JP4622187B2 - Cold-rolled steel sheet, cold-rolled steel sheet having excellent strain age hardening characteristics and no room temperature aging deterioration, and methods for producing them - Google Patents

Description

【００４６】
【表２】

【００４７】
表３に、調査の結果を示した。本発明の条件に適合したものは、引張特性、歪時効特性および室温時効特性のいずれも良好な特性を示しているのに対し、本発明の条件を外れるNo.15，16，17は、いずれかの特性を満たしていない。特に、No.16は、Ｃ量が高いため、スポット溶接性に劣るため自動車用鋼板としては不適である。
【００４８】
【表３】

【００４９】
（実施例２）
Ｃ：0.015mass％、Si：0.005mass％、Mn：0.55mass％、Ｐ：0.009mass％、Ｓ：0.005mass％、Al：0.015mass％、Ｎ：0.0126mass％、Ｎ(mass％)／Al(mass％)：0.84の鋼スラブを素材とし、表４に示したような製造条件で冷延鋼板とし、引き続き、溶融亜鉛めっき処理を常法に従って行い溶融亜鉛めっき鋼板を製造し、実施例１と同様にして機械的性質を調査した。その結果を表５に示す。
表５から、本願発明の製造方法に従えば、歪時効硬化特性、室温時効特性に優れ、かつ高加工性、高強度の溶融亜鉛めっき鋼板の製造が可能であることがわかる。
【００５０】
【表４】

【００５１】
【表５】

【００５２】
【発明の効果】
以上説明したように、本願発明によれば、化学組成、熱延条件および冷延、焼鈍条件を適正化し、最終の製品段階での固溶状態のＮを十分な量確保しつつ、固溶Ｃを制限することにより、加工するときは軟質で高い成形性を有しながら、加工後の歪時効により大きな強度上昇が得られ、かつ室温時効劣化のない冷延鋼板が得られる。また、この冷延鋼板を用いて、ほぼ同等の機械的性質を有する溶融亜鉛めっき鋼板などのめっき鋼板も製造できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cold-rolled steel sheet and a cold-rolled steel sheet that are excellent in strain age hardening characteristics and are not deteriorated by aging at room temperature, and methods for producing them.
[0002]
[Prior art]
In general, various thin steel sheets are used for the body of an automobile, and cold-rolled steel sheets with particularly excellent formability are used in many cases. Recently, however, the amount of high-tensile steel sheets is increasing. The reason for this is that reduction of the weight of the vehicle body is demanded due to automobile exhaust gas regulations derived from recent global environmental problems, and adopting a high-tensile steel plate can contribute to thinning of the steel plate. However, it is not sufficient to simply use high-tensile steel plates. For example, in the case of automotive parts, the required characteristics differ depending on the parts, but the static strength against bending and torsional deformation, fatigue strength, impact resistance characteristics, etc. Is required. Since these characteristics depend on the strength of the steel sheet, when reducing the weight (thinning) of parts, a lower limit strength is set for the steel sheet to be used, and it is required to satisfy this lower limit strength.
[0003]
In general, when a part is press-formed from a high-strength steel plate having a high yield stress or tensile strength, there are the following problems.
1) As the yield strength increases, the amount of springback increases and the shape freezeability decreases, resulting in problems such as twisting, twisting, and warping.
2) The ductility of the steel sheet is reduced, and problems such as cracking and necking occur during forming.
3) When thinning is achieved simply by applying a high-strength steel sheet, rigidity and dent resistance characteristics (resistance to dents caused by local compressive load loading) are reduced.
These problems hinder the application of high-strength steel sheets to automobile bodies.
[0004]
As a method for overcoming such problems, for example, in the cold-rolled steel sheet for automobile outer plates, a steel sheet that uses ultra-low carbon steel as a raw material, and finally controls the amount of C remaining in a solid solution state within an appropriate range. Manufacturing techniques have been proposed. This technique utilizes a hardening phenomenon caused by strain aging that occurs in a paint baking process of about 170 ° C. × 20 minutes performed after press molding. That is, it is soft at the time of press molding and does not cause a decrease in shape freezing property, and strength and dent resistance as a part are to be secured by strain age hardening by baking.
However, in the case of paint bake hardening type cold-rolled steel sheet, the hardening amount of which is increased by increasing the amount of solute C, aging deterioration at room temperature, that is, a remarkable increase in yield stress and a decrease in elongation occur over time. Is known and is difficult to apply in a wide range. Furthermore, the need to prevent stretcher strain, which is a surface defect, makes the amount of increase in strength limited, and there is a drawback that the contribution to thinning of the steel sheet is small.
[0005]
Japanese Patent Publication No. 8-23048 discloses a method of manufacturing a hot rolled steel sheet having a larger bake hardenability by making the structure a composite structure composed of ferrite and martensite. However, since the steel sheet manufactured by this method winds the coil at an extremely low temperature, the tensile strength increases due to strain aging hardening, but there is a problem that the mechanical properties fluctuate, in particular, the yield stress varies greatly. It was. In addition, in the case of a thin steel plate having a thickness of 2.0 mm or less, there is a problem that the shape of the steel plate is greatly deteriorated due to cooling strain, and press forming becomes extremely difficult. In the case of cold-rolled steel sheets, a technique for manufacturing a steel sheet having such a large strain age hardening characteristic is not disclosed.
