KR100478091B1 - Method for Manufacturing Galvanized Steel Sheet - Google Patents

Abstract

The present invention is a method for producing a high yield ratio type high strength hot-dip galvanized steel sheet used in deck plates, steel house studs, ceiling reinforcement, chief plate used in construction sites that require high strength The present invention relates to a method for manufacturing a cold-rolled hot-dip galvanized steel sheet which not only has excellent workability but also has high yield strength, high tensile strength and mechanical properties of appropriate ductility, as well as excellent surface properties, shape and plating adhesion. I want to do that, but there is a purpose,

The present invention provides a method for producing a hot-dip galvanized steel sheet,

By weight%, C: 50 ppm or less, Si: 0.03% or less, Mn: 0.8-1.0%, P: 0.08-1.0%, S: 0.015% or less, Al: 0.02-0.06%, Ti: 0.04-0.06%, B : Steel slab consisting of 5 ~ 15ppm, balance Fe and other unavoidable impurities is heated in the furnace to 1150 ~ 1250 ℃, hot rolled at 880 ~ 940 ℃ hot rolling finish temperature condition and wound up at 580 ~ 650 ℃. Next, the hot rolled steel sheet was pickled, cold rolled, degreased, heated to 650 to 700 ° C., cooled to 440 to 465 ° C., plated in a hot dip galvanizing bath, and elongated at an elongation of 0.7% or less. The present invention relates to a method of manufacturing hot-dip galvanized steel sheet by rough rolling.

Description

Method for manufacturing hot-dip galvanized steel sheet {Method for Manufacturing Galvanized Steel Sheet}

The present invention is a high yield ratio high strength hot-dip galvanized steel sheet used in deck plates, steel house studs, ceiling reinforcement, chief plate used in construction sites requiring high strength Iron (hereinafter also referred to as "CGI"), and more particularly, to a method of producing a high strength hot dip galvanized steel sheet having a high yield ratio tensile strength of 570 MPa or more.

CGI is a high yield ratio high strength hot-dip galvanized steel sheet, and is used in deck plates, steel house studs, ceiling reinforcements, and chief plates used in construction sites.

Recently, domestic and overseas customers have faced economic downturns, so as to reduce costs, lower the steel sheet thickness of existing CGI general steels and structural steels below 400 MPa, and increase the demand for high-strength steel plates with a higher tensile strength of more than 570 MPa and the expansion of Steel House. Steel demand has been increasing.

In particular, there is a need to develop a CGI steel sheet having an appropriate elongation at high yield ratio type having a high strength of 570 MPa or more and a yield strength of 560 MPa or more.

The steel sheet having such mechanical properties is easy to bend and be excellent in deformation resistance to external loads, impacts, fires, etc., after processing, so that the steel sheet thickness can be lowered.

Conventional methods for producing tensile strength 570 MPa grade CGI structural steels include the addition of Ti to low, medium and low carbon steels, and the addition of Ti as a basic component system. Line (hereinafter also referred to as "CGL") is a method of hot-dip galvanizing at a high line speed after heat treatment in a temperature range below the recrystallization temperature.

 As described above, when manufacturing high strength structural steel with a tensile strength of 570 MPa or more based on Ti addition, medium carbon steel or ultra low carbon steel as a basic component system, it is sensitive to CGL heat treatment temperature and line speed, and has large material defects and deviations. Since the heat treatment temperature is 540 ~ 600 ℃ for low and low carbon Ti additive steel and 580 ~ 620 ℃ for ultra low Ti added steel, it is difficult to connect with other steel grades, so a dummy coil is always needed to prevent material defects. In addition, CGL furnace atmosphere is unstable due to low temperature heat treatment and shape defection ability is inferior, resulting in a defective product rate of 20-30% or more.

In addition, in the above method, the dummy coil should be treated as a defective product that is difficult to sell normally, and the difference between the strip and roll temperature is large when the temperature is suddenly lowered while omitting the dummy coil and working with the general material and the 400MPa structural steel. As the strip meanders, there is a problem that eventually causes an accident called pan breaking, causing enormous damage such as factory shutdown for more than 12 hours.

In addition, according to the method, the full hard coil (cold rolled) after pickling the hot rolled steel sheet, each of the edge portion and the center portion of the steel sheet due to the roll force of 1,000 tons or more Or, there are wave shape defects on both parts, and CGL low-temperature heat treatment results in insufficient shape correction, so that defects frequently occur due to wave defects, and the average shape quality grade is also absolute compared to other steel grades of 680 ℃ or higher. Inferior to, and when the production rate is lowered or the heat treatment temperature is increased to secure the shape quality, the strength is reduced.

