JP2004076061A - Cold rolled steel sheet having reduced plane anisotropy and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet which has reduced plane anisotropy in r value, and is applicable to automobiles, house appliances or the like, and to provide a method for producing the same. <P>SOLUTION: The cold rolled steel sheet having reduced plane anisotropy has chemical components comprising, by mass, 0.01 to 0.08% C, ≤0.5% Si, ≤1.0% Mn, ≤0.05% P, ≤0.03% S, 0.01 to 0.1% Al, ≤0.01% N and 0.01 to 0.05% Ti, and the balance substantially iron, and in which ¾Δr¾<0.15 is satisfied. In the cold rolled steel sheet having excellent bulging formability as well, the plane average value of n value, n* satisfies inequality (1): n*≥0.1×¾Δr¾+0.175. In the method for producing the cold rolled steel sheet having reduced plane anisotropy, the steel is hot-rolled at an Ar<SB>3</SB>transformation point or higher, within 2 seconds after the rolling, cooling is started and is performed at a cooling rate of ≥70°C/s over the temperature range of ≥100°C, and the steel is subjected to cold rolling and continuous annealing to attain ¾Δr¾<0.15. <P>COPYRIGHT: (C)2004,JPO

Description

【００５０】
この表１に示すように、本発明例の鋼番１〜６は、いずれも化学成分が本発明の範囲内の本発明鋼であるが、鋼番７〜１０は、本発明の範囲から外れた比較鋼である。すなわち、鋼番７はＣ量が下限未満、鋼番８，９はＴｉ量が発明範囲外、鋼番１０は発明範囲外のＢが添加されている。
【００５１】
このスラブを１２００℃に加熱後、熱間圧延を行い、その後、種々の冷却条件により冷却し、通常の巻取温度の範囲内で巻取ることにより熱延鋼板を製造した。この熱延鋼板に酸洗、冷間圧延を行い、連続焼鈍により冷延鋼板、又は溶融亜鉛めっき鋼板もしくは電気亜鉛めっき鋼板とした。これらの冷延鋼板および亜鉛めっき鋼板に、圧下率０．５〜２．０％の調質圧延を施した。以上の熱延条件（仕上温度、冷却速度、冷却温度域の温度幅ΔＴ）および焼鈍（めっき）条件を表２に示す。
【００５２】
【表２】

【００５３】
これらの冷延鋼板および亜鉛めっき鋼板について、圧延方向に対して０゜、４５°、９０°方向のｒ値を測定し、Δｒを求めた。試験結果を表２に併せて示す。
【００５４】
表２に示すように、化学成分および製造条件が発明範囲内である本発明例Ｎｏ．１〜３，　７，　８，１０では、いずれも｜Δｒ｜＜０．１５を満足し、発明の目的が達成されている。一方、化学成分あるいは製造条件が発明範囲を外れている比較例では、面内異方性が増大し、本発明の目標とするΔｒの抑制効果が得られない。
【００５５】
例えば、比較例Ｎｏ．４〜６，　９は、化学成分は発明範囲内（鋼番３，　５）であるが、製造条件が発明範囲から外れているため、Δｒが目標範囲を超えている。Ｎｏ．４，５はそれぞれ圧延後の冷却速度、冷却温度幅ΔＴが発明範囲から外れており、Ｎｏ．６は仕上温度が、またＮｏ．９は冷却速度と冷却温度幅ΔＴが、それぞれ発明範囲から外れている。
【００５６】
