KR20100035835A - High strenth hot rolled steel sheet having excellent elongation-stretch flangeability property, and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high strength hot rolled steel sheet with a superior elongation ratio and elongation flange and a manufacturing method thereof are provided to make organization consisting of polygonal ferrite, pearlite and the minute precipitate by adding a small amount of vanadium and reducing the content of niobium. CONSTITUTION: A high strength hot rolled steel sheet with a superior elongation ratio and elongation flange is made from carbon 0.05~0.10 weight%, silicon 0.01~0.5 weight%, manganese 0.8~2.0 weight%, aluminum 0.005~0.05 weight%, vanadium 0.02~0.1 weight%, niobium 0.01~0.05 weight%, nitrogen less than 0.015 weight%, sulfur less than 0.006 weight%, and the rest which contains iron and inevitable impurity. The niobium and vanadium satisfy at a rate of 1:3~5, relatively.

Description

High-strength hot rolled steel sheet having excellent elongation-stretch flangeability property, and method for manufacturing the same

The present invention relates to a hot-rolled steel sheet, and more particularly, to a high tensile strength hot-rolled steel sheet having a tensile strength of 600MPa or more excellent in elongation and elongation flange, and a manufacturing method thereof.

The automobile industry, which accounts for the majority of steel demand, is trying to reduce the weight of the vehicle body and increase its rigidity in order to meet the regulations on automobile fuel efficiency and environmental regulations worldwide.

Recently, in the automotive industry, demand for hot rolled steel sheets having high strength and large body weight reduction effects is increasing to lighten automobiles and satisfy consumer demands. Accordingly, various studies have been conducted to manufacture high-processability high tensile strength hot rolled steel sheets according to the weight reduction of automobiles.

In general, the cold rolled steel sheet is mainly used in the part requiring the manufacture of a complex shape in the automobile, and the hot rolled steel sheet is mainly used in the structural member such as its reinforcement, wheel, chassis.

The machinability of hot rolled steel is divided into bendability, elongation property, and elongation flange property. The properties required for automotive chassis parts such as wheel, disk, and lower arm of an automobile are elongated. It is flangeable. The elongated flangeability, which is evaluated as the hole expandability, is known to be associated with the microstructure of the steel sheet.

However, the precipitation hardening hot rolled steel sheet, which is widely used in recent years, has a problem in that it is applied to automobile chassis parts because elongation and elongation flange deteriorate as strength increases.

Conventional techniques for improving this problem include Japanese Patent Laid-Open Nos. 57-70257, 6-49591, 6-293910, Japanese Patent JP91358007, and Korean Patent Publication No. 2003-55339.

Japanese Patent Application Laid-Open No. 57-70257 discloses a method of making a steel sheet structure into a three-phase structure of ferrite, martensite and bainite. This method is to replace part of martensite with bainite to improve the ductility and extension flange.

In the case of strength x extension flange balance, the ferritic-bainite composite steel proposed by NKK, a Japanese steel company, is known to be the best.

NKK Steel Company's patent is a three-stage controlled cooling pattern on a run-out table after hot rolling and cooling to 400 ~ 500 ℃ to form a ferrite and bainite phase to minimize the difference in hardness between phases and to suppress the formation of grain boundary cementite. Therefore, a method of manufacturing a hot rolled steel sheet having a good balance of strength and extension flange is proposed.

However, this composite tissue steel has a low elongation, and when the solid solution strengthening element or precipitation strengthening element is added to increase the strength of the hot rolled steel sheet has a problem that the ductility decrease is further increased. In addition, there is a problem that it is difficult to apply when the length of the run-out table is short, because a constant air cooling maintenance section is required during the hot rolling process.

The present invention is to solve the conventional problems as described above, it is an object of the present invention to provide a high-strength hot rolled steel sheet excellent in elongation and elongation flange by the control of the alloying element and the control of the hot rolling process and a manufacturing method thereof. .