The present inventors have conducted research and experiment to solve the above-mentioned problems of the prior art, and proposed the present invention based on the results, and the present invention not only has excellent workability but also high yield strength and high tensile strength. And to provide a method of manufacturing a cold-rolled hot-dip galvanized steel sheet that can ensure a good ductility of the mechanical properties as well as excellent surface properties, shape and plating adhesion, the purpose is,

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention provides a method for producing a hot-dip galvanized steel sheet,

By weight%, C: 50 ppm or less, Si: 0.03% or less, Mn: 0.8-1.0%, P: 0.08-1.0%, S: 0.015% or less, Al: 0.02-0.06%, Ti: 0.04-0.06%, B : Steel slab consisting of 5 ~ 15ppm, balance Fe and other unavoidable impurities is heated in the furnace to 1150 ~ 1250 ℃, hot rolled at 880 ~ 940 ℃ hot rolling finish temperature condition and wound up at 580 ~ 650 ℃. Next, the hot rolled steel sheet was pickled, cold rolled, degreased, heated to 650 to 700 ° C., cooled to 440 to 465 ° C., plated in a hot dip galvanizing bath, and elongated at an elongation of 0.7% or less. The present invention relates to a method of manufacturing hot-dip galvanized steel sheet by rough rolling.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

First, the composition of the inventive steel will be described.

The C is the most effective element in terms of securing strength, but there is a problem in that the rolling load occurs when cold rolling a hot rolled coil (steel sheet), and the low and medium carbon steel has a CGL heat treatment temperature of 600 ° C. or lower, so that the material is poor. There is a problem that the deviation is large and inferior to other steel grades and the work quality and shape.

Therefore, in the present invention, the C content is limited to 50 ppm or less in order to improve the material, shape, and workability by increasing the recrystallization temperature during the CGL heat treatment to be 650 ° C. or more.

Since Si has a problem in that the plating surface quality decreases as an element or an added amount increases in strength, the content thereof is preferably limited to 0.03% or less.

Since Mn is a solid solution strengthening element, together with C, it secures strength and lowers Ar ₃ transformation point, which is advantageous in preventing mixed structures and added to secure high strength and proper ductility of ferrous iron, and the amount thereof is limited to 0.8 to 1.0%. It is desirable to.

If the Mn content exceeds 1.0%, manufacturing cost is high and ductility and cold rolling load are generated. If the Mn content is less than 0.8%, it is difficult to secure sufficient strength.

P is a component added to secure strength as a precipitation strengthening element, and its content is preferably limited to 0.08 to 0.1%.

In general, when P is high, there is segregation and secondary processing brittleness problems, so that workability is lowered, but in the present invention, 5-15 ppm of B is added to solve such problems.

For simple bending and 90 degree machining, segregation and secondary processing brittleness are not a big problem.

Although S is inevitably present in steel as an impurity in steel, it is preferable to limit it to 0.015% or less because high component content adversely affects workability.

Al is an element necessary for deoxidation of steel, and is limited to 0.02% to 0.06% which is a normal level required for deoxidation in the present invention.

Ti is a carbonitride-forming element, which increases the CGL recrystallization temperature, and also advantageously increases the strength, so that the content is preferably limited to 0.04 to 0.06% in consideration of manufacturing cost.

The B is a component added to solve the secondary brittle fracture problem after the final processing of the customer, it is preferable to limit the content to 5 ~ 15ppm, because if the content of the B is too small the addition effect is insufficient In the case of exceeding 15 ppm, the cost increases and the material anisotropy increases.

Hereinafter, the manufacturing conditions of this invention are demonstrated.

In order to manufacture the hot-dip galvanized steel sheet according to the present invention, the steel slab of the present invention is heated to about 1150 ~ 1250 ℃ in a hot-rolled heating furnace.

When the heating temperature (extraction temperature) of the slab is high, many scales are generated on the surface of the hot rolled steel sheet to hinder the surface quality, and when the temperature is lower than 1150 ° C, it is difficult to secure the temperature above the Ar ₃ transformation point and the rolling load of the high strength 2.3 mm or less material Problems arise.

The slab heated as described above is hot rolled under the hot rolling finishing condition of 880-940 ° C. above Ar ₃ transformation point, and then cold-rolled at 580-650 ° C.,

The hot rolling step is preferably performed so that the thickness of the hot rolled steel sheet is 2.1 to 4.5 mm.

In general, hot-rolled steel sheet tends to decrease in strength as the coiling temperature increases. If the coiling temperature exceeds 650 ° C, grain size of the base iron increases, making it difficult to secure sufficient strength, and when the coiling temperature is 580 ° C or less, Since the fine grains are excessively fined, the strength is increased, and the cold rolling load is generated in the pickling and cold rolling process (PCM), the winding temperature of the hot rolled steel sheet is preferably limited to 580 to 650 ° C.