また、比較例Ｎｏ．１１〜１４は、化学成分が発明範囲を外れている（鋼番７〜１０）ため、異方性が大きくなっている。Ｎｏ．１１（鋼番７）は、Ｃ量が下限値より少ないため、面内異方性が目標範囲内から外れている。Ｎｏ．１２（鋼番８）は、Ｔｉ添加量が下限未満のため、面内異方性が目標範囲内から外れている。Ｎｏ．１３（鋼番９）は、Ｔｉ添加量が上限を超えており、面内異方性は目標範囲を僅かに外れた程度であるが、過剰なＴｉ添加が延性及び靭性を低下させるので、本発明が対象とする自動車用・家電用鋼板としては不適当である。Ｎｏ．１４（鋼番１０）は、添加されたＢが再結晶集合組織の形成に悪影響を及ぼし、面内異方性が非常に悪化している。
【００５７】
［実施例２］
表３に示す化学成分を有するＴｉ添加鋼を溶製し、連続鋳造によりスラブを製造した。
【００５８】
【表３】

【００５９】
この表３に示すように、本発明例の鋼番１１〜１６は、いずれも化学成分が本発明の範囲内の本発明鋼であるが、鋼番１７〜２０は、本発明の範囲から外れた比較鋼である。すなわち、鋼番１７はＣ量が下限未満、鋼番１８，１９はＴｉ量が発明範囲外、鋼番２０は発明範囲外のＢが添加されている。
【００６０】
このスラブを１２００℃に加熱後、熱間圧延を行い、その後、種々の冷却条件により冷却し、通常の巻取温度の範囲内で巻取ることにより熱延鋼板を製造した。この熱延鋼板に酸洗、冷間圧延を行い、連続焼鈍により冷延鋼板、又は溶融亜鉛めっきもしくは電気亜鉛めっきにより亜鉛めっき鋼板を製造した。これらの冷延鋼板および亜鉛めっき鋼板に、圧下率０．５〜２．０％の調質圧延を施した。以上の熱延条件（仕上温度、冷却速度、冷却温度域の温度幅ΔＴ）および焼鈍（めっき）条件を表４に示す。
【００６１】
【表４】

【００６２】
これらの冷延鋼板および亜鉛めっき鋼板について、圧延方向に対して０゜４５°、９０°方向のｎ値とｒ値を測定し、ｎ値の面内平均値ｎ＊と面内異方性Δｒを求めた。さらに、４００ｍｍ×４００ｍｍの試験片について、直径１６０ｍｍの球頭ポンチを用いた球頭張出し試験を行い、張出し成形性（限界張出し高さ）を調査した。試験結果を表４に併せて示す。
【００６３】
表４に示すように、化学成分および製造条件が発明範囲内である本発明例Ｎｏ．２１〜２３，２７，２８，３０では、いずれも｜Δｒ｜＜０．１５およびｎ＊とΔｒの関係式（１）を満足し、発明の目的が達成されている。
【００６４】
一方、化学成分あるいは製造条件が発明範囲を外れている比較例では、面内異方性が増大し、本発明の目標とするΔｒの抑制効果が得られず、ｎ値の面内平均値ｎ＊とΔｒの関係式（１）を満足していない。
【００６５】
例えば、比較例Ｎｏ．　２４〜２６，　２９は、化学成分は発明範囲内（鋼番１３，　１５）であるが、製造条件が発明範囲から外れているため、Δｒが目標範囲を超えている。Ｎｏ．２４，２５はそれぞれ圧延後の冷却速度、冷却温度幅ΔＴが発明範囲から外れており、Ｎｏ．２６は仕上温度が、またＮｏ．２９は冷却速度と冷却温度幅ΔＴが、それぞれ発明範囲から外れている。
【００６６】
また、比較例Ｎｏ．３１〜３４は、化学成分が発明範囲を外れている（鋼番１７〜２０）ため、ｎ＊とΔｒの関係式（１）を満足せず、面内異方性が大きくなっている。Ｎｏ．３１（鋼番１７）は、Ｃ量が下限値より少ないため、面内異方性が目標範囲内から外れている。Ｎｏ．３２（鋼番１８）は、Ｔｉ添加量が下限未満のため、面内異方性が目標範囲内から外れている。Ｎｏ．３３（鋼番１９）は、面内異方性は目標範囲を僅かに外れた程度であるが、過剰なＴｉ添加が延性及び靭性を低下させるので、本発明が対象とする自動車用・家電用鋼板としては不適当である。Ｎｏ．３４（鋼番２０）は、添加されたＢが再結晶集合組織の形成に悪影響を及ぼし、面内異方性が非常に悪化している。
【００６７】
【発明の効果】
この発明は、化学成分を特定の範囲内に制御するとともに、熱延仕上圧延およびその後の冷却条件を制御することにより、板厚０．５ｍｍ以上で面内異方性が小さい冷延鋼板あるいは亜鉛めっき鋼板を製造することに成功した。その結果、この発明の鋼板は、自動車用鋼板を始め、家庭用電器製品等に広く活用することが可能となる。
【図面の簡単な説明】
【図１】ｎ値の面内平均値ｎ＊および面内異方性｜Δｒ｜と張出し成形性の関係を示す図。
【図２】冷延鋼板の面内異方性に及ぼす冷却温度幅ΔＴの影響を示す図。
【図３】冷延鋼板の張出し成形性に及ぼす冷却温度幅ΔＴの影響を示す図。
【図４】冷延鋼板の面内異方性に及ぼす冷却速度の影響を示す図。
【図５】冷延鋼板の張出し成形性に及ぼす冷却速度の影響を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a cold-rolled steel sheet having a small in-plane anisotropy applicable to automobiles, home electric appliances, and the like, and a method for manufacturing the same.
