RU2530199C2 - High-strength hot-dip galvanized steel sheet with low yield point-to-breaking point ratio, high-strength annealed hot-dip galvanized steel sheet with low yield point-to-breaking point ratio, method of production of high-strength hot-dip galvanized steel sheet with low yield point-to-breaking point ratio and method of production of high-strength annealed hot-dip galvanized steel sheet with low yield point-to-breaking point ratio - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to high-strength hot-dip galvanised steel sheet used in motor-car production. Sheet is made of steel containing in wt %: 0.03-0.20 C, 1.0 or less Si, 1.5-3.0 Mn, 0.10 or less P, 0.05 or less S, 0.10 or less Al, 0.010 or less N, 0,5 or less Cr and 0.01-0.50 Mo, Fe and unavoidable impurities making the rest. Sheet microstructure comprises ferrite and secondary phase. Fraction of ferrite area makes 50% or more while fraction of secondary phase area including martensite makes 7-25%. Martensite crystal grain mean size makes 1-8 mcm. Sheet features yield point-to-breaking point ratio making 0.7 or less, high mouldability and coating properties.

EFFECT: high-strength hot-dip galvanized steel sheet.

10 cl, 3 tbl, 1 ex

Description

FIELD OF THE INVENTION

The present invention relates to a high strength with a low ratio of yield strength to tensile strength of a dipped galvanized steel sheet, high strength with a low ratio of yield strength to tensile strength of annealed galvanized dipped steel sheet, a method of manufacturing a high strength with a low ratio of yield strength to tensile strength of galvanized dipped steel sheet and of manufacturing high strength with a low ratio of yield strength to tensile strength of annealed zinc Nogo steel sheet by dipping.

Prior art

In the field of the automotive industry, in recent years, from the point of view of environmental protection, a decrease in the weight of the car body is largely demanded, for example, to improve fuel consumption aimed at reducing CO ₂ emissions. Meanwhile, from the point of view of ensuring the safety of the driver and passengers, the car body, obviously, should be more durable upon impact. For this reason, a more durable thin steel sheet is generally used as automotive steel sheet. Therefore, for the manufacture of dip galvanized sheet steel used as an automotive steel sheet, such as a high strength thin steel sheet, as described above, it is necessary to produce a coated main sheet that has a coating with excellent properties and also the required strength and ductility characteristics after galvanizing or after alloying.

In general, in order to strengthen sheet steel, a solid solution of reinforcing elements such as P, Mn and Si is added, or precipitation of reinforcing elements such as Ti, Nb and V is added. Moreover, when a steel sheet with these added alloying elements is processed on a continuous line galvanizing by immersion (CGL), it is necessary to add a large number of alloying elements for hardening, because it is difficult to obtain high strength as a result of annealing of sheet steel used at a temperature A _c1 or higher and a slow cooling rate waiting. However, the addition of a large number of alloying elements significantly affects the properties of the coating during galvanizing. Thus, the inclusion of alloying elements gives the opposite effect in hardening and improving the properties of the coating. Correspondingly, by the addition of alloying elements, hardening can be achieved, but good coating properties cannot be expected on a continuous dip galvanizing line (CGL). In addition, the disadvantage of adding alloying elements is poor formability characteristics, such as elongation, and therefore there are difficulties in achieving high strength thin steel sheet, which has both high strength and good formability, and the ability to achieve good coating properties during galvanizing dipping or alloying. The addition of a large number of alloying elements also causes the problem of a significant increase in costs.

Meanwhile, as a high-strength sheet steel with good formability, a biphasic steel sheet is usually proposed which includes a low-temperature transformation phase (including residual austenite) containing martensite as the main phase of the ferritic base. This biphasic steel sheet is not aged at room temperature and has a low ratio of yield strength to tensile strength (YR = yield strength [YS] / tensile strength [TS]), and thus is distinguished by formability and hardenability upon annealing after processing. As a method of manufacturing a sheet of biphasic steel, a method is known in which the steel sheet is heated to an intercritical temperature and then quenched by water or gas cooling. The advantage of this method is that the amount of alloying elements added can be reduced by increasing the cooling rate. For example, JP Patent Document No. 3,887,235 discloses a method of manufacturing, as the above-described sheet of biphasic steel, a high-strength dip galvanized sheet steel, which is distinguished by its suitability for flanging with tensile and impact resistance. In JP Patent Document No. 3,887,235, tensile flanging and impact resistance are improved by controlling the chemical composition of the steel and the maximum grain size and martensite area fraction.

SUMMARY OF THE INVENTION

The problem solved by the invention

Control of the content of components, manufacturing conditions, etc. described in JP Patent Document No. 3887235 provides insufficient dispersion of martensite in some cases, and does not take into account the improvement of ductility as an element of formability, which leads to cases in which good formability and good coating properties of high-strength dipped galvanized sheet steel with a low ratio of yield strength to tensile strength.

The present invention has been made in connection with the above problem. The aim of the present invention is to provide a high strength dipped galvanized sheet steel with a low yield strength to tensile strength and annealed high strength dipped galvanized sheet steel with a low yield strength to tensile strength, each of which has a low yield strength to tensile strength and high strength and stands out formability and coating properties. Another objective of the present invention is a method for manufacturing high strength dipping galvanized sheet steel with a low yield strength to tensile strength and a method for manufacturing annealed high strength dipping galvanized sheet steel with a low yield strength to tensile strength.

Problem Solving Tools

To solve the above problem, the authors of the present invention performed a number of studies and as a result found that a high-strength thin steel sheet with a low ratio of yield strength to tensile strength and with good formability and good coating properties is obtained by dispersing the lamellar structure formed by the secondary phase (for example, martensite, perlite and bainite) to a predetermined level and accurate selection, for example, of the martensite area fraction. Thus, the authors of the present invention have established the following.

High-strength with a low ratio of yield strength to tensile strength, the steel sheet includes, in% wt .: 0.03-0.20% C, 1.0% or less Si, from more than 1.5 to 3.0% Mn, 0, 10% or less P, 0.05% or less S, 0.10% or less Al, 0.010% or less N, 0.5% or less Cr, and 0.01-0.50% Mo and the rest Fe with inevitable impurities; has a structure including ferrite and a secondary phase as a microstructure, in which the ferrite area fraction is 50% or more, and the secondary phase includes martensite, whose area fraction is from 7 to less than 25%, and the thickness of the plate structure formed by the secondary phase satisfies the equation (one):

, где $$\text{Òb / Ò}\le \text{0}\text{, 005}\text{(one)}$$

where

Tb is the average thickness of the plate structure in the direction of the sheet thickness and T is the thickness of the sheet.