That is, stable and high strength can be obtained in the winding temperature range of 580-650 degreeC.

Next, the hot rolled steel sheet prepared as described above was removed to remove hot rolled scale, cold rolled to a target thickness, and then degreased, heated to 650 to 700 ° C., and then cooled to 440 to 465 ° C., followed by molten zinc. After plating in a plating bath, temper rolling at an elongation of less than 0.7% to produce a hot-dip galvanized steel sheet.

Examples of the degreasing method include alkaline degreasing or electrolytic degreasing.

The most important thing in the production of structural steel with a tensile strength of 570 MPa or more in CGL is that the target material must be obtained even when working in the high heat treatment temperature range.

If the heat treatment temperature is less than 650 ℃ shape correction is inferior, there is a problem that the furnace (workability) and workability is unstable because the difference in the heating temperature when connecting with other steel species.

In order to improve the shape due to low temperature heat treatment, when cold rolling is performed by lowering the thickness of the hot rolled coil, the final product shape is improved by lowering the cold reduction rate, but it must be forcedly accelerated to meet the final rolling target temperature during hot finishing rolling. In this case, there is a problem in that the width deviation occurs during the hot rolling, the twist mark defect and the equipment malfunction occur.

Therefore, in the present invention, the heat treatment temperature during the heat treatment before hot dip galvanizing is limited to 650 ~ 700 ℃.

As the hot dip galvanizing bath, it is preferable to use a plating bath containing 0.14 to 0.25% of Al, 50 ppm or less of Pb and Sb, and composed of the remaining Zn and other unavoidable impurities.

Since CGI should have good surface quality, there should be no fitting mark, flow pattern, and blackening phenomenon that steel plate surface turns black after 3 months after production. In general, when Pb and Sb are present in the sequin grain boundary, it segregates in the grain boundary after elapse of time and causes delamination over time.

When Pb and Sb components of the plating bath are included, ammonium phosphate solution is sprayed to prepare the ZERO sequins. As the spray marks show the fitting shape, the surface and roughness become rough and the surface becomes black after 3 months or after bending. do.

Therefore, by using a plating bath containing 50 ppm or less of Pb and Sb components, which are regular sequin forming elements, with Al: 0.14 to 0.25% and the remainder being Zn and other unavoidable impurities, it is possible to prevent over time peeling and By improving the surface quality, it is desirable to solve the plating peeling and blackening phenomenon over time during processing.

On the other hand, in the present invention, it is preferable to perform temper rolling at an elongation of not more than 0.7% in a skin pass mill in order to impart proper roughness to the molten zinc steel sheet and to correct the shape.

When the elongation at temper rolling exceeds 0.7%, a malfunction occurs and the elongation is lowered.

According to the present invention, by manufacturing a hot-dip galvanized steel sheet under appropriate conditions with a steel slab having an appropriate composition, the shape, surface and plating adhesion are good, and the yield strength is high as a high yield ratio type high strength structural steel, yielding strength of 560 MPa or more and tensile strength. It can meet the KS, JIS standards of strength 570MPa or more, elongation of 3% or more, and can produce a hot-dip galvanized steel sheet having less material deviation and defects than the conventional structural steel, improving ductility, and excellent shape.

In addition, the present invention can significantly improve the workability by raising the CGL heat treatment temperature.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

The steel prepared as shown in Table 1 was produced as a slab in the reproducing process, and then reheated for 2 hours in the range of 1150 ~ 1250 ℃, followed by hot rolling to prepare a hot rolled steel sheet. At this time, the hot finishing temperature was 910 ℃, the winding temperature was set to 581 ~ 645 ℃ range as shown in Table 2.

The hot rolled steel sheet manufactured as described above is cold rolled to a pickling and a target thickness in a pickling and cold rolling process to produce a full hard coil. The full hard coil was heated to 660˜695 ° C., cooled to 460 ° C., and then plated by plating in a hot dip galvanizing bath.

 As the zinc plating bath, a molten zinc bath containing 30 ppm of Pb and Sb components and 0.20% of Al, and the rest of which is composed of Zn and other unavoidable impurities was used.

Then, temper rolling was carried out at a skin pass mill at an elongation of 0.25% for proper roughness and shape correction.

The yield strength, tensile strength, elongation, shape quality and workability of the steel sheet manufactured as described above were evaluated, and the results are shown in Table 2 below.

