JP4332960B2 - Manufacturing method of high workability soft cold-rolled steel sheet - Google Patents

Description

【００４７】
［実施例２］
表２に示す成分の鋼を溶解・鋳造後、熱間圧延を行った。得られた熱延板を酸洗，冷間圧延後、７８０℃で焼鈍し板厚１．０ｍｍの焼鈍板を製造した。焼鈍板に伸長率１．０％で調質圧延を行った後に、引張試験とｒ値の測定を行った。鋼Ｎｏ．１〜Ｎｏ．７はＢを添加しない成分で、Ｎｏ．８以降はＢを添加した成分となっている。
【００４８】
鋼Ｎｏ．１〜Ｎｏ．６は圧延後、冷却を式（１）を満足する時間内に、開始した事例で、式（１）による冷却開始時間の上限を経過後、冷却を開始した鋼Ｎｏ．７と比較してｒ値が向上している。また、鋼Ｎｏ．８〜１３は圧延後、冷却を式（２）を満足する時間内に、開始した事例で、式（２）による冷却開始時間を経過後、冷却を開始した鋼Ｎｏ．１４と比較してｒ値が向上している。
【００４９】
【表２】

【００５０】
【発明の効果】
本発明は、低炭素鋼の熱間圧延後の冷却条件を、熱延板の組織が微細整粒組織となるように調整し、冷延後の焼鈍時に再結晶核を多数発生させることにより、粒成長のみに依存せずｒ値を向上させるので、加工性、且つ加工後の表面性状に優れ、自動車や家電製品等に適する冷延鋼板が製造可能で産業上極めて効果が大きい。
【図面の簡単な説明】
【図１】ｒ値に及ぼす冷却開始時間、仕上げ温度の影響を示す図
【図２】ｒ値に及ぼす冷却停止温度の影響を示す図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cold-rolled steel sheet used for automobiles, home appliances, and the like, and particularly relates to a method for producing a highly workable soft cold-rolled steel sheet having excellent workability and surface properties after processing.
[0002]
[Prior art]
Steel sheets used in automobiles, home appliances, and the like are required to have high formability, and soft and high r-values are being vigorously advanced. When manufacturing such a high workability cold-rolled steel sheet in continuous annealing, IF steel in which C and N in steel are reduced and carbonitride forming elements are added and C and N are completely fixed is used. However, since C and N in the steel are reduced to the limit and Ti, Nb and the like are added, there is a disadvantage that the manufacturing cost is high.
[0003]
Thus, as a technique for obtaining high workability without reducing C to the limit, B-added low carbon steel in which B is added to low carbon steel and N is fixed as BN has been studied. However, the r value, which is one of the indexes for workability, remains low due to the presence of solute C.
[0004]
On the other hand, paying attention to the fact that the r value of the annealed plate increases as the grain size of the hot-rolled plate decreases, JP-A No. 54-135616 discloses that steel with extremely reduced C, Mn, O is not less than Ar3. A method for producing a soft cold-rolled steel sheet is disclosed in which after hot rolling at a finishing temperature, cooling is performed at 100 ° C./second or more and winding is performed at 600 ° C. or less.
[0005]
However, when cooling to the coiling temperature at such a cooling rate, the ferrite structure of the hot-rolled sheet appears to have wavy grain boundaries, and distortion that improves the r value during cold rolling is less likely to accumulate. Does not rise as expected, and fine precipitates such as MnS are extremely reduced, so that the grain growth is extremely good, and the ferrite is coarsened to cause rough skin after processing.
[0006]
In JP-A-6-172869, a steel to which a small amount of B is added is rolled at a finishing temperature of 920 ° C. or higher, hot rolling is finished before BN precipitates, and cooling is performed at 20 ° C./second or higher after rolling. A manufacturing method of a soft cold-rolled steel sheet having good workability, which is characterized, is disclosed. However, in the examples, the finishing temperature is mainly 930 ° C. or more, and in this case, the crystal grains have a strong tendency to coarsen due to the recrystallization of γ before the γ → α transformation, and the roughening occurs after the processing.
[0007]
[Problems to be solved by the invention]
As described above, a method for producing a high workability cold-rolled steel sheet using a low-carbon steel sheet or the like has been disclosed, but a technique for achieving both excellent surface properties after processing and high workability has not been established. . The present invention provides a method for producing a highly workable soft cold-rolled steel sheet having excellent surface properties after processing.