According to a feature of the present invention for achieving the above object, the present invention is carbon (C): 0.05 ~ 0.10wt%, silicon (Si): 0.01 ~ 0.5wt%, manganese (Mn): 0.8 ~ 2.0wt%, Aluminum (Al): 0.005 to 0.05 wt%, Vanadium (V): 0.02 to 0.1 wt%, Niobium (Nb): 0.01 to 0.05 wt%, Nitrogen (N): 0.015 wt% or less, Sulfur (S): 0.006 wt It contains less than%, and has an alloy composition of the remaining iron (Fe) and other unavoidable impurities.

The niobium (Nb) and vanadium (V) are contained in a range satisfying the ratio of 1: 3-5.

Carbon (C): 0.05-0.10 wt%, Silicon (Si): 0.01-0.5 wt%, Manganese (Mn): 0.8-2.0 wt%, Aluminum (Al): 0.005-0.05 wt%, Vanadium (V): 0.02 ~ 0.1 wt%, niobium (Nb): 0.01 ~ 0.05 wt%, nitrogen (N): 0.015wt% or less, sulfur (S): 0.006wt% or less, consisting of the remaining iron (Fe) and other unavoidable impurities The niobium (Nb) and vanadium (V) are reheated to a steel slab having an alloy composition in a ratio of 1: 3 to 5 at a temperature of Ac3 or more and maintained for 1 to 2 hours, and hot finished at 840 to 900 ° C. It rolls and winds up between 600-640 degreeC.

The present invention manufactures a steel sheet having improved hole expandability represented by tensile strength, elongation and elongation flange by controlling the content of niobium and vanadium and controlling the hot rolling finish temperature and winding temperature, which are the process conditions of hot rolling. Therefore, it is possible to provide a steel sheet for a vehicle having a design that variously reflects the desires of the consumer, and has an effect of easily manufacturing into desired automotive parts using excellent processability.

Hereinafter, a preferred embodiment of a high tensile strength hot rolled steel sheet excellent in elongation and stretch flange according to the present invention and a method of manufacturing the same will be described in detail.

Hot-rolled steel sheet of the present invention is carbon (C): 0.05 ~ 0.10wt%, silicon (Si): 0.01 ~ 0.5wt%, manganese (Mn): 0.8 ~ 2.0wt%, aluminum (Al): 0.005 ~ 0.05 wt%, Vanadium (V): 0.02 to 0.1 wt%, niobium (Nb): 0.01 to 0.05 wt%, nitrogen (N): 0.015 wt% or less, sulfur (S): 0.006 wt% or less, and remaining iron (Fe) And other unavoidable impurities.

Then, the steel slab having the composition as described above is heated to 1200 ° C or more in a heating furnace and finished hot rolling at a finishing temperature of 840 to 900 ° C and wound up at a temperature range of 600 to 640 ° C.

The present invention is based on the component system satisfies 0.25 or less in order to ensure excellent weldability. The lower the Pcm value, the lower the likelihood of cold cracking in welding, so optimize the contents of carbon (C), silicon (Si), and manganese (Mn) so that the Pcm value is 0.25 or less for welding without forming cold cracks under welding conditions. .

And, in order to increase the strength in the steel lowering the Pcm value to 0.25 or less, the alloying ratio of niobium (Nb) and vanadium (V) is controlled and the winding temperature is controlled to secure the elongation flangeability.

As shown in Fig. 1, the steel sheet structure varies depending on the winding temperature after hot finishing rolling. Here, the structure of the steel sheet according to the coiling temperature is based on the case that contains vanadium in the range of 3 to 5 times of niobium.

In the cooling curve of FIG. 1, A region is austenite, B region is ferrite, C region is pearlite, D region is bainite, and E region is a martensite transformation section. Cooling in the C region after hot finish rolling forms a two-phase structure (a) of ferrite and perlite, and cooling in the D region is transformed organic plasticity (TRIP) with ferrite + bainite (b) or residual austenite (c). The steel is formed, and cooling to the E region forms a two-phase (DP) steel (d) having a martensite structure.

