JP2002146478A - HIGH TENSILE STRENGTH COLD ROLLED STEEL SHEET HAVING HIGH r-VALUE, EXCELLENT STRAIN AGE HARDENING CHARACTERISTIC AND COLD NON-AGING PROPERTY AND ITS PRODUCTION METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet having high formability and excellent strain age hardening characteristics and suitable for automobile parts requiring formability of a relatively high grade and to provide its inexpensive production method. SOLUTION: A steel slab having a composition containing 0.025 to 0.15% C, in which the contents of Si, Mn, P and S are controlled in optimum ranges, and containing <=0.02% Al and 0.0050 to 0.0250% N and also satisfying N/Al: >=0.3 is heated at >=1,000 deg.C and is subjected to hot finish rolling where FDT is >=800 deg.C. After the finish of the hot finish rolling, the steel is rapidly cooled at >=40 deg.C/s and is coiled at <=650 deg.C. The steel is next cold-rolled. The steel is thereafter subjected to box annealing at the recrystallization temperature to 800 deg.C and the subsequent continuous annealing in a two phase region. The steel is rapidly cooled to form a cold rolled steel sheet having a structure where the area ratio of ferrite with the average grain size of <=10 μm is >=80%, and that of martensite is >=2%, containing solid solution N by >=0.0010% and having an (r) value of >=1.3, excellent strain age hardening characteristics and non-aging properties.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-workability, high-tensile cold-rolled steel sheet and a plated steel sheet which are suitable mainly for use in an automobile body.
The present invention relates to a high-tensile cold-rolled steel sheet having a high r value of 0 MPa or less, excellent strain aging hardening characteristics and non-aging at room temperature, a plated steel sheet thereof, and a method for producing the cold-rolled steel sheet. The high-tensile cold-rolled steel sheet of the present invention is suitable for use in relatively severe drawing. In addition, the steel sheet in the present invention includes a steel sheet and a steel strip.

Further, in the present invention, "excellent strain aging hardening characteristics" means that after pre-deformation with a tensile strain of 5%, a temperature of 170.degree.
When the aging treatment is carried out for 20 minutes, the amount of deformation stress increase before and after this aging treatment (referred to as BH amount; BH amount = yield stress after aging treatment-pre-deformation stress before aging treatment) is 80 MPa or more, and The amount of increase in tensile strength before and after the strain aging treatment (pre-deformation + aging treatment) (referred to as ΔTS; ΔTS = tensile strength after aging treatment-tensile strength before pre-deformation) is 40 MPa or more. I do.

[0003]

2. Description of the Related Art Reduction of vehicle body weight in automobiles has become an extremely important issue in connection with recent emission regulations due to global environmental problems. In order to reduce the body weight of an automobile, it is effective to increase the strength of a steel plate used in large quantities, that is, to apply a high-tensile steel plate to reduce the thickness of the steel plate used.

[0004] However, even in the case of automobile parts using thin high-strength steel sheets, the performance imposed in accordance with their roles must be sufficiently exhibited. Such performance includes, for example, static strength against bending and torsion deformation, fatigue resistance, impact resistance, and the like. High-strength steel sheets applied to automotive parts need to have excellent properties after forming. These properties are related to the strength of the steel sheet after forming, and in order to achieve a reduction in thickness, it is necessary to set the lower limit of the strength of the high-tensile steel sheet used.

On the other hand, in the process of manufacturing automobile parts,
Press forming is performed on steel sheets, but if the strength of the steel sheets is too high, press freezing will deteriorate the shape freezing property, and the ductility will deteriorate, causing problems such as cracking and necking during forming. (Eg, resistance to dents caused by local compressive load) deteriorated, which hindered the application of high-tensile steel sheets to automobile bodies.

[0006] As a method for overcoming this, for example, in the case of cold-rolled steel sheets for outer panel, ultra-low carbon steel is used as a material.
There is known a steel sheet in which the amount of C finally remaining in a solid solution state is controlled to an appropriate range. This kind of steel sheet is kept soft during press forming, secures shape freezing and ductility, and yield stress of the strain aging hardening phenomenon that occurs in the paint baking process of 170 ° C × 20 minutes after press forming is performed. The aim is to secure the dent resistance by obtaining a rise. In this kind of steel sheet, C is dissolved in steel during press forming and is soft,
On the other hand, after press molding, in the coating baking process, solid solution C is fixed to dislocations introduced during press molding, and the yield stress increases.

However, in this type of steel sheet, the yield stress increase due to strain age hardening is suppressed low from the viewpoint of preventing the occurrence of a strainer strain that becomes a surface defect. For this reason, there has been a drawback that the part that actually contributes to the weight reduction of components is small. On the other hand, for applications where appearance does not matter much, baking hardenability can be improved by using a steel sheet with a further increased bake hardening amount using solid solution N or a composite structure composed of ferrite and martensite. Further improved steel sheets have been proposed.

For example, JP-A-60-52528 discloses that C:
0.02-0.15%, Mn: 0.8-3.5%, P: 0.02-0.15%,
Al: 0.10% or less, N: 550% steel containing 0.005 to 0.025%
A method for producing a high-strength thin steel sheet having both good ductility and spot weldability, in which hot-rolling at a temperature of not more than ° C. and annealing after cold-rolling as controlled cooling heat treatment is disclosed. The steel sheet manufactured by the technique described in JP-A-60-52528 has a mixed structure composed of a low-temperature transformation product phase mainly composed of ferrite and martensite, has excellent ductility, and is actively added. The purpose of the present invention is to obtain high strength by utilizing strain aging at the time of paint baking with N.

However, according to the technique described in Japanese Patent Application Laid-Open No. 60-52528, the increase in the yield stress YS due to strain age hardening is large, but the increase in the tensile strength TS is small.
Since there is a large variation in mechanical properties such as a large increase in the yield stress YS and a large variation, it is not possible to expect a steel sheet to be thin enough to contribute to a reduction in the weight of automobile parts currently demanded.

In Japanese Patent Publication No. 5-24979, C: 0.
08-0.20%, Mn: 1.5-3.5%, with the composition of the balance Fe and unavoidable impurities, the structure was composed of uniform bainite with a ferrite content of 5% or less or bainite partially containing martensite. Bake hardenable high tensile strength cold rolled steel sheets are disclosed. The cold-rolled steel sheet described in JP-B-5-24979 has a cooling range of 400 to 20 in the cooling process after continuous annealing.
By rapidly cooling the temperature range of 0 ° C. and gradually cooling it thereafter, the structure is changed to a structure mainly composed of bainite, and an unprecedented high bake hardening amount is to be obtained.

[0011] However, in the steel sheet described in Japanese Patent Publication No. 5-24979, the yield strength increases after baking paint, and a high bake hardening amount, which has never existed in the past, can be obtained, but the tensile strength can be increased. However, when applied to a strength member, improvement in fatigue resistance and impact resistance after molding cannot be expected. For this reason, there remains a problem that it cannot be applied to applications that require strong fatigue resistance and impact resistance.

