KR100573587B1 - Method for manufacturing Ultra High Strength Steel Sheet Having Excellent Bending Formability - Google Patents

Abstract

       The present invention relates to a method for producing an ultra high strength cold rolled steel sheet.

According to the present invention, by weight ratio of carbon 0.15 to 0.25%, silicon 0.6 to 1.5%, sulfur 0.01% or less, nitrogen 0.005% or less, aluminum 0.02 to 0.06%, manganese 1.8 to 2.8%, phosphorus 0.04 to 0.1%, chromium 0.1 to 0.5%, niobium 0.005 ~ 0.05%, and the rest is made of aluminum-kilted steel composed of iron and unavoidable impurities, homogenizing it, and finishing hot rolling on the Ar ₃ transformation point and hot-rolled to produce hot rolled steel sheet. Step, cold-rolling the hot rolled steel sheet after pickling, and annealing at a temperature of Ar ₃ or higher, followed by slow annealing to 600 to 730 ° C., quenching to 500 to 250 ° C., and then maintaining a temperature at that temperature for at least 1 minute. Provided is a method of manufacturing an ultra high strength cold rolled steel sheet, characterized in that

According to the present invention, it is possible to manufacture an ultra-high strength steel sheet having excellent bending workability with a tensile strength of 120kgf / mm ² or more and an elongation of 10% or more by using existing cooling equipment without the need for a special rapid cooling equipment for the continuous annealing equipment. This steel sheet is suitable for use in bumper reinforcement materials of automobiles mainly for roll forming.

Ultra high strength cold rolled steel, ferritic phase, continuous annealing heat treatment, bending workability

Description

Method for manufacturing Ultra High Strength Steel Sheet Having Excellent Bending Formability}

 1 is an optical micrograph of the present invention steel and comparative steel.

The present invention relates to a method for manufacturing an ultra high strength cold rolled steel sheet, and more particularly, to manufacture an ultra high strength cold rolled steel sheet having a tensile strength of 120kgf / mm ² or more and an elongation of 10% or more and excellent bending workability for use in a bumper reinforcement of an automobile. It is about a method.

Conventional high strength steel for automobile reinforcement is a low carbon steel containing a large amount of alloying elements and has been manufactured using a continuous annealing facility. In this case, a variety of phases such as precipitates, bainite, and retained austenite are generated in the microstructure of the steel sheet to secure strength and workability. Such steel sheet is secured in strength and workability, but has a disadvantage of poor weldability due to the addition of a large amount of alloying elements. If the weldability is poor, since the connection between the coil and the coil is difficult in the manufacturing process for producing the steel sheet is not only workability is worse but there is a problem that cannot be manufactured in the continuous annealing process.

Meanwhile, in order to reduce alloying elements, there is a method of manufacturing super high strength steel by quenching by water cooling and then tempering or overaging, but in this case, if the tempering temperature is too low, A large amount of carbides, which adversely affects bendability, is precipitated, and as a result, the bending workability of the steel sheet is deteriorated, and the steel sheet cannot be applied for roll processing, and if the tempering temperature is increased, the quenching effect is lost and the strength of the steel sheet is rapidly increased. There is a disadvantage of decreasing.

In order to solve this problem, Korean Patent Application No. 2002-84573 has a weight ratio of 0.15 ~ 0.20% carbon, 0.3 ~ 0.8% silicon, 0.02% or less sulfur, 0.005% or less nitrogen, 0.02 ~ 0.06% aluminum, 1.8 ~ 2.5% manganese. , Molybdenum 0.1 ~ 0.4%, niobium 0.03 ~ 0.06%, the rest is homogenized and hot rolled and cold rolled aluminum-kilted steel composed of iron and unavoidable impurities, and then hot-annealed at Ar ₃ or higher and then 350 ~ 300 ℃ A method of quenching up to and maintaining it for at least 1 minute while cooling slowly at this temperature range is proposed. According to this method, a steel sheet having a tensile strength of 120 kgf / mm ² or more and an elongation of 10% or more can be obtained without using a large amount of alloying components. However, there are some problems in bending processability, in particular, the process of quenching to 350 ~ 300 ℃ after high temperature annealing in Ar ₃ or more, and there is a problem that a special rapid cooling equipment is required for continuous annealing equipment.

It is an object of the present invention to manufacture an ultra-high strength steel sheet having excellent bending workability with a tensile strength of 120kgf / mm ² or more and an elongation of 10% or more by using existing cooling facilities without the need for a special rapid cooling system for continuous annealing facilities.

