KR100400864B1 - Automotive cold rolled sheet with excellent impact resistance and manufacturing method - Google Patents

Abstract

The present invention relates to a high-strength cold rolled steel sheet for automobiles having excellent roll processability and excellent impact energy absorption after processing, and a method for manufacturing the same, in terms of weight% of carbon: 0.10 to 0.20%, nitrogen: 0.01% or less, sulfur: 0.025% or less , Manganese: 1.5 ~ 3.0%, Silicon: 0.2 ~ 0.8%, Aluminum: 0.02 ~ 0.06%, Phosphorus: 0.03% or less, Niobium: 0.005 ~ 0.03%, but (10 × C + 2 × Si + Mn + 100 XNb + 10xMo) to 6 or more, Mn / Si ratio to 4 or more, and 1050-1300 steel slabs of steel containing elements inevitably contained in the production of other steels and aluminum-kilted steels having the above composition. Homogenizing treatment at about ℃; Finishing hot rolling at 850 ° C. to 950 ° C. directly above the Ar3 transformation point; Hot rolling in a temperature range of 450 ~ 700 ℃; Cold rolling at a cold reduction rate of 40 to 80%; Performing continuous annealing at a temperature range of 750 to 900 ° C; The present invention provides a method of manufacturing a cold rolled steel sheet having excellent impact resistance.

According to the present invention, the yield strength is high and the yield ratio is high, despite the high tensile strength, by appropriately adjusting the addition amount and the addition ratio of manganese (Mn), silicon (Si), niobium (Nb), etc. to the low carbon aluminum kicked steel. Since it is high, it has an effect that can be used as a steel sheet of the bumper reinforcement manufactured by roll processing.

Description

Automotive cold rolled steel sheet with excellent impact resistance and manufacturing method

The present invention relates to a high-strength cold-rolled steel sheet for automobiles having excellent roll workability and excellent impact energy absorption after processing and a method of manufacturing the same. More specifically, the addition amount and addition ratio of manganese, silicon, niobium, etc. Properly adjusted to precipitate bainite phase and fine carbide inside ferrite phase, it has high elongation and excellent workability. Also, it has high tensile strength and yield strength. The present invention relates to a high strength cold rolled steel sheet for automobiles which can be used as a component and has excellent impact resistance of flash butt weldability, and a method of manufacturing the same.

Conventional high strength cold rolled steel sheet has been improved in strength by utilizing the simple solid solution effect of these substituted elements by adding manganese (Mn), phosphorus (P), etc., which are solid solution strengthening elements, to low carbon aluminum-kilted steel. However, the tensile strength of such a steel sheet was about 60 Kg / mm ² .

On the other hand, low-carbon aluminum-kilted steel is added with manganese, chromium, molybdenum, etc., which have high reinforcing ability, and a composite steel structure in which martensite and ferrite are present at the same time by increasing the cooling rate in the quenching zone of the continuous annealing process.

However, the yield ratio of the steel thus produced is low, and in particular, the yield strength is low compared to the high tensile strength, it was difficult to apply for the bumper reinforcement processed by roll processing.

In addition, since martensite is generated only when the cooling rate is kept very high in the quenching zone of the continuous annealing process, an additional cooling facility is required for the continuous annealing facility, thereby increasing the manufacturing cost. In addition, since the martensite formed by quenching is decomposed during the overaging treatment in the continuous aging process of the continuous annealing process, it is difficult to manufacture the composite tissue steel in the conventional continuous annealing process.

In other words, since most martensite is decomposed in the overaging treatment usually performed at 450 to 400 ° C., it is difficult to secure high tensile strength in the existing component system.

In addition, the precipitation hardening type high tensile strength steel in which Ti is added to the low-carbon aluminum-kilted steel has a disadvantage of being difficult to use for processing because of its low elongation.

The present invention has been made to solve the above problems, an object of the present invention is to provide a high yield ratio high strength cold rolled steel sheet having a high elongation, high yield strength, excellent workability and good flash butt weldability and a method of manufacturing the same.

