JP3941264B2 - Method for producing soft cold-rolled steel sheet - Google Patents

Description

【００２８】
【表２】

【００２９】
【表３】

【００３０】
（実施例２）
表４に示す組成を有する鋼（Ａ〜Ｄ）を溶製して鋳造速度１．６〜２．２ｍｐｍで連続鋳造を行い厚さ２５０ｍｍ、長さ１３ｍのスラブとし、表５に示すようにＦ値を種々変化させた条件（Ｎｏ．１〜５：本発明例、Ｎｏ．６〜９：比較例）にて直送熱延を行い、７５％の冷間圧延を行った後に７５０℃で連続焼鈍を行って、各コイルの長手方向先端および後端からそれぞれ５０ｍの位置からサンプルを採取して引張試験を行い、ＴＳの変動量（ΔＴＳ）により材質均一性を評価した。
表６の結果に示すように、表５においてＦ値の変動量を本発明の範囲内の０．８以下（本発明例Ｎｏ．１〜５）に抑えることにより、鋼種によらず従来法（比較例Ｎｏ．６〜９）に比べΔＴＳが５〜１３程度と小さく、材質が均一な鋼板が得られることが解る。
【００３１】
一方、比較例Ｎｏ．６〜９は、粗圧延の圧下率及びＦ値の変動量が本発明の範囲から外れており、本発明例Ｎｏ．１〜５と比べた場合、ΔＴＳが大きくなっている。
【００３２】
【表４】