[0006]
[Problems to be solved by the invention]
As described above, the conventional technology is an industrially, inexpensively and stably manufactured steel sheet that satisfies various properties such as high workability, excellent strain age hardening characteristics, and low aging deterioration. It wasn't possible.
[0007]
An object of the present invention is to propose a cold-rolled steel sheet and a cold-rolled steel sheet that require high workability and have high strength and stable quality, and methods for producing the same.
[0008]
[Means for Solving the Problems]
Now, in order to solve the above-mentioned problems and realize the above object, the inventor manufactured steel sheets having various component compositions and conducted material evaluation experiments. As a result, the use of nitrogen, which has not been actively used for steel sheets that require workability, as a strengthening element, and the large strain age hardening ability of the nitrogen, can be applied to automobile paint baking treatment or further In order to make more effective use in combination with aging treatment by post heat treatment after forming, it has been found that it is effective to control the microstructure of the steel sheet. Also, in order to make the strain age hardening characteristics appear stably in cold rolled steel sheets, etc., it is particularly important to control the amount of Al added, and to reduce aging deterioration at room temperature. Obtained the knowledge that it is effective to suppress the solid solution C used for strain age hardening in the prior art rather low.
[0009]
In other words, solid solution N was used as a strengthening element, the Al content was controlled within an appropriate range, and hot rolling, cold rolling, and annealing conditions were optimized to optimize the microstructure and the amount of solid solution N. In this case, it has become clear that the present invention has remarkably superior formability and strain age hardening characteristics compared to conventional solid solution strengthened C-Mn steel and precipitation strengthened steel.
It has been found that the cold-rolled steel sheet according to the present invention developed based on such knowledge can stably obtain various characteristics particularly when applied to automotive parts.
Moreover, it turned out that the plated steel plate which uses the cold-rolled steel plate as the original plate also has excellent formability and strain age-hardening properties as in the cold-rolled steel plate.
[0010]
The present invention developed based on the above knowledge is basically
C: 0.01-0.15 mass%, Si: 0.1 mass% or less,
Mn: 3.0 mass% or less, P: 0.08 mass% or less,
S: 0.02 mass% or less, Al: 0.02 mass% or less,
N: including 0.0050-0.0250 mass%,
Including N as a solid solution state of 0.0030 mass% or more,
The content of C as a solid solution satisfies 3 mass ppm or less, and
N (mass%) / Al (mass%) is 0.3 or more,
It is a cold-rolled steel sheet having excellent strain age hardening characteristics and no room temperature aging deterioration, characterized in that the balance is composed of Fe and inevitable impurities and the ferrite phase has a structure with an area ratio of 50% or more.
Further, the present invention proposes a cold-rolled steel sheet having excellent strain age hardening characteristics and having no room temperature aging deterioration, which comprises a component composition containing the following group A and / or group B in addition to the above component composition.
Record
Group A: 1.0 mass% or less in total of any one or more selected from Cu, Ni, Cr and Mo
Group B: Any one or more selected from Nb, Ti, V and B in total 0.1 mass% or less
In addition to the above component composition, the present invention further includes:
We propose a cold-rolled steel sheet that has excellent strain age hardening characteristics and is free from room temperature aging deterioration, and has a component composition containing one or two of Ca and REM in total of 0.0010 to 0.010 mass%.
Moreover, the cold-rolled plated steel plate which has a plating layer on the surface of each said cold-rolled steel plate which concerns on this invention is proposed.
[0011]
  The present invention also provides
C: 0.01 to 0.15 mass%, Si: 0.1 mass% or less,
Mn: 3.0 mass% or less, P: 0.08 mass% or less,
S: 0.02 mass% or less, Al: 0.02 mass% or less,
N: 0.0050 to 0.0250 mass%, and
N (mass%) / Al (mass%) is 0.3 or moreWherein the balance is composed of Fe and inevitable impuritiesAfter heating the steel slab having the composition to 1000 ° C. or higher, hot rolling is performed at a finish rolling exit temperature of 800 ° C. or higher, winding at a temperature of 650 ° C. or lower, and then pickling and cold rolling. Then, by box annealing, it is excellent in strain aging hardening characteristics characterized by performing recrystallization annealing at a heating rate of 30 ° C./h or more, a maximum achieved temperature of 900 ° C. or less, and a cooling rate of 10 ° C./h or more. We propose a method for manufacturing cold-rolled steel sheets without cracks.
  The present invention also provides
C: 0.01 to 0.15 mass%, Si: 0.1 mass% or less,
Mn: 3.0 mass% or less, P: 0.08 mass% or less,
S: 0.02 mass% or less, Al: 0.02 mass% or less,
N: 0.0050 to 0.0250 mass%, and
N (mass%) / Al (mass%) is 0.3 or moreWherein the balance is composed of Fe and inevitable impuritiesAfter heating the steel slab having the composition to 1000 ° C. or higher, hot rolling is performed at a finish rolling exit temperature of 800 ° C. or higher, winding at a temperature of 650 ° C. or lower, and then pickling and cold rolling. Then, by a box annealing method, a recrystallization annealing is performed with a heating rate of 30 ° C./h or more, a maximum achieved temperature of 900 ° C. or less, and a cooling rate of 10 ° C./h or more to obtain a cold-rolled steel sheet, and then a plating treatment is performed. We propose a method for producing cold rolled galvanized steel sheets with excellent strain age hardening characteristics and no deterioration at room temperature.
