JP6579135B2 - Low yield ratio steel sheet for construction and manufacturing method thereof - Google Patents

Description

  The present invention relates to a low yield ratio steel sheet for construction, and in particular, to a low yield ratio steel sheet for construction having a low surface hardness and excellent toughness in a low yield ratio and a large heat input weld. Moreover, this invention relates to the manufacturing method of the said low yield ratio steel plate for construction.

  In recent years, with the increase in size of building structures, the use of steel materials has increased in thickness and strength, and in order to ensure the earthquake resistance of building structures, it has a low yield ratio of 80% or less. It is also required to be.

  A building structure is a welded structure that is assembled into a predetermined shape by welding, but if a large load is applied during an earthquake, brittle fracture may occur from the weld before sufficient plastic deformation occurs. In addition, the welded portion is required to have good toughness.

  In order to produce welded structures with high efficiency, large heat input welding with a heat input of 400 kJ / cm or higher is applied in the manufacture of box columns, such as submerged arc welding of corner joints and electroslag arc welding of diaphragm joints. The In general, since such a high heat input welded part has a slow cooling rate after welding, the residence time in the high temperature region of the region heated to the vicinity of the melting point becomes long. As a result, coarsening of the structure and generation of a hard embrittlement phase called island-like martensite (Martensite-Austenite constituent, MA) occur, and the weld heat affected zone becomes brittle.

  The weld heat-affected zone is more likely to be brittle as the strength of the steel increases in the amount of alloying elements, and often becomes a problem in the case of a tensile strength (TS) 590 MPa class steel for building structural steel. Therefore, various TS590MPa construction steels that can provide excellent weld heat affected zone toughness have been proposed.

  For example, in patent document 1, the manufacturing method of the high strength thick steel plate for construction which is excellent in the super-high heat input welding heat affected zone toughness exceeding the heat input of 400 kJ / cm is proposed. In the method, the microstructure of the weld heat-affected zone is refined by regulating the ACR value obtained by the formula consisting of Ca, O, and S and utilizing the fine composite sulfide generated during welding as a ferrite transformation nucleus. Has improved toughness.

Moreover, in patent document 2, the manufacturing method of the low yield ratio 600N / mm ² grade steel plate for construction excellent in the high heat input welding heat affected zone toughness is proposed. In the above method, the hardenability in the weld heat affected zone is lowered by using a steel composition having a low CB-free system, and the structure of the weld heat affected zone is refined by utilizing Ti oxide. Moreover, in the said method, the base material intensity | strength of 600 N / mm ² grade is ensured by precipitation strengthening by Cu.

  Patent Document 3 proposes a high-tensile steel plate excellent in toughness of a high heat input welding heat-affected zone. In the high-tensile steel sheet, by controlling the composition and controlling the morphology and finely controlling the block size of the transformation structure by making the component composition appropriately contain elements such as Mn, Ni, and Cr that are extremely low C and hardenability improved. As a result, the toughness improvement of the weld heat affected zone exceeding the heat input of 500 kJ / cm is achieved.

  Patent Document 4 proposes a low-yield-ratio high-tensile steel plate excellent in high heat input welding toughness. In the low-yield ratio high-tensile steel sheet, the base metal component composition is reduced to a low carbon equivalent (low Ceq), and the structure of the weld heat affected zone is refined by utilizing Ti carbonitride to obtain a heat input of 250 kJ. The toughness in welding at / cm or more is improved. Further, the ferrite fraction is adjusted for lower yield ratio, and Nb carbonitride is used for higher strength.

  Patent Document 5 proposes a steel plate for low-yield-ratio building structures that has a plate thickness of 30 mm or more and is excellent in super large heat input welding HAZ toughness that is welded at a heat input exceeding 400 kJ / cm and a manufacturing method thereof. In the thick steel sheet for building structure with low yield ratio, by controlling the ACR value in the component composition of the steel sheet, the composite sulfide with MnS precipitated on the CaS surface, which functions as a ferrite transformation nucleus during welding, is finely dispersed. Yes.

JP 2005-68519 A JP-A-6-128635 JP 2007-126725 A JP 2001-172736 A JP 2003-183767 A JP 2010-1111924 A

  In the conventional low yield ratio steel as described above, ferrite in steel is obtained by a method of performing a two-phase heat treatment (Patent Documents 1 and 2) or a method of quenching and tempering a low Ceq component system (Patent Document 4). As a result, the yield ratio is reduced.

