KR20100047001A - Hot-rolled steel sheet having ultra-high strength, and method for producing the same - Google Patents

Abstract

PURPOSE: A draft steel of a hot rolled steel sheet and a manufacturing method thereof are provided to minutely form grain by adding niobium, titanium and boron. CONSTITUTION: A draft steel of a hot rolled steel sheet is made of carbon 0.05~0.20 weight%, silicon 0.01~0.30 weight%, manganese 2.00~3.00 weight%, aluminum 0.10~0.50 weight%, niobium 0.01~0.10 weight%, titanium 0.01~0.10 weight%, nickel 0.02~0.30 weight%, copper 0.01~0.30 weight%, chrome 0.10~2.0 weight%, boron 0.0005~0.0030 weight%, phosphorus less than 0.05 weight%, sulfur less than 0.02 weight%, and nitrogen less than 0.01 weight%.

Description

Hot-rolled steel sheet having ultra-high strength, and method for producing the same

The present invention relates to an ultra high strength hot rolled steel sheet and a method for manufacturing the same, and more particularly, to a 980 to 1180 MPa class ultra high strength hot rolled steel sheet and a method for manufacturing the same.

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.

In particular, recent research interests in the steel industry and the automotive industry are concentrating on environmental pollution, high strength, and light weight, and as automobile designs become complicated and consumer needs are diversified, demands for high strength, processability, and formability are required. In the past, a method of manufacturing a new steel species which has improved strength while not significantly deteriorating workability has been steadily sought, and such a development direction can be distinguished as follows.

Firstly, by adding a hardenable element such as silicon (Si), manganese (Mn), chromium (Cr) and the like to form a low-temperature transformation structure after cooling, it is a method to increase the strength by this structure, prepared by this method Representative steels include metamorphic tissue steels (bainite steel, abnormal tissue steel, composite tissue steel, etc.), and the strength obtained at this time is controlled according to the cooling conditions and the amount of alloy added. Among them, the dual phase steel composed of two phases of ferrite and martensite is quenched in the austenite and ferrite two phase regions to produce about 10-30% (volume fraction) of martensite. Such metamorphic steel is superior to precipitation hardening steel and has excellent ductility and elongation processability, and is applied to members, bumpers and the like because of its high impact energy absorption capacity.

Ideal steel has been developed in the 1980s and has been actively applied recently. In Europe, the application rate is more than 50% of the total sheet. In addition, the ideal structure steel has excellent impact energy under high-speed deformation conditions such as vehicle collision, and is superior in ductility compared to TRIP steel because it has superior weldability and galvanizing property compared to equivalent TRIP (Transformation Induced Plasticity) steel. Overcoming some of the shortcomings, many components have been adopted in the automotive design process.

Second, there is a method in which residual austenite is distributed in polygonal ferrite or bainike tissue so that residual austenite generates metamorphic organic plasticity at the beginning of processing, thereby improving ductility. There is (TRIP) steel.

The transformation organic plastic (TRIP) steel is appropriately cooled according to the alloying amount so that pearlite transformation does not occur during the cooling of the austenite, and then delays the transformation of the austenite to bainite in the winding step again, thereby maintaining the austenite during final cooling. Part of the nitrate is transformed to martensite and the remainder is the remaining steel, where the retained austenite is transformed to martensite by initial processing, and ductility is formed in this process, which is called metamorphic organic plasticity. Refers to the steel grade that looks. However, in the case of dual phase steel or complex phase steel, when the fraction of martensite or precipitate is increased to increase the strength, the strength increases but the ferrite fraction decreases to decrease the ductility. However, by controlling the hot-rolled, cold-rolled, annealing heat treatment process to maintain austenite at room temperature can increase the strength and ductility at the same time. Induction of residual austenite into martensite during processing increases strength and increases ductility while locally relieving stress due to metamorphic organic plasticity. This is called transformed organic plastic (TRIP) steel.

On the other hand, excellent press formability is demanded because the internal parts of automobile parts are molded through press working. In order to secure excellent press formability, a method of improving the elongation must first be sought, and the point of securing shape freezing should not be overlooked.

Conventionally, Japanese Patent Application Laid-open No. Hei 8-134591 proposes a high-strength alloyed hot dip galvanized steel sheet having a resistive ratio and excellent press formability. In addition, the technique mentioned in Japanese Patent JP_B_35900 requires a cooling rate of 100 ° C./s or more in order to obtain a high strength, which makes it difficult to implement the gas jet cooling method.

