JP3435986B2 - Manufacturing method of high workability plated steel sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plated steel sheet for use in building materials, steel sheets for cooling / heating / hot water supply equipment, steel sheets for automobiles, etc., which require high corrosion resistance and drawability. And a method for producing the same. [0002] In the production of coated steel sheets, after hot-rolling a material for coated steel sheets, an iron oxide layer covering the surface of the steel sheets is removed by an pickling facility, cold-rolled, and then continuously melted. Generally, plating is performed by a plating apparatus, an electroplating apparatus, or the like. Here, the hot-rolled material cannot be recrystallized by accumulating working strain, and the drawability suitable for the above-mentioned use cannot be obtained. Therefore, it is necessary to make a plated steel sheet excellent in workability such as elongation and drawing characteristics. Further, the removal of the iron oxide layer is essential because the iron oxide layer inhibits plating and serves as a starting point of peeling of the plating layer, thereby deteriorating the adhesion of the plating. [0003] Japanese Patent Application Laid-Open Nos. 6-145937 and 6-279967 disclose the above-mentioned general production of plated steel sheets, in which the pickling step and the cold rolling step are omitted mainly for the purpose of cost reduction. A technique for performing this is disclosed. That is, it has been proposed to perform a hot dip galvanizing after performing a reduction treatment in a reducing atmosphere gas without removing an iron oxide layer on a hot-rolled sheet surface. [0004] However, the above-mentioned publication does not mention the deterioration of workability which is a concern due to the omission of cold rolling, and the damage (peeling) of plating caused by deformation such as bending is not described. Not resolved. Japanese Patent Application Laid-Open No. 6-145937 does not mention the adhesion of plating. On the other hand,
In Japanese Patent No. 279967, although the plating adhesion is improved by using a hot-rolled steel sheet having a relatively thin iron oxide layer of 1.1 to 4.6 μm, there is no mention of any means for obtaining the thin iron oxide layer. Not been. Furthermore, when using a steel type in which easily oxidizable components such as Si and Mn are present in the steel in order to increase the strength of the steel plate, these components are oxidized during annealing before plating and are concentrated on the steel plate surface, and In the treatment, there is also a problem that the reaction between the steel sheet and the molten metal is hindered, which eventually causes non-plating defects. Accordingly, the present invention relates to a method for producing a plated steel sheet excellent in workability and plating adhesion at a low cost even if cold rolling and pickling are omitted. The purpose is to propose. Means for Solving the Problems The inventors have eagerly studied the temperature and working conditions of hot rolling, the descaling conditions after rough rolling, the thickness of the iron oxide layer of the hot-rolled steel sheet, and the material of the steel sheet after annealing. The steel plate on which the iron oxide layer was formed was reduced under various conditions and then plated, and the plating characteristics were investigated. As a result, by incorporating processing strain into the hot-rolled sheet, it is possible to prevent deterioration in workability even if cold rolling is omitted, and to obtain a hot-rolled steel sheet with a sufficiently thin iron oxide layer. It has been found that the adhesion of the plating can be ensured without removing the iron layer. That is, the present invention relates to the following: C: 0.02 wt% or less, Si: 2 wt% or less, Mn: 3 wt% or less, Ti: 0.01 to 0.2
wt%, Nb: 0.001 to 0.2 wt%, B: 0.0001 to 0.003 wt
%, N: 0.01 wt% or less, P: 0.2 wt% or less, and S: 0.
A steel slab containing not more than 05 wt% and having a composition of each of C, Ti, Nb and N [C], [Ti], [Nb] and [N] that satisfies the following formula (1): Heat to a temperature range of ₃ transformation points or higher, perform hot rough rolling, and then discharge pressure is 300 kg / cm
After performing a descaling process with ^two or more high-pressure waters,
Hot finish rolling with a rolling reduction of 60% or more and a finishing temperature of 800 ° C or less in the temperature range of 500 ° C or more and the Ar ₃ transformation point or less is performed under lubrication. After winding, it is heated in an annealing furnace of continuous plating equipment. A method for producing a highly workable plated steel sheet, comprising reducing the iron oxide layer on the steel sheet surface by 50% or more at a steel sheet temperature of 750 ° C or higher and below the Acs transformation point, and then performing plating. Note that [C] / 12 + [N] / 14 ≦ [Ti] / 48 + [Nb] / 93--(1). Here, the Acs transformation point changes from α phase to γ
It is the temperature at which transformation to a phase begins. [0008] Next, the procedure of the present invention will be described specifically. First, C: 0.
02 wt% or less, Si: 2 wt% or less, Mn: 3 wt% or less, Ti: 0.