In high strength with a low ratio of yield strength to tensile strength by immersion galvanized steel sheet, martensite can have an average crystal grain size of 1-8 μm.

At least one element selected from 0.001-1.0% Cu, 0.001-1.0%, can be additionally included in the chemical composition, in mass%, of high strength with a low ratio of yield strength to tensile strength of dip galvanized steel sheet Ni, 0.001-1.0% V and 0.0003-0.0050% B.

In the chemical composition, in mass%, of high strength with a low ratio of yield strength to tensile strength of immersion galvanized steel sheet, 0.005-0.050% Ti can be additionally included.

In the chemical composition, in mass%, of high strength with a low ratio of yield strength to tensile strength of dip galvanized steel sheet, 0.001-0.005% Ca and / or 0.001-0.005% REM (REM) can be additionally included.

In a high-strength low yield strength to tensile strength annealed galvanized steel sheet, the zinc coating of the above-described high-strength low yield strength to low tensile strength galvanized steel sheet is a zinc alloy.

A method of manufacturing a high strength with a low ratio of yield strength to tensile strength of immersion galvanized steel sheet includes: heating a steel slab of the above chemical composition; hot rolling of a heated steel slab at a final temperature of 850-950 ° C; cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s; winding chilled steel at a temperature of 450-750 ° C, heating the resulting hot-rolled sheet or cold-rolled sheet obtained by the cold rolling process after winding at a temperature of 800 ° C or higher; cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher; and dipping galvanized chilled sheet.

A method of manufacturing a high strength with a low ratio of yield strength to tensile strength of immersion galvanized steel sheet includes: heating a steel slab of the above chemical composition; hot rolling of a heated steel slab at a final temperature of 850-950 ° C; cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s; winding chilled steel at a temperature of 450-750 ° C, a single heating of the obtained hot-rolled sheet or cold-rolled sheet obtained by the cold rolling process after winding to 800 ° C or higher; reheating the sheet to 750 ° C or higher after undergoing cooling and decapitation, cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, and galvanizing by immersion of the cooled sheet.

A method of manufacturing a high strength with a low ratio of yield strength to tensile strength of annealed zinc-plated steel sheet includes: heating a steel slab of the above chemical composition; hot rolling of a heated steel slab at a final temperature of 850-950 ° C; cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s; winding chilled steel at a temperature of 450-750 ° C, heating the resulting hot-rolled sheet or cold-rolled sheet obtained by the cold rolling process after winding to 800 ° C or higher; cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, dip galvanizing to create a zinc coating on the sheet; and alloying a zinc coating.

A method of manufacturing a high strength with a low ratio of yield strength to tensile strength of annealed zinc-plated steel sheet includes: heating a steel slab of the above chemical composition; hot rolling of a heated steel slab at a final temperature of 850-950 ° C; cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s; winding chilled steel at a temperature of 450-750 ° C, a single heating of the obtained hot-rolled sheet or cold-rolled sheet obtained by the cold rolling process after winding to 800 ° C or higher; reheating the sheet to 750 ° C or higher after undergoing cooling and decapitation, cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher; dip galvanizing to create a zinc coating on the sheet; and alloying a zinc coating.

Effect of the invention

In accordance with the present invention, it is possible to create a high strength low yield strength to tensile strength dipped galvanized steel sheet and a high strength low yield strength to low tensile strength annealed dipped galvanized steel sheet, each of which has a low yield strength to tensile strength and is distinguished by formability and coating properties and it is also possible to propose a method of manufacturing high strength with a low ratio of yield strength to tensile strength dip galvanized steel sheet and a method for manufacturing high strength with a low ratio of yield strength to tensile strength galvannealed steel sheet by dipping.

Best Mode (s) for Carrying Out the Invention

The following will describe the implementation of manufacturing high strength with a low ratio of yield strength to tensile strength of a dip galvanized steel sheet, high strength with a low ratio of yield strength to tensile strength of annealed galvanized steel sheet, a method of manufacturing a high strength with a low ratio of yield strength to tensile strength of galvanized steel sheet , and a method of manufacturing high strength with a low ratio of yield strength to tensile strength a dipped galvanized steel sheet in accordance with the present invention. In the description below, unless otherwise specified, the designation "%", representing the content of the elements of the steel composition, refers to "% mass.".

First, a description will be given of the experimental results that led to the present invention. In this experiment, a 30 mm thick sheet was prepared from a steel slab with a chemical composition including 0.09% C, 0.01% Si, 2.0% Mn, 0.009% P, 0.003% S, 0.041% Al, N 0.0026 %, 0.15% Mo, 0.02% Cr and the rest essentially consists of Fe. Then the sheet billet is heated to 1200 ° C and hot rolling is carried out in five passes to form a hot-rolled sheet 2.5 mm thick. Fine rolling is carried out at a temperature of 900 ° C and cooling is performed with an average cooling rate of 13 ° C / s. Subsequent winding is performed at 640 ° C. Then, the hot-rolled steel sheet thus obtained is decapitated and annealed. During the annealing process, some steel sheets are heated and incubated for one minute at a temperature of 800-900 ° C for preliminary annealing and after cooling to room temperature with an average cooling rate of 10 ° C / s, they are decapitated and then the steel sheets are heated and incubated for one minute at 780 ° C to be completely annealed and cooled from 700 ° C to 550 ° C with an average cooling rate of 10 ° C / s; after that, the steel sheets are held for one second at 470 ° C to simulate heat treatment during coating, then heated to 550 ° C to simulate alloying of the coating and then cooled to room temperature. Some other steel sheets are not preliminarily annealed, but heated and incubated for one minute at a temperature of 800-900 ° C for complete annealing and cooled from 700 ° C to 550 ° C with an average cooling rate of 10 ° C / s; after that, the steel sheets are held for one second at 470 ° C to simulate heat treatment during coating, then heated to 550 ° C to simulate alloying of the coating and then cooled to room temperature.

