JP2002256376A - Steel sheet having low deterioration in toughness caused by strain aging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet which has low deterioration in toughness caused by strain aging while maintaining strength required as steel for structural purpose, concretely, tensile strength. SOLUTION: The steel sheet having low deterioration in toughness caused by strain aging has a microstructure essentially consisting of ferrite or bainite, and contains C and N. When the average grain diameter of the crystal grains in which the crystal orientation difference by crystal orientation analysis using EBSP(Electron Back Scattering Pattern) is >=15 deg. is defined as D, the relation among the average grain diameter D and the contents of C and N satisfy the following inequality (1): (4.63×[C]+3.97×[N])×D/15<=0.4...(1); wherein, the [elements] respectively denote the content [mass%] of each element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a structural steel sheet,
More specifically, the present invention relates to a steel sheet having high strength and less toughness deterioration due to strain aging.

[0002]

2. Description of the Related Art When plastic deformation is applied to a steel sheet, mechanical properties,
In particular, it is generally known that toughness deteriorates. This is due to the strain aging characteristics, and the C element and N element dissolved in the steel interact with dislocations introduced by the application of strain, thereby hindering the movement of the dislocations, increasing the yield point, It is believed that embrittlement occurs as a result. Therefore,
In order to suppress the deterioration of toughness, there is a method of reducing the content of C element and N element.

[0003] As a technique for suppressing the degradation of toughness during plastic deformation, for example, Japanese Patent Application Laid-Open Nos. 10-88280 and 10-88281 have been proposed. These technologies aim at minimizing the degradation allowance of the toughness fracture performance due to plastic deformation,
By performing predetermined rough rolling and unrecrystallized zone rolling, or two-phase zone rolling on the steel sheet specified in the amount and the N amount, deterioration of arrest performance among brittle fracture resistance is small even when subjected to plastic strain. We provide structural steel sheets. In order to suppress toughness degradation during plastic deformation, the content of free N is set to 20%.
The amount of carbon is limited to suppress the content to less than ppm and to secure the ductility after strain application. However, no consideration is given to the relationship between the C content and toughness degradation.

[0004] Further, as cold-rolled steel sheets for deep drawing, I.
Use of F (Interstitial Free) steel or the like is known. IF steel is a steel that reduces the C element and other added elements as much as possible and adds Ti and Nb to fix the N element in the steel as a nitride. N adheres to dislocations introduced during processing. And a surface defect during processing (a wavy defect called a stretcher-strain) is thereby prevented. However, since the C content is minimized, when applied to a thick plate, it must be said that the strength is insufficient, and only a tensile strength level of 300 MPa can be obtained.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and it is an object of the present invention to maintain the strength (tensile strength) required for a structural steel sheet and to use strain aging. An object of the present invention is to provide a steel sheet with less toughness deterioration.

[0006]

Means for Solving the Problems A steel sheet having a small toughness deterioration due to strain aging, which can solve the above-mentioned problems, is a steel sheet having a microstructure mainly composed of ferrite or bainite, containing C and N, and containing EBSP. (Electron Back
When the average grain size of crystal grains having a crystal orientation difference of 15 ° or more by crystal orientation analysis using Scattering Pattern) is D,
The point is that the relationship between the average particle diameter D and the contents of C and N satisfies the following expression (1). (4.63 × [C] + 3.97 × [N]) × D / 15 ≦ 0.4 (1) Here, [element] is the content [mass%] of each element in the steel material.
(Hereinafter the same).

[0007] It is more preferable that the steel sheet contains C: 0.03 to 0.2% by mass% (hereinafter expressed as%) because the tensile strength is increased.

