JP5846113B2 - High strength thin steel sheet with excellent dent resistance and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength thin steel sheet suitable for use in building materials, home appliances, automobiles, cans, and the like, and particularly relates to improvement in dent resistance.

In the fields of building materials, electrical machinery, automobiles, cans, etc., there is a demand for weight reduction of products and members from the viewpoint of global environmental conservation. In order to meet such demands, it is conceivable to reduce the thickness of the material.
In YP220MPa class thin steel sheets (tensile strength TS: 330MPa class), which are widely used in the electrical and building materials field, for example, 0.7mm thickness or 0.6mm thickness is used instead of the 0.8mm thickness steel sheet that is normally used. There is a demand for further thinning of the steel sheet.

However, there is a concern that the reduction in the dent resistance of the member may be caused by reducing the thickness of the material according to the same steel type standard. Therefore, in many cases where the thickness is reduced, it is essential to improve the dent resistance of the member. The dent resistance A of the flat plate member is
A ∝ t × t × (yield strength of member)
(Where t: thickness)
It is said that According to this formula, the decrease in dent resistance due to the thinning can be compensated by the increase in strength. For example, a material that uses steel (current material) with a thickness of t: 0.8 mm and a thin steel material of t: 0.7 mm or t: 0.6 mm, with a yield point YP of 220 MPa. In order to maintain equivalent dent resistance A, yield strength YP: 287MPa for 0.7mm thick thin steel, and yield point YP: 391MPa for 0.6mm thick thin steel, increase strength. Will be necessary.

Conventionally, high strength methods for steel (steel plates), which are raw materials, can be broadly divided into solid solution strengthening by adding solid solution elements, precipitation dispersion strengthening that precipitates a large amount of precipitates, or quenching such as Mn. For example, transformation strengthening has been used in which a martensite phase is generated to increase the strength by the addition of a property improving element and dislocation strengthening to increase the strength by an unrecrystallized processed structure.
For example, Patent Document 1 describes a method for manufacturing a steel plate for cans. The technology described in Patent Document 1 includes C: 0.0015% or less, Si: 0.020% or less, Mn: 0.10% or less, N: 0.003% or less, Al: 0.150% or less, and Cr: 0.020-0.500%. Nb: 0.0020-0.0200%, Ti: 0.0050-0.0200%, B: 0.0002-0.0020% Slabs containing one or more types are reheated to 1050 ° C or lower, and the finishing mill entry temperature is 950 ° C. or less, total rolling reduction of 40% or more, final rolling reduction of 25% or more, winding at a temperature of 500 to 750 ° C., and after pickling, cold rolling with a rolling reduction of 50 to 98% is performed. It is a manufacturing method of the steel plate for cans. The technique described in Patent Document 1 is a method of increasing strength using so-called dislocation strengthening, in which an annealing step is omitted, and desired properties are ensured as a steel plate for cans while being cold-rolled.

  Patent Document 2 includes C: 0.08% or less, Si: 0.01 to 0.05%, Mn: 0.1 to 2.5%, acid-soluble Al: 0.010% or less, and N: 0.004 to 0.02%. A slab with a composition that contains acid-soluble Ti, N, and Al to satisfy a specific relationship is hot-rolled, wound into a coil, and immersed in a water bath when the coil temperature is 350 ° C or higher. A method for producing an iron plate excellent in ductility after cold rolling, which is subjected to cold rolling with a rolling reduction of 20% or more is described. According to the technique described in Patent Document 2, an iron plate having excellent ductility can be manufactured even if annealing is omitted. The technique described in Patent Document 2 is a method of increasing the strength using so-called dislocation strengthening, in which annealing is omitted and the desired ductility is ensured while still in cold rolling.

