JP3616472B2 - Method for producing high-strength hot-dip galvanized steel sheet with excellent workability - Google Patents

Description

【００１９】
表２に製造条件と機械的性質を示す。表中Ｎｏ１〜４、及びＮｏ１１が本発明例である。Ｎｏ５はＦＴが規定から外れている比較例であり引張強さが低い。Ｎｏ６は巻取温度が規定から外れている比較例で、Ｌ方向とＣ方向の全伸びの比である、ＲＥＬ＝（Ｃ方向の全伸び）／（Ｌ方向の全伸び）が小さく、異方性が大きい。Ｎｏ７、及び８は冷間圧延時の冷延率が規定から外れている比較例であり、Ｎｏ７は引張強さが小さく、Ｎｏ８はＲＥＬが不足している。また、Ｎｏ９、及び１０は溶融亜鉛めっきライン中での焼鈍温度が規定から外れている比較例であり、Ｎｏ９は引張強さが不足し、Ｎｏ１０では逆に引張強さが高過ぎ、またＲＥＬも不足している。また、Ｎｏ１２、及び１３は本発明鋼の成分規定から外れている比較例であり、ともに引張強さとＲＥＬが本発明の特許請求範囲から外れている。
【００２０】
【表２】

【００２１】
【発明の効果】
以上のように、本発明により得られる高強度溶融亜鉛めっき鋼板は、引張強さが４００〜５９０ＭＰａで、かつＣ、Ｌ方向の全伸びの異方性が小さいため、多くの分野に適用可能であり、しかも合金コストを抑制し、かつ製造コストも安価なため、経済性にも非常に優れており、産業上に与える効果は極めて大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and economy.
[0002]
[Prior art]
In recent years, high strength of steel has been demanded in various fields such as automobiles, various equipment members, and building materials from the viewpoint of economy. Many of them are molded products with a low degree of processing such as bending, mild overhang, and hole expansion, and do not require large ductility like recrystallized cold-rolled steel sheets. In such applications, due to the limitations of press equipment, molds, etc., high strength is not always sought. Tensile strength is about 400 to 590 MPa, (total elongation in the C direction) / (L There is a strong demand for a highly economical and stable steel plate with a total elongation in the direction) = REL> 0.7 or more.
[0003]
When manufacturing a high-strength steel sheet, generally, a strengthening method using precipitation, transformation, dislocation, or the like is used. For example, in Japanese Patent Publication No. 53-9167 and Japanese Patent Publication No. 54-41985, P is added to steel and precipitated during annealing after cold rolling to obtain high strength. Further, in the strengthening method by transformation to bainite, martensite or the like, a steel sheet having good elongation can be obtained, but a rapid cooling process is entered during production, and a large amount of alloying elements are required.
[0004]
The strengthening method by dislocation is a method to increase the strength by not completing recrystallization in the annealing process after cold rolling, compared with the strengthening method by precipitation and transformation that requires various alloy elements. However, the anisotropy between the elongation in the rolling direction (L direction) and the total elongation in the direction perpendicular to the rolling direction (C direction) is generally poor. As an improvement method thereof, a method characterized by performing extremely low temperature winding at 400 ° C. or lower as disclosed in Japanese Patent Laid-Open No. 56-119731, or during cold rolling after hot rolling as disclosed in Japanese Patent Publication No. 46-26217. A method of annealing after rolling at a light cold rolling rate has been known.
[0005]
[Problems to be solved by the invention]
The manufacturing method of the above-mentioned Japanese Patent Application Laid-Open No. 56-119731 is a steel plate in which carbon is dissolved in a solid carbide and fine carbide state by cold rolling after hot rolling to suppress voids during cold rolling. Is intended to improve the elongation anisotropy of the steel sheet, but is a method of manufacturing a steel sheet having a tensile strength of 600 MPa or more substantially, and is usually performed because the coiling temperature after hot rolling is extremely low. Since the recrystallization of the steel sheet is not completed within the hot rolling finishing to winding time, and a large amount of processed structure remains and hardens, the rolling load during the subsequent cold rolling is significantly increased. Therefore, the cold rolling rate cannot be increased, and there are disadvantages that not only the industrial restriction is caused, but the cold rolling power cost is remarkably increased.
[0006]
In the manufacturing method of Japanese Examined Patent Publication No. 46-26217, in order to obtain a cold-rolled sheet having the same thickness, the cold-rolling rate of hot rolling and cold rolling after pickling is as low as 10 to 35%. There is a fatal disadvantage that it is necessary to finish the hot rolling thinly, which causes a significant increase in hot rolling and pickling costs. Therefore, it has been difficult to produce a hot-dip galvanized steel sheet having a tensile strength of 400 to 590 MPa, a small anisotropy of total elongation in the C and L directions, and excellent economic efficiency.
[0007]
[Means for Solving the Problems]
The present inventor has intensively studied to solve such a problem, and has found that the anisotropy of elongation can be improved by controlling the precipitation direction of carbides by limiting the production conditions for specific steel components. . As a result, it was found that a hot-dip galvanized steel sheet having a tensile strength of 400 to 590 MPa and a small anisotropy of total elongation in the C and L directions and excellent in economic efficiency can be produced. That is, the gist of the manufacturing method of the high-strength hot-dip galvanized steel sheet according to the present invention is as follows:
[0008]
(1) By mass ratio, C: 0.041 to 0.1%, Si: 0.005 to 0.1%, Mn: 0.01 to 0.5%, P: 0.002 to 0.1% , S: 0.002-0.05%, Al: 0.005-0.1%, N: 0.001-0.01%, heating a slab having a steel composition consisting of the balance Fe and inevitable impurities , The hot rolling is completed at 785 to 830 ° C., followed by winding at a winding temperature of 500 ° C. to 650 ° C., and after normal pickling, cold rolling is performed at a cold rolling rate of 40 to 80%, Tensile strength obtained by applying hot dip galvanizing after annealing at a temperature of 500 to 630 ° C. in a plating line, and (total elongation in the C direction) / (total elongation in the L direction) = REL A method for producing a hot-dip galvanized steel sheet excellent in workability, characterized by being> 0.7 Yes, and
(2) A method for producing a high-strength hot-dip galvanized steel sheet having excellent workability as described in (1) above, containing B: 0.0002 to 0.003% by mass ratio.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The high-strength hot-dip galvanized steel sheet produced according to the present invention has a tensile strength of 400 to 590 MPa, a small anisotropy of total elongation in the C and L directions, and low cost and excellent workability. The reasons for limiting the components of the present invention will be described below.