[0002]
[Prior art]
2. Description of the Related Art In general, for rotationally symmetric parts such as automobiles and home electric appliances, a steel sheet having small in-plane anisotropy is demanded. By reducing the in-plane anisotropy, the ear formation after deep drawing is reduced, the quality such as the thickness distribution is made uniform, and the increase in the operation cost and the decrease in the material yield due to the trimming operation are suppressed. be able to. In addition, the direction in which the material is removed can be arbitrarily set, and the usability of the user is improved.
[0003]
In this case, as the in-plane anisotropy, particularly, the in-plane anisotropy of the plastic strain ratio r value has a large effect, and as parameters, r values in the 0 °, 45 °, and 90 ° directions with respect to the rolling direction. Δr = (r ₀ + r ₉₀ -2r ₄₅ ) / 2 calculated using r ₀ , r ₄₅ , and r ₉₀ is known. Generally, it is known that the absolute value of Δr of a cold-rolled steel sheet decreases with an increase in the cold-pressure ratio and reaches a minimum value, and then increases again when the cold-pressure ratio further increases.
[0004]
Regarding the reduction of the in-plane anisotropy of a cold-rolled steel sheet, for example, Japanese Patent Publication No. 61-7455 discloses a finish-rolled sheet thickness ratio (reduction ratio) of 13 or more in hot rolling, and a cooling rate of 20 after completion of hot rolling. ６５65 ° C./sec forced cooling at this time, by defining the finish rolling inlet temperature and the temperature range of the forced cooling after the end of hot rolling by the formula of C, Mn, and P content, the deep drawability and surface Methods for improving internal anisotropy have been proposed.
[0005]
On the other hand, in the field of materials for cans, in recent years, a shift to a two-piece can and a reduction in the thickness of the can have been promoted from the viewpoints of reducing the weight of the can, omitting steps, and reducing material costs. To reduce the thickness of the can body, it is conceivable to reduce the thickness of the hot-rolled sheet and increase the cold-pressure ratio. However, the former significantly impairs the productivity of hot rolling, and the latter increases earrings.
[0006]
In order to reduce the absolute value of Δr when the cooling rate is high, for example, Japanese Patent Application Laid-Open No. H8-3638 discloses a steel sheet having a small ear generation which is advantageous for improving the yield and the productivity in the can making process. , And a method for producing it with high productivity. According to this technology, low-carbon steel to which Ti, Nb, and B are added as necessary is hot-rolled and pickled, and then cold-rolled at a cold rolling reduction of 90% or more, and 50% or more of the cold-rolled steel is rolled. Rolling is performed at a temperature of 100 to 500 ° C. and annealing is performed.
[0007]
Further, Japanese Patent Application Laid-Open No. 10-330845 proposes a method of manufacturing a steel plate for a container in which the earring of a two-piece can is formed by deep drawing the bottom and body used in a beverage can and integrally forming by ironing to reduce the earring. I have. During hot rolling, the leading end of the preceding material is joined to the leading end of the following material during hot rolling, and the resulting material is subjected to finish rolling. Further, the finish rolling with a total draft of 50% or more is performed by lubrication, and the cold rolling is performed. This is a method for producing a steel sheet for a low-earring container, wherein cold rolling, recrystallization annealing, and skin pass rolling are performed under conditions where the total draft of cold rolling and skin pass rolling is 90 to 95%.
[0008]
Conventionally, the press formability of cold rolled steel sheets has been studied mainly from the viewpoint of deep drawability and stretch formability. With respect to the deep drawability, the main focus is on increasing the r-value. For example, Japanese Patent Application Laid-Open No. Hei 8-92656 proposes an ultra-low carbon steel sheet exhibiting a high r-value. In this technology, a hot-rolled steel sheet which has been hot-lubricated and rolled in the α region of hot-rolling is recrystallized, cold-rolled and annealed, whereby a cold-rolled steel sheet having an r value of 3.0 or more can be obtained. That you can get.
[0009]
In Japanese Patent Publication No. Hei 8-14003, a slab heating temperature, a hot-rolling finishing temperature, and a winding temperature are specified for a low-carbon cold-rolled steel sheet, and a cooling pressure ratio is specified according to a titanium content. Thereafter, a technique of manufacturing a quasi-isotropic cold-rolled steel sheet by performing batch annealing has been proposed. According to this technique, since titanium nitride is formed at an early stage, a pancake structure due to precipitation of aluminum nitride does not occur during recrystallization annealing.
[0010]
Regarding stretch formability, for example, “pressing of thin sheet” (published by Jikkyo) includes n values in a high strain region (for example, two points of 10% and 20%) obtained from a total elongation measurement and a load-elongation curve. It is stated that it is important to increase the n value measured by the method).