Another object of the present invention is to provide a steel sheet having a microstructure mainly composed of ferrite or bainite, which contains C and N, further contains Ti and / or Nb, and has an EBSP (Electron Back Scattering Pattern). When the average grain size of the crystal grains having a crystal orientation difference of 15 ° or more by the crystal orientation analysis used is D, the average grain size D and the C and N,
The relationship with the content of Ti and / or Nb is expressed by the following equation (2):
This can be achieved even with a steel sheet that satisfies the formula (4) and has little toughness deterioration due to strain aging. [N] − [Ti] /3.42≧0 (2) [C] − [Nb] /7.74≧0 (3) ｛4.63 × ([C] − [Nb] /7.74) +3. 97 × ([N]-[Ti] /3.42)｝ × D / 15 ≦ 0.4 (4)

Further, an object of the present invention is to provide a steel sheet having a microstructure mainly composed of ferrite or bainite, containing C and N, further containing Ti and / or Nb, and having an EBSP (Electron Back Scattering Pattern). When the average grain size of the crystal grains having a crystal orientation difference of 15 ° or more by the crystal orientation analysis used is D, the average grain size D and the C and N,
The relationship with the content of Ti and / or Nb is expressed by the following formula (5):
This can be achieved even with a steel sheet that satisfies the expression (7) and has little toughness deterioration due to strain aging. [N] − [Ti] /3.42 <0 (5) [C] − [Nb] /7.74≧0 (6) ｛4.63 × [C] − [Nb] /7.74 − ([Ti ] −3.42 × [N]) / 3.99｝ × D / 15 ≦ 0.4 (7)

[0010] In a steel sheet containing C and N and further containing Ti and / or Nb, mass% (hereinafter referred to as%)
And C: 0.03 to 0.2%, Ti: 0.05%
And / or Nb: 0.03% or less.

Further, when the tensile strength is 400 MPa or more, sufficient strength is obtained even when applied to a thick plate.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of pursuing the cause of toughness deterioration due to strain aging, the present inventors have found that the cause lies in solid solution C and solid solution N in steel, and it is only necessary to make these harmless. Knew.
However, if excessive addition of a carbonitride forming element such as Ti or Nb to render the solid solution C or solid solution N harmless, TiN
It has been found that a large amount of precipitates such as Nb and NbC are generated, and the toughness of the base material is easily deteriorated. Therefore, the solid solution C in steel
As a result of intensive studies on other techniques for detoxifying solid solution N, it has been found that the C element and the N element can be fixed to the crystal grain boundary portion, thereby reducing the toughness deterioration due to strain aging. Therefore, the relationship between the crystal grain boundaries in the steel and the solid solution C and solid solution N was repeatedly examined.

As a method for making the crystal grain boundaries appear, a method of observing using a corrosive liquid called nital (3% HNo ₃ alcohol solution) is generally adopted. However, it has been confirmed that even when the crystal grains confirmed by this method are fine, the amount of reduction in toughness degradation due to strain aging varies. However, after further research, EBSP (Electron Back Scattering Pattern)
As a result of observing the structure of the steel material using an analyzer, it was found that the large-angle grain boundaries effectively acted to reduce the toughness degradation. Here, the crystal orientation of the crystal adjacent to the large-angle grain boundary is
This means a grain boundary existing between crystal grains different by 15 ° or more. When C or N is fixed to the large-angle grain boundary, both elements are stably present at the grain boundary. That is, even if a steel sheet containing the C element and the N element is manufactured so as to generate a large number of large-angle grain boundaries in the structure, the C element and the N element are fixed to the large-angle grain boundaries and subjected to plastic deformation. It is considered that C and N do not adhere to the dislocations introduced into the alloy, so that deterioration of toughness due to strain aging is reduced.

As described above, the present invention has the largest point in finding out that the large-angle grain boundaries that can fix the C element and the N element are effective in reducing the deterioration of toughness. The intended purpose has been achieved by refining the crystal grains so that large-angle grain boundaries of 15 ° or more are generated in large quantities. The most important point of the present invention can be obtained only by clearly distinguishing between the large-angle grain boundaries and the small-angle grain boundaries, and the conventional observation means using a nital etching solution observes only the apparent crystallinity of the steel material. This is a finding that could not be obtained because it cannot be done.

