KR101009796B1 - Method for manufacturing high-strength hot-dip galvanized steel sheet - Google Patents

Abstract

The present invention relates to a method of manufacturing a high strength hot-dip galvanized steel sheet to control the microstructure by winding at low temperature after the multi-stage cooling to ensure excellent strength, ductility and plating properties.

By weight%, carbon (C) 0.08-0.2%, silicon (Si) 0.3-1.0%, manganese (Mn) 0.5-2.5%, aluminum (Al) 0.5-1.5%, phosphorus (P) 0.001-0.1%, sulfur (S) 0.001 to 0.005%, copper (Cu) 0.001 to 0.5%, nitrogen (N) 0.001 to 0.002% and the rest of the remainder additionally contains antimony in the steel (Fe) and the composition of inevitable impurities, niobium, At least one of vanadium and titanium is further contained, homogenized at 1100 to 1300 ° C, hot rolled at Ar3 to Ar3 + 100 ° C, and cooled to an intermediate cooling temperature of 680 to 750 ° C. It is quenched and wound up to a temperature of 400 ° C. or less to have a ˜70% ferrite and a 30 to 60% martensite fraction, followed by cold rolling and annealing.

According to the present invention, it is possible to adjust the balance between strength and ductility without adding expensive reinforcing elements, and also has a useful effect of securing mechanical properties with excellent plating properties.

Description

Manufacturing method of high strength hot-dip galvanized steel sheet {METHOD FOR MANUFACTURING HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET}

The present invention relates to a method for manufacturing a high strength hot-dip galvanized steel sheet, in particular, to improve the hot rolling heat treatment process of the modified organic plastic steel to have a microstructure of ferrite and martensite, and then cold-rolled, annealing heat-treated high-strength hot dip galvanizing It relates to a method for producing a steel sheet.

As the competition in the existing automobile industry intensifies, there is an increasing demand for quality and diversification of automobile quality.In order to satisfy the stricter regulations on safety and environmental regulations, efforts are being made to increase its own rigidity and improve fuel efficiency. have.

Recently, the research interests of the steel industry and the automotive industry are focused on environmental pollution, high strength, and light weight, and as the automobile design is complicated and the needs of consumers are diversified, the automotive industry demands steel having high strength and excellent workability and formability. .

In order to cope with this, in order to have high strength and high ductility at the same time, it can be applied to abnormal phase steel (DP: Dual Phase) obtained by alloying element and heat treatment condition or transformation organic plastic (TRIP) using residual austenite. Attention has been drawn to these two grades, with their strength and ductility being superior to those of their class.

Among them, TRIP steel controls the cooling rate and cooling end temperature during the hot rolling, cold rolling, and annealing heat treatment of slabs from the furnace to retain some austenite at room temperature and transform it into martensite during plastic deformation. By mitigating stress concentration, the ductility is increased to have high strength steel with excellent properties having both strength and ductility, and it is used in door impact bars, bumper reinforcements and shock absorbing parts of automobiles. (CAMP-ISIJ vol. 1, P. 877)

Existing metamorphic organic-plastic steels are produced by anustempering by reverse annealing and reverse transformation annealing to maintain a large amount of austenite in steel.

In this case, the Osstembering method adds about 0.2wt% of carbon (C), 1.0 ~ 2.0wt% of silicon (Si), more than 1.0wt% of manganese, and 1.0wt% of aluminum, and then maintains it for some time after cooling to bainite transformation temperature after annealing. To concentrate carbon in austenite.

At this time, the formation of cementite (Fe₃C) is suppressed by silicon, aluminum, etc. present in the steel, and carbon is concentrated to an adjacent austenite, thereby remaining at room temperature by stabilizing austenite.

However, in the above case, since the tensile strength of the steel grade is mainly dependent on the carbon content or the input amount of the solid solution element, the weldability is deteriorated when the carbon content is increased, and when the silicon is added in the solid solution element, plating is performed during hot dip galvanizing. Wetability (Wetability) is inhibited, and when phosphorus is added, segregation and alloying temperature are increased to act to delay alloying, so excessive addition has a problem of lowering workability.