Further, the above-mentioned conventional steel sheet is excellent in the strength evaluation after the paint baking treatment by a simple tensile test, but has a large variation in the strength when plastically deformed according to the actual pressing conditions. It was not always enough to be applied to parts that required reliability. As for hot rolled steel sheets, for example, Japanese Patent Publication No. 8-23048 proposes a steel sheet having a composite structure of ferrite and marnsite to further improve bake hardenability. . However, although the steel sheet manufactured by the technique described in Japanese Patent Publication No. 8-23048 was manufactured at an extremely low winding temperature, although the tensile strength increased due to strain age hardening, the yield stress increased. There is a problem that the dispersion is large and the fluctuation of the mechanical properties is also large. Further, when a thin steel plate having a thickness of 2.0 mm or less is manufactured, there is a problem that the shape of the steel plate is greatly disturbed, and press forming is extremely difficult. The r-value of the hot-rolled steel sheet was as low as 1.0 or less, and was not suitable for applications requiring deep drawability.

[0013]

SUMMARY OF THE INVENTION The present invention overcomes the limitations of the prior art described above, and has high moldability and stable quality characteristics suitable for automobile parts which require relatively high grade moldability. It is easy to mold into automotive parts with complicated shapes, and after molding into automotive parts, it has sufficient strength as an automotive part and can sufficiently contribute to the weight reduction of automobile bodies. Provided is a high-tensile cold-rolled steel sheet having a high r-value, excellent strain aging hardening characteristics and normal temperature non-aging property, a plated steel sheet, and a manufacturing method capable of manufacturing these steel sheets industrially at low cost and without disturbing the shape. The purpose is to: The strain age hardening characteristics in the present invention are as follows. Under the aging condition of preserving at a temperature of 170 ° C. for 20 minutes after pre-deformation at a tensile strain of 5%, the BH amount is 80 MPa or more and ΔTS is 40 MPa.
We aim at the above.

[0014]

Means for Solving the Problems In order to achieve the above object, the present inventors manufactured steel sheets with various compositions and manufacturing conditions, and conducted many material evaluation experiments. as a result,
In fields where high workability is required, N is added as a strengthening element, and by taking advantage of the large strain age hardening phenomenon developed by the action of this strengthening element, it is possible to improve moldability and increase strength after molding. Have been found to be easily compatible.

Further, the present inventors, in order to advantageously utilize the strain age hardening phenomenon due to N, use the strain age hardening phenomenon due to N under the conditions of baking paint for automobiles or more aggressively with the heat treatment conditions after molding. Must be combined advantageously
For this purpose, it has been found that it is effective to control the microstructure of the steel sheet and the amount of solute N in a certain range by optimizing the conditions of hot rolling, cold rolling and cold rolling annealing. In addition, it has been found that in order to stably develop the strain age hardening phenomenon due to N, it is important in terms of composition to control the Al content particularly in accordance with the N content. In addition, the present inventors have made the microstructure of the steel sheet a structure containing a ferrite phase having an average grain size of 10 μm or less in an area ratio of 80% or more without the problem of room-temperature aging deterioration, which was a conventional problem, We have found that N can be fully utilized.

The present inventors further applied box annealing after cold rolling to obtain a steel sheet having a high r value, and further subjected to continuous annealing to obtain a steel sheet having an area ratio of 2% or more in the ferrite phase. By forming a composite structure containing a site phase, an appropriate amount of solid solution N, a base ferrite having a high r value and a martensite having a high strength are advantageously compounded, and a high r value and excellent press formability are provided; and It has been found that a cold rolled steel sheet having excellent strain age hardening characteristics can be obtained.

That is, the present inventors used N as a strengthening element, controlled the Al content in an appropriate range according to the N content, and optimized the hot rolling conditions and the cold rolling annealing conditions to obtain fine powder. By optimizing the visual structure and solid solution N, compared to the conventional solid solution strengthened C-Mn-based steel sheet and precipitation strengthened steel sheet,
It has been found that a steel sheet having a high r value and high ductility, and having remarkably excellent formability and a strain aging hardening characteristic not found in the conventional steel sheet can be obtained.

In addition, the steel sheet of the present invention has a greater strength increase after painting and baking treatment by a simple tensile test than the conventional steel sheet, and has a small variation in strength when plastically deformed according to actual pressing conditions. As a result, stable component strength characteristics can be obtained, and application to components requiring reliability is possible. For example, the part where the plate thickness is reduced due to the large strain tends to be more uniform than the other parts when evaluated by the load capacity of (plate thickness) x (strength), and the strength as a part is stable. You do it.

The present invention has been completed based on the above findings and further studies. That is, in the first present invention, in mass%, C: 0.025 to 0.15%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: a composition containing 0.0050 to 0.0250%, N / Al of 0.3 or more, N as a solid solution of 0.0010% or more, the balance being Fe and unavoidable impurities, and an average crystal grain size of 10 μm or less. A high r-value characterized by having a structure containing a ferrite phase in an area ratio of 80% or more and a martensite phase in an area ratio of 2% or more as a second phase, and an r value of 1.3 or more; A high-tensile cold-rolled steel sheet with excellent strain age hardening properties and non-aging at room temperature.
In the first aspect of the present invention, in addition to the above composition, in addition to the composition, one or more of the following groups a to d: a, Cu, Ni, Cr, and Mo are combined in a total amount of 1.0%. Group b: one or more of Nb, Ti, and V in total
0.1% or less Group C: 0.0030% or less of B Group d: One or two of Ca and REM in total 0.0010 to 0.01
It is preferable to include one or more of 0%.

In the first aspect of the present invention, the high-tensile cold-rolled steel sheet may have an electroplating layer or a hot-dip coating layer on the surface. The second aspect of the present invention relates to a composition of the invention, in which C: 0.025 to
0.15%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.08%
Below, S: 0.02% or less, Al: 0.02% or less, N: 0.0050 ~
0.0250%, and N / Al is 0.3 or more, or further, the following groups a to d: a group: one or more of Cu, Ni, Cr, and Mo in total of 1.0% or less b Group: One or more of Nb, Ti, and V in total
0.1% or less Group C: 0.0030% or less of B Group d: One or two of Ca and REM in total 0.0010 to 0.01
A steel slab containing at least one group or two or more groups of 0% and having a balance of Fe and unavoidable impurities is heated to a slab heating temperature of 1000 ° C. or more, and rough-rolled to form a sheet bar. A hot-rolling process in which the bar is subjected to finish-rolling at a finish-rolling exit temperature of 800 ° C. or higher and a winding temperature of 650 ° C. or lower to obtain a hot-rolled sheet, and pickling and cold-rolling the hot-rolled sheet. And a cold rolling step of forming a cold-rolled sheet, and subjecting the cold-rolled sheet to box annealing at an annealing temperature: a recrystallization temperature or higher and 800 ° C. or lower, and then an annealing temperature: Ac ₁ transformation point to (Ac ₃ transformation point −20) C), followed by a cold-rolled sheet annealing step of cooling at a cooling rate of 10 to 300 ° C / s to a temperature range of 500 ° C or less. A method for producing a high-tensile cold-rolled steel sheet having r-value, excellent strain aging hardening characteristics and non-aging property at room temperature. In the present invention, following the cooling after the continuous annealing, it is preferable to perform an overaging treatment for a residence time of 20 s or more, preferably 120 s or less in a temperature range of 350 ° C. or less to the cooling stop temperature of the cooling, and In the second aspect of the present invention, it is preferable to further perform temper rolling or leveler processing at an elongation of 0.2 to 15% following the cold-rolled sheet annealing step.

In the second aspect of the present invention, it is preferable that successive sheet bars are joined to each other between the rough rolling and the finish rolling. It is preferable to use one or both of a sheet bar edge heater for heating the width end of the sheet bar and a sheet bar heater for heating the length end of the sheet bar during the finish rolling.

