JP2014074194A - Thick steel plate having small change of toughness before and after strain ageing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate having small change of toughness before and after strain ageing.SOLUTION: The thick steel plate satisfies a predetermined composition of chemical components, and satisfies ΔG/[Si]>0.4 and [Ti]×[N]≥4.0×10, and has a mixed structure having a ferrite fraction of 40 to 90 area% and a bainite fraction of 5 to 60 area%, has a sum of the ferrite fraction and the bainite fraction of 90 area% or more, contains a hard phase of 1 area% or more, has an effective crystal grain size of 3 to 25 μm and a solid solution C amount contained in a whole structure of 0.035 mass% or less.

Description

  The present invention relates to a thick steel plate suitably used for offshore structures and the like, and more particularly to a thick steel plate with little change in toughness before and after strain aging.

  It is known that the yield strength (YS) of steel materials increases due to strain aging. On the other hand, it is also known that the toughness decreases as the yield strength (YS) increases. That is, it can be said that the toughness is lowered by strain aging. The phenomenon that occurs during strain aging is said to be due to solid solution C / N fixing to dislocations that are included in the structure as it is rolled, or to dislocations that are introduced by imparting strain. From these facts, in order to reduce the change in characteristics due to strain aging, it is considered effective to control the dislocation state or reduce the amount of solid solution C / N.

  Focusing on such changes in toughness due to strain aging of steel materials, various proposals have conventionally been made regarding techniques related to steel materials that can suppress a decrease in toughness even after strain aging and ensure excellent toughness.

  The technique described in Patent Document 1 is a technique proposed to reduce the solid solution C / N amount as effective for obtaining a steel sheet with little change in toughness before and after strain aging. In addition, the content of C in the steel material is set to a low content (less than 0.030 wt%), and at least one of Ti and Nb is added to reduce the amount of dissolved C and toughness does not decrease. A steel sheet has been proposed in which the toughness after strain aging (after cold forming) can be ensured by coarsening the precipitated carbide so as not to increase the YR in the range.

  However, since the C content of the steel sheet described in Patent Document 1 is less than 0.030 wt% and the winding temperature is 550 ° C. or higher, the metal structure is approximately 100% polygonal ferrite, Since it becomes a structure containing almost no bainite or hard phase (martensite, MA), it is expected that YS before strain aging does not become sufficiently high.

  On the other hand, Patent Document 2 proposes a technique related to a steel material that suppresses changes in toughness due to strain aging, focusing on factors other than the amount of solute C / N. Specifically, by improving the mobility of dislocations introduced by applying strain, the TS level is 500 to 690 MPa, and the change in toughness (vTrs: fracture surface transition temperature) due to strain aging is small (within ΔvTrs of 25 ° C.). This makes it possible to produce steel plates with a thickness of 20-100 mm.

  However, in the production of this steel sheet, it is necessary to finish the rolling at 950 ° C. or higher, or to perform rolling at a reduction rate of 40% or higher in a temperature range of 950 ° C. or lower and Ar 3 point or higher. When the rolling end temperature is set to a high temperature of 950 ° C. or higher, there arises a problem that YS of the obtained steel sheet is insufficient. On the other hand, it is necessary to ensure a sufficient slab thickness when producing a steel sheet having a finished sheet thickness of 75 mm or more, to perform a reduction of a reduction ratio of 40% or more in a temperature range of 950 ° C. or lower and an Ar3 point or higher, There is a problem that slab manufacturing is difficult.

  Furthermore, the cooling stop temperature after rolling is about 600 ° C., and, like the technique described in Patent Document 1, the metallographic structure of the manufactured steel sheet is approximately 100% polygonal ferrite, and bainite or hard phase (martensite). Site, MA) is expected to become an organization that hardly includes.

  Further, Ni is a selective element, not the technical idea of controlling ΔG / [Si], and the balance of the component composition is different from that of the present invention. For example, the steel material described in Patent Document 2 has a change in toughness before and after strain aging. Even if it is a steel material with a small amount, the balance of toughness change due to strength (YS, TS) -toughness (vTrs: fracture surface transition temperature) -strain aging is different from the present invention.