In addition, after annealing or cooling directly to the bainite transformation temperature or slow cooling to an intermediate temperature, it has to be cooled at a high speed of 90 to 100 ° C / sec or more, and thus it is difficult to be applied to a general continuous annealing facility. It is difficult to work with steel grades.

On the other hand, reverse transformation annealing method forms an austenite in the lath boundary of the whole structure by cold annealing the martensite and bainite mixed structure obtained after hot rolling using a steel containing a large amount of manganese, an austenite stabilizing element. After cooling and remaining at room temperature.

The method of manufacturing cold rolled steel sheet by reverse transformation annealing method promotes the diffusion of carbon and manganese during high temperature annealing due to the large amount of potential introduced during cold rolling, so that the stability of austenite decreases in the cooling process and the ductility is relatively lowered. In low temperature annealing, there is a problem that the strength is reduced.

In addition, the use environment of the automobile is increasingly severe due to the use of snow removal salt, pollution, acid rain and the like, the importance of rust prevention in the steel sheet for automobile as the life is prolonged.

Therefore, demand for surface-treated (zinc-plated) steel sheet is increasing more than that of general cold-rolled steel sheet. Among galvanized steel sheets, EG (Electro Galvanized) or two-layer galvanized steel sheet is widely used. Increasingly, the use of hot-dip galvanized steel sheets (GA, Galva Annealed) is increasing.

In the case of TRIP steel, various alloying elements are added to the material in order to improve the strength and formability, and these additive elements are concentrated in the surface layer of the material, which greatly affects the reaction of Fe and Zn.

In particular, conventionally developed TRIP steels have a Si-based oxide layer (Mn ₂ SiO _4, etc.) formed on the surface of the material due to the high Mn and Si content, and the plating speed is lowered due to deterioration of plating wettability. .

Therefore, currently commercialized TRIP steel is being replaced by EG (Electro Galvanized) plating.

EG plating is superior in terms of quality and characteristics, but the manufacturing cost is increased by 5 to 10 times compared to other plating materials. Therefore, it is a TRIP steel using metamorphic organic plasticity, but it is difficult to actually apply it.

In Japanese Patent Laid-Open Nos. 1-230715 and 1-79345, a method of manufacturing a hot-dip galvanized steel sheet by heat-treating only a base alloy of 0.07 to 0.4 wt% of carbon, 0.3 to 2.0 wt% of silicon, and 0.2 to 2.5 wt% of manganese. However, the cooling conditions are difficult to apply variously to the general continuous annealing equipment and the thermal history during plating not only decomposes the retained austenite, but there is a concern that the good molten zinc plating characteristics may be inhibited by the silicon.

Korean Patent No. 10-1998-0060183 discloses a method for manufacturing TRIP steel by reverse transformation annealing method by adding carbon 0.05-0.2 wt%, silicon 0.2-0.5 wt% and manganese 4-7 wt%. Since the strength of the rolled material is high, there is a fear that the load is increased during cold rolling.

The present invention has been proposed to solve the above problems in view of the above problems, and its object is to provide a method of manufacturing high strength hot-dip galvanized steel sheet to satisfy the strength, ductility, and plating properties simultaneously without excessively adding reinforcing elements. There is.

The present invention for achieving the above object by weight%, carbon (C) 0.08-0.2%, silicon (Si) 0.3-1.0%, manganese (Mn) 0.5-2.5%, aluminum (Al) 0.5-1.5%, Alloy consisting of iron (Fe) and unavoidable impurities other than phosphorus (P) 0.001 ~ 0.1%, sulfur (S) 0.001 ~ 0.005%, copper (Cu) 0.001 ~ 0.5%, nitrogen (N) 0.001 ~ 0.002% An antimony is further contained in the steel having a composition, and at least one of niobium, vanadium, and titanium is further added, and after homogenizing at 1100 to 1300 ° C., hot rolling is finished at Ar ₃ to Ar _{3 +} 100 ° C. After cooling to an intermediate cooling temperature of 680 to 750 ° C., it is quenched and wound up to a temperature of 400 ° C. or less to have a 40 to 70% ferrite and 30 to 60% martensite fraction. It is characterized by.