  However, when performing the two-phase region heat treatment as described in Patent Documents 1 and 2, there is a problem that the manufacturing process becomes complicated and the productivity is lowered.

  On the other hand, as described in Patent Document 4, when quenching and tempering is performed without performing two-phase region heat treatment, it is necessary to use a high C component system in which C is 0.12% or more in order to ensure strength. . Therefore, even if a low yield ratio is achieved, the hardness of the steel sheet surface becomes excessively high, and the ductility in the vicinity of the surface layer portion is significantly deteriorated. Therefore, cracks may occur near the surface layer when stress is applied due to an earthquake or the like, and fracture may occur due to the notch effect.

  Therefore, Patent Document 6 proposes a low yield ratio steel sheet for construction that has improved earthquake resistance by reducing the hardness in the vicinity of the surface layer and improving the ductility in the vicinity of the surface layer without performing a two-phase region heat treatment. However, in the low yield ratio steel sheet for construction, the average value of Charpy absorbed energy vE (0 ° C.) at 0 ° C. remains at 100 J or more with respect to the toughness of the high heat input weld. In order to improve the safety of the structure, it is necessary to further improve the toughness of the high heat input weld.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a low-yield-ratio steel sheet for construction that has a low surface hardness and has a low yield ratio and excellent toughness in a large heat input weld zone. And Moreover, this invention provides the manufacturing method of the low yield ratio steel plate for construction which can manufacture the said low yield ratio steel plate for construction, without giving the two-phase area heat processing which causes productivity fall. Objective.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventors set the ACR value described later to 0.2 to 0.8, added Ti and N in appropriate amounts, and set the C amount to 0.07%. Hereinafter, by subjecting steel having a component composition of Si content less than 0.05% and Nb to 0.01% or less to accelerated cooling, surface hardness is low and low yield is obtained without performing a two-phase region heat treatment. It has been found that a low yield ratio steel sheet for construction can be produced that has both excellent ratio and high toughness in high heat input welds.

  The present invention has been made by further study based on the above findings, and the gist of the present invention is as follows.

1. % By mass
C: 0.03-0.07%,
Si: less than 0.05%,
Mn: 0.6 to 2.0%,
P: 0.020% or less,
S: 0.0005 to 0.0030%,
Cu: 0.5% or less,
Ni: 1.0% or less,
Ti: 0.005 to 0.030%,
Al: 0.100% or less,
N: 0.0025 to 0.0070%,
Ca: 0.0005 to 0.0040%,
Nb: 0.01% or less,
O: 0.0040% or less, and Cr: 0.5% or less, Mo: 0.2% or less, V: 0.08% or less, and B: 0.0003 to 0.0020% 2 or more
The balance Fe and inevitable impurities,
Ceq defined by the following formula (1) has a component composition of 0.40 to 0.46, and ACR defined by the following formula (2) is 0.2 to 0.8,
It has a microstructure in which the area fraction of ferrite is 2 to 30% and the area fraction of bainite is 65% or more,
A low yield ratio steel sheet for construction having a surface hardness of 350HV10 or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(However, the element symbols in the formulas (1) and (2) indicate the content (% by mass) of each element, and are zero when the element is not contained)

2. The component composition is further in mass%,
Mg: 0.0005 to 0.0050%,
The low yield ratio steel sheet for construction according to 1 above, which contains one or more selected from the group consisting of Zr: 0.001 to 0.020% and REM: 0.001 to 0.020%.

3. A steel material having the component composition described in 1 or 2 above is heated to 1000 to 1200 ° C.,
The heated steel material is subjected to hot rolling with a rolling end temperature equal to or higher than the Ar3 transformation point to form a hot rolled steel sheet having a thickness of 40 mm or more,
The hot-rolled steel sheet is accelerated and cooled at an average cooling rate of 3 to 15 ° C./s,
The method of manufacturing a low yield ratio steel sheet for building, wherein the hot-rolled steel sheet after the accelerated cooling is air-cooled or tempered,
The cooling stop temperature in the accelerated cooling is 450 to 200 ° C. when the tempering is not performed, and 450 to 50 ° C. when the tempering is performed.
The architectural low yield ratio steel sheet has a microstructure in which the area fraction of ferrite is 2 to 30%, the area fraction of bainite is 65% or more, and the surface hardness is 350HV10 or less. A method of manufacturing a steel sheet.