In addition, US Patent US2003 / 0129444A1 realizes 780Mpa grade tensile strength by controlling the content of vanadium, but it is difficult to add a relatively large amount because vanadium is expensive in terms of price, and it is problematic in weldability because of high content of silicon. Occurs.

In addition, Satshiro Hironaka et al., Published in GALVATECH 07 in November 2007, discloses a dual phase GA sheet of tensile strength of 590 to 980 MPa, including carbon, manganese and silicon. Although alloyed molten zinc steel sheet with excellent moldability was manufactured by adjusting alloying elements and manufacturing conditions, it is possible to produce a steel sheet having a good balance between strength and ductility only when the silicon content is more than 1.2wt%. The characteristics fall sharply. However, a solution for this has not been disclosed and it is common to generally reduce the silicon content as much as possible when developing a steel sheet.

Korean Patent Laid-Open Publication No. 10-2005-0063981 discloses a method for manufacturing a general cold rolled steel sheet using transformed organic plastic steel of 120kgf / mm2 strength, which increases the strength of steel sheet by adding more than 0.6% of silicon and forms a constant ferrite in the final structure. Suggesting. However, the above method has a problem in that manufacturing cost increases because it is difficult to perform the hot dip coating and the electroplating surface treatment should be used.

In addition, Korean Patent Laid-Open Publication No. 10-2005-0032721 also discloses a method of manufacturing a general cold rolled steel sheet using a transformation-oriented plastic steel of 120kgf / mm2 strength that adds 0.75% or more silicon and 4-7% manganese to secure residual austenite. Presenting. However, this method also has a problem that the hot dip surface treatment is difficult.

In addition, in Korean Patent Application No. 10-2005-7013049, nickel and silicon are required to satisfy the operating conditions of Ni ≧ 1/5 × Si (5) + 1/10 × Al (%) for hot dip plating. A method of improving the strength and plating performance of a steel sheet by controlling it is proposed. However, the above method has a problem in that an increase in the content of expensive nickel leads to an increase in manufacturing cost, thereby making commercial production impossible.

After forming austenite in the rolling process as described above, by controlling the cooling rate and the cooling end temperature in the cooling process to form ferrite, martensite at room temperature, and relatively small amount of alloying element added, the weldability is excellent, In addition, it is important to control the cooling conditions in order to improve the excellent elongation, and in order to obtain ferrite through air cooling during cooling, it is necessary to optimize the cooling pattern and optimize the components.

It is important to maintain ferrite more than a certain fraction of the ideal tissue steel. For this purpose, manganese, aluminum, chromium, etc. are added, and the crystal grains are added by adding niobium (Nb), titanium (Ti), and boron (B). It is important to make it fine.

The present invention has been invented in view of the various problems as described above, and an object of the present invention is to adjust the alloying elements, and to change the fraction of the structure through the control of hot rolling, pickling and plating processes to improve the parts processability and hole expandability. The present invention provides an excellent high strength hot rolled steel sheet having excellent plating properties and weldability, and a method of manufacturing the same.

Ultra high strength hot rolled steel sheet according to the present invention for achieving the above object, carbon (C) 0.05 ~ 0.20wt%, silicon (Si) 0.01 ~ 0.30wt%, manganese (Mn) 2.00 ~ 3.00wt%, aluminum (Al) 0.10 to 0.50 wt%, niobium (Nb) 0.01 to 0.10 wt%, titanium (Ti) 0.01 to 0.10 wt%, nickel (Ni) 0.02 to 0.30 wt%, copper (Cu) 0.01 to 0.30 wt%, chromium (Cr) 0.10 to 2.0 wt%, boron (B) 0.0005 to 0.0030 wt%, phosphorus (P) 0.05 wt% or less, sulfur (S) 0.02 wt% or less, nitrogen (N) 0.01 wt% or less and residual iron ( Fe) alloy composition, the microstructure has a three-phase structure of bainite, ferrite, residual austenite and martensite

The phase fraction of the bainite structure is 50 to 65 vol%.