【００３３】
【表５】

【００３４】
【表６】

【００３５】
【発明の効果】
以上説明したように、本発明によれば、鋼組成及び製造条件を特定することにより、長尺スラブを用いた直送圧延を用いて、低コストでかつコイル内の材質変動が小さい軟質冷延鋼板の製造方法を提供することができる。
【図面の簡単な説明】
【図１】 本発明の第１実施形態に係る直送圧延材の鋳造終了後から粗圧延終了までの所要時間（ｔｃｆ）と引張強さ（ＴＳ）との関係を示す図。
【図２】 本発明の第１実施形態に係る直送圧延材の冷延コイル内のＦ値の変動量と冷延コイル内の引張強さ（ＴＳ）の変動量との関係を示す図。
【図３】 本発明の第２実施形態に係る直送圧延材の鋳造終了後から粗圧延終了までの所要時間（ｔｃｆ）と引張強さ（ＴＳ）との関係を示す図。
【図４】 本発明の第２実施形態に係る直送圧延材の冷延コイル内のＦ値の変動量と冷延コイル内の引張強さ（ＴＳ）の変動量との関係を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a soft cold-rolled steel sheet with small material fluctuation by controlling the history from casting to hot rolling when performing direct rolling using a long slab, such as automobiles, home appliances, pipes, etc. The present invention relates to a method for producing a high-quality soft cold-rolled steel sheet at low cost for a wide range of applications.
[0002]
[Prior art]
Cold-rolled steel sheets are used in a wide range of fields such as automobiles, home appliances, and pipes. From the viewpoints of processing accuracy of structural members and product performance stability, the material fluctuation in the supplied coil should be reduced as much as possible. Is anxious.
[0003]
In order to reduce the longitudinal material fluctuation in the cold rolled coil, it is important to reduce the longitudinal fluctuation of the cold rolling and annealing conditions as much as possible. The application of continuous annealing technology has been improved to almost no problem level. However, if there is material variation in the hot-rolled coil, which is a cold-rolled material, the material variation also occurs in the cold-rolled coil due to this effect regardless of the cold-rolling and annealing processes, so this problem can be solved. It is an important issue. As a method of reducing the material fluctuation in the longitudinal direction in the hot-rolled coil, for continuous rolling in which a slab is heated to a rough bar after being heated in a heating furnace, and finish-rolling is performed by hot connection, JP-A-9- No. 241739, JP-A-9-241740, JP-A-9-241741, and JP-A-9-241742, the difference in finish rolling temperature in the coil is less than 50 ° C., and the tension in the finish mill is 0.1 kgf. / Mm ^{2 or} less, a strain rate in the finishing mill of 300 / sec or more in at least one pass, a difference in finishing rolling speed in the coil of less than 400 mpm, and a temperature difference between the entry and exit sides of the finishing mill of less than 100 ° C. Has been proposed. On the other hand, from the viewpoint of economy, compared to continuous rolling using a heating furnace such as the above-mentioned technique and heat-joining with a coarse bar, the continuous cast slab is reheated or simply heated or kept warm. Direct feed rolling, in which rolling is performed alone, is advantageous. Various technologies have already been proposed for direct rolling. For example, as a method for further improving the economy by making full use of the advantages of direct rolling, the slab is made longer than conventional heating furnaces and cropped. Japanese Laid-Open Patent Publication No. 9-253701 proposes a method for reducing the loss and the ratio of the unsteady part at the front and rear ends of the coil. This technology also takes into account the homogenization of the coarse bar by heat retention in the coil box.
[0004]
[Problems to be solved by the invention]
However, in the direct rolling using a long slab as described above, the heat history after casting differs greatly depending on the position in the longitudinal direction of the slab, so the temperature of the finish rolling process in the soaking by the coil box or the above-mentioned continuous rolling technique is significant. Moreover, there is a problem that the material fluctuation in the coil cannot be eliminated only by controlling the processing conditions.
[0005]
Thus, although the direct feed rolling method using a long slab has a great economic advantage, there is a problem that a uniform material cannot be obtained in the coil. Is not sufficiently put into practical use.
The present invention has been made in consideration of such circumstances, and provides a method for producing a soft cold-rolled steel sheet that is low-cost and has a small material fluctuation in the coil by using direct rolling using a long slab. It is the purpose.
[0006]
[Means for Solving the Problems]
In order to solve the above problems and achieve the object, the present invention uses the following means.
(1) The production method of the present invention is mass %, C: 0.08% or less, Si: 0.1% or less, Mn: 0.1-0.6%, P: 0.05% A steel plate containing the following, S: 0.03% or less, Al: 0.01 to 0.1%, and N: 0.01% or less, with the balance being Fe and inevitable impurities In the manufacturing method,
A step of continuous casting and immediately rough rolling of the steel;