  Patent Document 3 describes a method for producing an ultrathin cold-rolled steel sheet for a high-strength can. In the technique described in Patent Document 3, by using a rolling material containing C: 0.02% or less, Mn: 1.5% or less, Al: 0.01% or less, and N: 0.0050 to 0.0250% by mass%, the N amount is 90%. Rolling is started at a temperature at which at least% is in a solid solution state, hot rolling is performed to finish finish rolling at (Ar3 transformation point −30 ° C.) or more, and after the hot rolling is finished, strong cooling is started, and 600 ° C. Winding, winding and water-cooling after hot rolling, cold rolling, soaking at a temperature above the recrystallization temperature, and continuous annealing to cool to 40 ° C within 60 s after soaking It is a manufacturing method of the ultra-thin cold-rolled steel plate for cans. In the technique described in Patent Document 3, the yield strength after paint baking treatment is 550 MPa or more, the bake hardening amount is 100 MPa or more, and the increase in tensile strength by the paint baking treatment is 30 MPa or more. The technique described in Patent Document 3 is a method of increasing the strength using so-called solid solution strengthening in which (solid solution C + solid solution N) is present in an amount of 0.0050% or more after annealing.

  Patent Document 4 describes a nitrocarburized product using a carburizing and quenching method. The technology described in Patent Document 4 is a method in which an iron or iron alloy workpiece (material) is charged into a heat treatment furnace supplied with ammonia gas, heated to an austenite temperature, held, and then quenched and quenched. In this method, a hardened layer is formed on the surface of the workpiece to obtain a nitrocarburized product, which is a method of increasing the strength by utilizing so-called transformation strengthening.

  Patent Document 5 describes a method for producing a high-strength cold-rolled steel sheet. In the technique described in Patent Document 5, the mass% includes C: 0.016-0.2%, and Ti: 0.025-1%, Nb: 0.01-1.5%, V: 0.01-1%. After hot rolling, the steel containing is rolled up at 650 ° C or less, cold-rolled at a reduction rate of 85% or less, and then annealed at a heating rate of 30 ° C / s or more from 600 ° C to the end of recrystallization. Thus, a high strength cold-rolled steel sheet having a low yield ratio and excellent workability and weldability is obtained. The technique described in Patent Document 5 is a method for increasing the strength using so-called precipitation strengthening, in which carbides or nitrides of Ti, Nb, and V are finely precipitated.

Japanese Unexamined Patent Publication No. 08-176674 JP 2000-87184 A Japanese Patent Laid-Open No. 2001-107186 JP 2009-270155 A JP 2002-363649 A

  However, the increase in strength by dislocation strengthening used in the techniques described in Patent Documents 1 and 2 has a problem of not only increasing the tensile strength TS but also significantly reducing the ductility. In addition, the increase in strength by precipitation strengthening used in the technique described in Patent Document 5 requires a large amount of expensive elements such as Ti and Nb, which raises the problem of increasing the material cost. Further, the increase in strength by transformation strengthening used by the technique described in Patent Document 4 has a strong tendency to increase the tensile strength TS rather than the yield strength YP, and promotes mold wear during molding processing. There is a problem of increasing the spring back after molding. As described above, the increase in strength using precipitation strengthening, transformation strengthening, and dislocation strengthening has the above-described problems.

  The present invention advantageously solves the problems of the prior art, and is a high-strength thin steel sheet suitable in the fields of building materials, electrical machinery, automobiles, cans, etc., and has a yield strength of YP: 400 MPa or more and excellent dent resistance. Another object of the present invention is to provide a high-strength thin steel sheet having a low tensile strength TS of 1.25 YP or less and excellent in dent resistance and a method for producing the same. In the present invention, the target property is a yield strength YP of 400 MPa or higher and a high YP to ensure dent resistance, and a tensile strength TS of 1.25 YP or less in order to keep the springback after molding low. Limited to low TS. The “thin steel plate” here refers to a steel plate (steel strip) having a thickness of 1.5 mm or less.

  In order to achieve the above-described object, the present inventors first focused on increasing the strength by solid solution strengthening. In general, the solid solution strengthening has advantages such that the strength can be increased with a small addition amount, and the solid solution element is unlikely to hinder formability like a precipitate. In the technique described in Patent Document 3, after continuous annealing and soaking, for example, high-speed cooling at a cooling rate of 150 ° C./s or more is performed, and solid solution C and solid solution N remain in the steel sheet to increase the strength. I am trying. However, the amount of C and N that can be dissolved in the ferrite (steel plate) is at most about 0.017% by mass, and is small in achieving the desired high strength.