  Further, while focusing on the change in toughness due to strain aging, solid solution C / N control is not performed, but technologies relating to steel materials to which Cu and Ni are added are proposed as Patent Documents 3 to 6.

  The technology described in Patent Document 3 is a technology developed with the objective of improving ductility in the vicinity of the surface layer of steel for construction, high heat input HAZ toughness, and low yield ratio improvement. An example with Ni added is described. However, [Ti] × [N] in the steel materials described in the examples is at a low level, and it is considered that when a steel plate is produced by a process similar to that of the present invention, a specified structure cannot be created.

  Patent Document 4 describes a technique for improving the HAZ toughness of a welded joint, and in particular, although the manufacturing technique of the base material is not described in detail, there is an example in which Cu and Ni are added to the base material. Have been described. However, this base material contains 10 ppm or more of B, and it can be said that the metal structure is mainly bainite and the amount of solute C increases, and it is considered that the change in vTrs due to strain aging increases.

  Patent Document 5 describes a technique relating to a steel sheet that improves the properties of stopping the propagation of the brittle cracks that have occurred and also has excellent base material toughness at the center of the plate thickness. In addition, it is described that Cu and Ni are added to a steel plate. However, since the cooling stop temperature after rolling in the production process is high, it is considered that the metal structure does not become a specified structure and the TS or TS-toughness class is not sufficient.

  Patent Document 6 describes a technique related to a steel sheet for building that has excellent ductility near the surface layer and improved earthquake resistance, and describes an example in which Cu and Ni are added to the steel sheet in order to achieve the problem. Has been. However, the cooling stop temperature after rolling, which is an important requirement for controlling the amount of solute C, is as low as 280 ° C., and it is considered difficult to obtain sufficient toughness or to reduce the change in toughness due to strain aging. In many of the examples, Ca, which is important for securing the toughness of the thick steel plate, is not added, and it is considered that inclusions adversely affect the toughness.

JP 9-41035 A JP 2003-13174 A JP 2010-229442 A JP 2009-068078 A JP 2008-280600 A JP 2003-328080 A

  The present invention has been made as a solution to the above-described conventional problems, and is a thick steel plate with little change in toughness before and after strain aging. More specifically, the strength is YS (yield strength): 400 MPa or more, TS (tensile strength). : 500-700 MPa, vTrs (fracture surface transition temperature) after strain aging, −60 ° C. or less, vTrs change due to strain aging, within 30 ° C. Excellent strength-toughness—Tough steel plate having a toughness change balance due to strain aging It is a problem to provide.

The invention according to claim 1 is mass%, C: 0.03 to 0.06%, Si: 0.35% or less (excluding 0%), Mn: 1.25 to 1.75%, P : 0.010% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.025 to 0.035%, Cu: 0.1 to 0.4% , Ni: 0.45-0.75%, Nb: 0.01-0.05%, Ti: 0.005-0.025%, N: 0.0030-0.0060%, Ca: 0.0010 -0.0025%, the balance is iron and inevitable impurities, and ΔG / [Si]> 0.4 and [Ti] × [N] ≧ 4.0 × 10 ⁻⁵ are satisfied. And having a mixed structure with a ferrite fraction of 40 to 90 area% and a bainite fraction of 5 to 60 area%, and the ratio of ferrite fraction + bainite fraction. The total is 90 area% or more, the hard phase is contained 1 area% or more, the effective crystal grain size is 3 to 25 μm, and the solid solution C amount contained in the whole structure is 0.035 mass. % Thick steel plate with little change in toughness before and after strain aging.
However, in the above formula, ΔG = (A3−Bs) / A3, and 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 ] -83 [Mo]. In addition, in each above-mentioned formula, [] shows the mass%.

  The invention according to claim 2 further contains, by mass%, Cr: 0.5% or less (excluding 0%) and / or Mo: 0.5% or less (excluding 0%). It is a thick steel plate with little toughness change before and after strain aging of Claim 1.

  According to the thick steel plate of the present invention, a thick steel plate with little change in toughness before and after strain aging, particularly strength, YS (yield strength): 400 MPa or more, TS (tensile strength): 500-700 MPa, vTrs after strain aging ( Fracture surface transition temperature): −60 ° C. or less, vTrs change due to strain aging: within 30 ° C. An excellent strength-toughness—thick steel plate having a toughness change balance due to strain aging can be obtained.