[0011]
[Problems to be solved by the invention]
However, the above prior art has the following problems. For example, the technique disclosed in Japanese Patent Publication No. 61-7455 is merely a general method for producing a low-carbon cold-rolled steel sheet in which the amounts of C, Mn, and P are specified, and the obtained Δr is determined by the method of the embodiment. As can be seen in (Examples of Table 2), it is 0.15 to 0.25, and it cannot be said that in-plane anisotropy is sufficiently improved.
[0012]
In general, the prior art of the material for cans targets a thin steel plate having a plate thickness of 0.3 mm or less, all of which are technologies in a region where the cold pressure rate is high. For example, in the technique described in Japanese Patent Application Laid-Open No. 8-3638, it is necessary to perform cold rolling at a cold pressure rate of 90% or more, and when this is applied to a cold-rolled steel sheet having a sheet thickness of 0.5 mm or more, Since the plate thickness becomes 5 mm or more, commercial production is often difficult due to the limitations of rolling load, mill power, and the like in a normal tandem rolling mill.
[0013]
Further, in general, a material for a can is produced by performing cold rolling-annealing and then performing strong skin pass rolling (secondary cold rolling). The technique described in JP-A-10-330845 also produces a thin can material by combining cold rolling and skin pass rolling. However, cold-rolled steel sheets for automobiles and home electric appliances are usually produced by light skin pass rolling after annealing, and the production method is fundamentally different from that for can materials.
[0014]
Further, in this conventional technique, lubrication is performed in finish rolling of hot rolling, so that there is a high possibility that poor biting of the material into the roll bite, slip, or the like will occur. Therefore, after rough rolling, finish rolling is performed by joining the leading end of the succeeding material to the trailing end of the preceding material (of the rough bar). Such lubricating rolling and continuous hot rolling are both used for lubrication and Requires special equipment for coarse bar connection. In addition, operations require adjustment of lubrication conditions, joining of coarse bars, and the like. Therefore, it is difficult to implement with ordinary hot rolling equipment and operation.
[0015]
The technology described in Japanese Patent Publication No. Hei 8-14003 is a technology for reducing in-plane anisotropy on the premise of batch annealing, and has the advantage of reducing in-plane anisotropy as compared with continuous annealing. Since it takes a long time (several days), the productivity is low and it is not suitable for mass production.
[0016]
As described above, the in-plane anisotropy has not been sufficiently improved in the conventional technology of the cold-rolled steel sheet, and in the conventional technology of the material for cans, the cold pressure ratio is too high, or a strong skin pass or a special hot rolling method is used. It must be employed, and cannot be applied to the production of cold-rolled steel sheets (preferably, 0.5 mm or more in thickness) for automobiles and home electric appliances having small in-plane anisotropy, which is the object of the invention.
[0017]
Furthermore, in the conventional technology of cold-rolled steel sheets, stretch formability is improved by increasing the n-value. For example, when pressing an outer panel of an automobile, the n-value in the rolling direction or the width direction of the sheet is increased. It has been found that simply increasing the n value in one direction is not sufficient.
[0018]
An object of the present invention is to solve the above problems and provide a cold-rolled steel sheet having a small in-plane anisotropy of r value applicable to automobiles, home electric appliances, and the like, and a method for manufacturing the same.
[0019]
[Means for Solving the Problems]
The above problem is solved by the following invention. In the invention, the chemical components are mass%, C: 0.01 to 0.08%, Si: 0.5% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0. 0.03% or less, Al: 0.01 to 0.1%, N: 0.01% or less, Ti: 0.01 to 0.05%, the balance substantially consisting of iron, | Δr | <0 This is a cold-rolled steel sheet having a small in-plane anisotropy of 0.15.
[0020]
The present invention further provides a cold-rolled steel sheet having a small in-plane anisotropy, wherein the in-plane average value n * of the n value satisfies the following relational expression (1) and is excellent in stretch formability. You can also.
[0021]
n * ≧ 0.1 × | Δr | +0.175 (1)
These inventions produce a cold-rolled steel sheet having a thickness of 0.5 mm or more and a small in-plane anisotropy (| Δr | <0.15), which was extremely difficult in the prior art. A detailed study focused on the conditions was made. In the course of the study, the cooling condition after finish rolling is a very important factor affecting the in-plane anisotropy of the cold-rolled steel sheet, and the object has been achieved by finding the range of the optimum condition.
[0022]
Hereinafter, individual reasons for limitation of the invention will be described.