In order to achieve the above object, the present invention provides carbon: 0.10 to 0.20%, nitrogen: 0.01% or less, sulfur: 0.025% or less, manganese: 1.5 to 3.0%, silicon: 0.2 to 0.8%, aluminum: 0.02 by weight. ~ 0.06%, phosphorus: 0.03% or less, niobium: 0.005 ~ 0.03%, add (10 × C + 2 × Si + Mn + 100 × Nb) to 6 or more, Mn / Si ratio to 4 or more In addition, it is characterized by providing a cold rolled steel sheet for automobiles having excellent impact resistance including elements that are inevitably contained in the production of other steels.

In addition, the present invention comprises the steps of homogenizing the aluminum-kilted steel with the above composition at about 1050 ~ 1300 ℃ step, hot rolling finish at 850 ~ 950 ℃ directly above the Ar ₃ transformation point, and the temperature range of 450 ~ 700 ℃ The method of manufacturing cold rolled steel sheet having excellent impact resistance comprising the steps of hot rolling at cold rolling, cold rolling at a cold reduction rate of 40 to 80%, and cooling of continuous annealing at a temperature range of 750 to 900 ° C. To provide.

Hereinafter, the present invention will be described in more detail.

The present invention is characterized by the ability to manufacture a cold rolled steel sheet having high impact resistance and high yield strength by using the annealing cycle of the existing continuous annealing equipment by appropriately adjusting the addition amount and addition ratio of manganese, silicon, niobium have.

That is, according to the present invention, high-strength cold-rolled steel sheet for automotive parts requiring impact absorption, such as bumper stiffeners because of the high elongation and high yield strength using low carbon aluminum-kilted steel, carbon: 0.10 by weight ratio in the chemical composition 0.20%, nitrogen: 0.01% or less, sulfur: 0.025% or less, manganese: 1.5-3.0%, silicon: 0.2-0.8%, aluminum: 0.02-0.06%, phosphorus: 0.03% or less, niobium: 0.005-0.03% (10 × C + 2 × Si + Mn + 100 × Nb) to 6 or more, Mn / Si ratio of 4 or more, and the steel piece of aluminum-kilted steel containing the element inevitably contained in the steel production After homogenizing at the temperature of 1050 ~ 1300 ℃, roll it to the normal hot rolling temperature, but finish hot rolling temperature is 850 ~ 950 ℃, which is directly above Ar3 transformation point, and hot rolling is carried out in the temperature range of 450 ~ 700 ℃. In the cold rolling process, rolling is carried out at a cold rolling rate of 40 to 80%. Ignoring, it is prepared by carrying out the cooling of a continuous annealing at a temperature range of 750 ~ 900 ℃.

At this time, the cooling rate in the slow cooling zone of continuous annealing should be kept below -10 ℃ / sec, and the quenching start temperature should be performed at 600 ℃ or higher. The cooling rate in the quench zone should be maintained at the normal quench rate but maintained above -10 ℃ / sec. In addition, over-aging treatment of continuous annealing should be carried out at 500 ~ 350 ℃.

Hereinafter, the reason for limitation on the composition range and manufacturing conditions of the cold rolled steel sheet according to the present invention.

Since carbon (C) has a high solid solution strengthening effect, if the amount of carbon is less than 0.10% by weight (hereinafter referred to as "%"), the solid solution strengthening effect is small, and the amount of carbon is not sufficiently concentrated on the austenite in the annealing process. Since the strength of the second phase present in the ferrite phase is low, it is difficult to secure a tensile strength of 80 kgf / mm 2 or more. Therefore, in order to secure sufficient tensile strength, the amount of carbon added was limited to 0,10% or more. If the amount of carbon exceeds 0.20%, the tensile strength increases but the elongation decreases significantly. As the carbon content increases, the weldability deteriorates because the carbon equivalent increases. Therefore, the upper limit of carbon addition was limited to 0.20%.

Nitrogen (N), like carbon, is an element having a high solid solution strengthening effect, so the strength increases as the amount is increased, but since the effect of reducing the elongation is very large, the upper limit of nitrogen addition is limited to 0.010%.