Ultra high strength hot rolled steel sheet according to the present invention, carbon (C) 0.05 ~ 0.20wt%, silicon (Si) 0.01 ~ 0.30wt%, manganese (Mn) 2.00 ~ 3.00wt%, aluminum (Al) 0.10 ~ 0.50 wt%, Niobium (Nb) 0.01 ~ 0.10wt%, Titanium (Ti) 0.01 ~ 0.10wt%, Nickel (Ni) 0.02 ~ 0.30wt%, Copper (Cu) 0.01 ~ 0.30wt%, Chromium (Cr) 0.10 ~ 2.0 steel with alloy composition of wt%, boron (B) 0.0005 ~ 0.0030wt%, phosphorus (P) 0.05wt% or less, sulfur (S) 0.02wt% or less, nitrogen (N) 0.01wt% or less and residual iron (Fe) The slab is homogenized and heat treated at 1150 to 1250 ° C., hot rolled at 870 to 930 ° C. and wound up at 200 to 400 ° C., followed by pickling and plating.

After the hot rolling, after cooling to a range of 600 to 720 ° C. at a cooling rate of 30 to 100 ° C./sec, air cooling is performed for 5 to 10 seconds, and after the air cooling, multistage cooling to a winding temperature at a cooling rate of 30 to 100 ° C./sec. Is carried out.

In the plating treatment, the hot rolled steel sheet is pickled, heated to a temperature of 450 to 480 ° C., maintained within 30 seconds, hot-dip galvanized at 450 to 500 ° C., and subjected to alloying heat treatment at 480 to 550 ° C., followed by cooling. .

According to the ultra-high strength hot rolled steel sheet according to the present invention and a manufacturing method thereof, by increasing the residual austenite content through hot rolling control technology and plating heat treatment temperature control, the bending property and hole expandability and formability are superior to conventional steel sheet Is improved.

In addition, it is possible to replace the existing 590 ~ 780MPa grade high strength cold rolled steel sheet by securing important strength in automobile structural members by increasing the grain size and precipitation phase content by alloy element added to secure the strength. By reducing the thickness of the steel sheet, the total vehicle weight is reduced, and fuel efficiency is increased.

Therefore, according to the present invention, when the ultra-high strength hot rolled steel sheet is molded into an automobile structural member, and the component is applied, the formability is excellent, the complex part shape can be processed and the impact energy absorption capacity is improved in the crash due to the increased austenite fraction. Can be.

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

The present invention, by controlling the alloying element, by changing the fraction of the structure through the control of hot rolling, pickling and plating process, the ultra high strength to secure a tensile strength of 980MPa ~ 1180Mpa or more, elongation of 10% or more, formability and plating properties To prepare a steel sheet.

In the present invention, the desired tensile strength by increasing the manganese, chromium, copper, aluminum to reduce the carbon content at the same time to reduce the carbon content to improve the bendability and hole accuracy, and improve the weldability and plating properties, instead of improving the insufficient strength In order to secure elongation, and to minimize the silicon content in order to secure excellent plating property, niobium and titanium are added to maximize grain refinement to improve plating and elongation, and addition of a small amount of boron prevents bainite or pearlite transformation. Do not

Specific alloy composition of the present invention, carbon (C) 0.05 ~ 0.20wt%, silicon (Si) 0.01 ~ 0.30wt%, manganese (Mn) 2.00 ~ 3.00wt%, aluminum (Al) 0.10 ~ 0.50wt%, niobium ( Nb) 0.01 ~ 0.10wt%, Titanium (Ti) 0.01 ~ 0.10wt%, Nickel (Ni) 0.02-0.30wt%, Copper (Cu) 0.01 ~ 0.30wt%, Chromium (Cr) 0.10 ~ 2.0wt%, Boron ( B) 0.0005 ~ 0.0030wt%, phosphorus (P) 0.05wt% or less, sulfur (S) 0.02wt% or less, nitrogen (N) 0.01wt% or less and the balance iron (Fe) and other inevitable impurities.

The microstructure of the hot rolled steel sheet has a three-phase structure of bainite, ferrite, residual austenite and martensite. This three-phase structure is a three-phase structure by increasing the content of residual austenite through the temperature control of hot rolling and cold rolling. In this case, the ferrite has a grain size of 10 to 30 µm and a fraction of 20 to 30 vol%, and the retained austenite and martensite have a fraction of 5 to 10 vol% with a grain size of 5 µm or less. In addition, bainite has a grain size of 2 to 20 µm and a fraction of 50 to 65%. The residual austenite finally turns martensite in processing.

If bainite is 65 vol% or more, it is difficult to secure the target strength, and it is difficult to secure chemical stability of the austenite phase. If bainite is less than 50 vol%, there is little effect of strength.

