JP2009114529A - METHOD FOR MANUFACTURING 550 MPa CLASS LOW YIELD RATIO STEEL SHEET HAVING SMALL DISPERSION OF QUALITY IN STEEL SHEET - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently producing a steel sheet which secures the high strength having ≥550 MPa, reduces the unevenness in the material quality as much as possible and has excellent HAZ (heat-affected zone) toughness when the welding is performed with the high temperature heat. <P>SOLUTION: The steel material wherein the chemical compositions are suitably adjusted is turned into a steel slab with a continuous casting, and the steel slab is heated so that the temperature deviation in the slab is within 80°C and the maximum temperature in the slab becomes 1,000-1,250°C, and after rolling so that the maximum value in the steel sheet at the rolling finish temperature becomes 800-920°C, the rolled steel sheet is air-cooled for ≥30sec, and controlled so that cooling start temperature deviation in the steel sheet zone except the width edge part of 100 mm and the front and rear part of 300 mm, becomes within 50°C and successively, the steel sheet is cooled to ≤300°C at 1-100°C/sec cooling speed, and operation is performed so that a prescribed relation is satisfied in the steel sheet zone. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a steel sheet having a tensile strength of 550 MPa or more and a yield ratio of 80% or less, such as used for building structures and bridges, and in particular, manufacturing a steel sheet with as little material variation as possible in the steel sheet. It is about the method to do.

  In general, a steel sheet applied to a building structure, a bridge, or the like (hereinafter referred to as “building structure”) is welded to be a welded structure. In such a steel sheet, from the viewpoint of improving the seismic safety of the building structure, it is required that the yield ratio [(yield strength / tensile strength) × 100 (%)] is small (that is, the plastic deformability is high). (For example, 80% or less in the case of architectural use).

  Moreover, in the steel plate used for the above application, if the tensile properties (particularly the yield strength) in the plane direction of the steel plate vary, the entire building structure does not deform uniformly during an earthquake and is locally large. There is a possibility of causing deformation and collapse. For this reason, in order to ensure improved seismic safety of building structures, not only the yield ratio is low, but also the variation in tensile properties in the plane direction of the steel plate (hereinafter referred to as “material variation in the steel plate”). It is also an important requirement to reduce this.

  In the 490 MPa class and the 520 MPa class, the low-yield ratio high-tensile steel sheet is generally manufactured by air cooling after rolling, accelerated cooling after rolling, or the like. For steel sheets with a low yield ratio of 590 MPa, after direct rolling or online quenching after rolling, high strength is achieved by heating to the two-phase zone and quenching / tempering, or by adding B. Manufacturing is possible.

  On the other hand, in a conventional low yield ratio 550 MPa grade steel sheet, strength and a low yield ratio are ensured by adopting a mixed structure of ferrite and bainite. However, in reality, there is a problem in improving the seismic safety of the structure, such as a case where the standard value is not satisfied in a part of the region due to the material variation in the steel plate. Such a problem is an obstacle to securing a stable material at a low cost for a 550 MPa class steel plate as a mass-produced industrial product.

  In order to solve such a problem, various techniques have been proposed so far. For example, Patent Document 1 discloses a method for uniformly heating a slab in a heating furnace. However, simply heating the slab uniformly results in a temperature deviation during rolling, resulting in a situation where the steel sheet temperature deviation before starting cooling increases. As a result, the problem that the material in the steel sheet varies is not solved.

  Patent Document 2 discloses an accelerated cooling method that uniformly cools the temperature of the steel sheet, and can prevent warpage of the steel sheet during the cooling process. However, with such a technique, even if the steel sheet is uniformly cooled, the material variation is greatly influenced by the cooling start temperature deviation, and thus it is difficult to completely reduce the material variation.

  Patent Document 3 discloses a structure in which the carbon content in the steel sheet is less than 0.04%, the ferrite fraction is 90% or more, and is not affected by the cooling rate. It discloses that the hardness difference in the thickness direction of an object (for example, thickness: 80 mm) is reduced. However, in such a method, since it becomes a structure mainly composed of ferrite, it becomes difficult to secure a strength of 550 MPa or more. There is also a problem that a low yield ratio cannot be secured.

  In Patent Document 4, in order to produce a high-strength steel sheet having a low yield ratio, after performing quenching from an austenite region (direct quenching or normal quenching), quenching from a two-phase region and further tempering are performed. It is proposed to perform the heat treatment. In such a method, the operation becomes complicated and a lot of expensive alloy elements such as Ni and Cr are used, so that the manufacturing cost becomes high. In addition, there are problems that the delivery time is extended and the equipment load is increased due to many heat treatment processes.

Low yield ratio for the preparation of high-tensile steel sheet, after hot rolling from Ar ₃ transformation point or more of the temperature, by performing rapid cooling, bainite fraction by 50% or more, the reduction of material variations in the sheet thickness direction It has also been proposed to make it possible (Patent Document 5). However, this technique is the same rolling and cooling method as that of the 490 MPa class steel sheet, and is not strictly controlled in the manufacturing process in consideration of the reduction of the variation occurring in the case of higher strength, and the variation is particularly likely to occur. This is not an effective technique for reducing variation in steel sheets. Moreover, in this technique, B is added in order to improve the hardenability, and the addition of B greatly enhances the hardenability with a very small amount, so that the dispersion of the material in the steel sheet is promoted.

In Patent Document 6, air cooling is performed for a long time after rolling, the air cooling start temperature at this time is set to (Ar ₃ transformation point−20 ° C.) or less, and ferrite transformation is performed during air cooling, thereby increasing the ferrite fraction and reducing the low yield. It has been proposed to ensure the ratio. However, such a technique has a problem that the air cooling time becomes long and the productivity is extremely deteriorated.

