JP3391194B2 - Method for producing high-strength hot-dip galvanized steel sheet with excellent surface properties - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of cooling a column, a beam, a girder, a truss, a brace, a panel, and the like used for building materials, particularly for a steel house, by roll forming, pressing, or the like. The present invention relates to a technique for producing a high-strength hot-dip galvanized steel sheet having a sheet thickness of 1.6 mm or less and a tensile strength of 400 to 510 MPa, which is used as a material for a light-formed steel section formed between steel sheets. 2. Description of the Related Art In recent years, for building materials used for members such as columns, beams, girders, trusses, braces, panels and the like of steel houses, tensile strengths of 0.8 to 1.6 mm and a tensile strength of 400 to 1.6 mm are used.
The use of a 510 MPa high-strength hot-dip galvanized steel sheet has begun to be studied. Conventionally, when a steel sheet having a thickness of 1.6 mm or less is manufactured, the slab is temporarily made to have an intermediate thickness by hot rolling, the surface scale is removed by pickling, and then the final thickness is made by cold rolling. , And surface-treated by a continuous hot-dip galvanizing line to obtain a product. However, in recent years, inexpensive imported steel materials from NIES have been continuously entering, and it has become difficult to compete in terms of cost in the conventional process of plating cold-rolled steel sheets. Under these circumstances, in order to manufacture the above-mentioned steel sheet at lower cost, the cold rolling process is omitted, the final target thickness is completed in the hot rolling stage, pickling is performed, and surface treatment is performed on a plating line. Promising to reduce manufacturing costs. Conventionally, a number of techniques have been disclosed for a method of manufacturing a high-strength hot-dip galvanized steel sheet using a hot-rolled steel sheet as a material. For example, Japanese Patent Application Laid-Open No. 05-345953 and
In Japanese Patent Application Laid-Open No. 06-41682, solid-solution strengthening of ultra-low carbon steel having a C content of 0.008% or less is performed by adding Mn and P to the steel.
b, thickness of the sheet aimed at controlling the structure and increasing the strength by Ti.
A method for producing a hot-rolled hot-dip galvanized steel sheet having a thickness of 4 mm or less is disclosed, and JP-A-63-286523 discloses that
Tensile strength is 45-60 kgf / mm ² (441-588MP
A technique relating to a) a high-strength hot-rolled Zn-Al-based hot-rolled steel sheet is disclosed. [0006] However, Japanese Patent Application Laid-Open No. 05-345
No. 953 and Japanese Patent Application Laid-Open No. 06-41682 achieve the omission of the cold rolling process.However, in order to melt such ultra-low carbon steel at the steel making stage, decarburization refining equipment is required. A certain RH
Not only must a process be added, but also expensive alloying elements such as Nb and Ti are added, so that the cost of smelting steel becomes extremely high, so that the merit of omitting the cold rolling process is lost. In the technique disclosed in Japanese Patent Application Laid-Open No. 63-286523, since the hot rolling step is performed at a high temperature of 850 ° C. or more, a large amount of scale is formed on the surface of the hot-rolled coil, and the yield is reduced due to scale loss and acidity is reduced. This leads to a decrease in washing efficiency. In particular, in a hot-rolled steel sheet with a small thickness, the coil length at the same weight becomes longer, so that the scale loss or the decrease in pickling efficiency has a remarkable effect on the manufacturing cost as compared with a general case of a thicker sheet. growing. An object of the present invention is to provide a sheet material having a thickness of 0.8 to 0.8 for building materials.
Regarding the method of manufacturing a high-strength hot-dip galvanized steel sheet having a 1.6 mm tensile strength of 400 to 510 MPa, when manufacturing a thin hot-rolled steel sheet from a material having a low slab manufacturing cost,
By providing a hot-dip galvanized material with a high material yield that suppresses scale formation without adding reinforcing elements such as Nb and Ti, the surface properties can be improved by optimizing the hot-rolling conditions and the heating conditions during hot-dip galvanizing. An object of the present invention is to provide a method for producing an excellent high-strength hot-dip galvanized steel sheet. 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 method for producing a high-strength hot-dip galvanized steel sheet having excellent surface properties according to the present invention is as follows.