[0008]
[Means for Solving the Problems]
In the cold-rolled steel sheet, the inventors have achieved both high workability and prevention of rough surface after working by increasing the frequency of occurrence of recrystallized nuclei that contribute to the improvement of the r value. The effect of the structure of the hot-rolled sheet, which is the structure, on the r value was examined in detail. As a result, in order to improve the r value, it is important to refine the hot rolled sheet structure while maintaining the grain size. For that purpose, immediately after the rapid cooling after rolling, the ferrite grain size adjusting process is effective. I found it. That is, it has been found that a large number of recrystallized nuclei that contribute to the improvement of the r value during annealing after cold rolling are generated from the finely sized structure obtained by the sizing treatment of ferrite grains.
[0009]
In the grain-sizing treatment of the ferrite grains, the cooling stop temperature after hot rolling is set to the same temperature range as the annealing temperature range after cold rolling, and the cooling is stopped for 1 second or longer. By this treatment, the grain boundaries that are inevitably generated due to the transformation due to rapid cooling are recessed, and the portion where the interface energy is high is preferentially stretched, and the crystal grains are sized.
[0010]
Further, the present inventors have specified the cooling start time after hot rolling so that the γ → α transformation is performed before the dislocations introduced by hot rolling are sufficiently recovered. It has been found that refinement is further promoted while maintaining the sizing.
[0011]
The present invention has been made based on the above findings and further studies.
[0012]
1. % By mass, C ≦ 0.05%, Si ≦ 0.1%, Mn ≦ 0.5%, S ≦ 0.03%, P ≦ 0.03%, 0.04 ≦ Al ≦ 0.1%, The steel composed of N ≦ 0.004%, the balance Fe and inevitable impurities is subjected to hot rolling finish rolling, the finish rolling temperature (FT) is set to Ar 3 or more and 930 ° C. or less, and after finish rolling, the following formula (1) After the elapse of a time equal to or less than the calculated value t, cooling is started, and cooling is performed at a cooling rate of 120 ° C./second or more to 800 to 680 ° C. , Pickling, cold rolling, annealing at 800 to 680 ° C., a method for producing a highly workable soft cold-rolled steel sheet.
t ≦ 5.77−0.006 × FT (1)
Where t: waiting time for cooling (seconds), FT: finishing rolling temperature (° C.)
[0013]
2. % By mass, C ≦ 0.05%, Si ≦ 0.1%, Mn ≦ 0.5%, S ≦ 0.03%, P ≦ 0.03%, Al ≦ 0.1%, N ≦ 0. 004%, B ≦ 0.0035%, further satisfying −0.0015% ≦ B-11 / 14N ≦ 0.0010%, and the steel comprising the balance Fe and inevitable impurities is hot-rolled with Ar3 or more. Then, immediately cooled to 800 to 680 ° C. at a cooling rate of 120 ° C./second or more, allowed to cool for 1 second or more, wound up at 600 to 660 ° C., then pickled, cold rolled, 800 to 680 ° C. A method for producing a high workability soft cold-rolled steel sheet, characterized by annealing at a temperature.
[0015]
3. The finish rolling temperature (FT) is set to Ar3 or more and 920 ° C or less in finish rolling, and then cooling is started after the elapse of time equal to or less than a value t calculated by the following equation (2). A method for producing a highly workable soft cold-rolled steel sheet.
[0016]
t ≦ 5.77−0.006 × FT + 250 × B (2)
Where t: waiting time for cooling (seconds), FT: finish rolling temperature (° C.), B: B content (mass%)
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The reason for limitation of the component of this invention is demonstrated.
[0018]
C: C precipitates carbides and reduces ductility and aging resistance, so 0.05% or less.
[0019]
Si: If Si is added excessively, the strength is increased and the formability is deteriorated.
[0020]
Mn: Mn fixes S as MnS and improves hot ductility, so it is desirable to add 0.05% or more. However, excessive addition hardens the steel and deteriorates formability, so it is made 0.5% or less.
[0021]
P: P is a solid solution strengthening element, and if contained excessively, the steel is hardened, so the content is made 0.03% or less.
[0022]
S: S is a harmful element that hinders hot ductility and formability, and is fixed as MnS. However, when the amount of MnS increases, stretch flangeability decreases, so the content is made 0.03% or less.
[0023]
Al: Al is added as a deoxidizer. If it exceeds 0.1%, the effect is saturated, so 0.1% or less. When B is not added, N is fixed to be 0.04% or more. When B is added, N is fixed at B, so no lower limit is set. Moreover, when suppressing the amount of O in steel low, adding 0.010% or more is desirable.