If the retained austenite and martensite is less than 5 vol%, it is difficult to expect a desired improvement of elongation due to metamorphic organic plasticity, and the ferrite is a microstructure related to securing ductility, and if the fraction is 30 vol% or more to secure the chemical stability of the retained austenite. it's difficult.

Hereinafter, the function and content of the alloying elements of the present invention will be described in detail.

Carbon (C): 0.05-0.20 wt%

Carbon is an indispensable element for imparting high strength to steel. Carbon is concentrated on the austenite phase after air cooling in the two phase region and stabilizes the austenite in the bainite transformation temperature range. In addition, carbon is diffused and moved to austenite inside ferrite, and after cooling to room temperature, more than 10% of retained austenite and martensite exist to generate metamorphic organic plasticity when processing steel to improve formability.

When a small amount of carbon is added, austenite is transformed into ferrite, which makes it difficult to secure the residual austenite and martensite fractions, thereby degrading strength and elongation characteristics. Therefore at least 0.05 wt% or more is added. On the other hand, when carbon is excessively added, the weldability is lowered and the ductility and stretch-flange properties are reduced with increasing strength, so the upper limit is limited to 0.20 wt%.

Silicon (Si): 0.01 ~ 0.30wt%

Silicon (Si) contributes to the cleansing of steel as a solid solution strengthening element, promotes carbon concentration of austenite and increases the stability of austenite so that austenite remains at room temperature.

When added to steel with the addition of the appropriate manganese, silicon improves the flowability of molten metal during welding, reducing the inclusions in the weld as much as possible and improving the strength without compromising the yield ratio and balance of 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, when the silicon (Si) is excessively added, surface defects due to plating property and red scale are generated, and problems such as unplating and plating peeling occur due to deterioration of plating adhesion, so the upper limit is limited to 0.30 wt%.

Manganese (Mn) 2.00 ~ 3.00wt%

Manganese (Mn) is a solid solution strengthening element, which stabilizes austenite to lower the two-phase region temperature and prevents austenite from being decomposed into pearlite even at a low critical cooling rate, thereby making it easy to generate residual austenite and martensite.

If a small amount of manganese is added, the shape of the steel sheet is poor due to the thermal stress generated due to the fast cooling rate for obtaining martensite, and therefore, more than 0.8 wt% is required.

However, in order to obtain tensile strength of 980 MPa or more, at least 2.00 wt% must be added to obtain desired elongation and weldability. In addition, when excessively added, the hardenability increases and the workability is inferior. In the slab casting, the manganese band structure is formed at the center of the thickness, so that the bending workability is deteriorated, and at the time of spot welding, there is a tendency to break at the nugget weld. Is 3.00 wt%.

Manganese inhibits the formation of pearlite phase (ferrite + cementite) and promotes austenite formation and C-concentration inside, contributing to the formation of residual austenite and enhancing the hardenability to facilitate martensite formation upon cooling. If it is less than wt%, it is impossible to prevent the production of pearlite because it requires a very high cooling rate, and if it exceeds 2.5wt%, manganese band structure is formed and increases rapidly, which impairs the workability and weldability of the steel. Therefore, the content of manganese is particularly preferably contained in the range of 2.0 ~ 2.50wt%.

Aluminum (Al): 0.10 ~ 0.50wt%

Aluminum is an element mainly used as a deoxidizer. Aluminum, like silicon, suppresses precipitation of cementite after annealing, thereby delaying the progress of pearlite transformation and stabilizing ferrite grains. In addition, when aluminum is added, ferrite transformation starts at an early stage, so that the thickening of carbon in austenite increases, thereby increasing the stability of the austenite phase at room temperature.

And by combining with nitrogen (N) in the steel to precipitate the AlN to refine the crystal grains to improve the strength of the steel sheet and keep the dissolved oxygen content in the steel sufficiently low to prevent cracking during slab manufacturing.

When a small amount of aluminum is added, the oxygen content in the steel increases, leading to a decrease in ductility. Therefore, an amount of 0.01 wt% or more is required. However, addition of more than 0.1wt% is essential for securing strength and residual austenite phase. On the other hand, when the content exceeds 0.5wt%, it inhibits the plating performance similarly to silicon, so it is preferable to contain it in the range of 0.10 to 0.50wt%.