Then, the tensile strength TS and the ratio of the yield strength to the tensile strength YR of each of the steel sheets obtained as described above and the TSxEL value are determined. Then check the relationship between these values and the thickness of the plate structure after annealing in section in the direction of the thickness of the steel sheet. The thickness of the plate structure is defined as the ratio Tb / T of the average thickness Tb of the plate structure consisting of the secondary phase in the direction of the sheet thickness to the thickness of the sheet T obtained from the steel sheet. The actual average thickness Tb of the plate structure is determined as follows. The section in the direction of the thickness of the steel sheet is polished and then etched with a 3% nital solution. Then, the adjoining region, located at a quarter of the sheet thickness, was examined at a magnification of about 1500 using a scanning electron microscope (SEM), and the thickness of the columnar layered structure of the secondary phase at 20 points was measured using the image obtained using Image-Pro developed by Media Cybernetics. The average thickness Tb is defined as the average value at 20 points.

Moreover, in steel with a high content of C and Mn, concentrated layers of C and Mn, which aggregate along the boundaries of crystalline grains mainly at the stage of cooling the slabs, are stretched during hot rolling and subsequent cooling. Thus, the secondary phase group is formed in a columnar layered form in the rolling direction and in the sheet width direction of the annealed sheet. The lamellar structure consists of a secondary phase group. The lamellar structure (lamellar structure of the secondary phase), consisting mainly of C and Mn, is able to form thick when a large amount of manganese is added to provide strength. This phenomenon leads to a decrease in the concentration of manganese and other elements in austenite and, thus, is disadvantageous for the uniform dispersion of martensite.

As a result of this experiment, the authors of the present invention found that the ductility and the ratio of yield strength to tensile strength significantly change in the immediate vicinity of the Tb / T value of 0.005 in the steel sheet after annealing, and also found that when the Tb / T value is 0.005 or less the ratio of yield strength to tensile strength YR is only 70% or less, and the TSxEL ratio has a suitable value of 16,000 MPa ·% or more. Thus, the inventors have found that the Tb / T value is an approximate guideline for obtaining the following effect. By conducting heating (preliminary annealing) at a given temperature in the installation, such as a continuous annealing line before heating (main annealing) on a continuous galvanizing line by immersion, the thickness of the strip plate structure can be reduced and finely dispersed by heating in the continuous annealing line. In addition, as a result of this effect, even when the steel sheet is maintained at a predetermined temperature during the process of galvanizing by immersion on the continuous galvanizing line by immersion, or, for example, in the subsequent alloying process, martensite can be dispersed in the desired manner in a ferrite base after cooling, since the concentration C and Mn increase in the austenite phase.

It should be noted that the above effect is achieved not only in the case when the heating is carried out twice, once during preliminary annealing and the other in the main annealing, but also in the case of high-temperature heating, in which heating is carried out once, for example, on a continuous galvanizing line by immersion . However, in the latter case, the properties of the coating may deteriorate to some extent, because high-temperature heating can increase the Mn content on the surface of the steel sheet. Accordingly, in order to provide more stable coating properties, it is preferable to pre-anneal on the continuous annealing line and perform basic annealing on the continuous galvanizing line by immersion.

The following is a description of a high strength low yield strength to tensile strength galvanized dipped galvanized steel sheet, a high strength low yield strength to low tensile strength dipped annealed galvanized steel sheet, a method of manufacturing a high strength low yield strength to immersion galvanized steel sheet, and a manufacturing method high strength with a low ratio of yield strength to tensile strength of annealed galvanized dip sheet metal, in accordance with the present invention will be divided into a detailed description of the chemical composition, microstructure and methods of manufacturing steel sheets.

First, the chemical composition will be described.

(Content C)

Carbon (C) is one of the important main components of steel and, in particular, in the present invention is an important element, because carbon affects the volume fraction of the austenite phase when heated to intercritical temperature and, therefore, the amount of martensite after transformation. Mechanical properties, such as tensile strength obtained by a steel sheet, are highly dependent on the martensite fraction (area fraction) and the martensite phase hardness. In this case, the martensite phase is formed with difficulty when the C content is less than 0.03%. Meanwhile, spot weldability deteriorates when the C content exceeds 0.20%. Accordingly, the content of C is 0.03-0.20%. To achieve better properties, the content of C is preferably set in the range of 0.03-0.15%.

(Si content)

Silicon (Si) is an element that improves formability, for example elongation, by reducing the amount of dissolved carbon in the alpha phase. However, Si incorporation in excess of 1.0% impairs spot weldability and coating properties. Accordingly, the Si content is set to 1.0% or less, preferably 0.7% or less.

(Mn Content)

In the present invention, manganese (Mn) is concentrated in the austenite phase and has the effect of facilitating the martensitic transformation, thus, being an important element as the main component. However, the above effect is not achieved when the manganese content is 1.5% or less. Meanwhile, spot weldability and coating properties deteriorate significantly when the Mn content exceeds 3.0%. Accordingly, the Mn content is set equal to 1.5-3.0% and preferably 1.5-2.2%.

(Content P)

Phosphorus (P) is an effective element to achieve high strength at low cost. However, the inclusion of P in excess of 0.10%, significantly deteriorates the weldability by spot welding. Accordingly, the content of P is set to 0.10% or less, preferably limited to 0.050% or less.

(Content S)

Sulfur (S) can be a factor that causes hot cracks during hot rolling and also causes breaks at the weld point of a spot welded part. Therefore, it is preferable to reduce the S content as much as possible. Accordingly, the content of S is set to 0.05% or less, preferably limited to 0.010%) or less.

(Al content)

Aluminum (A1) is an effective element as a deoxidizer in the steelmaking process, and to stabilize N, which causes deterioration during aging, in the form of AlN. However, excessive addition of Al to a level of more than 0.10% leads to an increase in production costs. Accordingly, the Al content is set to 0.10% or less, preferably limited to 0.050% or less.

(Content N)

Nitrogen (N) leads to deterioration during aging and also leads to an increase in yield strength (ratio of yield strength to tensile strength) and elongation corresponding to yield strength. Accordingly, the N content is set to 0.010% or less, preferably limited to 0.0050% or less.