[0012]
  In addition, as a steel slab in the manufacturing method of the said cold-rolled steel plate or cold-rolled plated steel plate,In addition to the above component composition,Below (1)And / or (2)Described inConsists of component composition including componentsA steel slab is preferred.
(1) AFormation of group and / or group BMin
                        Record
Group A: Any one or two or more selected from Cu, Ni, Cr and Mo are 1.0 mass% or less in total.
Group B: Any one or more selected from Nb, Ti, V and B in total 0.1 mass% or less
(2) Ca, REM 1 type or 2 types in total 0.0010 to 0.010 mass%
[0013]
Here, in the present invention, “excellent strain age hardening characteristics” means that after applying a tensile pre-strain of 5% or more, heat treatment is performed at 100 ° C. or higher and 300 ° C. or lower for 30 seconds or longer and 20 minutes or shorter. This means that an increase in deformation stress and an increase in tensile strength can be obtained advantageously. In particular, the amount of deformation stress increase before and after aging treatment when applying aging treatment at 170 ° C. for 20 minutes after applying a tensile pre-strain of 5%, that is, (yield stress after aging treatment) − (pre-aging treatment before aging treatment). Deformation stress) is referred to as BH amount. Moreover, the difference between the tensile strength after strain aging treatment, that is, the tensile strength after (pre-strain deformation + aging treatment) and the tensile strength before applying pre-strain is referred to as ΔTS. And it is excellent in the strain age hardening characteristic that the amount of BH is 80 MPa or more and ΔTS is 40 MPa or more.
[0014]
Further, in the present invention, “no aging deterioration at room temperature” means that there is no increase in yield stress or decrease in elongation even if it is kept at room temperature for a long time after production. When evaluated by a decrease in total elongation after an aging experiment for storage, the amount of decrease is 2% or less.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The cold-rolled steel sheet according to the present invention has a feature that there is substantially no aging deterioration at room temperature despite remarkable strain age hardening by heat treatment after press forming (ordinary paint baking treatment or further aging treatment). . Since such a steel sheet is soft at the time of forming, it does not cause problems such as shape defects and cracks at the time of press forming accompanying the increase in strength. On the other hand, parts formed from such a steel sheet are subjected to heat treatment. Since it is remarkably strain age hardened and high rigidity is obtained, the same effect as that obtained by applying a high-strength steel sheet can be obtained, which is advantageous in realizing thinning of the steel sheet.
In addition, the steel plate which concerns on this invention includes from mild steel to what is called a high strength steel plate in terms of strength. Moreover, the use of the steel sheet is suitable for relatively light processing to relatively severe drawing.
Hereinafter, embodiments of the present invention will be specifically described.
[0016]
First, the reason for limiting the chemical composition of the steel sheet in the steel of the present invention will be described.
C: 0.01-0.15 mass%
If C exceeds 0.15 mass%, the fraction of carbides in the steel increases, and the ductility and further formability of the steel sheet are remarkably deteriorated. Furthermore, as a more important problem, when the C content exceeds 0.15 mass%, spot weldability, arc weldability, and the like are significantly reduced. Accordingly, the C content is 0.15 mass% or less, but 0.08 mass% or less is preferable for improving the moldability. Especially for applications where good ductility is required, 0.05 mass% or less is more preferable. On the other hand, if the amount of C is less than 0.01 mass%, it is difficult to reduce the amount of solid solution C, which is a feature of the present invention. This is presumably because the crystal grain size is remarkably coarsened and the fraction of carbides is reduced, resulting in a decrease in the precipitation sites of solid solution C.
[0017]
Si: 0.1 mass% or less
Si needs to be reduced to 0.1 mass% or less in order to ensure solid solution N after annealing for a relatively long time such as box annealing employed in the present invention. Furthermore, in order to stably obtain a large strain age hardening, it is desirable to set it to 0.05 mass% or less.
[0018]
Mn: 3.0mass% or less
Mn has the great advantage of significantly improving the mechanical properties of the steel sheet against fluctuations in hot rolling conditions, in particular the sensitivity of the excellent strain age hardening characteristics that the present invention is aimed at. Is preferred. Further, Mn is an effective element for preventing hot brittleness due to S, and is preferably added according to the amount of S. Further, Mn is an element effective in that it has the effect of refining crystal grains and also ensures stable solid solution N. In order to fix S stably and to secure solid solution N stably, addition of 0.2 mass% or more is more preferable.
However, if this Mn is added excessively, the formation of ferrite is suppressed and the ductility is significantly reduced. Moreover, although a detailed mechanism is unknown, there exists a tendency which increases the hot deformation resistance of a steel plate, and also exists in the tendency for weldability and the moldability of a weld part to deteriorate. From the above, the upper limit is set to 3.0 mass%. In applications where better corrosion resistance and formability are required, it is desirable that the content be 0.80 mass% or less.
