CN117165845A - 340 MPa-level alloying hot dip galvanized sheet for new energy automobile and preparation method thereof - Google Patents

Abstract

The invention provides a 340MPa grade alloyed hot-dip galvanized sheet for new energy vehicles and a preparation method thereof. The composition of the galvanized sheet substrate is as follows in weight percentage: C: 0.005% to 0.007%, Si: 0.012% to 0.017% , Mn: 0.22% ~ 0.34%, P: 0.03% ~ 0.05%, Nb: 0.04% ~ 0.07%, Ti: 0.003% ~ 0.005%, N ≤ 0.003%, S ≤ 0.005%, the balance is Fe and unavoidable Impurities, preferably, C-Nb/7.736 in the hot-dip galvanized outer panel is -5 to 15 ppm. The preparation method includes smelting, casting, hot rolling, pickling, cold rolling, and hot-dip galvanizing; the alloyed hot-dip galvanized sheet produced by the present invention has excellent comprehensive mechanical properties and room temperature resistance compared with 340-grade BH steel and IF steel. Timeliness performance, etc., have obvious performance advantages.

Description

340MPa grade alloyed hot-dip galvanized sheet for new energy vehicles and preparation method thereof

Technical field

The invention belongs to the field of metal materials, and in particular relates to a 340MPa grade alloyed hot-dip galvanized sheet for new energy vehicles and a preparation method thereof.

Background technique

As the automotive panel that uses the largest amount of steel in the entire vehicle, it requires the steel plate to have both excellent formability and good dent resistance, and it should also have good resistance to natural aging. However, the improvement of steel formability is often in conflict with the improvement of strength, and at the same time results in a decrease in dent resistance. At present, the most important types of steel for automobile sheets are interstitial-free (IF) steel and bake-hardened (BH) steel. IF steel has excellent formability, but its dent resistance is poor. BH steel has low yield strength during stamping and exhibits excellent formability. After high-temperature painting and baking, the strength of the steel plate is improved and it has good dent resistance, but it has certain aging problems. For this reason, it is necessary to develop high-strength automotive sheets with excellent formability, dent resistance and natural aging resistance.

The chemical composition of the steel plate disclosed in the invention "A high tensile strength substrate, hot-dip galvanized automobile plate and its manufacturing method" (CN101353755A) is (wt%): C: 0.01~0.08%, Si: ≤0.1%, Mn: 0.8～1.8%, Cr: ≤1.0%, Mo: ≤0.5%, T.Al: 0.02～0.08%, N: ≤0.006%, P+2S: ≤0.12%, 1.6%≤Mn+3Cr+2Mo≤3.8 %, Fe: balance. The composition design of this steel plate uses high carbon and adds precious metal elements such as Mn, Cr and Mo, which greatly increases the alloy cost. At the same time, its structure is the original structure of pearlite + ferrite, with high strength, but low elongation (32%-36%), which is not conducive to the stamping forming of automobile panels.

The invention "A kind of alloyed hot-dip galvanized steel with a tensile strength of 390MPa and its production method for automobiles" (CN201510455209.1) discloses the elemental composition (wt%) of the steel plate: C: 0.07~0.10%, Si: ≤ 0.03%, Mn: 0.7~1.0%, P: 0.010~0.025%, S≤0.010%, Als: 0.020~0.070%; the composition design of this steel plate has a high carbon content, which easily causes excessive solid solution C in the steel plate It affects the natural aging resistance of the steel plate, and adding more solid solution strengthening element Mn is not conducive to improving the formability of the steel plate.

Contents of the invention

The purpose of the present invention is to overcome the above problems and deficiencies and provide a 340MPa grade alloyed hot-dip galvanized sheet for new energy vehicles with excellent formability, dent resistance and natural aging resistance and a preparation method thereof.

The purpose of the present invention is achieved as follows:

A 340MPa grade alloyed hot-dip galvanized sheet for new energy vehicles. The composition of the galvanized sheet substrate is as follows in weight percentage: C: 0.005% ~ 0.007%, Si: 0.012% ~ 0.017%, Mn: 0.22% ~ 0.34% , P: 0.03% ~ 0.05%, Nb: 0.04% ~ 0.07%, Ti: 0.003% ~ 0.005%, N ≤ 0.003%, S ≤ 0.005%, the balance is Fe and inevitable impurities, preferably, the heat C-Nb/7.736 in galvanized sheets is -5~15ppm.