(Cr content)

Chromium (Cr) is an effective element for obtaining the martensite phase as well as Mn and Mo. However, the inclusion of Cr in excess of 0.5% affects the properties of the coating. Accordingly, the Cr content is set to 0.5% or less, preferably 0.35% or less.

(Mo content)

Molybdenum (Mo) is an effective means of obtaining the martensite phase without compromising coating properties and is an important element in the present invention. This effect is achieved by a Mo content of 0.01% or more. However, the addition of Mo to more than 0.50% is unlikely to provide a higher effect, but leads to an increase in production costs. Accordingly, the Mo content is 0.01-0.50%, and preferably 0.02-0.35%.

(Content of Cu, Ni, V and B)

Copper (Cu), nickel (Ni), vanadium (V) and boron (B) are effective elements for hardening steel by hardening of a solid solution or the formation of low-temperature phase transformations, such as the martensite phase. This effect is achieved by the inclusion of at least one element selected from these elements in a given range of their contents of 0.001-1.0% for Cu, Ni and V and 0.0003-0.0050% for B. However, the addition of any of the amount of these elements in excess of the above range saturates their effect and increases the cost. Accordingly, when at least one element of Cu, Ni, V and B is added, their content is 0.001-1.0% for Cu, Ni and V and 0.0003-0.0050% for B.

(Ti content)

Titanium (Ti) is an effective element for stabilizing N, causing aging deterioration in the form of TiN. This effect is achieved by creating a Ti content of 0.005% or more. Meanwhile, the addition of Ti of more than 0.050%) creates excess TiC and thereby significantly increases the ratio of yield strength to tensile strength YR. Accordingly, the Ti content upon addition is set equal to 0.005-0.050%.

(Content of Ca and REM)

Calcium (Ca) and rare earth metals (REM) are effective elements for increasing the formability of the morphological control of sulfides. This effect is achieved by the inclusion of calcium and / or REM with a content of each element of 0.001%) or more. However, excessive addition of any element of Ca and REM to a level of more than 0.005% may impair the purity of the steel. Accordingly, when added, the content of each of calcium and / or REM is set equal to 0.001-0.005%.

The residue, which differs from the components whose contents are described above, consists of Fe and inevitable impurities. The inclusion of components other than those described above is not excluded until their content is detrimental to the effects of the present invention.

Next will be described the microstructure of the sheet

(Fraction of martensite area)

A decrease in the fraction of martensite area to less than 7% significantly increases the ratio of yield strength to tensile strength YR. At the same time, increasing the fraction of martensite area to 25% or more reduces local ductility and thus reduces the overall elongation of EL. Accordingly, the proportion of martensite is from 7 to less than 25%. The fraction of martensite area is preferably set equal to 7-22% and more preferably 7-20%.

(Share of ferrite area)

Reducing the fraction of the ferrite area to less than 50% significantly reduces the total elongation of EL. Accordingly, the area fraction of ferrite is 50% or more, preferably 60% or more.

Here, the fraction of the ferrite area is the fraction of the area occupied by the ferrite phase in the observed region, and the fraction of the martensite area is the fraction of the area occupied by the martensite phase in the observed region. The actual values of the area fraction of ferrite and the area fraction of martensite are determined as follows. The cross section in the thickness direction of the obtained steel sheet is polished and etched with a 3% nital solution. Then, the adjoining region located at a quarter of the sheet thickness is examined at a magnification of about 1500 using a scanning electron microscope (SEM), and the resulting image is analyzed using the above Image-Pro. In this way, the area fraction of each of the phases is determined. In the image, ferrite can be recognized as a structure (main structure) of gray color and martensite can be recognized as a structure of white color.

(The average crystalline grain size of martensite)

Reducing the average crystalline size of martensite to less than 1 μm significantly increases the ratio of yield strength to tensile strength YR. Meanwhile, increasing the average crystalline grain size of martensite to more than 8 μm reduces local ductility and, thus, reduces the overall elongation of EL. Accordingly, the average crystalline grain size of martensite is preferably 1-8 microns, and more preferably 2-7 microns.

The actual value of the average crystalline grain size of martensite is determined as follows. The resulting steel sheet is studied at a magnification of about 1500 using a scanning electron microscope (SEM), and the total martensite area in the area under consideration is divided by the number of martensite areas to determine the average martensite area. Then, a certain average area is raised to a power of 1/2 to obtain the average crystalline grain size of martensite.

(Lamellar structure)

The thickness of the plate structure is set such that it satisfies equation (1) below. This is because, as described above, if the thickness of the lamellar structure does not satisfy equation (1) below and the Tb / T value exceeds 0.005, martensite cannot be dispersed as necessary in a ferritic basis and thus the ratio of yield strength to YR tensile strength and lower TSxEL. In equation (1) below, Tb represents the average thickness of the plate structure in the direction of the sheet thickness and T represents the thickness of the resulting steel sheet.

$$\text{Tb / Ò}\le \text{0}\text{, 005}\text{(one)}$$

In some cases, the microstructure includes structures such as bainite, perlite, and residual austenite that are different from the above structure. However, the purpose of the present invention can be achieved if the above conditions for the microstructure are satisfied. It should be noted, however, that from the point of view of the ratio of yield strength to tensile strength, the content of perlite and residual austenite should preferably be reduced, the content of perlite should preferably be 8% or less, and the content of residual austenite should preferably be 3% or less.

In the present invention, the steel of the above chemical composition is provided by the microstructure described above, and thus high-strength, low-yield to tensile strength, dipped galvanized sheet steel is obtained, for example, distinguished by formability and coating properties. Next, a description will be given of methods for obtaining high strength with a low ratio of yield strength to tensile strength of dipped galvanized sheet steel, etc., such as described above.

