KR101160001B1 - High strength steel sheet having excellent formability, and method for producing the same - Google Patents

Abstract

The present invention relates to a high strength steel sheet excellent in formability and a method of manufacturing the same. The present invention is 0.04 ~ 0.15wt% of carbon (C), 0.1wt% or less of silicon (Si), 0.05 ~ 1.9wt% of manganese (Mn), 0.01 ~ 0.2wt% of aluminum (Al), 0.02 ~ 0.2wt of tungsten (W) %, Calcium (Ca) 0.05-0.2wt%, phosphorus (P) 0.040wt% or less, sulfur (S) 0.003wt% or less, nitrogen (N) 0.005wt% or less and the remainder inevitably contained in the production of iron and steel It contains elements that are complex and has a complex structure of ferrite and martensite.

The present invention enables the production of high strength steel sheet which secures excellent tensile strength of 490 ~ 590MPa grade and excellent elongation of 28% or more without using expensive alloying elements. An alloyed hot dip galvanized steel sheet can be produced. Therefore, there is an advantage that can contribute to cost reduction and product quality improvement.

Hot-dip galvanized steel sheet, plating property, formability

Description

High strength steel sheet having excellent formability, and method for producing the same

The present invention relates to a high strength steel sheet and a manufacturing method thereof, and more particularly, to a 490 ~ 590MPa class high strength steel sheet excellent in hot-dip galvanizing properties and formability.

The steel industry and the automotive industry are making various efforts to develop ultra-low fuel consumption vehicles as environmental problems such as CO ₂ emissions are on the rise.

In particular, the automotive industry is focusing on research on high strength and light weight to improve fuel efficiency, and demands high strength and excellent formability as automobile designs become more complicated and consumer needs are diversified.

For example, a composite structure steel composed of ferrite and martensite phases is applied to structural members such as automobile bodies by using Si-Mn-based, Si-Mn-Cr-based, and Si-Mn-Cr-Mo-based alloys.

This steel is mainly applied to parts that require high energy absorption in collision of vehicles such as members, pillars, seal side, etc., and because it supports the body, high strength, processability and excellent fatigue characteristics are required.

Accordingly, the mechanical properties of the composite structure steel secure the tensile strength and the yield strength by controlling the fractions of the phases such as martensite and bainite, which are obtained at the high temperature phase of austenite quenching in addition to the basic structure of ferrite and pearlite.

However, these steels are suitable for aging due to over-aging existing immediately before and after plating when manufactured from alloyed hot-dip galvanized steel sheet and austenite in the abnormal region during alloying heat treatment as ferrite, pearlite (or cementite) and bainite. Since it is difficult to secure the site fraction, problems of yield point stretching and strength reduction occur.

Accordingly, by adding the hardenable elements manganese (Mn) and molybdenum (Mo) to suppress the transformation of austenite into ferrite, pearlite (or cementite) and bainite, so that the appropriate martensite fraction during secondary cooling is secured The development of composite tissue steel was attempted.

However, this method increases the strength, but due to the rapid increase in the content of manganese (Mn), the central segregation and dendritic segregation occurs during the play, resulting in a stripe band like manganese band in the final product. Therefore, the moldability is deteriorated and the plating property is deteriorated due to the manganese oxide (Mn ₂ SiO ₄ ) formed on the surface of the steel sheet during hot dip galvanizing.

Therefore, an attempt was made to satisfy the moldability and plating property by reducing the content of manganese (Mn) and adding molybdenum (Mo) and vanadium (V) instead. However, this method has a problem in that commercial production is impossible because of an increase in the production cost of molybdenum (Mo) and vanadium (V), which is expensive.

The present invention is to solve the conventional problems as described above, the object of the present invention is to reduce the content of molybdenum (Mo) and manganese (Mn) even in the form of 490 ~ 590MPa class to ensure formability and hot-dip galvanizing property It is to provide a high strength steel sheet excellent in formability having a tensile strength and a manufacturing method thereof.