Phosphorus (P): 0.05wt% or less

Phosphorus (P), like aluminum, is added to inhibit the formation of cementite and to increase strength, but is preferably not added. If it exceeds 0.05wt%, the weldability deteriorates and the final material deviation occurs due to the slab center segregation, so it is limited to the range of 0.05wt% or less.

Sulfur (S) 0.02wt% or less

Sulfur is an element that is inevitably contained in the production of steel, and is preferably not added because it inhibits toughness and weldability, increases the sulfide-based (MnS) base metal inclusions, and causes cracking. In particular, sulfur is regulated in the range of 0.02wt% or less because it increases the coarse inclusions to deteriorate the fatigue characteristics when over-added.

Niobium (Nb) Titanium (Ti): 0.01 ~ 0.10wt%

Niobium and titanium elements are elements that precipitate carbon or nitrogen in steel in the form of NbC, NbN, TiC, TiN, or improve the strength of steel through solid solution strengthening in iron (Fe). When added below 0.10wt%, the elements can contribute to the improvement of strength without impairing the gist of the present invention. Therefore, in some cases, one or more of niobium and titanium may be further contained in the range of 0.10 wt% or less.

Nitrogen (N) 0.01wt% or less

Nitrogen is refined by the formation of AlN, but is not preferably added because it is supersaturated during cooling in the alloying process of the galvanized layer during hot dip galvanizing and lowers the elongation, but it is limited to 0.01 wt% or less.

Nickel (Ni) 0.02 ~ 0.30wt%

Nickel is added as an element for preventing the red brittleness generated when copper is added for increasing strength and improving corrosion resistance. Usually, copper (Cu): nickel (Ni) = 1: 1 to 2 is the best effect when added. When it is difficult to secure quality such as red heat brittleness by adjusting process variables when adding copper (Cu), it is added within the range of 0.30 wt% or less according to the content of copper (Cu).

Copper (Cu) 0.01 ~ 0.30wt%

Copper (Cu), together with aluminum (Al), suppresses the precipitation of carbon in the bainite transformation zone and improves corrosion resistance along with the role of generating residual austenite. In addition, it has the effect of miniaturizing the ferrite grains has the function of increasing the strength. Copper is less than 0.30wt% because elongation decreases after 0.3wt%.

Chromium (Cr) 0.10 ~ 2.00wt%

Chromium is a ferrite-forming element that delays the transformation of austenite into pearlite or bainite, thereby austenite is transformed into bainite or martensite at room temperature and improved in strength after two-phase air cooling.

When chromium is added below 0.10wt%, it is difficult to obtain sufficient strength. When it is added above 2.00wt%, chromium decreases in ductility by causing an increase in strength, thereby causing a problem of a balance between strength and ductility.

Boron (B) 0.0005 ~ 0.0030wt%

Boron is an element that improves hardenability of steel and may be added as necessary. Boron is preferably added in more than 0.0005wt%, but if it is added in more than 0.0030wt% there is a problem of causing a deviation of the material by causing segregation in the grain boundary is preferably added in the 0.0005 ~ 0.0030wt% range.

The steel sheet of the present invention contains the above components, and the rest are substantially iron (Fe) and unavoidable elements, and fine amounts of inevitable impurities are also allowed as elements contained according to the situation of raw materials, materials, manufacturing facilities, and the like.

The slabs having the composition as described above are obtained by ingot or continuous casting process after obtaining molten steel through steelmaking process, and here, after hot rolling, the slabs are pickled and hot dip galvanized on the surface of the steel sheet. The following process is performed.

Each process is as follows.

[Heating 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. In addition, it is necessary to adjust the reheating temperature holding time according to the thickness of the slab, so that the thicker the thickness, the longer the reheating time, and the thinner the thickness, the shorter the holding time. The proper holding time is about 1 to 2 hours. If it is maintained for more than this time, it is uneconomical and if it is too short, the quality of the material may be degraded due to the homogenization of the material.

[Hot Rolling Process]

The slab reheated in the furnace process finishes the hot rolling in the temperature range of 870 ~ 930 ℃ so that the steel sheet structure has austenite structure before hot rolling.

When rolling below 870 ° C., excessive dislocations are introduced into the ferrite and grow during cooling or winding to form coarse grains on the surface. However, when the finish rolling at 930 ℃ or more increases the ferrite grain size decreases the strength.

After hot rolling, multi-stage cooling is carried out to the coiling temperature so that the steel sheet becomes a three-phase structure of ferrite-bainite-retained austenite and martensite.