First of all, in the case of the manufacture of dip galvanized steel sheet, the steel slab of the above chemical composition, obtained, for example, by a continuous casting process, is heated; then hot rolling of the heated steel slab is carried out in the temperature range corresponding to the finish rolling temperature of 850-950 ° C or lower and cooled with an average cooling rate of 7-60 ° C / s (hot rolling process); and then hot-rolled steel is wound at a temperature of 450-750 ° C (winding process). Then, the hot rolled sheet after decapitation and then descaling or the cold rolled sheet obtained by the cold rolling process after winding and decapitation is heated to 800 ° C or higher (annealing process) and then cooled to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, thus, the basis of the sheet for galvanizing. After this, galvanizing is carried out by immersion of the base of the sheet. This method yields high strength with a low ratio of yield strength to tensile strength by immersion galvanized sheet steel with excellent ductility and coating properties. In the case of manufacturing annealed galvanized dipped sheet steel, an additional alloying of the zinc coating is carried out after galvanizing by immersion. In this way, a high-strength with a low yield strength to tensile strength ratio annealed dipped galvanized sheet steel with excellent ductility and coating properties is obtained.

In the case of manufacturing a dip galvanized steel sheet by heating twice, the steel slab of the chemical composition described above is heated, followed by hot rolling of the heated steel slab in the temperature range corresponding to the finish rolling temperature of 850-950 ° C or lower, and cooled with an average cooling rate of 7 -60 ° C / s (hot rolling process); and then carry out the winding of chilled steel at a temperature of 450-750 ° C (winding process). Then, the hot-rolled sheet after decapitation and descaling, or the cold-rolled sheet obtained by the cold rolling process after winding and decapitation, is heated to 800 ° C or higher (pre-annealing process) and then cooling and decapitation processes are carried out, reheated to 750 ° C or higher ( main annealing process); and then the heated sheet is cooled to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, so that the base of the galvanized sheet is obtained. After this, galvanizing is carried out by immersion of the base of the sheet. In this way, high-strength with a low ratio of yield strength to tensile strength is obtained by immersion galvanized sheet steel with excellent ductility and coating properties. In the case of manufacturing annealed galvanized dipped sheet steel, an additional alloying of the zinc coating is carried out after galvanizing by immersion. In this way, a high-strength with a low yield strength to tensile strength ratio annealed dipped galvanized sheet steel with excellent ductility and coating properties is obtained.

Next will be described the temperature ranges and more in the respective processes.

(Hot rolling process)

The hot rolling process involves rough rolling and finishing rolling. After heating, the steel slab becomes a hot-rolled sheet by rough rolling and finishing rolling. When the heating temperature of the slab exceeds 1300 ° C, austenite significantly coarsens and, thus, the ductility decreases after annealing. Meanwhile, when the temperature of the slab is below 1150 ° C, defects such as bubbles and segregation in the surface layer of the slab can hardly be removed cracks, bumps, etc. on the surface of the steel sheet increase in size, which leads to a defective material in some cases. For this reason, the heating temperature of the slab is preferably 1150-1300 ° C. At a finish rolling temperature of more than 950 ° C, the austenite grain coarsens excessively and, thus, ductility decreases after annealing. Meanwhile, when the finish rolling temperature is lower than 850 ° C, the slab is rolled at an intercritical temperature, and thus an uneven voltage can be generated. For this reason, the formation of the plate structure is enhanced, and the plate structure remaining after annealing causes a decrease in ductility. Accordingly, the final temperature of hot rolling is set in the temperature range of 850-950 ° C.

At an average cooling rate after finishing rolling before winding in excess of 60 ° C / s, the shape of the sheet deteriorates significantly, which causes problems during cold rolling or annealing after cooling. Meanwhile, when the average cooling rate is less than 7 ° C / s, the formation of the plate structure increases significantly, and the plate structure remaining after annealing causes a drop in ductility. Accordingly, the average cooling rate from fine rolling to winding is set at 7-60 ° C / s, preferably 9-50 ° C / s.

(Winding process)

When the temperature during winding of the hot-rolled sheet after hot rolling exceeds 750 ° C, the thickness of the scale increases and the decapacity decreases. In addition, a situation arises in which the cooling rate after winding varies significantly from the leading end to the central part and the rear end in the longitudinal direction of the roll, or the cooling rate after winding varies significantly between the edges and the central part in the direction of the width of the roll. Thus, the change in the quality of the material increases. Accordingly, the winding temperature is set to 750 ° C or lower, and preferably 700 ° C or lower. Meanwhile, an excessively low winding temperature causes a deterioration in cold rolling. Accordingly, the winding temperature is limited to 450 ° C or higher.

(Decapitation and cold rolling processes)

In a subsequent decapitation process, black scale formed on the surface is removed. The decapitation conditions are not particularly limited. When carrying out cold rolling after decapitation, it is sufficient to use conventional methods and their conditions are not particularly limited.

(Pre-Annealing Process and Main Annealing Process)

In a manufacturing method in which the pre-annealing process and the main annealing process are carried out, the sheet metal is heated to a temperature of 800 ° C or higher (pre-annealing) and is cooled by galvanizing by immersion, and thus, as described above, C and Mn concentrated in a plate structure can be dispersed. In this way, the double phase of ferrite and martensite is effectively formed, and thus, ductility can be improved. In particular, the lamellar structure becomes thinner and finely dispersed upon heating during preliminary annealing, and thus, in the microstructure obtained ultimately after the completion of the main annealing process, the thickness of the lamellar structure can be reduced to a sufficiently small value to satisfy equation (1 ) above and the lamellar structure may be finely divided. In addition, when the steel sheet undergoes a dip galvanizing process on a continuous dip galvanizing line or, for example, a subsequent alloying process, the concentration of C and Mn in the austenite phase increases and, thus, the martensite phase can be dispersed as necessary in a ferrite base. This heating is preferably carried out on a continuous annealing line.

In addition, ductility can be improved by applying preliminary annealing. In particular, with a one-time heating of sheet metal to 800 ° C or higher and its cooling, recrystallization is facilitated, and the concentration of C and Mn in austenite is also facilitated, thereby creating the possibility of further improvement of formability. Although not explicitly indicated, the upper temperature limit is preferably about 950 ° C and lower in terms of performance. When the average cooling rate after heating in the preliminary annealing is less than 3 ° C / s, Mn is released again with the formation of a plate structure in some cases. For this reason, the average cooling rate after heating in the preliminary annealing is preferably 3 ° C / s or more, and more preferably 5 ° C / s or more.