[0022]

First, the reasons for limiting the composition of the high-tensile cold-rolled steel sheet of the present invention will be described. Below, mass% is simply%
Indicated by C: 0.025 to 0.15% C is an element that increases the strength of the steel sheet, and controls the structure, which is an important component of the present invention, uniformly and finely.
In order to secure a sufficient amount of the martensite phase, it must be contained at least 0.025%. On the other hand, when the content exceeds 0.15%, the carbide fraction in the steel sheet becomes excessively large, and the ductility and the formability are significantly reduced. As an even more important problem, when the C content exceeds 0.15%, spot weldability, arc weldability, and the like are significantly reduced. Therefore, C is 0.025
Limited to the range of ~ 0.15%. In addition, from the viewpoint of improvement in moldability, the content is preferably set to 0.08% or less. Also,
For applications requiring particularly good ductility, the content is more preferably 0.05% or less.

Si: 1.0% or less Si is a useful element capable of increasing the strength of a steel sheet without remarkably reducing the ductility of the steel.
The content is more preferably 0.1% or more. On the other hand, Si greatly raises the transformation point during hot rolling, making it difficult to ensure quality and shape, or also adversely affects the surface properties, chemical treatment, etc. Is also an adverse element,
In the present invention, the content is limited to 1.0% or less. When the content of Si is 1.0% or less, the above-described adverse effects can be suppressed. It should be noted that the Si content is desirably 0.5% or less for applications in which the required strength level is low, and in particular, where beautiful surface is required.

Mn: 2.0% or less Mn is an effective element for preventing hot cracking due to S.
It is preferable to add according to the amount of S contained, and Mn has a great effect on refinement of crystal grains, and it is desirable to add Mn and use it for improving the material quality. Further, Mn is an extremely effective element for stably generating martensite during rapid cooling after continuous annealing. From the viewpoint of stably fixing S, the content of Mn is desirably 0.2% or more. Mn is an element that increases the strength of the steel sheet. When a strength exceeding TS 500 MPa is required, Mn is desirably contained at 1.2% or more. It is more preferably at least 1.5%.

When the Mn content is increased to this level, there is a great advantage that the mechanical properties of the steel sheet with respect to the change of the hot rolling conditions, particularly the variation of the strain age hardening characteristics are remarkably improved. However, if Mn is excessively contained in excess of 2.0%, it becomes difficult to obtain a high r value, which is one of the important requirements of the present invention, and the ductility is significantly reduced.
Was limited to 2.0% or less. For applications requiring better corrosion resistance and moldability, the content is preferably 1.7% or less.

P: not more than 0.08% P is a useful element as a solid solution strengthening element for steel, and is not less than 0.001%, more preferably 0.015%, from the viewpoint of increasing strength.
It is preferable to contain the above. On the other hand, if contained excessively,
It embrittles the steel and further worsens the stretch flangeability of the steel sheet. In addition, P has a strong tendency to segregate in steel, which results in embrittlement of the weld. Therefore, P is 0.
Limited to 08% or less. In applications where stretch flangeability and weld toughness are particularly important, P is preferably set to 0.04% or less.

S: 0.02% or less S is an element that exists as an inclusion in the steel sheet, reduces ductility of the steel sheet, and further causes deterioration of corrosion resistance.
It is preferable to reduce as much as possible, and in the present invention, S is 0.02
% Or less. In particular, for applications requiring good workability, S is preferably set to 0.015% or less. Further, when particularly excellent stretch flange workability is required, S is preferably set to 0.008% or less. Also,
Although the detailed mechanism is unknown, it is effective to reduce S to 0.008% or less in order to stably maintain the strain aging hardening characteristics of the steel sheet at a high level.

Al: 0.02% or less Al is an element that acts as a deoxidizing agent, improves the cleanliness of steel, and further refines the structure of the steel sheet.
A content of 001% or more is desirable. In the present invention, N in a solid solution state is used as a strengthening element. However, an aluminum-killed steel containing Al in an appropriate range has excellent mechanical properties as compared with a conventional rimmed steel not adding Al. . on the other hand,
Excessive Al content deteriorates the surface properties of the steel sheet and significantly lowers N in the solid solution state, making it difficult to obtain an extremely large amount of strain age hardening. For this reason, in the present invention, the content of Al is limited to 0.02% or less. From the viewpoint of material stability, it is preferable that Al is set to 0.001 to 0.015%. In addition, there is a concern that the reduction of the Al content may lead to coarsening of crystal grains, but in the present invention, it is effective to limit other alloying elements to an optimum amount and to set annealing conditions in an optimum range. Has been prevented.

N: 0.0050 to 0.0250% N is an element that increases the strength of a steel sheet by solid solution strengthening and strain age hardening, and is the most important element in the present invention. Further, in the present invention, by containing an appropriate amount of N, further adjusting the Al content to an appropriate value as described above, further by controlling production conditions such as hot rolling conditions and annealing conditions,
The N in a solid solution state necessary and sufficient for a cold rolled product or a plated product is secured. As a result, the effect of increasing the strength (yield stress and tensile strength) due to solid solution strengthening and strain age hardening is sufficiently exhibited, the tensile strength is 440 MPa or more, and the bake hardening amount (BH
Amount) of 80 MPa or more, and a target value of the mechanical properties of the steel sheet of the present invention of not less than an increase amount ΔTS40 MPa of tensile strength before and after the strain aging treatment can be stably obtained.

If N is less than 0.0050%, the above-mentioned effect of increasing strength is unlikely to be stably exhibited. On the other hand, when N exceeds 0.0250%, the internal defect occurrence rate of the steel sheet increases, and slab cracks and the like during continuous casting also frequently occur. For this reason, N was limited to the range of 0.0050 to 0.0250%. From the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process, N is more preferably in the range of 0.0070 to 0.0170%. If the N content is within the range of the present invention, there is no adverse effect on weldability and the like.

N in the solid solution state: 0.0010% or more In order to ensure sufficient strength with a cold-rolled product and to effectively exert strain age hardening due to N, the N in the steel sheet must be in a solid solution state.
(Also referred to as solute N) must be present at least 0.0010% or more. Here, the amount of solid solution N is calculated from the total amount of N in the steel.
The value obtained by subtracting the amount of precipitated N is defined as solid solution N. Note that the precipitation N
As a method of analyzing the amount, as a result of comparative studies of various methods by the present inventors, it is effective to obtain the amount by an electrolytic extraction analysis method using a potentiostatic electrolysis method. In addition, as a method of dissolving ground iron used for extraction analysis, there are an acid decomposition method, a halogen method, and an electrolytic method. Among them, the electrolysis method can stably dissolve only ground iron without decomposing extremely unstable precipitates such as carbides and nitrides. Electrolyte is performed at a constant potential using an acetyl-acetone system as an electrolytic solution. In the present invention, the result of measuring the amount of deposited N using the potentiostatic electrolysis method showed a good correspondence with the actual material change.

Therefore, in the present invention, the residue extracted by the potentiostatic electrolysis method is subjected to chemical analysis to determine the N content in the residue, which is defined as the deposited N content. In addition, even higher BH
When the amount and ΔTS are required, the amount of solute N is preferably 0.0020% or more, and in order to obtain a higher value, it is preferably 0.0030% or more. Although the upper limit of the amount of solid solution N is not particularly limited,
Even if all the added N amounts remain, the decrease in mechanical properties is small.