According to a feature of the present invention for achieving the above object, the present invention is carbon (C) 0.04 ~ 0.15wt%, silicon (Si) 0.1wt% or less, manganese (Mn) 0.05 ~ 1.9wt%, aluminum ( Al) 0.01 to 0.2 wt%, tungsten (W) 0.02 to 0.2 wt%, calcium (Ca) 0.05 to 0.2 wt%, phosphorus (P) 0.040 wt% or less, sulfur (S) 0.003 wt% or less, nitrogen (N) 0.005 wt% or less and the remainder contain elements which are inevitably contained in the production of iron and steel, and have a composite structure of ferrite and martensite.

The ferrite has a fraction of 85 to 95% with a grain size of 20 µm or less, and the martensite has a fraction of 5 to 15% with a grain size of 5 µm or less.

The content of aluminum (Al) and tungsten (W) satisfies Equation 0.10 ≦ Al + W ≦ 0.35 [wt%].

0.04 ~ 0.15wt% carbon (C), 0.1wt% or less silicon (Si), 0.05 ~ 1.9wt% manganese (Mn), 0.01 ~ 0.2wt% aluminum (Al), 0.02-0.2wt% tungsten (W), calcium (Ca) 0.05 to 0.2 wt%, phosphorus (P) 0.040 wt% or less, sulfur (S) 0.003 wt% or less, nitrogen (N) 0.005 wt% or less, and the rest of the elements inevitably contained in the production of iron and steel Re-heated steels are re-heated and homogenized, finished hot rolled at a temperature of Ar 3 or higher, then wound at a temperature of 650 ° C. or lower, cold rolled at a reduction ratio of 50 to 70%, and then annealed.

The content of aluminum (Al) and tungsten (W) satisfies Equation 0.10 ≦ Al + W ≦ 0.35 [wt%].

The present invention reduces the content of silicon (Si) and manganese (Mn), and instead of controlling the content of aluminum (Al) and tungsten (W) alloying elements to secure tensile strength and elongation, calcium (Ca) is added to segregation And controlling inclusion formation to prevent cracking and elongation at break during processing.

Therefore, 490-590 MPa grade tensile strength and excellent elongation of 28% or more can be secured, and at the same time, a high strength steel sheet having improved hot-dip galvanizing properties can be manufactured. This solves the lowering of elongation due to segregation bands (eg, manganese bands) of the composite steel structure using conventional manganese (Mn) and molybdenum (Mo).

Therefore, it is possible to manufacture high-strength steel sheet with improved formability and plating property at a lower cost than conventional composite steel, so it can contribute to cost reduction and product quality improvement, which has the effect of further expanding the application range of automotive steel sheet. .

Hereinafter, a preferred embodiment of a high strength steel sheet excellent in formability according to the present invention and a manufacturing method thereof will be described in detail.

The present invention is 0.04 ~ 0.15wt% of carbon (C), 0.1wt% or less of silicon (Si), 0.05 ~ 1.9wt% of manganese (Mn), 0.01 ~ 0.2wt% of aluminum (Al), 0.02 ~ 0.2wt of tungsten (W) %, Calcium (Ca) 0.05-0.2wt%, phosphorus (P) above 0 0.040wt%, sulfur (S) above 0 0.003wt% or below, nitrogen (N) above 0 0.005wt% or below, balance iron (Fe) And alloy compositions of other unavoidable impurities.

In more detail, the steel having the alloy composition is characterized in that the hot-rolled, cold-rolled, alloyed hot-dip galvanizing treatment, after the alloyed hot-dip galvanizing treatment to have a composite structure composed of ferrite and martensite Ensure elongation of at least 28%.

Ferrite has a fraction of 5 to 15% with a grain size of 20 µm or less and martensite has a fraction of 5 to 15% with a grain size of 5 µm or less.

Ferrite is a microstructure associated with securing ductility. If the fraction is less than 85%, it is difficult to secure an elongation of more than 28%, and if it exceeds 95%, the strength is accompanied by a decrease in strength. Martensite is a microstructure related to securing strength. If the fraction is less than 5%, it is difficult to secure high strength, and if it exceeds 15%, the martensite is accompanied by a decrease in elongation due to the increase in strength.