[0024]
N: 0.004% or less N in a solid solution state inhibits recrystallization, lowers the r value, and degrades aging resistance. Further, if a large amount of nitride is present in the steel, the ductility is lowered, so 0.004% or less.
[0025]
With the above basic components, sufficient characteristics can be obtained in the present invention, but B can be added to further improve characteristics such as material stability.
[0026]
B: 0.0035% or less B is combined with N faster than Al, so it is added particularly when coil end properties are improved. During hot rolling, all of the additive B is dissolved, and if the amount of B is large, the rolling load during hot rolling increases and rolling cannot be performed stably, so the content is made 0.003% or less.
[0027]
-0.0015% ≦ B-11 / 14N ≦ 0.0010%
This parameter prevents B from being excessively added to N, and prevents the steel from becoming hard and low ductile due to solute B, or because the amount of B added is small, the effect of B is not fully exhibited. It is to prevent. The addition amount of B is regulated with respect to N so that this parameter is −0.0015% or more and 0.0010% or less.
[0028]
Next, the manufacturing conditions of the present invention will be described.
[0029]
Finishing temperature: Ar3 or higher When the finishing temperature is lower than Ar3, coarse grains are generated and it is difficult to refine the hot rolled structure. On the other hand, in the case of a composition not containing B, since the recrystallization of austenite occurs in a short time when the finishing temperature exceeds 930 ° C., there is a possibility that austenite recrystallizes before the start of cooling. desirable. Further, when B is added, the rolling strain is likely to accumulate, so that recrystallization is likely to occur in high temperature finishing, and therefore the finishing temperature is preferably 920 ° C. or lower.
[0030]
Cooling start time In the present invention, before the dislocations introduced by rolling recover, cooling is started and the hot rolled sheet is made into fine grains by utilizing the effect of strain accumulation to the maximum. In order to maximize the effects of the present invention, when B is not contained, cooling is started so as to satisfy the following formula (2) when B is contained.
[0031]
t ≦ 5.77−0.006 × FT (1)
Where t: waiting time for cooling (seconds), FT: finishing rolling temperature (° C.)
t ≦ 5.77−0.006 × FT + 250 × B (2)
Where t: waiting time for cooling (seconds), FT: finish rolling temperature (° C.), B: B content (mass%)
FIG. 1 shows the result of arranging r values of cold-rolled steel sheets by finishing temperature and cooling start time. C: about 0.015%, Si: about 0.01%, Mn: about 0.13%, S: about 0.005%, P: about 0.005%, Al: about 0.04%, N: The steel containing about 0.003% and B: about 0.002% is hot-rolled by changing the finishing temperature and the cooling start time, cooled to 750 ° C. at 200 ° C./second, and then allowed to cool for 2 seconds. The winding process was performed at 650 degreeC. Then, pickling, cold rolling, 750 ° C. annealing, and 1% temper rolling were performed to produce a 0.8 mmt cold rolled steel sheet.
[0032]
In the figure, ◯ indicates an experimental result where the finishing temperature is Ar3 or higher and 920 ° C or lower, and the cooling start time is equal to or less than the value calculated by Equation (2). The r value is an excellent value of 1.55 or higher. It has become. The Δ mark indicates the experimental result when the finishing temperature is Ar3 or higher and 930 ° C or lower, but the cooling start time exceeds the value calculated by the formula (2). The r value is 1.4 or higher and 1.55. Although it is less than the value of ○ mark, it is a good value. A cross indicates that the finishing temperature is less than Ar3, and the r value is less than 1.4. The cooling start time can also be adjusted by changing the rolling speed or the distance between the cooling bank and the final finishing stand.
[0033]
Cooling rate: 120 ° C./second or more If the cooling rate is slow, a supercooling temperature cannot be obtained during the γ → α transformation and it becomes difficult to refine the hot rolled structure. The faster the cooling rate, the better and the upper limit is the physical limit when cooling with cooling water. In conventional hot-rolled steel sheet production, the cooling rate is at most 70 ° C / second, and the boiling form of cooling water is a mixture of film boiling and nucleate boiling. It is necessary to. In this case, the cooling speed varies depending on the plate thickness, but 120 ° C./second can be secured if the plate thickness is usually made of a hot rolled material. The cooling method of the present invention is not particularly limited as long as the cooling water can uniformly nucleate on the plate surface.