Furthermore, generally, when the amount of C in steel is increased for the purpose of increasing the amount of solute C, at the same time, there is a problem that the formation of iron carbide in the steel sheet is promoted and a sufficient amount of solute C cannot be secured. On the other hand, the increase in the amount of N does not promote the formation of iron nitride as much as C. From this, it was considered that N is easily present in a sufficient amount and in a solid solution state in the steel, and that high strength can be achieved by solid solution strengthening.
However, in order to achieve high N by the normal steel making method, it is necessary to add a large amount of elements such as V and Cr at the same time, and this leads to high costs. It is said that it is not useful. As a method for increasing the amount of N in steel, for example, the pressurization type ESR (Electro Slag Remelting) method and the mechanical alloying method are known, but mass production such as manufacturing of high-strength thin steel sheet is required. It is unsuitable if you do.

Therefore, the present inventors paid attention to a nitrogen absorption treatment method as a method for increasing N. The nitrogen absorption treatment method adjusts the amount of nitrogen to be absorbed by changing the atmospheric gas, and has an advantage that it can be incorporated into an annealing process when manufacturing a thin steel sheet.
Patent Document 4 also describes a technique for diffusing and penetrating nitrogen into a workpiece and absorbing it during heat treatment. However, in the conventional high nitrogenization, any emphasis is placed on increasing the amount of absorbed nitrogen, and a specific steel sheet composition and manufacturing process for achieving high YP by increasing the amount of dissolved N is mentioned. There is no. For example, when Ti, Cr, Al, or the like is added, nitrogen precipitates as a nitride, and is difficult to exist in a solid solution state, which causes a problem of reducing ductility. In addition, in actual thin steel sheet production, there is a problem that the YP may be lowered in a shape correction process such as temper rolling.

  For these reasons, the inventors focused on increasing the strength by solid solution strengthening using solid solution N, and as a result of various investigations, the nitrogen absorption treatment method was applied to the annealing process after cold rolling. It was found that incorporating and using it is effective for achieving the above-described purpose. And to suppress the formation of nitrides, to make more solid solution N exist in the steel, and to secure the desired ductility, yield strength, tensile strength, after adjusting to the appropriate component range, It was found that appropriate heat treatment conditions exist according to the plate thickness.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.0005 to 0.0250%, Si: 1.0% or less, Mn: 0.1 to 2.5%, P: 0.10% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.015 to 0.500 %, A composition comprising the balance Fe and inevitable impurities, a ferrite phase with an area ratio of 70% or more, and a martensite phase with an area ratio of 5% or less, with the balance being other than the ferrite phase and the martensite phase. And a structure in which a precipitate containing N is adjusted to 0.010% by mass or less in terms of N, and has a yield strength YP in the rolling direction of 400 MPa or more and a tensile strength TS of 1.25 YP or less. A high-strength thin steel sheet with excellent dent resistance, characterized by having properties.
(2) A method for producing a thin steel sheet by subjecting a steel material to a hot rolling process, a cold rolling process, and an annealing process to obtain a thin steel sheet, wherein the steel material is in mass%, and C: 0.0005 to Contains 0.0250%, Si: 1.0% or less, Mn: 0.1-2.5%, P: 0.10% or less, S: 0.05% or less, Al: 0.05% or less, N: less than 0.015%, and the balance Fe and inevitable impurities A steel material having a composition, wherein the hot rolling step heats the steel material to a temperature of 1100 ° C. or higher, performs rough rolling and finish rolling at a finish rolling finish temperature of 850 ° C. or higher, and a coiling temperature : A step of winding at 650 ° C. or less, the cold rolling step is a step of performing cold rolling with a cold rolling ratio of 70 to 85%, and the annealing step is 0.03 to 50% in volume ratio In an atmosphere containing ammonia and the balance consisting of inert gas, 550 ° C or higher, the following formula (1)
TN (℃) = 723-10.7 × [Mn] (1)
(Where [Mn]: Mn content (mass%))
Is a process of holding for a predetermined holding time at an annealing temperature defined as TN temperature or less, and has tensile properties in which the yield strength YP in the rolling direction is 400 MPa or more and the tensile strength TS is 1.25 YP or less. A method for producing a high-strength thin steel sheet, characterized by being a thin steel sheet.
(3) In (2), the predetermined holding time th at the annealing temperature is expressed by the following equation (2): th (s) ≧ 4.5 × 10 ⁴ × (plate thickness [m]) ² × exp (9.5 × 10 ³ / T [K]) (2)
(Where T: annealing temperature (K))
A method for producing a high-strength thin steel sheet, characterized by satisfying
(4) In (2) or (3), after the annealing step, a tempering step in which temper rolling with an elongation of 2% or less is further performed, and an aging treatment step in which an aging treatment is performed at an aging temperature of 200 ° C. or less, A method for producing a high-strength thin steel sheet, characterized in that
(5) In (4), the holding time ta at the aging temperature is expressed by the following equation (3)
ta (s) ≧ 4.8 × 10 ⁻¹⁰ × T × exp (9500 / T) (3)
(Where, T: Aging temperature (K))
A method for producing a high-strength thin steel sheet, characterized by satisfying