N / Al (ratio between N content and Al content): 0.3
As described above, in order to stabilize 0.0010% or more of dissolved N in the product state, it is necessary to limit the amount of Al, an element that strongly fixes N. The N content within the composition range of the present invention (0.
0050-0.0250%) and the Al content (0.02% or less) were examined over a wide range of steel sheets.
By setting it to 3 or more, it was found that the solid solution N in the cold rolled product and the plated product can be stably made 0.0010% or more. For this reason, N / Al was limited to 0.3 or more.

In the present invention, in addition to the above-described composition, the following groups a to d: a group: one or more of Cu, Ni, Cr and Mo in total of 1.
0% or less b group: one or more of Nb, Ti, V is 0.1% in total
Group c: 0.0030% or less of B Group d: One or two of Ca and REM in total of 0.0010 to 0.01
It is preferable to contain 0% of one or more groups.

Elements of group a: Cu, Ni, Cr, and Mo are all elements that contribute to the increase in the strength of the steel sheet, and can be selectively used alone or in combination as necessary. Such an effect is recognized when Cu, Ni, Cr, and Mo are contained at 0.005% or more, respectively. However, if the content is too large, the hot deformation resistance increases, or the chemical conversion property and the surface treatment properties in a broad sense deteriorate, and the welded part is hardened and the welded part formability is deteriorated.
Also, the r value tends to decrease. For this reason, it is preferable that the total of the elements in group a be 1.0% or less. In addition, Mo is
If it is contained in a large amount of 0.05% or more, the r value may be remarkably reduced, and in the present invention, when Mo is contained, the content is preferably limited to less than 0.05%.

Elements of group b: Nb, Ti, and V are elements that contribute to the refinement and uniformity of crystal grains, and can be selectively used alone or in combination as necessary. Such an effect is recognized when Nb, Ti, and V are contained at 0.005% or more, respectively. However, if the content is too large, the hot deformation resistance increases, and the chemical conversion property and the surface treatment properties in a broad sense deteriorate. For this reason, it is preferable that the total of the elements of group b be 0.1% or less.

Element c group: B is an element having the effect of improving the hardenability of the steel, and is required for the purpose of increasing the fraction of the low-temperature transformation phase other than the ferrite phase and increasing the strength of the steel. It can be contained depending on. Such an effect is recognized at a B content of 0.0005% or more. However, if the amount is too large, the hot deformability decreases, and the amount of solute N is reduced by generating BN. Therefore, B is preferably set to 0.0030% or less.

Group d elements: Ca and REM are both elements that are useful for controlling the form of inclusions, and especially when stretch flangeability is required, they are preferably contained alone or in combination. In that case, the total of the elements of group d is 0.0010
%, The effect of controlling the morphology of inclusions is insufficient.
If it exceeds 0%, the occurrence of surface defects becomes noticeable. For this reason, it is preferable to limit the elements of group d to a total range of 0.0010 to 0.010%, whereby the stretch flange workability can be improved without the occurrence of surface defects.

Next, the structure of the steel sheet of the present invention will be described. Area ratio of ferrite phase: 80% or more The cold-rolled steel sheet of the present invention is intended for a steel sheet for automobiles that requires a certain degree of workability.
A structure containing a ferrite phase in an area ratio of 80% or more. If the area ratio of the ferrite phase is less than 80%, it becomes difficult to secure the required ductility as a steel sheet for automobiles requiring workability. In the case where better ductility is required, the area ratio of the ferrite phase is desirably 85% or more. The ferrite referred to in the present invention refers to a so-called polygonal ferrite in which no distortion remains.

Average grain size of ferrite phase: 10 μm or less In the present invention, the average grain size is defined as
A value calculated by the quadrature method specified in TM and A
The larger one of the nominal particle diameters determined by the cutting method specified in the STM (see, for example, Umemoto et al .: Heat Treatment, 24 (1984), 334) is employed.

In the cold-rolled steel sheet of the present invention, a predetermined amount of solute N is secured at the product stage. However, according to experiments and studies by the present inventors, even a steel sheet having the same amount of solute N In some cases, the strain age hardening characteristics varied, and it was found that one of the main factors was the crystal grain size. By setting the average crystal grain size to at least 10 μm or less, preferably 8 μm or less, a high BH amount and ΔTS can be obtained stably.
Although the detailed mechanism is unknown, it is presumed to be related to the segregation and precipitation of alloy elements at the crystal grain boundaries, and further to the effects of processing and thermal history on these.

Therefore, in order to stabilize the strain age hardening characteristics, the average crystal grain size of the ferrite phase needs to be 10 μm or less, preferably 8 μm or less. As described above, in order to ensure ductility as a steel sheet for automobiles, and to stabilize the strain aging hardening characteristics, in the present invention, the average crystal grain size 10
The structure should contain 80% or more ferrite of μm or less in area ratio.

Area ratio of martensite phase: 2% or more The cold-rolled steel sheet of the present invention contains, as a second phase, a martensite phase in an area ratio of 2% or more. 2% martensite phase
Due to the presence of the above, good ductility and a large amount of strain age hardening can be obtained. Although the detailed mechanism is unknown, it is presumed that the existence of the martensite phase causes the strain to be extremely effectively accumulated inside during prestrain processing before aging. Further, the presence of the martensite phase is effective for improving aging deterioration. In order to obtain a good strength-ductility balance and a low yield ratio, the area ratio of the martensite phase is preferably set to 5% or more. The presence of a large amount of martensite phase exceeding 20% has a problem that ductility is reduced. For this reason, the area ratio of the martensite phase is 2% or more, preferably 5% or more and 20% or less.

As the second phase, there is no problem that pearlite, bainite and retained austenite are present in addition to the above-mentioned martensite phase, but in the present invention, the ferrite phase fraction is set to 80% or more and the martensite phase fraction is set to 80% or more. It must be at least 2%, and the total area ratio of pearlite, bainite, and retained austenite is limited to less than 18%.

The cold-rolled steel sheet of the present invention having the above-described composition and structure has a tensile strength (TS) of 440 MPa or more and generally 780 MPa or less, and further has an r value of 1.3 or more by controlling the texture of the matrix ferrite. It is a cold rolled steel sheet having a high r value and excellent strain aging hardening characteristics. For steel sheets with TS below 440MPa,
It cannot be widely applied to members having structural members. To further expand the application range, TS is 500MPa
It is desirable to make the above. If the r value is less than 1.3,
Not applicable to a wide range of press-formed parts. The preferred range of the r value is 1.4 or more.

In the present invention, "excellent strain aging hardening characteristics"
As described above, after pre-deformation of 5% tensile strain, 170 ° C
When aging treatment is carried out at a temperature of 20 min, the amount of deformation stress increase before and after this aging treatment (referred to as BH amount; BH amount = yield stress after aging treatment-pre-deformation stress before aging treatment) is
80 MPa or more, and the amount of increase in tensile strength before and after strain aging treatment (pre-deformation + aging treatment) (denoted as ΔTS; ΔT
S = tensile strength after aging treatment-tensile strength before pre-deformation) is 40
It means above MPa.

When defining the strain age hardening characteristics, the amount of pre-strain (pre-deformation) is an important factor. The present inventors have investigated the effect of the amount of pre-strain on the strain age hardening characteristics, assuming a deformation mode applied to a steel sheet for automobiles. As a result, the deformation stress in the deformation mode is extremely deep drawing. Except in the case of (1), it is possible to arrange by the amount of strain (tensile strain) equivalent to one axis. In actual parts, the amount of strain equivalent to one axis is more than 5%. It has been found that they correspond well to the strengths (YS and TS) obtained later. Based on this finding, in the present invention, the pre-deformation of the strain aging treatment was set to 5% tensile strain.