  In addition to the various techniques described above, it has also been proposed that a strength of 550 MPa or more and a yield ratio of 80% or less can be obtained by defining the carbon equivalent Ceq, rolling end temperature, cooling rate, cooling stop temperature, and the like. (Patent Documents 7 to 9). However, such a method still has a problem in that there are still many variations in the quality of the steel sheet as a whole, and stable industrial production is not possible.

  In the shipping test of industrial products, it is common to evaluate by a tensile test at only one local point, but 550 MPa grade steel plate tends to vary, and thus material variations occur in the steel plate. In addition, there is a problem that it is difficult to determine shipping in a shipping test of industrial products.

In the welded structure as described above, for example, high heat input welding with a heat input of welding of 40 kJ / mm or more is directed from the viewpoint of inexpensive steel and improving welding efficiency. When high heat input welding is performed, the steel material is heated to a high temperature austenite region and then gradually cooled, so that the structure of the heat-affected zone (hereinafter, sometimes simply referred to as “HAZ”) is coarsened. There is a problem that the toughness of the steel tends to deteriorate. Therefore, in such a steel sheet, it is important to ensure such good toughness in HAZ (hereinafter sometimes referred to as “HAZ toughness”).
JP 2004-27332 A JP-A-8-215734 JP-A-8-85842 JP-A-8-92639 JP 2005-33651 A JP-A 63-286517 JP 2005-220379 A JP 2003-183767 A JP 2005-272951 A

  The techniques proposed so far have a problem that a large amount of alloy elements are required in order to achieve both a 550 MPa class and a low yield ratio, resulting in an increase in manufacturing cost. Further, although a ferrite-bainite structure is used in order to ensure high strength as a 550 MPa class, there is a problem that material variation occurs because the uniformity of the material in the steel sheet is not taken into consideration.

  The present invention has been made under such circumstances, and its purpose is to ensure a high strength of 550 MPa or more, to reduce variations in the material as much as possible, and to be excellent in HAZ toughness when welding with high heat input. It is providing the method which can manufacture a steel plate efficiently.

  The method of the present invention that can achieve the above object is C: 0.06 to 0.20% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 0.50%, Mn: 1.0 to 2.0%, Al: 0.01-0.05%, Nb: 0.005-0.025%, Ti: 0.005-0.020%, Ca: 0.0005-0.0030%, N : 0.0030-0.0080% and O: 0.0005-0.0030%, respectively, the balance consists of iron and inevitable impurities, and the carbon equivalent Ceq represented by the following formula (1) is 0.00. Steel which is 42 mass% or less and satisfies the relationship of the following formula (2) is made into a steel slab by continuous casting, and the temperature deviation in the slab is within 80 ° C and the maximum temperature in the slab is 1000 to 1250 ° C. The maximum value in the steel sheet of the rolling finish temperature is 800-9 After rolling to 0 ° C., air cooling is performed for 30 seconds or more, and the cooling start temperature deviation in the steel plate region excluding the width end portion 100 mm and the front and rear end 300 mm is controlled to be within 50 ° C., and the cooling rate is continued. : It has a gist in that it is cooled to 300 ° C. or less at 1 to 100 ° C./second, and is operated so as to satisfy the relationship of the following formulas (3) to (6) in the steel plate region. .

Ceq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
However, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents (mass of C, Si, Mn, Ni, Cr, Mo and V, respectively). %).
Ceq + 2 [Nb] ≧ 0.36 mass% (2)
However, [Nb] indicates the content (% by mass) of Nb.
SCT (MIN) ≧ Ar ₃ + 380−1000 (Ceq + 2 [Nb]) (3)
However, SCT (MIN): The minimum value (° C.) of the steel sheet surface temperature at the start of cooling, Ar ₃ : Ar ₃ transformation point of the steel sheet, and [Nb]: Nb content (mass%), respectively.
FRT (MAX) -10> SCT (MAX) (4)
Here, FRT (MAX): the maximum value in the steel sheet at the rolling finish temperature, SCT (MAX): the maximum value (° C.) of the steel sheet surface temperature at the start of cooling, respectively.
SCT (MIN) ≧ FRT (MAX) − (150−0.8t) (5)
However, SCT (MIN): the minimum value (° C.) of the steel sheet surface temperature at the start of cooling, FRT (MAX): the maximum value in the steel sheet at the rolling finishing temperature, and t: the thickness (mm) of the steel sheet, respectively.
SCT (MIN)> Ar ₃ -20 ° C. (6)
However, SCT (MIN): The minimum value (° C.) of the steel sheet surface temperature at the start of cooling, Ar ₃ : Ar ₃ transformation point of the steel sheet, respectively.

  In the said manufacturing method, after cooling to 300 degrees C or less at a cooling rate: 1-100 degrees C / sec, it is also useful to air-cool after reheating to 450-650 degreeC.

  Further, the steel slab used in the present invention is further Cu: 0.1-0.5%, Ni: 0.1-0.5%, Cr: 0.1-0.5%, Mo: 0.1-0. It is also preferable to contain at least one selected from the group consisting of 0.5% and V: 0.005 to 0.05%.

  According to the production method of the present invention, by appropriately adjusting the chemical composition of the steel slab and strictly defining the production conditions including the rolling conditions, a high strength of 550 MPa class and a low yield ratio are compatible. At the same time, it was possible to efficiently produce a steel sheet that reduced variations in the material as much as possible and also had excellent HAZ toughness when welded with high heat input.

  In the prior art, in order to achieve both the 550 MPa class and the low yield ratio, a large amount of alloy elements are required, resulting in an increase in manufacturing cost. Further, although it has been proposed to use a ferrite-bainite structure, a 550 MPa grade steel sheet has been proposed.