The microstructure of the hot-dip galvanized sheet substrate is critical zone ferrite, oriented epitaxial ferrite, diffusely distributed Nb(C,N) and TiN, and the volume percentage content of each microstructure is critical zone iron. 80% to 90% of prime body and 10% to 20% of oriented attached ferrite. Preferably, the hardness of critical zone ferrite is 160-180HV, and the hardness of orientation-attached ferrite is 190-210HV.

The hot-dip galvanized sheet has a yield strength of 220-260MPa, a tensile strength of 340MPa or more, an elongation after fracture of more than 40%, a plastic strain ratio r of 2.2-2.4, a strain hardening index n of 0.25 or more, and a bake hardening value BH2 of 40-40%. 60MPa.

The iron mass percentage of the hot-dip galvanized sheet coating is 8% to 10%.

The reasons for the component design of the present invention are as follows:

C: C is the most important additive element in the present invention, and it plays a vital role in the bake hardening value and aging resistance of the steel plate. When the C content increases, pearlite is easily generated in the steel and it is difficult to control the shape and distribution of pearlite, resulting in low elongation, r-value and n-value of the steel and excessive C content in the finished steel, causing aging problems. In addition, when the C element content is too low, the cost of steelmaking increases, and a certain amount of Nb-containing carbides cannot be guaranteed to be formed in the steel, causing the strength of the steel to be too low, and the bake hardening value of the steel plate to be too low. Therefore, in order to ensure the comprehensive mechanical properties, aging resistance and bake hardening value of the steel plate between 30-60MPa, the present invention requires that the C element content be 0.005% ~ 0.007% and C-Nb/7.736 be between -5 ~ 15ppm between.

Si: Si element is one of the important elements in the present invention and plays the role of solid solution strengthening. However, too high content of Si will reduce the plastic toughness of steel and seriously affect the surface quality of galvanized sheets. Therefore, the present invention requires the Si content to be 0.012% to 0.017%.

Mn: Mn element is one of the important elements in the present invention and plays the role of solid solution strengthening. However, when the added amount of Mn element is too high, the strength of the steel plate will be too high, affecting its formability. Therefore, the present invention requires the Mn content to be 0.22% to 0.34%.

P: P element is one of the important elements in the present invention and plays the role of solid solution strengthening. However, P is easily segregated at grain boundaries. When the P content is too high, the embrittlement performance of the steel plate will be reduced during secondary processing. At the same time, excessive P element makes the strength of the steel plate too high and affects its formability. Therefore, the present invention requires the P element content to be 0.03% to 0.05%.

Nb: Nb element is one of the important elements in the present invention. Nb mainly exists in the form of solid solution and precipitates in steel. The dragging effect of solid solution Nb atoms and the pinning effect of fine Nb (C, N) precipitation on the relative interface can affect the recrystallization of austenite. This behavior refines the final ferrite grains and improves the strength of the steel plate. In addition, the precipitation of Nb(C,N) can regulate the solid solution C content of the steel plate, thereby affecting the bake hardening performance of the steel plate. Therefore, when adding less than 0.05% Nb, the strength of the steel plate cannot be guaranteed and the bake hardening value is likely to be too high. At the same time, adding excessive Nb element results in poor bake hardening performance of the steel plate and increases production costs. Therefore, the present invention requires the Nb content to be 0.04% to 0.06%, and C-Nb/7.736 to be between -5 to 15ppm.

Ti: Ti element is one of the important elements in the present invention. Ti and the impurity element N in the steel form TiN. TiN is the most stable compound in the steel and can precipitate in the molten steel, so TiN can play the role of solidifying N. However, when the Ti content is too high, the TiN size will be too large and the performance of the steel plate will be deteriorated. At the same time, excessive Ti content will precipitate FeTiP in the steel, preventing the steel plate from forming a favorable texture during the recrystallization process and reducing the r value of the steel plate. In addition, the precipitation of this compound affects the solid solution strengthening effect of P and reduces the strength of the steel plate. Therefore, the present invention requires the Ti content to be 0.003% to 0.005%.

N: N is a magazine element in steel, which affects the aging resistance of the steel plate and is not conducive to the improvement of the r value. Therefore, the present invention requires that the N element content is ≤0.003%.