[0023]
C: 0.01 to 0.08%
C is an element necessary for securing the tensile strength of steel, but if it exceeds 0.08%, the ductility is significantly reduced. On the other hand, if the C content is less than 0.01%, the in-plane anisotropy tends to increase. Therefore, the C content is set in the range of 0.01 to 0.08%.
[0024]
Si: 0.5% or less Si is an element effective for ensuring strength. However, if it exceeds 0.5%, the surface properties are deteriorated, and when a steel sheet is plated, the adhesion of plating is significantly deteriorated. Therefore, the amount of Si is set to 0.5% or less.
[0025]
Mn: 1.0% or less Mn is an element effective for precipitating S in steel as MnS to prevent hot cracking of the slab and to increase strength without deteriorating plating adhesion. . However, when the Mn content exceeds 1.0%, not only does the slab cost increase significantly, but also the workability is degraded. Therefore, the Mn content is set to 1.0% or less.
[0026]
P: 0.05% or less P is an element effective for securing the strength. However, if added in excess of 0.05%, the secondary work brittleness after press forming is deteriorated, and when a galvanized steel sheet is formed, Causes deterioration in chemical processing property. Therefore, the P content is set to 0.05% or less.
[0027]
S: 0.03% or less S lowers the hot workability and increases the hot cracking susceptibility of the slab, and when it exceeds 0.03%, the workability is deteriorated due to precipitation of fine MnS. Therefore, the amount of S is set to 0.03% or less.
[0028]
Al: 0.01 to 0.1%
Al contributes to the deoxidation of the steel and has a role of fixing unnecessary solute N in the steel as a nitride. This effect is not sufficient if Al is less than 0.01%, and even if it exceeds 0.1%, the effect corresponding to the added amount cannot be obtained. Therefore, the Al content is set in the range of 0.01 to 0.1%.
[0029]
N: 0.01% or less N cannot be left in a solid solution state from the viewpoint of aging, and the smaller the content, the better. In addition, when the N content exceeds 0.005%, a large amount of the nitride-forming element is required, which is not only uneconomical, but also causes the ductility and toughness to deteriorate due to the presence of excess nitride, and further increases the content by 0.01%. , This tendency becomes remarkable. Therefore, the N content is set to 0.01% or less, preferably 0.005% or less.
[0030]
Ti: 0.01-0.05%
Ti has an effect of forming nitrides to refine crystal grains of a hot-rolled steel sheet, thereby reducing Δr of a cold-rolled steel sheet. This effect is not sufficient if the Ti content is less than 0.01%, and adding Ti below the lower limit rather increases Δr. On the other hand, if the amount of Ti exceeds 0.05%, the effect of reducing Δr is not only saturated, but also the ductility is reduced. Therefore, Ti is set in the range of 0.01 to 0.05%.
[0031]
Sheet thickness: preferably 0.5 mm or more The present invention is directed to cold-rolled steel sheets for automobiles and home electric appliances. The thickness of these cold-rolled steel sheets is preferably 0.5 mm or more from the viewpoint of component strength such as panel rigidity. Therefore, the plate thickness is preferably limited to 0.5 mm or more.
[0032]
In-plane anisotropy Δr: By reducing the absolute value | Δr | of the in-plane anisotropy index Δr having an absolute value of less than 0.15 r, a rotationally symmetric part can be uniformly formed. When | Δr | is 0.15 or more, ear formation after deep drawing becomes large, and quality such as plate thickness distribution becomes non-uniform. Further, the trimming operation increases the operating cost and lowers the material yield. Therefore, | Δr | is less than 0.15.
[0033]
Relationship between in-plane average value n * of n values and Δr: n * ≧ 0.1 × | Δr | +0.175
The influence of the n value and the r value on the stretch formability was examined. The in-plane average value n * of the n value and the absolute value | Δr | of the in-plane anisotropy of the r value greatly affected the stretch formability. I found out. Therefore, a ball head overhang test using a ball head punch having a diameter of 160 mm was performed on a test piece having a plate thickness of 1 mm and 400 mm × 400 mm, and the overhang formability (critical overhang height) was investigated. As a result, it was found that sufficient stretch formability could not be obtained simply by increasing the n value, and it was necessary to reduce in-plane anisotropy at the same time. FIG. 1 shows the relationship between the above n * and | Δr | and the stretch formability. Analysis of the results of the investigation revealed that satisfying the above-mentioned relational expression (1) would provide very good stretch formability.
[0034]
The invention of a manufacturing method capable of obtaining the cold-rolled steel sheet described above is as follows. According to the invention, a steel having the chemical composition of the above-mentioned invention is hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point, cooling is started within 2 seconds after finishing rolling, and the cooling is performed at 70 ° C./s. An in-plane anisotropy characterized by | Δr | <0.15 by performing the above-mentioned cooling rate over a temperature range of 100 ° C. or more, and subjecting the obtained hot-rolled steel sheet to cold rolling and continuous annealing. This is a method for producing a cold-rolled steel sheet having low resistance.