Further, in the present invention, it is important to appropriately control the amounts of C and N in the steel material in addition to the control of the large-angle grain boundaries. In other words, even if the crystal grains can be refined so as to generate large-angle grain boundaries, if the C and N contents are large, the C and N amounts that can be fixed to the grain boundaries exceed the allowable range, and excessive amounts of elements cause distortion. This is because it concentrates on the part and causes toughness degradation due to strain aging. When C or N is added in a large amount, Ti or Nb which is a carbonitride forming element may be added to precipitate as TiN or NbC. N not trapped by Ti
(Hereinafter sometimes referred to as "free N") and C not trapped by Nb (hereinafter sometimes referred to as "free C"), and the strength characteristics can be obtained only by appropriately controlling the amount of large-angle grain boundaries. It is possible to provide a steel sheet which is excellent and has little toughness deterioration due to strain aging. Hereinafter, the requirements of the present invention will be described in detail.

First, the steel sheet according to the present invention, which has less toughness deterioration due to strain aging, has a ferrite-based or bainite-based microstructure.

Here, “mainly composed of ferrite” means that the microstructure of the steel sheet is substantially composed of a ferrite structure. Specifically, the area ratio of ferrite is preferably 80% by area or more, more preferably 90% by area.
That is all. Examples of the structure other than ferrite include pearlite, bainite structure, martensite, and the like, and the ratio of these is preferably 10 area% or less. By the way, if there are too many of these other structures, the toughness of the base material (material to which no strain is applied) will deteriorate,
There are disadvantages such as a decrease in ductility.

"Bainite mainly" means that the microstructure of the steel sheet is substantially constituted by a bainite structure. Specifically, it means that the occupancy ratio of the bainite structure is about 60% by area or more. Examples of the structure other than the bainite include a ferrite structure, a pearlite structure, and martensite.

Next, the steel sheet of the present invention is made of EBSP (Electron Bac
The relationship between the average grain size D and the contents of C and N in steel, where D is the average grain size of crystal grains having a crystal orientation difference of 15 ° or more by crystal orientation analysis using k scattering pattern). It is important that the following formula (1) is satisfied. here,
The average particle diameter means a diameter of a circle assumed to have an equal area for each crystal grain of a predetermined structure.
[Element] in the following formula represents the content [% by mass] of each element in the steel material. (4.63 × [C] + 3.97 × [N]) × D / 15 ≦ 0.4 (1)

First, in the present invention, the above-mentioned microstructure is
Observe the crystal orientation difference by crystal orientation analysis using EBSP (Electron Back Scattering Pattern). Specifically, a TexSEM Laboratories device was used as an EBSP analyzer, and crystal grains with a crystal orientation difference of 15 ° or more were observed by color change based on the crystal orientation criteria shown in Fig. 1 (a). I do. An example of a photograph (color map) of a cross section in the thickness direction observed by the EBSP analyzer is shown in FIG. 1 (b). Figure
From FIG. 1 (b), it is understood that the crystal grain having a crystal orientation difference of 15 ° or more can be identified by a change in color tone when EBSP is used.
The above equation (1) is applied to steel containing substantially no Ti or Nb, and the number of free C and free N occupying in the large angle grain boundary is calculated by the numerical value of the above equation (1). This indicates that the desired characteristics can be obtained by controlling the range.

Here, in the above formula (1), 4.63 × [C] and 3.97 × [N] represent the amount of C element and the amount of N element (mass%) present in the steel in atomic% (atom.%), Respectively. ) Is expressed as the number of atoms. The above equation (1) expresses the ratio of the total amount of atoms calculated in this way to the large-angle crystal grain boundaries, and is expressed by the above equation (1). And the value derived from the grain boundary volume: D / 15). In setting the above equation (1), it is necessary to specify the grain boundary thickness. However, in the crystal grain boundaries existing between crystal grains having a crystal orientation difference of 15 ° or more, Fe atoms and the like are not present. Exist in the rules,
Since the thickness (the thickness of the crystal grain boundary) is about 5 to 10 °, in the present invention, the thickness was calculated by approximating the thickness of the grain boundary to 5 °.