Therefore, in the present invention, the second phase, which is known to have a great influence on the hole expandability, is controlled by the winding temperature after general cooling without intermediate air cooling section, not the bainite structure. Be sure to

Niobium and vanadium are added at a ratio of 1: 3-5. Niobium, which has a great influence on the grain refining effect, is reduced, and vanadium element which is active in low temperature precipitation is added to promote low temperature precipitation. Therefore, in the case of winding temperature, winding is performed at the temperature at which the precipitation of the temperature range of 600 to 640 ° C. is most active. In addition, vanadium is difficult to secure high strength when added to less than three times the niobium content, and when added to more than five times the niobium content causes a cost increase due to the effect saturation.

And, by controlling the winding temperature in the active temperature range of vanadium is to be a structure consisting of polygonal ferrite, pearlite and fine precipitates.

The microstructure is formed of 80 to 85% of polygonal ferrite having an average grain size of 2 to 20 µm, and 15 to 20% of pearlite as the second phase. Polygonal ferrite is a round polygonal structure that is more effective for stretching flange than ferrite.

The function and content of the alloying elements which are the basic components of the present invention are as follows.

Carbon (C): 0.05-0.10wt%

Carbon is an indispensable element for securing the strength of the steel sheet, and the upper limit thereof is made 0.1 wt% because the elongated flange property and the spot weldability deteriorate when a large amount is added. However, more than 0.05wt% should be added to secure the strength of the material.

Silicon (Si): 0.01 ~ 0.5wt%

Silicon is a ferrite stabilizing element that is a solid solution element that raises the strength without deterioration of ductility as a ferrite stabilizing element employed in ferrite. Since the strength of ferrite decreases when silicon is added below 0.01 wt%, it is necessary to add 0.01 wt% or more. However, when the additive is excessively added, the surface defect of the hot-rolled steel sheet is generated by oxidation scale and the weldability is lowered, so the upper limit thereof is 0.5 wt%.

Manganese (Mn): 0.8-2.0 wt%

Manganese gains strength through strengthening employment and improving hardenability. Manganese is an indispensable component for inhibiting pearlite transformation and obtaining bainite structure, but if the content is less than 0.8 wt%, the effect is insufficient. In addition, when the content exceeds 2.0wt%, the polygonal ferrite is not sufficiently produced and the workability is degraded during hot rolling, so the content is limited to 2.0wt% or less.

Aluminum (Al): 0.005 ~ 0.05wt%

Aluminum is a deoxidizer and a carbide forming element. Aluminum combines with nitrogen in the steel to form AlN to refine the structure and remove oxygen in the steel to prevent cracking in the manufacture of the slab. When aluminum is added at less than 0.005 wt%, its effect is insignificant, and when added excessively, workability is inhibited, so the upper limit thereof is limited to 0.05 wt%.

Vanadium (V): 0.02 to 0.1 wt%

Vanadium is a carbonitride element, such as niobium, added to increase the strength of the steel. If the amount of vanadium is less than 0.02 wt%, the amount of finely dispersed complex carbides is not sufficient. If the amount of vanadium is added more than 0.1 wt%, the amount of precipitation is saturated and no increase in strength is observed. As a result, the composite carbide becomes coarse and the strength decreases, so the upper limit of vanadium is limited to 0.1%.

Niobium (Nb): 0.01 to 0.05 wt%

Niobium combines with C or N to precipitate NbC and NbN precipitates. These precipitates prevent grain boundary growth during hot rolling, thereby miniaturizing the grain size. Niobium, an element of employment, acts as a strength enhancer and prevents austenite from transforming to a lower temperature than martensite.

When niobium is added less than 0.01wt%, the precipitation amount is too small to expect the effect of strength improvement due to precipitation hardening, and when the addition amount exceeds 0.05wt%, the yield strength is increased to decrease the ductility. Therefore, the content of niobium is preferably added in the range of 0.01 ~ 0.05 wt%.

Nitrogen (N): 0.015 wt% or less

Nitrogen increases austenite formation and forms aluminum nitride (AlN) or titanium nitride (TiN) to increase its strength, so it is advantageous to keep the addition amount as low as possible. In particular, nitrogen is preferably limited to the range of 0.015 wt% or less since the elongation is reduced when excessively added to inhibit workability.