In the cold rolling step, it is preferable to maintain a cold reduction ratio of 30 to 50%.

Also preferably, the total content of at least one of the antimony, niobium, vanadium and titanium may be added in an amount of 0.01 to 0.6 wt% based on the steel having the alloy composition.

The present invention relates to a method for manufacturing a high strength hot-dip galvanized steel sheet to control the microstructure by winding at low temperature after the multi-stage cooling to ensure excellent strength, ductility and plating properties, according to the present invention is an expensive reinforcing element It is possible to adjust the balance between strength and ductility without adding, and also has a useful effect to ensure excellent mechanical properties of the plating property.

Method for producing a high strength hot-dip galvanized steel sheet according to the present invention, in weight%, carbon (C) 0.08 ~ 0.2%, silicon (Si) 0.3 ~ 1.0%, manganese (Mn) 0.5 ~ 2.5%, aluminum (Al) 0.5 ~ 1.5%, phosphorus (P) 0.001 ~ 0.1%, sulfur (S) 0.001 ~ 0.005%, copper (Cu) 0.001 ~ 0.5%, nitrogen (N) 0.001 ~ 0.002% In addition, one or two or more of the reinforcing elements niobium (Nb), vanadium (V), and titanium (Ti) are additionally added to the steel having the alloy composition, and antimony (Sb) is added to improve hot dip galvanizing properties. The steel is hot rolled to have martensite and ferrite structure, cold rolled to 30-50% reduction rate, followed by annealing heat treatment and hot dip plating.

In the hot rolling process, the slab having the composition as described above is homogenized at 1100 to 1300 ° C, the hot rolling is finished at Ar3 to Ar3 + 100 ° C, and the hot rolled material microstructure is 40 to 70% ferrite and 30 to 30% by volume fraction. In order to make up 60% martensite, it is cooled to the intermediate cooling temperature of 680 ~ 750 ℃, maintained for 6-8 seconds, then cooled to the temperature of 400 ℃ or less at the cooling rate of 150 ℃ / second or more, and finally pickled to remove the oxide. do.

If the homogenization temperature is less than 1100 ℃, there is a problem that the hot rolling load is increased because the slab temperature is low, and if the temperature exceeds 1300 ℃, the workability of the cold rolled steel sheet is lowered due to the generation of precipitates, the homogenization treatment temperature is limited to 1100 ~ 1300 ℃ It is desirable to.

In addition, if the finish hot rolling temperature is less than Ar3, there is a problem that the productivity decreases due to the increase in the rolling load during rolling, and the production cost increases if it exceeds Ar3 + 100 ℃, the finish hot rolling temperature is limited to Ar3 ~ Ar3 + 100 ℃ It is preferable.

When the intermediate cooling temperature is less than 680 ° C, coarse pearlite is produced, and if the 750 ° C or more is difficult to secure the ferrite fraction, the intermediate cooling temperature is preferably limited to 680 to 750 ° C.

Since the cooling rate and the winding temperature is preferably wound at a temperature of 400 ℃ or less at a cooling rate of 150 ℃ / sec or more in order to obtain a martensite volume fraction of 30 ~ 60%.

In the cold rolling process, the wound hot rolled sheet is cold rolled at a reduction ratio of 30 to 50% to obtain a final desired thickness and continuously annealed at 780 to 860 ° C.

At this time, if the reduction ratio is less than 30%, there is a problem of increasing the recrystallization temperature, and if it exceeds 50%, there is a problem that rolling is too difficult, so the cold reduction ratio is preferably limited to 30 to 50%.

In addition, when the annealing temperature is less than 780 ℃, it is difficult to secure sufficient processability, and the austenite transformation is not sufficient. If the annealing temperature exceeds 860 ℃, the ferrite transformation occurs when cooling after full austenite transformation. Because of this, the annealing temperature is preferably limited to 780 to 860 ° C.