The rough bar obtained by rough rolling is heated to adjust the finish rolling entry temperature, and finish rolling is performed so that the fluctuation amount of the F value given by the following formula (1) is 0.8 or less in the coil. Winding process,
Cold rolling the rolled hot-rolled steel sheet at a reduction rate of 50% or more and annealing at 700 to 900 ° C. for 30 seconds or more;
A method for producing a soft cold-rolled steel sheet having a small material variation in the coil.
F = In (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 (1)
However, Vf: Rolling speed (mpm) of final finishing stand, Tf: Finishing rolling finish temperature (° C.), tcf: Required time (minutes) from the end of casting to the end of rough rolling.
[0007]
(2) The production method of the present invention is mass %, C: 0.08% or less, Si: 0.1% or less, Mn: 0.1-0.6%, P: 0.05% A steel plate containing the following, S: 0.03% or less, Al: 0.01 to 0.1%, and N: 0.01% or less, with the balance being Fe and inevitable impurities In the manufacturing method,
A step of continuously casting the steel and immediately subjecting it to rough rolling at a reduction rate of 80% or less;
The rough bar obtained by rough rolling is heated to adjust the finish rolling entry temperature, and finish rolling is performed so that the fluctuation amount of the F value given by the following formula (1) is 0.8 or less in the coil. Winding process,
Cold rolling the rolled hot-rolled steel sheet at a reduction rate of 50% or more and annealing at 700 to 900 ° C. for 30 seconds or more;
A method for producing a soft cold-rolled steel sheet having a small material variation in the coil.
F = In (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 (1)
However, Vf: Rolling speed (mpm) of final finishing stand, Tf: Finishing rolling finish temperature (° C.), tcf: Required time (minutes) from the end of casting to the end of rough rolling.
[0008]
(3) In the production method of the present invention, in the rough rolling step, rough rolling is performed using a continuous cast slab corresponding to at least two coil weights, and the steel sheet is divided and wound after the finish rolling step. The method for producing a soft cold-rolled steel sheet having a small material variation in the coil according to the above (1) or (2).
(4) The production method of the present invention further includes Ti: 0.06 to 0.08 mass% and / or B: 0.002 to 0.003 mass% as a component of the steel sheet. (1) It is a manufacturing method of the soft cold-rolled steel plate with the small material fluctuation | variation in the coil in any one of (3).
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have conducted a detailed investigation on direct feed rolling using a long slab, paying attention to the relationship between heat and processing history and material at each position in the longitudinal direction of the slab.
As a result, the time required from the time when the slab produced by continuous casting exits the mold to the end of rough rolling and the rolling reduction of the rough rolling greatly affect the material, and finish rolling is finished according to this required time. It has been found that the material fluctuation in the cold-rolled coil can be significantly reduced by using the hot-rolled coil obtained by regulating the temperature and the rolling speed of the final finishing stand and regulating the rolling reduction of the rough rolling.
[0010]
Based on this knowledge, the present inventors have determined the hot rolling conditions (i.e., the time required from the end of casting to the end of rough rolling, the rolling reduction of rough rolling, and finish rolling when direct rolling the steel with the specified chemical composition. The end temperature and the rolling speed of the final finishing stand) are controlled within a certain range, and in direct feed rolling using a long slab, a method for producing a soft cold-rolled steel sheet with a small material fluctuation in the coil is found. Completed the invention.
[0011]
That is, the present invention provides a method for producing a high-quality soft cold-rolled steel sheet at a low cost for a wide range of uses such as automobiles, home appliances, and pipes by limiting the steel composition and production conditions to the following ranges. be able to.
Below, the reason for component addition of the present invention, the reason for component limitation, and the reason for limitation of production conditions will be described.
[0012]
(1) Component composition range C: 0.08% or less C is preferable because it lowers the hardness and ductility of the steel sheet, and the upper limit is 0.08%.
Si: 0.1% or less Since Si hardens the steel sheet and causes deterioration of the surface properties due to the occurrence of red scale, it is 0.1% or less.
Mn: 0.1 to 0.6%
Mn is added in an amount of 0.1% or more from the viewpoint of securing S as MnS and ensuring hot ductility, but if added over 0.6%, the steel sheet is hard and ductile, so 0.6% is the upper limit. It is.