Manganese (Mn) is less than 1.5% is difficult to secure tensile strength when added. Since manganese is known to have a relatively high solid solution effect, the amount of manganese added is required more than 1.5% to secure the strength of phosphorus. However, if the amount of manganese exceeds 3.0%, the strength increases due to the effect of solid solution of manganese, but the elongation decreases, resulting in the deterioration of workability, and the addition of a large amount of manganese implies a large increase in manufacturing cost. Is difficult to use commercially. In addition, the increase in manganese increases the carbon equivalent, thereby also weakening the weldability. Therefore, the range of addition of manganese was limited to 1.5 ~ 3.0%.

Sulfur (S) is generally an element that is inevitably contained in the production of steel, so the addition range was limited to 0.025% or less.

Since silicon (Si) is an element having a high solid solution strengthening effect, it should be added at least 0.2% to secure sufficient tensile strength. However, when a large amount is added, there is a drawback in that the elongation is decreased and the workability is worsened. In addition, when it is added in excess of 0.8%, martensite is formed, so the yield strength is lowered. In addition, when the content exceeds 0.8%, the flash butt weldability is greatly deteriorated, so the upper limit amount is limited to 0.8%.

The flash butt weldability is greatly affected by the ratio of Mn / Si. When the Mn / Si ratio is 4 or less, the flash butt weldability is greatly deteriorated, so the Mn / Si ratio is set to 4 or more.

Phosphorus (P) is often added as a solid solution element in high strength steels, but the upper limit of phosphorus (P) is limited to 0.03% because the bending workability is degraded when a large amount of phosphorus is added.

Aluminum (Al) is added to the steel for deoxidation. If the amount of aluminum is less than 0.02%, oxygen is present in the steel to form manganese oxide and silicon oxide when oxidizing elements such as manganese and silicon are added during steelmaking. This makes it difficult to control the components of manganese and silicon. And, if the amount of aluminum exceeds 0.06%, the amount of aluminum is added more than necessary to increase the manufacturing cost and generate a large amount of surface defects of the steel sheet, so the upper limit of the aluminum addition is limited to 0.06%.

Niobium (Nb) has an effect of increasing the tensile strength by forming a fine precipitate by adding a small amount in the steel. Niobium has the effect of improving ductility by fixing some carbon in the steel and reducing the carbon concentration in the ferrite. Therefore, the minimum addition amount of niobium was limited to 0.005%. However, when a large amount of niobium is added, a large amount of carbides, such as niobium carbide (NbC), is precipitated and the recrystallization temperature is increased, which leads to incomplete annealing during annealing, thereby deteriorating the ductility of the steel. Therefore, the upper limit of niobium addition amount was limited to 0.03%.

The reason why (10 x C + 2 x Si + Mn + 100 x Nb) is 6 or more is that a sufficient amount of alloying elements must be added to secure sufficient tensile strength and yield strength under ordinary annealing conditions. That is, under normal annealing conditions, the overaging temperature should be maintained at 350 ℃ or higher. At this temperature, since martensite formed during the quenching process is decomposed, a sufficient amount of alloying elements should be included to prevent the yield strength and tensile strength. It can be secured.

According to the present invention, the slab of the steel dissolved in the composition is homogenized at about 1050 to 1300 ° C. If the homogenization temperature is less than 1050 ℃, slab temperature is low, hot rolling is difficult, and if the homogenization temperature exceeds 1300 ℃ fine precipitates are precipitated and workability worsens. Therefore, homogenization temperature is maintained at 1050-1300 degreeC.

Homogenized specimens are subjected to hot-rolling at 850-950 ° C., directly above Ar ₃ temperature, and wound at 450-700 ° C. to obtain hot-rolled sheet structure in which fine precipitates are distributed.

If the finishing temperature is less than 850 ° C., the grains are formed and workability is greatly reduced. If the finish temperature is more than 950 ° C., the grain growth rate is increased immediately after hot rolling to obtain coarse grains, thereby degrading workability. In addition, the hot rolled coiling temperature is 700 ° C. or less so that the size of the carbide precipitated on the hot rolled sheet can be reduced to facilitate decomposition in the annealing process. That is, since the fine carbide is easily dissolved in the annealing process and the carbon is concentrated into austenite, the second phase at room temperature is increased to increase the tensile strength and clean the ferrite phase, thereby increasing the elongation.