  According to the present invention, it is possible to reduce the yield strength: 400 MPa or more without adding an excessive alloying element, to reduce the spring back after forming, and to reduce the high-strength thin steel sheet having excellent dent resistance. Moreover, it can be manufactured easily and has a remarkable industrial effect. Further, according to the present invention, the thickness of the member can be reduced by about 0.1 mm to 0.2 mm without impairing the dent resistance of the member, and the effect of contributing to the weight reduction of the member can be achieved.

First, the reason for limiting the composition of the thin steel sheet of the present invention will be described. Hereinafter, mass% is simply expressed as%.
C: 0.0005% or more and 0.0250% or less
C is an element that increases the strength of steel, but if it is contained in a large amount exceeding 0.0250%, the amount of carbide increases and the ductility decreases. For this reason, in the present invention, C is limited to 0.0250% or less. In addition, since excessive reduction causes the refining cost to rise, it was limited to 0.0005% or more.

Si: 1.0% or less
Si is an element that dissolves to stabilize ferrite and further increase the strength of steel. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, excessive content exceeding 1.0% lowers chemical conversion treatment properties, plating properties, and ductility. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.2% or less.

Mn: 0.1-2.5%
Mn is an element that combines with S to form MnS and suppresses hot brittleness due to S. Mn also dissolves and increases the strength of the steel. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, an excessive content exceeding 2.5% generates a martensite structure during annealing and impairs ductility. For these reasons, Mn was limited to a range of 0.1 to 2.5%. In addition, from the viewpoint of suppressing an increase in material cost, it is preferably 1.0% or less.

P: 0.10% or less
P is a ferrite stabilizing element and has the effect of increasing the strength. However, P is preferably segregated at grain boundaries and the like, and decreases ductility. In addition, if it exceeds 0.10% and it contains excessively, plateability will fall. For this reason, P was limited to 0.10% or less. In addition, Preferably it is 0.03% or less.

S: 0.05% or less
S exists as a sulfide in steel and reduces ductility. For this reason, it is desirable to reduce S as much as possible, but 0.05% is acceptable. For this reason, S was limited to 0.05% or less. In addition, Preferably it is 0.02% or less.
A1: 0.05% or less
Al acts as a deoxidizer, reduces oxides (inclusions) in steel, and contributes effectively to improving ductility and brittleness. In order to acquire such an effect, it is desirable to contain 0.001% or more. Moreover, since Al has strong affinity with N, the amount of solid solution N is reduced, and the solid solution strengthening ability by solid solution N is reduced. For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.001 to 0.04%.

N: 0.015-0.500%
N has a strong solid solution strengthening ability and is the most important element in the present invention in order to ensure a desired high YP. In the present invention, N is contained as much as possible. In order to ensure the desired high YP, it is necessary to contain 0.015% or more. In the present invention, N is absorbed by the nitrogen absorption treatment method in the annealing process because the N addition limit by the normal steel making method is about 0.015%. On the other hand, an excessive annealing time is required to absorb a large amount of N exceeding 0.500% in the annealing process. For this reason, N was limited to 0.500% or less from the viewpoint of productivity. For these reasons, N is limited to a range of 0.015 to 0.500%.