01-0.2 wt%, Nb: 0.001-0.2 wt%, B: 0.0001-0.
003 wt%, N: 0.01 wt% or less, P: 0.2 wt% or less, S: 0.05 wt% or less, and the contents of C, Ti, Nb and N [C], [Ti], [Nb] And [N] are the above formulas
Use a steel slab with a component composition that satisfies (1). That is, C and N are interstitial solid-solution elements and are effective in increasing the strength of a steel sheet. On the other hand, C and N reduce workability typified by elongation and r value. Then, by strengthening with components other than C and N, C and N are suppressed to 0.02 wt% or less and 0.01 wt% or less, respectively, in the steelmaking stage. Further, in order to render the adverse effects of the remaining C and N on workability harmless, Ti and Nb are added to cause C and N to react and precipitate, thereby ensuring excellent workability. Specifically, Ti: 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 wt%
It is important to add% to the content of C and N under the above formula (1). This is because Ti is highly reactive, especially compared to Nb, and is selectively consumed for the precipitation of N and C, and is easily oxidized and consumed also by oxygen in steel. If it is less than 30, the effect of addition does not appear. On the other hand, if it is added in excess of 0.2 wt%, the effect will be saturated and the cost will increase. Nb exhibits an effect from a small amount of addition because it has a lower reactivity with elements other than C as compared with Ti.
If the amount is less than 1 wt%, the number of atoms as the number of atoms is too small as compared with the amounts of C and N, so that no effect is exhibited. On the other hand, if the content exceeds 0.2 wt%, the effect is saturated and the cost increases. Further, by adding Ti and Nb in a range that satisfies the above formula (1) in accordance with the amounts of C and N, it is possible to secure sufficient amounts to precipitate C and N. Further, Si and Mn are components that can increase the strength of a steel sheet without impairing workability, and 2 wt% and 3 wt% can be added to the upper limit, respectively. If the upper limit is exceeded, cracks tend to be formed in the steel sheet edge during hot working, and scales are generated abnormally, so that a clean steel sheet surface cannot be obtained. Note that the lower limits of Si and Mn are not particularly limited. Adjusted for the required strength. However, in order to avoid an increase in steel cost, the content is preferably 0.001% and 0.01% by weight, respectively. If B is less than 0.0001 wt%, improvement in brittleness due to grain boundary strengthening of steel cannot be expected. On the other hand, when the content exceeds 0.003 wt%, the brittleness improving effect is saturated and the workability of the steel is rapidly deteriorated. [0015] P is relatively inexpensive, can maintain high workability, and can increase the strength. However, in steel in which C and N are reduced, P precipitates at grain boundaries and causes embrittlement. %
Is the upper limit. The lower limit is not particularly limited, but is preferably 0.001 wt% or more from the viewpoint of not increasing the steelmaking cost. Further, by coexisting P with B, it becomes possible to measure high strength while maintaining high workability without embrittlement of the steel sheet at a relatively small amount of addition and at low cost. Next, in the hot rolling step, prior to the rough rolling, the steel slab adjusted to the above-mentioned components is heated to the Ac ₃ transformation point or higher. Specifically, the slab is heated to about 1200 ° C. to reduce the deformation resistance of the steel sheet in the subsequent rough rolling. The slab cast by continuous casting or the like may be directly subjected to a rough rolling process before cooling without heating the slab again. Thereafter, after rough rolling, descaling with high-pressure water is performed prior to finish rolling. As a result, the scale generated before the rough rolling is removed. Here, the descaling using high-pressure water is used in order to almost completely remove the oxide scale generated and grown up to the rough rolling. The reason why high-pressure water is used is that scale can be efficiently removed without leaving any scratches on the steel sheet surface. The discharge pressure of the high-pressure water needs to be 300 kgf / cm ² or more. This is because, it is less than 300 kgf / cm ^2,
The scale removal is not complete, resulting in finish rolling,
This is because the scale on the hot-rolled steel sheet after winding becomes thick and non-uniform. Therefore, by reducing the thickness of the oxidized scale by performing descaling with high-pressure water, the surface of the hot-rolled steel sheet can be made beautiful. By reducing in a heating furnace of a hot-dip galvanizing apparatus, a beautiful plated steel sheet with good plating adhesion can be manufactured. On the other hand, from hot-rolled steel sheets with a thick oxide scale and poor sheet surface properties manufactured without using high-pressure water descaling, produce beautiful plated steel sheets with good plating adhesion without descaling by pickling. It is difficult. In the next finish rolling step, rolling at a rolling reduction of 60% or more below the Ar ₃ transformation point is performed under lubrication, and the final finish rolling temperature is 800 ° C. or less. In other words, by setting the rolling reduction under the Ar ₃ transformation point to 60% or more under lubrication, hot-rolled steel sheets after cooling can incorporate processing strain with low shear stress, and then can be reduced in a reduction furnace. At the time of recrystallization, a structure advantageous for imparting high workability appears. Therefore, workability can be ensured without performing cold rolling. The reason why the final finishing temperature is set to 800 ° C. or less is to suppress the growth of the iron oxide layer immediately after hot rolling. Specifically, the thickness of the iron oxide layer in the hot-rolled sheet is set at 4 ° C.