The conventional paint baking condition is 170 ° C. × 20
min is adopted as a standard. A large amount of solid solution N
When a strain of 5% or more is applied to the steel sheet of the present invention, the hardening is achieved even with a milder (lower temperature) treatment, in other words, the aging condition can be broadened. In general, in order to increase the amount of curing, it is advantageous to hold at a higher temperature and for a longer time as long as the material is not softened by excessive aging.

More specifically, in the steel sheet of the present invention, the lower limit of the heating temperature at which hardening is remarkable after pre-deformation is approximately 100 ° C. On the other hand, if the heating temperature exceeds 300 ° C., the curing hardens, and on the contrary, it tends to soften slightly, and the occurrence of heat distortion and temper color becomes conspicuous. If the holding time is about 30 seconds or more when the heating temperature is about 200 ° C., almost sufficient curing can be achieved. In order to obtain even larger stable curing, the holding time is preferably 60 seconds or more. However, if the holding time exceeds 20 minutes, not only no further curing can be expected, but also the production efficiency is significantly reduced, which is disadvantageous in practical use.

From the above, in the present invention, the aging treatment condition is the heating temperature of the conventional paint baking treatment condition of 170 ° C.
It was determined that the evaluation was performed at 20 ° C. and a holding time of 20 min. Low-temperature heating and
Even under the condition of aging treatment for a short time, large hardening is stably achieved in the steel sheet of the present invention. The method of heating is not particularly limited, and in addition to atmospheric heating by a furnace employed for normal coating baking, any of, for example, induction heating, heating by non-oxidizing flame, laser, plasma, and the like can be preferably used.

It is necessary that the strength of automobile parts be able to withstand complicated external stress loads. Therefore, not only the strength characteristics in a small strain range but also the strength characteristics in a large strain range are important for a material steel plate. . In view of this point, the present inventors set the BH amount of the steel sheet of the present invention to be used as a material for automobile parts to be 80 MPa or more and the ΔTS amount to be 40 MPa or more. More preferably, the BH amount is 100 MPa or more and ΔTS50 MPa or more. Also, the heating temperature during aging treatment is set to a higher temperature,
By setting the holding time to a longer time side, the BH amount and the ΔTS amount can be further increased.

Further, the steel sheet of the present invention has an advantage that the strength can be expected to be increased by about 40% of the complete aging simply by leaving the steel sheet at room temperature for about one week without heating after forming. Further, the steel sheet of the present invention does not undergo aging deterioration (phenomenon in which YS increases and El (elongation) decreases) even when left for a long time at room temperature in a state where it is not formed, which is not found in conventional aging steel sheets. There are benefits too. In addition, in order to prevent defects in actual press forming, the aging at room temperature for 3 months before press forming increases the YS by 30 MPa or less, decreases the elongation by 2% or less, and recovers the yield point elongation. It must be less than 0.2%.

In the present invention, there is no problem if the surface of the cold-rolled steel sheet of the present invention is electroplated or hot-dip plated. These plated steel sheets also exhibit the same amount of TS, BH and ΔTS as before plating. As the type of plating, any of electrogalvanizing, hot-dip galvanizing, alloyed hot-dip galvanizing, electrotin plating, electrochromic plating, and electronickel plating can be preferably applied.

Next, a method for manufacturing the steel sheet of the present invention will be described. The steel sheet of the present invention is basically a steel slab having a composition in the above-described range, which is subjected to rough rolling after heating to form a sheet bar, subjected to finish rolling on the sheet bar, cooled after finishing rolling, and wound up. A hot rolling step of forming a hot rolled sheet, a cold rolling step of performing pickling and cold rolling on the hot rolled sheet to form a cold rolled sheet, performing box annealing on the cold rolled sheet, and then performing continuous annealing It is manufactured by sequentially performing a cold-rolled sheet annealing step.

The slab used in the production method of the present invention is desirably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. In addition, after the slab was manufactured, in addition to the conventional method of once cooling to room temperature and then heating again, direct-feed rolling in which a slab was charged and rolled into a heating furnace as it was without cooling, or slight heat retention was performed. Energy saving processes such as direct rolling, which immediately rolls later, can be applied without any problem. In particular,
Direct rolling is one of useful techniques for effectively securing N in a solid solution state.

First, the reason for limiting the conditions of the hot rolling step will be described. Slab heating temperature: 1000 ° C or higher The slab heating temperature is 1000 ° C as an initial state of hot rolling, in order to secure a necessary and sufficient amount of solid solution N and satisfy the target value of solid solution N in the product. The above is preferable. In addition, 1280 due to the increase in loss due to the increase in oxidation weight
It is desirably set to not more than ° C.

The slab heated under the above conditions is converted into a sheet bar by rough rolling. The conditions for the rough rolling need not be particularly defined, but may be performed according to a conventional method. However, from the viewpoint of securing the amount of dissolved N, it is desirable to perform the treatment in as short a time as possible. Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet. In the present invention, it is desirable to join the successive sheet bars between the rough rolling and the finish rolling and perform continuous rolling. As joining means,
Also in the pressure welding method, it is preferable to use a laser welding method, an electron beam welding method, or the like.

The continuous rolling eliminates the so-called rolling unsteady portions at the leading and trailing ends of the coil (material to be processed), and enables stable hot rolling conditions over the entire length and width of the coil (material to be processed). Become. This is extremely effective in improving the cross-sectional shape and dimensions of not only hot-rolled steel sheets but also cold-rolled steel sheets. In addition, even when cooling on a hot run table after rolling, tension can always be applied, so that the steel plate shape can be kept good.

Further, since the coil tip can be stably threaded by performing continuous rolling, lubricated rolling, which could not be applied due to the problem of threading and biting properties, is applied in single-shot rolling for each normal sheet bar. can do. As a result, the rolling load can be reduced and, at the same time, the surface pressure of the roll can be reduced, and the life of the roll can be extended. Also, in the present invention, on the entrance side of the finishing rolling mill between rough rolling and finish rolling,
Using either or both of a sheet bar edge heater that heats the width end of the sheet bar and a sheet bar heater that heats the length end of the sheet bar, the temperature distribution in the width direction and the length direction of the sheet bar can be obtained. It is preferable to make them uniform. Thereby, the material variation in the steel plate can be further reduced. It is preferable that the sheet bar edge heater and the sheet bar heater are of an induction heating type.

It is desirable to use the sheet bar edge heater to compensate for the temperature difference in the width direction. The heating amount at this time depends on the steel composition and the like, but is preferably set so that the temperature distribution range in the width direction on the finish-rolling exit side is approximately 20 ° C. or less. Next, the temperature difference in the longitudinal direction is compensated by the sheet bar heater. The amount of heating at this time is preferably set such that the temperature at the end of the length is approximately 20 ° C. higher than the temperature at the center.