In the present invention, the impurities Ti and Nb are adjusted to 0.003% or less, respectively.
Ti and Nb are strong nitride-forming elements, reduce the amount of solute N, and lower the solid solution strengthening ability of solute N. In addition, Ti and Nb precipitate nitride during hot rolling and refine the crystal grains, so the crystal interfacial area that becomes the AlN precipitation site is increased, the amount of solid solution N is reduced, and the solid solution strengthening is reduced. Let For this reason, impurities Ti and Nb are adjusted to 0.003% or less, respectively. If each is 0.003% or less, the above-mentioned adverse effects can be tolerated.

The balance other than the above components is Fe and inevitable impurities.
Next, the reason for limiting the structure of the thin steel sheet of the present invention will be described.
The thin steel sheet of the present invention has a structure whose main phase is a ferrite phase with an area ratio of 70% or more. Since the diffusion amount of the solid solution N in the ferrite phase is larger than that in the austenite phase, the solid solution N can be more uniformly dispersed as the ferrite phase increases. If the ferrite phase is less than 70%, the steel sheet structure becomes non-uniform and the workability is lowered, and the desired high YP cannot be secured by utilizing the solid solution strengthening by the solid solution N. For this reason, the ferrite phase was limited to 70% or more by area ratio. In addition, Preferably, it is 95% or more.

  The second phase other than the main phase is a martensite phase with an area ratio of 5% or less. Since the martensite phase is a hard phase, it tends to reduce ductility. If the martensite phase exceeds 5% in area ratio, voids may be generated during transformation, and the ductility is significantly reduced, and the yield point (yield strength) YP compared to the tensile strength TS. It becomes a factor to lower. For this reason, the martensite phase which is the second phase is limited to 5% or less in terms of area ratio.

  The balance other than the main phase and the second phase is a phase (other phase) other than the ferrite phase and the martensite phase. There is an austenite phase as another phase. Note that the austenite phase becomes a site that absorbs N (nitrogen), and tends to reduce the solid solution strengthening ability of the ferrite phase by the desired solid solution N. For this reason, in the present invention, it is desirable that the austenite phase contains 0% and is minimized.

The austenite phase may be formed on the steel sheet surface layer as the amount of N penetration into the ferrite phase increases. The formation of a large amount of the surface austenite phase tends to inhibit N diffusion to the center of the plate thickness, but even if a large amount of the surface austenite phase is formed, the ductility is not excessively lowered.
Moreover, in this invention, the precipitate (nitride) containing N is adjusted to 0.010 mass% or less in N conversion.

Precipitates (nitrides) containing N, for example, Fe ₄ N, AlN, etc., become the starting point of cracks during bending and reduce ductility. For this reason, in this invention, it was decided to reduce the precipitate (nitride) containing N as much as possible at 0.010 mass% or less in terms of the amount of N (also referred to as the amount of precipitated N) involved in the precipitation. When the amount of precipitated N exceeds 0.010% by mass, workability such as bending characteristics is deteriorated. In addition, Preferably it is 0.005% or less. Here, the amount of precipitated N was expressed by mass% with respect to the total amount of the electrolytic extract by analyzing the amount of N contained in the electrolytic residue extracted by the electrolytic extraction method.

Below, the preferable manufacturing method of this invention high strength thin steel plate is demonstrated.
First, the hot rolling process which makes hot rolling which consists of rough rolling and finish rolling is given to the steel raw material (slab) which has the above-mentioned composition except N content, and the hot rolling process which makes a hot-rolled sheet is given.
The method for producing the steel material is not particularly limited. It is preferable to melt a molten steel having the above-described composition other than N content by a generally known melting method such as a converter, and to make a steel material such as a slab by a generally known casting method such as a continuous casting method. . The N content of molten steel is desirably up to about 0.015%, which is the maximum possible by a normal melting method, and is preferably less than 0.015%. The N amount is preferably 0.0005% or more from the viewpoint of refining costs.