  The present inventors obtain a thick steel plate having a small toughness change before and after strain aging, in particular, a steel plate having an excellent balance of strength (YS, TS) -toughness (vTrs: fracture surface transition temperature) -toughness change due to strain aging. Therefore, earnest research was conducted from various angles such as component composition and metal structure. As a result, after defining the component composition of the thick steel plate with Cu and Ni as essential additive elements, ΔG / [S] and [Ti] × [N] are in an appropriate range, and bainite occupying the metal structure By appropriately controlling the ratio of ferrite and hard phase (area%), and further keeping the effective crystal particle diameter and the amount of solute C contained in the entire structure within an appropriate range, a desired thick steel plate is obtained. As a result, the present invention has been completed. In addition, [] shows the mass% of each element. (The same applies to the following descriptions.)

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

  As described above, in the present invention, the component composition of the thick steel plate, ΔG / [S] and [Ti] × [N] obtained from the component composition, bainite and ferrite in the metal structure, and the ratio of the hard phase ( Area%), effective crystal particle diameter, and the amount of dissolved C contained in the entire structure are defined. First, the component composition will be described in detail. Hereinafter, the content of each element (chemical component) is simply described as%, but all indicate mass%.

(Component composition)
C: 0.03-0.06%
C is an essential element for ensuring the strength of the steel sheet. If the C content is less than 0.03%, the strength required by the steel sheet cannot be secured. The minimum with preferable content of C is 0.02%. On the other hand, when the content of C is excessive, the amount of dissolved C increases, bainite is not generated, and a large amount of hard island martensite (MA) is generated, resulting in toughness deterioration. Therefore, the C content is 0.06% or less. The upper limit with preferable content of C is 0.05%.

Si: 0.35% or less (excluding 0%)
Si is an element useful for securing the strength of the steel sheet as in the case of C. However, when its content is excessive, precipitation of solute C into carbide is promoted, and hard martensite is hard. The production of (MA) is promoted and the toughness of the steel sheet is deteriorated. Therefore, the Si content is set to 0.35% or less. A preferable upper limit is 0.20%, and a more preferable upper limit is 0.10%. In addition, although the minimum of content of Si is not prescribed | regulated in particular, a preferable minimum is 0.01%.

Mn: 1.25 to 1.75%
Mn is also an element useful for securing the strength of the steel sheet, and it is necessary to contain Mn in an amount of 1.25% or more in order to effectively exhibit such effects. The minimum with preferable content of Mn is 1.35%, and a more preferable minimum is 1.45. On the other hand, in order to suppress segregation when it is contained excessively exceeding 1.75%, the Mn content is 1.75% or less. The upper limit with preferable content of Mn is 1.65%, and a more preferable upper limit is 1.55%.

P: 0.010% or less (excluding 0%)
Since P is an impurity element that easily causes grain boundary fracture and adversely affects toughness, its content is preferably as small as possible. From the viewpoint of ensuring toughness, the P content needs to be suppressed to 0.010% or less, preferably 0.007% or less. The lower limit of the P content is not particularly defined, but it is difficult to make P in steel 0% industrially.

S: 0.003% or less (excluding 0%)
Since S is an element that forms Mn sulfide and deteriorates toughness, its content is preferably as small as possible. From the viewpoint of securing toughness, the S content needs to be suppressed to 0.003% or less. The lower limit of the S content is not particularly specified, but it is difficult to industrially make S in steel 0%.

Al: 0.025 to 0.035%
Al is an element useful for fixing the impurity N and reducing a change in toughness due to strain aging, and it is necessary to contain 0.025% or more in order to effectively exhibit such an effect. On the other hand, if the content exceeds 0.035%, the toughness is reduced, so the Al content is 0.035% or less.

Cu: 0.1 to 0.4%
Cu is an element useful for ensuring the strength of a steel sheet and for ensuring toughness after strain aging. In order to exhibit such an effect effectively, 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, it is set to 0.4 or less so as not to have a predetermined ratio or more with respect to the added Ni. Preferably it is 0.3% or less. In particular, the combined addition of Ni—Cu reduces the increase in cost and is effective in securing toughness after strain aging. Although not certain, it is considered that the addition of Cu contributes to securing the toughness after strain aging because the dislocation distribution introduced at the time of applying strain is changed by the addition of Cu.