Further, the following equations (2) to (4) or the following equations (5) to (7) apply to steel materials containing Ti and / or Nb in the steel. [N] − [Ti] /3.42≧0 (2) [C] − [Nb] /7.74≧0 (3) ｛4.63 × ([C] − [Nb] /7.74) +3. 97 × ([N] − [Ti] /3.42)｝ × D / 15 ≦ 0.4 (4) [N] − [Ti] /3.42 <0 (5) [C] − [Nb] /7.74≧0 (6) {4.63 × [C] − [Nb] /7.74 − ([Ti] −3.42 × [N]) / 3.99} × D / 15 ≦ 0.4 (7)

This is specified to precipitate C and N in steel as carbides and nitrides by adding Ti and Nb. In the steel sheet, the above-mentioned "Amount of C and N in steel" In addition to the above control, it is important to appropriately control the "amount of Ti and / or Nb". Excessive addition exceeding the amount of Ti or Nb that can combine with C or N adversely affects the toughness and the weldability.

Among them, the above equations (2) to (4) show that the amount of N is larger than that of added Ti and that there is a lot of free N that does not bond with Ti [the above equation (2)]. In the case where the amount of C is large and there is a lot of free C that is not bonded to Nb [Equation (3)], the amount of free C or free N occupying in the large-angle grain boundary as in the above equation (4). It is shown that a desired effect can be obtained by appropriately controlling. Some of the free N and free C that do not bond with Ti and Nb combine with other elements in the steel to form nitrides and carbides, but the free N and free C that do not bond still have toughness due to strain aging. It causes deterioration. In such a case, the amount of free C or free N occupying in the large-angle grain boundaries may be appropriately controlled so as to satisfy the relationship of the above equation (4).

On the other hand, in the above equations (5) to (7), the amount of N is smaller than that of the added Ti, and no free N is present [the above equation (5)]. Many,
In the case where there are many free Cs that do not bind to Nb,
This shows that a desired effect can be obtained by appropriately controlling the amount of free C occupying in the large-angle grain boundaries as in the above equation (7). That is, Ti present in the steel combines with free C and precipitates as carbide.

In the present invention, the case where the added Nb is larger than the amount of C and free C hardly exists is not particularly assumed, but this is because it is difficult to assume such an aspect at the actual operation level. .

The steel sheet of the present invention does not suppress the addition of C and N, but can be added as needed, so that the suppression of the addition of C does not cause a decrease in tensile strength. Therefore, a material having a strength of 400 MPa or more can be provided.

The requirements that characterize the present invention have been described above. In the present invention, the method of refining the crystal to produce a desired large-angle grain boundary depends on the type of steel, etc. , The cumulative equivalent plastic strain during finish rolling, the cooling temperature, etc. may be appropriately controlled.

Specifically, if the heating temperature is high, the initial austenite grains are likely to become coarse, so that a lower heating temperature is desirable. However, even when the heating temperature is high, by increasing the cumulative rolling reduction of the rough rolling, the recrystallization of austenite progresses to make the austenite fine, and as a result, the ferrite structure and the bainite structure can be made fine. Furthermore, it is desirable to increase the amount of cumulative equivalent plastic strain during finish rolling because the ferrite structure and the bainite structure are more likely to be refined. Also, when water cooling is adopted as the cooling method, the ferrite structure and the bainite structure are easily refined, and water cooling is recommended when the refinement of crystal grains is insufficient. However, these methods
There is no need to satisfy all, and it is important to make each item appropriate according to the amount of the added component.

Next, the chemical components in the steel of the steel sheet of the present invention will be described. As described above, the steel sheet of the present invention has a crystal orientation
Generate crystal grains of 15 ° or more, and C present at the crystal grain boundaries
And the most important point in appropriately controlling the solid solution amount of N and the chemical composition in the steel is not particularly limited, but a preferred chemical composition for obtaining a ferrite-based or bainite-based microstructure is It is as follows.