When the preliminary annealing is carried out in accordance with the above-described conditions, it is sufficient that the heating temperature in the main annealing is 750 ° C or higher, because the plate structure is suppressed by preliminary annealing. However, at temperatures below 750 ° C austenite is not formed sufficiently and it is difficult to obtain the required amount of martensite. Accordingly, the heating temperature in the main annealing is 750 ° C or higher in the case of preliminary annealing. Although not explicitly stated, the upper temperature limit is preferably 850 ° C or lower, because exceeding the temperature of 850 ° C may allow elements such as Cr, Mn, Si and re-concentrate on the surface, and thus, the properties of the coating may deteriorate. The upper temperature limit is more preferably 825 ° C or lower.

At an average cooling rate after the main annealing in the temperature range 700-550 ° C less than 3 ° C / s, cooling produces ferrite and perlite excessively and, thus, the required amount of martensite is not obtained. Accordingly, the average cooling rate after the main annealing in the temperature range of 700-550 ° C is 3 ° C / s or more. Although not explicitly indicated, the upper limit of the average cooling rate is preferably about 100 ° C / s or less, because an average cooling rate in excess of 100 ° C / s can degrade the sheet shape due to rapid heat shrinkage and thus can cause technological problems. such as tortuosity.

In the case of preliminary annealing, elements that inhibit the properties of the coating, such as Cr, Si and Mn, are excessively concentrated on the surface during the main annealing, and thus degrade the properties of the coating. Therefore, the surface concentrated layer must be removed, for example, by decapitation after preliminary annealing. However, the removal of scale by decapitation after winding after hot rolling does not affect the effect of the present invention. In order to increase the productivity of the continuous galvanizing line by immersion, performing the post-process, training can be performed in the period after preliminary annealing and before decapitation.

The dip galvanizing process is preferably carried out by immersing the steel sheet obtained by the above processes in a zinc bath at a temperature of 440-500 ° C, followed by adjusting the coating weight, for example, by gas removal. In addition, when doping a zinc coating, the zinc coating is preferably alloyed at a temperature of 460-580 ° C for 1-40 seconds. It is preferable to use a zinc bath containing 0.08-0.18% of the mass. Al for galvanizing.

Steel sheet training can be applied after galvanizing by immersion or after additional alloying of the zinc coating with the aim, for example, of straightening the shape or adjusting the surface roughness. Various types of coating treatments can also be applied, such as applying resin or oil.

(Annealing process)

In the manufacturing method in which the annealing process is carried out, the steel sheet is heated to a high temperature of 800 ° C or higher. Setting an annealing temperature of 800 ° C or higher eliminates the segregation of C and elements that form a solid substitution solution, such as Mn, which are concentrated in the plate state, and thus the plate structure is suppressed, resulting in improved formability. After heating, the steel sheet is cooled to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher. At an average cooling rate after heating to a temperature of 700-550 ° C less than 3 ° C / s, ferrite and perlite are excessively formed upon cooling, and thus the required amount of martensite is not obtained. Accordingly, the average cooling rate after heating to a temperature of 700-550 ° C is set equal to 3 ° C / s and higher. Although not explicitly stated, the upper limit of the average cooling rate is preferably about 100 ° C / s or less, because an average cooling rate in excess of 100 ° C / s can degrade sheet shape due to rapid heat shrinkage and thus can cause technological problems such as tortuosity. It should be noted that the conditions for galvanizing by immersion and alloying are the same as described above.

In the annealing process described above, the same effect can be obtained as in the case of preliminary annealing and basic annealing described above, although with a single heating.

(Examples)

Table 1 shows the chemical composition (in mass%) of steel examples of the present invention and comparative examples serving as samples. Table 2 presents the manufacturing conditions of the examples of the present invention and comparative examples. It should be noted that in the “Present Structures (Type)” paragraphs of Table 2, “B” stands for bainite, “P” stands for perlite, “TM” stands for tempering martensite, and “γ” stands for residual austenite.

Table 1 Steel Chemical composition (% wt.) Note FROM Si Mn P S Al N Cr Mo V Ni Si Ti AT Ca Rem BUT 0,085 0.01 1.93 0.011 0.002 0,044 0.0033 0.01 0.15 - - - - - - - In the scope of the invention AT 0.114 0,03 2.11 0.007 0.002 0,063 0.0038 0.02 0.20 0.02 - - 0.02 - - - In the scope of the invention FROM 0,067 0.02 1,60 0.014 0.004 0,035 0.0025 0.01 0.14 - - - - - - - In the scope of the invention D 0,057 0.02 2,39 0,021 0.001 0,034 0.0033 0.01 0.08 - 0.1 - - - - - In the scope of the invention E 0.058 0.04 2.03 0.015 0.007 0,038 0.0034 0.02 0.12 - - - - - - - In the scope of the invention F 0,044 0.01 1.88 0.019 0.003 0,041 0.0022 0,03 0.16 - - 0.1 - - - - In the scope of the invention G 0,076 0.02 1,52 0.008 0.002 0,033 0.0029 0.01 0.19 0,03 - - - - - - In the scope of the invention N 0,036 0.02 1.81 0.012 0.004 0,045 0.0032 0.01 0.19 - - - - 0.003 - - In the scope of the invention I 0.151 0.05 1,53 0.012 0.001 0,044 0.0032 0,03 0.17 0.02 - - 0.04 - - - In the scope of the invention J 0,074 0.02 1.51 0.008 0.001 0,031 0.0036 0.02 - - - - - - - - Beyond the scope of the invention TO 0.008 0.01 1,57 0.010 0.006 0,029 0.0027 0,03 0.08 - - - - - - - Beyond the scope of the invention L 0,069 0.01 0.73 0.014 0.004 0.024 0.0029 0.01 0.21 - - - - - - - Beyond the scope of the invention M 0,089 0,03 1.71 0.135 0.001 0.051 0.0035 0,03 0.09 - - - - - - - Beyond the scope of the invention N 0,071 0.33 2.00 0.005 0.005 0,035 0.0036 0.32 0.11 - - - - - 0.001 - In the scope of the invention 0 0.058 0.22 1,64 0.013 0.006 0,060 0.0028 0.49 0.02 - - - - - - 0.001 In the scope of the invention R 0,061 0.30 1.80 0,025 0.005 0,041 0.0031 0.85 0.02 - - - - - - - Beyond the scope of the invention Q 0.059 1.21 1.75 0,029 0.004 0,034 0.0034 0.31 0.01 - - - - - - - Beyond the scope of the invention

