KR20100047012A - Bake-hardenable steel sheet and method for producing the same - Google Patents

Abstract

PURPOSE: A baking hardness steel sheet and a manufacturing method thereof are provided to generate the minute carbon nitride by performing a multi stage heat maintenance process before a hot milling process. CONSTITUTION: A baking hardness steel sheet is made of carbon 0.0015~0.0030 weight%, manganese 0.1~0.3 weight%, silicon 0.02~0.05 weight%, niobium 0.008~0.015 weight%, titanium 0.003~0.008 weight%, phosphorus 0.01~0.06 weight%, aluminum 0.03~0.07 weight%, nitrogen 0.003~0.005 weight%, boron 0.0005~0.0020 weight%, and sulfur less than 0.006 weight.

Description

Bake-hardenable steel sheet and method for producing the same

The present invention relates to a hardened hardened steel sheet and a method for manufacturing the same, and more particularly, to a hardened hardened steel sheet and a method of manufacturing the same by applying multi-stage heating.

Beo hardened steel, which is used as an exterior material for automobiles, requires excellent formability and shape freezing property before molding due to the continuous demands of customers for various shapes, and the final product of the car does not have plastic deformation due to external applied force. Dent property is required. The hardened hardened steel is easy to be processed by press due to its low yield strength and excellent formability before forming, and it is widely used as automotive exterior plate material because it has excellent dent resistance property due to increase of yield strength after coating and baking. .

The baking hardening phenomenon is a phenomenon in which the yield strength is increased because the solid solution carbon and nitrogen which are activated at the time of coating baking are adhered to the potential generated during the press. Compared with high strength steel, which increases the strength and decreases in formability and elongation as the strength increases, the hardened hardened steel is an ideal material for automotive exterior materials because it has the characteristics of easy molding and improvement of dent resistance in the final product.

Excessive employment of elements in the hardened hardened steel increases the hardened hardenable properties, but increases the natural aging phenomena resulting in loss of the characteristics of the hardened hardened steel. According to the results of the previous study, the characteristics of the grain boundary and the solid solution carbon in the grain hardening and the aging resistance are different in the case of hardly hardened steel using solid carbon. In Korean Patent Laid-Open No. 10-2000-0018794, the total dissolved carbon content is 0.0015wt% with 0.0003 ~ 0007wt% of dissolved carbon in the mouth and 0.0005 ~ 0.001wt% of grain boundary carbon in order to increase the amount of hardening of baking It is limited to%.

Since the solid carbon remaining in the grain is relatively free to move, it affects room temperature aging in combination with the operating potential, but the solid carbon present in a more stable position, such as grain boundaries or surroundings of precipitates, is activated at high temperatures such as coating baking. This will affect the baking hardening characteristics. As grain size becomes finer, grain-bound solid-solution carbon content is increased, and room-temperature inaging is ensured, and the baking hardening characteristic is improved. In addition, even when fine precipitates are evenly distributed in the grain and grain boundaries, the solid solution carbon may be maintained at a stable state at room temperature.

In the conventional slab reheating process, the target temperature is set to perform uniform heating and hot rolling. In the case of the ultra low carbon steel and the hardened hardened steel, the reheating is performed at a high temperature of 1200 ° C or higher to secure a uniform material and completely dissolve the precipitate. Although reheating was performed as a process, there was a problem in that satisfactory hardening hardening steel was not obtained.

The present invention has been invented in view of the above problems, and the object of the present invention is to control the alloying elements while applying multi-stage heating and maintaining the hardening hardening ability and aging resistance, moldability and elongation excellent hardening steel sheet and its manufacture To provide a method.

The bake hardened steel sheet according to the present invention for achieving the above object, carbon (C) 0.0015 ~ 0.0030wt%, manganese (Mn) 0.1 ~ 0.3wt%, silicon (Si) 0.02-0.05wt%, niobium ( Nb) 0.008 ~ 0.015wt%, Titanium (Ti) 0.003 ~ 0.008wt%, Phosphorus (P) 0.01 ~ 0.06wt%, Aluminum (Al) 0.03-0.07wt%, Nitrogen (N) 0.003-0.005wt%, Boron ( B) 0.0005 ~ 0.0020wt% Sulfur (S) has an alloy composition of 0.006wt% or less and the balance iron (Fe).