[0034]
Cooling stop temperature: 680-800 ° C
The cooling stop temperature is very important in the present invention. When cooled to the coiling temperature at a high cooling rate, the ferrite grains are wavy, the grain boundary is uneven, and the r value is low. In order to allow only the grain boundary recesses with high interfacial energy to protrude and to regulate the ferrite grains, the temperature at which slight grain growth occurs is the normal annealing temperature after cold rolling of low-carbon steel in the present invention. 680 degreeC or more and 800 degrees C or less are made into cooling stop temperature. The cooling stop temperature of 680 to 800 ° C. is a temperature at which AlN and BN are likely to precipitate, and the adverse effect of N is reduced.
[0035]
FIG. 2 shows the relationship between the r value of the cold-rolled steel sheet and the cooling stop temperature when quenched at 150 ° C./second after hot rolling. C: about 0.02%, Si: about 0.01%, Mn: about 0.2%, S: about 0.01%, P: about 0.01%, Al: about 0.04%, N: Steel containing about 0.003% and B: about 0.002% is hot-rolled, cooled at 150 ° C./second, then allowed to cool for 3 seconds, then restarted, and wound at 620 ° C. did. Furthermore, pickling, cold rolling, 750 ° C. annealing, and 1% temper rolling were performed to produce a 0.8 mmt cold rolled steel sheet.
[0036]
When the cooling stop temperature is 680 to 800 ° C., the r value is the best. When the cooling stop temperature is less than 680 ° C., the crystal grains are not sized and the r value is lowered. On the other hand, if it exceeds 800 ° C., the grain growth property is good, the crystal grains become coarse, the effect of rapid cooling is lost, and the r value decreases. In addition, the figure also shows the r value of the cold-rolled steel sheet when the cooling rate after hot rolling, which is a normal manufacturing method, is 30 ° C./second.
[0037]
Cooling time: After stopping cooling for 1 second or longer, in order to make the grain boundary recesses protrude and adjust the particle size, it is allowed to cool for at least 1 second before winding. The cooling time is preferably within 30 seconds in order to prevent coarse particles. After cooling, when cooling to the coiling temperature, the cooling rate is not specified.
[0038]
When B is not added to the coiling temperature steel, the coiling temperature is set to 660 ° C. or higher in order to completely fix N with AlN. When B is added to the steel, the precipitation rate of BN is faster than that of AlN, and the precipitation is completed immediately after winding, so there is no need for high-temperature winding, and the temperature is 660 ° C. or lower.
[0039]
There is no problem even if the hot rolling according to the present invention is continuous hot rolling in which rough bars are joined and finish rolling is continuously performed after rough rolling. After rough rolling, there is no problem even if the rough bar is heated or wound around a coil box for the purpose of temperature control. You may combine heating of a rough bar and continuous rolling. The continuous cast slab may be rolled as it is, or may be supplemented or heated within 100 minutes for the purpose of slab soaking without cooling to room temperature. Furthermore, even if rough rolling is omitted using a thin slab, the effect of the present invention does not change.
[0040]
The cold rolling after pickling is preferably performed at a rolling rate of 30% to 90% in view of workability, particularly deep drawability. Although there is no restriction | limiting about the conditions of temper rolling, 2% or less is desirable from the fall of El being remarkable when it exceeds 2%. Both the converter and the electric furnace can be used for adjusting the components of the steel of the present invention. Scrap may be used as a raw material, and there is no restriction on impurities to be mixed. Even if the steel plate of the present invention is subjected to chemical conversion treatment such as galvanization, tin plating, chromate, zinc phosphate, etc., the effect of the present invention is not affected.
[0041]
【Example】
Examples of the present invention will be described in detail.
[0042]
[Example 1]
Hot-rolling was performed after melting and casting the steels having the components shown in Table 1. Cooling started 1 second after hot rolling. The obtained hot-rolled sheet was pickled and cold-rolled and then annealed at 760 ° C. to produce an annealed sheet having a thickness of 0.8 mm. The temper rolling was performed on the annealed sheet at an elongation rate of 1.0%, and then a tensile test and an r value were measured.
[0043]
Steel No. 1 to 8 are components to which B is not added. The components after 9 are components to which B is added. Steel No. Nos. 1 to 5 are examples of the present invention. The r value is improved as compared with the conventional example 8 (the cooling rate of cooling after hot rolling is slow).
[0044]
Steel No. In No. 6, the cooling rate was slow, the particle size of the hot-rolled sheet was increased, and the r value was not improved. Steel No. No. 7 had a low cooling stop temperature, the hot-rolled plate had an uneven grain shape, and the r value did not improve. Steel No. Nos. 9 to 13 are examples of steel No. The r value is improved compared to 16 conventional examples (no water cooling after hot rolling).