S: S is an impurity element in steel. It easily reacts with Mn to form MnS, which deteriorates the performance of the steel plate, so the lower its content, the better. Therefore, the present invention requires that the S element content is ≤0.005%.

The second technical solution of the present invention is to provide a method for preparing a 340MPa grade alloyed hot-dip galvanized sheet for new energy vehicles, including smelting, casting, hot rolling, pickling, cold rolling, and hot-dip galvanizing;

1. Smelting and casting: Smelting, refining and casting into billet according to the above chemical composition.

Middle bag temperature: The middle bag temperature is set at 1500~1600℃ to ensure uniform structure and composition.

2. Hot rolling: heating temperature is between 1250~1280℃, furnace time is >120min, opening rolling temperature is between 1060~1120℃, final rolling temperature is between 893~924℃, coiling temperature is between 400~490 between ℃. The thickness of hot-rolled coils is between 3 and 4mm.

Heating temperature: Ti element is added to steel to fix N element, so in order to achieve a more ideal N fixation effect, the heating temperature is usually controlled above 1250°C. At the same time, this temperature ensures that the Nb-containing compound Nb(C,N) does not precipitate in the steel, which is conducive to uniform grain growth, and allows Ti(C,N) to precipitate in the subsequent rolling stage of the recrystallization zone to pin the grain boundaries. , the effect of refining the original austenite grains. In addition, if the heating temperature is too high, the grains will grow too much and the effect of grain refinement will not be achieved, and energy will be wasted. The heating time is longer than 120 minutes to ensure uniform distribution of alloy elements. Therefore, the heating temperature is controlled at 1250~1280℃, and the heating time is >120min.

Final rolling temperature: The final rolling temperature is controlled between 893 and 924°C. This temperature is higher than Ar3, which can ensure that deformation bands are formed within the austenite grains during rolling, providing more space for the subsequent ferrite phase transformation. nucleation points to refine the grains.

Coiling temperature: When the coiling temperature is low, the supersaturation of the alloy elements in the matrix phase is large, which is conducive to precipitation, but low temperature is not conducive to the diffusion of elements. While higher temperatures are conducive to the diffusion of elements, the driving force for precipitation is low. . Therefore, the curling temperature is strongly related to the degree of carbonitride precipitation, which has a direct impact on the final solid solution C content and BH value in the steel. In addition, the curling temperature is too high and there is too much iron scale on the surface of the steel plate, which increases the cost of phosphorus removal and affects the surface quality of the steel plate. At the same time, low-temperature coiling is used to effectively suppress laminar cooling or dislocation recovery that occurs from final rolling to coiling, promote more dislocations to be retained inside the matrix, improve deformation energy storage, and promote recrystallization nucleation in subsequent annealing. Therefore, according to the relationship between the element content of C and Nb, the present invention stipulates that the curling temperature is when C-Nb/7.736 is between -5 to 5ppm, the curling temperature is controlled at 400 to 450°C; when C-Nb/7.736 is between 5 to 15ppm When between, excluding 5ppm, the curling temperature is controlled at 450~490℃.

3. Pickling: Pickling is used to remove the iron scale oxide present on the steel surface after hot rolling and coiling.

4. Cold rolling: The cold-rolled product specifications are maintained at 0.6~0.8mm thick, corresponding to the target automotive panel thickness corresponding to the invented product, and the rolling reduction rate is controlled at 80~85%. The stored energy of cold rolling deformation is the driving force for annealing recrystallization. Sufficient reduction rate can ensure the effect of ferrite recrystallization and refine the ferrite grains. At the same time, as the cold rolling reduction increases, a strong γ fiber texture with texture components of {111}<110> and {111}<112> is mainly formed in the steel plate. The grains with this orientation have large The stored energy is easy to grow during the subsequent annealing process, so that the steel plate forms a strong {111}//ND texture, which is beneficial to the improvement of deep drawing performance. However, excessive rolling reduction increases the load on the cold rolling mill and cannot guarantee the realization of the target thickness.