Finish-rolling outlet temperature: 800 ° C. or higher The finish-rolling outlet temperature FDT is set to 800 ° C. or higher in order to obtain a uniform and fine hot-rolled base plate structure. If the FDT is lower than 800 ° C., the structure of the steel sheet becomes non-uniform, a part of the processed structure remains, and after the cold rolling annealing step, the non-uniform structure of the steel remains without disappearing. For this reason, the danger that various troubles occur during press molding increases. Further, if a high winding temperature is employed in order to avoid the remaining of the processed structure, coarse crystal grains are generated and the same problem occurs. In addition, when the winding temperature is set to a high temperature, the amount of solid solution N is remarkably reduced, so that it is difficult to obtain a target tensile strength of 440 MPa or more. From this, the finish-rolling exit temperature F
DT was 800 ° C. or higher. In order to further improve the mechanical properties, it is desirable that the FDT be 820 ° C. or higher. In particular, the upper limit of the FDT is not specified, but when it is excessively high, the occurrence of scale flaws and the like becomes remarkable. Note that FDT
Is preferably up to about 1000 ° C.

The cooling after finish rolling is not particularly strictly limited, but the following conditions are desirable from the viewpoint of the uniformity of the material in the longitudinal and width directions of the steel sheet. That is, in the present invention, it is desirable that cooling be started immediately (within 0.5 seconds) after finishing rolling, and that the average cooling rate during cooling be 40 ° C./s or more. By satisfying this condition, the high temperature region where AlN precipitates can be rapidly cooled, and N in a solid solution state can be effectively secured. If the cooling start time or the cooling rate does not satisfy the above conditions, the grain growth is so advanced that it is difficult to reduce the crystal grain size, and the precipitation of AlN is accelerated by the strain energy introduced by rolling. There is a possibility that the amount of dissolved N may be lacking, and the structure tends to be non-uniform. In addition, from the viewpoint of ensuring uniformity of the material and shape, the cooling rate should be 300 ° C /
s or less is preferable.

Winding temperature: 650 ° C. or less As the winding temperature CT decreases, the strength of the steel sheet tends to increase. In order to secure the target tensile strength of TS440 MPa or more, it is preferable that CT is 650 ° C. or less. Note that C
When T is less than 200 ° C., the shape of the steel sheet is easily disturbed, and there is a high risk of causing a problem in actual operation, and the uniformity of the material tends to be reduced. For this reason, CT is desirably 200 ° C. or higher. If more uniform material is required, the CT is preferably set to 300 ° C. or higher. The temperature is more preferably 350 ° C. or higher.

In the present invention, lubricating rolling may be performed in finish rolling to reduce the hot rolling load. By performing lubricating rolling, there is an effect that the shape and material of the hot rolled sheet are made more uniform. The coefficient of friction during lubricating rolling is preferably in the range of 0.25 to 0.10.
The operation of hot rolling is further stabilized by combining lubrication rolling and continuous rolling.

The hot-rolled sheet that has been subjected to the above-mentioned hot rolling step is then subjected to pickling and cold rolling in a cold-rolling step to become a cold-rolled sheet. The conditions for pickling may be generally known conditions, and are not particularly limited. When the scale of the hot-rolled sheet is extremely thin, cold rolling may be immediately performed without performing pickling.

[0066] The cold rolling conditions may be generally known conditions, and are not particularly limited. In addition, it is preferable that the cold rolling reduction is 40% or more from the viewpoint of ensuring the uniformity of the structure. Next, the reasons for limiting the conditions of the cold rolling step will be described. The cold-rolled sheet is then subjected to a cold-rolled sheet annealing step including box annealing and continuous annealing. Box annealing temperature: not less than recrystallization temperature and not more than 800 ° C. In the present invention, box annealing is performed on a cold-rolled sheet to control the texture of a ferrite phase as a base material. By controlling the texture of the ferrite phase, it is possible to increase the r value of the product sheet. By this box annealing, a (111) texture desirable for increasing the r-value is easily formed on the product plate.

If the box annealing temperature is lower than the recrystallization temperature, recrystallization is not completed, the texture of the ferrite phase cannot be adjusted, and a high r-value cannot be achieved. On the other hand, if box annealing is performed at a temperature exceeding 800 ° C., the occurrence of surface defects on the steel sheet becomes remarkable, and the initial purpose cannot be achieved. In addition, box annealing,
The annealing is preferably performed in an annealing atmosphere mainly containing nitrogen gas and containing 3 to 5% hydrogen gas. In this case, the heating / cooling rate may be the same as that of ordinary box annealing, and is about 30 ° C./hr.
Further, a higher heating / cooling rate may be obtained by using an annealing atmosphere gas of 100% hydrogen gas.

Continuous annealing temperature: not less than the Ac ₁ transformation point (Ac ₃ transformation point −20 ° C.) and less If the continuous annealing temperature is less than the Ac ₁ transformation point, a martensitic phase is not formed after annealing, while the (Ac ₃ transformation point) If the temperature exceeds (−20 ° C.), a desired texture formed by box annealing is lost due to transformation, so that a product sheet having a high r value cannot be obtained. For this reason, the continuous annealing temperature is preferably set to the Ac ₁ transformation point or more (Ac ₃ transformation point −20 ° C.) or less.

It is preferable that the holding time of the continuous annealing time is as short as possible from the viewpoints of production efficiency, microstructuring of the structure, and securing the amount of dissolved N. On the other hand, from the viewpoint of operation stability, the holding time is preferably set to 10 s or more, and from the viewpoint of refining the structure and securing the amount of dissolved N, 120 s is preferable.
It is preferable to set the following. In addition, from the viewpoint of stabilizing the material, it is more preferable to set the length to 20 s or longer.

Cooling after continuous annealing: Cooling at a cooling rate of 10 to 300 ° C./s to a temperature range of 500 ° C. or less Cooling after soaking in continuous annealing is performed by refining the structure, forming martensite, and dissolving N It is important from the viewpoint of securing quantity. In the present invention, at least up to a temperature range of 500 ° C. or less.
Continuous cooling at a cooling rate of 10 ° C / s or more. Cooling rate is 10 ℃
If it is less than / s, a required amount of martensite, a uniform and fine structure, and a sufficient amount of solid solution N cannot be obtained. On the other hand, when the cooling rate exceeds 300 ° C./s, the amount of supersaturated solid solution C remarkably increases, so that the uniformity of the material in the width direction of the steel sheet decreases. If the cooling stop temperature at a cooling rate of 10 to 300 ° C./s after continuous annealing exceeds 500 ° C., fine structure cannot be achieved.

Overaging treatment conditions: After cooling after continuous annealing, residence time of 20 seconds or more in a temperature range of 350 ° C. or lower below the cooling stop temperature of the cooling. An overaging treatment with a residence time of 20 s or more may be performed in a temperature range of 350 ° C. or less below the temperature. By performing the overaging treatment, the amount of solute C can be selectively reduced while maintaining the amount of solute N. If the residence temperature range is lower than 350 ° C, it takes a long time to reduce solid solution C, which leads to a decrease in productivity.
Preferably, the temperature range is 0 ° C. or higher.

In the temperature range below the cooling stop temperature and above 350 ° C.,
By staying for s or more, the amount of dissolved C can be reduced, and a higher degree of non-aging at room temperature can be achieved. Further improvement can be expected by increasing the residence time, but generally 1
Since the effect tends to be saturated at about 20 s, the residence time is preferably set to 120 s or less. In order to obtain a large amount of strain age hardening, it is advantageous to use both solid solution C and solid solution N. However, when solid solution C is used, aging deterioration at room temperature becomes remarkable, Will be limited. Therefore, in order to manufacture a versatile strain-age hardened steel sheet, it is preferable to perform an overaging treatment after securing a sufficient amount of solid solution N.