It can be suppressed to about μm or less. After the hot-rolled steel sheet is wound up, it is reduced at the same time as recrystallization annealing in an annealing furnace of a continuous hot-dip coating apparatus, and then plated. That is, in the continuous hot-dip coating apparatus, the iron oxide layer is reduced in the annealing furnace, and recrystallization is generated in the steel sheet. Here, in order to carry out both reactions quickly, it is important to adjust the temperature of the steel sheet to 750 ° C. or higher.
s If the transformation point is exceeded, the structure is randomized and a structure advantageous for workability cannot be obtained. The reduction of the iron oxide layer must be carried out at least 50%, preferably less than 100%. If the reduction ratio is less than 50%, the iron oxide layer remains largely, and peels off when subjected to impact or processing, which is not practical. On the other hand, when the content becomes 100%, the occlusion of hydrogen atoms starts, and if this increases, hydrogen is discharged from the steel after plating and loses its place to go, and vaporizes at the plating interface and progresses to local plating exfoliation. In particular, when Si or Mn is contained at a high concentration, 100% reduction causes surface concentration due to oxidation of Si or Mn, which impairs wettability in the subsequent plating process and induces non-plating defects. Will do. The atmosphere is a general reducing gas of 3
Although N ₂ containing at least H ₂ % can be used, the H ₂ concentration is preferably at least 7% for efficient reduction. The steel sheet which has been subjected to the predetermined reduction and recrystallization annealing is cooled to a plating bath temperature and then introduced into a plating bath for plating. For example, as a zinc-based plating bath, in addition to Zn and Fe, for the purpose of improving various performances,
It is possible to contain Al, Mg, Mn, Ni, Co, Cr, Si, Pb, Sb, Bi, Sn and the like singly or in combination. Finally, the steel plate plated by immersion is
After adjusting to the required basis weight of ²⁰ to 250 g / m ² by gas wiping or the like, the product is cooled by air cooling, air cooling, or water cooling, and then subjected to leveler or temper rolling as necessary to produce the product. . Further, in order to improve the corrosion resistance and the like, it is possible to perform a chromate treatment, a phosphate treatment or the like after cooling or temper rolling, and it is also effective to further paint. Similarly, a lubrication treatment can be performed as a post-treatment. On the other hand, in a use of assembling a steel plate by spot resistance welding or the like, plating is performed in a molten Zn bath containing 0.1 to 0.2 wt% of Al, and after adjusting the basis weight, heat alloying is performed. It is valid. The basis weight here is 20g
/ M corrosion is insufficient less than ^3, since the plating layer is easily peeled off during working of the bending and squeezing more than 80 g / m ^3,
It is preferably in the range of 20 to 80 g / m ³ . Similarly, the Fe content in the plating is 7 to 12 wt%. Because 7wt
%, The unalloyed pure Zn layer remains on the plating surface, impairing spot resistance weldability, and the pure Zn layer is easily eluted from flaws after painting. Is rapidly embrittled, and peeling of plating becomes remarkable during processing. Although the description has been given mainly of the hot-dip galvanized steel sheet, the present invention is not limited to the hot-dip galvanized steel sheet, but is similarly applicable to other hot-dip galvanized steel sheets or electroplated steel sheets. For example, 55% Al-Zn plating, Al plating, Pb
Suitable for plated steel plates such as plating, Sn plating or Zn-Ni plating. In any case, it is sufficient to perform plating after a reduction treatment of 50% or more and less than 100%, and a steel sheet having excellent plating characteristics can be obtained without being restricted by a plating type. In a hot-dip galvanizing line, it is usual to arrange a plating tank continuously to an annealing furnace, so that it is particularly suitable for the present invention. EXAMPLE A slab having a steel composition shown in Table 1 was heated to 1200 ° C., and then hot-rolled according to Table 2 to obtain a hot-rolled sheet having a thickness of 0.8 mm. In finish rolling, lubrication with mineral oil was performed.