The method for producing a hardened hardened steel sheet according to the present invention includes carbon (C) 0.0015 to 0.0030 wt%, manganese (Mn) 0.1 to 0.3 wt%, silicon (Si) 0.02 to 0.05 wt%, and niobium (Nb) 0.008 to 0.015 wt %, Ti (Ti) 0.003 ~ 0.008wt%, Phosphorus (P) 0.01 ~ 0.06wt%, Aluminum (Al) 0.03 ~ 0.07wt%, Nitrogen (N) 0.003-0.005wt%, Boron (B) 0.0005 ~ 0.0020wt %, Sulfur (S) 0.006wt% or less and steel slab having an alloy composition of the balance iron (Fe), after heating and maintaining in three stages in the temperature range of 1130 ~ 1250 ℃, temperature range of 880 ~ 930 ℃ Hot rolling is finished at, and wound up at 600 ~ 750 ℃.

The step of maintaining the heating in three stages, the first heating maintenance step of maintaining the heating for 30 minutes to 1 hour at 1130 ~ 1160 ℃, and the heating and maintaining at 1180 ~ 1250 ℃ 30 minutes to 1 hour after the first heating maintenance step A second heating maintenance step and a third heating maintenance step of maintaining the heating for 30 minutes to 1 hour at 1130 ~ 1180 ℃ after the second heating maintenance step.

After the hot rolling, cold rolling at a reduction ratio of 70 to 80%, annealing at 780 to 880 ° C, and temper rolling at a reduction ratio of 0.8 to 1.2%.

According to the present part hardened steel sheet and the manufacturing method thereof, by controlling the alloying elements and by applying a multi-stage heating maintenance, it is possible to obtain a hardened hardened steel sheet having excellent hardening hardening ability, age resistance, formability and elongation.

Hereinafter, the preferred embodiments of the hardened hardened steel sheet according to the present invention and a manufacturing method thereof will be described in detail.

According to the present invention, a bake hardened steel sheet having excellent bake hardenability, age resistance, formability and elongation is produced by adjusting the alloying element, heating and maintaining it in multiple stages (three stages) and hot rolling.

Specific alloy composition of the present invention, carbon (C) 0.0015 ~ 0.0030wt%, manganese (Mn) 0.1 ~ 0.3wt%, silicon (Si) 0.02 ~ 0.05 wt%, niobium (Nb) 0.008 ~ 0.015 wt%, titanium ( Ti) 0.003 to 0.008 wt%, phosphorus (P) 0.01 to 0.06 wt%, aluminum (Al) 0.03 to 0.07 wt%, nitrogen (N) 0.003 to 0.005 wt%, boron (B) 0.0005 to 0.0020 wt%, sulfur ( S) 0.006 wt% or less and the balance of iron (Fe) and other inevitable impurities.

Referring to the function and content of the alloying elements in detail as follows.

Carbon (C): 0.0015 ~ 0.0030wt%

Carbon is an important element that causes hardening and together, it is essential to control the content of natural aging. Depending on the carbon content, the formability may be degraded or natural aging may be accelerated. Therefore, strong carbonitride-forming elements such as niobium (Nb) and titanium (Ti) are added to control solid solution nitrogen, and the higher the content, the more expensive alloying elements are added. As necessary, the carbon content is limited to 0.0015 ~ 0.0030wt% in order to reduce cost, secure hardening and aging resistance. This component range is a range that can reduce the failure rate of hardening hardened steel by carbon content in the carbon content range of high hit rate in the manufacturing process of ultra low carbon steel.

Manganese (Mn) 0.1 ~ 0.3wt%

Manganese binds to sulfur in the steel to precipitate MnS to prevent hot brittleness due to the formation of FeS and to refine the particles without ductile damage. Increasing the amount of manganese deteriorates moldability and hardening hardenability by Mn-C dipole formation, coarsening of MnS precipitates, and also accelerates hardening hardening by reducing the amount of solid carbon by accelerating the cementite formation. When manufacturing hot-dip steel sheet, a large amount of oxides such as MnO are formed on the surface in the annealing process, which degrades the adhesion of the plating, and a large amount of plating defects occur to deteriorate the surface quality as the outer plate material. Therefore, the content is limited to 0.1 to 0.3%. do.

Silicon (Si): 0.02 ~ 0.05wt%

Silicon (Si) is a solid solution strengthening element, which contributes to the cleanliness of steel, and when added to steel with the addition of appropriate manganese, it improves the fluidity of molten metal during welding, reducing the inclusions in the weld as much as possible and balancing the yield ratio, strength and elongation. Improves strength without inhibiting 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 silicon (Si) causes surface defects due to plating property and red scale when excessively added, and decreases the adhesion of plating, problems such as unplating and plating peeling occur, so the content is limited to 0.02 ~ 0.05wt%. do.