[0035]
The present invention has been made as a result of studying the manufacturing conditions under which the cold-rolled steel sheet of the above invention can be obtained, and the details thereof will be described below.
[0036]
Finishing Temperature: finish rolling Ar ₃ between transformation point or more hot rolling is conducted at a temperature at which the plate temperature is Ar ₃ transformation point or more. If the finishing temperature is lower than the Ar ₃ transformation point, the rolling load greatly fluctuates due to the discontinuity of the deformation resistance of the material (difference in deformation resistance between austenite and ferrite), and stable threading cannot be performed. As a result, a uniform and good material and plate shape cannot be obtained. Therefore, from the viewpoint of making the grain size of the hot-rolled steel sheet uniform and reducing the grain size, the finishing temperature is set to the Ar ₃ transformation point or higher.
[0037]
Cooling start time after rolling: Within 2 seconds after finish rolling The time from finishing rolling to cooling start is particularly important for suppressing the growth of austenite crystal grains before transformation, and this time is 2 seconds. If it exceeds 300, grain growth becomes remarkable. Therefore, cooling is started within 2 seconds after finishing rolling. Further, in order to reduce the in-plane anisotropy, it is effective to further shorten the time until the start of cooling, and it is desirable that the time be within 1 second.
[0038]
Cooling conditions after rolling: In the cooling after the hot rolling at a cooling rate of 70 ° C./s or more in a temperature range of 100 ° C. or more, it is extremely important to control the temperature width ΔT and the cooling rate of the temperature range in which cooling is performed. This is a finding obtained as a result of producing hot-rolled steel sheets from steel having the chemical composition of the present invention under various cooling conditions using an actual machine and examining the cold-rolled steel sheets in detail.
[0039]
FIGS. 2 and 3 are diagrams showing the effect of the cooling temperature width ΔT on the in-plane anisotropy | Δr | of the cold-rolled steel sheet and the stretch formability (critical stretch height). These figures show that when the cooling temperature width ΔT is 100 ° C. or more, the in-plane anisotropy | Δr | is significantly reduced.
[0040]
FIGS. 4 and 5 are diagrams showing the effect of the cooling rate on the in-plane anisotropy | Δr | and stretch formability (critical stretch height) of the cold-rolled steel sheet. From these figures, when the cooling rate is 70 ° C./s or more, | Δr | and stretch formability are significantly reduced, which is considered to be due to the microstructure of the hot-rolled steel sheet.
[0041]
As described above, in the present invention, the cooling rate after rolling is a cooling rate of 70 ° C./s or more in a temperature range of 100 ° C. or more.
[0042]
Annealing method after cold rolling: continuous annealing An object of the present invention is to produce a cold-rolled steel sheet having small in-plane anisotropy by continuous annealing that has high productivity and is suitable for mass production. Therefore, the method of annealing after cold rolling is continuous annealing.
[0043]
Needless to say, the cold rolled steel sheet of the invention described above can also achieve the desired effects even when it is used as an electro-galvanized steel sheet or a hot-dip galvanized steel sheet. In these galvanized steel sheets of the present invention, an organic coating treatment may be further performed after plating. In the present invention, since Si and P are kept low, there is no adverse effect on the surface properties of the galvanized steel sheet, and the present invention can be applied to an outer panel of an automobile.
[0044]
In these inventions, "the balance is substantially iron" means that those containing other trace elements, including unavoidable impurities, are included in the scope of the present invention, as long as the functions and effects of the invention are not impaired. Means that
[0045]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, when hot rolling a slab, the slab can be rolled after being heated in a heating furnace, or can be directly rolled without heating. The coiling temperature of hot rolling can be set to a normal range of about 550 to 700 ° C, and particularly preferably 600 to 680 ° C.
[0046]
As is well known, there are appropriate ranges for the cold-pressure ratio and the annealing temperature of the cold-rolled steel sheet according to the chemical composition. If a hot-rolled steel sheet is manufactured according to the invention of the above-described manufacturing method, the cooling pressure ratio may be within a normal range (less than 90%), and the annealing temperature may be the normal temperature of continuous annealing. However, in order to the tissue of the steel sheet with ferrite single-phase structure, the annealing temperature is preferably set to a temperature of less than Ac ₃ transformation point.