In multi-stage cooling, after hot rolling, it is cooled to the range of 600 ~ 720 ℃ at the cooling rate of 30 ~ 100 ℃ / sec, followed by air cooling for 5 ~ 10 seconds, and 200 ~ 400 ℃ at the cooling rate of 30 ~ 100 ℃ / sec after air cooling. It adopts cooling method to the range and winding up. Then, a part of austenite is transformed into ferrite and bainite in the air cooling process so that the structure of the final hot rolled steel sheet becomes a three-phase structure of ferrite-bainite-martensite and residual austenite.

In this case, when the intermediate temperature of the multi-stage cooling is lower than 600 ° C., perlite is produced instead of ferrite, which makes it difficult to concentrate carbon in the austenite, and thus, it is difficult to retain austenite at room temperature.

When the coiling temperature is lower than 200 ° C., the stress is too high when the coil is wound, which causes a problem of unwinding. When the coiling temperature is higher than 400 ° C., the martensite fraction which cannot exhibit sufficient strength cannot be formed, and thus the mechanical properties cannot be satisfied. .

Pickling (PO) and Plating Process

The hot rolled steel sheet was immersed in 15-25% diluted hydrochloric acid solution heated to a temperature range of 60-85 ° C. for 10-30 seconds to remove the surface oxide layer, and then heated to 450-480 ° C. and maintained within 30 seconds. After hot dip galvanizing at ~ 500 ℃, alloying heat treatment is performed at 480 ~ 550 ℃.

If the heating temperature is lower than 450 ℃, the quality of the hot-dip galvanized steel sheet due to dross occurs.If the heating temperature is higher than 480 ℃ or the holding time exceeds 30 seconds, the material is rapidly deteriorated in the hot rolling process. It is difficult to obtain mechanical properties. Therefore, the heating temperature is 450 ~ 480 ℃, the holding time is preferably limited to within 30 seconds.

On the other hand, when the alloying heat treatment temperature is lower than 480 ° C, the alloy reaction rate of the plating layer decreases and the iron (Fe) concentration in the plating layer decreases, thereby making it difficult to secure stable growth of the plating layer. And higher than 550 ℃ can not achieve the object of the present invention to improve the workability of the steel sheet due to the transformation organic plasticity effect is easy to occur due to excessive alloying, peeling of the plating layer during molding and the fraction is reduced by decomposition of residual austenite. .

Hereinafter, the ultra-high strength hot rolled steel sheet and a method of manufacturing the same will be described in comparison with the invention and other comparative examples.

Table 1 shows the component ratio of the invention example of this invention and another comparative example.

division Chemical composition (wt%, balance Fe)  Remarks C Si Mn P S Al Cu Nb Ti Ni Cr B N Inventive Example 1 0.07 0.15 2.45 0.011 0.002 0.20 0.15 0.025 0.020 0.05 0.98 0.0015 0.0045 Comparative Example 1 0.14 0.21 1.55 0.016 0.002 - 0.64 - - 0.72 - - 0.0044 Comparative Example 2 0.21 1.50 2.10 0.010 0.003 - - 0.024 0.053 - - - 0.0041 Inventive Example 2 0.09 0.10 2.25 0.011 0.002 0.20 0.10 0.022 0.030 0.02 0.60 0.0015 0.0045 Inventive Example 3 0.10 0.12 2.33 0.011 0.002 0.20 0.13 0.045 - 0.06 1.12 0.0015 0.0045 Comparative Example 3 0.20 0.50 2.05 0.011 0.002 1.01 - 0.024 0.053 - 0.52 0.0015 0.0042 Comparative Example 4 0.25 0.50 2.02 0.011 0.002 0.99 - 0.027 0.052 - 0.52 0.0015 0.0049 Comparative Example 5 0.24 0.48 2.54 0.010 0.002 0.99 - 0.026 - - 0.51 0.0015 0.0035 Inventive Example 4 0.09 0.11 2.70 0.011 0.002 0.20 0.20 0.025 0.020 0.05 0.47 0.0015 0.0042 Inventive Example 5 0.12 0.10 2.45 0.011 0.002 0.15 0.20 0.022 0.040 0.08 0.85 0.0015 0.0044

Table 2 shows the results of measuring the mechanical properties of the specimens manufactured by the following heat treatment and rolling conditions using the slabs composed as shown in Table 1 above.