[0019]
P: 0.08 mass% or less
P is effective as a solid solution strengthening element of steel, and is an element that may be added as appropriate according to the required strength. However, if excessively contained, it embrittles the steel and further increases the stretch flangeability of the steel sheet. make worse. Further, P is not preferable because it has a strong tendency to segregate in the steel, resulting in embrittlement of the weld. From the above, the upper limit is set to 0.08 mass%. When these characteristic deteriorations are regarded as particularly important, it is necessary to be 0.04 mass% or less.
[0020]
S: 0.02 mass% or less
S is an element that exists as an inclusion in the steel, reduces the ductility of the steel sheet, and further deteriorates the corrosion resistance. Therefore, the upper limit is set to 0.02 mass%. In particular, in applications where good workability is required, the content is preferably 0.015 mass% or less. Furthermore, when the stretch flange characteristic sensitive to the amount of S is particularly required, it is desirable to set it to 0.008 mass% or less. Moreover, although a detailed mechanism is unknown, reducing S to 0.008 mass% or less is effective in order to stably maintain the strain age hardening characteristics at a high level.
[0021]
Al: 0.02 mass% or less
Al is added as a deoxidizing element for steel, is an element useful for improving the cleanliness of steel, and is a component that plays an important role in the present invention. In addition, it is a desirable element in terms of material in order to refine the steel structure. In the present invention, solid solution N is used as a strengthening element, but aluminum killed steel to which Al in an appropriate range is added has better mechanical properties than rimmed steel to which no Al is added. . When the Al content is increased, the surface properties are deteriorated and the solid solution N is remarkably lowered, and it is difficult to ensure extremely large age-hardening characteristics, which is an object of the present invention. For this reason, the upper limit was made 0.020 mass% lower than that of the conventional steel. From the viewpoint of the stability of the material, 0.001 to 0.015 mass% is preferable. In addition, although there is a concern that the reduction of the Al addition amount may cause the crystal grains to become coarse and lead to deterioration of the material, in the present invention, an appropriate amount of other alloy elements such as Mn is added and the annealing condition is set to an optimum range. This can be prevented.
[0022]
N: 0.0050-0.0250 mass%
N is one of the most important additive elements in the present invention. That is, by adding N in an appropriate range and controlling the production conditions, it becomes possible to secure the necessary and sufficient solid solution N, and increase in yield stress and tensile strength due to solid solution strengthening and strain age hardening. The effect can be obtained stably. Further, N has an effect of lowering the transformation point of steel, and is an effective additive element when it is desired to avoid hot rolling below a transformation point with a thin material. Such an effect can be stably obtained by adding 0.0050 mass% or more. However, if added over 0.0250 mass%, the rate of occurrence of internal defects in the steel sheet increases and the occurrence of slab cracking during continuous casting becomes significant, so the upper limit was made 0.0250 mass%. The range of 0.0070 to 0.0170 mass% is preferable from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process. In addition, if it is the addition range of nitrogen of this invention, there will be no bad influence on weldability etc.
[0023]
Solid solution N: 0.0030 mass% or more
In order to ensure sufficient strength of the steel sheet and to effectively exhibit strain age hardening by N, N in a solid solution state needs to be 0.0030 mass% or more. Here, the solid solution N is a value obtained by subtracting the precipitation N obtained by the dissolution method by electrolytic extraction from the total N amount in the steel. This is based on the fact that the method employed in the present invention best copes with a change in material as a result of various examinations of the analysis method for precipitate N. When a larger strain age hardening is required, it is effective to make the solid solution N 0.0050 mass% or more.
[0024]
C content as a solid solution: 3 mass ppm or less
In the present invention, the content of C as a solid solution state (also referred to as a solid solution C amount) needs to be reduced as much as possible, and the solid solution C amount needs to be suppressed to a very small range of 3 mass ppm or less. C is different from N, and when it exists in a large amount in a solid solution state, aging deterioration at room temperature is remarkable. In order to obtain such a low solute C amount, addition of an alloy element in an appropriate range and box annealing under an appropriate condition are essential conditions.
In addition, it is preferable that the analysis method of such a trace amount C is based on the internal friction method.
[0025]
N (mass%) / Al (mass%) ratio: 0.3 or more
Al is an element having an effect of strongly fixing N, and it is desirable to limit the addition amount of Al low according to the N content. As a result of investigating the influence of Al on the amount of solute N, in the case of the steel composition of the present invention, the value of N (mass%) / Al (mass%) is set to 0.3 or more, so that the solid solution is stably formed. It was confirmed that the N amount can be secured by 0.0030 mass% or more and the target strain aging effect can be obtained. Therefore, the ratio of N (mass%) / Al (mass%) is 0.3 or more.
[0026]
In order to obtain the following effects, the cold-rolled steel sheet according to the present invention preferably contains the following components of Group A and / or Group B in addition to the components described above. In addition, even if each element of Group A and Group B is added singly or in combination, or combined over A and B groups, these desirable effects are not offset.
-Group A: Any one or more selected from Cu, Ni, Cr and Mo total 1.0 mass% or less
-Group B: Any one or more selected from Nb, Ti, V and B in total 0.1 mass% or less
[0027]
Each of the elements of Group A and Group B has an effect of making the steel structure uniform and fine, and is effective in increasing the strength. These effects are respectively Cu: 0.01 mass% or more, Ni: 0.01 mass% or more, Cr: 0.01 mass% or more, Mo: 0.01 mass% or more, Nb: 0.002 mass% or more, Ti: 0.002 mass% or more, V : 0.01 mass% or more, B: 0.0002 mass% or more are particularly recognized.
