / 4 (t: plate thickness), the ferrite fraction occupies 60 to 85 area% of the entire structure, and the island martensite fraction is 5%. (Including 0%) and the balance is a mixed structure of bainite structure, and the average crystal of the ferrite A low-yield ratio high-strength steel sheet excellent in low temperature toughness of a weld heat-affected zone and a base material, wherein the grain size is 14 μm or less and the average Vickers hardness Hv of the second phase is 265 to 400.
  2. The low yield ratio high tensile strength steel sheet according to claim 1, wherein the chemical composition satisfies the following formula (1).
    −20 ≦ (B-NT / 1.3) ≦ 10 (1)
    {In formula, B shows B content (mass ppm).
    NT is
    The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
    When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
    (N-Ti / 3.4) <0 indicates NT = 0}
  3. Furthermore, Cu: 0.4% or less (not including 0%), Ni: 0.4% or less (not including 0%), Cr: 0.4% or less (not including 0%), Mo: 0 The composition according to claim 1 or 2, comprising at least one selected from the group consisting of .4% or less (not including 0%) and V: 0.02% or less (not including 0%). Low yield ratio high strength steel sheet.
  4. In producing the low-yield ratio high-tensile steel sheet according to any one of claims 1 to 3 , after hot rolling, the steel sheet is heated from a temperature exceeding (Ar 3 transformation point-40 ° C) to 10 ° C / second. Cooling to the temperature below (Ar 3 transformation point −40 ° C.) at the above average cooling rate, cooling is temporarily interrupted at the temperature, and air cooling is performed for 30 to 150 seconds, and subsequently t / 4 (t: plate thickness) The temperature at the position is cooled at an average cooling rate of 10 ° C./second or more from a temperature range of (Ar 3 transformation point−80 ° C.) to (Ar 3 transformation point−190 ° C.) to a temperature range of more than 350 ° C. and less than 550 ° C. A method for producing a low-yield-ratio high-tensile steel sheet, characterized in that
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