Grain size is related to strength gain. The strength decreases when the ferrite grain size exceeds 20 µm and the martensite grain size exceeds 5 µm.

In order to secure the above-described fractions of ferrite and martensite, aluminum (Al) and tungsten (W) are used instead of conventional manganese (Mn), silicon (Si), and expensive molybdenum (Mo) to improve plating properties and reduce segregation. It contains.

Aluminum increases the hardness of martensite by stabilizing ferrite as an element having a silicon-like effect and concentrating carbon in austenite, while tungsten increases martensite fraction to improve strength.

However, aluminum contains low melting point alloying elements, causing excessive cracking, and tungsten forms tungsten carbide to increase yield strength. Therefore, aluminum and tungsten are contained in a range having a resistance yield ratio (yield strength / tensile strength) of 60% or less to prevent springback phenomenon due to an increase in yield strength.

Therefore, aluminum (Al) and tungsten (W) are contained in a range satisfying the formula 0.10 ≦ Al + W ≦ 0.35 [wt%].

The above equation is for suppressing the increase in yield strength due to the formation of cracks and tungsten carbide by the low melting point alloy element.

If the sum of aluminum and tungsten is less than 0.10wt%, it is difficult to secure the martensite fraction related to high strength.If the sum of aluminum and tungsten exceeds 0.35wt%, aluminum explosively increases the amount of inclusions in the steel sheet, thereby reducing elongation or tungsten carbide To increase the yield strength by forming a resistance resistance ratio and hot-dip galvanizing properties are inhibited.

Contains calcium (Ca) to inhibit segregation band formation in steel species. Calcium is mainly used for the purpose of preventing the nozzle clogging during the continuous casting process, in the present invention is contained in order to inhibit the segregation band and spheroidizing the inclusions to improve the elongation.

Hereinafter, the function and content of the alloying elements of the present invention are as follows.

Carbon (C) 0.04 ~ 0.15wt%

Carbon (C) is an indispensable element for imparting high strength to the steel sheet. When the carbon content is less than 0.04w%, austenite is transformed into ferrite, making it difficult to secure the martensite fraction of the final structure.On the other hand, when the carbon content exceeds 0.15wt%, the weldability decreases and the ductility and stretch-flange property decreases as strength increases. do.

Silicon (Si) 0.1wt% or less

Silicon (Si) contributes to the cleansing of steel as a solid solution element, promotes carbon enrichment in austenite and increases the stability of austenite.

When silicon is added to the steel to which the appropriate manganese (Mn) is added, it improves the flowability of molten metal during welding to reduce the inclusions in the weld as much as possible, and improves the strength without inhibiting the balance between yield ratio and strength and elongation. In addition, silicon slows the diffusion rate of carbon in the ferrite, inhibits the growth of carbides and stabilizes the ferrite to improve the elongation.

However, since excessively added silicon generates surface defects due to plating property and red scale, the upper limit of the silicon is limited to 0.1 wt% or less.

Manganese (Mn) 0.05 ~ 1.9wt%

Manganese (Mn) is a solid solution strengthening element, which stabilizes austenite to lower the two-phase reverse annealing temperature and prevents austenite from being decomposed into pearlite even at a low critical cooling rate, thereby making martensite easily formed. In the present invention, manganese is added to enhance the strength by solid solution strengthening.

If the content of manganese is less than 0.05wt%, the shape of the steel sheet becomes poor due to the thermal stress generated due to the fast cooling rate to obtain martensite, whereas if it exceeds 1.9wt%, the hardenability increases and the workability becomes poor. During slab casting, manganese bands are developed in the center of thickness, and bending workability is reduced. Therefore, manganese is added in the range 0.05 ~ 1.9 wt%.

Aluminum (Al) 0.01 ~ 0.2wt%

Aluminum (Al) is an element that stabilizes ferrite grains to improve elongation and improve plating properties. Aluminum is mainly used as a deoxidizer, but in the present invention, aluminum is used as an alternative element of silicon, which inhibits plating properties.

Aluminum improves elongation by stabilizing ferrite, but increasing elongation is more effective because it reduces the amount of segregation present in the steel species when 0.01wt% or more is added.

