KR101553748B1 - Thick steel plate with little change in toughness before and after strain againg - Google Patents

Abstract

Disclosure of the Invention An object of the present invention is to provide a steel sheet having little change in toughness before and after deformation aging. At the same time satisfying the composition specified chemical composition, a △ G / [Si]> 0.4 and [Ti] × [N] satisfy the ≥4.0 × 10 ^-5, and also, a ferrite fraction is 40 to 90% by area, a bainite fraction Wherein the total of the ferrite fraction and the bainite fraction is 90% or more by area, the hard phase contains 1% or more by area, the effective crystal grain diameter is 3 to 25 占 퐉, Is 0.035 mass% or less.

Description

{THICK STEEL PLATE WITH LITTLE CHANGE IN TOUGHNESS BEFORE AND AFTER STRAIN AGAIN}

TECHNICAL FIELD The present invention relates to a steel sheet which is suitably used for an offshore structure or the like, and particularly to a steel sheet having little change in toughness before and after deformation.

It is known that the yield strength (YS) of a steel material increases due to strain aging. On the other hand, it is also known that the toughness is lowered by increasing the yield strength (YS). That is, it can be said that the toughness is deteriorated by the strain aging. It is said that the phenomenon occurring during the deformation aging is attributed to the fact that the solid solution C / N is adhered to the potential introduced by the dislocation and deformation imparted in the rolled state texture. For these reasons, it is considered effective to control the potential state or reduce the amount of solid solution C / N in order to reduce the characteristic change due to strain aging.

A variety of proposals have been made for a technique relating to a steel material capable of suppressing a decrease in toughness even after the aging of strain and thus securing excellent toughness by paying attention to the change in toughness caused by the strain age of such a steel material .

The technique described in Patent Document 1 is a technique proposed to reduce the amount of solid solution C / N to obtain a steel sheet with little change in toughness before and after deformation aging. Considering the amount of solid solution C, (Less than 0.030 wt%), and then at least one of Ti and Nb is added to reduce the amount of solid solution C and to prevent the formation of a high YR within a range where the toughness is not lowered , It is possible to ensure toughness after deformation aging (after cold forming).

However, since the content of C in the steel sheet described in Patent Document 1 is less than 0.030 wt% and the coiling temperature is 550 DEG C or higher, the metal structure is preferably 100 wt% or more of bismuth and hard phase (martensite, MA) is hardly included, so that YS before deformation aging is not expected to be sufficiently high.

On the other hand, Patent Document 2 proposes a technique relating to a steel material in which a toughness change due to deformation aging is suppressed by paying attention to factors other than the solid solution C / N ratio. More specifically, it is possible to improve the tacticity of the potential introduced by the application of strain so that the TS level is 500-690 MPa and the change in toughness (vTrs: wave-face transition temperature) Thereby making it possible to manufacture a steel sheet having a thickness of 20-100 mm.

However, in the production of this steel sheet, it is necessary to finish rolling at 950 占 폚 or higher, or to reduce the rolling reduction by 40% or more at a temperature range of 950 占 폚 or lower and Ar3 point or higher. When the rolling finish temperature is set to a high temperature of 950 DEG C or more, there arises a problem that the YS of the obtained steel sheet is insufficient. On the other hand, the reduction of not less than 40% in the temperature range of not more than 950 DEG C and the Ar3 point or more is required to secure a sufficient slab thickness when producing a steel sheet having a finish plate thickness of 75 mm or more, Lt; / RTI >

Further, as in the case of the technique described in Patent Document 1, the metal structure of the steel sheet to be produced is such that the polygonal ferrite is approximately 100%, the bainite and the hard phase (martensite, MA) It is expected that the organization will be almost not included.

Further, since Ni is a selective element and is not a technical concept to control? G / [Si], the balance of the composition of the component is different from that of the present invention. For example, when the steel described in Patent Document 2 has a toughness change The strength (YS, TS) -toughness (vTrs: wavefront transition temperature) - balance of toughness change due to strain aging is different from the present invention.

Further, attention is focused on toughness change due to strain aging, and employment C / N control is not performed, but techniques relating to a steel material to which Cu and Ni are added have been proposed as Patent Documents 3 to 6.