C: 0.03% to 0.2% The C content is preferably 0.03% or more, more preferably 0.05% or more, in order to secure necessary strength. However, excessive content adversely affects weldability and base metal toughness.
It is desirable that the content be suppressed to 0.17% or less.

N: 0.01% or less N forms nitrides with additional elements such as Al, Ti, Nb, and V contained in steel, and has an effect of refining the base metal structure. However, if the content exceeds 0.01% and becomes too large, an increase in solid solution N is caused, and particularly, the toughness of a welded portion is deteriorated.
It is preferable to keep the content to 0.01% or less.

In order to make C and N harmless, Ti
It is also effective to add Nb or Nb to precipitate as TiN or NbC. The amount of Ti or Nb added in this case is not particularly limited, but one example is shown below.

[0034] Ti: 0.05% or less and / or Nb: 0.03%
Hereinafter, these elements are elements that have an effect of suppressing the coarsening of austenite grains during heating of a slab, an action of promoting the formation of ferrite transformation nuclei after rolling, or an effect of suppressing the recrystallization of austenite grains, thereby reducing the size of ferrite grains. Specifically, Ti can reduce free N in steel by forming nitrides. However, even if added in excess of 0.05%, the base material toughness is deteriorated.
Is preferable.

Nb exerts an austenite grain coarsening effect during rolling and a recrystallization suppressing effect due to the formation of carbonitrides, and is an element effective in refining ferrite grains after the rolling is completed. Since it combines with free C to form carbide, the amount of free C can be reduced. However, an excessive addition exceeding 0.03% deteriorates the weldability. Therefore, it is preferable to set the upper limit to 0.03%.

The steel sheet of the present invention contains C and N elements as essential elements, and Ti and Nb are added as necessary. Therefore, the balance may include elements conventionally added as structural steel, Fe, inevitable impurities, and trace allowable elements. Examples of commonly added elements and their amounts are shown below.

Si: 0.5% or less Si is an element useful as a component for improving the strength of the base material and deoxidizing the molten steel.
% Is desirably contained. However, if the content is too large, the weldability and base metal toughness will deteriorate,
0.5% or less, more preferably 0.45% or less.

Mn: 1.8% or less Mn is useful as an element for increasing the strength of the base material, and for that purpose it is preferable to contain 0.5% or more, and more preferably 0.7% or more. However, excessive content causes deterioration of weldability and base metal toughness, so that the content is 1.8% or less, more preferably 1.6% or less.
%.

Al: 0.01 to 0.1% Al is not only useful as a deoxidizing agent, but also forms a nitride and contributes to the refinement of the base metal structure. These effects
0.01% or more, more preferably 0.015% or more is effective, but if it exceeds 0.1% excessively, the base material toughness deteriorates, so 0.1% or less, more preferably 0.05% or less.
% Is desirable.

It is also effective to positively add the following elements as other elements for the purpose of imparting further characteristics.

V: 0.05% or less and / or B: 0.002% or less
Lower V, like Nb, exhibits austenite grain coarsening during rolling and suppresses recrystallization by forming carbonitrides,
It is an element effective for refining ferrite grains after rolling.
To effectively exert such an effect, it is preferable to add 0.002% or more. However, if added in excess of 0.05%, the weldability deteriorates, so the upper limit is preferably set to 0.05%.

B is an element effective for improving the toughness of the heat affected zone (HAZ), and is preferably added in an amount of 0.0002% or more in order to effectively exert such an effect. However, if added in excess of 0.002%, hardenability increases,
Since the low-temperature toughness of the base material deteriorates, the upper limit is 0.002.
% Is preferable.

Cu: 0.5% or less and / or Ni: 0.5% or less
Under these elements are all elements contributing to the improvement of the miniaturization and low-temperature toughness of the austenite crystal grains.

Specifically, Cu is an element effective for refining crystal grains, and it is preferable to add 0.2% or more in order to effectively exert such an effect. However, the addition of a large amount deteriorates the weldability of the base material, so the upper limit is preferably set to 0.5%.