Sulfur (S): 0.006wt% or less

Sulfur inhibits toughness and weldability, increases MnS non-metallic inclusions in finishing rolling, causing cracks during processing of steel, and increases coarse inclusions when excessively added, thereby causing fatigue properties. Sulfur weakens the elongation flange when added more than 0.006wt%, so the content is limited to 0.006wt% or less. However, it is best not to add sulfur.

The present invention contains the components of the steel sheet, with the remainder being substantially iron (Fe) and unavoidable elements, where the incorporation of unavoidable impurities is minimized.

According to the above-described composition, two-phase tissue steel, preferably based on polygonal ferrite and pearlite, is prepared, and the two-phase tissue steel generates precipitates consisting of NbC and VC to secure strength and sufficiently grow ferrite particles. This prevents the extension of the extension flange, which is the hole expansion property, from being lowered.

The slabs having the composition as described above are obtained by ingot or continuous casting process after obtaining molten steel through the steelmaking process, the elongation and elongation flange of the steel sheet produced here is greatly changed according to the process of controlling the structure of the metal Hereinafter, manufacturing conditions for each process will be described in detail.

[Heating process]

In order to reclaim segregated components during casting, the slabs are heated at a temperature of Ac 3 or higher in a furnace. If the reheating temperature is low, the segregated components are not reusable. If the reheating temperature is excessively high, the austenite grain size increases and the ferrite grain size is coarsened, thereby decreasing the strength.

The slab is adjustable in heating temperature holding time according to the thickness. For example, the thicker it is, the longer the reheating time is. The thinner the thickness, the shorter the time. The proper holding time is 1 ~ 2 hours, and if it is kept longer, it may be economically inefficient. If it is too short, the quality may be deteriorated due to the lack of homogenization.

Hot rolling, cooling and winding processes;

The slab reheated in the furnace process is hot rolled at 840 ~ 900 ℃, quenched at a cooling rate of 20 ~ 150 ℃ / sec, and wound up at 600 ~ 640 ℃.

The finish hot rolling temperature affects the particle size of the ferrite after transformation. If the finish rolling temperature is 840 ℃ or less, excessive rolling force during rolling has an adverse effect on the rolling sheetability, and if it exceeds 900 ℃, the structure becomes too coarse, leading to a decrease in strength and ductility. Therefore, it is preferable to control hot rolling in the temperature range of 840 degreeC-900 degreeC.

In order to improve the hole expandability, in the conventional case, the coiling temperature was wound at 450 to 500 ° C. to form a two-phase structure of ferrite bainite. However, in the case of the inventive material, the structure of the hot rolled steel sheet was wound at 600 to 640 ° C. so as to have polygonal ferrite and pearlite having fine precipitates. As shown in Figure 1, if the coiling temperature is less than 400 ℃ all austenite phase can be changed to martensite, if it exceeds 640 ℃ cementite may occur at the grain boundary,

That is, when the coiling temperature is lower than the reference value, the elongated flange property represented by workability and hole expandability is not secured due to the rapid increase in strength.

Hereinafter, the high-strength hot-rolled steel sheet excellent in elongation and extension flange described above and a method of manufacturing the same will be described by comparing the present invention with a comparative example.

Table 1 shows the component ratios of the inventive examples and comparative examples, Table 2 shows the results of measuring the mechanical properties of the specimen prepared by the inventive examples and comparative examples of Table 1.

(Alloy component wt%, balance Fe) division C Si Mn P S Al Cu Nb V N Comparative Example 1 0.08 0.10 1.5 0.02 0.06 0.025 0.01 0.04 - 0.00 Inventive Example 1 0.06 0.15 1.2 0.02 0.06 0.025 - 0.02 0.06 0.010 Inventive Example 2 0.06 0.15 1.2 0.02 - 0.025 - 0.02 0.09 0.010

division FDT CT TS (MPa) YS (MPa) Yield fee EL (%) λ Comparative Example 1 870 660 602 517.7 0.86 21 42 Inventive Example 1 870 660 596 524.4 0.88 26 55 Inventive Example 2 870 660 601 534.8 0.89 27 72 Comparative Example 1 870 620 610 524.6 0.86 23 48 Inventive Example 1 870 620 616 529.7 0.86 27 65 Inventive Example 2 870 620 624 542.8 0.87 28 78 Comparative Example 1 870 580 610 512.4 0.84 22 47 Inventive Example 1 870 580 605 520.3 0.86 27 60 Inventive Example 2 870 580 612 532.4 0.87 27 70