[0047]
In addition, it is desirable that the r value r _{90 in the} direction of 90 ° with respect to the rolling direction is 1.3 or less. This is because if this _is became magnitude relation of _{r 0 <r 45 <r 90} , Δr is reduced on calculations, the difference between _{r 0} and _{r 90} (LC difference) is _enlarged, the upper limit on _{r 90} This is to suppress the LC difference to be low by providing. In practice, it can be said that if r _{90 is set} to 1.3 or less, the in-plane anisotropy in consideration of this LC difference is sufficiently small.
[0048]
【Example】
[Example 1]
A Ti-added steel having the chemical components shown in Table 1 was melted, and a slab was manufactured by continuous casting.
[0049]
[Table 1]

[0050]
As shown in Table 1, steel numbers 1 to 6 of the present invention are all steels of the present invention whose chemical components are within the scope of the present invention, but steel numbers 7 to 10 are out of the scope of the present invention. Comparative steel. That is, steel number 7 has a C content less than the lower limit, steel numbers 8 and 9 have a Ti content outside the invention range, and steel number 10 has a B amount outside the invention range.
[0051]
The slab was heated to 1200 ° C., hot-rolled, then cooled under various cooling conditions, and rolled within a normal winding temperature to produce a hot-rolled steel sheet. The hot-rolled steel sheet was subjected to pickling and cold rolling, and was subjected to continuous annealing to obtain a cold-rolled steel sheet, a hot-dip galvanized steel sheet or an electro-galvanized steel sheet. These cold-rolled steel sheets and galvanized steel sheets were subjected to temper rolling at a rolling reduction of 0.5 to 2.0%. Table 2 shows the above hot rolling conditions (finishing temperature, cooling rate, temperature range ΔT of cooling temperature range) and annealing (plating) conditions.
[0052]
[Table 2]

[0053]
For these cold-rolled steel sheets and galvanized steel sheets, r values were measured in the 0 °, 45 °, and 90 ° directions with respect to the rolling direction, and Δr was determined. The test results are also shown in Table 2.
[0054]
As shown in Table 2, Invention Example No. in which the chemical components and production conditions were within the scope of the invention. In each of 1-3, 7, 8, and 10, | Δr | <0.15 was satisfied, and the object of the invention was achieved. On the other hand, in the comparative examples in which the chemical components or the production conditions are out of the range of the invention, the in-plane anisotropy increases, and the target effect of suppressing Δr of the present invention cannot be obtained.
[0055]
For example, in Comparative Example No. In Nos. 4 to 6 and 9, the chemical components are within the scope of the invention (steel Nos. 3 and 5), but Δr exceeds the target range because the manufacturing conditions are out of the scope of the invention. No. In Nos. 4 and 5, the cooling rate and the cooling temperature width ΔT after rolling were out of the range of the invention, respectively. No. 6 has a finishing temperature, and No. 6 In No. 9, the cooling rate and the cooling temperature width ΔT are out of the range of the invention, respectively.
[0056]
In Comparative Example No. Nos. 11 to 14 have large anisotropy because the chemical components are out of the range of the invention (steel numbers 7 to 10). No. In No. 11 (Steel No. 7), the in-plane anisotropy is out of the target range because the C content is smaller than the lower limit. No. In No. 12 (Steel No. 8), the in-plane anisotropy is out of the target range because the amount of Ti added is less than the lower limit. No. No. 13 (Steel No. 9) has a Ti addition amount exceeding the upper limit and the in-plane anisotropy is slightly out of the target range, but the excessive Ti addition lowers the ductility and toughness. It is unsuitable as a steel sheet for automobiles and household appliances to which the present invention is directed. No. In No. 14 (Steel No. 10), the added B exerted a bad influence on the formation of the recrystallized texture, and the in-plane anisotropy was extremely deteriorated.
[0057]
[Example 2]
A slab was produced by smelting Ti-added steel having the chemical components shown in Table 3 and continuous casting.
[0058]
[Table 3]

[0059]
As shown in Table 3, steel numbers 11 to 16 of the present invention are steels of the present invention whose chemical components are all within the scope of the present invention, but steel numbers 17 to 20 are out of the scope of the present invention. Comparative steel. That is, steel No. 17 has a C content less than the lower limit, steel Nos. 18 and 19 have a Ti content outside the invention range, and steel No. 20 has a B content outside the invention range.
[0060]
The slab was heated to 1200 ° C., hot-rolled, then cooled under various cooling conditions, and rolled within a normal winding temperature to produce a hot-rolled steel sheet. The hot-rolled steel sheet was pickled and cold-rolled to produce a cold-rolled steel sheet by continuous annealing or a galvanized steel sheet by hot-dip galvanizing or electro-galvanizing. These cold-rolled steel sheets and galvanized steel sheets were subjected to temper rolling at a rolling reduction of 0.5 to 2.0%. Table 4 shows the above hot rolling conditions (finish temperature, cooling rate, temperature width ΔT of the cooling temperature range) and annealing (plating) conditions.