4). A steel material having the component composition described in 1 or 2 above is heated to 1000 to 1200 ° C.,
The heated steel material is subjected to hot rolling with a rolling end temperature equal to or higher than the Ar3 transformation point to form a hot rolled steel sheet having a thickness of 40 mm or less,
The hot-rolled steel sheet is accelerated and cooled at an average cooling rate of 15 ° C./s or more,
The method of manufacturing a low yield ratio steel sheet for building, wherein the hot-rolled steel sheet after the accelerated cooling is air-cooled or tempered,
The cooling stop temperature in the accelerated cooling is 650 to 450 ° C.,
The architectural low yield ratio steel sheet has a microstructure in which the area fraction of ferrite is 2 to 30%, the area fraction of bainite is 65% or more, and the surface hardness is 350HV10 or less. A method of manufacturing a steel sheet.

  The low yield ratio steel sheet for construction of the present invention has a surface hardness as low as 350 HV10 or less, and also has a low yield ratio and excellent toughness in a large heat input weld, so that the heat input is 400 kJ / cm or more. Even when used for building structures subjected to heat input welding, it can exhibit extremely excellent earthquake resistance. Further, the low yield ratio steel sheet for construction of the present invention can be manufactured without performing a two-phase region heat treatment that causes a decrease in productivity, and therefore can be manufactured extremely economically, and is extremely industrially Useful.

  Hereinafter, the present invention will be specifically described. In the low yield ratio steel sheet for building in one embodiment of the present invention, the component composition and the microstructure are controlled as described above.

[Ingredient composition]
First, the reason for limiting the component composition as described above will be described. In addition, "%" regarding a component composition shall mean "mass%" unless there is particular notice.

C: 0.03-0.07%
C is an important element that affects strength, surface hardness, and toughness. In order to ensure the strength, 0.03% or more is necessary. On the other hand, in order to reduce the surface hardness, improve the ductility near the surface, and improve the toughness of the weld heat affected zone, it is necessary to be 0.07 or less, so the C content is 0.03 to 0.07% .

Si: Less than 0.05% When the Si content is 0.05% or more, island-like martensite is generated in the heat-affected zone of high heat input welding, and toughness deteriorates. Therefore, in the present invention, it is extremely important to suppress the Si content to less than 0.05%. On the other hand, from the viewpoint of suppressing the generation of island martensite, it is desirable to make the Si content as low as possible, so the lower limit of the Si content is not particularly limited and may be zero, but excessively low Si Siization may cause a decrease in production efficiency and an increase in manufacturing cost, so the Si content is preferably 0.005% or more.

Mn: 0.6 to 2.0%
Mn is an element effective for securing strength by solid solution strengthening, and 0.6% or more is necessary to exert its effect. Moreover, if it exceeds 2.0%, weldability deteriorates. Therefore, it regulates to 0.6 to 2.0%. The Mn content is preferably 0.6 to 1.6%.

P: 0.020% or less P is an element mixed as an impurity. When the mixed amount increases, the base material toughness deteriorates. Therefore, it is suppressed to 0.020% or less. The P content is preferably 0.015% or less. On the other hand, the lower limit of the P content is not particularly limited and may be zero. However, excessive reduction may cause an increase in manufacturing cost, so the P content is preferably 0.0005% or more.

S: 0.0005 to 0.0030%
S is an element necessary for controlling the ACR value, which will be described later, and forms CaS as a nucleus of MnS formation. The generated MnS is effective in generating intragranular ferrite and refining the structure of the high heat input weld. Effect. In order to obtain the effect, 0.0005% or more is necessary. On the other hand, if the MnS content exceeds 0.0030%, the material in the plate thickness direction is deteriorated due to MnS generation. Therefore, it is suppressed to 0.0030% or less. The S content is preferably 0.0010 to 0.0030%.

Cu: 0.5% or less Cu is an element effective for solid solution strengthening, but addition exceeding 0.5% causes manufacturing problems such as deterioration of hot ductility and increase of surface defects. Therefore, the Cu content is 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.05% or more from the viewpoint of the effect of addition of Cu.