The technique described in Patent Document 3 is a technique developed for the purpose of improving the ductility, the substitution thermal HAZ toughness and the resistance reduction ratio in the vicinity of the surface layer of a steel material for construction, and in order to achieve the object, . However, [Ti] x [N] in the steels described in the examples is at a low level, and it is considered that if a steel sheet is produced by the same process as that of the present invention, a specified structure can not be produced.

Patent Document 4 describes a technique for improving the HAZ toughness of a welded joint, and in particular, there is no description of a technique for manufacturing a base material in detail, but an example in which Cu and Ni are added to a base material is described. However, it is considered that the base material contains B of 10 ppm or more, the metal structure becomes bainite-based, and the amount of solid solution C is increased, and the change of vTrs due to strain aging is increased.

In the description of Patent Document 5, there is disclosed a technique relating to a steel sheet which improves the property of stopping the propagation of the generated brittle cracks and also has excellent toughness at the central portion of the plate thickness. In order to achieve the object, Is added. However, since the cooling stop temperature after rolling in the manufacturing process is high, the metal structure does not become a prescribed structure, and it is considered that TS or TS-toughness class is not sufficient.

Patent Document 6 discloses a technique relating to a steel sheet for construction which is excellent in ductility near the surface layer and improved in vibration resistance. In order to attain the object, an example in which Cu and Ni are added to a steel sheet is described. However, it is considered that it is difficult to obtain sufficient toughness or to reduce change in toughness due to strain aging because the cooling stop temperature after rolling, which is an important requirement in the control of the amount of solid solution C, is as low as 280 deg. Further, in many of the examples, Ca is not added to the steel sheet in securing toughness of the steel sheet, and it is considered that the inclusions adversely affect the toughness.

Japanese Patent Application Laid-Open No. 9-41035 Japanese Patent Application Laid-Open No. 2003-13174 Japanese Patent Application Laid-Open No. 2010-229442 Japanese Patent Application Laid-Open No. 2009-068078 Japanese Patent Application Laid-Open No. 2008-280600 Japanese Patent Application Laid-Open No. 2003-328080

(YS): 400 MPa or more, TS (tensile strength): 50 MPa or more, and a steel sheet having a tensile strength 500 to 700 MPa and vTrs (wave-front transition temperature) after strain aging of -60 deg. C or lower and vTrs change due to strain aging are within 30 deg. And to provide an image processing method.

The invention according to claim 1 is a steel sheet comprising, as mass%, 0.03 to 0.06% of C, 0.25% or less of Si (not including 0%), 1.25 to 1.75% of Mn and 0.010% 0.003% or less (not including 0%), Al: 0.025 to 0.035%, Cu: 0.1 to 0.4%, Ni: 0.45 to 0.75%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.025 [Si] > 0.4 and [Ti] x [N] > = 4.0 x 10 < 3 >, and the balance of iron and inevitable impurities, ^-5 , and has a mixed structure in which the ferrite fraction is 40 to 90% by area and the bainite fraction is ⁵ to 60% by area, and the sum of the ferrite fraction and the bainite fraction is 90% , The effective crystal grain diameter is 3 to 25 占 퐉, and the solid content of solid solution contained in the whole structure is 0.035 mass% or less.

A3 = 894.5-269.4 [C] +37.4 [Si] -31.6 [Mn] -19.0 [Cu] -29.2 [Ni] -11.9 [ Cr] +18 [Mo] +22.2 [Nb]) and Bs = 830-270 [C] -90 [Mn] -37 [Ni] -70 [Cr] -83 [Mo]. In the above formulas, [] represents mass%.

The invention according to claim 2 is characterized in that the strain age according to claim 1 further contains 0.5% or less of Cr (not including 0%) and / or Mo: 0.5% or less (excluding 0% It is a steel sheet with little change in toughness before and after.

According to the steel sheet after the present invention, the strength, YS (yield strength): 400 MPa or more, TS (tensile strength): 500-700 MPa, and vTrs The fracture transition temperature): -60 DEG C or less, vTrs change due to strain aging: 30 DEG C or less, excellent strength-toughness-strain steel sheet having balance of toughness due to strain aging can be obtained.

