KR20010060651A - A TENSILE STRENGTH OF 780MPa GRADE HOT ROLLED STEEL SHEET FOR STRUCTURAL USE AND A METHOD FOR MANUFACTURING IT - Google Patents

Abstract

PURPOSE: Provided is a method for manufacturing hot rolled steel sheets having high tensile strength of 780MPa for the application to structural member such as automobile frame and deck, crane boom, etc. CONSTITUTION: The hot rolled steel sheet is manufactured by reheating a steel slab comprising C 0.06-0.10wt.%, Si 0.2wt.% or less, Mn 1.4-2.0wt.%, P 0.02wt.% or less, S 0.005wt.% or less, Al 0.010-0.050wt.%, Ti 0.050-0.15wt.%, Nb 0.020-0.040wt.%, Mo 0.1-0.3wt.%, N 60ppm or less, a balance of Fe, and other inevitable impurities; finish hot rolling the steel slab at an austenite temperature zone; cooling it to 550-640deg.C; and then coiling.

Description

Tensile strength 780MPA class hot rolled steel sheet and its manufacturing method {A TENSILE STRENGTH OF 780MPa GRADE HOT ROLLED STEEL SHEET FOR STRUCTURAL USE AND A METHOD FOR MANUFACTURING IT}

The present invention relates to a high-strength hot-rolled steel sheet and a method for manufacturing the same, which are applied to structural strength members such as frames and decks of commercial vehicles, crane booms, and more specifically, to adjust steel components and to control hot rolling and winding temperatures. The present invention relates to a hot-rolled steel sheet having a tensile strength of 780 MPa class having excellent bendability and welding characteristics, and a method of manufacturing the same.

The workability of the hot rolled steel sheet is divided into bendability, elongation property and stretch flange property, and the main required property of structural hot rolled steel sheet is bendability, which is mainly affected by nonmetallic inclusions and band-like perlite structure. In addition, welding and riveting may be considered in terms of joining the structure. In order to secure stability to the structure, the softening amount of the weld portion should be as small as possible.

Recently developed methods for producing high strength hot rolled steel sheets with a tensile strength of 780 MPa or more use a structure suitable for each deformation property, rather than simultaneously addressing the above-described ductility and stretching flange properties. Examples include the following techniques. .

That is, the dual phase steel disclosed in Japanese Patent Application Laid-Open No. 61-15128 has superior strength-elongation balance characteristics compared with precipitation-reinforced hot-rolled steel sheets, but its elongated flange property is superior to bainite single phase steel. It is known to be inferior.

In Japanese Patent Laid-Open Publication No. 91-358007, unlike the conventional ideal steel, by controlling the ferrite and the second phase precipitated and strengthened by TiC carbide with martensite, it is possible to manufacture a high-strength ratio type high strength hot rolled steel sheet having excellent elongation and fatigue characteristics. It is proposed.

Japanese Patent Application Laid-Open No. 3-10049 proposes a retained austenite steel having a tensile strength of 780 to 980 MPa, which is more ductile than the above ideal steel. The residual austenite steel has a tensile strength-elongation balance of Although extremely excellent, the tensile properties are largely dependent on the amount of retained austenite, and thus, it is difficult to obtain a uniform material in the width direction and the length direction of the hot rolled steel sheet.

However, the above ideal steel or residual austenite steel is very excellent in terms of strength × elongation balance, but since the Si is added in the range of 0.5 to 2.5 wt% for early deposition of ferrite after hot rolling, there is a possibility of occurrence of surface oxide scale defects. It is high, and the hardness difference between constituent phases increases in the process of organic transformation of retained austenite to martensite in the forming step, and thus has poor elongation flangeability compared to conventional ferrite-perlite tissue steel or ferrite single phase steel, and shear processing is final. Bendability is also known to be inferior in the case of machined surfaces.

On the other hand, the ferrite-bainite composite tissue steel proposed by Nippon Steel Pipe Co., Ltd. (NKK) and the like is known to be the most excellent in terms of strength x extension flange balance. The steel is manufactured by applying a three-stage controlled cooling pattern on a run out table after hot rolling and winding in a temperature range of 450 to 500 ° C., so that the hardness difference between the ferrite matrix and the second phase is small and the grain boundary cement is produced. Tight formation is suppressed, and the strength x stretching flange balance is excellent. However, it is pointed out that having a elongation of about 18%, which is a relatively low value in processing applications, has a drawback.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the above problems and propose the present invention based on the results, and the present invention adds an appropriate amount of manganese, titanium, niobium and molybdenum, and hot rolls and By appropriately controlling the coiling temperature, it is an object of the present invention to provide a structural hot rolled steel sheet having a high tensile strength, an elongation of at least 18%, and excellent bendability, and a method of manufacturing the same.