Ni: 1.0% or less Ni is an element effective for solid solution strengthening, but when the Ni content exceeds 1.0%, the alloy cost increases and the manufacturing cost increases. Therefore, the Ni content is 1.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but is preferably 0.10% or more from the viewpoint of the effect of adding Ni.

Ti: 0.005-0.030%
Ti is an element effective for refining the structure of the heat affected zone by generating TiN. In order to exert this effect, 0.005% or more is necessary. On the other hand, if added over 0.030%, the base metal toughness and the heat-affected zone toughness deteriorate due to TiC precipitation. Therefore, the Ti content is set to 0.005 to 0.030%. The Ti content is preferably 0.008 to 0.020%.

Al: 0.100% or less When the Al content exceeds 0.100%, Al ₂ O ₃ is generated, and the cleanliness of the steel is lowered. Therefore, the Al content is 0.100% or less. On the other hand, the lower limit of the Al content is not particularly limited, but in order to obtain the effect of Al as a deoxidizer, it is preferably 0.030% or more, and more preferably 0.055%.

N: 0.0025 to 0.0070%
N is an element effective for refining the structure of the heat affected zone by generating TiN. In order to exhibit this effect, 0.0025% or more is necessary. On the other hand, if added over 0.0070%, the solid solution N of the weld heat affected zone increases, and the toughness of the heat affected zone deteriorates. Therefore, the N content is 0.0025 to 0.0070%. The N content is preferably 0.0030 to 0.0065%.

Ca: 0.0005 to 0.0040%
Ca is an element necessary for ACR value control. In addition, Ca forms CaS that serves as a production nucleus of MnS, and the produced MnS exerts an effective action on the formation of intragranular ferrite and refinement of the structure of the high heat input weld. In order to obtain the effect, 0.0005% or more is necessary. On the other hand, if the Ca content exceeds 0.0040%, Ca-based oxides increase and the cleanliness of the steel decreases. Therefore, the Ca content is set to 0.0005 to 0.0040%. The Ca content is preferably 0.0005 to 0.0040%.

Nb: 0.01% or less Nb increases the hardenability of the weld heat-affected zone by adding a small amount, and as a result, suppresses the formation of ferrite, causes upper bainite, and deteriorates toughness. When the Nb content exceeds 0.01%, the deterioration of the weld heat affected zone toughness due to the above action becomes a problem, so the Nb content is set to 0.01% or less. On the other hand, the lower limit of Nb is not limited and may be zero, but excessive reduction leads to an increase in manufacturing cost in the melting process. Therefore, the Nb content is preferably 0.001% or more.

O: 0.0040% or less O is an element mixed as an impurity. If the O content exceeds 0.0040%, oxide inclusions increase and the cleanliness of the steel deteriorates. Therefore, the O content is suppressed to 0.0040% or less. On the other hand, since it is desirable that the O content is low, the lower limit thereof is not limited and may be zero, but excessive reduction leads to an increase in manufacturing cost in the melting step. Therefore, the O content is preferably 0.0005% or more.

  Furthermore, the component composition contains two or more selected from the group consisting of Cr, Mo, V, and B. When these elements are contained, the content of the elements is in the following range.

Cr: 0.5% or less Cr is an element effective for securing the strength of the base material, but addition exceeding 0.5% lowers weldability. Therefore, when it contains Cr, Cr content shall be 0.5% or less.

Mo: 0.2% or less Mo is an element effective for securing the strength of the base material, but increases the hardenability of the weld heat affected zone by adding a small amount, and as a result, suppresses the formation of ferrite, It becomes upper bainite and deteriorates toughness. When it exceeds 0.2%, such an action is caused, and the weld heat-affected zone toughness is deteriorated. Therefore, when adding Mo, Mo content is made 0.2% or less.

V: 0.08% or less V is an element effective for ensuring the strength of the base material. However, addition of more than 0.08% increases the alloy cost and the manufacturing cost. Therefore, when V is added, the V content is set to 0.08% or less.

B: 0.0003 to 0.0020%
B is an element effective in improving hardenability and ensuring the strength of the base material. In order to exert the effect, the B content needs to be 0.0003% or more. On the other hand, addition exceeding 0.0020% deteriorates weldability. Therefore, when adding B, B content shall be 0.0003 to 0.0020%.