5. Hot-dip galvanizing: The annealing temperature before hot-dip galvanizing is 865~893℃, and the annealing time is 30~60s; then, the steel plate is slowly cooled to 785~813℃ at a cooling rate of 10~16℃/s, and then ≥ The cooling rate of 30℃/s is reduced to 440~460℃; then the steel plate is heated to 450~460℃ and then enters the zinc pot. The galvanizing process takes 1~3 seconds, and then the steel plate enters the alloying furnace for alloying treatment. The alloying temperature The temperature is 473~530℃, and the alloying time is 18-23s; finally, the steel plate is cooled to room temperature. The key parameters are described as follows:

Annealing temperature before hot-dip galvanizing: The annealing stage can form a small amount of austenite in the steel to introduce oriented attached ferrite during the slow cooling stage, and at the same time regulate the solid solution C content in the final steel through the redissolution of carbides. If the temperature is too high, the degree of carbide dissolution will increase the solid solution C content in the steel, causing the BH value to be too high and causing aging problems. At the same time, too high annealing temperature reduces the strength of the steel plate and affects the surface quality of the steel plate. The annealing temperature is too low and the degree of carbide dissolution is low, making the final BH value too low. Therefore, according to the relationship between the element content of C and Nb in the steel, the heating temperature is controlled at 880-893°C when C-Nb/7.736 is between -5 and 5 ppm; when C-Nb/7.736 is between 5 and 15 ppm time, excluding 5ppm, and the heating temperature is controlled at 865~880℃.

Annealing time before hot-dip galvanizing: In order to ensure that the steel plate can adjust the solid solution C content in the steel through the dissolution of Nb (C, N) at a certain annealing temperature. If the time is too long, the solid solution C content will be too high and the ferrite will be reduced. The size of the recrystallized grains is uneven and the time is too short, resulting in too low solid solution C content. Therefore, the present invention requires the annealing time before hot-dip galvanizing to be 30 to 60 seconds.

Cooling rate: In the slow cooling stage, the cooling rate is 10-16°C/s to 785-813°C, which can form oriented attached ferrite in the steel plate and improve the strength of the steel plate. Finally, rapid cooling at ≥30°C/s can ensure that a certain amount of solid solution C content is retained in the steel. Therefore, the present invention requires that the cooling rates of the slow cooling stage and the rapid cooling stage are 10-16°C/s and ≥30°C/s respectively.

Temperature of the steel plate before galvanizing: The temperature of the steel plate before galvanizing is 450~460℃ to ensure that the steel plate will not condense or drag zinc during the subsequent galvanizing process.

Alloying annealing temperature: The alloying annealing process is a process in which the iron content in the coating continues to increase under a constant temperature environment. Research shows that when the iron content on the surface of the coating reaches 9%, the alloying process is completed. At this time, the ideal dense δ phase (FeZn10) is present in the coating. Therefore, the present invention requires that the iron content of the coating be controlled at 8%-10%. In order to meet this requirement, the annealing temperature is controlled at 473~530℃. When the annealing temperature exceeds 530°C, Fe3Zn10 (Γ phase) or Fe5Zn21 (Γ1 phase) is easily formed in the coating, which deteriorates the powder resistance of the coating and the surface quality of the galvanized sheet. In addition, the annealing temperature is too low to effectively complete alloying. Therefore, the annealing temperature is controlled to 473~530°C.

Alloying annealing time: The annealing time is less than 18s, and the iron content in the coating is too low to form sufficient delta phase. If the annealing time is too long, a medium Γ1 phase will be formed in the coating, which will deteriorate the powder resistance of the steel plate and affect the surface quality of the steel plate. Therefore, the annealing time is controlled at 18-23s.

The final structure consists of: 80-90% critical zone ferrite (160-180HV) + 10-20% oriented epitaxial ferrite (190-210HV) + diffusely distributed Nb (C, N) and TiN.

Through the above method, the alloyed hot-dip galvanized bake-hardened steel plate can be obtained with a yield strength between 220-260MPa, a tensile strength of more than 340MPa, an elongation after fracture of more than 40%, a plastic strain ratio r of 2.2-2.4, and a strain hardening index. n is above 0.25, and the baking hardening value BH2 is between 40-60MPa.

The beneficial effects of the present invention are:

(1) The 340-grade automotive sheet involved in the present invention is designed with a microcarbon (0.005% to 0.007%) component. Compared with ultra-low carbon BH steel, it reduces the cost of steelmaking and does not add Mo or Cr. , V, Sb, Pb and other high-cost alloy elements, so the alloy cost is effectively controlled.