Ni: 0.45-0.75
Ni is a useful element for improving the strength-toughness balance and securing toughness after strain aging. In order to exhibit such an effect effectively, it is necessary to contain 0.45% or more, preferably 0.5% or less. On the other hand, since Ni is an expensive element, its content is 0.75% or less, preferably 0.65% or less. Although not certain, it is considered that the addition of Ni contributes to securing the toughness after strain aging because the dislocation distribution introduced at the time of strain application is changed by the addition of Ni.

Nb: 0.01 to 0.05%
Nb is a useful element for securing the strength of the steel sheet, so it needs to be contained in an amount of 0.01% or more. On the other hand, excessive addition of Nb leads to the formation of coarse Nb crystals in the slab stage and lowers toughness. In addition, in the range of 0.01 to 0.05%, it is preferable that the amount of Nb added is large because the amount of dissolved C can be reduced and the TS level can be improved. Preferably it is 0.020% or more, More preferably, it is 0.023% or more, More preferably, it is 0.025% or more.

Ti: 0.005-0.025%
Since Ti has an action of fixing the impurity N, it is necessary to contain 0.005% or more. On the other hand, the content is set to 0.025% or less in order to suppress coarse TiN that can be a starting point of fracture and to secure toughness. In addition, in the range of 0.005 to 0.025%, it is preferable that the amount of Ti added is large, because the amount of dissolved C can be reduced and the TS level can be improved. Preferably it is 0.015% or more, More preferably, it is 0.018% or more.

N: 0.0030 to 0.0060%
N is an impurity element inevitably mixed in, but it has an effect of suppressing the coarsening of old γ grains by utilizing AlN and TiN, so 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 necessary to be 0.0060% or less.

Ca: 0.0010 to 0.0025%
Ca is preferably added in a small amount to ensure toughness, and also has an effect of detoxifying MnS. In order to exhibit such an effect effectively, it is necessary to contain 0.0010% or more. However, if it is contained excessively, coarse inclusions are formed and the toughness is lowered, so it is necessary to keep it to 0.0025% or less.

  The above are the essential elements specified in the present invention, and the balance is iron and inevitable impurities. As an inevitable impurity, mixing of elements such as Sn, As, and Pb brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. Further, it is effective to further contain the following elements, and the characteristics of the thick steel plate are further improved. Also, B is not actively added.

Cr: 0.5% or less (not including 0%) and / or Mo: 0.5% or less (not including 0%)
Cr and Mo are effective elements for improving the strength. However, if excessively added, the bainite fraction becomes excessive and / or the solid solution C becomes excessive.

B: Not added Basically, B is not added except for impurities contained in the steel. If 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 cannot be obtained.

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

A3 and Bs described in ΔG = (A3−Bs) / A3 in the previous equation can be obtained from the following equation. That is, ΔG can be controlled by all components contained in the steel. In addition, although the steel plate of this invention is not a bainite single phase, it judged that Bs which Steven and others calculated | required was effective for the comparison of the transformation point of each steel type, and was used as a reference. About A3, the bcc precipitation start temperature of each component was calculated | required with the thermocalc, and the regression equation was created.
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] -83 [Mo]

[Ti] × [N] ≧ 4.0 × 10 ⁻⁵
Moreover, it is necessary to satisfy [Ti] × [N] ≧ 4.0 × 10 ⁻⁵ . When this formula is not satisfied, that is, when [Ti] × [N] <4.0 × 10 ⁻⁵ , the number density of fine TiN that suppresses grain growth is insufficient. In particular, in the present invention that targets thick steel plates to which a high rolling reduction cannot be imparted, it is important to assist the fine graining effect by the fine TiN.

(Organization)
Ferrite fraction: 40 to 90%, bainite fraction: 5 to 60%, etc. In order to achieve the target strength (YS, TS) of the steel plate of the present invention, the ferrite fraction is 40 to 90 area%, A mixed structure having a bainite fraction of 5 to 60% by area, a total of ferrite fraction + bainite fraction being 90% by area or more, and a metal structure containing 1% by area or more of a hard phase (martensite, MA). There must be.