C is an element necessary for ensuring high strength. If it is less than 0.041% , high strength cannot be obtained, the decarburization cost increases, and if it is 0.1% or more, the workability deteriorates and the alloy cost increases, so 0.041% to 0.1 % %.
Si is a deoxidizer in the steel smelting process, and is effective as a solid solution strengthening element for strengthening steel. However, it becomes a factor of scale flaws during hot rolling. It also adversely affects the surface quality, and it is economically difficult to make it 0.005% or less, so it was made 0.005% to 0.5%.
[0010]
Mn is a solid solution strengthening element that is effective for strengthening steel and is added to prevent hot brittleness due to S. However, if it is less than 0.01%, it is ineffective. Since it becomes a cause of high, it was made into 0.01%-0.5%.
P is an element that affects the strength and workability, but if it exceeds 0.1%, the brittleness and workability deteriorate significantly, and if it is less than 0.002%, there is no significant influence. 002 to 0.1%.
[0011]
S is inevitably included, but if it is 0.05% or more, the surface deteriorates due to hot embrittlement, and if it is less than 0.002%, the effect is small and the desulfurization cost increases. 0.002 to 0.05%.
Al is added for deoxidation and denitrification, but if it is less than 0.005%, there is no effect. On the other hand, if it exceeds 0.1%, there is a risk of nozzle clogging in the steelmaking process, and the surface properties of the product. Is 0.005 to 0.1%.
[0012]
As for N, since the solid solution N causes stretcher strain and the surface properties deteriorate, it is desirable that N is less, and when it exceeds 0.01%, the effect becomes remarkable. However, it is 0.001 to 0.01% because it is industrially difficult to make it less than 0.001%.
B is an element used for improving secondary work brittleness, but if less than 0.0002%, there is no effect of addition, and if 0.003%, the effect is saturated, so 0.0002% to 0.003%.
[0013]
Next, manufacturing conditions will be described. The present inventor has found that the precipitation anisotropy can be controlled and the anisotropy of elongation can be improved by controlling the slab containing the above-described components to the following production conditions.
The finishing temperature of the hot rolling is important for the present invention, and the hot rolling is finished at 785 ° C to 830 ° C. This meaning lies in precipitation control of precipitated carbides and reduction of cold rolling load. By performing finish rolling in this temperature range, precipitation of carbides precipitated at the grain boundaries from finishing to after winding is increased in the L direction, and precipitation in the C direction can be reduced. This precipitation affects the elongation after cold rolling and annealing described later, and in particular, in the C direction, the effect of improving the elongation due to the reduction of precipitates becomes significant. The structure after cold rolling extends in the L direction, and the elongation in the L direction is not deteriorated.
[0014]
In addition, when rolling in this temperature range, a texture with low deformation resistance develops parallel to the plate surface, so the cold rolling load is reduced and the load on the precipitated carbide is reduced, which has adverse effects such as crushing during rolling. Can be suppressed. Rolling at a temperature higher than 830 ° C. is not preferable because not only this effect is not obtained, but also coarse crystal grains and fine grains are likely to coexist, the plate-through property is deteriorated and the hot-rolled sheet becomes rough. Moreover, if it is less than 720 degreeC, a process structure will remain after hot rolling, and the load of a cold rolling process will become large rapidly.
[0015]
The coiling temperature after hot rolling has two aspects of suppressing precipitated carbides and obtaining a complete recrystallized structure, and it is necessary to achieve both. When the temperature is 650 ° C or higher, cementite after cold rolling annealing is markedly grain boundary precipitated parallel to the rolling surface and deteriorates the total elongation. When the temperature is lower than 500 ° C, the work structure at the time of hot rolling is restored. It remained at 500 to 650 ° C. because it remained without crystallizing, and the plateability during cold rolling deteriorated.
Then, after normal pickling, cold rolling is performed. When the rolling rate of the cold rolling is less than 40%, a predetermined tensile strength cannot be obtained, and when it is 80% or more, the absolute value of elongation becomes small.
[0016]
Next, hot dip galvanization is performed, but if the annealing temperature before plating is 630 ° C. or higher, recrystallization proceeds and a predetermined strength cannot be obtained, and if it is less than 500 ° C., recovery does not proceed sufficiently and the tensile strength is predetermined. Since it was higher and the anisotropy of elongation was large, it was set to the range of 500 ° C to 630 ° C.
As described above, the present invention is based on a completely new viewpoint of reducing the anisotropy of elongation by actively controlling the precipitation direction of the carbide, and simply makes the precipitated carbide into a solid solution state or a fine carbide state. There is a significant difference from the technique described in Japanese Patent Laid-Open No. 56-1197331.
[0017]
【Example】
The steels having chemical components A to F shown in Table 1 are converted into steel in a converter and steel pieces are obtained by continuous casting. Then, the finish rolling exit temperature is 780 to 905 ° C, and the winding temperature is 550 to 670 ° C. After hot rolling to produce a 4.5 mm thick hot rolled coil, it is cold rolled at a cold rolling rate of 35 to 85%, and then 480 to 650 ° C. and an annealing time of 60 seconds in a continuous hot dip galvanizing line. Then, after the annealing, a material survey was performed on the material that was hot-dip galvanized at 470 ° C. The tensile test piece was a JIS No. 5 test piece cut from 0 ° and 90 ° with respect to the rolling direction.
[0018]
[Table 1]