  In order to suppress variations in the material in the steel sheet, the present inventors optimize the steel sheet component, heating temperature during rolling, temperature deviation in the slab, rolling end temperature, air cooling time, temperature deviation in the steel sheet, cooling start temperature, cooling rate Then, an experiment was conducted in which the cooling stop temperature and the like were appropriately controlled. As a result, the material variation is affected by components such as an increase in the yield ratio accompanying the increase in the yield point and the tensile strength, and the effect of uniforming the crystal grains by alloy elements such as Nb, and the steel plate before the start of cooling. It became clear that the influence of temperature deviation etc. is large.

Based on the above findings, the present inventors further investigated the optimization of the component composition, the appropriate cooling start temperature, and the like. As a result, in particular, the cooling start was determined by rolling finishing temperature FRT, Ar ₃ transformation point, carbon equivalent Ceq + 2 [Nb It has been clarified that the variation in the material in the steel sheet can be effectively reduced by defining the parameters such as the above in combination with the manufacturing conditions.

  As a product of the steel plate, it is necessary to satisfy the standard value at any position in the steel plate, but since the material test of the entire area in the steel plate is impossible, by managing the process conditions as in the present invention, It is necessary to guarantee the characteristics of the entire area in the steel sheet by a tensile test with a representative value at one point after suppressing the material variation of the steel sheet.

  In the present invention, the chemical component composition satisfying the above formulas (1) and (2) is used to define the components necessary for obtaining a low yield ratio 550 MPa grade steel sheet, and the formulas (3) to ( 6) The cooling start temperature is controlled to minimize material variations using the formula, and the temperature difference in the steel sheet is reduced (50 ° C or less) by air cooling for 30 seconds or more after rolling, and the residual strain is made uniform. Thus, the bainite-based structure is obtained. The “bainite-based structure” means, for example, a structure having a bainite fraction of 90 area% or more.

  Details of the reasons for defining the formulas (1) to (6) will be described later. The outline of these formulas is as follows. First, the equation (1) defines the carbon equivalent Ceq in order to realize weldability and a low yield ratio. In the formula (2), the material variation is suppressed by using Nb as an essential component. Moreover, Nb can implement | achieve an intensity | strength increase, suppressing deterioration of weld crack resistance, As a result, the intensity | strength of 550 Mpa or more can be implement | achieved.

On the other hand, in the formula (3), in the case of a low alloy component, even if the cooling starts from exceeding (Ar ₃ -20 ° C.), material variation occurs due to the generation of ferrite. The starting temperature (minimum value of the steel sheet surface at the start of cooling) is limited as much as possible. Equation (4) has a role of making the temperature of the steel sheet surface uniform immediately after the end of rolling, and suppresses the generation of ferrite due to uneven cooling. Equation (5) suppresses the non-uniform formation of ferrite due to an extreme temperature drop by considering the difference in cooling rate depending on the plate thickness. The expression (6) suppresses the generation of ferrite due to the steel sheet temperature at the start of cooling being lower than (Ar ₃ -20 ° C.). Since this ferrite is generated non-uniformly in the steel sheet, material variations occur, so that the generation of such ferrite is suppressed as much as possible.

  As described above, the chemical composition of the steel sheet is specified, and the temperature deviation of the steel sheet before the start of cooling is strictly controlled, and the temperature deviation in the entire area of the steel sheet excluding the width end portion 100 mm and the front and rear ends 300 mm is reduced as much as possible. By starting the cooling, it is possible to produce a 550 MPa class low yield ratio steel sheet in which non-uniform formation of ferrite is suppressed and material variation is suppressed.

  Next, requirements defined in the present invention will be described sequentially. In the present invention, it is necessary to appropriately define the chemical component composition of the steel material (steel slab). The reasons for limiting the ranges of these components are as follows.

[C: 0.06-0.20%]
C is an element having an effect of increasing the strength of the steel sheet, and it is necessary to contain 0.06% or more in order to ensure the necessary strength. However, if the C content is excessive, the weldability of the steel sheet and the toughness of the heat affected zone (HAZ) are remarkably reduced, so it is necessary to make it 0.20% or less. In addition, the minimum with preferable C content is 0.10%, and a preferable upper limit is 0.16%.

[Si: 0.05 to 0.50%]
Si is an element effective for securing the strength of the steel sheet, deoxidation, and the like, and in order to exert its effects, the Si content needs to be 0.05% or more. However, if the Si content is excessive, the HAZ toughness is lowered, so it is necessary to set it to 0.50% or less. In addition, the minimum with preferable Si content is 0.2%, and a preferable upper limit is 0.4%.

[Mn: 1.0 to 2.0%]
Mn is an element effective for improving the hardenability and ensuring the strength of the steel sheet. In order to exhibit such an effect, it is necessary to contain 1.0% or more. However, if the Mn content becomes excessive, the HAZ toughness deteriorates on the contrary, so it is necessary to make it 2.0% or less. In addition, the minimum with preferable Mn content is 1.2%, and a preferable upper limit is 1.6%.

[Al: 0.01 to 0.05%]
Al is an element effective for deoxidation of steel and refinement of crystal grains, and for that purpose, it is necessary to contain 0.01% or more. However, if the Al content is excessive, Al oxide inclusions are generated and the toughness of the base material deteriorates, so it is necessary to make it 0.05% or less. In addition, the minimum with preferable Al content is 0.02%, and a preferable upper limit is 0.04%.