When manufacturing a high-tensile cold-rolled steel sheet having a hot-dip coating layer on the surface of the high-tensile cold-rolled steel sheet of the present invention, continuous annealing after box annealing is performed in a continuous hot-dip plating line, and continuous annealing is performed. Subsequent to the subsequent cooling, hot-dip galvanizing or further alloying may be performed to produce a hot-dip galvanized steel sheet. Temper rolling or leveler processing: elongation 0.2 to 15% In the present invention, subsequent to the cold rolling annealing step, temper rolling or leveler processing may be performed for the purpose of shape correction and roughness adjustment. If the total elongation of the temper rolling or leveling is less than 0.2%, the intended purposes of shape correction and roughness adjustment cannot be achieved. On the other hand, if it exceeds 15%, the ductility is significantly reduced. In addition, although the processing types are different between the temper rolling and the leveler processing, it has been confirmed that there is no significant difference between the two. Temper rolling, leveler processing,
It is effective even after plating.

[0074]

(Embodiment 1) Next, an embodiment of the present invention will be described. Molten steel having the composition shown in Table 1 was smelted in a converter and made into a steel slab by a continuous casting method. These steel slabs were heated under the conditions shown in Table 2 (with some flakes charged), rough-rolled into sheet bars, and then subjected to a hot-rolling step of finish rolling under the conditions shown in Table 2 to obtain a hot-rolled sheet. did. In some sheet bars, successive sheet bars were joined to each other by a melt pressure welding method, and continuous rolling was performed.

These hot-rolled sheets were formed into cold-rolled sheets by a cold rolling process comprising pickling and cold rolling under the conditions shown in Table 2.
Subsequently, box annealing and subsequent continuous annealing were performed on these cold-rolled sheets under the conditions shown in Table 2. Part of the steel sheet was subjected to temper rolling after the cold rolling annealing step. In addition, it carried out also in the case without box annealing. The annealing temperatures of box annealing were all higher than the recrystallization temperature.

With respect to the obtained cold-rolled annealed sheet,
The microstructure, tensile properties, r value measurement, strain aging hardening properties, and aging properties were investigated. The survey method is as follows. (1) Investigation of the amount of solute N The amount of solute N was determined by subtracting the amount of precipitated N from the total amount of N in steel determined by chemical analysis. The amount of precipitated N was determined by an analytical method using a potentiostatic electrolysis method. (2) Microstructure A specimen is collected from each cold-rolled annealed plate, and a microstructure is imaged using a light microscope or a scanning electron microscope for a cross section (C cross section) orthogonal to the rolling direction, and an image analyzer is used. Was used to determine the structure fraction of ferrite and the type and structure fraction of the second phase.

The average grain size of the ferrite was determined by rolling
From the micrograph of the cross section (C cross section) perpendicular to the direction
Value calculated by the quadrature method specified in ASTM or AST
Any of the nominal particle sizes obtained by the cutting method specified in M
Or the larger one was adopted. (3) Tensile properties A JIS No. 5 test piece was sampled from each cold-rolled annealed plate in the rolling direction.
Crosshead speed 10mm / mi according to JIS Z 2241
n, a tensile test was performed, yield stress YS, tensile strength TS,
The elongation El was determined. (4) Strain aging hardening characteristics A JIS No. 5 test piece was sampled from each cold-rolled annealed sheet in the rolling direction.
As a pre-deformation, a tensile pre-strain of 5% is given here.
A heat treatment equivalent to a paint baking treatment at 170 ° C for 20 minutes was performed.
The tensile test was performed at a crosshead speed of 10 mm / min.
Predeformation-Tensile properties after paint baking (yield stress YS_BH,
Tensile strength TS_BH), And BH amount = YS _BH-YS_Five%, Δ
TS = TS_BH-TS was calculated. YS_Five%Is the product
The deformation stress when the plate is pre-deformed by 5%, YS_BH, T
S_BHIs the pre-deformation-yield stress and tensile strength after paint baking
Yes, TS is the tensile strength of the product plate. (5) R value measurement The rolling direction (L direction) of each cold-rolled annealed sheet, 45
° direction (D direction), 90 ° direction to rolling direction (C direction)
JIS No. 5 test pieces were collected from 15% for these specimens
The width and thickness strains of each specimen when uniaxial tensile strain of
The ratio of width strain to thickness strain is calculated according to the definition formula, r = ln (w / w₀) / Ln (t / t₀(Where w₀, T₀Is the width and thickness of the specimen before the test
And w and t are the width and thickness of the test piece after the test.
You. ) Was used to determine the r value in each direction. For details, see
In order to improve the degree, measure the elongation strain and width strain and make the volume constant and process
Then, it was calculated from the above defined formula. The obtained r value in each direction
From the following equation r_mean= (R_L+2 r_D+ R_c) / 4 gives the average r value r_meanI asked. Where r_LRolled
R value in the direction (L direction), and r_DIs the rolling direction (L direction
R) in the 45 ° direction (D direction) with respect to _cIs
The r value in the 90 ° direction (C direction) with respect to the rolling direction (L direction)
is there. (6) Aging characteristics A JIS No. 5 test piece was collected from each cold-rolled annealed sheet and
After aging at 50 ° C x 200, a tensile test was performed.
gave. From the results obtained, the difference in yield elongation before and after aging treatment
ΔY-El was determined, and aging resistance was evaluated. ΔY-El is 0.3 or less
If it was, it was evaluated that it was excellent in aging resistance as non-aging property.
If ΔY-El is 0.2 or less under the above-mentioned conditions, the temperature
It was evaluated as having aging properties.

The surfaces of No. 17 and No. 18 steel sheets were subjected to hot dip galvanizing in-line after continuous annealing as shown in the table to form galvanized steel sheets, and various properties were similarly evaluated. Table 3 shows the results.

[0079]

[Table 1]

[0080]

[Table 2]

[0081]

[Table 3]

In each of the examples of the present invention, excellent ductility, a remarkably high BH content and ΔTS are exhibited, and excellent strain aging hardening characteristics, high r value with an average r value of 1.3 or more, and non-aging at room temperature are obtained. Have. The properties of the hot-dip galvanized steel sheets No. 17 and No. 18 shown in Table 3 were almost the same as those of the cold-rolled steel sheets continuously annealed. On the other hand, the comparative examples out of the range of the present invention show that the ductility is deteriorated, the BH amount,
Depending on whether the ΔTS is small or the aging deterioration is remarkable, the steel sheet does not have all the target properties and cannot be said to be a steel sheet having sufficient properties.

In steel sheet No. 11, the amounts of C and N were out of the ranges of the present invention, and the amounts of solute N and martensite were out of the ranges of the present invention. Therefore, the amounts of BH and ΔTS decreased, and ΔY-El decreased. It has increased. In addition, in steel sheet No. 12, Al, N / Al, and N were out of the range of the present invention, and the amount of solute N was out of the range of the present invention.
The average crystal grain size of ferrite deviates from the range of the present invention,
Therefore, the BH amount and ΔTS decrease, and ΔY-El increases.

In the steel sheet No. 13, the slab heating temperature and the FDT were out of the preferred ranges of the present invention, the amount of solute N and the amount of martensite deviated from the range of the present invention, and the average ferrite grain size was within the range of the present invention. , And therefore, the r value, the BH amount,
ΔTS has decreased. In addition, for steel sheet No. 14, the coiling temperature after hot rolling deviated from the range of the present invention, the amount of solute N departed from the range of the present invention, and the average crystal grain size of ferrite departed from the range of the present invention. , R value, BH amount, and ΔTS decrease.