However, if both elements of Group A and Group B are contained in large quantities, deformation resistance during hot rolling tends to increase, surface treatment characteristics such as chemical conversion treatment tend to deteriorate, and hardening of the welded portion further occurs. As a result, the decrease in formability becomes significant. Therefore, the added amount of these elements needs to be 1.0 mass% or less in total for each element in the A group, and 0.1 mass% or less in total for each element in the B group.
[0028]
In particular, when stretch flange formability is required, it is also effective to control the form of inclusions by adding Ca and REM. The amount of each added is 0.0010 to 0.010 mass% in total, so that the stretch flange characteristics can be improved without causing surface defects.
[0029]
Next, the reason for limiting the structure of the cold-rolled steel sheet according to the present invention will be described.
Ferrite phase area ratio: 50% or more
The present invention is directed to steel sheets for automobiles and the like that require high workability. In such a steel sheet, if the area ratio of ferrite is less than 50%, it becomes difficult to ensure the required ductility. When better ductility is required, a ferrite fraction of 75% or more is desirable. Here, the ferrite includes not only so-called polygonal ferrite but also bainitic ferrite and acicular ferrite which do not include carbide precipitation. As the second phase structure other than ferrite, pearlite, cementite, bainite, retained austenite, martensite and the like can be included, but from the viewpoint of strain age hardening, it is preferable to include martensite.
In addition, since refinement of the structure, particularly refinement of the ferrite structure as the main phase, is effective in improving the strain age hardening characteristics, the average crystal grain size of ferrite is preferably about 15 μm or less.
[0030]
Next, measurement conditions for strain aging and strain aging characteristics will be described.
5% plastic deformation
The amount of pre-strain is an important factor when defining strain age hardening characteristics. Assuming the deformation mode to which automotive steel plates are applied, we investigated the effect of strain on the subsequent strain age hardening characteristics. Except for extremely deep drawing, it can be organized with strain equivalent to approximately one axis. It was revealed. In actual parts, the amount of strain equivalent to this uniaxial axis is more than 5%, and the strength of the parts is the strength obtained by heat treatment such as paint baking after applying 5% strain. It became clear that it responded well. Therefore, in the present invention, 5% plastic strain was set as the pre-strain condition.
[0031]
Heat treatment temperature and time
In the steel sheet according to the present invention, sufficient hardening can be achieved even at a low temperature and in a short time, in which sufficient hardening cannot be achieved with the conventional steel sheet, but as a heat treatment condition for bake hardening property evaluation, it has been conventionally standard 170. Adopt ℃ x 20 minutes.
In general, the aging temperature has an upper limit, but higher temperatures and longer times are more advantageous for curing.
However, as in the case of the cold-rolled steel sheet according to the present invention, when a large amount of solute N remains and imparts a strain of 5% or more, hardening is achieved even by a more gradual treatment, and heating to 100 ° C. Curing becomes significant. Further, if the temperature exceeds 300 ° C., the effect is saturated, and on the contrary, it tends to soften somewhat, and problems such as thermal distortion and temper color become remarkable, which is not desirable.
The heat treatment time is from 30 seconds to 20 minutes. At an aging temperature of 200 ° C., almost sufficient curing can be obtained by holding for 30 seconds or more. In order to achieve a larger and more stable strain age hardening, holding for 60 seconds or more is desirable. However, a long-time heat treatment of 20 minutes or more is not preferable because practically the change in strength is small.
As a heating method, not only heating in an atmosphere in a furnace as in ordinary paint baking, but also heating by induction heating, non-oxidizing flame, laser, plasma, or the like is naturally effective.
[0032]
Strength increase due to strain aging (BH amount: 80 MPa or more, ΔTS: 40 MPa or more)
In the steel sheet according to the present invention, the increase in yield stress (BH amount) obtained by strain age hardening is 80 MPa or more, and the increase in tensile strength (ΔTS) is 40 MPa or more. Either of these is not sufficient. That is, because parts for automobiles are subjected to complicated external stress, not only strength characteristics in a small strain region but also strength properties in a large strain region are important. These values are more desirably 100 MPa and 50 MPa, respectively. Further, higher strength can be obtained by raising the aging temperature and lengthening the aging time.
The steel sheet of the present invention can be expected to increase in strength simply by leaving it to stand at room temperature after forming, even if it is not heated and subjected to accelerated aging (artificial aging). About 40% of the minimum can be estimated. This is one of the advantages of the steel sheet of the present invention.
However, on the other hand, it is a feature of the steel sheet of the present invention that aging deterioration due to so-called holding at room temperature does not occur in a state where plastic working is not applied. In the steel sheet of the present invention, the decrease in elongation (El) due to room temperature aging is 2% or less.
[0033]
Next, the reason for limiting the manufacturing conditions will be described.
In order to prevent macro segregation of components, the slab manufacturing method is preferably manufactured by a continuous casting method, but can also be performed by an ingot forming method or a thin slab casting method.