  The component composition of the low yield ratio steel sheet for construction in one embodiment of the present invention is composed of the above elements, the remaining Fe and inevitable impurities.

  In other embodiment of this invention, the said component composition can contain 1 or more selected from the group which consists of Mg, Zr, and REM further for the toughness improvement of a welding heat affected zone. When these elements are contained, the content of the elements is in the following range.

Mg: 0.0005 to 0.0050%
Mg is an element that produces oxysulfide and has the effect of improving the toughness of the weld heat affected zone by generating ferrite and refining the structure in the weld heat affected zone. When adding Mg, in order to acquire the said effect, Mg content shall be 0.0005% or more. On the other hand, when Mg is added excessively, the cleanliness of the steel is lowered.

Zr: 0.001 to 0.020%
Zr is an element having an effect of improving toughness due to dispersion of oxides. In order to exert the effect, it is necessary to contain at least 0.001% or more. On the other hand, if the content exceeds 0.020%, the effect is saturated. Therefore, when adding Zr, the Zr content is set to 0.001 to 0.020%.

REM: 0.001-0.020%
REM (rare earth metal) is an element that produces oxysulfide and has the effect of improving the toughness of the weld heat affected zone by generating ferrite and refining the structure in the weld heat affected zone. When adding REM, in order to acquire the said effect, content shall be 0.001% or more. On the other hand, when added excessively, the cleanliness of the steel decreases, so when added, the content is made 0.020% or less.

In the above component composition, Ceq defined by the following formula (1) should be 0.40 to 0.46, and ACR defined by the following formula (2) should be 0.2 to 0.8. .
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(However, the element symbols in the formulas (1) and (2) indicate the content (% by mass) of each element, and are zero when the element is not contained)

Ceq: 0.40 to 0.46
From the viewpoint of strength and toughness of high heat input welding heat-affected zone, Ceq (carbon equivalent) is controlled within the above range. When Ceq is less than 0.40, excessive generation of ferrite and / or generation of bainite is suppressed, and strength cannot be ensured. On the other hand, when Ceq exceeds 0.46, the toughness of the high heat input welding heat-affected zone deteriorates. Therefore, Ceq is set to 0.40 to 0.46.

ACR: 0.2-0.8
From the viewpoint of high heat input welding heat-affected zone toughness, the ACR (Atomic Concentration Ratio) is controlled within the above range. If the ACR is less than 0.2, the amount of Ca-based sulfide necessary for ferrite formation decreases, and the high heat input welding heat affected zone toughness improving effect cannot be obtained. On the other hand, when the ACR exceeds 0.8, Ca-based sulfides are generated, but MnS having the cores is not generated, so that the heat affected zone refinement effect due to ferrite generation cannot be obtained. Therefore, ACR is set to 0.2 to 0.8.

[Microstructure]
The low yield ratio steel sheet for building according to the present invention has a microstructure in which the area fraction of ferrite is 2 to 30% and the area fraction of bainite is 65% or more.

Ferrite: 2-30%
From the viewpoint of reducing the yield ratio, the area fraction of ferrite is set to the above range. If the ferrite area fraction is less than 2%, a yield ratio of 80% or less cannot be obtained. On the other hand, when the ferrite area fraction exceeds 30%, it is difficult to ensure the strength. Therefore, the ferrite area fraction is set to 2 to 30%.

Bainite: 65% or more From the viewpoint of securing the strength of the base material, the area fraction of bainite is set to the above range. When the bainite area fraction is less than 65%, TS590 MPa or more cannot be obtained.

  In addition, if the said conditions are satisfy | filled, it will be accept | permitted that structures other than a ferrite and a bainite exist. Examples of the other structures include martensite, MA (island martensite), pearlite (including pseudo pearlite), and the like. However, the total area fraction of the structure other than ferrite and bainite is preferably 10% or less, and preferably 5% or less.

[surface hardness]
Surface hardness: 350 HV10 or less In the present invention, from the viewpoint of improving the surface ductility, the surface hardness of the low yield ratio steel sheet for building is set to 350 HV10 or less. If the surface hardness is higher than 350 HV10, the ductility near the surface layer is lowered and the earthquake resistance is deteriorated. Therefore, it shall be 350HV10 or less. On the other hand, the lower limit of the surface hardness is not particularly limited, but is preferably 220 HV10 or more in order to ensure high strength of the base material. In addition, the said surface hardness can be measured by the method as described in an Example.