In steel sheet No. 15, the continuous annealing temperature was out of the preferred range of the present invention, martensite was not generated, and the average crystal grain size of ferrite was out of the range of the present invention.
The BH amount and ΔTS decrease and ΔY-El increases. Also,
In steel sheet No. 16, the box annealing was not performed, and a desirable texture was not developed, so that the r value was particularly lowered. The average grain size of ferrite and the area ratio of martensite are also outside the scope of the present invention.

(Example 2) Steel having the composition shown in Table 4 was
A slab was formed in the same manner as in Example 1, and the slab was prepared as shown in Table 5.
The sheet was heated under the conditions shown in (1) and subjected to rough rolling to form a sheet bar having a thickness of 30 mm, and then into a hot-rolled sheet by a hot rolling step of performing finish rolling under the conditions shown in Table 5. In addition, about a part, after rough rolling, the adjacent sheet bars on the finishing rolling entry side were joined by the fusion welding method, and were continuously rolled. In addition, the sheet bar temperature was adjusted at the width end and the end in the length direction of the sheet bar by using an induction heating type sheet bar edge heater and a sheet bar heater.

These hot-rolled sheets were cold-rolled to a thickness of 1.6 mm by a cold rolling process including pickling and cold rolling under the conditions shown in Table 5. Next, these cold-rolled sheets were subjected to the conditions shown in Table 5,
Box annealing was performed, followed by continuous annealing in a continuous annealing furnace. The annealing temperatures in the box annealing were all higher than the recrystallization temperature. About the obtained cold-rolled annealed sheet, the amount of dissolved N, microstructure, tensile properties, r value measurement, and strain aging hardening properties were examined in the same manner as in Example 1. In addition, tensile properties were examined at 10 locations in each of the width direction and the longitudinal direction of each cold-rolled annealed sheet, and variations in yield strength, tensile strength, and elongation were examined. The variation is represented by the difference between the maximum value and the minimum value in all the measured locations, for example, δYS = (maximum value of YS) − (minimum value of YS).

Table 6 shows the results.

[0089]

[Table 4]

[0090]

[Table 5]

[0091]

[Table 6]

Each of the examples of the present invention has excellent strain aging hardening characteristics and a high r value, and despite the fluctuations in the manufacturing conditions.
A stable and extremely high BH amount, ΔTS, and average r value were shown. Further, in the example of the present invention, it was confirmed that by performing continuous rolling and adjusting the temperature in the longitudinal direction and the width direction of the sheet bar, the thickness accuracy and shape of the product steel plate were improved, and the variation in the material was reduced.

[0093]

According to the present invention, the tensile strength (TS) 44
High strain cold-rolled steel sheet having a strain-age hardening property at which the yield stress increases with pre-deformation-paint baking treatment at 80 MPa or more and the tensile strength increases with 40 MPa or more, and an average r value of 1.3 or more and a high r value. Can be manufactured inexpensively and without disturbing the shape, and it has a remarkable industrial effect. Further, when the high-tensile cold-rolled steel sheet of the present invention is applied to an automobile part, the tensile strength together with the yield stress is increased by 1 to 1.5 grades from 440 MPa to 540 to 780 MPa, for example, by paint baking treatment, and a stable high part is obtained. Characteristics can be obtained, and the thickness of the steel plate to be used is, for example, 2.0 mm to 1.6 mm
It is also possible to reduce the thickness and the grade by one grade as compared with the prior art, and there is also an effect that it is possible to sufficiently contribute to reducing the weight of the vehicle body. Further, the cold-rolled steel sheet of the present invention is suitable as a plating base sheet such as a hot-dip galvanized steel sheet.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 F term (Reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA31 EA32 EA36 EB06 EB07 EB08 EB09 EB11 FA02 FA03 FB03 FC03 FC04 FD04 FE01 FE02 FH01 FH03 FJ02 FJ05 FK03 FL01 FM01 FM02 GA05 HA01 JA06 JA07

Claims (7)

[Claims] 1. Mass%, C: 0.025 to 0.15% Si: 1.0% or less, Mn: 2.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.0050 to 0.0250 % And N / Al is 0.3 or more,
A composition containing 0.0010% or more, with the balance being Fe and unavoidable impurities, and a ferrite phase having an average crystal grain size of 10 μm or less, containing 80% or more in area ratio, and a second phase containing 2% or more of martensite in area ratio. A structure including a site phase, and
Value: 1.3 or more, a high-tensile cold-rolled steel sheet having a high r value, excellent strain aging hardening characteristics, and non-aging property at room temperature. 2. The high-tensile cold-rolled steel sheet according to claim 1, further comprising one or more of the following groups a to d in mass% in addition to the composition. Group a: 1.0% or less in total of one or more of Cu, Ni, Cr, and Mo Group b: One or two or more of Nb, Ti, V in total
0.1% or less Group C: 0.0030% or less of B Group d: One or two of Ca and REM in total 0.0010 to 0.01
0% 3. A high-tensile cold-rolled steel sheet having an electroplating layer or a hot-dip coating layer on the surface of the high-tensile cold-rolled steel sheet according to claim 1. 4. In mass%, C: 0.025 to 0.15% Si: 1.0% or less, Mn: 2.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.0050 to 0.0250 %, And N / Al is not less than 0.3, a steel slab having a composition of not less than 0.3 is heated to a slab heating temperature of not less than 1000 ° C., rough-rolled to form a sheet bar, and a finish-rolling exit temperature on the sheet bar is:
A hot rolling process in which finish rolling is performed at 800 ° C. or more and a winding temperature: 650 ° C. or less to obtain a hot rolled sheet, and a cold rolling is performed by pickling and cold rolling the hot rolled sheet to form a cold rolled sheet. Cold rolling, annealing the cold-rolled sheet with box annealing at an annealing temperature: not lower than the recrystallization temperature and not higher than 800 ° C., and then performing continuous annealing at an annealing temperature: Ac ₁ transformation point to (Ac ₃ transformation point −20 ° C.) Thereafter, a cold rolled sheet annealing step of cooling at a cooling rate of 10 to 300 ° C./s to a temperature range of 500 ° C. or less is sequentially performed. R value: high r value of 1.3 or more and excellent strain age hardening A method for producing a high-tensile cold-rolled steel sheet having characteristics and non-aging property at room temperature. 5. The overaging treatment according to claim 4, wherein, after the cooling after the continuous annealing, an overaging treatment for a residence time of 20 s or more is performed in a temperature range of 350 ° C. or less below the cooling stop temperature of the cooling. Manufacturing method of high tensile cold rolled steel sheet. 6. The high-tensile cold-rolled steel sheet according to claim 4, further comprising, after the cold-rolled sheet annealing step, temper rolling or leveling at an elongation of 0.2 to 15%. Manufacturing method. 7. The method according to claim 4, further comprising one or more of the following groups a to d in mass% in addition to the composition. Manufacturing method of tension cold-rolled steel sheet. Group a: 1.0% or less in total of one or more of Cu, Ni, Cr, and Mo Group b: One or two or more of Nb, Ti, V in total
0.1% or less Group C: 0.0030% or less of B Group d: One or two of Ca and REM in total 0.0010 to 0.01
0%