JP5919812B2 - High strength thin steel sheet with excellent formability and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength thin steel sheet excellent in formability (particularly bending workability) suitable for a material of a member such as a building material, a home appliance, and an automobile, and a manufacturing method thereof.

  In the fields of building materials, home appliances, automobiles, etc., the thickness of materials used has been reduced from the viewpoint of reducing the weight of products, etc., and the 0.8mm thick TS340MPa grade steel plate widely used as a material for these materials is 0.7 It is being replaced by steel sheets with a thickness of mm or even 0.6 mm.

However, the thickness of the steel plate used as a raw material greatly affects the characteristics of the member. Therefore, it is not easy to reduce the thickness of the member (steel plate) while maintaining the desired characteristics required for the member.
For example, the dent resistance A of a flat plate member becomes better as the plate thickness t and tensile strength TS of the member (or steel plate as a material) increases, and is generally evaluated by the relational expression of “A∝t × t × TS”. The Therefore, the thinning of the steel plate (member) according to the same steel type standard causes a decrease in the dent resistance of the member, which causes a problem.

  Therefore, increasing the strength is essential for reducing the thickness of the member. Specifically, according to the above relational expression (A∝t × t × TS), while maintaining the same dent resistance as the current material “0.8mm thickness, TS340MPa class” steel sheet, the sheet thickness of the steel sheet is 0.7 When thinning to mm and 0.6 mm, it is necessary to increase the tensile strength to TS: 440 MPa and 600 MPa.

  In order to increase the strength of steel sheets, there are roughly used solid solution strengthening by increasing the solid solution strengthening and dispersion strengthening that precipitates the second phase finely and in large quantities. However, in these strengthening mechanisms, it is necessary to add many solid solution strengthening elements and precipitation strengthening elements, which increases the manufacturing cost. Moreover, in many cases, a steel sheet that is a member of a building material, a home appliance, an automobile, or the like is subjected to a plating treatment for the purpose of ensuring corrosion resistance. However, when many elements are added, there is a problem that the plating adhesion is impaired.

In order to solve such a problem, there has been established a technique for increasing the strength of a steel sheet by work hardening in a cold rolling process at the time of manufacturing the steel sheet without increasing the additive elements.
For example, Patent Document 1 and Patent Document 2 propose a technique for increasing the strength of a steel sheet by performing cold rolling at the end of the manufacturing process of the steel sheet. However, work-hardened steel is very poor in workability although it is high strength. For this reason, it is extremely difficult to obtain a steel plate with excellent workability by these techniques that omits the annealing step after cold rolling, and there are various obstacles when forming a steel plate into a desired member shape by pressing or the like. Bring

  In contrast to these technologies, Patent Document 3 relates to a method for producing steel sheets for cans used for various containers, and hot rolling is performed on a rolling material having a predetermined composition to form a hot rolled sheet, and then the hot rolled sheet. A technology has been proposed in which the sheet is cold-rolled and then soaked in a continuous annealing process at a temperature range of 500 ° C or higher and a recrystallization rate of less than 90%, preferably 60% or more and less than 90%. . And according to the technique concerned, it is supposed that the ultra-thin cold-rolled steel plate for high-strength cans excellent in workability will be obtained by performing continuous annealing in a partial recrystallization state.

  However, in the partially recrystallized state, variation in strength increases, and it is difficult to obtain a high-strength steel plate with stable strength.

On the other hand, Patent Document 4 relates to a method for producing a cold-rolled steel sheet excellent in bending workability used in the fields of electric machinery, building materials, automobiles, etc., in mass%, C: 0.025% or less, Si: 0.1% or less, Mn: 0.05 to 0.5%, P: 0.03% or less, S: 0.02% or less, sol.Al: 0.01 to 0.1%, steel having a component composition consisting of Fe and inevitable impurities, the Ar ₃ transformation point After hot rolling at the above finishing temperature, winding at a coiling temperature of 500 ° C. or more and 650 ° C. or less, pickling, rolling rate within the range of 85% or less, and tensile strength TS of the steel sheet after cold rolling is A technique has been proposed in which cold rolling is performed so that the sheet thickness is 390 MPa or more and the plate thickness is 0.4 mm or more, or recovery annealing is further performed after cold rolling.

  And according to the technique proposed in Patent Document 4, by adjusting the rolling rate (also referred to as the reduction rate) of cold working to increase the strength by work hardening, the subsequent recovery annealing is performed to improve the bending process. It is said that both high strength with a tensile strength TS of 390 MPa and higher and excellent bending workability that enables severe 90 degree bending with a punch tip curvature of 2R or less can be achieved. In the technique proposed in Patent Document 4, it is possible to perform adhesive bending by making the steel sheet into a ferrite rolled structure and suppressing the C content of the steel to 0.0040% or less and suppressing the precipitation amount of cementite. It is supposed to be.

JP-A-8-176664 JP 2000-87184 A JP 2001-107187 A JP 2010-229545 A

  However, in the technique proposed in Patent Document 4, when the obtained steel plate is subjected to a 90-degree bending portion or a contact bending portion and the presence or absence of rough skin is confirmed by observing the processed portion, the thickness of the steel plate is 0.8. In the case of mm, rough surface is not confirmed, but roughening may be confirmed if the rolling rate of cold working (also referred to as the rolling reduction) exceeds 75% and the sheet thickness is reduced to 0.7 mm or less. It was. Therefore, application to members that require designability is limited, and problems are seen in terms of versatility. Furthermore, in the technique proposed in Patent Document 4, that is, a technique for adjusting the rolling reduction ratio of the cold working to increase the strength of the steel sheet, the rolling reduction ratio of the cold working is increased to over 70% for the purpose of ensuring the strength. However, when the steel sheet produced by setting to be subjected to close contact bending, cracking does not occur, but the thickness of the processed part may become extremely thin. When the plate thickness of the processed portion is significantly reduced, the rigidity of the processed portion is extremely lowered. Therefore, when such a steel plate is applied to a member that requires a predetermined strength, there is a problem that a crack or the like is caused when the member is used. There was also a problem of breaking when the bent part was returned to the flat state.

  As described above, in the prior art, when thinning a steel sheet, it has been extremely difficult to achieve high strength while maintaining workability that can withstand severe processing conditions such as tight bending. Therefore, in the prior art, excellent dent resistance is required and complex shaped members that are molded under severe processing conditions, such as building materials, home appliances, automobiles, etc., have various troubles and are improved. There was room for it.

  The present invention advantageously solves the above-mentioned problems of the prior art, and is suitable for a material of a member such as a building material, a home appliance, and an automobile, and is a high strength thin steel sheet excellent in formability (particularly bending workability), specifically The tensile strength TS is 400MPa or more, more preferably 500MPa or more. Even if it is subjected to close contact bending, the bending part is not cracked, the plate thickness is reduced extremely, and the skin is not rough. It aims at providing a strength thin steel plate and its manufacturing method.