On the other hand, if the hot rolled coil temperature exceeds 700 ° C, coarse pearlite is formed in the steel and it is difficult to decompose during annealing, making carbon concentration to austenite difficult. Further, since the ferrite is not cleaned and the elongation is lowered, workability in the cold rolled steel sheet is greatly deteriorated. Therefore, the upper limit of the hot rolled winding temperature was limited to 700 ° C. When the coiling temperature is less than 450 ° C, the quenching should be performed after the finish rolling, so that the shape of the hot rolled sheet is poor and the winding after the hot rolling makes it difficult to limit the lower limit temperature to 450 ° C.

The cold rolling reduction effect on the material is large, but if the rolling reduction is not sufficient, it is difficult to secure sufficient strength, so the lower limit of the rolling reduction is 40% .As the rolling reduction increases, the grain size becomes fine and the strength increases, but in the case of high strength steel When the reduction ratio exceeded 80%, the load of the rolling mill was greatly increased during cold rolling, so the upper limit of the reduction ratio was set to 80%.

After cold rolling, annealing is performed. At this time, cooling of the continuous annealing is performed in the annealing temperature range of 750 to 900 ° C. When the continuous annealing is cooled in the two-phase region, the solid solution carbon in the ferrite is released to austenite. Therefore, in order to increase the amount of carbon and manganese of austenite to increase the amount of the second phase present after quenching, the lower and upper temperatures of the continuous annealing cooling were limited to 750 ° C and 900 ° C, respectively.

By performing the cracking treatment at this temperature before starting cooling, the ferrite is cleaned and the carbon and manganese of austenite are concentrated. In order to further increase the carbon content of austenite, a slow cooling process is required after the cracking process in the two-phase region. This is because some of the austenite formed during cracking transforms into saltpeter ferrite, which further increases the amount of austenite carbon.

At this time, the rapid quenching of the continuous annealing should be performed at a temperature of 600 ° C. or higher, because when the end cooling temperature is less than 600 ° C., a large amount of austenite is transformed into ferrite and some austenite is transformed into pearlite. That is, since the amount of austenite remaining after quenching decreases, the strength decreases and the elongation decreases, the lower limit of the slow cooling end point temperature is set below 600 ° C.

In the slow cooling zone, the cooling rate should be as low as possible so that austenite does not transform into pearlite to release carbon in the ferrite into austenite. Therefore, if the cooling rate is too fast, the austenite is not sufficiently transformed into ferrite, so it is difficult to clean the ferrite, and because the carbon concentration of the austenite is not sufficiently achieved, the slow cooling upper limit rate is limited to -10 ° C / sec.

The cooling rate in the quench zone should be at the normal quench rate, but should be maintained above -10 ℃ / sec. This is for retaining the austenite remaining after slow cooling until the overaging step.

The quench end temperature must be maintained above the martensitic transformation temperature (Ms) of the steel. When the quench end temperature is below the martensite transformation temperature, most of the austenite is transformed to martensite, so the elongation is greatly reduced, and the yield strength of the cold rolled steel sheet is drastically reduced due to the operation potential caused by martensite formation. For this reason, the quench end point temperature should be maintained above Ms or above 400 ℃.

According to the present invention, overaging plays an important role in improving the elongation. This is because, in the overageing process, a part of the austenite remaining after quenching converts to bainite, thereby further concentrating carbon in the austenite. Therefore, overaging must be performed between 500 and 350 ° C, which is the normal bainite transformation temperature range of steel.

Hereinafter, the present invention will be described in detail through examples.