In addition, aluminum combines with nitrogen in the steel to precipitate AlN to refine the grains, thereby improving the strength of the steel sheet and removing oxygen in the steel to prevent cracking during slab manufacture.

If the aluminum content is less than 0.01wt%, the oxygen content increases, causing ductility deterioration. If the aluminum content exceeds 0.2wt%, the slab cracks occur during playing, and the amount of inclusions in the steel species is exploded and the elongation is deteriorated. Plating performance is also impaired.

Tungsten (W) 0.02 ~ 0.2wt%

Tungsten (W) is a strong hardenable element that increases the martensite fraction to improve strength. If the amount of tungsten is added less than 0.02wt%, the effect is insignificant, and if it exceeds 0.2wt%, it is difficult to secure the elongation of 28% or more required by the present invention because it promotes the formation of tungsten carbide to increase the yield strength.

Calcium (Ca) 0.05 ~ 0.2wt%

Calcium (Ca) is added for the purpose of spheroidizing inclusions and segregation to improve elongation. Calcium is added more than 0.05wt% in order to obtain a spheroidizing effect, when excessively added, there is a problem that the quality of the product is degraded because there are many inclusions in the steel, limit the addition amount to 0.05 ~ 0.2wt% range.

Phosphorus (P) greater than 0 and less than 0.040 wt%

Phosphorus (P) is an element that increases the rigidity of the steel sheet by solid solution strengthening. Phosphorus acts to stabilize residual austenite due to the increase of solid solution carbon by suppressing carbide formation. When the phosphorus content is excessively contained, the alloying degree is decreased in the plating process, weldability and local ductility deterioration occur, and the development of cubic orientation is promoted and the development of (111) // RD orientation is inhibited. Restrict.

Sulfur (S) greater than 0 and less than 0.003wt%

Sulfur (S) is an element that is inevitably contained in the production of steel, inhibits toughness and weldability, and increases the MnS non-metallic inclusions, causing cracking and the like. In particular, sulfur is limited to the upper limit of 0.003wt% or less because it increases the coarse inclusions to deteriorate the fatigue characteristics.

Nitrogen (N) greater than 0 0.005wt%

Nitrogen is refined by the formation of AlN but limited to 0.005wt% or less since it is supersaturated during cooling in the alloying process of the zinc plating layer during hot dip galvanizing to lower the elongation.

The present invention contains the components of the steel sheet, the remainder is substantially iron (Fe) and unavoidable elements, and the incorporation of fine amounts of inevitable impurities as the elements contained in accordance with the situation of raw materials, materials, manufacturing facilities, etc. is also allowed.

The microstructure of the steel sheet is composed of a second phase including ferrite and martensite, and Ceq (carbon equivalent) is limited to 0.24 or less to secure spot weldability of automotive steel sheets.

The slabs having the composition as described above are obtained by ingot or continuous casting process after obtaining molten steel through the steelmaking process, in this case hot-rolled, cold-rolled and manufactured in the form of steel sheet, then alloyed molten zinc on the surface of the steel sheet The plating process is performed as follows.

Each process is as follows.

[Heating furnace process]

The process of reheating the slab is to reclaim segregated components during casting. Reheat is heated to a temperature range of 1200 ± 50 ° C. This is because the low reheating temperature prevents segregation of the segregated components, while excessively high recrystallization increases the austenite grain size and decreases the strength as the grain size of the ferrite is coarsened.

[Hot Rolling Process]

The slab reheated in the furnace process finishes hot rolling above the Ar3 temperature. Hot rolling finish temperature is less than Ar3, the rolling load is increased to reduce the productivity, so it is carried out above the Ar3 temperature.

After hot rolling is finished, it is wound up to a temperature of 650 ℃ or less in order to suppress the formation of tungsten carbide.

Thereafter, pickling is performed to descale the hot rolled steel sheet and applied with oil to prevent oxidation.

[Cold rolling process]

Cold rolling is performed to obtain the final desired thickness of the steel sheet and to obtain the desired material. Cold rolling is performed at a reduction ratio of 50 to 70% at room temperature.