  In the hot rolling step, first, the steel material having the above composition is heated to a temperature in the range of heating temperature: 1100 ° C. or higher, preferably 1280 ° C. or lower. Heating temperature: If it is less than 1100 ° C., AlN precipitates and remains until after the annealing step, thereby reducing ductility. The heating temperature is preferably higher from the viewpoint of reducing the deformation resistance. However, if the heating temperature is excessively higher than 1280 ° C, the oxide scale is formed thick on the steel surface, the oxidation loss increases, and the yield decreases. In addition, the cost of pickling treatment increases. For this reason, the heating temperature of the steel material is limited to a temperature in the range of 1100 ° C. or higher, preferably 1280 ° C. or lower.

In the hot rolling process, the heated steel material is subjected to rough rolling. The rough rolling conditions to be applied are not particularly limited as long as the sheet bar can have a predetermined size and shape. In the hot rolling process, the sheet bar is then subjected to finish rolling.
In the finish rolling, the finish rolling finish temperature (exit temperature) is an austenite region of 850 ° C. or higher. When the finish rolling finish temperature (exit temperature) is less than 850 ° C., rolling is performed in a two-phase region, and the structure of the hot-rolled sheet becomes uneven and refined, and the material becomes unstable. Moreover, excessive rolling in the ferrite region increases the strength of the steel sheet, leads to an increase in rolling load during cold working, and causes an increase in manufacturing cost. For this reason, the finish rolling finish temperature (exit temperature) of finish rolling is limited to 850 ° C. or higher.

After finishing rolling, it is cooled by air cooling or the like, and wound at a winding temperature of 650 ° C. or lower. When the coiling temperature exceeds 650 ° C., AlN precipitates and solid solution N decreases. In addition, Preferably it is 450 degreeC or more and 600 degrees C or less.
The hot-rolled sheet is then subjected to a cold rolling process.
In the cold rolling step, the hot-rolled sheet is pickled by a normal method and cold-rolled to obtain a cold-rolled sheet having a desired thickness. The cold rolling rate of cold rolling is 70% or more. The higher the cold rolling rate, the more the recrystallization proceeds at a low temperature during the subsequent annealing treatment, and the dislocations that become AlN precipitation sites that cause the low ductility can be eliminated. On the other hand, when it exceeds 85%, work hardening progresses and a rolling load increases. For this reason, the cold rolling rate of the cold rolling is set in the range of 70 to 85%.

Next, the cold rolled sheet is subjected to an annealing process.
In the annealing process, ammonia is contained in an amount of 0.03 to 50% by volume and the balance is made of inert gas.
TN (℃) = 723-10.7 × [Mn] (1)
(Where [Mn]: Mn content (mass%))
And a step of performing a process of holding for a predetermined holding time at an annealing temperature not higher than the TN temperature defined in.

  When the annealing temperature exceeds the TN temperature, the martensite phase increases excessively after annealing and the ductility decreases. The TN temperature is a temperature (transformation point) at which the austenite phase starts to be generated, and depends on the amount of alloy elements. In addition, when the annealing temperature is less than 550 ° C., recrystallization does not occur sufficiently, an unrecrystallized structure remains even after annealing, and the ductility is extremely reduced. For this reason, the annealing temperature was limited to 550 ° C. or higher and TN temperature or lower. As nitrogen absorption proceeds, the austenite phase is stably generated as the amount of N in the steel increases. Therefore, the annealing temperature is preferably 700 ° C. or lower.

  Moreover, the atmosphere of annealing treatment shall be in the atmosphere which contains ammonia of 0.03 to 50% by volume, and consists of remainder inert gas. If the ammonia gas content is less than 0.03%, the desired N content in steel cannot be ensured by annealing treatment. On the other hand, if the ammonia gas content exceeds 50%, many voids are generated on the surface of the steel sheet, and the amount of solute N decreases. Note that the remainder of the annealing atmosphere is an inert gas. The inert gas is preferably nitrogen gas, argon gas, or a mixture thereof. Further, hydrogen gas, oxygen gas, and carbon dioxide gas may be contained as long as the amount does not affect the action of ammonia gas.