The inventors of the present invention have found that, in order to obtain a post-steel sheet excellent in balance of toughness due to a change in toughness caused by a change in toughness before and after deformation aging, particularly a steel sheet, in particular strength (YS, TS) toughness (vTrs Composition, metal structure, and the like. As a result, it was found that, after setting the composition of the steel sheet as an essential additive element of Cu and Ni, the values of ΔG / [S] and [Ti] × [N] were set in a suitable range, and bainite and ferrite And the ratio (area%) of the hard phase is appropriately controlled and the effective crystal grain diameter and the amount of solute C contained in the whole structure are kept within an appropriate range. It was completed. In addition, [] represents the mass% of each element (the same applies to the following description).

Hereinafter, the present invention will be described in detail based on embodiments.

As described above, in the present invention,? G / [S] and [Ti] x [N] determined from the composition of the steel sheet and the composition of the steel sheet, the ratio of bainite to ferrite and hard phase %), The effective crystal grain size, and the amount of solid solution C contained in the whole structure. First, the component composition will be described in detail. Hereinafter, the content of each element (chemical component) is simply expressed in% but all represent% by mass.

(Composition of components)

C: 0.03 to 0.06%

C is an essential element for securing the strength of the steel sheet. When the content of C is less than 0.03%, the strength required for the steel sheet can not be secured. The lower limit of the content of C is preferably 0.02%. On the other hand, if the content of C is excessive, the amount of solid solution C increases, bainite is not produced, and a large amount of hard island-shaped martensite (MA) is generated, leading to deterioration of toughness. Therefore, the content of C is 0.06% or less. The preferable upper limit of the content of C is 0.05%.

Si: not more than 0.35% (not including 0%)

Si is a useful element for securing the strength of the steel sheet as in C, but if the content of Si is excessive, precipitation of the solid solution C in the carbide is promoted and the production of hard island martensite (MA) , Resulting in deterioration of toughness of the steel sheet. Therefore, the Si content should be 0.35% or less. The preferred upper limit is 0.20%, and the more preferable upper limit is 0.10%. The lower limit of the Si content is not specifically defined, but the lower limit is preferably 0.01%.

Mn: 1.25 to 1.75%

Mn is also an element useful for securing the strength of the steel sheet. In order to effectively exhibit such effect, it is necessary to contain Mn of 1.25% or more. The lower limit of the content of Mn is 1.35%, and the lower limit is more preferably 1.45. On the other hand, if it is contained in excess of 1.75%, the segregation is suppressed, so the content of Mn is 1.75% or less. The preferable upper limit of the Mn content is 1.65%, and the more preferable upper limit is 1.55%.

P: not more than 0.010% (not including 0%)

P is an impurity element which tends to cause grain boundary destruction and adversely affects the toughness, so that the content of P is preferably as small as possible. From the viewpoint of securing toughness, the content of P must be suppressed to 0.010% or less, preferably 0.007% or less. The lower limit of the content of P is not particularly specified, but it is difficult to industrially make the P content of steel 0%.

S: not more than 0.003% (not including 0%)

Since S is an element that forms Mn sulfides and deteriorates toughness, the content of S is preferably as small as possible. From the viewpoint of securing toughness, it is necessary to suppress the S content to 0.003% or less. The lower limit of the content of S is not specifically defined, but it is difficult to industrially make S of steel 0%.

Al: 0.025 to 0.035%

Al is an element useful for fixing the impurity N and reducing the change in toughness due to strain aging. To effectively exhibit such effect, Al must be contained in an amount of 0.025% or more. On the other hand, if it is contained in excess of 0.035%, the toughness is lowered, so the content of Al is 0.035% or less.

Cu: 0.1 to 0.4%

Cu is a useful element for securing the strength of a steel sheet and securing toughness after aging. In order to effectively exhibit such an effect, it is necessary to contain 0.1% or more, preferably 0.2% or more. On the other hand, in order to suppress embrittlement due to segregation, the content of Ni is adjusted to 0.4 or less so as not to exceed a predetermined ratio with respect to Ni added. It is preferably 0.3% or less. Particularly, the composite addition of Ni-Cu is effective in reducing cost increase and securing toughness after deformation aging. The reason why the addition of Cu contributes to ensuring the toughness after the aging of the strain is thought to be that the potential distribution introduced at the time of imparting the strain by the addition of Cu changes.

Ni: 0.45-0.75

Ni is a useful element for improving strength-to-wear balance and securing toughness after aging. In order to exhibit such effect effectively, it is necessary to contain 0.45% or more, preferably 0.5% or less. On the other hand, Ni is an expensive element and its content is 0.75% or less, preferably 0.65% or less. The reason why the addition of Ni contributes to ensuring the toughness after aging of the strain is thought to be attributable to the fact that the potential distribution introduced at the time of imparting the deformation by Ni addition changes.