In the alloying hot dip galvanizing or hot dip galvanizing process, the annealed steel sheet is rapidly quenched and maintained for more than 30 seconds in a section of 450 to 350 ° C., followed by plating and heat treatment at a normal hot dip galvanizing temperature of 450 to 550 ° C. within 2 minutes. And finally cooling.

When the quenching end temperature exceeds 450 ℃, the ductility decreases because all the austenite phase changes to bainite phase, and if it is lower than 350 ℃, the austenite phase changes to martensite phase, so the rapid rise in strength and workability decrease, so the end of quenching The temperature is preferably limited to the 450 to 350 ° C range.

Hereinafter, the compositional elements of the present invention and their contents will be described.

Carbon [C]: 0.08 to 0.20 wt  %

Carbon (C) is concentrated on austenite upon annealing in an ideal zone, stabilizes austenite in the bainite transformation temperature range, diffuses and transfers to austenite in ferrite, and remains 3 to 20% after cooling to room temperature. Due to the presence of austenite, transformation organic plasticity is generated during processing to improve moldability.

If the carbon content is less than 0.08 wt%, sufficient residual austenite is not secured, making it difficult to obtain elongation characteristics and lowering.

On the contrary, when the carbon content exceeds 0.20 wt%, the tensile strength is increased due to the solid solution strengthening effect, and a large amount of residual austenite is formed, such as delayed fracture, but also poor weldability.

Therefore, the carbon content is preferably in the range of 0.08 to 0.20 wt%.

silicon[ Si ]: 0.3 ~ 1.0 wt %

Silicon is an important element that increases the strength of the steel sheet by the solid solution strengthening effect and increases the carbon activity in the ferrite, thereby increasing the amount of carbon in the austenite to increase the stability of the austenite. When the content is less than 0.3 wt% If the effect is not obtained and is 1.0 wt% or more, the plating wettability is greatly lowered, and the molten plating properties such as unplating and plating peeling are inhibited, so the Si content is preferably in the range of 0.3 to 1.0 wt%.

manganese[ Mn ]: 0.5-2.5 wt  %

Manganese (Mn) is a component that inhibits the formation of pearlite phase (ferrite + cementite) and promotes austenite formation and C concentration inside, contributing to the formation of residual austenite and increasing the hardenability to facilitate the formation of martensite upon cooling. When the Mn content is less than 0.5 wt%, a very fast cooling rate is required to prevent pearlite production, and when it exceeds 2.5 wt%, manganese band structure is formed and segregation increases rapidly, which impairs the workability and weldability of the steel. The content is preferably in the range of 0.5 to 2.5 wt%.

aluminum[ Al ]: 0.5-1.5 wt %

Aluminum (Al) is mainly used as a deoxidizer, but in the present invention, as an alternative to silicon (Si), which inhibits plating, it promotes ferrite formation and carbon enrichment in the austenite phase to suppress pearlite formation and promote residual austenite formation. It is used to If the amount of aluminum is less than 0.5 wt%, it is impossible to obtain a sufficient effect of promoting the formation of retained austenite. If the amount of aluminum exceeds 1.5 wt%, the surface defects increase during the production of slabs during the casting process, so that the content is 0.5 to 1.5 wt%. It is preferable to limit to the range of.

Phosphorus [P]: 0.001 ~ 0.1 wt %

Phosphorus (P) is sometimes added to enhance solid solution, but like Si, it is added to suppress the formation of carbides and to increase strength. When it exceeds 0.1 wt%, there is a problem in that weldability is deteriorated and material deviation occurs due to central segregation, so it is preferable to limit the amount to 0.1 wt% or less.

Sulfur [S]: 0.001-0.005 wt %

Sulfur (S) is an element that is inevitably contained in the manufacture of steel, and forms an emulsion-based (MnS, etc.) inclusions, and causes cracks, etc., and therefore, it is preferable to limit it to 0.005 wt% or less.