In order to solve the above-described problems, the present inventors diligently studied various factors affecting the strength and bending workability (particularly, crack resistance and surface properties of the closely bent portion) of a thin steel sheet having a ferrite structure.
First, the present inventors performed adhesion bending processing on a thin steel plate having a tensile strength TS: 500 MPa class obtained by utilizing work hardening, and investigated the formability of the thin steel plate. And it was confirmed that as the work hardening amount is larger, the bent portion has a tendency to significantly reduce the thickness of the bent central portion although it does not cause rough skin, cracks, or cracks during close contact bending. In addition, it was confirmed that by subjecting the thin steel plate to an annealing treatment, surface roughness and cracking in the contact bending processed portion, or a reduction in plate thickness at the center of bending was significantly suppressed.

  Therefore, the present inventors made a steel sheet having a ferrite structure (sheet thickness: 0.6 mm thin steel sheet) by performing cold working under various conditions, collected bending test pieces from these steel sheets, Adhesion bending processing (bending direction: C direction) was performed, and the adhesion bending processed portion was observed to confirm the degree of roughness of the skin and the presence or absence of cracks, and the thickness reduction rate at the center of bending was measured. In addition, after the cold working, annealing under various conditions is performed, and the adhesive bending process (bending direction: C direction) is performed in the same manner as the cold-worked steel sheet, and the adhesion bending process part is observed to observe the degree of rough skin and cracks. The presence or absence was confirmed, and the thickness reduction rate at the center of bending was measured. Here, the C direction is a direction orthogonal to the rolling direction of the steel sheet.

In addition, as shown in FIG. 5, the roughness degree of the rough surface is obtained by observing the cross section (C direction cross section) in the width direction of the test piece after the contact bending process, The point A in FIG. 5 is connected with a straight line, and the maximum distance L between the straight line and the convex portion (point B in FIG. 5) (the maximum value of L ₁ , L ₂ , L ₃ ... In FIG. 5). Quantified with.
Further, as shown in FIG. 6, the plate thickness reduction rate was measured by observing the cross section (C direction cross section) in the width direction of the test piece after the contact bending process, and the thickness at the bending center section (t in FIG. 6). It was measured, - the value obtained by (the thickness t ₀ of the unprocessed plate thickness t of the bent central portion of the adhesion after bending) / (plate thickness t ₀ of the previous processing).

  As a result, it was confirmed that all of the cold-worked steel sheets had a plate thickness reduction rate of 0.10 (10%) or more, and were in a state in which cracks were likely to occur in the contact bending process portion. Moreover, in all the steel sheets as cold-worked, the roughness of the skin of the tightly bent portion was 5 μm or more. Furthermore, even steel sheets that have been annealed following cold working tend to be more prone to cracking and roughening in the tightly bent parts as the rolling rate (work hardening amount) of cold working increases. It became clear. For these steel sheets, the reduction rate of cold working (work hardening amount) is reduced, and the thickness reduction rate of all steel sheets that have been annealed following cold working is 0.05 (5%) or less. It was confirmed that cracks did not occur and good adhesion bending workability was exhibited. In addition, in each of these steel plates, the roughness degree of the tightly bent portion was 3 μm or less.

  As described above, the inventors of the present invention reduced the cold rolling reduction rate (work hardening amount) and produced an annealing process at an appropriate temperature when manufacturing a thin steel sheet having a sheet thickness reduced to 0.6 mm. As a result, it was found that a thin steel sheet having excellent bending workability that can withstand severe processing such as contact bending is obtained. However, there remains a problem that a desired strength cannot be obtained with a thin steel sheet having a reduced cold work rolling rate (work hardening amount) as described above.

  Therefore, the present inventors sought a means to increase the strength of the thin steel sheet while reducing the work hardening amount (rolling rate of cold working) as much as possible, and utilized N, which is an inexpensive element, as a solid solution strengthening element. I came up with the idea. According to the solid solution strengthening mechanism using N as a solid solution strengthening element, the plate thickness reduction rate and skin roughness during close contact bending are maintained at a relatively low value, and the steel plate can be made high without impairing close contact bending workability. It was found that it was possible to increase the strength.

In addition, as described above, when manufacturing a thin steel sheet, a thin steel sheet having excellent bending workability is obtained by performing an annealing treatment at an appropriate temperature following cold working, but the strength of the thin steel sheet is reduced due to the annealing treatment. Is inevitable. Therefore, the present inventors secure a desired tensile strength TS (400 MPa or more) for a thin steel sheet having a ferrite structure utilizing a solid solution strengthening mechanism using N as a solid solution strengthening element together with a strengthening mechanism by work hardening. Annealing conditions were investigated. As a result, when manufacturing a thin steel sheet, the steel composition is prescribed to a predetermined composition, and the annealing temperature AT (° C.) is set so as to satisfy the following formula, so that the annealed thin steel sheet has a pressure of 400 MPa or more. It was found that the tensile strength TS was maintained.
Record
AT (℃) ≦ (400−152−7.3 × CR−155 × [N] × 100 / (3.8 × Nb−0.26)
(CR: Rolling ratio in cold rolling process (%))
(Nb: Nb content (mass%). However, Nb = 0 when steel does not contain Nb.)
([N]: content of solid solution N contained in the cold-rolled sheet before the annealing process (mass%))

  Experiments leading to the above formula, that is, each of the cold work rolling rate CR (%), solute N content (mass%), Nb content (mass%) and annealing temperature AT (° C.) An experiment for investigating the influence on the tensile strength TS (MPa) of a thin steel sheet after annealing is described below.

C: Hot rolled to a steel containing 0.003% to obtain a hot rolled sheet, and the hot rolled sheet was subjected to cold rolling at various rolling rates (40 to 70%) to obtain a cold rolled sheet having a ferrite structure (plate) Thickness: 0.6mm). From the obtained cold-rolled sheet, a JIS No. 5 tensile test piece with the rolling direction as the tensile direction was sampled and subjected to a tensile test based on the provisions of JIS Z 2241 to measure the tensile strength TS ₀ in the rolling direction. The measurement results are shown in FIG. Figure 1 shows the effect of rolling ratio of cold rolling on the rolling direction tensile strength TS _0, as rolling ratio CR is increased 1% rolling direction tensile strength TS ₀ of the cold-rolled sheet is increased 7.3MPa I understand that.

Moreover, the cold-rolled sheet obtained above was subjected to annealing treatment at various annealing temperatures AT (° C.) (annealing time: 100 s) to obtain an annealed sheet. Next, a JIS No. 5 tensile test piece with the rolling direction as the tensile direction was taken from the obtained annealed plate, a tensile test based on the provisions of JIS Z 2241 was performed, and the tensile strength TS in the rolling direction was measured. And from these measurement results, the recovery rate ρ during annealing treatment (reduced amount of strength of annealed sheet (MPa / ° C) with increasing annealing temperature) was calculated, and the rolling direction of the unannealed cold rolled sheet previously obtained The relationship between the tensile strength TS ₀ and the recovery rate ρ was determined. 2, the recovery rate ρ (MPa / ℃) to exert shows the effect of rolling direction tensile strength TS ₀ of the unannealed cold-rolled sheet, as the rolling direction tensile strength TS ₀ of the unannealed cold-rolled sheet is increased 100MPa It can be seen that the recovery rate ρ increases by 0.08 MPa / ° C.