[Nb: 0.005 to 0.025%]
Nb is an element that suppresses recrystallization of austenite in a solid solution state, enhances the effect of controlled rolling, and further promotes transformation strengthening by accelerated cooling after rolling to contribute to improvement of strength and toughness. In order to exert these effects, the Nb content needs to be 0.005% or more. However, if the Nb content becomes excessive, the toughness of the welded portion deteriorates, so the upper limit was made 0.025%. In addition, the minimum with preferable Nb content is 0.010%, and a preferable upper limit is 0.020%.

[Ti: 0.005 to 0.020%]
Ti is an element that forms a hardly soluble nitride, suppresses the coarsening of austenite grains during welding, and becomes a ferrite transformation nucleus from within the austenite grains, thereby improving the HAZ toughness. In order to exert such effects, the Ti content needs to be 0.005% or more. However, when the Ti content is excessive, a large amount of coarse inclusions are generated and the toughness is decreased, so that the content needs to be 0.020% or less. In addition, the minimum with preferable Ti content is 0.008%, and a preferable upper limit is 0.017%.

[Ca: 0.0005 to 0.0030%]
Ca is an element effective in controlling the form of sulfide and improving the thickness direction characteristics of the steel sheet. In order to exert such effects, the Ca content needs to be 0.0005% or more. However, if the Ca content is excessive, inclusions such as CaS and CaO are produced in large quantities, resulting in large inclusions, which not only affect the toughness of the steel sheet but also the cleanliness, and also adversely affect the weldability. Therefore, the Ca content needs to be 0.0030% or less. In addition, the minimum with preferable Ca content is 0.0010%, and a preferable upper limit is 0.0025%.

[N: 0.0030 to 0.0080%]
N is an element effective for improving the HAZ toughness as Ti nitride particles (TiN). In order to exert such an effect, the N content needs to be 0.0030% or more. However, if the N content is excessive, the HAZ toughness deteriorates on the contrary, so it is necessary to make it 0.0080% or less. In addition, the minimum with preferable N content is 0.0040%, and a preferable upper limit is 0.0070%.

[O: 0.0005 to 0.0030%]
O is contained as an unavoidable impurity and exists as an oxide in the steel, which lowers the cleanliness of the steel sheet. Therefore, the smaller the amount, the better. When the O content is excessive, oxides such as Al, Ti, and Ca are coarsened, which adversely affects toughness. Therefore, the upper limit of the O content is set to 0.0030% (preferably 0.0025% or less). However, since O is an impurity inevitably mixed in the steel manufacturing process, it is difficult to make the amount 0% industrially, and the lower limit is set to 0.0005%.

  The basic component composition of the steel slab used in the present invention is as described above, and the balance is substantially iron. However, it is naturally allowed that unavoidable impurities (for example, P and S) brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like are included in the steel sheet. Moreover, the slab used in the present invention may contain the following selective elements as necessary. These elements are all effective elements for increasing the strength of the steel sheet, but the detailed effects are as follows.

[Cu: 0.1-0.5%, Ni: 0.1-0.5%, Cr: 0.1-0.5%, Mo: 0.1-0.5% and V: 0.005 ~ At least one selected from the group consisting of 0.05%]
Cu is an element effective for increasing the strength without incurring toughness deterioration, and is contained as required. In order to exert such effects, the Cu content is preferably set to 0.1% or more. However, if the Cu content is excessive, the cost increases and the weldability deteriorates.

  Ni is an element effective for increasing the strength without causing toughness deterioration, and is contained as necessary. In order to exert such effects, the Ni content is preferably set to 0.1% or more. However, the cost of excessive Ni content increases, so 0.5% or less is preferable.

  Cr is an element effective for increasing the strength without significantly increasing the alloy cost, and is contained as necessary. In order to exert such effects, the Cr content is preferably 0.1% or more. However, if the Cr content is excessive, the weldability is deteriorated, so 0.5% or less is preferable.

  Mo is an element effective for increasing the hardenability and increasing the strength, and is contained as necessary. In order to exert such effects, the Mo content is preferably set to 0.1% or more. However, if the Mo content is excessive, the weldability is deteriorated, so 0.5% or less is preferable.

  V is an element effective for increasing the strength by precipitation strengthening, and is contained as necessary. In order to exhibit such an effect, the V content is preferably 0.005% or more. However, if the V content is excessive, weldability deteriorates, so 0.05% or less is preferable.

  The steel slab used in the present invention needs to adjust the chemical composition as described above, but the carbon equivalent Ceq represented by the following formula (1) is 0.42% by mass or less and the following formula (2) It is also necessary to satisfy this relationship. The reasons for defining these equations are as follows.

Ceq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
However, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents (mass of C, Si, Mn, Ni, Cr, Mo and V, respectively). %) (When no alloying element is added, calculation is made assuming that the term is not present).

  When the carbon equivalent Ceq represented by the above formula (1) exceeds 0.42% by mass, the weldability is adversely affected. In addition, the yield ratio increases with increasing yield point and tensile strength. For these reasons, the carbon equivalent Ceq represented by the above formula (1) was set to 0.42 mass% or less.

Ceq + 2 [Nb] ≧ 0.36 mass% (2)
However, [Nb] indicates the content (% by mass) of Nb.

  The above formula (2) defines the requirements necessary to obtain a 550 MPa grade steel sheet. Normally, the strength level is determined by the carbon equivalent Ceq, but in the present invention, it contains Nb which is a strengthening element, and 2 [Nb] obtained by carbonizing Nb to carbon equivalent is added to the carbon equivalent Ceq. The strength of 550 MPa or more is secured by setting the value to 0.36% by mass or more. Here, the reason for including Nb is that Nb contributes to the homogenization of the material by not generating extremely large crystal grains partially during rolling in order to expand the non-recrystallization temperature range in austenite. is there. Nb has the advantage that the strength can be increased without degrading the weld crack resistance.