[FDT: Hot Rolling Finish Temperature, CT: Winding Temperature, TS (MPa): Tensile Strength, YS (MPa): Yield Strength, EL (%): Elongation, λ: Hole Extensibility (Elongation Flange)]

Table 2 shows the strength, elongation, and hole expandability of steel slabs having the composition shown in Table 1, followed by finishing hot rolling at 840 to 900 ° C., followed by cooling to 600 to 640 ° C ..

Inventive Examples 1 and 2 are niobium (Nb) and vanadium (V) composite additive steels which are not added with niobium (Nb) alone but instead reduce the content of niobium that affects recrystallization during hot rolling and instead increase the content of vanadium. Winding temperature is controlled to 600 ~ 640 ℃ to maximize precipitation.

Looking at Table 2, it can be seen that the elongation, hole expandability, and strength characteristics of the hot rolled steel sheet are divided according to the content distribution of niobium and vanadium based on Comparative Example 1, and the control of the winding temperature.

That is, when comparing the comparative example and the invention example under the same conditions, niobium and vanadium are added in combination, the content ratio of niobium and vanadium is contained in the range of 1: 3-5, and satisfying the winding conditions of 600 ~ 640 ℃ It can be confirmed that excellent elongation, hole accuracy and tensile strength are secured. This is also confirmed through a graph measuring tensile strength according to the winding temperature of FIG. 2.

In addition, in the tissue photograph of FIG. 3, in the case of Comparative Example 1, the ferrite structure was formed in accordance with the addition of 400 ppm of niobium, whereas in the case of Inventive Example 2, the content of niobium was reduced and elongation was added by adding vanadium which has little effect on tissue micronization. It can be seen that a structure consisting of polygonal ferrite and pearlite effective for flangeability and fine precipitates effective in improving strength is formed.

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

1 is a cooling curve predicting the transformation structure according to the coiling temperature after hot finishing rolling.

Figure 2 is a graph measuring the tensile strength according to the coiling temperature according to the present invention.

Figure 3 is a microscopic histogram showing the microstructure of Comparative Example 1 and Inventive Example 2 of Table 1.

Claims (3)

Carbon (C): 0.05-0.10 wt%, Silicon (Si): 0.01-0.5 wt%, Manganese (Mn): 0.8-2.0 wt%, Aluminum (Al): 0.005-0.05 wt%, Vanadium (V): 0.02 ~ 0.1 wt%, niobium (Nb): 0.01 to 0.05 wt%, nitrogen (N): 0.015 wt% or less, sulfur (S): 0.006 wt% or less, alloy of the remaining iron (Fe) and other unavoidable impurities High tensile hot rolled steel sheet having excellent elongation and elongation flangeability, characterized in that it has a composition. The method according to claim 1, The niobium (Nb) and vanadium (V) is a high-strength hot-rolled steel sheet excellent in elongation and elongation flange, characterized in that it is contained in a range satisfying the ratio of 1: 3-5. Carbon (C): 0.05-0.10 wt%, Silicon (Si): 0.01-0.5 wt%, Manganese (Mn): 0.8-2.0 wt%, Aluminum (Al): 0.005-0.05 wt%, Vanadium (V): 0.02 ~ 0.1 wt%, niobium (Nb): 0.01 ~ 0.05 wt%, nitrogen (N): 0.015wt% or less, sulfur (S): 0.006wt% or less, consisting of the remaining iron (Fe) and other unavoidable impurities The niobium (Nb) and vanadium (V) is a steel slab having an alloy composition contained in a ratio of 1: 3-5. A method for producing a high tensile strength hot rolled steel sheet having excellent elongation and elongation flangeability, characterized in that it is reheated at a temperature of Ac 3 or more, maintained for 1 to 2 hours, and hot finished rolling at 840 to 900 ° C. for winding at 600 to 640 ° C.

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