[Annealing process]

The cold rolled steel sheet is subjected to annealing heat treatment in the ferrite-austenite two-phase zone of Ar1 to Ar3 and cooled. Cooling is cooled to the temperature range of 400 ~ 500 ℃ at a cooling rate of 10 ~ 50 ℃ / sec. At this time, if the cooling rate is too slow, austenite is transformed into ferrite ferrite (cementite), bainite in the cooling process, and when the cooling rate is too fast, a problem of material unevenness occurs.

[Alloyed hot dip galvanizing or hot dip galvanizing process]

The annealed steel sheet is hot dip galvanized, and after hot dip galvanizing, alloying heat treatment is performed at 460 to 520 ° C. for stable growth of the plating layer. When the alloying heat treatment temperature is lower than 460 ° C, it is difficult to secure stable growth of the alloying degree and the plating layer, and when the alloying heat treatment temperature is higher than 520 ° C, a problem of deterioration of the steel sheet material occurs.

Hereinafter, the high-strength steel sheet excellent in formability described above and a manufacturing method thereof will be described in detail with reference to Examples.

Table 1 below shows the invention examples and comparative examples of each component element is different.

division
Chemical composition (wt%), balance Fe
Remarks C Si Mn P S Al W Ca N (ppm) Al + W One 0.059 0.11 1.83 0.015 0.029 0.02 0.02 0.02 30 0.04 Comparative example 2 0.060 0.12 1.83 0.017 0.028 0.02 0.05 0.02 30 0.07 Comparative example 3 0.058 0.11 1.82 0.016 0.027 0.02 0.15 0.02 30 0.17 Comparative example 4 0.057 0.13 1.79 0.015 0.027 0.02 0.30 0.02 29 0.32 Comparative example 5 0.062 0.1 1.82 0.014 0.029 0.15 0.02 0.15 28 0.17 Inventive Example 6 0.061 0.12 1.79 0.016 0.025 0.15 0.05 0.15 32 0.20 Inventive Example 7 0.063 0.11 1.82 0.013 0.027 0.15 0.15 0.15 30 0.30 Inventive Example 8 0.059 0.1 1.81 0.015 0.031 0.15 0.30 0.15 32 0.45 Comparative example 9 0.060 0.11 1.79 0.015 0.029 0.30 0.02 0.15 31 0.32 Comparative example 10 0.061 0.11 1.82 0.015 0.028 0.30 0.05 0.15 29 0.35 Comparative example 11 0.062 0.12 1.79 0.015 0.028 0.30 0.15 0.15 28 0.45 Comparative example 12 0.060 0.11 1.83 0.015 0.030 0.30 0.30 0.15 27 0.60 Comparative example

division
Hot Rolling Condition
Cold Rolling Condition Mechanical properties Organization characteristics Plating characteristics
Remarks
FDT (℃) CT (℃) AT (℃) YS TS EL YR Vm One 890 600 790 311 466 25 67 3 Good Comparative example 2 890 600 790 312 486 24 64 4 Good Comparative example 3 890 600 790 312 512 22 61 6 Good Comparative example 4 890 600 790 316 546 22 58 8 Bad Comparative example 5 890 600 790 295 496 33 59 5 Good Inventive Example 6 890 600 790 296 512 33 58 6 Good Inventive Example 7 890 600 790 298 534 32 56 7 Good Inventive Example 8 890 600 790 338 544 30 62 8 Good Comparative example 9 890 600 790 345 539 26 64 7 Bad Comparative example 10 890 600 790 346 551 26 63 9 Bad Comparative example 11 890 600 790 346 626 25 55 12 Bad Comparative example 12 890 600 790 386 666 24 58 14 Bad Comparative example

[FDT: Hot finishing rolling temperature, CT: Winding temperature, AT: Annealing heat treatment temperature, YS (MPa): Yield strength, TS (MPa): Tensile strength, EL (%): Elongation, YR: Resistance ratio, Vm (% ): Martensite fraction]

Table 2 shows the mechanical properties of the specimens subjected to hot rolling, cold rolling, annealing, hot dip galvanizing and alloy heat treatment after reheating the slab having the alloy composition of Table 1 in a heating furnace at 1150 ° C. for 2 hours.