Niobium (Nb): 0.008 to 0.015 wt%

Niobium is a strong carbonitride-forming element, which forms fine precipitates with carbon and nitrogen present in steel during hot rolling, thereby suppressing grain growth after production. In general, the grain refinement effect shows the effect of improving the strength and suppressing the secondary work brittleness. Grain refinement acts effectively to improve the hardening hardening and aging resistance.In the hardening hardening steel, a certain amount of solid solution carbon is required, so that the content of Nb / C (atomic ratio) should be considered in order to retain or re-dissolve the solid solution carbon when necessary. It is important to control. Niobium is one of steel grades with easy control of its content. In the present invention, Nb / C (atomic ratio) is controlled to limit the content to 0.008 to 0.015 wt% in order to adjust the total dissolved carbon content to 0.006 to 0.0015 wt%. .

Titanium (Ti): 0.003 ~ 0.008wt%

Titanium is a strong carbonitride element that forms nitride with nitrogen (N) at high temperature or carbides during hot rolling. In the present invention, titanium is formed in the form of TiC and TiN during slab reheating to finely grain size during hot rolling. It remains in the final material and serves to improve the material of the final material. If the content is more than 0.008%, coarsening of precipitates occurs during reheating, so the content is limited to 0.003 to 0.008 wt%.

Phosphorus (P): 0.01 ~ 0.06wt%

Phosphorus (P) is the substitution type alloy element having the largest solid solution strengthening effect, and serves to improve in-plane anisotropy and strength, and as the phosphorus is added to the spot competition with carbon, the bake hardenability by carbon increases. In addition, since the grain size decreases due to the addition of phosphorus, it is expected to improve the hardening hardening resistance and aging resistance according to the grain size reduction. However, if the content of phosphorus increases secondary brittleness occurs, the content is limited to 0.01 ~ 0.06wt%.

Sulfur (S) 0.006wt% or less

Sulfur is an element that is inevitably contained in the production of steel, inhibits toughness and weldability, and increases the sulfide-based (MnS) non-metallic inclusions, causing cracking and the like. In particular, sulfur is regulated in the range of 0.006wt% or less because it increases the coarse inclusions to deteriorate the fatigue characteristics.

Aluminum (Al): 0.03 ~ 0.07wt%

Aluminum is an element mainly used as a deoxidizer. Nitrogen (N) and nitride (AlN) are precipitated during hot rolling to show grain restraining effect. Below 0.03%, it is difficult to obtain the deoxidation effect. If the added amount is increased, it helps to secure the strength by refining grains, but when it is added more than 0.07wt%, the inclusions are excessively formed during steelmaking operation, and the possibility of defects on the plating surface increases. Therefore, the content is limited to 0.03 to 0.07wt%.

Nitrogen (N) 0.003 ~ 0.005wt% or less

Nitrogen is an alloying element that causes quench hardening and aging with carbon.It has a greater ability to enhance quench hardening than carbon, but deterioration and aging of elongation and formability also increase rapidly, and the yield point extension zone rapidly increases. It is an alloy element that is difficult to apply. In addition, if the content is large, the size of the nitride rapidly increases and moldability is reduced, so the content is limited to 0.003 to 0.005wt%. In the present invention, it is used as an alloying element causing precipitation phenomenon together with titanium.

Boron (B) 0.0005 ~ 0.0020wt%

Boron is an alloying element having a large effect compared to the amount of addition, and suppresses secondary work brittleness of phosphorus (P) -added steel. In addition, it has high affinity with carbon and high ratio at the grain boundary, which enhances the effect of carbon diffusion through the grain boundary and improves the hardening characteristic by carbon. However, when a certain amount or more is added, the material deteriorates, so its content is limited to 0.0005 ~ 0.0020wt%.

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 slab having the composition as described above is obtained by ingot or continuous casting process after obtaining the molten steel through the steelmaking process, where it is manufactured in the form of steel sheet through the multi-stage heating and holding process, hot rolling and winding process, cold rolling and Through the annealing process and temper rolling, the following process is carried out.

Each process is as follows.

[Multistage heating maintenance process]

A process for reheating and maintaining a slab in multiple stages, the first heating holding step of maintaining heating at a temperature section of 1130 to 1160 ° C. for 30 minutes to 1 hour together with the component ratio of the alloying elements forming slab carbonitride, and the first heating holding step After the second heating maintenance step of maintaining the heating for 30 minutes ~ 1 hour at 1180 ~ 1250 ℃, and the third heating maintenance step of maintaining the heating for 30 minutes ~ 1 hour at 1130 ~ 1180 ℃ after the second heating maintenance step.