[0061]
[Table 4]

[0062]
For these cold-rolled steel sheets and galvanized steel sheets, the n value and the r value in the 0 ° 45 ° and 90 ° directions with respect to the rolling direction were measured, and the in-plane average value n * of the n-values and the in-plane anisotropy Δr I asked. Further, a test piece of 400 mm × 400 mm was subjected to a ball head overhang test using a ball head punch having a diameter of 160 mm, and the overhang formability (critical overhang height) was investigated. The test results are shown in Table 4.
[0063]
As shown in Table 4, this invention example No. in which the chemical components and the production conditions were within the scope of the invention. 21 to 23, 27, 28, and 30 all satisfy | Δr | <0.15 and the relational expression (1) between n * and Δr, thereby achieving the object of the invention.
[0064]
On the other hand, in the comparative examples in which the chemical components or the production conditions are out of the range of the invention, the in-plane anisotropy is increased, the effect of suppressing the target Δr of the present invention is not obtained, and the in-plane average value n of the n values is not obtained. The relational expression (1) between * and Δr is not satisfied.
[0065]
For example, in Comparative Example No. In Nos. 24 to 26 and 29, the chemical components are within the range of the invention (steel numbers 13 and 15), but since the manufacturing conditions are out of the range of the invention, Δr exceeds the target range. No. In Nos. 24 and 25, the cooling rate and the cooling temperature width ΔT after rolling were out of the range of the invention, respectively. No. 26 is the finishing temperature. Reference numeral 29 indicates that the cooling rate and the cooling temperature width ΔT are out of the range of the invention, respectively.
[0066]
In Comparative Example No. In Nos. 31 to 34, the chemical components are out of the range of the invention (Steel Nos. 17 to 20), so that the relational expression (1) between n * and Δr is not satisfied, and the in-plane anisotropy is large. No. No. 31 (Steel No. 17) has an in-plane anisotropy outside the target range because the C content is smaller than the lower limit. No. In No. 32 (Steel No. 18), the in-plane anisotropy is out of the target range because the amount of Ti added is less than the lower limit. No. No. 33 (steel No. 19) has an in-plane anisotropy slightly out of a target range, but excessive addition of Ti lowers ductility and toughness. It is not suitable as a steel plate. No. In No. 34 (Steel No. 20), the added B has an adverse effect on the formation of a recrystallized texture, and the in-plane anisotropy is extremely deteriorated.
[0067]
【The invention's effect】
The present invention controls a chemical composition within a specific range, and controls a hot-rolling finish rolling and a subsequent cooling condition so that a cold-rolled steel sheet or a zinc sheet having a thickness of 0.5 mm or more and a small in-plane anisotropy is obtained. We succeeded in producing plated steel sheets. As a result, the steel sheet of the present invention can be widely used for automobile steel sheets and household electric appliances.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between an in-plane average value n * and an in-plane anisotropy | Δr | of an n value and stretch formability.
FIG. 2 is a view showing the effect of a cooling temperature width ΔT on in-plane anisotropy of a cold-rolled steel sheet.
FIG. 3 is a view showing the effect of a cooling temperature width ΔT on stretch formability of a cold-rolled steel sheet.
FIG. 4 is a view showing the effect of a cooling rate on in-plane anisotropy of a cold-rolled steel sheet.
FIG. 5 is a view showing the effect of the cooling rate on the stretch formability of a cold-rolled steel sheet.

Claims (3)

Chemical component is mass%, C: 0.01 to 0.08%, Si: 0.5% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.03% or less , Al: 0.01-0.1%, N: 0.01% or less, Ti: 0.01-0.05%, the balance being substantially composed of iron, | Δr | <0.15 Cold rolled steel sheet with small in-plane anisotropy. 2. The cold-rolled steel sheet having a small in-plane anisotropy according to claim 1, wherein the in-plane average value n * of the n value satisfies the following relational expression and is excellent in stretch formability.
n * ≧ 0.1 × | Δr | +0.175 The steel comprising the chemical composition according to claim 1 is subjected to hot rolling at a finishing temperature not lower than the Ar ₃ transformation point, cooling is started within 2 seconds after finishing rolling, and the cooling is performed at 70 ° C./s or more. At a speed of 100 ° C. or higher, and cold-rolling the obtained hot-rolled steel sheet for continuous annealing to control the temperature in a range satisfying | Δr | <0.15. Manufacturing method of cold-rolled steel sheet with small anisotropy.

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