  In addition to appropriately adjusting the chemical composition of the steel slab, the present invention strictly controls the production conditions (particularly hot rolling and subsequent cooling conditions), thereby reducing material variations in the steel sheet, It is characterized in that a steel sheet exhibiting both high strength and low yield ratio characteristics can be produced. Below, the manufacturing conditions of the manufacturing method of this invention are demonstrated in detail.

[Temperature deviation in steel slab is within 80 ℃]
The temperature deviation in the steel slab means a temperature difference between the slab end portion, which is the maximum temperature in the slab at the time of slab extraction, and the center in the width / length direction or the skid portion, which is the lowest temperature. If the temperature deviation in the slab is large at the time of slab heating, not only will the Nb solid solution amount and the cooling start temperature be affected, but also the tensile strength at each position of the steel sheet after rolling will vary. Moreover, since the temperature deviation in a slab is directly linked to the temperature variation in the steel plate before cooling, the temperature deviation in the slab was set to 80 ° C. or less. In addition, what is necessary is just to make in-furnace time long in order to make the temperature deviation in a steel slab into 80 degrees C or less.

[Heating so that the maximum temperature in the slab is 1000 to 1250 ° C]
When the maximum temperature in the slab (the temperature at the end of the slab is substantially equal to the temperature of the heating furnace) exceeds 1250 ° C. during the heating of the steel slab, the toughness deteriorates due to the coarsening of the austenite grains. On the other hand, when the maximum temperature in the slab at the time of heating is less than 1000 ° C., the amount of Nb solid solution decreases, leading to insufficient strength. For these reasons, the maximum temperature in the slab was set to 1000 to 1250 ° C.

[Maximum rolling finish temperature in steel sheet is 800-920 ° C]
When the rolling finish temperature (FRT) exceeds 920 ° C., the strength level increases, and the yield ratio increases accordingly, so that it is difficult to achieve a low yield ratio. For these reasons, the maximum temperature in the steel sheet for the rolling finish temperature (FRT) was set to 920 ° C. or lower. On the other hand, when the rolling finish temperature (FRT) is less than 800 ° C., not only is it difficult to obtain 550 MPa grade steel, but material variations are likely to occur due to the low cooling start temperature. For these reasons, the rolling finishing temperature is set to 800 ° C. or higher at the maximum temperature in the steel sheet.

[Air cooling for 30 seconds or more after rolling]
The steel sheet after rolling has a temperature deviation at the time of slab extraction, and is in a temporary temperature unstable state due to descaling during rolling. For this reason, if cooling is started immediately after rolling, variations in material tend to occur. For these reasons, air-cooling for 30 seconds or more after rolling has the effect of suppressing material variations. The air cooling time is not particularly limited as long as it is 30 seconds or longer, but it is preferably 5 minutes or shorter from the viewpoint of productivity.

[Cooling start temperature deviation in the steel plate region excluding the width end portion 100 mm and the leading and trailing end 300 mm of the steel plate is within 50 ° C.]
The cooling start temperature deviation in the steel plate region means {[maximum value SCT (MAX) of steel plate surface temperature just before the start of cooling] − [minimum value SCT (MIN) of steel plate surface temperature just before the start of cooling]} ( However, the width end portion of the steel plate and the front and rear end of 300 mm are excluded). Since the structure of the 550 MPa grade steel sheet is a ferrite bainite transition region, if there is temperature variation in the steel sheet before the start of cooling, the tensile strength in the steel sheet becomes unstable. Therefore, the temperature deviation in the steel plate before the start of cooling is set to 50 ° C. or less.

[Cooling rate: 1 to 100 ° C / sec.
After controlling the temperature difference in the steel sheet as described above, it is necessary to cool to 300 ° C. or less at a cooling rate of 1 to 100 ° C./second. If the cooling rate at this time is higher than 100 ° C./sec, a martensite-based structure is obtained. In such a structure, the yield ratio increases and the toughness deteriorates. On the other hand, when the cooling rate is less than 1 ° C./second, the strength is insufficient.

[Cooling rate: after cooling to 300 ° C. or less at 1 to 100 ° C./second, if necessary, re-cooling to 450 to 650 ° C. and air cooling]
After cooling to 300 ° C. or lower as described above, air cooling may be performed after reheating to 450 to 650 ° C. if necessary. By adding such conditions, toughness can be improved. If the reheating temperature at this time exceeds 650 ° C., the required strength is lowered. On the other hand, when the reheating temperature is less than 450 ° C., the effect (toughness improving effect) obtained by the reheating is small.

  In the method of the present invention, in addition to the basic production conditions as described above, the following formulas (3) to (6) are used in the steel plate region (that is, the region excluding the width end portion 100 mm and the front and rear end 300 mm of the steel plate). It is necessary to operate to satisfy the relationship. The reasons for specifying these requirements are as follows.

SCT (MIN) ≧ Ar ₃ + 380−1000 (Ceq + 2 [Nb]) (3)
However, SCT (MIN): The minimum value (° C.) of the steel sheet surface temperature at the start of cooling, Ar ₃ : Ar ₃ transformation point of the steel sheet, and [Nb]: Nb content (% by mass), respectively.

  The material variation occurring in the conventional 550 MPa grade steel sheet is due to non-uniform generation of ferrite. The above equation (3) is defined in order to suppress the non-uniform formation of ferrite, which is the cause of insufficient strength and material variation within the specified component.