In the first heating maintenance step, fine TiN is formed, in the second heating maintenance step, a uniform austenite structure is formed, while carbonitrides other than TiN are completely dissolved, and in the third heating maintenance step, fine NbC and TiC are precipitated. Let's do it.

FIG. 1 is a photograph of a precipitate observed by TEM after quenching after slab reheating (1150 ° C. → 1200 ° C. → 1150 ° C. multistage heating) through general slab reheating (1200 ° C., 2 hours uniform heating) and multistage heating holding. As shown in FIG. 1, the fine carbonitride may be generated in the steel before the hot rolling process through the multi-stage heating and maintaining process.

[Hot Rolling and Winding Process]

The slab, which has undergone a multi-stage heating and maintenance process, is finished hot rolling in the temperature range of 830 ~ 930 ℃. If the filament rolling temperature of hot rolling is too high, the grain size of the final material rises and strength deterioration occurs. Therefore, limit the temperature to 930 ℃ or less.If the temperature is too low, uneven microstructure is generated by the width of the plate, resulting in in-plane anisotropy. This increases and deteriorates such as moldability, so the temperature is limited to 880 ° C or higher.

After finishing hot rolling, it is wound up at 600 ~ 750 ℃. If the coiling temperature is higher than 750 ℃, abnormality of grain growth or excessive grain growth causes deterioration of formability and strength of final material. Limited to

[Cold rolling, annealing and temper rolling process]

The steel sheet wound after hot rolling is subjected to cold rolling. As the combined ratio of cold rolling increases, the grain size of the final material decreases, and the <110> // RD rolling aggregate structure is developed to optimize moldability after annealing. // ND recrystallization aggregates develop. In practice, problems of productivity and rolling load occur, so the reduction ratio is limited to 70 to 80%.

In the case of cold rolled material, the deformation state during rolling acts differently depending on the thickness of the rolled material. In this case, if the annealing temperature is too low, the in-plane anisotropy increases due to the development of uneven microstructures and aggregates depending on the thickness of the rolled material, and may cause surface defects of the uneven pattern due to uneven deformation during press molding. In addition, since the development of the recrystallized structure is limited to formability and ductile deterioration occurs, annealing is performed at 780 ℃ or more. If the annealing temperature is increased, the hardening hardening capacity is increased by re-dissolution of NbC precipitates, and the moldability is increased by the growth of {111} // ND grains. However, if the annealing temperature is too high, it is caused by abnormal grain growth and development of uneven microstructure according to the thickness layer. The material is deteriorated, and the tension applied to the plate due to the annealing at high temperature and the expansion of the high temperature annealing equipment (burner, etc.) are problematic, so it is limited to 880 ° C. or less.

After annealing, temper rolling is carried out to improve the hardening and aging resistance of the bake, and to remove the yield point stretching caused by the employment element. It is preferable to perform the temper rolling in the range of 0.8 to 1.2% according to the literature information and the test results.

After the annealing, before the temper rolling, the alloy may be hot dip galvanized in the temperature range of 450 ~ 550 ℃.

Through the above manufacturing method, the tensile strength is 350MPa or more, the plastic anisotropy index (R value) is 1.6 or more, the baking hardening amount is 30MPa or more, the Aging Index is 30MPa or less during artificial aging treatment, the yield point It is possible to produce a hardened hardened steel having an elongation of 0.2% or less and a yield strength of the final material of 300 MPa.

Table 1 shows the component ratio which compared the invention example of this invention with the comparative example.

division Chemical composition (wt%, balance Fe)  Remarks C Mn Si Nb Ti P Al N B S Comparative Example 1 0.0024 0.34 0.03 0.0010 - 0.02 0.04 0.0044 0.0010 0.005 Inventive Example 0.0025 0.35 0.04 0.0011 0.0045 0.02 0.05 0.0039 0.0015 0.006 Comparative Example 2 0.0026 0.33 0.03 0.0012 0.0100 0.02 0.04 0.0039 0.0010 0.005

Comparative Example 1 is a component system in which only Nb is added to target 0.001% in order to retain about 0.0012% of solid solution carbon, and Inventive Example 1 is a component system satisfying the optimized conditions of the present invention, and Comparative Example 2 relates to Ti according to the present invention. It is the component system added so that it might exceed the range of.