%, Si: 0.03% or less, and Mn: 0.4 to 1%
And S: 0.01 to 0.04%, with the balance being a method of producing a hot-dip galvanized steel sheet from steel having a composition substantially consisting of Fe. After rolling, when performing finish rolling, the rolling reduction in the final stand: R%, finish rolling end temperature: FT ° C, average cooling rate for 3 seconds from the end of finish rolling: v ° C / sec.
c, and a step of performing hot rolling at a finishing temperature of FT ° C. and a final draft of R% at a winding temperature of CT ° C. satisfying the following formulas (1) and (2); Average cooling rate for 3 seconds from the end: cooling at v ° C / sec.
And pickling the rolled steel plate after 600
A step of heating to a temperature range exceeding ° C. and applying a hot-dip galvanized plating. F (T) = {4.57 × 10 ¹¹ exp (−28456 / (T + 273))} ^0.5 (1) 1.4 f (FT + 50) × (1−R / 100) + 30f (CT) +1.7 f ( FT-1.5 v) ≦ 4.0 (2) DETAILED DESCRIPTION OF THE INVENTION The present inventor made a thin hot rolled steel sheet from a material having a low slab production cost without adding a reinforcing element such as Nb or Ti. Has the desired strength when manufacturing
In order to provide a hot-dip galvanized steel sheet with suppressed surface scale formation, the relationship between the hot rolling conditions of the steel material, the strength of the steel sheet and the thickness of the surface scale after winding, the heating conditions during hot-dip galvanizing, and the steel sheet strength We conducted intensive research on the relationship with. As a result, the surface scale thickness of the hot-rolled steel sheet after completion of the winding is proportional to the rolling reduction in the final final stand, the finish rolling finish temperature, the average cooling rate for 3 seconds after the finish rolling finish, and the winding temperature. Was obtained.
Further, it has been found that a desired strength can be obtained by heating the steel sheet to a certain temperature or higher before hot-dip galvanizing. [0013] When hot rolled coils of the same weight are compared, when the surface scale thickness is the same, in a hot rolled steel sheet having a small thickness, the reduction in material yield and the pickling efficiency due to the scaling of the base iron significantly decrease. For example, a plate thickness of 1.2 mm and 2.4 m
m, the time required for scale loss and pickling is 2 if the scale thickness on the steel sheet surface is the same.
Double. Therefore, in order to improve the material yield and the pickling efficiency, reducing the amount of surface scale generated at the end of rolling compared to the conventional hot-rolled steel sheet having a large thickness greatly contributes to the reduction in manufacturing cost. . The scale formation is determined by the temperature at which the scale is formed and the holding time at that temperature, and the final scale thickness in the hot rolling process subjected to continuous reduction is the final rolling and the run-out before winding. The temperature is dominant because the cooling is short. In addition, during the finish rolling, the scale is thin and stretched by rolling because the scale has ductility. On the other hand, the cooling after winding is slow cooling, so that the time contributes more than at the time of finish rolling. Based on the above findings, the present inventor has proposed that Nb,
Index values indicating the hot rolling conditions that affect the surface scale thickness of the hot-rolled steel sheet after winding without adding reinforcing elements such as Ti (rolling reduction in final finishing stand, finish rolling end temperature, finish rolling The average cooling rate and the function of the winding temperature for 3 seconds from the end, the formula (2)) are controlled within a certain range, and the steel sheet is heated to a certain temperature or more before hot-dip galvanizing. A method for producing a hot-dip galvanized steel sheet according to the present invention having excellent strength and excellent surface properties was found, and the present invention was completed. That is, the present invention limits the steel composition and the manufacturing conditions of the hot-dip galvanized steel sheet to the following ranges, so that the sheet thickness for building materials is 0.8 to 0.8.