[Mechanical properties]
In the low yield ratio steel sheet for building of the present invention, a low yield ratio is achieved by satisfying the above conditions. The specific range of the yield ratio (YR) is not particularly limited, but is preferably 80% or less. Further, the tensile strength (TS) is not particularly limited, but is preferably 590 MPa or more. Similarly, the yield stress (YS) is not particularly limited, but is preferably 440 MPa or more. The TS and YS can be measured by the method described in Examples, and YR can be obtained as (YS / TS).

[Thickness]
Although the board thickness of the said low yield ratio steel plate for construction can be made into arbitrary values, without being limited in particular, generally it is preferred to set it as 19-100 mm.

  Further, in the low yield ratio steel sheet for building of the present invention, by satisfying the above-mentioned compositional composition, generation of island martensite when performing large heat input welding is suppressed, and as a result, the weld heat affected zone The toughness is significantly improved. The characteristics are not particularly limited. For example, when the plate thickness is 40 mm or more, in the heat affected zone structure in the vicinity of the bond when large heat input welding with a welding heat input of about 1000 kJ / cm is performed, island martensite The area fraction is preferably 2% or less. Here, “in the vicinity of the bond” refers to a range (bond + 1 mm) from the boundary (bond) between the weld metal and the base steel plate to the base steel plate side of 1 mm.

  Next, the manufacturing method of the low yield ratio steel sheet for construction in one embodiment of the present invention is explained.

The low yield ratio steel sheet for construction of the present invention can be produced by sequentially applying the following treatment to a steel material (steel slab or the like) having the above component composition.
(1) Heating (2) Hot rolling (3) Accelerated cooling (4) Air cooling or tempering

[heating]
Heating temperature: 1000-1200 ° C
When the heating temperature is less than 1000 ° C., the hot deformation resistance is high and rolling becomes difficult. On the other hand, if the heating temperature exceeds 1200 ° C., the initial structure during heating becomes coarse, the base material structure becomes coarse, and the toughness deteriorates. Therefore, heating temperature shall be 1000-1200 degreeC.

Rolling end temperature: Ar3 transformation point or higher If the rolling end temperature in the hot rolling process is lower than the Ar3 transformation point, the ferrite produced during rolling is refined and the yield ratio is increased. Therefore, the rolling end temperature is set to the Ar3 transformation point or higher. Note that the Ar ₃ transformation point can be obtained by the following equation (3), for example.
Ar ₃ (° C.) = 910-310 × C-80 × Mn-20 × Cu-55 × Ni-15 × Cr-80 × Mo (3)

  After hot rolling, accelerated cooling is performed on the hot-rolled steel sheet, and the conditions vary depending on the thickness of the hot-rolled steel sheet as follows.

(When plate thickness is 40mm or more)
Average cooling rate: 3-15 ° C / s
If the cooling rate of accelerated cooling is less than 3 ° C./s, the ferrite fraction exceeds 30%, and it is difficult to ensure the strength. On the other hand, if it is higher than 15 ° C./s, the ferrite fraction becomes less than 2%, and it becomes difficult to reduce the yield ratio. Therefore, an average cooling rate shall be 3-15 degrees C / s.

  Furthermore, even in the case where the plate thickness is 40 mm or more, the cooling stop temperature in the accelerated cooling is set to be different depending on whether the tempering is performed after the accelerated cooling or not.

(When not tempering)
Cooling stop temperature: 450-200 ° C
When tempering is not performed, the cooling stop temperature of accelerated cooling is set to 450 to 200 ° C. When the cooling stop temperature is higher than 450 ° C., excessive formation of ferrite and / or formation of bainite is suppressed, and it becomes difficult to ensure strength. On the other hand, if the cooling stop temperature is less than 200 ° C., it is difficult to ensure the steel plate shape due to cooling strain or the like.

(When tempering)
Cooling stop temperature: 450-50 ° C
When performing tempering, the cooling stop temperature of accelerated cooling is set to 450 to 50 ° C. As in the case where tempering is not performed, when the cooling stop temperature is higher than 450 ° C., it is difficult to ensure the strength. On the other hand, when performing tempering, it becomes possible to correct the steel plate shape during tempering heating, so the cooling stop temperature in accelerated cooling can be expanded to a lower temperature, specifically 50 ° C. or higher. can do.