That is, after reheating, hot finish rolling was performed at 840 ° C. using a shear press method and a post step down method at 840 mm, and then wound at 580 ° C., and the steel plate was cold rolled to a thickness of 1.2 mm after pickling.

Thereafter, the cold rolled steel sheet was annealed at 700 to 800 ° C., quenched to 460 ° C., and hot-dipped galvanized, followed by alloy heat treatment at 500 ° C. FIG.

Looking at Table 1 and Table 2, tungsten increases the strength, but when the content is high, the yield strength shows a tendency to lower the resistance yield ratio and plating properties. And aluminum showed an effect of increasing the tensile strength and elongation, but when the addition of 0.2wt% or less, the elongation increase effect was higher.

However, as in the case of Comparative Examples 9 to 12, when aluminum is added in excess of 0.2wt%, the elongation is lowered and the plating property is poor. This can be seen that when the aluminum is excessively added to more than 0.2wt%, the amount of inclusions in the steel species is exploded and the elongation is lowered, resulting in poor moldability and poor plating properties.

And looking at the case of Comparative Examples 5 to 12, it is confirmed that the elongation is more improved than other comparative examples when 0.15wt% of calcium is added. This is due to the fact that calcium inhibits segregation band formation in the steel species and spheroidizes inclusions, thereby improving elongation. `

From the above experiments, even if conventional silicon (Si), manganese (Mn) and expensive molybdenum (Mo) are reduced or omitted, aluminum (Al) 0.01-0.2wt%, tungsten (W) 0.02-0.2wt% and the formula 0.10 When ≤ Al + W ≤ 0.35 [wt%] and calcium (Ca) is added within the 0.05 ~ 0.2wt% range, it can be seen that it is possible to manufacture a composite steel of 490 ~ 590MPa while satisfying the resistivity ratio (YR ≤ 60) Can be.

This is confirmed through Inventive Examples 5 to 7, and satisfies both the elongation of 28% or more and the resistivity ratio and plating properties of 60% or less.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

Claims (5)

0.04 ~ 0.15wt% carbon (C), 0.1wt% or less silicon (Si), 0.05 ~ 1.9wt% manganese (Mn), 0.01 ~ 0.2wt% aluminum (Al), 0.02-0.2wt% tungsten (W), calcium (Ca) 0.05 to 0.2 wt%, phosphorus (P) greater than 0 and 0.040 wt% or less, sulfur (S) greater than 0 and 0.003 wt% or less, nitrogen (N) greater than 0 and 0.005 wt% or less and the remainder in the production of iron and steel Inevitably contains elements, The content of the aluminum (Al) and tungsten (W) satisfies the formula 0.10≤Al + W≤0.35 [wt%], and has a high formability, characterized in that it has a composite structure of ferrite and martensite. The method according to claim 1, The ferrite has a grain size of less than 20㎛ 85 to 95%, the martensite has a grain size of 5㎛ or less having a fraction of 5 to 15%, high strength steel sheet excellent in formability. delete 0.04 ~ 0.15wt% carbon (C), 0.1wt% or less silicon (Si), 0.05 ~ 1.9wt% manganese (Mn), 0.01 ~ 0.2wt% aluminum (Al), 0.02-0.2wt% tungsten (W), calcium (Ca) 0.05 to 0.2 wt%, phosphorus (P) greater than 0 and 0.040 wt% or less, sulfur (S) greater than 0 and 0.003 wt% or less, nitrogen (N) greater than 0 and 0.005 wt% or less and the remainder in the production of iron and steel Inevitably contains elements, The aluminum (Al) and tungsten (W) content is a steel that satisfies the formula 0.10≤Al + W≤0.35 [wt%] Reheating and homogenization treatment, finishing hot rolling at a temperature of Ar3 or higher, then winding at a temperature of 650 ° C or lower, cold rolling at a reduction ratio of 50 to 70%, and then annealing. Excellent method for producing high strength steel sheet. delete