A hot-dip galvanized steel sheet having a 1.6 mm tensile strength of 400 to 510 MPa and excellent surface properties can be obtained. The reasons for adding the components, the reasons for limiting the components, and the reasons for limiting the production conditions of the present invention are described below. (1) Component composition range C: 0.08 to 0.2% C is an inexpensive element for strengthening steel.
%, It is difficult to secure a tensile strength of 400 to 510 MPa, and even if it exceeds 0.2%, a significant increase in strength cannot be obtained, and the cost of addition increases, which is not economical. . Therefore, the C content is 0.08 to 0.2%. Si: 0.03% or less Si is an element effective for increasing the strength of steel.
If added in excess of 03%, non-plating occurs during hot-dip galvanizing, and the surface properties of the steel sheet deteriorate, so in the present invention, Si is not added positively and is generally used in the steel making stage. The upper limit is 0.03% from the upper limit that can be reduced in the refining process. Mn: 0.4 to 1% Mn is an element that increases the strength of steel like C,
If it is less than 0.4%, it is difficult to secure a tensile strength of 400 to 510 MPa, and if it exceeds 1%, it is necessary to add metal Mn which is more expensive than ordinary ferromanganese in the steel making stage. Not only does the cost rise, but the strength also exceeds 510 MPa, so the Mn content is 0.4-1%. S: 0.01% to 0.04% S exists in a solid solution state and becomes brittle at the time of hot working, causing cracks on the sheet surface and causing surface defects on the steel sheet at the hot rolling step. Since an abnormal alloying reaction occurs near the surface defect during hot-dip galvanizing and the surface appearance becomes non-uniform, the upper limit is made 0.04%. Regarding the lower limit, the frequency of occurrence of surface defects that are small from the viewpoint of the surface properties of the steel sheet decreases, but in order to obtain a low S component of less than 0.01%,
In the steelmaking stage, a desulfurization refining process is added, which significantly increases the cost of smelting steel and is not economical. Therefore, the amount of S is 0.1.
01 to 0.04%. Further, P, sol. Al and N are allowed to be mixed as long as the effects of the present invention are not impaired. (2) Hot-dip galvanized steel sheet manufacturing process After the steel adjusted to the above-mentioned components is melted in a converter, it is made into a slab by continuous casting. The slab is once turned into a rough bar by rough rolling. The slab heat history up to hot rolling after slab casting may be either direct hot rolling in which rough rolling is performed immediately without cooling after casting, or a reheating method in which the slab is cooled once to Ar ₃ points or less and reheated. Next, when performing the finish rolling, the entry side is descaled by descaling with high-pressure water or the like, and the finishing end temperature is FT ° C. by a continuous hot rolling mill.
Then, hot rolling is performed at the final stand at a rolling reduction of R%, and an average cooling rate for 3 seconds from the end of the finish rolling: v ° C./sec.
And wind at CT ° C. Here, the rolling reduction at the final stand: R%,
Finish rolling finish temperature: FT ° C., average cooling rate for 3 seconds from finish roll finish: v ° C./sec, and winding temperature: CT
C is a condition that satisfies the following equations (1) and (2). F (T) = {4.57 × 10 ¹¹ exp (−28456 / (T + 273))} ^0.5 (1) 1.4 f (FT + 50) × (1-R / 100) + 30f (CT) +1.7 f (FT-1.5v) ≦ 4.0 (2) The conditions are as follows: in the hot rolling step of the slab, after rough rolling and then finish rolling, the surface scale thickness of the hot-rolled coil is reduced and the pickling step is performed. It has been set for the purpose of improving the material yield by improving the efficiency and reducing the scale loss after finish rolling. The inventor of the present invention has conducted intensive studies on the reduction of the surface scale thickness of the hot-rolled steel sheet. It became clear that the rolling end temperature was in proportion to FT ° C., the average cooling rate for 3 seconds from the end of finish rolling: v ° C./sec, and the winding temperature was CT ° C. That is, in the finish rolling, the lower the FT, the smaller the amount of scale produced, and the higher the final finishing draft, the thinner the scale produced in the finish rolling by rolling. In run-out cooling after finishing rolling, the larger the average cooling rate in the first three seconds at a higher temperature, the smaller the amount of scale generated in run-out. Further, the amount of scale generated in the winding stage is the highest at the winding temperature. Therefore, the lower the winding temperature, the lower the amount of scale generated in the slow cooling stage after winding. In the present invention, FT, R, CR, CT
Is not particularly specified, and if the above expression (2) is satisfied, the thickness of the surface scale at the hot-rolled sheet stage is reduced, and the pickling efficiency and the material yield are improved. In the cooling after the finish rolling, the scale formation can be effectively suppressed by the rapid cooling and the cutoff of the steel sheet and the air by the run-out water cooling control technology by controlling the run-out intermediate temperature. Winding may be performed by a conventional method, but mist cooling in a cooling stage or cooling in a non-oxidizing atmosphere such as Ar gas can also effectively reduce scale formation. Next, after the rolled steel sheet is pickled,
Heat to a temperature exceeding ℃ and apply hot-dip galvanized plating.