Nb: 0.01 to 0.05%

Since Nb is a useful element for securing the strength of the steel sheet, it is necessary to contain Nb at 0.01% or more. On the other hand, excessive addition of Nb leads to formation of coarse Nb pellets in the slab stage to lower the toughness, so that it is 0.05% or less. Within the range of 0.01 to 0.05%, it is preferable that the amount of Nb added is large, since the amount of solid solution C can be reduced and the TS level can be improved. , Preferably not less than 0.020%, more preferably not less than 0.023%, and still more preferably not less than 0.025%.

Ti: 0.005 to 0.025%

Ti has an action of fixing the impurity N, and therefore it is necessary to contain Ti in an amount of 0.005% or more. On the other hand, in order to suppress coarse TiN which can be a starting point of fracture and ensure toughness, the content is set to 0.025% or less. Within the range of 0.005 to 0.025%, the amount of Ti to be added is preferably large because the amount of solid solution C can be reduced and the TS level can be improved. , Preferably not less than 0.015%, and more preferably not less than 0.018%.

N: 0.0030 to 0.0060%

N is an impurity element which is inevitably incorporated, but it has an effect of inhibiting? -Lip coarsening by the use of AlN and TiN, and therefore it is necessary to contain 0.0030% or more. On the other hand, in order to suppress toughness deterioration due to strain aging, it is required to be 0.0060% or less.

Ca: 0.0010 to 0.0025%

Ca is preferably added in a small amount in order to secure toughness, and it also acts to detoxify MnS. In order to exhibit such an effect effectively, it is necessary to contain 0.0010% or more. However, if it is contained in an excess amount, coarse inclusions are formed and the toughness is lowered, so it is necessary to suppress the content to 0.0025% or less.

The above is an essential containing element defined in the present invention, and the remaining part is iron and inevitable impurities. As the inevitable impurities, incorporation of elements such as Sn, As, Pb, etc. to be brought in according to the conditions of raw materials, materials, manufacturing facilities, etc. is allowed. It is also effective to actively contain the following elements, and the properties of the post-steel sheet are further improved. Further, B is not actively added.

Cr: not more than 0.5% (not including 0%) and / or Mo: not more than 0.5% (not including 0%)

Cr and Mo are effective elements for improving the strength. However, if it is excessively added, the bainite fraction becomes excessive and / or the solid solution C becomes excessive, so that the content is made 0.5% or less.

B: Not added

B is basically not added except for the impurities contained in the steel. When B is further added to a steel material to which Nb is added in an amount of 0.01 to 0.05%, the delay of ferrite transformation becomes significant, and the target ferrite fraction can not be obtained.

? G / [Si] > 0.4

In the present invention, it is necessary to satisfy? G / [Si] > 0.4 in addition to the content of each element contained in the steel after plate. By satisfying the above formula, it is possible to promote the reaction of the solid solution C to the precipitation of the carbide upon cooling after the rolling. Preferably,? G / [Si]? 0.5, more preferably? G / [Si]? 1.5. Further, the numerator: ΔG of the above formula approximates the driving force of the reaction, and the denominator: [Si] approximates the reaction rate.

A3 and Bs described in? G = (A3-Bs) / A3 in the above formula can be obtained from the following formulas. That is, ΔG can be controlled by the total components contained in the steel. The steel sheet of the present invention is not a bainite single phase, but it was determined that Bs determined by Steven et al. Are effective for comparison of transformation points of steel types. For A3, bcc precipitation start temperature of each component was determined by thermocarc, and a regression equation was prepared.

A3 = 894.5-269.4 [C] +37.4 [Si] -31.6 [Mn] -19.0 [Cu] -29.2 [Ni] -11.9 [Cr] +19.5 [Mo] +22.2 [Nb]

Bs = 830-270 [C] -90 [Mn] -37 [Ni] -70 [Cr]

[Ti] x [N] > = 4.0 x 10 < ^-5 >

In addition, [Ti] × [N] it has to satisfy the ≥4.0 × 10 ^-5. When this formula is not satisfied, that is, when [Ti] x [N] < 4.0 x 10 < ^{-5 &} gt ;, the number density of fine TiN suppressing grain growth is insufficient. Particularly, in the present invention in which the steel sheet is subjected to a high rolling reduction rate, it is important to assist in the effect of atomization by the fine TiN.