When hot-rolling slabs, in addition to the conventional method of cooling to room temperature after manufacturing the slab, and then reheating it, it is charged in a heating furnace without cooling or with little heat retention. Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately after, can be applied without any problem. In particular, direct feed rolling is one of the useful techniques for effectively securing solid solution N.
[0034]
The hot rolling conditions are defined as follows.
Slab heating temperature: 1000 ℃ or more
The lower limit of the slab heating temperature is defined in terms of securing N in a solid solution state. The upper limit is not particularly limited, but is preferably 1280 ° C. or less because of an increase in oxidation loss.
[0035]
Finishing rolling delivery temperature: 800 ℃ or more
A uniform and fine hot-rolled sheet structure can be obtained by hot rolling at a finish rolling outlet temperature of 800 ° C. or higher. However, if the finish rolling finish temperature is below 800 ° C, the processed structure remains, the structure of the steel sheet becomes uneven, and a uniform structure cannot be obtained even after cold rolling and annealing. The risk of occurring increases. This non-uniformity cannot be eliminated even when high-temperature winding is adopted, and conversely, a problem due to the generation of coarse grains occurs. Moreover, the solid solution N also falls and it becomes difficult to obtain the target amount of the solid solution N. Accordingly, the finish rolling outlet temperature is set to 800 ° C. or higher. Further, in order to improve mechanical properties, 820 ° C. or higher is preferable. Although the upper limit temperature is not particularly restricted, scale wrinkles occur when rolling at an excessively high temperature, so the upper limit is about 1000 ° C.
[0036]
Hot coiling temperature: 650 ° C or less
By lowering the hot rolling coiling temperature, the strength and strain aging characteristics tend to be improved. In order to obtain a sufficiently high tensile strength and strain age hardening amount that contributes to the weight reduction of the vehicle body, it is necessary to set the coiling temperature to 650 ° C. or less. The lower limit temperature is not strictly defined in terms of material, but if it is below 200 ° C., the shape of the steel sheet is disturbed and the possibility of causing problems in use increases. In addition, the uniformity of the material tends to decrease. Therefore, the hot rolling coiling temperature is preferably 200 ° C. or higher. When higher material uniformity is required, a winding temperature of 300 ° C. or higher is more preferable.
[0037]
Pickling, cold rolling
Pickling may be performed according to a conventional method. In addition, if it is in a very thin scale state, it can be cold-rolled without pickling. The cold rolling may be performed according to a conventional method, and the rolling reduction of the cold rolling may be 40 to 90% that is normally performed.
[0038]
Recrystallization annealing
The present invention is characterized in that recrystallization annealing is performed by a box annealing method. Thereby, solid solution C can be 3 mass ppm or less, and room temperature aging deterioration can be prevented. The heating rate during annealing needs to be 30 ° C / h or more. If it is less than 30 ° C / h, the structure becomes coarse and the workability deteriorates. The upper limit does not need to be particularly limited, and is acceptable if it is about 300 ° C./h that can be achieved with ordinary box annealing equipment. The maximum temperature achieved during box annealing is set to 900 ° C. or lower in order to prevent solute N from precipitating and decreasing. Note that the maximum temperature achieved in the box annealing may be equal to or higher than the temperature at which recrystallization is completed, but the strength can be secured below the recrystallization temperature. . Further, if the cooling rate is less than 10 ° C./h, ferrite crystal grains tend to be coarsened and the strain aging characteristics tend to be lowered, so it is necessary to make the cooling rate 10 ° C./h or more. Further, when the cooling rate is 300 ° C./h or more, the amount of dissolved C increases, and aging deterioration at room temperature becomes remarkable, which is not preferable.
In addition, although the effect of the present invention is manifested regardless of the plate thickness, in the present invention, the box annealing method is adopted as the annealing method, so the flexibility with respect to the plate thickness is high compared to the continuous annealing method, and the steel plate There is no need to particularly limit the thickness.
[0039]
Plating treatment
Moreover, in this invention, the plating layer, such as electroplating or hot dipping, is formed on the surface of the cold rolled steel sheet of the present invention to form a plated layer, and there is no problem as a cold rolled steel sheet. These cold-rolled plated steel sheets also exhibit the same TS, BH amount, and ΔTS amount as before plating. As the kind of plating, any of electrogalvanizing, hot dip galvanizing, alloying hot dip galvanizing, electrotin plating, electrochromic plating, electronickel plating, etc. can be preferably applied. In the present invention, the recrystallization annealing is performed to obtain a cold-rolled steel sheet, and then the plating process is performed. However, the plating conditions at this time do not need to be changed from those of the conventional steel sheet, and may be performed according to a conventional method.
[0040]
Temper rolling or leveler processing
The temper rolling is preferably performed not only for the conventional purposes of shape correction and roughness adjustment, but also for stably improving the strain age hardening characteristics of the steel sheet. In the steel sheet of the present invention, the same effect as temper rolling can be obtained by leveler processing. These effects can be evaluated by the rolling reduction and elongation, and if it is approximately 0.5% or more, it is sufficient. However, if it is applied in excess of 5%, the workability deteriorates, which is not preferable.
[0041]
Next, other manufacturing conditions that are desirably applied will be described.