  Also, hot rolled steel (sheet thickness: 1.5 mm) was hot rolled on steel containing C: 0.003% similar to the above, and the hot rolled sheet was cold rolled at a rolling rate of 60% to obtain various solids. A cold-rolled sheet (thickness: 0.6 mm) having a ferrite structure with a dissolved N content (mass%) was used. In addition, the amount of solid solution N (mass%) contained in a cold-rolled sheet was adjusted by changing hot rolling conditions (rolling temperature, winding temperature, etc.). Further, the tensile strength TS in the rolling direction was measured in the same manner as described above for the annealed plates obtained by annealing the obtained cold-rolled plates at various annealing temperatures (450 to 500 ° C.). Moreover, the solid solution N content of the cold rolled sheet and the annealed sheet was measured, and it was found that there was no change in the solid solution N content before and after annealing. The measurement result of TS is shown in FIG. FIG. 3 shows the effect of the solute N amount (mass%) of the annealed sheet (cold rolled sheet) on the tensile strength TS in the rolling direction of the annealed sheet, and the annealed sheet increases as the solute N amount increases by 0.01 mass%. It can be seen that the tensile strength TS in the rolling direction increases by 155 MPa.

  Further, steels with various Nb contents are prepared by adding 0 to 0.035% by mass of Nb to the above-mentioned C: 0.003% steel, and these steels are hot-rolled to form hot-rolled sheets. The steel sheet was cold-rolled (rolling rate: 60%) to obtain a cold-rolled sheet having a ferrite structure (thickness: 0.6 mm). Subsequently, the various cold-rolled plates obtained were subjected to annealing treatment at various annealing temperatures (annealing time: 100 s) to obtain annealed plates. From the obtained annealed plate, a JIS No. 5 tensile test piece with the rolling direction as the tensile direction was sampled and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to measure the tensile strength TS in the rolling direction. And from these measurement results, the recovery rate ρ during annealing (the amount of decrease in the strength of the annealed plate (MPa / ° C.) accompanying the increase in the annealing temperature) was determined. FIG. 4 shows the influence of the steel plate Nb content (mass%) on the recovery rate ρ (MPa / ° C.). As the Nb content contained in the steel plate increases by 0.01 mass%, the recovery rate ρ becomes 0.038 MPa / ° C. It turns out that it decreases.

1-4, the rolling direction tensile strength TS (MPa) of the annealed sheet, the rolling rate (%) of the cold rolling, and the solid solution N content (mass of the annealed sheet (cold rolled sheet)) %), Nb content (% by mass) in steel, and annealing temperature AT (° C), a proportional relationship is recognized, and the rolling direction tensile strength TS (MPa) of the annealed sheet is expressed by the following formula: Can do.
TS = 7.3 (MPa) × CR + 155 (MPa) × [N] × 100 + (3.8 × Nb−0.26) × AT + Const.
In the above formula, [N] is the solid solution N content (mass%) of the annealed sheet (cold rolled sheet), Nb is the Nb content (mass%) in the steel, and Const. Is a constant.

Also, by substituting each data (TS, CR, [N], Nb, AT) obtained by the above experiment into the above formula, a constant (Const.) Is calculated for each data, and the average value of these is constant. When used as (Const. = 152 (MPa)) and further transforming the above equation, an equation representing the annealing temperature AT (° C.) is obtained as shown below.
AT (° C) = (TS−152−7.3 × CR−155 × [N] × 100) / (3.8 × Nb−0.26)… (1A)
Therefore, when the tensile strength TS in the rolling direction of the annealed sheet is set to 400 MPa or more, that is, when the lower limit value (TS _L ) of the annealed sheet is set to 400 MPa, the annealing temperature AT (° C. that satisfies the following formula: ) To be annealed.
AT (° C) ≦ (400−152−7.3 × CR−155 × [N] × 100) / (3.8 × Nb−0.26)

  Furthermore, the present inventors performed the structure observation (structure before the adhesive bending process) of the thin steel sheet having a rolling direction tensile strength TS of 400 MPa or more among the thin steel sheets obtained by the above experiment. Subsequently, after performing close contact bending (bending direction: C direction), the degree of skin roughness and the plate thickness reduction rate were measured. As a result, it is clear that all the thin steel sheets that showed excellent adhesion bending workability (skin roughness: 3 μm or less, sheet thickness reduction rate: 0.05 or less) have a recovery structure mainly composed of a ferrite phase. It became.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%
C: 0.0005% or more and 0.0080% or less, Si: 0.4% or less,
Mn: 0.1% to 0.5%, P: 0.08% or less,
S: 0.04% or less, sol.Al: 0.05% or less,
N: 0.0060% or more and 0.0200% or less containing N and sol.Al so that the following formula (1) is satisfied, and solid solution N: 0.0035% or more, with the balance being Fe and inevitable impurities And a cold rolling recovery structure with a ferrite phase as the main phase, with a tensile strength in the rolling direction of 400 MPa or more and less than 600 MPa , and a plate thickness of 0.7 mm or less. High strength thin steel sheet with excellent properties.
Record
N / sol.Al ≧ 0.2… (1)
(N, sol.Al: content of each element (% by mass))

[2] In [1], in addition to the above-mentioned composition, it further contains, by mass%, one or two of Ti and B so as to satisfy the following formulas (2) and (3): A high-strength thin steel sheet with excellent adhesion bending workability and formability.
Record
0.0060 ≦ N− (Ti / 48 + B / 11) × 14… (2)
{N− (Ti / 48 + B / 11) × 14} /sol.Al≧0.2… (3)
(N, sol.Al, Ti, B: content of each element (mass%))

[3] In the above [1] or [2], in addition to the above composition, Nb: less than 0.05% by mass% is contained so as to satisfy the following formula (4): And high strength thin steel sheet with excellent formability.
Record
{N− (Ti / 48 + B / 11) × 14} / Nb ≧ 0.5… (4)
(N, Ti, B, Nb: content of each element (mass%))