FIG. 1 shows the range of suppression of non-uniform ferrite that causes insufficient strength and material variations. FIG. 1 is a summary of data of Examples described later (numerical values in the figure indicate test Nos. In Tables 2 and 3 described later). The horizontal axis in FIG. 1 is (Ceq + 2 [Nb]), the vertical axis is SCT (MIN), and the area indicated by hatching indicates the range where fine ferrite generation is suppressed. That is, in a portion other than hatching, it becomes a region where minute ferrite is generated. When Ceq + 2 [Nb] is less than 0.40, even if the cooling start temperature satisfies the relationship of the formula (6) [that is, SCT (MIN)> Ar ₃ −20 ° C.] Will be generated. Therefore, the ferrite generated due to the low component is suppressed by increasing SCT (MIN) by the equation (3). The above equation (3) is defined to increase SCT (MIN) in order to prevent generation of minute ferrite when Ceq + 2 [Nb] is 0.40 or less.

FRT (MAX) -10> SCT (MAX) (4)
Here, FRT (MAX): the maximum value in the steel sheet at the rolling finish temperature, SCT (MAX): the maximum value (° C.) of the steel sheet surface temperature at the start of cooling, respectively.

  The above expression (4) defines the upper limit of the cooling start temperature necessary for temperature equalization by air cooling before the start of cooling. FIG. 2 shows the range. FIG. 2 is a summary of data of Examples described later (numerical values in the figure indicate test Nos. In Tables 2 and 3 described later). Immediately after the end of rolling, a temperature difference is generated on the surface of the steel plate due to descaling, and there is a possibility that the temperature rises due to recuperation. If cooling is started with this temperature deviation left after rolling, the material becomes unstable. If the relationship of the above formula (4) is not satisfied, the temperature in the steel sheet is sufficiently uniformed by reheating. Therefore, temperature variation occurs in the steel sheet. In order to avoid such a situation, in the present invention, in addition to satisfying the relationship of the above expression (4), the air cooling time before the start of cooling is set to 30 seconds or more.

SCT (MIN) ≧ FRT (MAX) − (150−0.8t) (5)
However, SCT (MIN): the minimum value (° C.) of the steel sheet surface temperature at the start of cooling, FRT (MAX): the maximum value in the steel sheet at the rolling finishing temperature, and t: the thickness (mm) of the steel sheet, respectively.

The lower limit of the cooling start temperature needs to be regulated by the relationship between the rolling finishing temperature (FRT) and the sheet thickness t, in addition to the relationship with the Ar ₃ transformation point and the relationship with the carbon equivalent Ceq. In FIG. 2, such a range is shown at the same time based on the data of the examples described later. This is due to the difference in temperature drop between the thick and thin objects when simultaneously cooled. When the plate thickness is thin, the temperature drop due to air cooling is large, so the FRT needs to be increased accordingly. By setting FRT (MAX)-(150-0.8t) as the lower limit value of the cooling start temperature, the required strength is ensured from a thin object to a thick object, and in combination with the above equation (4), cooling in the steel sheet It was possible to eliminate the starting temperature deviation and to make a steel plate with less material variation.

SCT (MIN)> Ar ₃ -20 ° C. (6)
However, SCT (MIN): The minimum value (° C.) of the steel sheet surface temperature at the start of cooling, Ar ₃ : Ar ₃ transformation point of the steel sheet, respectively.

When the steel plate temperature at the start of cooling is lower than Ar ₃ -20 ° C, ferrite is generated after cooling. This ferrite is generated non-uniformly in the steel sheet, resulting in material variations. It is necessary to suppress the generation of such ferrite as much as possible. For these reasons, when the minimum temperature in the steel sheet exceeds (Ar ₃ -20 ° C.), the generation of ferrite is suppressed in the entire steel sheet, and material variations in the entire steel sheet are reduced. However, as described above, when Ceq + 2 [Nb] is less than 0.40 (mass%), a small amount of ferrite is generated and causes variation. Therefore, priority is given to the range given by the expression (3). To do.

  In the present invention, the highest value among the SCTs (MIN) given by the above equations (3), (5) and (6) is the SCT (MIN) effective in suppressing material variations.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  The slab having the chemical composition shown in Table 1 below is subjected to the conditions shown in Table 2 {the thickness of the steel sheet after hot rolling [t (mm)]; heating temperature before hot rolling (maximum temperature in the slab); Temperature deviation; maximum value FRT (MAX) of rolling finish temperature; air cooling time; maximum value SCT (MAX) of steel sheet surface temperature at the start of cooling; minimum value SCT (MIN) of steel sheet surface temperature at the start of cooling; cooling start temperature Deviation [SCT (MAX) -SCT (MIN)]; cooling stop temperature FCT; cooling rate; tempering temperature}, hot rolling and cooling (tempering in some cases) to perform steel sheet (sheet width W: 2500 to 3500 mm, Length L: 7000-20000 mm) was manufactured.

Table 1 also shows the value of the formula (1) (carbon equivalent Ceq), the value of the formula (2) (Ceq + 2 [Nb]), and the Ar ₃ transformation point represented by the following formula (7). Table 2 also shows the value [Ar ₃ + 380−1000 (Ceq + 2 [Nb])] on the right side of the expression (3), the value [FRT (MAX) −10] on the left side of the expression (4), and the expression (5). The value on the right side [FRT (MAX) − (150−0.8t)], the value on the right side of formula (6) (Ar ₃ −20 ° C.), and the Ar ₃ transformation point are also shown. The temperature deviation in the slab is a temperature difference between the slab end portion that is the highest temperature in the slab at the time of slab extraction and the width / length center or the skit portion that becomes the lowest temperature. Each temperature shown in Table 2 is measured by a radiation thermometer.