P: 0.05% or less P is 0.05% or less because P is an element that makes the steel sheet hard and ductile.
S: 0.03% or less S not only lowers hot ductility and increases crack susceptibility of slabs, but also inhibits grain growth through precipitation of fine MnS, resulting in deterioration of workability. % Or less.
Al: 0.01 to 0.1%
Al is an element that fixes N and renders it harmless. For this purpose, addition of 0.01% or more is necessary. However, since the effect is saturated when it exceeds 0.1%, the upper limit is 0.1%.
N: 0.01% or less N is an element that hardens and lowers the ductility of the steel sheet, and needs to be 0.01% or less.
In addition, you may add component elements, such as Ti and B, in the range which does not inhibit the effect of this invention as needed.
By adjusting to the above component composition range, it is possible to obtain a soft cold-rolled steel sheet with low cost and small material fluctuation in the coil by using direct feed rolling using a long slab.
[0013]
A steel plate having such characteristics can be manufactured by the following manufacturing method.
(2) Steel plate manufacturing process (2-1) Manufacturing conditions of manufacturing mode 1 (manufacturing method)
The steel adjusted to the above component composition range is melted in a converter and continuously casted and immediately subjected to rough rolling, and then the resulting rough bar is heated by a heating device arranged in the rolling line and finished rolling is entered. The temperature is adjusted, and finish rolling is performed so that the fluctuation amount of the F value given by the following formula (1) is 0.8 or less in the coil. Next, the obtained hot-rolled steel sheet is cold-rolled at a reduction rate of 50% or more and annealed at 700 to 900 ° C. for 30 seconds or more. F = ln (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 (1)
However, Vf: Rolling speed (mpm) of final finishing stand, Tf: Finishing rolling finish temperature (° C.), tcf: Required time (minutes) from the end of casting to the end of rough rolling.
[0014]
Further, in the rough rolling step, rough rolling may be performed using a continuous cast slab corresponding to the weight of at least two coils, and the steel plate may be divided and wound after the finish rolling step.
[0015]
Time required from the time the slab produced by continuous casting exits the mold to the end of rough rolling: tcf (minutes), rolling speed of the final finishing stand: Vf (mpm), finishing rolling finishing temperature: Tf (° C.) The reason for limitation will be described.
[0016]
As a result of investigating in detail the relationship between material fluctuations in a cold-rolled coil manufactured from a direct-rolled rolled material and manufacturing conditions, the present inventors have found that the influence of tcf is large. Therefore, as a result of various analyzes to extract factors that are effective in reducing the influence of fluctuations in tcf, it was concluded that controlling Vf and Tf is most effective.
[0017]
FIG. 1 shows (0.040% to 0.043%) C- (0.01% to 0.02%) Si- (0.21% to 0.25%) Mn steel under various conditions. In the cold-rolled coil that was subjected to direct rolling and 75% cold-rolled and then subjected to continuous annealing at 740 to 760 ° C, the results of hot rolling finish rolling conditions were Vf = 700 to 800 mpm, Tf = 850 to 860 ° C. This is the result of examining the tensile strength (TS, MPa) and organizing by tcf (min). It can be seen that TS increases as tcf increases. The details of this mechanism are unknown, but with the increase of tcf, the form of sulfide and nitride that are strain-induced precipitation during hot rolling rough rolling becomes more finely dispersed, which hinders recrystallization and grain growth in finish rolling. It is presumed that it is due to being done.
[0018]
FIG. 2 shows the fluctuation amount of TS in the cold-rolled coil with the fluctuation amount of F = ln (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 for the test coil. It has been organized. Here, the fluctuation amount of TS was evaluated by collecting samples from the positions of the front and rear ends of the coil in the longitudinal direction of the coil. The fluctuation amount of TS in the coil increases as the fluctuation amount of F value increases. As can be seen from this figure, in order to stabilize the TS fluctuation amount to a low level, the F value fluctuation amount needs to be 0.8 or less.
[0019]
Further, the rolling reduction when the hot-rolled steel sheet wound after the finish rolling is cold-rolled is preferably 50% or more from the viewpoint of workability, particularly deep drawability. About annealing, it is 700 degreeC or more for softening, and 900 degrees C or less for coarse grain suppression. The soaking time is preferably 30 seconds or longer in order to stabilize the structure.