In the present example, each slab obtained by continuous casting of the chemical composition shown in table 1 and a thickness of 220 mm is heated to 1200 ° C with the manufacturing conditions shown in table 2, and after rough rolling in two passes, the hot-rolled coil 2 is wound, 3 mm thick on a finish mill with seven stands. Then, after subsequent decapitation, a portion of the hot-rolled coil is cold rolled to a thickness of 1.0 mm. After that, part of each hot-rolled coil and cold-rolled coil thus obtained is annealed under the conditions given in table 2 on the continuous annealing line, and after decapitation, they are annealed (main annealed) and galvanized, as well as alloying on the continuous galvanizing line by immersion. Some steel sheets are not preliminarily annealed, but subjected to annealing and galvanizing, as well as alloying under the conditions given in table 2 on the continuous galvanizing line by immersion. Here, in the process of galvanizing, steel sheets are immersed in a zinc bath at a temperature of 460 ° C and a two-side coating is applied with a coating weight of 35-45 g / m. In the case of alloying, the zinc coating is alloyed at a temperature of 480-540 ° C and the steel sheets are cooled to room temperature with an average cooling rate of 10 ° C / s after galvanizing or alloying.

Each of the steel sheets obtained as described above is used as a sample for evaluating the mechanical properties, coating properties, suitability for alloying and spot weldability. The results are presented in table 3.

Table 3 Galvanized Steel Sheet No. Tb / T YS (MPa) TS (MPa) EL (%) YR (%) TS × EL (MPa%) Coating properties Alloying Spot weldability Note one 0.003 372 600 32 62 19200 Excellent Excellent Excellent Example 2 0.008 418 588 26 71 15288 Good ones Good ones Excellent Comp. example 3 0.007 418 572 27 73 15444 Excellent Excellent Excellent Comp. example four 0.003 404 493 28 82 13804 Good ones Good ones Excellent Comp. example 5 0.005 389 570 29th 68 16530 Excellent Excellent Excellent Example 6 0.003 378 605 thirty 62 18150 Excellent Excellent Excellent Example 7 0.003 346 591 33 59 19503 Excellent Excellent Excellent Example 8 0.006 425 599 25 71 14975 Excellent Excellent Excellent Comp. example 9 0.008 459 611 24 75 14664 Excel. Excel. Excellent Comp. example 10 0.003 370 630 thirty 59 18900 Excellent - Excellent Example eleven 0.002 342 595 31 57 18445 Excellent - Excellent Example 12 0.002 432 541 27 80 14607 Excellent - Excellent Comp. example 13 0.005 344 590 32 58 18880 Good ones Excellent Excellent Example fourteen 0.007 435 596 25 73 14900 Good ones Excellent Excellent Comp. example fifteen 0.004 365 561 31 65 17391 Excellent - Excellent Example 16 0.004 332 544 32 61 17408 Good ones Good ones Excellent Example 17 0.004 433 513 22 84 11286 Excellent - Excellent Comp. example eighteen 0.003 330 594 31 56 18414 Excellent - Excellent Example 19 0.003 335 602 31 56 18662 Excellent - Excellent Example twenty 0.002 355 637 29th 56 18473 Excellent Excellent Excellent Example 21 0.005 428 662 26 65 17212 Excellent - Excellent Example 22 0.001 424 580 28 73 16240 Excellent Excellent Excellent Comp. example 23 0.001 366 480 28 76 13440 Excellent - Excellent Comp. example 24 0.003 349 488 26 72 12688 Excellent Excellent Excellent Comp. example 25 0.002 441 638 25 69 15950 Good ones Good ones The bad Comp. example 26 0.004 401 652 32 62 20864 Good ones - Excellent Example 27 0.002 353 639 thirty 55 19170 Good ones - Excellent Example 28 0.003 368 619 31 59 19189 The bad Good ones Excellent Comp. example 29th 0.002 395 679 32 58 21728 The bad The bad Excellent Comp. example

The mechanical properties determined by the tensile test are evaluated in accordance with JIS Z 2241 (2011). The yield strength YS, the tensile strength TS and the total elongation EL are measured and the ratio of the yield strength to the tensile strength YR TSxEL (balance TSxEL) is determined. The structure of the steel sheet was studied by the same method as described above, and thus obtained the fraction of the area of ferrite, the fraction of the area of martensite and the average crystalline grain size of martensite. In addition, measure the average thickness of the plate structure in the direction of the sheet thickness and determine the ratio Tb / T of the average thickness Tb of the plate structure to the thickness of the sheet T.

Coating properties are rated as “excellent” when there are no areas without coverage, as “good” when there are small areas without coverage, but their number is acceptable, and as “bad” when there are many areas without coverage, and are visually evaluated. Suitability for doping is assessed as “excellent” when there is no uneven doping, as “good” when there is slightly uneven doping, but its level is acceptable, and as “bad” when there is significant unevenness in doping, and is evaluated visually. Spot weldability is evaluated by testing tensile shear strength of a welded joint in accordance with JIS Z 3136 (1999). Assuming a tensile shear strength of 6700 N as a lower tensile strength, spot weldability is rated “excellent” when the tensile shear strength is equal to or greater than the lower tensile strength, and “poor” when the shear strength is lower than lower tensile strength.

As shown in table 3, examples of steels of the present invention are evaluated as follows. The ratio of yield strength to tensile strength YR is low; TSxEL balance is good and coating properties, alloyability and spot weldability are also good.

As described above, in accordance with the present invention, the content of elements such as Mn and the manufacturing conditions are controlled and thus the lamellar structure can be thinned to the desired thickness and obtain fine. Thus, martensite can be dispersed to the necessary degree in a ferrite base. In addition, for example, the fraction of the martensite area can be changed accordingly. Accordingly, when applying galvanizing by immersion or even when alloying is additionally applied, on a plant such as a continuous galvanizing line by immersion, it is possible to stably produce high-strength with a low ratio of yield strength to tensile strength, galvanized by immersion galvanized steel sheet and high-strength with a low ratio of yield strength to tensile strength annealed dip galvanized steel sheet, each of which has high strength and low tensile strength ratio the yield strength and is distinguished by the formability and coating properties, and it is also possible to stably apply a method of manufacturing a high-strength low yield strength to the tensile strength galvanized by dipping galvanized steel sheet and a method of manufacturing a high-strength low yield strength ratio to the tensile strength of galvanized dipped galvanized steel sheet. In particular, it is possible to obtain high strength with a low ratio of yield strength to tensile strength by immersion galvanized steel sheet, etc., which has good coating properties, while satisfying, as an indicator representing strength, the ratio of yield strength to tensile strength and ductility , the condition that the ratio of yield strength to tensile strength YR is 70% or less and TSxEL is 16000 MPa ·% or more.