Ar ₃ transformation point (° C.) = 910-310. [C] -80. [Mn] -20. [Cu] -15. [Cr] -55. [Ni] -80. [Mo] (7)
However, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] indicate the contents (mass%) of C, Mn, Cu, Cr, Ni and Mo, respectively. When no element is added, calculation is made assuming that the term is not present.

  About the manufactured steel plate, the tensile test characteristic (yield strength YS, tensile strength TS, yield ratio YR), base material toughness, HAZ toughness, and weldability were evaluated as follows. The results are collectively shown in Table 3 below.

[Tensile test properties]
At each position in the width direction (1/4) W of the steel plate of length L, a test piece [JIS Z2201 No. 1A test piece (length direction perpendicular to the rolling direction) from a position 500 mm from the front end of the steel plate rolling direction. )] Was collected, a tensile test was performed, and the yield strength YS, the tensile strength TS, and the yield ratio YR were measured. Further, in order to measure the material variation, at each position in the width direction (1/2) W portion and (1/4) W in the steel plate having the steel plate length L, 500 mm, 2000 mm, (1 / 2) L, 500 mm from the rear end of the steel plate, specimens are taken from 2000 mm positions (10 places in total), a tensile test is performed, and the maximum and minimum values of yield strength YS, tensile strength TS, and yield ratio YR are measured. The differences (ΔYS, ΔTS, ΔYR) were measured. Tensile strength TS: 550 to 700 MPa, yield strength YS: 385 MPa or more, and yield ratio YR: 80% or less were evaluated as acceptable. In addition, the evaluation criteria for the material variation were ΔYS: 60 MPa or less, ΔTS: 30 MPa or less, and ΔYR: 4% or less.

[Base material toughness evaluation method]
A V-notch test piece was taken from the position of the steel plate in the width direction (1/4) W and 500 mm from the tip, and subjected to a Charpy impact test in accordance with JIS Z 2242. The fracture surface transition temperature (vTrs) was less than −20 ° C. Was evaluated as passing.

[HAZ toughness]
A test piece was sampled from the position in the width direction (1/4) W of the steel sheet and 500 mm from the tip, and a reproducible HAZ thermal cycle V-notch Charpy test was performed. The reproduced HAZ thermal cycle conditions were: holding time at 1400 ° C .: 30 seconds, cooling time from 800 to 500 ° C .: 730 seconds, and simulating the heat history of the bond part in high heat input welding with a heat input of 80 kJ / mm. . As for vE ₀ (absorption energy at 0 ° C.) of HAZ toughness, 27 J or more was evaluated as acceptable.

[Weld crack resistance]
In accordance with the y-type weld cracking test method specified in JIS Z 3158, carbon dioxide gas welding was performed at a heat input of 1.7 kJ / mm, and “○” indicates that no root crack occurred at room temperature. Evaluated as “x”.

  From this result, it can be considered as follows. First, it is understood that 550 MPa class low yield ratio steel sheets with small variations in the material in the steel sheet can be realized by those manufactured under the requirements (chemical composition and manufacturing conditions) defined in the present invention (Test Nos. 1 to 10). On the other hand, it can be seen that one manufactured under manufacturing conditions deviating from the requirements (or preferred requirements) defined in the present invention (Test Nos. 11 to 37) does not satisfy any of the characteristics.

  Among these, test No. Although the thing of 11-23 satisfies the chemical component composition prescribed | regulated by this invention, manufacturing conditions remove | deviate from the range prescribed | regulated by this invention, and any characteristic has deteriorated. Specifically, Test No. In the case of No. 11, the strength is insufficient because the cooling rate is low. Test No. In No. 12, since the requirement prescribed | regulated by (5) Formula is not satisfied, the material variation in a steel plate is large. Test No. In the case of No. 13, since the rolling finish temperature (FRT) is high, the strength is high and the yield ratio is not low. Test No. In the case of No. 14, since the air cooling time after rolling is short, the temperature becomes unstable, the cooling start temperature deviation becomes large, and the material variation in the steel sheet becomes large. Test No. In the case of No. 15, since the air cooling time after rolling is extremely short, the relationship defined by the equation (4) is not satisfied, and the material variation in the steel sheet is large.

  Test No. In the case of No. 16, since the rolling finish temperature FRT is low, the strength is lowered, and the cooling start temperature is extremely low so as not to satisfy any of the relations of the expressions (3) and (6). The variation is large. Test No. In the case of No. 17, since the material varies greatly due to the large temperature deviation in the slab being taken over by the cooling start temperature deviation, the yield ratio largely fluctuates, and the low yield ratio characteristics are not stably obtained. Test No. In the case of No. 18, the cooling stop temperature is high, the bainite structure is not obtained, the strength level is low, and the yield ratio is not low. Test No. In No. 19, since the tempering temperature is high, the strength is low and the yield ratio is not low.

Test No. In No. 20, since the heating temperature is high, the base material toughness is poor, and the yield ratio is not low. Test No. In No. 21, the heating temperature is low, so the amount of Nb dissolved is insufficient, and the strength is low. Test No. In the case of No. 22, the cooling starts from a temperature exceeding (Ar ₃ −20 ° C.), but since the relationship of the formula (3) is not satisfied, the ferrite is generated non-uniformly, and the material variation in the steel sheet is Has occurred. Test No. In the case of No. 23, since the cooling is performed from (Ar ₃ -20 ° C.) or less, the relationship of the formula (6) is not satisfied, and the material variation in the steel plate occurs.