In Table 1, conventional steel (1) is a low carbon steel + Ti steel, conventional steel (2) is a medium-low + Ti steel, conventional steel (3) is a medium carbon steel, conventional steel (4) is very low + Ti steel, comparative steel ( 1) is very low P + Ti steel, comparative steel (2) is very low + Mn + P + Ti steel, while invention steel is very low + high Mn + high P + Ti + B additive steel

Steel grade Chemical composition (wt%.) C Si Mn P S Sol.Al Ti B (ppm) Conventional Steel 1 0.040 0.01 0.25 0.012 0.006 0.035 0.061 - Conventional Steel 2 0.020 0.01 0.15 0.012 0.006 0.035 0.030 - Conventional Steel 3 0.010 0.01 0.50 0.011 0.007 0.036 - - Conventional Steel 4 0.004 0.01 0.40 0.030 0.006 0.034 0.050 - Comparative Steel 1 0.003 0.01 0.45 0.065 0.007 0.032 0.013 - Comparative Steel 2 0.004 0.01 0.60 0.040 0.008 0.036 0.055 5 Inventive Steel 1 0.003 0.01 0.88 0.086 0.006 0.030 0.052 13 Inventive Steel 2 0.003 0.01 0.91 0.084 0.007 0.032 0.047 11 Inventive Steel 3 0.004 0.01 0.92 0.084 0.006 0.035 0.043 10

Psalm No. Winding temperature (℃) CGL heat treatment temperature (℃) Characteristics Steel grade Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Shape quality Workability (hot rolled, PCM, CGL) KS-SGC570 Material Standard - - Over 560 More than 570 3 or more - - - Conventional material 1 620 650 440 460 15 Bad Bad Conventional Steel 1 Conventional material 2 620 540-600 601 644 9.2 Bad Bad Conventional Steel 2 Conventional Materials 3 600 550 650 710 6.0 Bad Bad Conventional Steel 3 Conventional material 4 620 570-600 596 636 11.0 Bad Bad Conventional Steel 4 Comparative material 1 620 620 440 470 5.0 Bad Bad Comparative Steel 1 Comparative material 2 620 620 600 630 3.0 Bad Bad Comparative Steel 2 Invention 1 645 660 632 708 12.2 Good Good Inventive Steel 1 Invention Material 2 581 695 628 724 11.6 Good Good Inventive Steel 2 Invention 3 617 680 620 710 12.0 Good Good Inventive Steel 3

As shown in Table 2, the conventional materials (1 to 4) and the comparative materials (1 to 2) are subjected to CGL heat treatment at a heat treatment temperature of 620 ° C. or lower, so that the materials and shapes are each poor or both the materials and shapes are poor. It can be seen that poor or poor workability.

On the other hand, in the case of the inventive materials (1 to 3) according to the present invention, the shape quality and workability are good as the CGL heat treatment temperature is relatively high at 660 to 695 ° C., and the target materials are also satisfied. Can be.

As described above, according to the present invention, CGI steel sheet having high yield strength (560 MPa or more), high tensile strength (570 MPa or more), and appropriate workability, used for deck plates, steel house studs, fabric steels, and chief plates. In manufacturing, compared with the conventional method, material deviation, material defect, and shape defect can be greatly improved, and workability can be significantly improved.

In addition, according to the present invention, it is possible to supply CGI high-strength structural steel to the customer in the future to contribute to the cost reduction of the customer, it is possible to save resources and energy.

Claims (3)

In the method of manufacturing a hot-dip galvanized steel sheet, By weight%, C: 50 ppm or less, Si: 0.03% or less, Mn: 0.8-1.0%, P: 0.08-1.0%, S: 0.015% or less, Al: 0.02-0.06%, Ti: 0.04-0.06%, B : Steel slab consisting of 5 ~ 15ppm, balance Fe and other unavoidable impurities is heated in the furnace to 1150 ~ 1250 ℃, hot rolled at 880 ~ 940 ℃ hot rolling finish temperature condition and wound up at 580 ~ 650 ℃. Next, the hot rolled steel sheet was pickled, cold rolled, then degreased, heated to 650 to 700 ° C., cooled to 440 to 465 ° C., plated in a hot dip galvanizing bath, and refined to an elongation of 0.7% or less. Manufacturing method of rolling hot-dip galvanized steel sheet The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein the hot rolling step is performed so that the thickness of the hot rolled steel sheet is 2.1 to 4.5 mm. 3. The molten zinc plating bath according to claim 1 or 2, wherein the hot dip galvanizing bath is a plating bath containing 0.14 to 0.25% of Al, 50 ppm or less of Pb and Sb, and composed of the remaining Zn and other unavoidable impurities. Manufacturing method of galvanized steel sheet