(When plate thickness is 40mm or less)
On the other hand, when the plate thickness of the hot-rolled steel sheet is reduced, the same structure form as that when the plate thickness is thick can be obtained even if the cooling rate and the cooling stop temperature are increased as compared with the case where the plate thickness is thick. Therefore, accelerated cooling is performed under the following conditions for steel plates having a thickness of 40 mm or less.

Average cooling rate of 15 ° C./s or more If the average cooling rate in accelerated cooling is less than 15 ° C./s, it is difficult to ensure strength. Therefore, an average cooling rate shall be 15 degrees C / s or more.

Cooling stop temperature: 650-450 ° C
If the cooling stop temperature exceeds 650 ° C., the ferrite fraction exceeds 30%, so that the strength cannot be secured. On the other hand, if the cooling stop temperature is lower than 450 ° C., ferrite is not generated, and a low yield ratio cannot be achieved. Therefore, the cooling stop temperature is set to 650 to 450 ° C.

  Even if tempering is not performed, the surface hardness becomes 350 HV10 or less due to the combination of the above component composition and accelerated cooling conditions, and further decreases when tempering is performed. The tempering conditions are not particularly limited. However, if the tempering temperature exceeds 450 ° C., the strength may decrease and the yield ratio may increase, so the tempering temperature is preferably 450 ° C. or lower. When tempering is not performed, air cooling is performed after completion of accelerated cooling.

Example 1
A steel plate having a thickness of 40 to 100 mm was produced by the following procedure, and its characteristics were evaluated.

  Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace to obtain a slab as a steel material. The obtained slab was hot-rolled to obtain a hot-rolled steel sheet, and the obtained hot-rolled steel sheet was subjected to accelerated cooling. Next, air cooling or tempering was performed to obtain a test material. Table 2 shows the manufacturing conditions in each step.

  Next, for each of the obtained test materials, the area ratio of ferrite, the area fraction of bainite, and the surface hardness were measured.

(Area fraction of ferrite and bainite)
Test specimens were collected so that the 1/4 position of each steel plate would be the measurement position, and the optical microscope was used to observe five fields of 500 ×, 180 μm × 150 μm range, and the average value of the ferrite and bainite area fractions in each field of view. The area fraction of ferrite and bainite was used.

(surface hardness)
The surface hardness was a Vickers hardness test with a load of 10 kgf in accordance with JISZ2244, 20 points were measured at 0.5 mm below the surface layer, and the maximum value was taken as the representative value.

  Furthermore, in order to evaluate the characteristics of the obtained steel sheet, MA fraction measurement and Charpy impact test in the heat-affected zone of high heat input welding, and tensile test of the steel sheet were performed.

(MA fraction of heat-affected zone with large heat input welding)
In order to evaluate the generation of island martensite (MA) in the heat affected zone (Heat Affected Zone, HAZ) when large heat input welding was performed, electroslag welding with heat input of 960 kJ / cm was performed. Test specimens were collected so that the weld heat affected zone 1 mm away from the bond portion was the observation position, and five 2000-magnification photographs of a scanning electron microscope (SEM) were traced. The area fraction was determined and the average value was calculated.

(HAZ toughness)
In order to evaluate the toughness (HAZ toughness toughness) in the heat-affected zone when large heat input welding was performed, the Charpy impact test was performed to measure the absorbed energy. Specifically, a test piece was collected from the sample subjected to the high heat input welding so that the weld heat affected zone 1 mm away from the bond portion was a notch position, and a Charpy impact test was performed. The test temperature was 0 ° C., and the average value of the absorbed energy in three test samples was vE (0 ° C.).

(Tensile test)
In order to evaluate the mechanical properties of the steel sheet, a tensile test was performed according to the following procedure. In accordance with JIS Z2201, JIS No. 4 test specimens were collected from two locations, the thickness 1/4 (1/4 t) position and the thickness center (1/2 t) position of the steel sheet, and tensile properties (yield stress (YS ), Tensile strength (TS), and yield ratio (YR)). The test piece was taken in the L direction.

