KR20100047016A - High tensile hot -rolled steel sheet having excellent stretch flanging formability, and method for producing the same - Google Patents

Abstract

PURPOSE: A high strength hot rolled steel sheet with a superior hole enlargeability and a manufacturing method thereof are provided to manufacture the hot rolled steel sheet with the superior elongation ratio by controlling the contain amount of vanadium and niobium. CONSTITUTION: A high strength hot rolled steel sheet is made of carbon 0.05~0.15 weight%, silicon 0.3~1.0 weight%, manganese 0.8~3.0 weight%, aluminum 0.3~1.0 weight%, vanadium 0.02~0.20 weight%, niobium 0.01~0.06 weight%, boron 0.0001~0.002 weight%, phosphorus 0.02 weight%, sulfur 0.006 weight%, nitrogen 0.015 weight% and the rest iron. Micro-structure has the dual phase structure of bainite and ferrite.

Description

High tensile hot-rolled steel sheet having excellent stretch flanging formability, and method for producing the same

The present invention relates to a high tensile strength hot rolled steel sheet having excellent hole expandability and a method of manufacturing the same, and more particularly, to a 1GPa class high tensile strength hot rolled steel sheet having excellent hole expandability and weldability.

The automobile industry, which accounts for a large portion of the steel demand, is intensifying competition due to global oversupply, increasing the demand for quality and diversification of automobile quality, and increasing interest in global environmental protection. Various studies are underway to reduce the gas. In addition, in order to promote passenger safety in the event of a car crash and to produce a car with a complex design according to diversification of consumer desires, the automobile industry needs a high-strength, workability and formability steel.

Hot rolled steel sheet used in automobile structural parts has a greater effect of reducing the weight of the body compared to cold rolled steel sheet, so the high tensile strength of the steel sheet is rapidly developing and the demand is expanding. In the case of hot-rolled steel sheet, the elongation is generally lowered with increasing strength. Therefore, it is recognized that there is a limit to the application for automobiles requiring high processability.

Recently, hot-rolled steel sheets have been developed in which various types of low-temperature transformation tissues and untransformed austenite phases are dispersed in an appropriate amount to suppress or improve degradation of ductility or extension flanges due to high strength. In particular, when forming a hot rolled steel sheet, ductility and hole expandability are important. Japanese Patent Laid-Open No. 6-387685 and Japanese Patent Laid-Open No. Hei 6-200351 disclose that titanium (Ti) and niobium ( The technique which improves hole expansion property by adding and adjusting Nb) is proposed. In the case of hot rolled steel sheet, as the strength increases, a large amount of precipitate added elements are added, so as the strength increases, both the elongation and the hole expandability deteriorate. Also, as the strength increases, the elongation increases and the elongation increases. It is difficult to manufacture the steel sheet having the properties. In addition, in case of the precipitation hardened steel that is used for general purpose, since the elongation and elongation flange property are decreased as the strength is increased, it is necessary to improve these problems in order to apply to automobile chassis parts.

At present, the tensile strength of 590MPa grade is mainly developed by adding niobium (Nb) to form precipitation effect and transformation tissue steel. In addition, in the case of the tensile strength of 780MPa class, various alloy components are added and developed, mainly by adding titanium (Ti) to promote high temperature precipitates to increase the strength and secure the hole expandability using the transformation structure. However, when a large amount of titanium is added, the TiN precipitate is formed during the continuous casting process, which tends to deteriorate castability.

In general, the steel plate is composed of three-phase structure of ferrite, martensite and bainite, and since martensite steel has a large phase difference between ferrite, which is a base structure, compared with other steels, it is not only used for elongation flanges but also for hole expansion. There is a problem.