Here, in order to remove the surface scale of the hot-rolled steel sheet and improve the plating adhesion in the subsequent hot-dip galvanizing, pickling is performed before heating the wound steel sheet. In pickling, hydrochloric acid pickling in accordance with a conventional method may be used, but the method of the present invention can significantly reduce the pickling time, so even in a facility where a pickling line is installed on the entrance side of the continuous galvanizing line, Effective descaling is possible without reducing the line speed. Prior to hot-dip galvanizing, the steel sheet is once heated to 600 ° C. or higher because the heat treatment activates the surface and improves the plating adhesion, and the strength level of the steel sheet is increased to 400 ° C. This is for adjusting the pressure to ５１０510 MPa. Heating above 600 ° C. is performed in a hydrogen atmosphere of 3% or more, whereby the steel sheet surface can be more effectively activated, and the heating time is preferably 30 seconds or more and 3 minutes or less. In the present invention, the hot-dip zinc-based plating is hot-dip galvanizing, hot-dip zinc-5% Al plating, hot-dip zinc-55% Al
It is intended for hot-dip plating containing zinc such as plating. As described above, according to the production method of the present invention, the surface properties are excellent and the strength level is 400 to 510 MPa.
It is possible to obtain a high-strength hot-dip galvanized steel sheet having a. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention. EXAMPLES The steels of the present invention (Nos. 1 to 6) and comparative steels (Nos. 7 to 10) having the component compositions shown in Table 1 were melted to form slabs.
After heating to 0 ° C., rough rolling was performed once, then finish rolling was performed under the conditions shown in Table 2, and after winding, the coil was cooled. Then, the scale of the steel sheet surface is removed by pickling for 10 seconds in a pickling line having a 9% hydrochloric acid concentration and a bath temperature of 90 ° C., followed by heating at 700 ° C. for 30 seconds in a hot-dip galvanizing line. And hot-dip galvanized. In Table 2, the sample No. Sample Nos. 1 to 6, 11 to 14 are hot-dip galvanized steel sheets of the present invention. 7-10,15-1
7 is a hot-dip galvanized steel sheet of a comparative example. Next, from this plated material, J
IS5 test pieces were collected and evaluated for mechanical properties. Samples were taken from these hot-rolled coils, and the steel sheet was immersed in a 9% hydrochloric acid bath at a bath temperature of 90 ° C. in a laboratory. A constant current was passed between the steel sheet and the platinum electrode to measure the potential difference. The complete descaling time by pickling was measured by the method. Furthermore, these hot-rolled sheets were pickled in a 9% hydrochloric acid bath at a bath temperature of 90 ° C. for 10 seconds in a laboratory, and the surface properties after hot-dip galvanizing were examined. ○ indicates good surface properties, and X indicates poor surface properties. Table 3 shows the evaluation results. In addition, the descaling time ratio in the table is steel No. 9% of the scale under the condition calculated as 4.0 in the above formula (2) out of 2.