[4] A steel material is heated, subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, and hot rolled into a hot rolled sheet, and the hot rolled sheet is cooled. A cold rolling step for performing cold rolling to form a cold rolled sheet, and an annealing treatment step for further heating the cold rolled plate to a predetermined temperature, holding the heated temperature at a predetermined time, and thereafter performing an annealing treatment for cooling. In making thin steel plate,
The steel material in mass%,
C: 0.0005% or more and 0.0080% or less, Si: 0.4% or less,
Mn: 0.1% to 0.5%, P: 0.08% or less,
S: 0.04% or less, sol.Al: 0.05% or less,
N: 0.0060% or more and 0.0200% or less containing N and sol.Al so that the following formula (1) is satisfied, with the balance being composed of Fe and inevitable impurities, and the heating temperature in the hot rolling step is 1000 ° C or higher, the finish rolling finish temperature of the finish rolling is 850 ° C or higher, the winding temperature CT of the winding is set to a temperature that satisfies the following formula (5), and the rolling rate CR of the cold rolling step is 40 % To 65 %, the heating temperature AT in the annealing treatment step is 450 ° C. to 620 ° C. and satisfies the following formula (6), and the holding time at the heating temperature in the annealing treatment step is 5 seconds or more. It has excellent adhesive bending workability and formability, characterized in that it is 600 s or less, and the steel sheet is a high strength steel sheet having a tensile strength in the rolling direction of 400 MPa or more and less than 600 MPa and a thickness of 0.7 mm or less. A method for producing high strength thin steel sheets.
Record
N / sol.Al ≧ 0.2… (1)
CT （℃） ≦ 700−10 × (sol.Al/N)… (5)
(CT: coiling temperature in hot rolling process (℃))
(N, sol.Al: content of each element (% by mass))
AT (° C) ≤ (TS _L −152−7.3 × CR−155 × [N] × 100) / (− 0.26)… (6)
(AT: Heating temperature in annealing process (℃))
(TS _L : Lower limit of target tensile strength of thin steel sheet (MPa), TS _L ≧ 400MPa)
(CR: Rolling ratio in cold rolling process (%))
([N]: content of solid solution N contained in the cold-rolled sheet before the annealing process (mass%))

[5] In [4], in addition to the above composition, the composition further contains one or two of Ti and B so as to satisfy the following formulas (2) and (3) by mass%. A method for producing a high-strength thin steel sheet having excellent adhesion bending workability and formability.
Record
0.0060 ≦ N− (Ti / 48 + B / 11) × 14… (2)
{N− (Ti / 48 + B / 11) × 14} /sol.Al≧0.2… (3)
(N, sol.Al, Ti, B: content of each element (mass%))

[6] In [4] or [5], in addition to the above composition, Nb: less than 0.05% by mass is contained so as to satisfy the following formula (4), and the coiling temperature in the hot rolling step CT is changed to the temperature satisfying the following equation (7) instead of the above equation (5), the heating temperature AT in the annealing treatment step is 450 ° C. or more and 620 ° C. or less, and the equation (6) is replaced with the following (8 A method for producing a high-strength thin steel sheet excellent in close contact bending workability and formability, characterized in that the temperature satisfies a formula.
Record
{N− (Ti / 48 + B / 11) × 14} / Nb ≧ 0.5… (4)
(N, Ti, B, Nb: content of each element (mass%))
CT (℃) ≦ 700−z (7)
Here, z is a larger value of 10 × (sol.Al/N) and 50 × (Nb / N).
(CT: coiling temperature in hot rolling process (℃))
(N, sol.Al, Nb: content of each element (mass%))
AT (° C) ≤ (TS _L −152−7.3 × CR−155 × [N] × 100) / (3.8 × Nb−0.26)… (8)
(AT: Heating temperature in annealing process (℃))
(TS _L : Lower limit of target tensile strength of thin steel sheet (MPa), TS _L ≧ 400MPa)
(CR: Rolling ratio in cold rolling process (%))
(Nb: Nb content (% by mass))
([N]: content of solid solution N contained in the cold-rolled sheet before the annealing process (mass%))

[7] In any one of [4] to [6], a heating temperature AT in the annealing treatment step is set to 450 ° C. or more and 620 ° C. or less, a cooling stop temperature in the annealing treatment step is set to 500 ° C. or less, and the annealing treatment step A method for producing a high-strength thin steel sheet excellent in adhesion bending workability and formability, characterized by providing a hot dip galvanizing process later.

[8] In [7], an alloying treatment step is provided after the hot dip galvanizing treatment step, and the alloying treatment temperature GT of the alloying treatment step is set to 550 ° C. or less, and the adhesive bending workability and molding For producing high-strength thin steel sheets with excellent properties.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength thin steel plate suitable for the raw material of members, such as building materials, household appliances, and an automobile, can be obtained with a tensile strength of 400 MPa or more and excellent in formability, particularly bending workability. That is, according to the high-strength thin steel sheet of the present invention, even when the thickness is less than 0.8 mm, for example, 0.7 mm, 0.6 mm, the dent resistance is equal to or better than the current 0.8 mm thick TS340MPa class steel sheet. Therefore, it contributes greatly to weight reduction of building materials, home appliances, and automobiles. Moreover, since the high-strength thin steel sheet of the present invention has processability that can withstand severe processing conditions such as tight bending, it can be applied to a member having a complicated shape, and has a remarkable industrial effect.

Shows the effect of rolling direction tensile strength TS ₀ of the unannealed cold-rolled sheet rolling ratio CR (%) of cold rolling on the (MPa). Figure showing the effect of rolling direction tensile strength TS ₀ (MPa) of unannealed cold-rolled sheets on the recovery rate ρ (annealed sheet strength decrease (MPa / ° C) with increasing annealing temperature) during annealing treatment is there. It is a figure which shows the influence of the amount of solute N (mass%) which has on the rolling direction tensile strength TS of an annealed sheet. It is a figure which shows the influence of steel plate Nb content (mass%) which has on the recovery rate (rho) (the amount of reduction of the intensity | strength of an annealing board accompanying the high temperature of annealing temperature (MPa / degreeC)) in annealing treatment. It is a figure explaining the rough skin degree of the steel plate which gave the close contact bending process. ((A) is a structure photograph, (b) is a schematic diagram.) It is a figure explaining the plate | board thickness reduction | decrease rate of the steel plate which performed the adhesion bending process. ((A) is a structure photograph, (b) is a schematic diagram.)

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the component composition of the steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.0005% or more and 0.0080% or less
C is an element effective in strengthening steel. However, if the C content exceeds 0.0080%, the precipitation amount of coarse carbides that become the starting point of cracking during close-contact bending of steel sheets increases, and bending workability is increased. Damage. On the other hand, when the C content is less than 0.0005%, the production cost is remarkably increased. Therefore, the C content is 0.0005% or more and 0.0080% or less. Preferably it is 0.0010% or more and 0.0050% or less.

Si: 0.4% or less
Si is a solid solution strengthening element and is an element effective for increasing the strength of steel. In order to acquire such an effect, it is desirable to contain 0.01% or more, but when Si content exceeds 0.4%, the ductility of a steel plate will fall. Therefore, the Si content is 0.4% or less. Preferably it is 0.1% or less.

Mn: 0.1% to 0.5%
Mn is an element that forms sulfides and improves hot brittleness. In the present invention, the Mn content is 0.1% or more. Preferably, it is 0.16% or more. On the other hand, if the Mn content is excessively high, the amount of MnS that becomes the starting point of cracking at the time of tight bending of the steel sheet increases, and cracking is likely to occur during processing, so the Mn content is set to 0.5% or less. Preferably it is 0.3% or less.