In the case of strength x extension flange balance, ferritic-bainite composite steel proposed by JFE (NKK), a Japanese steel company, is known to be the best. The method of manufacturing this steel is to use a 3-stage controlled cooling pattern on the run-out table after hot rolling. In the case of a multi-stage cooling pattern, which requires a constant air cooling maintenance section in the middle, the length of the run out table (ROT) is short during the hot rolling process. There is a problem that it is difficult to apply. In addition, the coiling temperature is performed in the bainite region of 400 to 500 ° C. to form ferrite and bainite phases to minimize the difference in hardness between phases and to suppress the formation of grain boundary cementite, thus producing a method for producing a hot rolled steel sheet having a good balance of strength x elongation flangeability. Although it has been proposed, there is a problem that the low elongation does not satisfy the various moldability requirements of the hot rolled steel sheet.

The present invention optimizes the steel component to minimize the decrease in elongation due to the increase in strength, which is a problem of the prior art, and also adds the compound of precipitation hardening elements (Nb, V, etc.) to obtain optimal hot rolling process conditions and cooling conditions. It is an object of the present invention to provide a 1GPa class high tensile strength hot rolled steel sheet having high strength and excellent elongation, hole expandability, and optimum balance, and a method of manufacturing the same.

High tensile hot rolled steel sheet having excellent hole expandability according to the present invention for achieving the above object, carbon (C) 0.05 ~ 0.15wt%, silicon (Si) 0.3 ~ 1.0wt%, manganese (Mn) 0.8 ~ 3.0 wt%, aluminum (Al) 0.3 ~ 1.0wt%, vanadium (V) 0.02 ~ 0.20wt%, niobium (Nb) 0.01 ~ 0.06wt%, boron (B) 0.0001 ~ 0.002wt%, phosphorus (P) 0.02wt% It has a alloy composition of sulfur (S) 0.006wt% or less, nitrogen (N) 0.015wt% or less and balance iron (Fe), and the microstructure is composed of 60-70vol% ferrite and 30-40vol% bainite. Has a phase organization.

The method for manufacturing a high tensile strength hot rolled steel sheet having excellent hole expandability according to the present invention includes carbon (C) 0.05 to 0.15 wt%, silicon (Si) 0.3 to 1.0 wt%, manganese (Mn) 0.8 to 3.0 wt%, and aluminum (Al). ) 0.3 ~ 1.0wt%, Vanadium (V) 0.02 ~ 0.20wt%, Niobium (Nb) 0.01 ~ 0.06wt%, Boron (B) 0.0001 ~ 0.002wt%, Phosphorus (P) 0.02wt% or less, Sulfur (S) After reheating the steel slab having an alloy composition of 0.006wt% or less, nitrogen (N) 0.015wt% or less, and iron (Fe) at a temperature of 1200 ± 50 ° C, hot rolling is performed and the final rolling temperature is 840 to 880. After finishing the hot rolling at ℃, it is characterized in that the multi-stage cooling to the coiling temperature.

The multi-stage cooling is one-stage cooling to cool down to 650-700 ° C. at a cooling rate of 20 to 150 ° C./s after completion of hot rolling, followed by two-stage cooling to air-cool for 4 to 15 seconds, and then 20 to It consists of three stages of cooling to 400 ~ 500 ℃ at a cooling rate of 100 ℃ / s.

The present invention has a tensile strength of 1Gpa or more and has excellent elongation, so that it is possible to manufacture a high tensile strength steel sheet having excellent hole expandability (or elongation flangeability) and weldability, and thus it can be applied to various structural steel sheets including automobiles.

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

In the present invention, vanadium (V) and niobium are added to optimize the content of carbon (C), silicon (Si), and manganese (Mn), which are the main components of steel, in order to obtain excellent weldability, and to secure strength through grain refinement. An appropriate amount of (Nb) is added. In addition, the content of aluminum (Al), silicon (Si), boron (B), manganese (Mn), etc. was optimized to facilitate the fraction of bainite phase having a large influence on the hole expandability.