(2) The present invention designs different process systems based on the addition amounts of C and Nb. By coupling the curling and annealing systems, the final solid solution C content in the steel is effectively controlled to ensure the bake hardening performance and room temperature aging resistance of the steel plate.

(3) The grade 340 alloyed hot-dip galvanized sheet involved in the present invention introduces oriented attached ferrite and dispersed precipitate phases on the critical zone ferrite base, so that the yield strength is between 220-260MPa and the tensile strength is The strength is above 340MPa, the elongation after fracture is above 40%, the plastic strain ratio r is 2.2-2.4, the strain hardening index n is above 0.25, and the bake hardening value BH2 is between 40-60MPa. Compared with grade 340 BH steel and IF steel, it has excellent comprehensive mechanical properties and room temperature aging resistance, and has obvious performance advantages.

Detailed ways

The present invention will be further described below through examples.

The embodiment of the present invention performs smelting, casting, hot rolling, pickling, cold rolling, and hot-dip galvanizing according to the component ratio of the technical solution;

(1) Hot rolling:

The heating temperature is 1250~1280℃, the furnace time is >120min, the opening rolling temperature is 1060~1120℃, the final rolling temperature is 893~924℃, the coiling temperature is 400~490℃; the thickness of the hot rolled coil is 3~4mm;

(2) Cold rolling: The rolling reduction is controlled at 80% ~ 85%; the thickness of the cold rolled steel plate is 0.6 ~ 0.8mm.

(3) Hot dip galvanizing:

a) Heating the cold-rolled steel plate to 865~893℃, isothermal time 30~60s; then slowly cooling the steel plate to 785~813℃ at a cooling rate of 10~16℃/s, and then cooling at a cooling rate of ≥30℃/s Drop to 440~460℃;

b) The steel plate is heated to 450~460℃ and then enters the zinc pot. The galvanizing process takes 1~3 seconds.

c) Alloying treatment of the steel plate, the alloying temperature is 473~530℃, the alloying time is 18-23s; finally, the steel plate is cooled to room temperature.

Further; during the coiling process,

When C-Nb/7.736 is -5~5ppm, the curling temperature is controlled at 400~450℃;

When C-Nb/7.736 is between 5 and 15 ppm, excluding 5 ppm, the curling temperature is controlled at 450 to 490°C.

Further; during the hot-dip galvanizing process,

When C-Nb/7.736 is -5~5ppm, the heating temperature is controlled at 880~893℃;

When C-Nb/7.736 is between 5 and 15 ppm, excluding 5 ppm, the heating temperature is controlled at 865 to 880°C.

The composition of the steel in the embodiment of the present invention is shown in Table 1. The main process parameters of steel continuous casting, hot rolling and cold rolling according to the embodiment of the present invention are shown in Table 2. The main process parameters of cold rolling and annealing of steel according to the embodiment of the present invention are shown in Table 3. The properties of the steels in the embodiments of the present invention are shown in Table 4. The microstructure of the steel according to the embodiment of the present invention is shown in Table 5.

Table 1 Composition of steel according to the embodiment of the present invention (wt%)

Table 2 Main process parameters of steel continuous casting, hot rolling and cold rolling according to the embodiment of the present invention

Example Heating temperature/℃ Opening rolling temperature/℃ Final rolling temperature/℃ Coiling temperature/℃ Cold rolling reduction/% 1 1261 1113 911 424 81 2 1252 1076 904 472 80 3 1271 1053 896 457 83 4 1243 1108 897 411 85 5 1278 1098 903 404 82 6 1256 1119 921 482 81 7 1267 1109 897 427 83 8 1256 1082 912 446 84 9 1278 1079 908 482 81 10 1269 1074 916 437 82

Table 3 Main process parameters of cold rolling annealing of steel according to the embodiment of the present invention

Table 4 Properties of Steels in Examples of the Invention

Table 5 Microstructure of steel according to the embodiment of the present invention

It can be seen from the above examples that by using the component design and production process of the present invention, the yield strength is between 220 and 260MPa, the tensile strength is more than 340MPa, the elongation after fracture is more than 40%, and the plastic strain ratio r is 2.2-2.4 , the strain hardening index n is above 0.25, and the bake hardening value BH2 is between 40 and 60MPa, achieving the high performance indicators of alloyed galvanized high-strength automobile sheets.