P: 0.08% or less
P is a solid solution strengthening element and is an element effective in increasing the yield strength of steel, but is also an element that adversely affects the ductility of the steel sheet. Therefore, the P content is 0.08% or less. Preferably it is 0.015% or less.

S: 0.04% or less
S exists as a sulfide in steel and causes deterioration of the ductility and stretch flangeability of the steel sheet. Therefore, in the present invention, it is preferable to reduce S as much as possible, and to be 0.04% or less. Preferably it is 0.01% or less.

sol.Al: 0.05% or less
Al acts as a deoxidizer and is an effective element for improving the brittleness of steel. In order to acquire such an effect, it is desirable to contain 0.005% or more of acid-soluble Al (sol. Al). However, since Al has a strong affinity for N, if the sol.Al content is excessive, the amount of solid solution N during the cold rolling process is reduced, and the solid solution strengthening ability due to the solid solution N deteriorates. For this reason, sol.Al content shall be 0.05% or less. Preferably it is 0.04% or less, More preferably, it is 0.019% or less.

N: 0.0060% or more and 0.0200% or less
N must be contained in an amount of 0.0060% or more from the viewpoint of securing solid solution N contained in the hot-rolled sheet before the cold rolling process. On the other hand, when the N content is excessive, it causes an increase in the occurrence rate of slab cracking during continuous casting. Therefore, the N content is 0.0200% or less. Preferably it is 0.016% or less.

Solid solution N: 0.0035% or more
In the present invention in which N is used as a solid solution strengthening element, the solid solution N amount needs to be 0.0035% or more in order to give the steel sheet a desired strength (tensile strength in the rolling direction TS: 400 MPa or more). In order to set the tensile strength TS in the rolling direction to 440 MPa or more, the solid solution N content is preferably 0.0065% or more.

  Here, the solute N amount is a value obtained by subtracting the precipitated N amount measured by electrolytic extraction from the total N amount in the steel. The amount of precipitated N is a value determined by a dissolution method by electrolytic extraction using a constant potential electrolysis method. As the electrolytic solution, it is preferable to use an acetylacetone-based electrolytic solution.

The steel sheet of the present invention contains N and sol.Al so as to satisfy the above-mentioned range and the expression (1).
N / sol.Al ≧ 0.2… (1)
(N, sol.Al: content of each element (% by mass))
The above formula (1) is a requirement that must be satisfied in order to obtain a desired content (0.0035% or more) of solute N in the steel sheet. If sol.Al is excessively contained relative to N, the amount of precipitated AlN in the steel sheet increases (the amount of solute N is insufficient), and the desired strength cannot be imparted to the steel sheet. Therefore, N / sol.Al is 0.2 or more. Preferably it is 0.3 or more.

The above is the basic composition in the present invention. In addition to the basic composition, one or two of Ti and B can be contained so as to satisfy the expressions (2) and (3). In the formulas (2) and (3), Ti = 0 when Ti is not contained, and B = 0 when B is not contained.
0.0060 ≦ N− (Ti / 48 + B / 11) × 14… (2)
{N− (Ti / 48 + B / 11) × 14} /sol.Al≧0.2… (3)
(N, sol.Al, Ti, B: content of each element (mass%))

As will be described later, the steel sheet of the present invention is a steel sheet having a composition having a ferrite phase as a main phase, and is manufactured by subjecting the steel to hot rolling and cold rolling and then annealing.
Here, Ti and B have an effect of increasing the recrystallization temperature of ferrite and suppressing the steel sheet from being softened during the annealing treatment. On the other hand, these elements have a very strong affinity with N. Therefore, if their content becomes excessive, precipitates (nitrides) are formed at high temperatures, and the amount of solute N during the cold rolling process is reduced. There is a risk of significant reduction.

  Therefore, in the present invention, when Ti and B are contained, they are contained so as to satisfy the expressions (2) and (3) in order to ensure the amount of solute N in the steel sheet.

  Further, if sol.Al is excessively contained with respect to N that does not form precipitates together with Ti and B, the amount of AlN precipitation in the steel sheet increases (that is, solute N is insufficient), so {N- (Ti /48+B/11)×14}/sol.Al is 0.2 or more. Preferably it is 0.3 or more.

Further, in addition to the basic composition, Nb may be contained so as to be less than 0.05% and satisfy the formula (4). In the formula (4), Ti = 0 is set when Ti is not contained, and B = 0 is set when B is not contained.
{N− (Ti / 48 + B / 11) × 14} / Nb ≧ 0.5… (4)
(N, Ti, B, Nb: content of each element (mass%))

Nb: less than 0.05%
Nb, like Ti and B, has an effect of increasing the recrystallization temperature of ferrite and suppressing the steel sheet from being softened during the annealing treatment. In order to express such an effect, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content is excessive, the steel sheet may be excessively strengthened to deteriorate ductility. Therefore, the Nb content is adjusted to be less than 0.05% and to satisfy the formula (4) in order to secure the solid solution N amount in the steel sheet.

  In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. Inevitable impurities include Ni, Cr, Cu and the like. The total content of these is 0.3% or less.

Next, the reason for limiting the structure of the steel sheet of the present invention will be described.
The steel sheet of the present invention has a cold rolling recovery structure having a ferrite phase as a main phase.
In the present invention, in order to ensure the formability (particularly bending workability) of the steel sheet, the steel sheet structure has a ferrite phase as a main phase. Here, the main phase refers to a case where the volume ratio with respect to the entire structure is 92% or more, preferably 95% or more. Examples of the second phase other than the main phase include cementite and pearlite. The second phase is 8% or less by volume. When the volume fraction of the second phase exceeds 8%, the ductility of the steel sheet is significantly reduced. When particularly good ductility is required, the volume ratio of the second phase is preferably 5% or less.

  Further, as described above, in the steel sheet as cold-rolled, the reduction rate of the thickness at the time of the close contact bending process becomes large, and the rough surface of the close contact bending process part becomes remarkable. Therefore, in the present invention, for the purpose of improving the bending workability of the steel sheet, the steel sheet is annealed as described later, and the steel sheet structure is set as a cold rolled recovery structure. In addition, when the steel sheet structure becomes a recrystallized structure, the steel sheet strength is remarkably lowered, and the variation in the steel sheet strength increases, which is not preferable.

  Here, the “cold rolling recovery structure” means a structure in which the strain accumulated in the crystal grains of the steel sheet such as ferrite grains is recovered by the heat treatment by the cold rolling process, and the structure recrystallized by the heat treatment. Different. In addition, the judgment as to whether or not the crystal grains have a cold-rolled recovery structure is based on the fact that the steel sheet of the present invention has a ferrite phase as a main phase, so that the recrystallized grains of equiaxed ferrite are 10% or less. If the proportion of the recrystallized grains in the total ferrite grains is 10% or less, the recovery structure is obtained.