The present invention is particularly suitable for use as an automotive steel sheet used, for example, for interior panels and exterior panels of a car body. The use of the present invention for automotive sheet steel can lead to weight loss and hardening of the structural elements of the vehicle, and can lead to the global conservation of nature by improving fuel consumption and ensuring the safety of the driver and passengers.

The implementation of the present invention has been described above. However, the present invention is not limited to the description of the constituent parts of the disclosure of the present invention by implementation. That is, other implementations, examples, process steps, and the like that are made based on the present embodiment by those skilled in the art are included in the scope of the present invention. For example, in a series of heat treatments in the manufacturing method described above, it is sufficient that conditions are met with respect to, for example, the temperature range in the respective processes and the means for performing heat treatment, for example, are not particularly limited.

Claims (10)

1. High strength with a yield strength to tensile strength ratio of 0.7 or less, immersion galvanized steel sheet having:
chemical composition, in wt.%: 0.03-0.20 C, 1.0 or less Si, more than 1.5 to 3.0 Mn, 0.10 or less P, 0.05 or less S, 0 10 or less Al, 0.010 or less N, 0.5 or less Cr and 0.01-0.50 Mo and the rest Fe with inevitable impurities, and
a structure comprising ferrite and a secondary phase as a microstructure, in which the fraction of the ferrite area is 50% or more and the fraction of the secondary phase area including martensite is from 7 to less than 25% and the thickness of the lamellar structure formed by the secondary phase satisfies the following equation (1 ):
$$\text{Tb / Ò}\le \text{0}\text{, 005}\text{(one)}$$

,
where Tb is the average thickness of the lamellar structure in the direction of the thickness of the sheet and T is the thickness of the sheet, and the average crystalline grain size of martensite is 1-8 μm. 2. The steel sheet according to claim 1, which further includes in% wt .: at least one element selected from 0.001-1.0% Cu, 0.001-1.0% Ni, 0.001-1.0% V, and 0.0003-0.0050% B. 3. A steel sheet according to any one of claims 1 or 2, which further comprises 0.005-0.050 wt.% Ti. 4. The steel sheet according to any one of claims 1 or 2, which additionally includes in wt%: 0.001-0.005 Ca and / or 0.001-0.005 REM. 5. The steel sheet according to claim 3, which further includes in% wt .: 0.001-0.005 Ca and / or 0.001-0.005 REM. 6. High strength with a yield strength to tensile strength ratio of 0.7 or less, dipped galvanized steel sheet, in which the dipped galvanized steel sheet according to any one of paragraphs. 1-5 are annealed, and the zinc coating is a doped zinc coating. 7. A method of manufacturing high strength with a yield strength to tensile strength ratio of 0.7 or less galvanized by immersion of a steel sheet, including:
heating a steel slab having the composition specified in any one of claims 1 to 5;
hot rolling of a steel slab heated to a final temperature of 850-950 ° C;
cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s;
winding chilled steel at a temperature of 450-750 ° C;
heating the resulting hot-rolled sheet or cold-rolled sheet obtained by cold rolling after winding to a temperature of 800 ° C or higher;
cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, and
galvanizing of the cooled sheet by immersion. 8. A method of manufacturing high strength with a yield strength to tensile strength ratio of 0.7 or less galvanized by immersion of a steel sheet, including:
heating a steel slab having a composition in% wt .: 0.03-0.20 C, 1.0 or less Si, more than 1.5 to 3.0 Mn, 0.10 or less P, 0.05 or less than S, 0.10 or less Al, 0.010 or less N, 0.5 or less Cr and 0.01-0.50 Mo and the rest Fe with inevitable impurities;
hot rolling of a steel slab heated to a final temperature of 850-950 ° C;
cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s;
winding chilled steel at a temperature of 450-750 ° C;
heating the resulting hot-rolled sheet or cold-rolled sheet obtained by cold rolling after winding to a temperature of 800 ° C or higher;
reheating the sheet to 750 ° C or higher after undergoing cooling and decapitation;
cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, and
galvanizing of the cooled sheet by immersion. 9. A method of manufacturing high strength with a ratio of yield strength to tensile strength of 0.7 or less, galvanized by immersion with annealing of a steel sheet, including:
heating a steel slab having the composition specified in any one of claims 1 to 5;
hot rolling of a steel slab heated to a final temperature of 850-950 ° C;
cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s;
winding chilled steel at a temperature of 450-750 ° C;
heating the resulting hot-rolled sheet or cold-rolled sheet obtained by cold rolling after winding to 800 ° C or higher;
cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, and
dip galvanizing to create a zinc coating on the sheet, and
alloying of a zinc coating. 10. A method of manufacturing high strength with a ratio of yield strength to tensile strength of 0.7 or less, galvanized by immersion with annealing of a steel sheet, including:
heating a steel slab having a composition in% wt .: 0.03-0.20 C, 1.0 or less Si, more than 1.5 to 3.0 Mn, 0.10 or less P, 0.05 or less than S, 0.10 or less Al, 0.010 or less N, 0.5 or less Cr and 0.01-0.50 Mo and the rest Fe with inevitable impurities;
hot rolling of a steel slab heated to a final temperature of 850-950 ° C;
cooling of hot rolled steel with an average cooling rate of 7-60 ° C / s;
winding chilled steel at a temperature of 450-750 ° C;
heating the resulting hot-rolled sheet or cold-rolled sheet obtained by cold rolling after winding to a temperature of 800 ° C or higher;
reheating the sheet to 750 ° C or higher after undergoing cooling and decapitation;
cooling the heated sheet to a temperature of 700-550 ° C with an average cooling rate of 3 ° C / s or higher, and
dip galvanizing to create a zinc coating on the sheet, and
alloying of a zinc coating.