In the present invention, carbon: 0.06 to 0.10%, silicon: 0.2% or less, manganese: 1.4 to 2.0%, phosphorus: 0.02% or less, sulfur: 0.005% or less, aluminum: 0.010 to 0.050%, titanium: 0.050 to 0.15 %, Niobium: 0.020% to 0.040%, molybdenum: 0.1% to 0.3%, nitrogen: 60 ppm or less, residual Fe and other unavoidable impurities, and a tensile strength 780 MPa class hot rolled steel sheet having a ferrite single phase structure.

In the present invention, carbon: 0.06 to 0.10%, silicon: 0.2% or less, manganese: 1.4 to 2.0%, phosphorus: 0.02% or less, sulfur: 0.005% or less, aluminum: 0.010 to 0.050%, titanium: 0.050 ~ 0.15%, niobium: 0.020 to 0.040%, molybdenum: 0.1 to 0.3%, nitrogen: 60 ppm or less, reheat the steel slab containing residual Fe and other unavoidable impurities, and finish hot roll at the austenite temperature range, It relates to a method for producing a tensile strength 780 MPa class hot rolled steel sheet characterized by cooling to a temperature range of 640 ℃ and wound at this temperature.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The carbon (C) is basically an element necessary for obtaining the strength of the hot rolled steel sheet, and in the present invention steel, titanium and niobium carbides are used for inhibiting the formation of the ferrite structure, which deteriorates the bendability, that is, to reduce the ferrite single-phase organization and grain boundary cementite. In order to control to the minimum amount necessary for the precipitation of, the content is preferably limited to 0.06 to 0.10%.

The manganese (Mn) is an effective element for solid solution strengthening. If the content is less than 1.4%, the expected strength is not obtained. If the content exceeds 2.0%, the strength is 780 MPa or more, but the elongation is drastically reduced. In other words, when the manganese content is increased, the hardenability of the steel is increased to increase the possibility of bainite single phase, the precipitation of titanium carbide is suppressed and the strength is lowered. Therefore, the content is preferably set to 1.4 to 2.0%.

The silicon (Si) is a solid solution element that raises the strength without deterioration of ductility, but causes an oxide scale defect on the surface of the hot rolled steel sheet, in the present invention, the content of the silicon (Si) is inevitably added to control the shape of the emulsion. It is desirable to limit it to 0.20% or less. When silicon is added in the above range, good surface properties can be obtained.

The niobium (Nb) is an element that is effective in strengthening the precipitation of ferrite phase as well as contributing to the refinement of grains of austenite while being precipitated or being in solid solution during hot rolling. However, if the addition amount is excessive, the possibility of generating slag cracks during the manufacturing process of the continuous casting slab becomes high, so it is preferable to regulate it to 0.04% or less.

The titanium (Ti) is an important element in the present invention steel, precipitates as titanium-niobium carbide or titanium-niobium-molybdenum composite carbide in the ferrite structure during the hot rolling process to contribute to the strengthening of the ferrite matrix. If the content is small, the precipitation strengthening effect cannot be sufficiently expected. On the contrary, the content may exceed 0.15%, but the precipitation strengthening effect is saturated. Therefore, the amount of titanium added is preferably added at 0.05 to 0.15%.

The molybdenum (Mo) is an element that increases the hardenability of the steel together with Mn, and contributes to precipitation strengthening together with titanium and niobium in the winding temperature range after rolling of the present invention. In the present invention, it is preferable to add the tensile strength of 780MPa grade hot rolled steel sheet in consideration of the strength level and weldability of 0.1 ~ 0.3%. If the content exceeds 0.3%, the likelihood of occurrence of weld cracking, which is a dysfunction of the increase in hardenability due to molybdenum, is increased.

In the present invention, the steel slab formed as described above is reheated and hot rolled. Preferably, it is important to finish hot roll at 840 ° C. or higher, which is Ar ₃ or higher, and to wind the steel slab in a temperature range of 550 to 640 ° C.

That is, in the production of conventional HSLA steel, in order to contribute to the increase in strength while the precipitated reinforced element precipitates as fine carbide, it was wound around 600 ° C after hot rolling, but the present invention also basically deposits carbide on ferrite. Because of the concept, the winding temperature was set in the range of 550 ~ 640 ℃. If the temperature is lower than this, precipitation is suppressed. On the contrary, if the temperature is high, the precipitate size becomes coarse and the tensile strength is lowered.

The hot rolled steel sheet of the present invention has a ferrite single phase structure precipitated and strengthened with titanium-niobium-molybdenum composite carbide in the final structure, and in mechanical properties, tensile strength of 780 MPa or more, yield strength of 690 MPa or more, tensile strength x elongation balance Can provide excellent characteristics of 15000 MPa ·% or more.