(2-2) Manufacturing conditions (production method) of aspect 2
The steel adjusted to the above component composition range is melted in a converter and continuously casted, and immediately subjected to rough rolling with a reduction rate of 80% or less, and then the obtained coarse bar is heated by a heating device arranged in the rolling line. Then, the finish rolling entry temperature is adjusted, and finish rolling is performed so that the fluctuation amount of the F value given by the following formula (1) is 0.8 or less in the coil. Next, the obtained hot-rolled steel sheet is cold-rolled at a reduction rate of 50% or more and annealed at 700 to 900 ° C. for 30 seconds or more.
[0020]
F = ln (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 (1)
However, Vf: Rolling speed (mpm) of final finishing stand, Tf: Finishing rolling finish temperature (° C.), tcf: Required time (minutes) from the end of casting to the end of rough rolling.
[0021]
Further, in the rough rolling step, rough rolling may be performed using a continuous cast slab corresponding to the weight of at least two coils, and the steel plate may be divided and wound after the finish rolling step.
[0022]
Rolling reduction ratio, time required from the time when the slab produced by continuous casting exits the mold to completion of rough rolling: tcf (minutes), rolling speed of the final finishing stand: Vf (mpm), finishing rolling finished The reason for limiting the temperature: Tf (° C.) will be described.
[0023]
FIG. 3 shows that (0.040% -0.043%) C- (0.01% -0.02%) Si- (0.21% -0.25%) Mn steel is used under various conditions. In the cold-rolled coil that was subjected to direct rolling and 75% cold-rolled and then subjected to continuous annealing at 740 to 760 ° C, the results of hot rolling finish rolling conditions were Vf = 700 to 800 mpm, Tf = 850 to 860 ° C. This is the result of examining the tensile strength (TS, MPa) for the portion of, and distinguishing between those with a rolling reduction ratio of over 80% and those with a rolling reduction of 80% or less, and arranging them in tcf (min). Similar to the example of FIG. 2, the TS increased with the increase of tcf, but the result of the increase in TS was small when the rolling reduction of rough rolling was 80% or less. This is considered to be because the tcf dependency of the strain-induced precipitation form of sulfide or nitride during rough rolling is lowered by lowering the rolling reduction.
[0024]
FIG. 4 shows the fluctuation amount of TS in the cold-rolled coil with F = ln (Vf−350) · (tcf + 95) ^1/2 / These are arranged with a variation of exp (−5450 / (Tf + 273)) / 1000. Here, the fluctuation amount of TS was evaluated by collecting samples from the positions of the front and rear ends of the coil in the longitudinal direction of the coil. The fluctuation amount of TS in the coil increases as the fluctuation amount of F value increases. As can be seen from this figure, in order to stabilize the TS fluctuation amount to a low level, the F value fluctuation amount needs to be 0.8 or less.
[0025]
The reasons for limiting the rolling reduction, annealing temperature, and soaking time when the hot-rolled steel sheet wound up after the finish rolling is cold-rolled are the same as the production conditions of the above-described aspect 1.
In the present invention, sufficient effects can be obtained even when various electroplating treatments are performed after annealing or in addition to organic coating treatments, and when annealing and plating treatments are performed by a continuous hot dip galvanizing line after cold rolling. .
Examples of the present invention will be given below to prove the effects of the present invention.
[0026]
【Example】
Example 1
Steels (A to D) having the composition shown in Table 1 are melted and continuously cast at a casting speed of 1.6 to 2.2 mpm to form a slab having a thickness of 250 mm and a length of 13 m. As shown in Table 2, F Continuous annealing at 750 ° C. after direct hot rolling under conditions with various values changed (No. 1-5: invention examples, No. 6-9: comparative examples) and 75% cold rolling Then, samples were taken from positions 50 m from the front and rear ends in the longitudinal direction of each coil, a tensile test was performed, and the material uniformity was evaluated by the amount of TS variation (ΔTS).
As shown in the results of Table 3, in Table 2, the fluctuation amount of the F value is suppressed to 0.8 or less (invention example Nos. 1 to 5) within the range of the present invention, so that the conventional method ( It can be seen that a steel plate having a uniform material and a small ΔTS of about 5 to 18 compared with Comparative Examples No. 6 to 9) can be obtained.
On the other hand, Comparative Example No. Nos. 6 to 9 show that the fluctuation amount of the F value is out of the scope of the present invention. When compared with 1 to 5, ΔTS is larger.
[0027]
[Table 1]