Specific alloy composition of the present invention, carbon (C) 0.05 ~ 0.15wt%, silicon (Si) 0.3 ~ 1.0wt%, manganese (Mn) 0.8 ~ 3.0wt%, aluminum (Al) 0.3 ~ 1.0wt%, vanadium ( V) 0.02 to 0.20 wt%, niobium (Nb) 0.01 to 0.06 wt%, boron (B) 0.0001 to 0.002 wt%, phosphorus (P) 0.02 wt% or less, sulfur (S) 0.006 wt% or less, nitrogen (N) 0.015 wt% or less and the balance of iron (Fe) and other inevitable impurities.

The microstructure of the hot rolled steel sheet to which the present invention is applied has a biphasic structure of bainite and ferrite. The phase fraction of the bainite is 30-40 vol%, the phase fraction of the ferrite is 60-70 vol%, and the structure is determined by the alloy component and the cooling pattern.

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

Carbon (C): 0.05-0.15 wt%

Although it is an indispensable element to secure the required strength as a high tensile strength steel, the elongated flange property and the spot weldability deteriorate when a large amount is added, so that the content thereof should be 0.15 wt% or less, and when it is added below 0.05 wt%, the desired strength can be obtained. none.

Silicon (Si): 0.3 ~ 1.0wt%

Silicon (Si) is a ferrite stabilizing element employed in ferrite, which is a solid solution element that increases strength without deterioration of ductility, but also causes defects due to oxidation scale on the surface of hot rolled steel sheet, and also deteriorates weldability. If the content is less than 0.30wt%, the strength of ferrite decreases, so more than 0.30wt% should be added. And, if it is more than 1.0wt% weldability is not good.

Manganese (Mn) 0.8 ~ 3.0wt%

Manganese (Mn) is an indispensable component for securing the strength of steel sheet, suppressing pearlite transformation and obtaining bainite structure. However, the content of manganese (Mn) is not effective at 0.8 wt% or less, and is continuous at 3.0 wt% or more. In the slab rolling of the casting, cracks are severely generated, resulting in a decrease in productivity, poor weldability, and insufficient polygonal ferrite being produced, resulting in workability deterioration.

Aluminum (Al): 0.3 ~ 1.0wt%

Aluminum is a deoxidizer and carbide forming element. If the amount of aluminum added is less than 0.3wt%, the above effect according to the addition cannot be obtained. If the amount of aluminum is added more than 1.0wt%, workability is impaired and strength is lowered.

Vanadium (V): 0.02 ~ 0.20wt%

Vanadium is precipitated with carbonitrides, such as niobium, to increase the strength of the steel, and is added as an effective element. If the amount is too large, the amount of vanadium is saturated and the strength does not increase further, so the content is limited to 0.20 wt%. Carbide of vananium can dissolve at relatively low temperatures and readily dissolve upon slab reheating. If the added amount of vanadium is 0.02 wt% or less, the amount of finely dispersed composite carbide cannot be sufficiently exhibited. If the added amount of vanadium exceeds 0.2 wt%, the composite carbide coarsens to lower the strength.

Niobium (Nb): 0.01 to 0.06 wt%

Niobium element precipitates in the form of Nb (CN), NbC, TiC, TiN together with carbon or nitrogen in the steel to prevent grain boundary growth during hot rolling and annealing annealing to form fine grain size. Niobium, a solid solution, acts as a strength enhancer and prevents austenite from transforming to a lower temperature than martensite. When niobium is added in an amount of 0.06 wt% or more, the ductility decreases, and when it is added in an amount of 0.01 wt% or less, a reinforcing effect cannot be obtained.

Boron (B) 0.0001 ~ 0.002wt%

Boron is a component added to the steel in a small amount to increase the hardenability of the steel, and facilitates the formation of low-temperature transformation phase, such as ecicular ferrite and bainite. If it is added more than 0.0001wt%, it is segregated at the austenite grain boundary at high temperature, which contributes to the hardenability of the steel by suppressing the formation of ferrite, and if it is added more than 0.002wt%, the weldability is degraded and the recrystallization temperature is excessively increased, which lowers the drawing ability. The content is preferably limited to the range 0.0001 ~ 0.002wt%.