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

Example

The steel slab having the chemical composition of Table 1 was hot-rolled and cooled under the conditions of Table 2 below, and then the winding temperature was changed to prepare a hot rolled steel sheet having a final plate thickness of 3.2 to 6.0 mm.

Thereafter, the mechanical properties of the manufactured hot rolled steel sheet were measured, and the results are shown in Table 2 below.

At this time, the tensile strength was evaluated by preparing a JIS No. 5 tensile test specimen in a direction perpendicular to the rolling direction and pulling at a speed of 10 mm / min at room temperature.

In the bending test, a burr generation part of a specimen having a size of 35W × 240L × t (mm) in a right angle direction of rolling was placed outside the bend to measure the minimum radius at which the bend crack was generated.

Steel grade Chemical composition (% by weight) C Mn Si P S Al Ti Nb Mo N (ppm) Conventional Steel 1 0.075 1.45 0.16 0.009 0.006 0.043 － 0.035 － 14 Conventional Steel 2 0.078 1.68 0.08 0.013 0.001 0.031 0.105 0.033 40 Invention steel 0.073 1.69 0.05 0.017 0.002 0.034 0.088 0.030 0.20 38 Comparative Steel 1 0.074 1.65 0.10 0.010 0.005 0.037 0.082 0.031 0.10 36 Comparative Steel 2 0.075 1.70 0.01 0.010 0.003 0.032 0.050 0.030 0.22 40

division Winding temperature (℃) Thickness (mm) Mechanical property Tensile Strength × Elongation Balance (MPa ×%) Bending radius Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Conventional Materials 1 Conventional Steel 1 580 3.6 510 617 27 16659 1T Conventional material 2 Conventional Steel 2 594 1.0 657 745 21 15649 Conventional Materials 3 585 5.0 666 735 21 15435 Comparative Material 1 Invention steel 650 5.0 627 774 23 17802 Invention 1 620 3.2 764 833 21 17493 Invention 2 580 4.5 735 794 20 15880 Comparative Material 2 550 4.0 666 764 18 13752 Comparative Material 3 Comparative Steel 1 600 696 754 18 13752 Comparative Material 4 Comparative Steel 2 725 774 21 16254

As shown in Table 2, the invention materials (1), (2) made of the present invention steel is both 780MPa or more tensile strength, 18% or more elongation, tensile strength x elongation balance is 15000 (MPa x%) or more, Satisfies the condition that no crack occurs even in 1t bending.

On the other hand, conventional materials (1), (2) and comparative materials (3) and (4) made of conventional steel and comparative steel that do not satisfy all the component ranges of the present invention steel do not reach the level of the present invention. I could not.

In the case of the comparative material (1) which uses the invention steel but the coiling temperature is higher than the scope of the present invention, as the precipitates coarsen, the precipitation strengthening effect is lowered and the tensile strength is lowered. In addition, in the case of the comparative material (2) which uses the invention steel but the winding temperature is lower than the range of the present invention, precipitation of (Ti, Nb, Mo) C-based precipitates is suppressed, and low-temperature transformation structures such as bainite are formed and mechanical The properties fell short of the present level.

On the other hand, because all the specimens of Table 2 is a ferrite single-phase structure steel containing low carbon of less than 0.1% C, bendability was very excellent. That is, no crack was generated in the 180 ° complete close bending test.

As described above, the present invention provides a tensile strength 780MPa grade hot rolled steel sheet having excellent mechanical properties by adjusting the steel component and controlling the hot rolling and winding temperature, and having ferritic single phase structure steel precipitated and strengthened with titanium-niobium-molybdenum composite carbide. There is an effect that can be provided.

Claims (2)

By weight%, carbon: 0.06 to 0.10%, silicon: 0.2% or less, manganese: 1.4 to 2.0%, phosphorus: 0.02% or less, sulfur: 0.005% or less, aluminum: 0.010 to 0.050%, titanium: 0.050 to 0.15%, niobium : 0.020 ~ 0.040%, Molybdenum: 0.1 ~ 0.3%, Nitrogen: 60ppm or less, Residual Fe and other unavoidable impurities. Hot rolled steel By weight%, carbon: 0.06 to 0.10%, silicon: 0.2% or less, manganese: 1.4 to 2.0%, phosphorus: 0.02% or less, sulfur: 0.005% or less, aluminum: 0.010 to 0.050%, titanium: 0.050 to 0.15%, niobium : 0.020 ~ 0.040%, Molybdenum: 0.1 ~ 0.3%, Nitrogen: 60ppm or less, Re-heating steel slab containing remainder Fe and other unavoidable impurities, and finish hot rolling in austenite temperature range, then temperature range of 550 ~ 640 ℃ Method for producing a hot-rolled steel sheet for structural strength 780 MPa class, characterized in that cooled to and wound at this temperature