  If the ferrite fraction exceeds 90 area%, at least one of YS (yield strength) and TS (tensile strength) will be insufficient. On the other hand, when the ferrite fraction falls below 40 area%, or when the bainite fraction exceeds 60 area%, TS (tensile strength) becomes excessive. Furthermore, when the bainite fraction is less than 5 area% or the hard phase ratio is less than 1 area%, at least one of YS (yield strength) and TS (tensile strength) is insufficient. Further, when the sum of the ferrite fraction and the bainite fraction is less than 90% by area, a relatively hard third phase is formed in the remainder, so that the toughness is lowered.

  Incidentally, most of the metal structure of the thick steel plate of the present invention is occupied by a bainite structure and a ferrite structure, but the balance may include pearlite and pseudo-pearlite in addition to the hard phase.

Effective crystal grain size: 3-25 μm
In the present invention, the effective crystal grain size is also defined. The effective crystal grain size must be 25 μm or less in order to ensure the base material toughness before aging. On the other hand, in order to reduce the toughness deterioration amount due to strain aging to a predetermined value or less, it is set to 3 μm or more. In addition, the measuring method of an effective crystal grain diameter is demonstrated in the column of an Example.

Solid solution C amount: 0.035% by mass or less In the present invention, the amount of solid solution C contained in the entire structure of the thick steel plate is further defined. In order to keep the vTrs change due to strain aging within a predetermined temperature difference, it is necessary to define the upper limit of the solid solution C amount. In the present invention, the vTrs change is within 30 ° C. 0.035% by mass or less.

(Manufacturing requirements)
The steel plate of the present invention uses steel that satisfies the above-mentioned composition, and is melted by a normal melting method to form a slab, followed by normal heating, hot rolling (rough rolling, finish rolling), and cooling. In particular, the slab heating temperature, total rolling reduction from slab to finishing, rolling reduction during rough rolling, rolling end temperature (FRT), time to start cooling after rolling, cooling start temperature By setting the cooling rate after rolling, the cooling stop temperature, the cooling rate after stopping, and the tempering to the conditions described below, respectively, it is possible to reliably manufacture a thick steel plate that satisfies the requirements of the present invention.

Slab heating temperature: 1000 to 1250 ° C
The required amount of solid solution Nb can be ensured by setting the heating temperature of the slab before hot rolling to 1000 to 1250 ° C. When heating temperature is less than 1000 degreeC, solid solution Nb cannot be ensured, As a result, bainite formation will become inadequate in a subsequent cooling process. On the other hand, when the heating temperature exceeds 1250 ° C., the old γ grain size becomes coarse during heating, which adversely affects toughness.

Total reduction ratio: 50% or more, reduction ratio during rough rolling: 20% or more In order to reduce the old γ grain size to a predetermined value or less in the recrystallization region, it is necessary to perform a predetermined reduction by rough rolling before temperature adjustment. is there. In the present invention, the rolling reduction during rough rolling is 20% or more. Further, in order not to make the crystal grain size coarse, it is necessary to perform rolling at a total reduction ratio from slab to finish of 50% or more. When the rough rolling reduction is less than 20%, or the total reduction is less than 50%, the crystal grain size becomes coarse.

  In addition, the structure | tissue (bainite + ferrite) which has a desired particle size is securable by adding the distortion more than predetermined to unrecrystallized (gamma). In particular, considering the balance of the rolling reduction when performing rough rolling and finish rolling within a predetermined total rolling reduction, even if it is a thick steel plate having a thickness of 75 mm or more, the rolling reduction can be ensured after temperature adjustment. The rough rolling reduction is preferably 40% or less.

Rolling end temperature (FRT): 700-900 ° C
The rolling end temperature (FRT) is 700 to 900 ° C. The reason why the lower limit is set to 700 ° C. or more is to reduce rolling load, and the upper limit is set to 900 ° C. in order to introduce a desired transformation structure (bainite + ferrite) by introducing strain in the non-recrystallized γ region. It is. When the rolling end temperature (FRT) exceeds 900 ° C., not only the cooling load in the subsequent process is increased, but a large amount of coarse ferrite is generated.