In hot rolling, it is desirable to join the sheet bars on the entry side of the finish rolling mill and perform continuous rolling. By performing continuous rolling, so-called unsteady portions at the front and rear ends of the coil are eliminated, and stable hot rolling is possible over the entire length of the coil. In addition, even when the rolled steel sheet is cooled on a hot run table, tension can always be applied to the steel sheet, so that the shape of the steel sheet can be kept good. Furthermore, in ordinary batch rolling, it is also possible to apply lubrication rolling that could not be applied from the viewpoints of sheet feeding and biting, and at the same time as reducing rolling load, roll surface pressure can be reduced, so roll life Can be extended. In addition, continuous rolling is extremely effective for improving the cross-sectional shape and dimensions of not only hot-rolled steel sheets but also cold-rolled steel sheets. There are no particular restrictions on the method of joining the sheet bars, and the same applies to the pressure welding method, laser welding method, electron beam welding method, and other joining methods.
[0042]
Furthermore, it is desirable to apply an edge heater and a sheet bar heater to the sheet bar at the finishing mill entry side. Heating the edge portion of the sheet bar with an edge heater is desirable to compensate for the difference in rolling temperature in the width direction. It is recommended that the heating amount at this time be such that the temperature difference in the width direction in finish rolling is 20 ° C. or less, but it varies depending on the steel composition and the like. In addition, the application of the sheet bar heater can make the temperature-decreasing portion at the rear end of the coil tip more uniform. The heating amount in this case is recommended to be within a range of + 20 ° C. with respect to the central portion in the longitudinal direction from the viewpoint of uniform material.
[0043]
Further, performing lubrication rolling to reduce the hot rolling load is also effective from the viewpoint of uniform shape and uniform material. Lubrication rolling is preferably performed in the range of a friction coefficient of 0.25 to 0.10, and it is further desirable from the viewpoint of operational stability of hot rolling to apply the above-described continuous rolling process.
Furthermore, in cooling after hot rolling, it is also effective to use a cooling water masking technique for the edge portion in order to prevent overcooling of the edge portion and suppress variation in the material in the width direction.
[0044]
【Example】
Next, examples of the present invention will be described.
Example 1
A steel containing the composition shown in Table 1 and the balance being substantially Fe is melted in a converter, the steel slab is hot-rolled under the conditions shown in Table 2, and pickled, cold-rolled, and boxed. Recrystallization annealing in annealing was performed to produce a cold rolled steel sheet. About these steel plates, the amount of solid solution N was calculated | required by the above-mentioned method, and the amount of solid solution C was measured by the internal friction method. Further, the microstructure, tensile properties and strain age hardening properties were evaluated by the following methods.
Microstructure: Test specimens were collected from each cold-rolled annealed plate, and the microstructure was observed for the cross section (C cross section) perpendicular to the rolling direction, and the area ratio of ferrite and the type of the second phase were investigated. Further, the average crystal grain size of the ferrite as the main phase is obtained from a structural photograph of a cross section (C cross section) orthogonal to the rolling direction by a value calculated by a quadrature method prescribed in ASTM or by a cutting method prescribed by ASTM. The larger of the nominal particle diameters was adopted.
・ Tensile properties: JIS No. 5 test specimens were sampled in the rolling direction from each cold-rolled annealed plate and subjected to a tensile test in accordance with the provisions of JIS Z2241, yield strength (YS), tensile strength (TS), Elongation (El) was determined.
・ Strain age hardening characteristics: JIS No. 5 test specimens were taken from each test material in the rolling direction, subjected to the 5% tensile pre-strain mentioned above and heat treatment at 170 ° C for 20 minutes. BH amount and ΔTS were determined.
・ Aging characteristics at room temperature: After each cold-rolled steel sheet is manufactured and stored at room temperature (room temperature) for one year, the tensile test is performed to determine the elongation in the same manner as the investigation of the tensile characteristics, and the elongation immediately after manufacturing (El1). Evaluation was based on the difference (El1−El2) in elongation (El2) after storage for one year. In addition, if the difference in elongation before and after holding for one year at room temperature (amount of decrease) is 2% or less, it can be said that there is substantially no aging deterioration at room temperature.
[0045]
[Table 1]

[0046]
[Table 2]

[0047]
Table 3 shows the results of the survey. The ones suitable for the conditions of the present invention showed good characteristics in all of the tensile characteristics, strain aging characteristics and room temperature aging characteristics, whereas Nos. 15, 16, and 17 outside the conditions of the present invention Does not meet these characteristics. In particular, No. 16 is not suitable as a steel plate for automobiles because of its high C content and poor spot weldability.
[0048]
[Table 3]

[0049]
(Example 2)
C: 0.015 mass%, Si: 0.005 mass%, Mn: 0.55 mass%, P: 0.009 mass%, S: 0.005 mass%, Al: 0.015 mass%, N: 0.0126 mass%, N (mass%) / Al ( mass%): A steel slab of 0.84 was used as a raw material, and a cold-rolled steel sheet was produced under the production conditions as shown in Table 4. Subsequently, a hot-dip galvanizing process was performed in accordance with a conventional method to produce a hot-dip galvanized steel sheet. Similarly, the mechanical properties were investigated. The results are shown in Table 5.