Table 2 shows the results of measuring the mechanical properties of the specimen prepared by the slab having the above components through the process of the following conditions.

division Heating, rolling and heat treatment conditions Mechanical properties  Remarks Heating condition ℃, Hr Rolling finish temperature ℃ Winding temperature ℃ Cold rolling reduction rate (%) Annealing temperature ℃ Tensile strength (Mpa) Yield strength (MPa) r value BH (MPa) AI (MPa) Elongation (%) Comparative Example 1 1230, 2 900 640 75 840 346 214 1.91 25.1 19.7 43 Comparative Example 1 Multi-stage heating 900 640 75 840 351 220 1.95 32.3 21.2 43 Inventive Example 1230, 2 900 640 75 840 352 216 2.01 24.2 18.3 44 Inventive Example Multi-stage heating 900 640 75 840 362 235 2.22 43.5 18.1 46 Comparative Example 2 1230, 2 900 640 75 840 354 215 2.14 13.4 4.7 47 Comparative Example 2 Multi-stage heating 900 640 75 840 365 228 2.09 19.1 11.5 48

In the heating conditions shown in Table 2, the multi-stage heating is a three-stage heating holding step of the multistage heating holding step, and the temperature and the holding time are values obtained by taking an intermediate value. The r value represents the plastic anisotropy index, the BH represents the baking hardening amount during coating baking, and the AI represents the aging index during artificial aging treatment.

In Comparative Example 1, when the multi-stage heating is applied than when the general reheating method is applied, the material properties are higher in the hardening amount, formability, and strength. This may be due to the reduction of grain size during hot rolling and cold annealing due to the influence of fine NbC carbides produced by multi-stage heating.

Looking at the invention example, a significant difference was not found with Comparative Example 1 to which the general reheating method was applied, but it can be seen that the material properties are greatly improved when the multistage heating is applied. This may be due to the fact that in addition to NbC carbide, fine TiN generated in the first heating maintenance step prevented grain growth during hot rolling and cold rolling annealing.

Comparative Example 2 is a case where Ti is added beyond the component system of the present invention. When Ti exceeds the range of the present invention, TiN seems to grow rapidly and does not show a great effect.

The multi-stage heating method of the present invention can be applied to the production of ultra low carbon (IF) steel using carbonitrides such as Ti, Nb, and V.

Figure 1 is a photograph of the precipitate observed by TEM after quenching after slab reheating through the general slab reheating and multistage heating holding.

Claims (4)

0.0015 ~ 0.0030wt% carbon (C), 0.1 ~ 0.3wt% manganese (Mn), 0.02-0.05wt% silicon (Si), 0.008 ~ 0.015wt% niobium (Nb), 0.003-0.008wt% titanium (Ti), Phosphorus (P) 0.01 ~ 0.06wt%, Aluminum (Al) 0.03 ~ 0.07wt%, Nitrogen (N) 0.003 ~ 0.005wt%, Boron (B) 0.0005 ~ 0.0020wt% Sulfur (S) 0.006wt% or less and balance iron A hardened steel sheet characterized by having an alloy composition of (Fe). 0.0015 ~ 0.0030wt% carbon (C), 0.1 ~ 0.3wt% manganese (Mn), 0.02-0.05wt% silicon (Si), 0.008 ~ 0.015wt% niobium (Nb), 0.003-0.008wt% titanium (Ti), Phosphorus (P) 0.01 ~ 0.06wt%, Aluminum (Al) 0.03 ~ 0.07wt%, Nitrogen (N) 0.003 ~ 0.005wt%, Boron (B) 0.0005 ~ 0.0020wt%, Sulfur (S) 0.006wt% or less and balance Steel slab having an alloy composition of iron (Fe), After the step of maintaining the heating in three stages in the temperature range of 1130 ~ 1250 ℃, finish the hot rolling in the temperature range of 880 ~ 930 ℃, winding method at 600 ~ 750 ℃ . The method according to claim 2, The step of maintaining the heating in three stages, A first heating maintenance step of maintaining heating at 1130-1160 ° C. for 30 minutes to 1 hour, A second heating maintenance step of maintaining heating at 1180 to 1250 ° C. for 30 minutes to 1 hour after the first heating maintenance step; Method of producing a hardened hardened steel sheet, characterized in that the third heating holding step consisting of maintaining the heating for 30 minutes to 1 hour at 1130 ~ 1180 ℃ after the second heating holding step. The method according to claim 2 or 3, After the hot rolling, cold rolling at a reduction ratio of 70 to 80%, annealing at 780 to 880 ° C., and then temper rolling at a reduction ratio of 0.8 to 1.2%.

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