Copper[ Cu ]: 0.001-0.5 wt %

Copper (Cu), together with Al, serves to suppress the precipitation of C in the bainite transformation region as a substitute element for Si and to generate residual austenite, and to improve corrosion resistance. In addition, there is an effect of miniaturizing the ferrite grains to increase the strength, but when exceeding 0.5wt% elongation is reduced, it is preferable to limit to 0.5wt% or less.

nickel[ Ni ]: 0.5 wt %Below

Nickel (Ni) is added as an element to prevent the red heat brittleness generated when Cu is added to increase strength and improve corrosion resistance. Usually, it is known that the effect is best when added in a ratio of Cu: Ni = 1: 1. It is preferable to regulate in the range of 0.5 wt% or less according to copper (Cu) addition content, and may not add.

Nitrogen [N]: 0.001 ~ 0.02 wt %

Nitrogen (N) can be used as a substitute for Si and Al because it increases austenite formation when the trace amount is added, and increases strength by forming AlN or TiN. However, when exceeding 0.02 wt%, since elongation is reduced and workability is inhibited, it is preferable to limit to 0.02 wt% or less.

antimony[ Sb ]: 0.01 ~ 0.4 wt %

Antimony (Sb) is more precious than Fe and is concentrated on the surface and grain boundaries without being oxidized during hot rolling, thereby inhibiting the formation of Si-based oxide layer (Mn2SiO4) that inhibits hot dip galvanizing properties. It is preferable to limit the content to 0.01 to 0.4 wt% since it affects.

Niobium [ Nb ]: 0.1 wt  % Below

Niobium (Nb) forms a precipitate of Nb (C, N) to refine the grains of ferrite during transformation, thus increasing the amount of cornerstone ferrite and suppressing cementite precipitation, thus adding to the formation of residual austenite. However, when adding more than 0.1 wt%, there is a problem in that the precipitation strengthening effect is increased so much that the ductility is lowered.

titanium[ Ti ]: 0.1 wt % Below

Like niobium (Nb), titanium (Ti) may be added to increase the strength due to grain refinement and precipitation strengthening. However, due to the strong oxidizing property, many non-metallic inclusions are formed during steelmaking, causing surface defects on the steel sheet, and increasing the recrystallization temperature, thereby increasing the manufacturing cost.

Vanadium [V]: 0.1 wt % Below

Vanadium (V) is added as a reinforcing element like niobium (Nb). However, it is preferable to limit the content to 0.1 wt% or less because there is a problem of deterioration in moldability and production cost.

Since niobium, titanium, and vanadium are added as reinforcing elements in consideration of the properties of steel grades, the lower limit thereof is not described, but if it is stated, it can be defined as 0.001 wt% as the minimum amount of addition that can see the effect.

Hereinafter, a method of manufacturing a hot-dip galvanized steel sheet of the present invention having the composition described above will be described in comparison with a comparative example.

Ingot of the inventive steel (steel Nos. 1 to 5) having the chemical composition shown in Table 1 was prepared and reheated at 1250 ° C. for 1 hour, followed by hot rolling to a thickness of 2.4 mm. Hot rolling finish temperature was 900 ° C., winding The temperature is 300 ° C. and 650 ° C. for multistage cooling and general cooling, followed by furnace cooling for 1 hour. After pickling, cold rolling to 1.4mm thickness and continuous annealing were performed with annealing temperature of 820 ° C and aging temperature of 460 ° C and GA temperature of 520 ° C.

Steel grade Chemical composition (wt%) C Si Mn P S Al Co Cu Ni Nb Ti V Sb N (ppm) One 0.092 0.29 1.58 0.026 0.007 0.96 - 0.11 - - 0.009 - 0.05 64 2 0.098 0.31 1.65 0.042 0.007 1.03 - 0.12 - - - 0.011 0.05 85 3 0.092 0.26 1.54 0.038 0.006 0.96 0.31 0.10 - - 0.009 0.009 0.05 70 4 0.088 0.30 1.55 0.042 0.007 0.99 - 0.11 - 0.023 - - 0.10 66 5 0.091 0.27 1.57 0.068 0.007 0.95 - 0.10 - 0.023 - - 0.10 60

Table 2 shows the results of evaluation of the mechanical properties and the plating properties according to the manufacturing conditions of the inventive steel and the comparative steel.