(group)

Ferrite fraction: 40 to 90%, Bainite fraction: 5 to 60%, etc.

In order to achieve the intended strength (YS, TS) of the steel sheet according to the present invention, it is preferable to use a mixed structure having a ferrite fraction of 40 to 90% by area and a bainite fraction of 5 to 60% (Martensite, MA) is not less than 90% by area and the hard phase (martensite, MA) is not less than 1% by area.

If the ferrite fraction exceeds 90% by area, at least one of YS (yield strength) and TS (tensile strength) becomes insufficient. On the other hand, when the ferrite fraction is less than 40% by area, or when the bainite fraction exceeds 60% by area, the TS (tensile strength) becomes excessive. When the bainite fraction is less than 5% by area or the ratio of the hard phase is less than 1% by area, at least one of YS (yield strength) and TS (tensile strength) becomes insufficient. When the total of the ferrite fraction and the bainite fraction is less than 90% by area, the relatively hard third phase is formed in the remaining portion, so that the toughness is lowered.

In addition, most of the metal structure of the steel sheet of the present invention is occupied by the bainite structure and the ferrite structure, and the remaining portion may include pearlite and pseudo-pearlite in addition to the hard phase.

Effective crystal grain diameter: 3 to 25 탆

The present invention also defines the effective grain diameter. The effective crystal grain diameter should be 25 占 퐉 or less in order to ensure toughness of the base material before aging. On the other hand, in order to reduce the tearing deterioration due to strain aging to a predetermined value or less, the thickness is set to 3 탆 or more. The method of measuring the effective crystal grain diameter will be described in the column of Examples.

C solids content: not more than 0.035% by mass

In the present invention, the amount of solid solution C contained in the entire structure of the steel sheet is also specified. In order to keep the change in vTrs due to the strain aging within a predetermined temperature difference, it is necessary to define the upper limit of the solute C amount. In the present invention, the solute C amount is set to 0.035 mass% or less in order to change the vTrs within 30 占 폚.

(Manufacturing requirements)

The steel sheet of the present invention can be obtained by a process which comprises the steps of using a steel satisfying the above-mentioned component composition, forming a slab by a usual casting method, and then subjecting it to ordinary heating, hot rolling (rough rolling and finish rolling) The rolling reduction end temperature (FRT), the time from the start of the cooling after the rolling, the cooling start temperature, the cooling after the rolling By making the conditions such as the speed, the cooling stop temperature, the cooling rate after stopping, and the quality of the steels respectively as described below, it is possible to manufacture a steel sheet satisfactorily satisfying the requirements of the present invention.

Slab heating temperature: 1000 ~ 1250 ℃

By setting the heating temperature of the slab before hot rolling to 1000 to 1250 占 폚, it is possible to secure the required amount of solidified Nb. When the heating temperature is lower than 1000 占 폚, solid Nb can not be secured, and as a result, bainite formation becomes insufficient in the subsequent cooling step. On the other hand, when the heating temperature is higher than 1250 ° C, the spherical γ-particle size becomes coarse during heating, and toughness is adversely affected.

Total reduction ratio: 50% or more, reduction rate during rough rolling: 20% or more

In order to reduce the spherical a-particle size in the recrystallized region to a predetermined value or less, it is necessary to perform a predetermined rolling down to the temperature regulation by rough rolling. In the present invention, the rolling reduction during rough rolling is set to 20% or more. Further, in order not to coarsen the crystal grain diameter, it is necessary to perform rolling at a total reduction rate of 50% or more from the slab to the finishing. If the rough rolling reduction ratio is less than 20% or the total reduction ratio is less than 50%, the crystal grain diameter becomes coarse.

In addition, a structure having a desired grain size (bainite + ferrite) can be secured by applying a predetermined deformation to the non-recrystallized?. Particularly, considering the balance of the reduction rate in the case of rough rolling and finish rolling within a predetermined total reduction ratio, even in the case of a backing plate having a thickness of 75 mm or more, It is preferable that the reduction rate is 40% or less.