Sulfur (S) 0.006wt% or less

Sulfur is not included because it increases the fine sulfide-based (MnS) non-metallic inclusions in the course of finishing rolling, causing cracks and the like, which adversely affects the stretch flangeability. It is limited to the range of 0.006 wt% or less.

Phosphorus (P): 0.02wt% or less

Phosphorus (P), like aluminum, is added to inhibit the formation of cementite and to increase strength, but less is preferred. If it exceeds 0.02wt%, there is a problem that deterioration of weldability and final material deviation occurs due to slab center segregation, so it is preferable to limit it to the range of 0.02wt% or less.

Nitrogen (N): 0.015wt% or less

In the case of the carbonitride-forming element N, it is advantageous to keep it as low as possible as an element adversely affecting ductility. When nitrogen is present in excess, nitride is precipitated in a large amount, and ductile deterioration is likely to occur, so it is preferable to suppress it to 0.015 wt% or less.

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 present invention focuses on precipitates composed of ferrite and ferrite which improves ductility in TiN, NbC, and VC, which ensures good ductility in substantially two-phase tissue steels. It is to improve the strength, and to produce a precipitate to secure the strength. By obtaining the metal structure of such a steel plate, the high strength hot rolled steel sheet which satisfy | fills hole expandability, ductility, and elongation flange property simultaneously can be obtained. (Ti, Nb, V) means containing one or two or more specific amounts of Ti, Vb and V. In the present invention, Ti having the same content as Nb may be added.

The slabs having the composition as described above are obtained through molten steel through a steelmaking process and then manufactured through an ingot or continuous casting process, where the steel slab is hot-rolled to be cooled and wound.

Each process is as follows.

[Heating process]

The slab heating process is used to reclaim segregated components during casting. If the reheating temperature is low, segregated components are not reusable. Excessively high slab size increases austenite grain size, resulting in coarsening of ferrite grain size and decreasing strength. do. Therefore, the temperature of 1200 ± 50 ℃ is preferred for the present invention steel.

It is also necessary to adjust the reheat temperature holding time according to the thickness of the slab. The thicker the thickness, the longer the reheating time needs to be. The thinner the thickness, the shorter the reheating time needs to be maintained. The proper holding time is 1-2 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, cooling and winding process]

The slab reheated in the furnace process is hot rolled and then hot rolled in the 840 ~ 880 ℃ temperature range before cooling. Hot rolling finish temperature affects the particle size of ferrite after transformation. If the finish rolling temperature is 840 ° C. or less, the rolling force is excessively reduced during rolling, which adversely affects the rolling sheetability. When the finish rolling temperature is higher than 880 ° C., the structure becomes coarse to reduce the strength and ductility.

The hot rolled slab is composed of one stage cooling to cool down to 650 to 700 캜 at a cooling rate of 20 to 150 캜 / s after the completion of hot rolling, followed by two stage cooling to air cool for 4 to 15 seconds, and then 20 Multistage cooling with three stage cooling to 400 ~ 500 ℃ with cooling rate of ~ 100 ℃ / s and wound up. The steel sheet thus wound and wound is a two-phase structure of bainite and ferrite, the bainite phase fraction is 30 to 40 vol%, and the ferrite phase fraction is 60 to 70 vol%.

When the end temperature of the first stage cooling reaches 650 ° C., the ferrite fraction of the cyclic type increases, and in the case of 700 ° C. or more, the ferrite fraction decreases.

In addition, the coiling temperature (CT) was found to represent the highest hole expandability based on 450 ℃.