  The steel sheet of the present invention may have a plating film on the surface. As the plating film, a hot dip galvanizing film or an alloyed hot dip galvanizing film is preferable.

Next, the manufacturing method of the thin steel plate of this invention is demonstrated.
In the present invention, the steel material having the above composition is heated, subjected to hot rolling consisting of rough rolling and finish rolling, cooled after completion of finish rolling, wound, and hot rolled into a hot rolled sheet, A cold rolling process in which the hot-rolled sheet is cold-rolled to form a cold-rolled sheet, and further, the cold-rolled sheet is heated to a predetermined temperature, held at the heating temperature for a certain time, and then subjected to an annealing process for cooling. The processing steps are sequentially performed to obtain a thin steel plate. At this time, the heating temperature of the hot rolling step is 1000 ° C. or higher, the finish rolling finish temperature of the finish rolling is 850 ° C. or higher, and the winding temperature CT of the winding is a temperature satisfying the formula (5). . Further, when the steel material contains Nb, the winding temperature CT of the winding is set to a temperature satisfying the expression (7).
CT （℃） ≦ 700−10 × (sol.Al/N)… (5)
CT (℃) ≦ 700−z (7)
Here, z is the larger value of 10 × sol.Al / N and 50 × Nb / N.
(CT: coiling temperature in hot rolling process (℃))
(N, sol.Al, Nb: content of each element (mass%))

Further, the rolling rate CR in the cold rolling step is 40% or more and 70% or less, the heating temperature AT in the annealing treatment step is 620 ° C. or less, and the temperature satisfies the formula (6). The holding time at the heating temperature is 5 s or more and 600 s or less. Further, when the steel material contains Nb, the heating temperature AT in the annealing treatment step is set to a temperature that is 620 ° C. or lower and satisfies the equation (8) instead of the above.
AT (° C) ≤ (TS _L −152−7.3 × CR−155 × [N] × 100) / (− 0.26)… (6)
AT (° C) ≤ (TS _L −152−7.3 × CR−155 × [N] × 100) / (3.8 × Nb−0.26)… (8)
(AT: Heating temperature in annealing process (℃))
(TS _L : Lower limit of target tensile strength of thin steel sheet (MPa), TS _L ≧ 400MPa)
(CR: Rolling ratio in cold rolling process (%))
(Nb: Nb content (% by mass))
([N]: content of solid solution N contained in the cold-rolled sheet before the annealing process (mass%))

  In the present invention, the method for melting the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. In addition, after melting, it is preferable to use a slab (steel material) by a continuous casting method because of problems such as segregation, but it may also be used as a slab by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. good.

Heating temperature of steel material: 1000 ° C. or higher The steel material obtained as described above is subjected to hot rolling consisting of heating, rough rolling and finish rolling to form a hot rolled sheet. Heat the steel material to 1000 ℃ or higher. When the heating temperature is less than 1000 ° C., N remains undissolved in the steel material, so that the solid solution strengthening ability due to the solid solution N cannot be sufficiently expressed. From this point of view, the higher the heating temperature, the better. However, if the heating is performed excessively, a thick oxide scale is generated on the steel surface, and the pickling cost increases. Therefore, the heating temperature of the steel material is preferably 1280 ° C. or less.

  In addition, when the steel material before casting and before rough rolling is maintained at a temperature of a predetermined temperature or higher (1000 ° C or higher) and N in the steel material is in solid solution, the steel material is not heated. Direct rolling may be used. Moreover, it is not necessary to specifically limit the rough rolling conditions.

Finishing finish temperature: 850 ° C or higher If the finish rolling finish temperature is less than 850 ° C, it will be difficult to finish rolling in the austenite region, and coarse grains will be generated on the hot rolled sheet, resulting in uneven strength and rough skin during bending. It becomes easy. Accordingly, the finish rolling end temperature is set to 850 ° C. or higher. Preferably it is 880 degreeC or more. Note that if the finish rolling finish temperature becomes excessively high, the secondary scale becomes thick and scale defects are likely to occur. Therefore, the finish rolling finish temperature is preferably 920 ° C. or lower.

Winding temperature CT (℃)
The coiling temperature is a temperature that satisfies the above formula (5) when the steel material does not contain Nb, and a temperature that satisfies the above formula (7) when the steel material contains Nb. When the winding temperature becomes excessively high and the above formulas (5) and (7) are not satisfied, Al or Nb forms a nitride during winding, which is not preferable from the viewpoint of securing solid solution N. It becomes difficult to ensure a desired amount of dissolved N (0.0035% or more). On the other hand, from the viewpoint of maintaining a good plate shape, the winding temperature is preferably 400 ° C. or higher.

  In addition, from the viewpoint of securing a desired solid solution N amount (0.0035% or more), it is preferable that the cooling after finish rolling is water cooling, and the average cooling rate from the finish rolling finish temperature to the winding temperature is 10 ° C. It is preferable to be at least 200 s / s.

Rolling ratio CR in the cold rolling process: 40% or more and 70% or less The hot-rolled sheet obtained as described above is pickled according to a normal method and cold-rolled to obtain a cold-rolled sheet. Here, when the rolling rate CR in the cold rolling process is less than 40%, the work hardening amount becomes too small, and it becomes difficult to secure a desired steel plate strength. On the other hand, when the rolling reduction CR exceeds 70%, work hardening proceeds excessively, and the ductility of the steel sheet is significantly reduced. Therefore, the rolling rate CR in the cold rolling process is 40% or more and 70% or less. Preferably they are 45% or more and 65% or less.

Heating temperature AT in annealing process
The cold-rolled sheet obtained as described above is subjected to an annealing treatment for the purpose of improving ductility by recovery. In the present invention, it is important to set the annealing temperature (heating temperature of the annealing treatment) AT (° C.) to a temperature that satisfies the above formula (6) or (8). Here, [N] in the formula is the content of solid solution N contained in the cold-rolled sheet before the annealing treatment step, and does not change before and after the annealing treatment step within the scope of the present invention. Therefore, the solute N content contained in the cold-rolled sheet may be analyzed directly from the cold-rolled sheet, or the existing annealed sheet such as when the cold rolling reduction ratio (CR) is changed from the existing manufacturing conditions. The solid solution N content may be regarded as the solid solution N content in the cold-rolled sheet.

If it does not satisfy the above expression (6) or (8), the desired steel sheet strength TS _L (rolling direction tensile strength: more than 400 MPa) can not be ensured. Even when the above formula (6) or (8) is satisfied, if the heating temperature (annealing temperature) AT of the annealing process exceeds 620 ° C., recrystallization occurs and the steel sheet strength softens rapidly. . Therefore, the heating temperature (annealing temperature) AT of the annealing treatment is a temperature that satisfies the above formula (6) when the steel material does not contain Nb, and the above formula (8) when the steel material contains Nb. The temperature is 620 ° C. or lower. In particular, in the case of a steel composition that does not contain Ti or Nb, the recrystallization temperature becomes low, so the heating temperature (annealing temperature) AT in the annealing treatment is preferably set to 580 ° C. or lower.