Tensile test was performed at room temperature tensile test in JIS No. 5, and the residual austenite fraction was determined by X-ray diffraction intensity after chemical polishing of 1/4 layer of plate thickness from surface layer with HF + H2O2 mixed solution.

The appearance after plating was evaluated by visually determining the unplated state after peeling only the plating layer with a 20% NaOH solution, exposing the alloy layer. Evaluation criteria are divided into 1: 3 / dm2 2: 4 ~ 10 / dm2 3: 11 ~ 15 / dm2 4: 16 or more / dm2 or more, 1,2 and pass and 3, 4 are classified as fail. The adhesion of plating was carried out after the 60 degree V bending test, and the taping test was carried out. The rate of dropping was evaluated based on the following criteria, and 1.2 was passed and 3 and 4 were rejected.

1: 0 to 10% 2: 10 to 20% 3: 20 to 30% 4: 30% or more

Steel grade Winding temperature (℃) Cooling method Tensile Strength (MPa) Elongation (%) Tensile Strength × Elongation Residual austenite fraction (%) Plating appearance Plating adhesion division One 300 Multistage 700 33 23100 7 2 2 Example 650 Normal 584 32 18688 4 2 2 Comparative example 2 300 Multistage 720 35 25200 8 2 2 Example 650 Normal 632 33 20856 5 2 2 Comparative example 3 300 Multistage 650 32 20800 7 2 2 Example 650 Normal 601 30 18030 5 2 2 Comparative example 4 300 Multistage 680 33 22440 8 One One Example 650 Normal 612 30 18360 4 One One Comparative example 5 300 Multistage 703 31 21793 6 One One Example 650 Normal 629 30 18870 4 One One Comparative example

Invention examples of the present invention is to improve the elongation due to the increase of the residual austenite fraction and the excellent strength-ductility balance by increasing the residual austenite by forming a microstructure consisting of ferrite and martensite after the multi-stage cooling to control the hot-rolled microstructure In order to secure the plating property, silicon (Si) is reduced and aluminum (Al), copper (Cu), and nitrogen (N) content are controlled to secure desired mechanical properties and antimony (Sb) is added. It can be seen that more excellent plating characteristics can be obtained.

In addition, the content of micro-alloying elements such as Nb, V, Ti, etc. is added in a range that satisfies the formula of (Nb + V + Ti) ≤ 0.3% to balance strength and ductility so that strength-ductility balance and plating properties It can be seen that a high-strength hot-dip galvanized steel sheet which can be used as an excellent 590MPa grade automotive steel sheet can be produced.

Claims (3)

By weight%, carbon (C) 0.08-0.2%, silicon (Si) 0.3-1.0%, manganese (Mn) 0.5-2.5%, aluminum (Al) 0.5-1.5%, phosphorus (P) 0.001-0.1%, sulfur (S) 0.001 to 0.005%, copper (Cu) 0.001 to 0.5%, nitrogen (N) 0.001 to 0.002%, and the rest of the remainder added to the steel having an alloy composition composed of iron (Fe) and unavoidable impurities. And niobium (Nb), vanadium (V), and titanium (Ti), further containing at least one, and homogenizing treatment at 1100 ~ 1300 ℃, hot rolling finish at Ar3 ~ Ar3 + 100 ℃ After cooling to an intermediate cooling temperature of 680 to 750 ° C., it is quenched and wound to a temperature of 400 ° C. or lower to have a 40 to 70% ferrite and 30 to 60% martensite fraction, cold rolled and annealed, followed by hot dip plating. Method for producing a high strength hot dip galvanized steel sheet, characterized in that. The method according to claim 1, The method of manufacturing a high-strength hot-dip galvanized steel sheet, characterized in that for maintaining the cold reduction rate 30 to 50% in the cold rolling. The method according to claim 1 or 2, The antimony, niobium, vanadium, the total content of one or more added in the titanium is a method for producing a high strength hot-dip galvanized steel sheet, characterized in that 0.01 to 0.6wt% with respect to the steel having the alloy composition.