  The evaluation results were as shown in Table 2. The target characteristics were YS ≧ 440 MPa, TS ≧ 590 MPa, YR ≦ 80%, high heat input HAZ toughness vE0 ° C. ≧ 180 J.

(Example 2)
In the same procedure as in Example 1, a steel plate having a thickness of 19 to 40 mm was manufactured, and its characteristics were evaluated. The production conditions are shown in Table 3.

  However, since the plate thickness was thin in Example 2, the MA fraction of the large heat input weld and the welding for the Charpy impact test were electroslag welding with a heat input of 400 kJ / cm. In the tensile test, a test was performed using a full-thickness JIS No. 5 test piece collected in the L direction in accordance with JIS Z2201. Other conditions and evaluation methods were the same as in Example 1. The evaluation results were as shown in Table 3.

  As can be seen from the results shown in Tables 2 and 3, all of the examples satisfying the conditions of the present invention have high strength of TS of 590 MPa or more, YR of 80% or less, absorbed energy vE0 of the weld heat affected zone. The temperature was 180 J or more, and the weld heat affected zone toughness was also excellent. On the other hand, the comparative example which does not satisfy | fill the conditions of this invention was inferior in any one or more of said characteristics.

Claims (4)

% By mass
C: 0.03-0.07%,
Si: less than 0.05%,
Mn: 0.6 to 2.0%,
P: 0.020% or less,
S: 0.0005 to 0.0030%,
Cu: 0.5% or less,
Ni: 1.0% or less,
Ti: 0.005 to 0.030%,
Al: 0.100% or less,
N: 0.0025 to 0.0070%,
Ca: 0.0005 to 0.0040%,
Nb: 0.01% or less,
O: 0.0040% or less, and Cr: 0.5% or less, Mo: 0.2% or less, V: 0.08% or less, and B: 0.0003 to 0.0020% 2 or more
The balance Fe and inevitable impurities,
Ceq defined by the following formula (1) has a component composition of 0.40 to 0.46, and ACR defined by the following formula (2) is 0.2 to 0.8,
It has a microstructure in which the area fraction of ferrite is 2 to 30% and the area fraction of bainite is 65% or more,
A low yield ratio steel sheet for construction having a surface hardness of 350HV10 or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(However, the element symbols in the formulas (1) and (2) indicate the content (% by mass) of each element, and are zero when the element is not contained) The component composition is further in mass%,
Mg: 0.0005 to 0.0050%,
The low yield ratio steel sheet for construction according to claim 1, comprising one or more selected from the group consisting of Zr: 0.001 to 0.020% and REM: 0.001 to 0.020%. A steel material having the component composition according to claim 1 or 2 is heated to 1000 to 1200 ° C,
The heated steel material is subjected to hot rolling with a rolling end temperature equal to or higher than the Ar3 transformation point to form a hot rolled steel sheet having a thickness of 40 mm or more,
The hot-rolled steel sheet is accelerated and cooled at an average cooling rate of 3 to 15 ° C./s,
The method of manufacturing a low yield ratio steel sheet for building, wherein the hot-rolled steel sheet after the accelerated cooling is air-cooled or tempered,
The cooling stop temperature in the accelerated cooling is 450 to 200 ° C. when the tempering is not performed, and 450 to 50 ° C. when the tempering is performed.
The architectural low yield ratio steel sheet has a microstructure in which the area fraction of ferrite is 2 to 30%, the area fraction of bainite is 65% or more, and the surface hardness is 350HV10 or less. A method of manufacturing a steel sheet. A steel material having the component composition according to claim 1 or 2 is heated to 1000 to 1200 ° C,
The heated steel material is subjected to hot rolling with a rolling end temperature equal to or higher than the Ar3 transformation point to form a hot rolled steel sheet having a thickness of 40 mm or less,
The hot-rolled steel sheet is accelerated and cooled at an average cooling rate of 15 ° C./s or more,
The method of manufacturing a low yield ratio steel sheet for building, wherein the hot-rolled steel sheet after the accelerated cooling is air-cooled or tempered,
The cooling stop temperature in the accelerated cooling is 650 to 450 ° C.,
The architectural low yield ratio steel sheet has a microstructure in which the area fraction of ferrite is 2 to 30%, the area fraction of bainite is 65% or more, and the surface hardness is 350HV10 or less. A method of manufacturing a steel sheet.