Table 1 shows the component ratio of the invention example of this invention and another comparative example. At this time, the vanadium (V) showed a similar tendency to the mechanical properties in Table 2 when added up to four times niobium (Nb).

division Chemical composition (wt%, balance Fe) C Si Mn P max S max Al Cr Cu V Nb Ti Ni B (ppm) N (ppm) Comparative Example 1 0.05 1.00 1.5 0.01 0.005 0.06 0.71 - - 0.04 0.06 - - 40 Inventive Example 1 0.15 0.30 2.5 0.01 0.006 0.30 0.20 - - 0.05 0.02 - 18 80 Inventive Example 2 0.15 0.30 2.5 0.01 0.006 0.30 0.20 0.1 - 0.05 0.02 0.1 18 80 Inventive Example 3 0.15 0.30 2.5 0.01 0.006 0.30 0.20 0.1 0.15 0.04 - 0.1 19 80

The results of measuring the mechanical properties of the specimens prepared by the following heat treatment and rolling conditions using the slab composition as shown in Table 1 are shown in Table 2.

Inventive example: After heating at 1215 ° C. for 1 hour, the final hot rolling was performed at 840˜880 ° C., and then cooled to 460 ° C. and wound up in a winder. Cooling rate was water-cooled or air-cooled at 20 ~ 100 ℃ / sec after rolling.

Coiling temperature Cooled to 460 ℃  Remarks division FDT TS YS EL λ Comparative Example 1 870 780 764 17 42 Inventive Example 1 870 1020 987 12.8 50 Inventive Example 2 870 1002 955 12.5 68 Inventive Example 3 870 1014 968 13.9 48

In Table 2, FDT is hot finish temperature (° C.), TS is tensile strength (MPa), YS is yield strength (MPa), EL is elongation (%), and λ is porosity (%). .

As shown in Table 2 it can be seen that the hot-rolled steel sheet to which the present invention is applied exhibits ultra high strength of 1G MPa or more and shows excellent hole expandability.

Claims (4)

0.05 ~ 0.15wt% carbon (C), 0.3 ~ 1.0wt% silicon (Si), 0.8 ~ 3.0wt% manganese (Mn), 0.3 ~ 1.0wt% aluminum (Al), 0.02-0.20wt% vanadium (V), Niobium (Nb) 0.01 ~ 0.06wt%, Boron (B) 0.0001 ~ 0.002wt%, Phosphorous (P) 0.02wt% or less, Sulfur (S) 0.006wt% or less, Nitrogen (N) 0.015wt% or less and balance iron ( Fe) alloy composition, Microstructure has a high tensile strength hot rolled steel sheet, characterized in that it has a two-phase structure of ferrite and bainite. The method according to claim 1, The phase fraction of the ferrite tissue is 60 ~ 70vol%, High tensile strength hot rolled steel sheet, characterized in that the phase ratio of the bainite structure is 30 to 40 vol%. 0.05 ~ 0.15wt% carbon (C), 0.3 ~ 1.0wt% silicon (Si), 0.8 ~ 3.0wt% manganese (Mn), 0.3 ~ 1.0wt% aluminum (Al), 0.02-0.20wt% vanadium (V), Niobium (Nb) 0.01 ~ 0.06wt%, Boron (B) 0.0001 ~ 0.002wt%, Phosphorous (P) 0.02wt% or less, Sulfur (S) 0.006wt% or less, Nitrogen (N) 0.015wt% or less and balance iron ( Steel slab having an alloy composition of Fe), After reheating at a temperature of 1200 ± 50 ℃, hot rolling is performed and the final rolling temperature is 840 ~ 880 ℃, and hot rolling is performed, followed by multi-step cooling to coiling temperature. Method of manufacturing steel sheet. The method according to claim 3, The multi-stage cooling is, After the end of hot rolling, 1st stage cooling to 650 ~ 700 ℃ at the cooling rate of 20 ~ 150 ℃ / s, followed by 2 stage cooling for 4 ~ 15 seconds air cooling, and then 20 ~ 100 ℃ / s Method for producing a high tensile strength hot rolled steel sheet having excellent hole expansion, characterized in that consisting of three stage cooling to 400 ~ 500 ℃ at a cooling rate.