[0045]
Steel No. No. 14 had a high cooling stop temperature, the grain size of the hot-rolled sheet became coarse, and no improvement in the r value was observed. Steel No. No. 15 had a low cooling stop temperature, the hot-rolled plate had an uneven grain shape, and the r value did not improve. In addition, the conventional example is a case where the cooling rate is 50 ° C./S or less after hot rolling and water cooling mainly of film boiling, and the comparative example is a case where any of the steps of the present invention is performed under conditions other than the regulations of the present invention. Examples are shown.
[0046]
[Table 1]

[0047]
[Example 2]
Hot rolling was performed after melting and casting the steels having the components shown in Table 2. The obtained hot-rolled sheet was pickled and cold-rolled, and then annealed at 780 ° C. to produce an annealed sheet having a thickness of 1.0 mm. The temper rolling was performed on the annealed sheet at an elongation rate of 1.0%, and then a tensile test and an r value were measured. Steel No. 1-No. 7 is a component to which B is not added. From 8 onwards, B is added.
[0048]
Steel No. 1-No. No. 6 is an example in which cooling was started within a time that satisfies the formula (1) after rolling, and after the upper limit of the cooling start time according to the formula (1) had elapsed, the steel No. 6 that started cooling was used. Compared to 7, the r value is improved. Steel No. Nos. 8 to 13 are examples in which the cooling was started within the time satisfying the formula (2) after rolling, and after the cooling start time according to the formula (2) had elapsed, the steel No. Compared to 14, the r value is improved.
[0049]
[Table 2]

[0050]
【The invention's effect】
The present invention adjusts the cooling conditions after hot rolling of the low carbon steel so that the structure of the hot-rolled sheet becomes a fine grained structure, and generates a large number of recrystallization nuclei during annealing after cold rolling. Since the r value is improved without depending only on the grain growth, a cold-rolled steel sheet suitable for automobiles, home appliances and the like can be manufactured, which is excellent in workability and surface properties after processing, and is extremely effective in industry.
[Brief description of the drawings]
FIG. 1 is a diagram showing the effect of cooling start time and finishing temperature on r value. FIG. 2 is a diagram showing the effect of cooling stop temperature on r value.

Claims (3)

% By mass, C ≦ 0.05%, Si ≦ 0.1%, Mn ≦ 0.5%, S ≦ 0.03%, P ≦ 0.03%, 0.04 ≦ Al ≦ 0.1%, The steel composed of N ≦ 0.004%, the balance Fe and inevitable impurities is subjected to hot rolling finish rolling, the finish rolling temperature (FT) is set to Ar 3 or more and 930 ° C. or less, and after finish rolling, the following formula (1) After the elapse of a time equal to or less than the calculated value t, cooling is started, and cooling is performed at a cooling rate of 120 ° C./second or more to 800 to 680 ° C. , Pickling, cold rolling, annealing at 800 to 680 ° C., a method for producing a highly workable soft cold-rolled steel sheet.
t ≦ 5.77−0.006 × FT (1)
Where t: waiting time for cooling (seconds), FT: finishing rolling temperature (° C.) % By mass, C ≦ 0.05%, Si ≦ 0.1%, Mn ≦ 0.5%, S ≦ 0.03%, P ≦ 0.03%, Al ≦ 0.1%, N ≦ 0. 004%, B ≦ 0.0035%, further satisfying −0.0015% ≦ B-11 / 14N ≦ 0.0010%, and the steel comprising the balance Fe and inevitable impurities is hot-rolled with Ar3 or more. Then, immediately cooled to 800 to 680 ° C. at a cooling rate of 120 ° C./second or more, allowed to cool for 1 second or more, wound up at 600 to 660 ° C., then pickled, cold rolled, 800 to 680 ° C. A method for producing a high workability soft cold-rolled steel sheet, characterized by annealing at a temperature. The hot rolling finish rolling is characterized in that the finishing rolling temperature (FT) is set to Ar3 or more and 920 ° C or less, and then cooling is started after the elapse of a time t or less calculated by the following equation (2). The manufacturing method of the high workability soft cold-rolled steel plate of Claim 2 .
t ≦ 5.77−0.006 × FT + 250 × B (2)
Where t: waiting time for cooling (seconds), FT: finish rolling temperature (° C.), B: B content (mass%)

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