In the annealing process, the holding time th (s) at the above-described annealing temperature is expressed by the following equation (2).
th (s) ≥ 4.5 x 10 ⁴ x (plate thickness [m]) ² x exp (9.5 x 10 ³ / T [K]) (2)
(Where T is the annealing temperature (K))
Is preferably satisfied. If the retention time th (s) does not satisfy the formula (2), the desired amount of N cannot be absorbed into the steel sheet, and the nitrogen N absorbed from the steel sheet surface layer is uniformly distributed in the thickness direction. I can't let you. The holding time th was defined as the time required for N absorbed from the steel sheet surface layer to diffuse in the thickness direction to at least a quarter of the thickness. In addition, the value described in the literature (Metal Data Book 4th edition, edited by the Japan Institute of Metals, Maruzen) was used as the diffusion coefficient.

It is more desirable to satisfy the holding time th so as to satisfy the time required for diffusing from the plate thickness surface layer to 1/2 position in the plate thickness direction. In such a case, the retention time th (s) is given by
th (s) ≧ 1.8 × 10 ⁵ × (plate thickness [m]) ² × exp (9.5 × 10 ³ / T [K])
(Where T is the annealing temperature (K))
It is more preferable to adjust so as to satisfy the above.

In addition, temper rolling and leveling may be performed after the annealing process for the purpose of correcting the plate shape, etc. However, excessive temper rolling imparts deterioration of ductility, so the elongation rate should be 2% or less. Is preferred.
However, when temper rolling is performed, the yield point decreases due to the introduction of movable dislocations, and the desired yield point YP cannot be secured. Therefore, it is necessary to perform an aging treatment step in order to fix the movable dislocations with solute nitrogen or carbon to obtain a high yield point.

The aging treatment is preferably performed at a temperature (aging temperature) T of 200 ° C. or lower. And the holding time ta (s) at the temperature (aging temperature) T is the following equation (3)
ta (s) ≧ 4.8 × 10 ⁻¹⁰ × T × exp (9500 / T) (3)
(Where, T: Aging temperature (K))
It is preferable to adjust so as to satisfy the above.

  As the aging temperature T is higher, the diffusion of nitrogen is promoted, and dislocations can be fixed in a shorter time. However, when it is excessively high, nitrides with Fe are formed, and solid solution elements are reduced. . Therefore, the aging temperature T is preferably 200 ° C. or lower. Further, when the holding time ta (s) at the aging temperature T does not satisfy the expression (3), the dislocations are not sufficiently fixed, and a desired high YP cannot be secured.

  Needless to say, the surface of the steel plate may be plated with zinc, nickel or the like, or a chemical conversion treatment may be performed to improve corrosion resistance, slidability, or the like.

  A steel material (slab) having the composition shown in Table 1 is heated under the conditions shown in Table 2, subjected to finish rolling consisting of finish rolling end temperatures shown in Table 2, and wound in a coil shape at the winding temperature shown in Table 2. Then, a hot rolling process for forming a hot-rolled sheet (steel strip) was performed. The obtained hot-rolled sheet was subjected to conventional pickling treatment, and then cold-rolled under the conditions shown in Table 2 to give a cold-rolling step to obtain a cold-rolled sheet having the thickness shown in Table 2. Subsequently, the obtained cold-rolled sheet was subjected to an annealing process for annealing under the conditions shown in Table 2 to obtain a cold-rolled annealed sheet. Some cold-rolled annealed sheets were subjected to temper rolling under the conditions shown in Table 2 and aging treatment under the conditions shown in Table 2.