Ni is an element effective for improving the low-temperature toughness, but is expensive, so its upper limit is preferably 0.5%.

These elements may be used alone,
Alternatively, they may be used in combination. However, since hot cracking may occur when Cu is added alone, it is preferable to add Ni at the same time to prevent hot cracking.

Cr: 0.1% or less and / or Mo: 0.1% or less
Under these elements, either precipitated carbonitrides, is an element contributing to the increase in strength, in order to develop this function effectively, it is preferable to add any of the elements even 0.03% or more. However, excessive addition deteriorates weldability and base metal toughness, so the upper limit is preferably set to 0.1%.

[0048] Ca: 0.01% or less and / or Zr: 0.01%
Hereinafter, these elements have the effect of increasing the toughness of the base material by spheroidizing the form of inclusions in the steel.

Of these, Ca has the effect of improving the toughness of the base material by spheroidizing the form of inclusions in the steel. 0.0005% to effectively exert such effects
It is preferable to add the above. However, an excessive addition adversely degrades the toughness of the base material, so the upper limit is preferably set to 0.01%.

Like Ca, Zr has the effect of improving the toughness of the base metal by making the form of inclusions in the steel spheroid. In order to exert such effects effectively, it is necessary to use 0.
It is preferable to add 003% or more. However, excessive addition adversely degrades the toughness of the base material, so the upper limit is 0.01%.
It is preferable that

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention, and any design changes based on the above and following gist will be described. Are included within the technical scope of

[0052]

EXAMPLES Using steel types 1 to 5 shown in Table 1, the steel sheets were heated under the conditions shown in Table 2, rolled in two stages, and then cooled to produce steel sheets having the structure shown in Table 3. Using an EBSP analyzer manufactured by TexSEM Laboratories, each of the obtained steel sheets was observed for crystal grains having a crystal orientation difference of 15 ° or more in the surface layer region (at a depth of 1.5 mm from the surface), and the average crystal grain size was measured.
D was calculated.

In the case of steel types 1 and 2 shown in Table 1, the above (1)
Calculate the value on the left side of the equation. For steel types 3 and 4, above (4)
The value on the left side of the equation was calculated, and in the case of steel type 5, the value on the left side of the above equation (7) was calculated. Table 3 shows the values of these equations as parameter values.

In order to measure the properties of the obtained steel material, the amount of deterioration in toughness when a steel material to which a strain of 10% was previously applied was subjected to an aging treatment at 250 ° C. for 1 hour was calculated and evaluated. Here, the toughness deterioration amount (° C.) means a difference in brittle fracture transition temperature (vTrs), and is calculated from “brittle fracture transition temperature when 10% strain is applied” to “brittle fracture transition temperature of base material”. It is the value subtracted. A small amount of toughness deterioration indicates that there is little toughness deterioration due to strain aging. The tensile test was performed using a JIS No. 1A test piece.
In the table, YP or YS (MPa) is the yield strength and TS (M
Pa) indicates the tensile strength.

Table 3 shows the results. In addition, the value (parameter value) of the above formula (1) or (4) or (7)
Fig. 2 shows the relationship between the toughness degradation amount (° C).

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

The following can be considered from Table 3 and FIG.

Nos. 1 to 12 are examples satisfying the requirements of the present invention, and the value (parameter value) of the above equation (1), (4) or (7) is 0.4 or less. The transition temperature of toughness fracture surface of the base metal varies depending on the test piece, but even when the aging treatment is performed after 10% strain imparting, the toughness deterioration amount is small and 20 ° C.
It can be seen that: That is, those satisfying the requirements of the present invention had good balance of solid solution amounts of C and N in the large-angle grain boundaries, so that the balance was good and the deterioration of toughness due to strain aging could be reduced. Furthermore, in the example of the present invention, since a sufficient amount of C for imparting strength is contained, the tensile properties are also good.