Holding time: 5 s or more and 600 s or less When the holding time at the heating temperature (annealing temperature) AT in the annealing treatment step is less than 5 s, the recovery does not proceed sufficiently and the ductility of the steel sheet is not improved. On the other hand, when the holding time exceeds 600 s, it is easy to shift from recovery to recrystallization. Therefore, the holding time is set to 5 s or more and 600 s or less. Preferably, it is 10 seconds or more and 400 seconds or less.

  In addition, the cooling conditions after hold | maintaining for predetermined time with the heating temperature (annealing temperature) AT of an annealing treatment process are not prescribed | regulated. In addition, after annealing, temper rolling and leveling may be performed for the purpose of straightening the plate shape, etc., but excessive temper rolling causes ductile deterioration of the steel sheet, so the elongation should be 1% or less. Is preferred.

  As described above, a thin steel plate (plate thickness of 0.4 to 1.6 mm) excellent in bending workability, that is, a thickness reduction rate when subjected to contact bending: 0.05 or less, skin roughness: 3 μm or less, and tensile in the rolling direction High strength steel sheet with strength TS: 400MPa or more is obtained. The rolling direction tensile strength TS is preferably 680 MPa or less, and more preferably less than 600 MPa, from the viewpoint of reducing the load during shearing and punching. Note that the tensile strength TS in the rolling direction is, for example, according to the formula (1A), annealing temperature AT (° C.), rolling rate CR (%) in the cold rolling process, steel composition (solid solution N amount (%), Nb content By adjusting the amount (%, etc.), it can be controlled to a desired value (400 to 680 MPa).

  In the present invention, a plating film may be formed on the surface of the steel sheet by plating the thin steel sheet manufactured as described above. For example, a hot dip galvanizing process may be performed as a plating process to form a hot dip galvanized film, or an alloyed hot dip galvanized film may be formed on the surface of the steel sheet by further alloying after the hot dip galvanizing process. .

  When performing the hot dip galvanizing treatment, if the heating temperature (annealing temperature) AT in the annealing treatment step is less than 450 ° C., there is a concern that non-plating may occur. Therefore, it is preferable that the heating temperature (annealing temperature) AT satisfies the above formula (6) or (8) and is set to 620 ° C. or lower and 450 ° C. or higher. Further, after being kept at the heating temperature (annealing temperature) for a predetermined time (5 s or more and 600 s or less), it is cooled to a temperature of 500 ° C. or less, and then subjected to hot dip galvanizing treatment and cooled. The reason for cooling to a temperature of 500 ° C. or lower in the above is to suppress the occurrence of surface defects. In addition, the hot dip galvanization process conditions are not specifically limited, A hot dip galvanization process can be performed according to a conventional method.

  Moreover, you may give an alloying process to the thin steel plate after the said hot dip galvanization process. When the alloying treatment is performed, the alloying treatment temperature GT is preferably set to 550 ° C. or less. This is because if the alloying treatment is performed in a high temperature region exceeding 550 ° C., the plating adhesion is likely to deteriorate.

  Molten steel having the components shown in Table 1 was melted and cast by a generally known technique to obtain a slab (steel material) having a thickness of 300 mm. These slabs are heated to 1250 ° C, roughly rolled, and finish-rolled at a finish rolling finish temperature of 900 ° C. After finish rolling, the temperature range from finish finish temperature to winding temperature is 30 ° C / It cooled at the average cooling temperature of s, and wound up at the winding temperature shown in Table 2, and it was set as the hot rolled sheet. Next, the hot-rolled sheet was pickled, and then cold-rolled at a rolling rate CR shown in Table 2 to obtain a cold-rolled sheet (plate thickness 0.6 mm).

The cold-rolled sheet obtained as described above was annealed. Table 2 shows the heating temperature (annealing temperature) AT during the annealing treatment and the holding time at the heating temperature (annealing temperature). The lower limit TS _L in the rolling direction tensile strength of steel sheet to be targeted is 400 MPa. Some of the cold-rolled sheets were annealed, cooled to 470 ° C., and then subjected to hot dip galvanizing or further alloying. The alloying treatment was performed by maintaining the alloying treatment temperature at 500 ° C. for 10 seconds.

  Samples are taken from the steel plate after annealing (or after hot dip galvanizing or alloying) obtained as described above, and microstructure observation, solid solution N content measurement, tensile test, adhesion bending work test are performed. The volume ratio of the ferrite phase, the content of solute N, the tensile strength TS in the rolling direction, the plate thickness reduction rate after the contact bending process, and the skin roughness of the contact bending process part were determined. The test method is as follows. In addition, it was considered that content of the solid solution N contained in the cold-rolled sheet before an annealing treatment process was equal to the solid solution N content of the annealed sheet calculated | required by the above.

<Tissue observation>
A specimen is taken from the steel plate after annealing (or after hot dip galvanizing or alloying), and the cross section in the rolling direction of the specimen is mechanically polished and corroded with nital, followed by a scanning electron microscope (SEM). ) Magnification: Using a structure photograph (SEM photograph) taken at a magnification of 1000 times, the type of the structure other than the ferrite phase and the ferrite phase and the area ratio thereof were obtained by an image analyzer, and the volume ratio of each phase was obtained. .
In addition, the determination of whether or not the (ferrite phase) crystal grains have a recovery structure is performed by measuring the area ratio of equiaxed ferrite using the structure photograph (SEM photograph), and if the area ratio is 10% or less Judged to be a recovery organization.

<Measurement of solid solution N amount>
A specimen is collected from the steel sheet after annealing (or after hot dip galvanizing treatment and after alloying treatment), and the value obtained by subtracting the precipitation N amount measured by electrolytic extraction from the total N amount in the specimen is dissolved. N amount. The amount of precipitated N was determined by a dissolution method by electrolytic extraction using a constant potential electrolysis method. As the electrolytic solution, an acetylacetone-based electrolytic solution was used.

<Tensile test>
A specimen of JIS No. 5 size with the rolling direction as the length direction (tensile direction) was taken from the steel sheet after annealing (or after hot-dip galvanizing and alloying), and a tensile test in accordance with JIS Z 2241. To determine the tensile strength TS in the rolling direction.

<Adhesion bending test>
A specimen with a rolling direction of 25 mm, a rolling vertical direction of 60 m, and a thickness of 0.6 mm is taken from the steel plate after annealing (or after galvanizing treatment and alloying treatment), and the vertical direction of rolling becomes the bending direction. In this way, close contact bending was performed to check for cracks. In addition, the specimen after adhesion bending is observed after being polished in the width direction (that is, observed after polishing the central section in the width direction of the specimen), and the thickness after adhesion bending according to the method described above. The weight loss rate and the degree of skin roughness of the contact bending process were determined. As for the degree of skin roughness, those having a skin roughness of 3 μm or less were designated as “skin roughness: good (◯)”, and those having a skin roughness of more than 3 μm were designated as “skin roughness: poor (×)”.
The results are shown in Table 3.