A test piece was collected from the obtained cold-rolled annealed plate and subjected to structure observation, precipitate analysis, and tensile test. The test method was as follows.
(1) Microstructure observation From the obtained cold-rolled annealed sheet, a specimen for microstructural observation is collected, the cross section in the rolling direction is polished and corroded (Nital solution), and the position is 1/4 to 3/4 of the sheet thickness. The number of fields of view: 3 or more fields of view was observed with an optical microscope (magnification: 200 times) or a scanning electron microscope (magnification: 200 times), and the tissues were identified by imaging. The tissue fraction was determined.
(2) Precipitate analysis From the obtained cold-rolled annealed plate, a test piece for electrolytic extraction was collected, and the amount of N was analyzed for the electrolytic extraction residue extracted by the constant potential electrolysis method in the AA-based electrolytic solution. The amount of precipitated N was defined as the amount of N that is a precipitate containing N. The obtained amount of precipitated N was subtracted from the total amount of N in the steel to obtain a solid solution N amount. In addition, it analyzed also about N content after an annealing process.
(3) Tensile test From the obtained cold-rolled annealed plate, a JIS 13B tensile test piece was collected so that the tensile direction was the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241. Tensile properties (yield point YP, tensile strength TS) were determined.

  The obtained results are shown in Table 3.

  Each of the inventive examples has a yield strength YP: high YP of 400 MPa or more and a low tensile strength TS of 1.25 YP or less, which is a high strength thin steel sheet having excellent dent resistance and less spring back. Yes. On the other hand, in the comparative example that is out of the scope of the present invention, the yield strength YP is less than 400 MPa, or the tensile strength TS is higher than 1.25 YP, and the steel plate has a large spring back.

Claims (5)

% By mass
C: 0.0005 to 0.0250%, Si: 1.0% or less,
Mn: 0.1-2.5%, P: 0.10% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.015-0.500%
A composition comprising the balance Fe and inevitable impurities,
It contains a ferrite phase with an area ratio of 70% or more and a martensite phase with an area ratio of 5% or less, and the balance consists of phases other than the ferrite phase and martensite phase, and the N-containing precipitate is 0.010% by mass in terms of N With the organization adjusted below,
A high strength thin steel sheet with excellent dent resistance, characterized by having a tensile strength with a yield strength YP in the rolling direction of 400 MPa or more and a tensile strength TS of 1.25 YP or less. It is a manufacturing method of a thin steel sheet that is subjected to a hot rolling process, a cold rolling process, and an annealing process on a steel material,
The steel material in mass%,
C: 0.0005 to 0.0250%, Si: 1.0% or less,
Mn: 0.1-2.5%, P: 0.10% or less,
S: 0.05% or less, Al: 0.05% or less,
N: A steel material containing less than 0.015% and having the balance Fe and inevitable impurities,
In the hot rolling step, the steel material is heated to a temperature of 1100 ° C. or higher, subjected to rough rolling and finish rolling at a finish rolling finish temperature of 850 ° C. or higher, and wound at a winding temperature of 650 ° C. or lower. Process,
The cold rolling step is a step of performing cold rolling with a cold rolling ratio of 70 to 85%,
The annealing step includes 0.03 to 50% by volume of ammonia and the remainder is made of an inert gas, and the annealing process is performed at an annealing temperature not lower than 550 ° C. and not higher than TN temperature defined by the following formula (1). It is a process that is a process for holding the holding time,
A method for producing a high-strength thin steel sheet, characterized by producing a thin steel sheet having tensile properties with a yield strength YP in the rolling direction of 400 MPa or more and a tensile strength TS of 1.25 YP or less.
Record
TN (℃) = 723-10.7 × [Mn] (1)
Where [Mn]: Mn content (% by mass) The method for producing a high-strength thin steel sheet according to claim 2, wherein the predetermined holding time th at the annealing temperature satisfies the following expression (2).
Record
th (s) ≥ 4.5 x 10 ⁴ x (plate thickness [m]) ² x exp (9.5 x 10 ³ / T [K]) (2)
Where T: annealing temperature (K)   4. The tempering step of performing temper rolling with an elongation rate of 2% or less and the aging treatment step of performing an aging treatment at a temperature of 200 ° C. or less are performed following the annealing step. The manufacturing method of the high strength thin steel plate as described. The method for producing a high-strength thin steel sheet according to claim 4, wherein the holding time ta at the aging temperature satisfies the following expression (3).
Record
ta (s) ≧ 4.8 × 10 ⁻¹⁰ × T × exp (9500 / T) (3)
Where T: Aging temperature (K)