For comparison, according to the conventional method, pickling was performed after hot rolling and then cold rolling was performed under the conditions shown in Table 3 to produce a cold rolled sheet. Next, the hot-rolled sheet was cut into a test piece of 60 × 200 mm, washed with acetone, subjected to reduction treatment and recrystallization annealing with a vertical molten metal plating simulator, and then zinc-based plating was performed. Table 2 shows the hot rolling and annealing conditions and the iron oxide layer thickness of the hot-rolled sheet, and Table 4 shows the plating conditions. With respect to the plated steel sheet thus obtained, the reduction ratio of the iron oxide layer was measured, and the mechanical properties and plating adhesion were evaluated. Table 2 shows the reduction ratio and mechanical properties of the iron oxide layer, and Table 4 shows the evaluation results of the plating adhesion. Here, the reduction rate of the iron oxide layer was determined in advance by separately calculating the amount of iron oxide dissolved and removed by pickling, calculating the amount of reduced iron oxide from the weight reduced by reduction annealing, and determining the ratio thereof. The plating adhesion was evaluated by a ball impact test and a 180 ° outer bending test. That is, in the ball impact test, a hammer having a hemispherical convex surface having a diameter of 1/2 inch was applied to the back side of the test surface, and a hemispherical concave saucer was placed on the test surface side, and a 2 kg weight was applied to a 70 cm. The cell was dropped from the height of the test piece, the hitting core was hit, the cellophane adhesive tape was attached to the protruding test surface, and then peeled off, and the surface of the plated steel sheet was observed. In the 180-degree outer bending test, a vinyl adhesive tape was applied to the test surface in advance, a 0.9 mm steel plate was put in the spacer, and the test surface was bent 180 degrees with the hydraulic press to the outside, and then flattened again. After bending back, the vinyl tape was peeled off, and the surface of the plated steel sheet was observed. [Table 1] [Table 2] [Table 3] [Table 4] The alloyed hot-dip Zn plating was evaluated in the same manner. That is, a test piece similar to the above was prepared using a slab having the steel composition shown in Table 1. Here, the hot rolling conditions and the annealing conditions before plating are as shown in Table 5, the pickling and cold rolling conditions according to the conventional method are as shown in Table 6, and the alloyed zinc plating conditions are as shown in Table 7. . The mechanical properties and plating adhesion of the plated steel sheet thus obtained were evaluated. The results of each evaluation are also shown in Tables 5 to 7, respectively. The plating adhesion was measured by a 90-degree inner bending test and a 180-degree outer bending test. That is, a vinyl adhesive tape is applied to the test surface in advance, and in the 90-degree internal bending test,
After bending at 90 degrees with the test surface inside along a 1 mm radius die, and then bending it back to a flat state again, in the 180 degree outer bending test, put a 0.9 mm steel plate in the spacer and use a hydraulic press. After bending by 180 degrees with the test surface outside, the plate was bent back to a flat state again, the vinyl tape was peeled off, and the surface of the plated steel sheet was observed. [Table 5] [Table 6] [Table 7] According to the present invention, a plated steel sheet obtained by plating without removing the iron oxide layer can provide both excellent workability and plating adhesion.
A plated steel sheet can be provided at low cost.

──────────────────────────────────────────────────続 き Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/00 301 C22C 38/00 301T 38/14 38/14 C23C 2/40 C23C 2/40 (56) References 6-212385 (JP, A) JP-A-5-98354 (JP, A) JP-A-4-127912 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/02 -2/40 B21B 3/00 C21D 1/76 C21D 8/02 C21D 9/46 C22C 38/00 301 C22C 38/14

Claims (1)

(57) [Claims] [Claim 1] C: 0.02% by weight or less, Si: 2% by weight or less, M
n: 3 wt% or less, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2
wt%, B: 0.0001 to 0.003 wt%, N: 0.01 wt% or less,
P: 0.2 wt% or less and S: 0.05 wt% or less, and the respective contents of C, Ti, Nb and N [C], [Ti], [Nb]
And a steel slab having a component composition in which [N] satisfies the following formula is subjected to hot rough rolling by heating to a temperature range not lower than the Ac ₃ transformation point, and then depressurized by high-pressure water having a discharge pressure of 300 kg / cm ² or more. After performing the scaling process, the rolling reduction in the temperature range between 500 ° C and the Ar ₃ transformation point is 60% or more and the end temperature is 800 ° C.
The following hot finish rolling is performed under lubrication, and after winding, after reducing 50% or more of the iron oxide layer on the steel sheet surface at a steel sheet temperature of 750 ° C or higher and the Acs transformation point or lower in an annealing furnace of a continuous plating facility. A method for producing a highly workable plated steel sheet, characterized by applying plating. [C] / 12 + [N] / 14 ≦ [Ti] / 48 + [Nb] / 93