[0019]
Table 2 shows manufacturing conditions and mechanical properties. Table in No1~ 4, and No 11 are the invention examples. No. 5 is a comparative example in which FT is out of regulation, and the tensile strength is low. No. 6 is a comparative example in which the coiling temperature is out of regulation, and the ratio of the total elongation in the L direction and the C direction, REL = (total elongation in the C direction) / (total elongation in the L direction), is small and different. Isotropic. No. 7 and No. 8 are comparative examples in which the cold rolling rate during cold rolling is out of regulation, No. 7 has a low tensile strength, and No. 8 is deficient in REL. In addition, No 9 and 10 are comparative examples in which the annealing temperature in the hot dip galvanizing line is out of regulation, No 9 has insufficient tensile strength, and No 10 has excessively high tensile strength, REL is also lacking. Nos. 12 and 13 are comparative examples that deviate from the component specifications of the steel of the present invention, and both the tensile strength and REL are out of the scope of the claims of the present invention.
[0020]
[Table 2]

[0021]
【The invention's effect】
As described above, the high-strength hot-dip galvanized steel sheet obtained by the present invention has a tensile strength of 400 to 590 MPa and has a small total anisotropy in the C and L directions, and thus can be applied to many fields. In addition, since the alloy cost is suppressed and the manufacturing cost is low, the cost is very excellent and the effect on the industry is extremely large.

Claims (2)

By mass ratio, C: 0.041 to 0.1%, Si: 0.005 to 0.1%, Mn: 0.01 to 0.5%, P: 0.002 to 0.1%, S: 0.002~0.05%, Al: 0.005~0.1%, N: 0.001~0.01%, heating the slab having the steel composition and the balance Fe and unavoidable impurities, 785 ~ Finish the hot rolling at 830 ° C, then wind it at a coiling temperature of 500 ° C to 650 ° C, perform normal pickling, perform cold rolling at a cold rolling rate of 40 to 80%, and further to the hot dip galvanizing line The tensile strength obtained by performing hot dip galvanization after annealing at a temperature of 500 to 630 ° C. is 400 to 590 MPa, and (total elongation in the C direction) / (total elongation in the L direction) = REL> 0. 7. A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability, characterized in that it is 7. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in workability of Claim 1 containing B: 0.0002 to 0.003% by mass ratio.