[0028]
[Table 2]

[0029]
[Table 3]

[0030]
(Example 2)
Steels (A to D) having the composition shown in Table 4 are melted and continuously cast at a casting speed of 1.6 to 2.2 mpm to form a slab having a thickness of 250 mm and a length of 13 m. As shown in Table 5, F Continuous annealing at 750 ° C. after direct hot rolling under conditions with various values changed (No. 1-5: invention examples, No. 6-9: comparative examples) and 75% cold rolling Then, samples were taken from positions 50 m from the front and rear ends in the longitudinal direction of each coil, a tensile test was performed, and the material uniformity was evaluated by the amount of TS variation (ΔTS).
As shown in the results of Table 6, the conventional method (regardless of the steel type) by suppressing the fluctuation amount of the F value to 0.8 or less (invention example Nos. 1 to 5) within the scope of the invention in Table 5. It can be seen that a steel plate having a uniform material and a small ΔTS of about 5 to 13 compared with Comparative Examples No. 6 to 9) can be obtained.
[0031]
On the other hand, Comparative Example No. In Nos. 6 to 9, the rolling reduction of the rough rolling and the fluctuation amount of the F value are out of the scope of the present invention. When compared with 1 to 5, ΔTS is larger.
[0032]
[Table 4]

[0033]
[Table 5]

[0034]
[Table 6]

[0035]
【The invention's effect】
As described above, according to the present invention, by specifying the steel composition and production conditions, the soft cold-rolled steel sheet with low cost and small material fluctuation in the coil using direct feed rolling using a long slab. The manufacturing method of can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between a required time (tcf) from the end of casting of a direct-rolled material according to the first embodiment of the present invention to the end of rough rolling and the tensile strength (TS).
FIG. 2 is a view showing the relationship between the fluctuation amount of the F value in the cold-rolled coil and the fluctuation amount of the tensile strength (TS) in the cold-rolled coil according to the first embodiment of the present invention.
FIG. 3 is a view showing a relationship between a required time (tcf) from the end of casting of the direct-rolled material according to the second embodiment of the present invention to the end of rough rolling and the tensile strength (TS).
FIG. 4 is a view showing the relationship between the fluctuation amount of the F value in the cold-rolled coil and the fluctuation amount of the tensile strength (TS) in the cold-rolled coil according to the second embodiment of the present invention.

Claims (4)

In mass %, C: 0.08% or less, Si: 0.1% or less, Mn: 0.1-0.6%, P: 0.05% or less, S: 0.03% or less And, in a method for producing a steel sheet containing Al: 0.01 to 0.1%, N: 0.01% or less , the balance being Fe and inevitable impurities ,
A process of continuously casting and immediately rough rolling the steel;
The rough bar obtained by rough rolling is heated to adjust the finish rolling entry temperature, and finish rolling is performed so that the fluctuation amount of the F value given by the following formula (1) is 0.8 or less in the coil. Winding process,
Cold rolling the rolled hot-rolled steel sheet at a reduction rate of 50% or more and annealing at 700 to 900 ° C. for 30 seconds or more;
A method for producing a soft cold-rolled steel sheet having a small material variation in the coil.
F = In (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 (1)
However, Vf: Rolling speed (mpm) of final finishing stand, Tf: Finishing rolling finish temperature (° C.), tcf: Required time (minutes) from the end of casting to the end of rough rolling. In mass %, C: 0.08% or less, Si: 0.1% or less, Mn: 0.1-0.6%, P: 0.05% or less, S: 0.03% or less And, in a method for producing a steel sheet containing Al: 0.01 to 0.1%, N: 0.01% or less , the balance being Fe and inevitable impurities ,
A step of continuously casting the steel and immediately performing rough rolling at a reduction rate of 80% or less;
The rough bar obtained by rough rolling is heated to adjust the finish rolling entry temperature, and finish rolling is performed so that the fluctuation amount of the F value given by the following formula (1) is 0.8 or less in the coil. Winding process,
Cold rolling the rolled hot-rolled steel sheet at a reduction rate of 50% or more and annealing at 700 to 900 ° C. for 30 seconds or more;
A method for producing a soft cold-rolled steel sheet having a small material variation in the coil.
F = In (Vf−350) · (tcf + 95) ^1/2 / exp (−5450 / (Tf + 273)) / 1000 (1)
However, Vf: Rolling speed (mpm) of final finishing stand, Tf: Finishing rolling finish temperature (° C.), tcf: Required time (min) from the end of casting to the end of rough rolling.   In the rough rolling step, rough rolling is performed using a continuous cast slab corresponding to the weight of at least two coils, and the steel plate is divided and wound after the finish rolling step. Or the manufacturing method of the soft cold-rolled steel plate with a small material fluctuation | variation in the coil of 2. The coil according to any one of claims 1 to 3, further comprising Ti: 0.06 to 0.08 mass% and / or B: 0.002 to 0.003 mass% as a component of the steel sheet. Of soft cold-rolled steel sheet with small material fluctuation.

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