Time to start cooling after rolling: within 120 seconds Time to start cooling after rolling is within 120 seconds. The reason for setting it to 120 seconds is to suppress coarse ferrite formed by ferrite transformation in a high temperature region. When the time until the start of cooling exceeds 120 seconds, the crystal grain size becomes coarse.

Cooling start temperature: 650 ° C. or higher The cooling start temperature is 650 ° C. or higher. The reason why the temperature is set to 650 ° C. or higher is to suppress ferrite transformation in a high temperature range and suppress coarse ferrite from the viewpoint of driving force.

Cooling rate after rolling: 2-30 ° C./s
Cooling after rolling is performed at a cooling rate of 2 to 30 ° C./s. The lower limit of the cooling rate is set to 2 ° C./s in order to suppress generation of ferrite more than necessary and not to reduce productivity. On the other hand, the upper limit of the cooling rate is set to 30 ° C./s in order to secure the strength by making the structure a mixed structure of ferrite and bainite.

Cooling stop temperature: 350-450 ° C
In order to make the structure a mixed structure of ferrite and bainite, the cooling stop temperature must be 350 to 450 ° C. When the cooling stop temperature is lower than 350 ° C., the entire surface becomes bainite or a martensite structure. Moreover, the amount of solid solution C increases. On the other hand, when the cooling stop temperature exceeds 450 ° C., bainite cannot be obtained sufficiently. In addition, the crystal grain size becomes coarse.

Cooling rate after stopping: 1 ° C./s or less By setting the cooling rate after stopping cooling to 1 ° C./s or less, precipitation of solute C can be promoted.

Tempering: None In producing the thick steel plate of the present invention, tempering such as tempering is basically not performed from the viewpoint of improving productivity.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  Using steel of each component composition shown in Table 1 and Table 2, it is melted by a normal melting method to form a slab, and then finished through steps of heating, hot rolling (rough rolling, finish rolling), and cooling. Plate thickness: A thick steel plate of 100 mm was obtained. Slab heating temperature, rolling reduction during rough rolling, total rolling reduction from slab to finishing, rolling finish temperature (FRT), time to start cooling after rolling, cooling start temperature, cooling rate after rolling, cooling stop temperature, The cooling rate after stopping was the conditions shown in Table 3 and Table 4. The rolling temperature for rough rolling was 900 ° C. or higher, and the finishing temperature was 720 ° C.

  Each thick steel plate manufactured according to the above requirements has a ferrite fraction, a bainite fraction, a hard phase fraction, an effective crystal grain size, a solid solution C amount, YS (yield strength), TS (tensile strength), and strain. Changes in vTrs due to aging and vTrs (fracture surface transition temperature) before strain aging were determined by measurement or the like. These measurement results are shown in Tables 5 and 6.

(Ferrite fraction, bainite fraction, hard phase fraction, effective crystal grain size)
From the surface of each thick steel plate, a test piece is cut out from a position of depth t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in a cross section parallel to the rolling direction. Etching and observing 3 fields or more in the range of 800 μm × 600 μm at 100 times, ferrite fraction (area%), bainite fraction (area%), and hard phase fraction (area%) in the metal structure ) In this example, crystal grains having an aspect ratio of less than 2 were defined as ferrite after nital corrosion. In addition, crystal grains having an aspect ratio of 2 or more, which are not white contrast after repeller corrosion, are defined as bainite, and a white contrast portion is defined as a hard phase. The effective crystal grain size and aspect ratio were measured by the line segment method. Since it becomes more detrimental than the surroundings due to corrosion, the area surrounded by the black line contrast is regarded as the effective crystal grain size in the optical microscope observation, and 100 or more major axes and minor axes are obtained for each field of view, and the average of these is obtained. Was the effective crystal grain size.