  On the other hand, test no. Nos. 24 to 37 are out of the chemical composition defined in the present invention, and any of the characteristics is deteriorated. Specifically, Test No. In the case of 24, since the C content exceeds the range specified in the present invention, the relationship of the formula (1) is not satisfied, the strength is higher than 700 MPa, and the yield ratio is not low. Moreover, since Ceq was high, HAZ toughness was bad, and the crack was observed by the y-type weld cracking test. Test No. In the case of No. 25, the requirement defined by the equation (2) is not satisfied, and the strength is insufficient. Test No. In the case of No. 26, since the Mn content exceeds the range specified in the present invention, it has a high strength exceeding 700 MPa due to the increase in hardenability. Further, the yield ratio was not low, and Ceq was high, so HAZ toughness was poor, and cracks were observed in the y-type weld crack test. Test No. In the case of No. 27, since the Mn content is less than the range specified in the present invention, the strength is insufficient. Moreover, since the relationship of Formula (3) is not satisfied, the material variation in the steel sheet is large.

  Test No. In the case of No. 28, since the Al content is less than the range specified in the present invention, the effect of refining the crystal grains is small, the vicinity of the specified strength and the HAZ toughness are also deteriorated. Test No. In No. 29, since the Al content exceeds the range specified in the present invention, the base material toughness and the HAZ toughness are deteriorated. Test No. In the case of 30, since the Nb content exceeds the range specified in the present invention, the strength is as high as exceeding 700 MPa and the yield ratio is not low. Since the Ceq is high, the HAZ toughness is poor, and cracks were observed in the y-type weld crack test. Test No. In the case of No. 31, since the Nb content is less than the range specified in the present invention, the relationship of the formula (2) is not satisfied, so that the strength is insufficient.

  Test No. Since the thing of 32 does not contain Ti, the deoxidation effect becomes small, the amount of oxygen increases, and the HAZ toughness is deteriorated. Test No. In No. 33, since the Ti content exceeds the range specified in the present invention, the base material toughness and the HAZ toughness are deteriorated. Also, the yield ratio is not low. Test No. Since the thing of 34 does not contain Ca, HAZ toughness is getting worse. Test No. In 35, since Ca content exceeds the range prescribed | regulated by this invention, HAZ toughness is getting worse. Test No. In the case of 36, since B is contained, the hardenability is increased, the strength is higher than 700 MPa, and the yield ratio is not low. Moreover, HAZ toughness is bad. Moreover, since the strength of B is greatly changed with a small amount, material variations are likely to occur. Test No. In No. 37, since the N content exceeds the range specified in the present invention, the HAZ toughness is deteriorated.

It is a graph which shows the suppression range of the non-uniform | heterogenous ferrite which causes insufficient strength and material dispersion. It is a graph which shows the range of the upper limit and the minimum of the cooling start temperature by (4) Formula and (5) Formula.

Claims (3)

C: 0.06 to 0.20% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 0.50%, Mn: 1.0 to 2.0%, Al: 0.01 to 0. 05%, Nb: 0.005-0.025%, Ti: 0.005-0.020%, Ca: 0.0005-0.0030%, N: 0.0030-0.0080% and O: 0 .0005 to 0.0030%, the balance being iron and inevitable impurities, and the carbon equivalent Ceq represented by the following formula (1) is 0.42% by mass or less, and the following formula (2) The steel satisfying the above relationship is made into a steel slab by continuous casting, and is heated so that the temperature deviation in the slab is within 80 ° C. and the maximum temperature in the slab is 1000 to 1250 ° C. 30 seconds after rolling to a value of 800-920 ° C Air-cooled and controlled so that the cooling start temperature deviation in the steel plate region excluding the width end portion 100 mm and the leading and trailing ends 300 mm of the steel plate is within 50 ° C., and subsequently the cooling rate: 1 to 100 ° C./sec. A method for producing a 550 MPa class low yield ratio steel sheet having a small variation in the material in the steel sheet, wherein the steel sheet is cooled and operated so as to satisfy the following expressions (3) to (6) in the steel sheet region: .
Ceq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
However, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents (mass of C, Si, Mn, Ni, Cr, Mo and V, respectively). %).
Ceq + 2 [Nb] ≧ 0.36 mass% (2)
However, [Nb] indicates the content (% by mass) of Nb.
SCT (MIN) ≧ Ar ₃ + 380−1000 (Ceq + 2 [Nb]) (3)
However, SCT (MIN): The minimum value (° C.) of the steel sheet surface temperature at the start of cooling, Ar ₃ : Ar ₃ transformation point of the steel sheet, and [Nb]: Nb content (% by mass), respectively.
FRT (MAX) -10> SCT (MAX) (4)
Here, FRT (MAX): the maximum value in the steel sheet at the rolling finish temperature, SCT (MAX): the maximum value (° C.) of the steel sheet surface temperature at the start of cooling, respectively.
SCT (MIN) ≧ FRT (MAX) − (150−0.8t) (5)
However, SCT (MIN): the minimum value (° C.) of the steel sheet surface temperature at the start of cooling, FRT (MAX): the maximum value in the steel sheet at the rolling finish temperature, and t: the thickness (mm) of the steel sheet, respectively.
SCT (MIN)> Ar ₃ -20 ° C. (6)
However, SCT (MIN): The minimum value (° C.) of the steel sheet surface temperature at the start of cooling, Ar ₃ : Ar ₃ transformation point of the steel sheet, respectively.   Cooling rate: The manufacturing method of Claim 1 which air-cools after cooling to 450-650 degreeC after cooling to 300 degrees C or less at 1-100 degreeC / sec. The steel slab further includes Cu: 0.1 to 0.5%, Ni: 0.1 to 0.5%, Cr: 0.1 to 0.5%, Mo: 0.1 to 0.5%, and V: The manufacturing method according to claim 1 or 2, which contains at least one selected from the group consisting of 0.005 to 0.05%.

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