  All of the examples of the present invention have excellent bending workability of sheet thickness reduction rate of 0.05 or less and skin roughness: 3 μm or less when subjected to close contact bending, and a high tensile strength TS in the rolling direction of 400 MPa or more. It is a strength thin steel plate. Further, as a result of the structure observation, it is confirmed that in all of the examples of the present invention, the area ratio of the equiaxed ferrite is 10% or less, and the crystal grains of the ferrite phase as the main phase have a recovery structure. On the other hand, in the comparative example that is out of the scope of the present invention, the predetermined high strength cannot be ensured or the bending workability cannot be ensured.

Claims (8)

% By mass
C: 0.0005% or more and 0.0080% or less, Si: 0.4% or less,
Mn: 0.1% to 0.5%, P: 0.08% or less,
S: 0.04% or less, sol.Al: 0.05% or less,
N: 0.0060% or more and 0.0200% or less containing N and sol.Al so that the following formula (1) is satisfied, and solid solution N: 0.0035% or more, with the balance being Fe and inevitable impurities And a cold rolling recovery structure with a ferrite phase as the main phase, with a tensile strength in the rolling direction of 400 MPa or more and less than 600 MPa , and a plate thickness of 0.7 mm or less. High strength thin steel sheet with excellent properties.
Record
N / sol.Al ≧ 0.2… (1)
(N, sol.Al: content of each element (% by mass)) 2. In addition to the composition, the composition further contains one or two of Ti and B so as to satisfy the following formulas (2) and (3) by mass%. High-strength thin steel sheet with excellent adhesion bending workability and formability.
Record
0.0060 ≦ N− (Ti / 48 + B / 11) × 14… (2)
{N− (Ti / 48 + B / 11) × 14} /sol.Al≧0.2… (3)
(N, sol.Al, Ti, B: content of each element (mass%)) In addition to the above composition, Nb: less than 0.05% by mass is contained so as to satisfy the following expression (4). Excellent high-strength thin steel sheet.
Record
{N− (Ti / 48 + B / 11) × 14} / Nb ≧ 0.5… (4)
(N, Ti, B, Nb: content of each element (mass%)) The steel material is heated, subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, hot rolled into a hot rolled sheet, and the hot rolled sheet is subjected to cold rolling. A thin steel sheet is sequentially subjected to a cold rolling process for applying a cold-rolled sheet, and an annealing process for heating the cold-rolled sheet to a predetermined temperature, holding the heated temperature for a certain period of time, and thereafter performing an annealing process for cooling. In the case of
The steel material in mass%,
C: 0.0005% or more and 0.0080% or less, Si: 0.4% or less,
Mn: 0.1% to 0.5%, P: 0.08% or less,
S: 0.04% or less, sol.Al: 0.05% or less,
N: 0.0060% or more and 0.0200% or less containing N and sol.Al so that the following formula (1) is satisfied, with the balance being composed of Fe and inevitable impurities, and the heating temperature in the hot rolling step is 1000 ° C or higher, the finish rolling finish temperature of the finish rolling is 850 ° C or higher, the winding temperature CT of the winding is set to a temperature that satisfies the following formula (5), and the rolling rate CR of the cold rolling step is 40 % To 65 %, the heating temperature AT in the annealing treatment step is 450 ° C. to 620 ° C. and satisfies the following formula (6), and the holding time at the heating temperature in the annealing treatment step is 5 seconds or more. 600s or less ,
A high-strength thin steel sheet excellent in adhesive bending workability and formability, characterized in that the thin steel sheet is a high-strength thin steel sheet having a tensile strength in the rolling direction of 400 MPa or more and less than 600 MPa and a thickness of 0.7 mm or less. Manufacturing method.
Record
N / sol.Al ≧ 0.2… (1)
CT （℃） ≦ 700−10 × (sol.Al/N)… (5)
(CT: coiling temperature in hot rolling process (℃))
(N, sol.Al: content of each element (% by mass))
AT (° C) ≤ (TS _L −152−7.3 × CR−155 × [N] × 100) / (− 0.26)… (6)
(AT: Heating temperature in annealing process (℃))
(TS _L : Lower limit of target tensile strength of thin steel sheet (MPa), TS _L ≧ 400MPa)
(CR: Rolling ratio in cold rolling process (%))
([N]: content of solid solution N contained in the cold-rolled sheet before the annealing process (mass%)) 5. In addition to the composition, the composition further contains one or two of Ti and B so as to satisfy the following formulas (2) and (3) by mass%. A method for producing a high-strength thin steel sheet having excellent adhesion bending workability and formability.
Record
0.0060 ≦ N− (Ti / 48 + B / 11) × 14… (2)
{N− (Ti / 48 + B / 11) × 14} /sol.Al≧0.2… (3)
(N, sol.Al, Ti, B: content of each element (mass%)) In addition to the above composition, Nb: less than 0.05% by mass is contained so as to satisfy the following formula (4), and the coiling temperature CT in the hot rolling step is replaced by the above formula (5) and the following ( 7) is a temperature satisfying the formula, the heating temperature AT in the annealing treatment step is 450 ° C. or more and 620 ° C. or less, and the temperature satisfies the following formula (8) instead of the formula (6): The manufacturing method of the high strength thin steel plate excellent in the adhesion bending workability and formability of Claim 4 or 5.
Record
{N− (Ti / 48 + B / 11) × 14} / Nb ≧ 0.5… (4)
(N, Ti, B, Nb: content of each element (mass%))
CT (℃) ≦ 700−z (7)
Here, z is a larger value of 10 × (sol.Al/N) and 50 × (Nb / N).
(CT: coiling temperature in hot rolling process (℃))
(N, sol.Al, Nb: content of each element (mass%))
AT (° C) ≤ (TS _L −152−7.3 × CR−155 × [N] × 100) / (3.8 × Nb−0.26)… (8)
(AT: Heating temperature in annealing process (℃))
(TS _L : Lower limit of target tensile strength of thin steel sheet (MPa), TS _L ≧ 400MPa)
(CR: Rolling ratio in cold rolling process (%))
(Nb: Nb content (% by mass))
([N]: content of solid solution N contained in the cold-rolled sheet before the annealing process (mass%))   The heating temperature AT in the annealing treatment step is 450 ° C. or more and 620 ° C. or less, the cooling stop temperature in the annealing treatment step is 500 ° C. or less, and a hot dip galvanizing treatment step is provided after the annealing treatment step. Item 7. A method for producing a high-strength thin steel sheet having excellent adhesion bending workability and formability according to any one of Items 4 to 6.   An alloying treatment step is provided after the hot dip galvanizing treatment step, and an alloying treatment temperature GT of the alloying treatment step is set to 550 ° C. or less. An excellent method for producing high strength thin steel sheets.