(Solution C amount)
The amount of carbide (cementite) was measured by X-ray diffraction (XRD). After the measurement, the amount of solid solution C was calculated from the following formula, assuming that the amount of carbide = the amount of precipitated C. In addition, since the carbide | carbonized_material smaller than a fixed value cannot be detected by X-ray diffraction, the amount of carbide | carbonized_material is measured slightly. Therefore, the calculated solid solution C amount is considered to be a value larger than the actual solid solution C amount. On the other hand, it is thought that it is correlated with the actual amount of solute C, and the following formula was adopted.
Solid C amount = Total C amount-Precipitated C amount

(Evaluation of yield strength and tensile strength)
From the surface of each thick steel plate, from the position of depth t / 4 (t: thickness), a JIS Z 2201 No. 4 test piece was sampled at right angles to the rolling direction, and a tensile test of JIS Z 2241 was conducted. The yield strength (YS) and tensile strength (TS) in the rolling direction of the piece were determined by measurement. In the present Example, the thick steel plate which satisfy | fills the conditions that YS is 400 Mpa or more and TS is 500-700 Mpa was evaluated as satisfying pass conditions.

(VTrs change due to strain aging, evaluation of vTrs before strain aging)
Three Charpy impact test pieces (JIS Z 2242 No. 4 test piece) were collected from the position of depth t / 4 (t: plate thickness) from the surface of each thick steel plate before and after strain aging (the axis of the test piece) Was taken so as to pass through the t / 4 position), and a V-notch Charpy impact test was conducted. About each test piece, the brittle fracture surface ratio was measured on conditions more than 3 temperature, and the temperature from which the average value of a brittle fracture surface ratio will be 50% was calculated | required in increments of 5 degreeC. In this example, a change in vTrs due to strain aging was within 30 ° C., and a vTrs before strain aging was −60 ° C. or less was evaluated as acceptable.

(Test results)
No. 1 to 20 are invention examples that satisfy the requirements of the present invention. In addition to the component composition, ΔG / [Si]> 0.4, and [Ti] × [N] ≧ 4.0 × 10 ⁻⁵ , The bainite fraction, ferrite fraction, hard phase fraction, and solid solution C content satisfy the requirements of the present invention. As a result, the yield strength (YS), tensile strength (TS), vTrs change due to strain aging, and vTrs before strain aging all satisfied the evaluation criteria of this example.

  In contrast, no. 21 to 30 are comparative examples in which the component composition deviates from the requirements defined in the present invention (No. 21 to 24 and 27 are any of bainite fraction, ferrite fraction, and hard phase fraction, Even the amount of solute C does not satisfy the requirements of the present invention.). As a result, yield strength (YS), tensile strength (TS), vTrs change due to strain aging, and all evaluation criteria for vTrs before strain aging were satisfied.

  No. 31-37, although the component composition satisfies the requirements specified in the present invention, the requirements specified in the present invention in any one or more of bainite fraction, ferrite fraction, hard phase fraction, and solid solution C amount. This is a comparative example that deviates. As a result, one or more items of yield strength (YS), tensile strength (TS), vTrs change due to strain aging, and vTrs before strain aging did not satisfy the evaluation criteria.

Claims (2)

In mass%, C: 0.03 to 0.06%, Si: 0.35% or less (excluding 0%), Mn: 1.25 to 1.75%, P: 0.010% or less (0 %), S: 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: 0.01-0.05%, Ti: 0.005-0.025%, N: 0.0030-0.0060%, Ca: 0.0010-0.0025% The balance is iron and inevitable impurities,
ΔG / [Si]> 0.4 and [Ti] × [N] ≧ 4.0 × 10 ⁻⁵ are satisfied,
And it has a mixed structure with a ferrite fraction of 40 to 90 area% and a bainite fraction of 5 to 60 area%, the total of the ferrite fraction and the bainite fraction is 90 area% or more, and the hard phase is 1 More than area%,
The effective crystal grain size is 3-25 μm,
Further, a thick steel plate having a small change in toughness before and after strain aging, wherein the amount of solute C contained in the entire structure is 0.035% by mass or less.
However, in the previous equation, ΔG = (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 [Nb]),
Bs = 830-270 [C] -90 [Mn] -37 [Ni] -70 [Cr] -83 [Mo].
In addition, in each above-mentioned formula, [] shows the mass%.   Furthermore, before and after strain aging according to claim 1, containing, in mass%, Cr: 0.5% or less (not including 0%) and / or Mo: 0.5% or less (not including 0%) Steel plate with little toughness change.