In order to achieve the above object, the present invention, by weight ratio of carbon 0.15 ~ 0.25%, silicon 0.6 ~ 1.5%, sulfur 0.01% or less, nitrogen 0.005% or less, aluminum 0.02 ~ 0.06%, manganese 1.8 ~ 2.8%, phosphorus 0.04 ~ 0.1 %, Chromium 0.1 to 0.5%, niobium 0.005 to 0.05%, and the remainder are made of aluminum-kilted steel consisting of iron and inevitable impurities and homogenized at 1050 to 1300 ° C, 850 to 950 just above the Ar ₃ transformation point. Manufacturing hot rolled steel sheet by finishing hot rolling at ℃ and hot rolled at a temperature range of 550 to 700 ° C., and then cold rolling the hot rolled steel sheet to a cold reduction rate of 30 to 80% after pickling, and at a temperature of Ar ₃ or more. After the high temperature annealing to 600 ~ 730 ℃ slow cooling to 500 ~ 250 ℃ after annealing provides a method for producing a super high strength cold rolled steel sheet having excellent workability, characterized in that it consists of a continuous annealing step that is maintained for more than 1 minute.

At this time, the cooling rate is preferably -10 ~ -50 ℃ / sec.

The present invention controls the amount of manganese, silicon, chromium, phosphorus, niobium, etc. added to the low carbon aluminum-kilted steel, thereby remaining in the final steel after continuous annealing heat treatment using the existing cooling equipment without the need for special rapid cooling equipment. Distributing the austenite or martensite phase and the ferrite phase has a characteristic of producing a high strength cold rolled steel sheet having high tensile strength and excellent bending workability.

Hereinafter, the present invention will be described in detail.

In this invention, adjustment of the component added to steel is very important, and each component is adjusted as follows.

Carbon is an essential element in steel, and in the present invention, the amount of carbon is limited to 0.15% to 0.25% by weight (hereinafter referred to as%). When 0.15% or less is used, not only grains grow but also solid solution effect and precipitation strengthening by carbon. It is because the effect is small and sufficient tensile strength cannot be secured, and when the amount of carbon is 0.25% or more, the tensile strength increases but the workability deteriorates. In particular, as the amount of carbon increases, the weldability of steel decreases drastically, and thus, when continuously working like continuous annealing equipment, breakage occurs at the welds connecting the work coils before and after. Also, since it is greatly reduced, it is necessary to limit the above range.

Silicon (Si) has the effect of improving the strength of the steel sheet by the solid-solution strengthening effect, and clean the ferrite to improve the ductility of the steel. And because it is a ferrite transformation promoting element to form a ferrite phase in the final structure after annealing to form a certain amount of ferrite phase in the final structure. Therefore, the minimum addition amount of Si was limited to 0.6%. However, as the amount of Si added increases, the rolling load increases during hot rolling, Si oxides are formed, and the pickling property is deteriorated, and the surface quality of the steel sheet is deteriorated by being concentrated on the surface of the cold rolled steel sheet during annealing. There is a disadvantage that the weldability is also bad. Therefore, the upper limit of the addition amount of Si was limited to 1.5%.

Manganese (Mn) has a high effect of delaying the transformation of austenite to ferrite and lowering the transformation temperature of austenite to ferrite, together with a solid solution strengthening effect. When the amount of Mn added is less than 1.8%, the temperature of Ar ₃ increases, making it difficult to anneal the austenite single phase in annealing. If the austenite single phase reverse annealing is not performed during annealing, the bending property is greatly reduced because the target structure (Mn banded structure) of the hot-rolled steel sheet remains as it is. Therefore, the minimum addition amount of Mn was made into 1.8%. Since Mn has a high solid-solution strengthening effect, an appropriate amount must be added to secure sufficient tensile strength, but when Mn addition amount exceeds 2.8%, the hardenability is so high that martensite phase is easily formed even at low cooling rate. When the phase is formed, the load is applied during cold rolling, and the delayed structure phenomenon occurs when a large amount of martensite is formed. Therefore, the amount of Mn should be controlled to 2.8% or less.

Aluminum (Al) is added for deoxidation of the steel. When the amount of aluminum added is less than 0.02%, oxygen is present in the steel. When oxygen is present in the steel, when oxide forming elements such as manganese and silicon are added during steelmaking, manganese oxide, silicon oxide, and the like are formed, which makes it difficult to control the components of manganese and silicon. When the amount of aluminum is more than 0.06%, the amount of aluminum is added more than necessary, which increases the manufacturing cost and generates a large amount of surface defects of the steel sheet. Therefore, the upper limit of aluminum is limited to 0.06%.