Rolling finish temperature (FRT): 700 to 900 占 폚

The rolling finish temperature (FRT) is 700 to 900 占 폚. The reason why the lower limit is set to 700 占 폚 or more is to reduce the rolling load and the upper limit is set to 900 占 폚 in order to obtain a desired transformation structure (bainite + ferrite) by introducing deformation in the non recrystallized? Region. If the rolling finishing temperature (FRT) exceeds 900 캜, not only the cooling load in the subsequent step becomes large but also a large amount of coarse ferrite is produced.

Time to start cooling after rolling: 120 seconds or less

The time until the start of cooling after rolling is set to 120 seconds or less. The reason for setting it to 120 seconds is to suppress the coarse ferrite formed in the ferrite transformation in the high temperature region, and when the time until the start of cooling exceeds 120 seconds, the crystal grain diameter becomes coarse.

Cooling start temperature: 650 ° C or more

The cooling start temperature should be 650 ° C or higher. The reason why the temperature is set to 650 占 폚 or more is to suppress the ferrite transformation in the high temperature region and suppress the coarse ferrite in view of the driving force.

Cooling speed after rolling: 2 to 30 ° C / s

The cooling after rolling is carried out at a cooling rate of 2 to 30 DEG C / s. The reason why the lower limit of the cooling rate is set to 2 캜 / s is to suppress the generation of ferrite more than necessary and to prevent the productivity from being lowered. On the other hand, the reason why the upper limit of the cooling rate is set to 30 DEG C / s is that the structure is made of a mixed structure of ferrite and bainite and the strength is secured.

Cooling stop temperature: 350 ~ 450 ℃

In order to make the structure of the mixture structure of ferrite and bainite, the cooling stop temperature should be 350 to 450 占 폚. When the cooling stop temperature is lower than 350 占 폚, the entire surface becomes a bainite or martensite structure. Also, the employment C is increased. On the other hand, when the cooling stop temperature exceeds 450 캜, bainite is not sufficiently obtained. In addition, the crystal grain diameter becomes coarse.

Cooling speed after stopping: 1 ℃ / s or less

The precipitation of the solid solution C can be promoted by setting the cooling rate after the cooling stop to 1 DEG C / s or lower.

Conditioning: None

In the production of the post-steel sheet of the present invention, tempering such as tempering is basically not performed from the viewpoint of productivity improvement.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples, and it is also possible to carry out the present invention while appropriately changing it within a range suitable for the purpose of the present invention , All of which are included in the technical scope of the present invention.

The slabs were subjected to a solvent treatment in accordance with a usual casting method using the steels having the respective compositional compositions shown in Tables 1 and 2 and then subjected to the steps of heating, hot rolling (rough rolling, finish rolling) A plate thickness of 100 mm was obtained. The rolling reduction end temperature (FRT), the time from the start of the cooling after the rolling, the cooling start temperature, the cooling rate after the rolling, the cooling stop temperature, The cooling rates were set as shown in Tables 3 and 4. The rolling reduction temperature of the rough rolling was 900 ° C or higher and the finishing temperature was 720 ° C.

The ferritic fraction, the bainite fraction, the hard phase fraction, the effective crystal grain diameter, the solid solution amount, YS (yield strength), TS (tensile strength), vTrs change due to strain aging, VTrs (wave-front transition temperature) before strain aging was obtained by measurement or the like. The results of these measurements are shown in Tables 5 and 6.

(Ferrite fraction, bainite fraction, hard phase fraction, effective crystal grain diameter)

The test pieces were cut out from the surface of the steel sheet after each time at a depth of t / 4 (t: sheet thickness) (the axial center of the test piece passed through the position of t / 4) And the ferrite percentage (area%), bainite fraction (area%), and hard phase fraction (area%) of the metal structure were determined by observing at least 3 fields of view at 800 times the area of 800 μm × 600 μm. Further, in this embodiment, the crystal grains having an aspect ratio of less than 2 after the receding corrosion are defined as ferrite. Further, in crystal grains having an aspect ratio of 2 or more, bainite which is not a white contrast after repera erosion and white phase is defined as a hard phase. The effective grain diameter and aspect ratio were measured by line segment method. The area surrounded by the black line contrast is regarded as an effective crystal grain diameter, and more than 100 long and short axes are determined for each field, and the average thereof is defined as the effective crystal grain diameter .