On the other hand, Nos. 13 to 22 are comparative examples which do not satisfy the requirements of the present invention. Since the parameter values defined by the above formula (1) or (4) or (7) exceed 0.4, the amounts of C and N atoms are not properly present in the large-angle grain boundaries. When the amount of free N is large and strain is applied, C and N adhere to introduced dislocations and the like, and toughness deteriorates. In addition, the tensile properties also deteriorated.

No. 1-3 and No. 13-14, No. 4-5 and No. 15-1
6, No.6-7 and No.17-18, No.8-9 and No.19-20, No.10-
Comparing No. 12 with Nos. 21 to 22, respectively, the chemical composition of the steel type is the same, but there are cases where the requirements of the present invention are satisfied and cases where the requirements are not satisfied. In other words, it can be seen that the balance between the volume of the large-angle grain boundaries and the solid solution amounts of C and N changes depending on the manufacturing conditions. In such a case, the heating temperature, the cumulative rolling reduction of the rough rolling, the cumulative equivalent plastic strain of the finish rolling, the cooling method, and the like may be appropriately changed.

[0063]

According to the present invention, it is possible to provide a steel sheet which maintains the tensile strength required for structural steel and has less toughness deterioration due to strain aging.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an EBSP color map of a cross section in the thickness direction of a steel sheet obtained by the present invention.

FIG. 2 is a graph showing a relationship between a parameter value and a toughness deterioration amount.

Claims (6)

[Claims] 1. A steel sheet having a microstructure mainly composed of ferrite or bainite, comprising C and N,
When the average grain size of crystal grains having a crystal orientation difference of 15 ° or more as determined by crystal orientation analysis using an SP (Electron Back Scattering Pattern) is defined as D, the relationship between the average grain size D and the contents of C and N described above. Satisfies the following expression (1): a steel sheet with less toughness deterioration due to strain aging. (4.63 × [C] + 3.97 × [N]) × D / 15 ≦ 0.4 (1) Here, [element] is the content [mass%] of each element in the steel material.
Represents each. 2. C: 0.0% by mass (hereinafter referred to as%)
The steel sheet according to claim 1, which contains 3 to 0.2%. 3. A steel sheet having a microstructure mainly composed of ferrite or bainite, containing C and N, further containing Ti and / or Nb, and comprising an EBSP (Electron Backing).
When the average grain size of crystal grains having a crystal orientation difference of 15 ° or more by a crystal orientation analysis using Scattering Pattern is D, the average grain size D is compared with the average grain size of C, N, Ti and / or Nb.
Characterized by satisfying the following formulas (2) to (4) with respect to the content ratio of: [N] − [Ti] /3.42≧0 (2) [C] − [Nb] /7.74≧0 (3) ｛4.63 × ([C] − [Nb] /7.74) +3. 97 × ([N] − [Ti] /3.42)｝ × D / 15 ≦ 0.4 (4) where [element] is the content [mass%] of each element in the steel material.
Represents each. 4. A steel sheet having a microstructure mainly composed of ferrite or bainite, containing C and N, further containing Ti and / or Nb, and comprising an EBSP (Electron Backing).
When the average grain size of crystal grains having a crystal orientation difference of 15 ° or more by a crystal orientation analysis using Scattering Pattern is D, the average grain size D is compared with the average grain size of C, N, Ti and / or Nb.
Characterized by satisfying the following formulas (5) to (7): [N] − [Ti] /3.42 <0 (5) [C] − [Nb] /7.74≧0 (6) ｛4.63 × [C] − [Nb] /7.74 − ([Ti ] −3.42 × [N]) / 3.99｝ × D / 15 ≦ 0.4 (7) where [element] is the content of each element in the steel material [mass%]
Represents each. 5. C: 0.0% by mass (hereinafter referred to as%)
The steel sheet according to claim 3, wherein the steel sheet contains 3 to 0.2%, and contains 0.05% or less of Ti and / or 0.03% or less of Nb. 6. The steel sheet according to claim 1, having a tensile strength of 400 MPa or more.