Since chromium (Cr) not only increases the hardenability but also increases the toughness of the steel sheet, it is very effective when added to a steel sheet requiring high collision energy absorption. Therefore, the addition of chromium is important in order to prevent the deterioration of the bendability due to the formation of the second phase. The addition range is limited to 0.1 to 0.5%. In order to obtain excellent bendability, more than 0.1% of chromium should be added. However, if the amount of chromium is increased to more than 0.5%, a large amount of martensite is produced, which may cause deterioration of bending workability and mar-de-bact by martensite, thereby causing bumpers. This is because it becomes difficult to use the impact absorbing member such as the reinforcement.

Niobium (Nb) acts as a precipitation hardening element, but refines ferrite grains and refines a second phase such as martensite or residual austenite to improve the workability of the steel. Therefore, Nb is added, but the range of addition is limited to 0.005 to 0.05%. When niobium is added at 0.05% or more, cracks are likely to occur in the slab during the playing process, and the recrystallization temperature rises rapidly, resulting in high temperature annealing. This is because the workability of the steel sheet is reduced because no complete recrystallization occurs at.

Phosphorus (P) is also a solid solution element, but in order to solve the problem caused by the addition of a large amount of silicon as an element added instead if the addition amount is too small the formation of a ferrite phase, the residual austenite phase is difficult to form. However, if the addition amount is too large, the weldability is sharply worsened, so the addition amount of phosphorus was limited to 0.04 to 0.10%.

 In general, sulfur (S) is an element that is inevitably contained in the production of steel, the amount was limited to less than 0.01%, nitrogen also inevitably contained as an element is limited to less than 0.005%.

After dissolving the steel of the composition to make a slab and performing a homogenization treatment at about 1050 ~ 1300 ℃ similar to the conventional conditions, the homogenized treatment is finished under normal conditions in the range of 850 ~ 950 ℃ immediately above the Ar ₃ temperature Finish hot rolling is performed and hot rolling wound at 550 to 700 ° C is performed. If the hot rolling temperature is too low, bainite or martensite is formed in the hot rolled steel sheet, which increases the strength of the hot rolled steel sheet, making cold rolling difficult, and cracking occurs at the edge during cold rolling, causing plate breakage. On the other hand, when the coiling temperature is increased, the strength of the hot rolled steel sheet is lowered, but coarse pearlite is formed, and during cold rolling, cracking occurs at the edge and plate breaking occurs. Therefore, the hot rolled winding temperature was limited to 550 ~ 700 ℃.

After hot rolling, cold rolling is performed to match the shape and thickness of the steel plate after pickling. Although the reduction rate does not have a significant effect on the material, if the reduction rate is not sufficient, sufficient strength cannot be obtained, and when the reduction rate increases, the grains become finer and the strength increases, but in the case of high strength steel, when the rolling rate is too high, cold rolling As the load on the rolling mill increases, the rolling reduction is limited to 30 ~ 80% in consideration of securing the required strength and ease of operation of cold rolling.

When the steel sheet is cold rolled, it enters the annealing process. In order to ensure sufficient workability and bending workability, annealing is carried out at an Ar ₃ temperature or higher to be annealed in the austenitic single phase region. After high temperature annealing above Ar ₃ temperature, it is cooled slowly to 600 ~ 730 ℃ and quenched to 500 ~ 250 ℃ to maintain at least 1 minute at that temperature. At this time, the cooling rate is preferably -10 ~ -50 ℃ / sec. The cooling rate in this way is to allow a small amount of ferrite phase to be generated during cooling, because the ferrite phase formed in a small amount together with the cornerstone ferrite greatly improves the workability of the steel. Since the cooling rate is a cooling rate that can be obtained in a typical continuous annealing facility without additional cooling facility reinforcement, ultra high strength steel can be manufactured without modification of the continuous annealing facility.

Hereinafter, embodiments of the present invention will be described.

Table 1 shows the steel composition of the inventive steel and the comparative steel (in this case, nitrogen is all 20-40ppm level) used in the examples. The molten steel ingot was heated at 1250 ° C. for one hour and then hot rolled. At this time, the hot rolling finish temperature was 900 ° C and the winding temperature was 620 ° C. The hot rolled steel sheet was subjected to a tensile test using a universal tensile tester to evaluate the cold rolling property, and the results are shown in Table 3. The hot rolled steel sheet was cold rolled at 50% of cold rolling after pickling. . Cold-rolled specimens were annealed at 830 ° C (maintained for 100 seconds annealing time), quenched slowly to 680 ° C, and quenched to -30 ° C / sec to 350 ° C (350 ° C and 450 ° C in the present invention). It was. It was maintained at this temperature for 300 seconds to perform overaging. After the continuous annealing, a tensile test was performed using a universal tensile tester, and a minimum radius of cracking during bending was obtained using a V-bend tester, and the results are shown in Table 2.