From Table 5, it can be seen that according to the production method of the present invention, it is possible to produce a hot-dip galvanized steel sheet having excellent strain age hardening characteristics and room temperature aging characteristics and having high workability and high strength.
[0050]
[Table 4]

[0051]
[Table 5]

[0052]
【The invention's effect】
As described above, according to the present invention, the chemical composition, hot rolling conditions, cold rolling, and annealing conditions are optimized, and a sufficient amount of N in the solid solution state in the final product stage is secured, while the solid solution C By limiting the strength, a large strength increase is obtained by strain aging after processing, and a cold-rolled steel plate free from room temperature aging deterioration is obtained while being soft and having high formability. Moreover, using this cold-rolled steel sheet, a plated steel sheet such as a hot-dip galvanized steel sheet having substantially the same mechanical properties can be produced.

Claims (10)

C: 0.01-0.15 mass%,
Si: 0.1 mass% or less,
Mn: 3.0 mass% or less,
P: 0.08 mass% or less,
S: 0.02 mass% or less,
Al: 0.02 mass% or less,
N: 0.0050-0.0250 mass% is included,
N as a solid solution state is included 0.0030 mass% or more,
A component composition in which the content of C as a solid solution satisfies 3 mass ppm or less and N (mass%) / Al (mass%) is 0.3 or more, and the balance is Fe and inevitable impurities; A cold-rolled steel sheet having excellent strain age hardening characteristics and no room temperature aging deterioration, wherein the ferrite phase has a structure with an area ratio of 50% or more. In addition to the component composition of claim 1,
The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet comprises a component composition including the following group A and / or group B.
Group A: Any one or two or more selected from Cu, Ni, Cr and Mo are 1.0 mass% or less in total. Group B: Any one selected from Nb, Ti, V and B 0.1 mass% or less of seeds or 2 or more In addition to the component composition according to claim 1 or 2,
The cold-rolled steel sheet according to claim 1 or 2, comprising a component composition containing 0.0010 to 0.010 mass% of one or two of Ca and REM in total. A cold-rolled plated steel sheet having excellent strain age hardening characteristics and no room temperature aging deterioration, comprising a plated layer on the surface of the cold-rolled steel sheet according to any one of claims 1 to 3. C: 0.01-0.15 mass%,
Si: 0.1 mass% or less,
Mn: 3.0 mass% or less, P: 0.08 mass% or less,
S: 0.02 mass% or less,
Al: 0.02 mass% or less,
N: 0.0050-0.0250 mass% is contained,
And N (mass%) / Al (mass%) is 0.3 or more , and the steel slab which has the component composition which consists of Fe and an unavoidable impurity in the remainder is heated to 1000 degreeC or more, Then, the finish rolling exit side Hot rolling is performed at a temperature of 800 ° C. or higher, winding is performed at a temperature of 650 ° C. or lower, pickling and cold rolling are performed, and then a heating rate of 30 ° C./h or higher and a maximum temperature of 900 ° C. are obtained by box annealing. A method for producing a cold-rolled steel sheet having excellent strain age hardening characteristics and no room temperature aging deterioration, characterized by performing recrystallization annealing at a cooling temperature of 10 ° C./h or lower. In addition to the component composition, the steel slab is further
It consists of a component composition containing following A group and / or B group, The manufacturing method of the cold-rolled steel plate of Claim 5 characterized by the above-mentioned.
                        Record
Group A: Any one or two or more selected from Cu, Ni, Cr and Mo are 1.0 mass% or less in total.
Group B: Any one or more selected from Nb, Ti, V and B in total 0.1 mass% or less In addition to the component composition, the steel slab is further
The method for producing a cold-rolled steel sheet according to claim 5 or 6, comprising a component composition containing 0.0010 to 0.010 mass% of one or two of Ca and REM in total. C: 0.01-0.15 mass%,
Si: 0.1 mass% or less,
Mn: 3.0 mass% or less,
P: 0.08 mass% or less,
S: 0.02 mass% or less,
Al: 0.02 mass% or less,
N: A steel slab containing 0.0050 to 0.0250 mass%, N (mass%) / Al (mass%) being 0.3 or more , and the balance being a composition composed of Fe and inevitable impurities Is heated to 1000 ° C. or higher, hot rolled at a finish rolling exit temperature of 800 ° C. or higher, wound at a temperature of 650 ° C. or lower, pickled and cold rolled, and then box annealed. Is subjected to recrystallization annealing at a heating rate of 30 ° C./h or higher, a maximum achieved temperature of 900 ° C. or lower, and a cooling rate of 10 ° C./h or higher to form a cold-rolled steel sheet, and then subjected to plating treatment. A method for producing cold-rolled steel sheets with excellent properties and no aging deterioration at room temperature. The said steel slab consists of a component composition containing the following A group and / or B group in addition to the said component composition, The manufacturing method of the cold-rolled plated steel plate of Claim 8 characterized by the above-mentioned.
                        Record
Group A: Any one or two or more selected from Cu, Ni, Cr and Mo are 1.0 mass% or less in total.
Group B: Any one or more selected from Nb, Ti, V and B in total 0.1 mass% or less The said steel slab consists of a component composition which further contains 0.0010-0.010 mass% of 1 type or 2 types of Ca and REM in addition to the said component composition in total. Manufacturing method of cold rolled galvanized steel sheet.