In order to describe the present invention, the present invention has been appropriately and fully described through the examples above. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can do so without departing from the present invention. Various changes and modifications can be made without departing from the spirit and scope of the invention. Any modifications, equivalent substitutions, improvements, etc. shall be included in the protection scope of the invention. The patent protection scope of the invention shall be determined by Claim limitations.

Claims (9)

1. The 340 MPa-level alloying hot-dip galvanized sheet for the new energy automobile is characterized in that the galvanized sheet comprises the following substrate components in percentage by weight: c:0.005% -0.007%, si:0.012 to 0.017 percent, mn:0.22 to 0.34 percent, P:0.03 to 0.05 percent of Nb:0.04 to 0.07 percent of Ti:0.003 to 0.005 percent, N is less than or equal to 0.003 percent, S is less than or equal to 0.005 percent, and the balance is Fe and unavoidable impurities.

2. The 340 MPa-level alloying hot galvanized plate for a new energy automobile according to claim 1, wherein the C-Nb/7.736 in the hot galvanized outer plate is-5-15 ppm.

3. The 340 MPa-level alloying hot galvanized sheet for a new energy automobile according to claim 1, wherein the hot galvanized substrate microstructure is ferrite in a critical area, oriented auxiliary ferrite, and dispersed Nb (C, N) and TiN, wherein the volume percentage of each microstructure is 80-90% of ferrite in the critical area and 10-20% of oriented auxiliary ferrite.

4. The 340 MPa-grade alloyed hot-galvanized sheet for a new energy automobile according to claim 3, wherein the ferrite hardness of the critical zone is 160-180HV and the ferrite hardness of the oriented attached ferrite is 190-210 HV.

5. The 340 MPa-level alloyed hot-dip galvanized sheet for a new energy automobile according to claim 1, wherein the yield strength of the hot-dip galvanized sheet is 220-260MPa or more, the tensile strength is 340MPa or more, the elongation after break is 40% or more, the plastic strain ratio r is 2.2-2.4, the strain hardening index n is 0.25 or more, and the bake hardening value BH2 is 40-60 MPa.

6. The 340 MPa-grade alloying hot galvanized plate for the new energy automobile according to claim 1, wherein the mass percentage of iron in the hot galvanized plate coating is 8-10%.

7. A method for preparing a 340 MPa-grade alloyed hot-galvanized sheet for a new energy automobile according to any one of claims 1 to 6, comprising smelting, casting, hot rolling, pickling, cold rolling, hot galvanizing; the method is characterized in that:

(1) And (3) hot rolling:

the heating temperature is 1250-1280 ℃, the furnace duration is more than 120min, the initial rolling temperature is 1060-1120 ℃, the final rolling temperature is 893-924 ℃, and the coiling temperature is 400-490 ℃; the thickness of the hot rolled coil is 3-4 mm;

(2) Cold rolling: rolling reduction is controlled to 80% -85%; the thickness of the cold-rolled steel plate is 0.6-0.8 mm,

(3) Hot galvanizing:

a) Heating the cold-rolled steel plate to 865-893 ℃ and keeping the isothermal time for 30-60 s; slowly cooling the steel plate to 785-813 ℃ at a cooling speed of 10-16 ℃/s, and then cooling to 440-460 ℃ at a cooling speed of more than or equal to 30 ℃/s;

b) Heating the steel plate to 450-460 ℃ and then entering a zinc pot, wherein the zinc plating process is carried out for 1-3 s,

c) Alloying treatment is carried out on the steel plate, the alloying temperature is 473-530 ℃, and the alloying time is 18-23s; finally, the steel plate is cooled to room temperature.

8. The method for preparing 340MPa grade alloyed hot-galvanized sheet for new energy automobile according to claim 7, wherein in the hot rolling process,

when the C-Nb/7.736 is-5 ppm, the crimping temperature is controlled at 400-450 ℃;

when C-Nb/7.736 is 5-15 ppm, 5ppm is excluded, and the crimping temperature is controlled at 450-490 ℃.

9. The method for preparing the 340 MPa-grade alloyed hot-dip galvanized sheet for the new energy automobile according to claim 7, wherein in the hot dip galvanizing process,

when the C-Nb/7.736 is between-5 and 5ppm, the heating temperature is controlled between 880 and 893 ℃;

when the C-Nb/7.736 is 5-15 ppm, 5ppm is not included, and the heating temperature is controlled to be 865-880 ℃.