Steel No. 1 to No. 2 of the present invention steel is a cold rolled steel sheet having a tensile strength of 120kgf / mm ² or more and an elongation of 10% or more as a result of tensile test after continuous annealing. In addition, since the minimum radius of cracking does not occur is 0mm can be said to be a suitable material for the steel sheet of the product subjected to bending processing such as roll processing. In spite of the high strength of the invention steel, the excellent ductility is attributed to the fact that as shown in Fig. 1, the addition of a large amount of silicon and phosphorus produced a large number of ferrite phases, which are advantageous for workability. It can be seen that a steel sheet having good high strength and workability can be obtained even under conditions that can be easily obtained in a conventional continuous annealing heat treatment facility.

Sample No. 3 steel is added with molybdenum, a hard hard element, to secure sufficient tensile strength in cold rolled steel sheets, but it is not suitable as a bumper reinforcement for automobiles because it has a minimum bending radius of 3mm and is liable to crack during roll forming. And, as can be seen in Table 3, because the strength of the hot rolled steel sheet is high, the rolling load is increased during cold rolling, and edge cracks are generated, making it difficult to produce in actual production facilities.

Steel No. 4 is not suitable for the bumper reinforcement of automobiles that require high strength because it does not secure sufficient tensile strength because the amount of carbon and silicon is low and the amount of chromium, which is a hardenable element, is low.

Steel No. 5 has a small amount of silicon and phosphorus in the steel, and a large amount of titanium, which is a precipitation-reinforced element, is added but is not suitable for the bumper reinforcement of automobiles because of its low tensile strength.

Sample No. 6 steel was not added with niobium, a precipitation-reinforcing element, and thus it was judged that the precipitation strengthening effect and grain refinement were not strengthened by niobium precipitates. Is not suitable.

Sample No. 7 steel can secure the strength suitable for the bumper reinforcement material of automobiles, but it is not suitable for the bumper reinforcement material mainly for roll forming process due to its low bending workability. And, as can be seen in Table 3, because the strength of the hot rolled steel sheet is high, the rolling load is increased during cold rolling, and edge cracks are generated during rolling, making it difficult to produce in actual production facilities.

According to the present invention from the above it can be seen that in the conventional continuous annealing heat treatment equipment, it is possible to produce an ultra-high strength steel sheet having a tensile strength of 120kgf / mm ² or more, elongation 10% or more and very excellent bending workability.

Table 1

TABLE 2

TABLE 3

According to the present invention, it is possible to manufacture an ultra-high strength steel sheet having excellent bending workability with a tensile strength of 120kgf / mm ² or more and an elongation of 10% or more by using existing cooling equipment without the need for a special rapid cooling equipment for the continuous annealing equipment. This steel sheet is suitable for use in bumper reinforcement materials of automobiles mainly for roll forming.

Claims (2)

By weight ratio, carbon 0.15 to 0.25%, silicon 0.6 to 1.5%, sulfur 0.01% or less, nitrogen 0.005% or less, aluminum 0.02 to 0.06%, manganese 1.8 to 2.8%, phosphorus 0.04 to 0.1%, chromium 0.1 to 0.5%, niobium 0.005 ~ 0.05%, and the remainder is made of aluminum-kilted steel composed of iron and inevitable impurities, homogenized at 1050 ~ 1300 ℃, then hot-rolled finish at 850 ~ 950 ℃ immediately above Ar ₃ transformation point, 550 ~ Hot rolled winding in a temperature range of 700 ℃ to make a hot rolled steel sheet; Cold-rolling the hot rolled steel sheet at a cold rolling reduction of 30 to 80% after pickling; And a continuous annealing step after the high temperature annealing at a temperature of Ar ₃ or higher and then slowly cooled to 600 to 730 ° C. and quenching at a cooling rate of −10 to −50 ° C./sec to 500 to 250 ° C. and then maintaining at that temperature for at least 1 minute. A method of manufacturing an ultra-high strength steel sheet having excellent bending workability with a tensile strength of 120 kgf / mm 2 or more and an elongation of 10% or more The method of manufacturing an ultra-high strength cold rolled steel sheet having excellent workability according to claim 1, wherein the cooling rate in the continuous annealing step is -10 to -50 ° C / sec.

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