Table 1 shows the chemical composition of the present invention steel and comparative steel, the steel strip of the molten steel was subjected to hot rolling after homogenization treatment for 1 hour in a 1250 ℃ heating furnace. At this time, the hot rolling finish temperature was 900 ℃, the coiling temperature was 620 ℃, and the cold rolling was performed at 48% cold rolling rate, and then the cold start temperature of the annealing process was 800, 830, 860 ℃ for 60 seconds. Was carried out. During continuous annealing, the cooling rate of the slow cooling zone was -6 ° C / sec, and the cooling rate of the quenching zone was -30 ° C / sec. Subsequently, overaging treatment was 420 degreeC. Annealing time was 60 second. The specimens after continuous annealing were subjected to a tensile test using a universal tensile tester.

Table 2 shows the change in mechanical properties according to the annealing temperature of the inventive steel and the comparative steel. Steel Nos. 1 to 3 of the present invention exhibit a high yield ratio type high strength with a yield strength of 50kgf / mm2 or more, a tensile strength of 80kgf / mm2 or more, a yield ratio (yield strength / tensile strength) of 0.67 or more and an elongation of 20% or more. Is showing.

However, in the case of steel No. 4, which is a comparative steel, niobium was not added, and thus carbides were not precipitated in the annealing process, thereby ensuring sufficient tensile strength.

For steel Nos. 5 and 6 of the comparative steels, silicon and molybdenum were added to show the properties of the composite steel, resulting in reduced yield strength, which is not suitable for bumper reinforcement materials. In addition, since the addition amount of silicon is high and Mn / Si is high, the flash butt weldability is poor, and thus there is a high possibility of plate breaking in the rolling line during field production.

In addition, steel No. 7 of the comparative steel is a precipitation hardening type high tensile strength steel in which strengthening by titanium precipitates occurs. Since the amount of manganese is not sufficient, the tensile strength is not high, and the elongation is low, so that it is difficult to apply for processing.

As described above, according to the present invention, the elongation is high and yields in spite of the high tensile strength by appropriately adjusting the addition amount and the addition ratio of manganese (Mn), silicon (Si), niobium (Nb) and the like to the low carbon aluminum killed steel. Because of high rain yield and high yield strength, it has an effect that can be used as a steel sheet of bumper reinforcement manufactured by roll processing.

Claims (4)

In weight%, carbon: 0.10 to 0.20%, nitrogen: 0.01% or less, sulfur: 0.025% or less, manganese: 1.5 to 3.0%, silicon: 0.2 to 0.8%, aluminum: 0.02 to 0.06%, phosphorus: 0.03% or less, Add 0.005% to 0.03% niobium, (10 × C + 2 × Si + Mn + 100 × Nb) to be 6 or more, Make Mn / Si ratio 4 or more, Cold rolled steel sheet for automobiles having excellent impact resistance, comprising an element inevitably contained in the production of other steels. In weight%, carbon: 0.10 to 0.20%, nitrogen: 0.01% or less, sulfur: 0.025% or less, manganese: 1.5 to 3.0%, silicon: 0.2 to 0.8%, aluminum: 0.02 to 0.06%, phosphorus: 0.03% or less, Niobium: 0.005 ~ 0.03% is added, but (10 × C + 2 × Si + Mn + 100 × Nb) is 6 or more, Mn / Si ratio is 4 or more, and is inevitably contained in the production of other steels Homogenizing the steel slabs of aluminum-kilted steel containing elements at 1050 to 1300 ° C; Finishing hot rolling at 850 ° C. to 950 ° C. directly above the Ar ₃ transformation point; Hot rolling in a temperature range of 450 ~ 700 ℃; Cold rolling at a cold reduction rate of 40 to 80%; A method for producing a cold rolled steel sheet for automobiles having excellent impact resistance, comprising the step of performing continuous annealing at a temperature range of 750 to 900 ° C. The method of claim 2, The cooling rate in the slow cooling zone of the continuous annealing is maintained at -10 ° C / sec or less, the slow cooling start temperature is performed at 600 ° C or higher, and the cooling rate in the quenching zone is maintained at -10 ° C / sec or more. Method for producing cold rolled steel sheet for automobiles having excellent impact resistance. The method of claim 2, The over-aging treatment of the continuous annealing is a method of manufacturing a cold rolled steel sheet for automobiles, characterized in that the impact resistance is carried out in a temperature range of 500 ~ 350 ℃.