The ratio of the pickling time of the hot rolled sheet under other conditions to the hydrochloric acid concentration and the pickling end time at a bath temperature of 90 ° C. is shown. Table 3 shows that the samples of the present invention (Nos. 1 to 6,11 to
It can be seen that each of 14) secures a tensile strength of 400 MPa (41 kgf / mm ² ) or more and has excellent surface properties. On the other hand, the comparative sample No. 7, 9 are each C,
The tensile strength is insufficient because the amount of Mn added is insufficient. On the other hand, Comparative Sample No. In Nos. 8 and 10, since the addition amounts of Si and S are beyond the range of the present invention, the surface properties after hot-dip galvanizing are deteriorated. Also, N
o. 15, 16, and 17 indicate that the index value of the hot rolling condition (the value of the formula (2)) affecting the surface scale thickness exceeds the range of 4.0 of the present invention, and the scale remains even after the pickling treatment. Since the surface properties at the time of plating have deteriorated, the surface properties after hot-dip galvanizing have deteriorated. Next, steel no. Steel slabs were collected from the slabs Nos. 2 and 4 and subjected to a scale generation test during hot rolling by a hot rolling mill in a laboratory. 2, 4.0 in the equation (2).
9% hydrochloric acid concentration and bath temperature 9
FIG. 1 shows the ratio of the pickling time of the hot-rolled sheet under other conditions to the pickling end time at 0 ° C. From FIG. 1, it can be seen that the pickling end time changes according to the above equation (2), and that the smaller the value of the equation (2), the shorter the pickling time. [Table 1] [Table 2] [Table 3] According to the present invention, a sheet thickness of 0.8 for building materials is used.
Regarding the method for producing a high-strength hot-dip galvanized steel sheet having a tensile strength of 400 mm to 1.6 mm and a tensile strength of 400 to 510 MPa, Nb, Ti
When manufacturing thin hot-rolled steel sheets from materials with low slab manufacturing cost without adding reinforcing elements such as hot-rolled steel, we provide hot-dip galvanized plating materials with high material yield with reduced scale generation, By optimizing the heating conditions during plating, a high-strength hot-dip galvanized steel sheet having excellent surface properties can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a relationship between hot rolling conditions and a time ratio required for complete descaling by pickling according to an embodiment of the present invention.

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C23C2 / 40 C23C2 / 40 (56) References JP-A-63-149321 (JP, A) JP-A-5-311244 (JP, A) JP-A-5-125449 (JP, A) JP-A-60-165320 (JP, A) JP-A-4-314828 (JP, A) JP-A-5-105596 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (1)

(57) [Claims 1] C: 0.08 to 0.2% by weight%;
Si: 0.03% or less, Mn: 0.4 to 1%, S:
0.01 to 0.04%, with the balance being substantially Fe
In the method for producing a hot-dip galvanized steel sheet from steel having a composition consisting of: a steel sheet produced by continuous casting, after rough rolling, and then to finish rolling, a rolling reduction in a final stand: R%, finish rolling end temperature : FT ° C., average cooling rate for 3 seconds from the end of finish rolling: v ° C./sec, and winding temperature: CT ° C. under the conditions satisfying the following formulas (1) and (2): Finishing end temperature: FT ° C. A step of performing hot rolling at a final rolling reduction of R%, and an average cooling rate of 3 seconds from the end of the finish rolling: v ° C / sec.
c) a step of cooling at a temperature of c and winding at CT ° C .; a step of pickling the wound steel sheet, heating the steel sheet to a temperature exceeding 600 ° C., and applying hot-dip galvanized plating. Manufacturing method of high strength hot-dip galvanized steel sheet with excellent properties. f (T) = {4.57 × 10 ¹¹ exp (−28456 / (T + 273))} ^0.5 (1) 1.4 f (FT + 50) × (1−R / 100) + 30f (CT) +1.7 f (FT−1.5 v) ≦ 4.0… (2)