(Employment C amount)

The amount of carbide (cementite) was measured by X-ray diffraction (XRD). After the measurement, the amount of carbonized water = the amount of precipitated C, and the amount of solid solution C was calculated from the following formula. Further, in the X-ray diffraction, the amount of carbide can be measured to be slightly less since a carbide smaller than a predetermined value can not be detected. Therefore, the calculated amount of solute C is considered to be larger than the actual solute C amount. On the other hand, it is considered to be correlated with the actual amount of solid solution C, and the following equation was adopted.

Solvent C content = total C content - precipitation C content

(Evaluation of yield strength and tensile strength)

Four test specimens of JIS Z 2201 were taken from a position of depth t / 4 (t: sheet thickness) perpendicular to the rolling direction from the surface of each subsequent steel sheet, and a tensile test of JIS Z 2241 was carried out. The yield strength (YS) and the tensile strength (TS) of the steel sheet were measured. In the present embodiment, the steel sheet satisfying the conditions of YS of 400 MPa or more and TS of 500 to 700 MPa was evaluated as satisfying the acceptance condition.

(Change in vTrs due to strain aging, evaluation of vTrs before strain aging)

Three Charpy impact test specimens (JIS Z 2242, No. 4 test specimen) were sampled from the positions of depth t / 4 (t: sheet thickness) from the surface of each steel sheet before and after deformation aging Position), and a V-notch Charpy impact test was conducted. For each of the test pieces, the brittle fracture ratio was measured under conditions of 3 or more, and the temperature at which the average value of the brittle fracture ratio became 50% was determined at a pitch of 5 캜. In the present example, it was evaluated that the change in vTrs due to strain aging was within 30 占 폚 and the vTrs before strain aging were below -60 占 폚.

(Test result)

Nos. 1 to 20 are inventive examples satisfying the requirements of the present invention. In addition to the composition of the components,? G / [Si]> 0.4 and [Ti] x [N]? 4.0 x 10 ^-5 , bainite fraction, Fraction of hard phase, and solid content of C satisfy the requirements of the present invention. As a result, the yield strength (YS), the tensile strength (TS), the vTrs change due to the strain aging, and the vTrs before the strain aging all satisfied the evaluation criteria of this embodiment.

On the other hand, Nos. 21 to 30 are comparative examples (Nos. 21 to 24 and 27 in any one of bainite fraction, ferrite fraction and hard phase fraction, No.21 , And 23 does not satisfy the requirements of the present invention even in the amount of solute C). As a result, all of the evaluation criteria of yield strength (YS), tensile strength (TS), vTrs change due to strain aging, and vTrs before strain aging were satisfied.

In Nos. 31 to 37, the composition satisfies the requirements specified in the present invention. However, in the case of any of the bainite fraction, the ferrite fraction, the hard phase fraction and the solid solution C, Yes. As a result, the evaluation criteria were not satisfied at any one item among the yield strength (YS), tensile strength (TS), vTrs change due to strain aging, and vTrs before strain aging.

Claims (2)

(Inclusive of 0%), Mn: 1.25 to 1.75%, P: 0.010% or less (not including 0%), S: 0.003% or less, (Inclusive of 0%), Al: 0.025 to 0.035%, Cu: 0.1 to 0.4%, Ni: 0.45 to 0.75%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.025% %, Ca: 0.0010 to 0.0025%, the balance being iron and inevitable impurities,
Satisfies △ G / [Si]> 0.4 and [Ti] × [N] ≥4.0 × 10 -5 , and
The ferrite composition of the present invention has a mixed structure in which the ferrite fraction is from 40 to 90% by area and the bainite fraction is from 5 to 60% by area, and the sum of the ferrite fraction and the bainite fraction is 90% ,
The effective crystal grain diameter is 3 to 25 占 퐉,
Further, the steel sheet has a small change in toughness before and after deformation aging, characterized in that the amount of solid solution C contained in the whole structure is 0.035 mass% or less.
In the above formula,? G = (A3-Bs) / A3,
A3 = 894.5-269.4 [C] +37.4 [Si] -31.6 [Mn] -19.0 [Cu] -29.2 [Ni] -11.9 [Cr] +19.5 [Mo] +22.2 [
Bs = 830-270 [C] -90 [Mn] -37 [Ni] -70 [Cr] -83 [Mo].
In the above formulas, [] represents mass%. The method according to claim 1,
The steel sheet according to any one of claims 1 to 3, further comprising at least one of Cr: not more than 0.5% (not including 0%) and Mo: not more than 0.5% (not including 0% .