Inventive Example: After reheating at 1250 ° C. for 2 hours, hot rolling was performed at 900 mm or more to a thickness of 2.4 mm, followed by water cooling to 650 ° C. medium temperature, and then maintained for 8 seconds, followed by winding at 350 ° C. After pickling, hot dip galvanizing at 470 ° C. and then alloying heat treatment at 490 ° C. were performed.

Comparative Example: The hot rolling was finished at 900 ° C. and wound up at 650 ° C., followed by holding for 1 hour, followed by quenching. Thereafter, pickling was performed, and the 2.4 mm hot rolled sheet was cold rolled to a 1.4 mm thickness, and subjected to continuous annealing at an annealing temperature of 820 ° C, an aging temperature of 460 ° C, and a GA temperature of 520 ° C.

 division Mechanical properties  Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Hall expandability (%) Plating Weldability Inventive Example 1 799 993 14 46 Good Good Comparative Example 1 700 996 15 28 Bad Good Comparative Example 2 685 986 16 32 Bad Bad Inventive Example 2 820 1070 11 41 Good Good Inventive Example 3 930 1190 10 42 Good Good Comparative Example 3 709 1090 13 34 Bad Bad Comparative Example 4 790 1186 13 44 Bad Bad Comparative Example 5 782 1202 13 33 Bad Bad Inventive Example 4 820 1125 12 43 Good Good Inventive Example 5 895 1078 12 42 Good Good

As shown in Table 2, in the case of the present invention in which the multi-stage cooling was performed after the hot rolling process and the plating temperature was controlled, tensile strength of 980 MPa or more was secured, and the elongation also satisfied the standard range. In addition, it can be seen that both the hole expandability, the plating property, and the weldability of the present invention are good.

Claims (5)

0.05 to 0.20 wt% of carbon (C), 0.01 to 0.30 wt% of silicon (Si), 2.00 to 3.00 wt% of manganese (Mn), 0.10 to 0.50 wt% of aluminum (Al), 0.01 to 0.10 wt% of niobium (Nb), Titanium (Ti) 0.01 ~ 0.10wt%, Nickel (Ni) 0.02 ~ 0.30wt%, Copper (Cu) 0.01 ~ 0.30wt%, Chromium (Cr) 0.10 ~ 2.0wt%, Boron (B) 0.0005 ~ 0.0030wt%, 0.05 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.01 wt% or less of nitrogen (N), and balance of balance iron (Fe), Microstructure is super high strength hot rolled steel sheet, characterized in that it has a three-phase structure of bainite, ferrite, residual austenite and martensite. The method according to claim 1, Ultra-high strength hot rolled steel sheet, characterized in that the phase fraction of the bainite structure is 50 ~ 65 Vol%. 0.05 to 0.20 wt% of carbon (C), 0.01 to 0.30 wt% of silicon (Si), 2.00 to 3.00 wt% of manganese (Mn), 0.10 to 0.50 wt% of aluminum (Al), 0.01 to 0.10 wt% of niobium (Nb), Titanium (Ti) 0.01 ~ 0.10wt%, Nickel (Ni) 0.02 ~ 0.30wt%, Copper (Cu) 0.01 ~ 0.30wt%, Chromium (Cr) 0.10 ~ 2.0wt%, Boron (B) 0.0005 ~ 0.0030wt%, Steel slab having an alloy composition of phosphorous (P) 0.05 wt% or less, sulfur (S) 0.02 wt% or less, nitrogen (N) 0.01 wt% or less, and balance iron (Fe), Homogenizing heat treatment at 1150 ~ 1250 ℃, hot rolling finish at a temperature range of 870 ~ 930 ℃, wound at 200 ~ 400 ℃ after the pickling and plating process characterized in that the hot-rolled steel sheet. The method according to claim 3, After the hot rolling, after cooling to a range of 600 to 720 ° C. at a cooling rate of 30 to 100 ° C./sec, air cooling is performed for 5 to 10 seconds, and after the air cooling, multistage cooling to a winding temperature at a cooling rate of 30 to 100 ° C./sec. Method for producing a super high strength hot rolled steel sheet, characterized in that for carrying out. The method according to claim 3 or 4, In the plating treatment, the hot rolled steel sheet is pickled, heated to a temperature of 450 to 480 ° C., maintained within 30 seconds, hot-dip galvanized at 450 to 500 ° C., and then cooled after performing an alloy heat treatment at 480 to 550 ° C. Ultra high strength hot rolled steel sheet manufacturing method characterized in that.