KR20090068989A - Dual phase ultra-high strength hot rolled steel sheets and method for manufacturing process the same - Google Patents

Abstract

The present invention relates to one or more tissue steel ultra-high strength hot rolled steel sheet and a method of manufacturing the same so as to produce an abnormal steel-steel hot-rolled steel sheet with a greatly improved strength.

The production method of the present invention in weight%, carbon (C) 0.15 ~ 0.30%, silicon (Si) 0.3 ~ 2.0%, manganese (Mn) 1.5 ~ 4.0%, sulfur (S) 0.01% or less, phosphorus (P) 0.03 % Or less, titanium (Ti) 0.03 ~ 0.06%, boron (B) 0.001 ~ 0.003%, the remaining content is dissolved in the converter so as to have a composition consisting of iron (Fe) and impurities inevitable in the iron and continuous casting To form a slab, to reheat the slab to hot roll in the form of a steel sheet, and to cool the hot rolled steel sheet to have martensite and ferrite abnormal structures.

According to the present invention, the strength of the present invention is greatly improved so that the present invention can be applied to a part requiring rigidity for impact energy among automotive parts, and thus has a useful effect of achieving the purpose of safety and weight reduction of an automobile.

Description

Dual phase ultra-high strength hot rolled steel sheets and method for manufacturing process the same

The present invention relates to an ultra-high strength hot rolled steel sheet and a method for manufacturing the same, in particular, by increasing the content of carbon, silicon, and manganese, and by greatly increasing the strength by adding elements of titanium and boron, automobile safety and weight can be reduced. The present invention relates to an ultra-high strength hot rolled steel sheet and a method of manufacturing the same.

Recently, the tendency to increase the strength of the steel sheet currently applied for the purpose of reinforcing the body and weight reduction for improving the fuel cost of the collision safety of the automobile.

In particular, steel sheets used for bumper reinforcement of automobiles or shock absorbers in doors that have a direct relationship with passenger safety in a vehicle crash are mainly used for hot rolled steel sheets having very high strength of 780 MPa or more, and require high elongation and high elongation. I'm doing it.

In addition, the use of high strength steel of higher strength parts is increasing in order to increase fuel efficiency in order to cope with increasingly severe environmental pollution regulations, and recently, research on commercialization of high strength steel of 780 MPa or more has been developed.

In the past, methods of manufacturing new steel species have been steadily sought while improving the strength and the workability is not significantly deteriorated. Such development directions can be distinguished as follows.

There is a method in which residual austenite is distributed in polygonal ferrite or bainite structure so that residual austenite generates metamorphic organic plasticity at the beginning of processing, thereby improving ductility. TRIP steel is a representative steel manufactured by this method.

This TRIP (Transformation Induced Plasticity) steel performs appropriate cooling according to the alloying amount so that pearlite transformation does not occur during the cooling of the austenite, and then delays the transformation of the austenite to bainite in the winding step again, thereby maintaining the austenite during final cooling. Part of the knight is transformed into martensite and the remainder is the remaining steel, at which time the retained austenite is transformed to martensite by initial processing, and ductility is formed in this process, which is called metamorphic organic plasticity, and this behavior Refers to the steel grade that looks.

However, in this steel grade, only bainite and residual austenite exist in the tissue, and even if the residual austenite turns to martensite during deformation, the amount is small in 5-20%, so there is a limit to further improving the strength. ..

On the other hand, by adding a hardenable element Si, Mn, Cr, etc. to form a low-temperature transformation structure after cooling, it is a method of increasing the strength by this structure, a typical steel manufactured in this way as a metamorphic structure steel (bainite Steel, abnormal tissue steel, composite steel, etc.), and the strength obtained here is to adjust the strength according to the cooling conditions and the amount of alloy added.

Among them, dual phase steel, which is a two-phase phase of ferrite and martensite, is quenched in the austenite and ferrite two-phase regions to produce about 10-30% (volume fraction) of martensite. Compared with precipitation strengthened steel, it has excellent ductility and elongation processability, and is applied to members, bumpers, etc. because of its high impact energy absorption capacity.

Two-phase tissue steels were developed in the 1980s and have been actively applied in recent years. In Europe, the application rate is more than 50% of the total sheet.

In addition, the ideal structure steel has excellent impact absorption energy under high-speed deformation conditions such as vehicle collision, and has superior weldability and galvanizing property compared to similar TRIP steels. Many parts are adopted in the process. However, the abnormally developed steel sheet, which was developed and mass-produced, was quenched in the abnormal region of austenite and ferrite.

As the fraction of martensite in the whole tissue increases, the strength increases, and as the ferrite fraction increases, the ductility increases. Therefore, the martensite fraction must be sufficiently obtained to increase the strength.

Therefore, this steel sheet has been developed as a situation in which tissue steel is required to significantly increase the strength of the steel sheet and maximize the safety and light weight of the automobile.

The present invention has been proposed to solve the above problems in view of the above problems, the object of which is that the hot rolled steel super high strength hot rolled steel has significantly increased the strength to be applied to parts receiving a lot of impact energy among hot rolled steel sheet for automotive parts It is to provide a steel sheet and a method of manufacturing the same.

The ideal structure steel ultra high strength hot rolled steel sheet of the present invention for achieving the above object is by weight%, carbon (C) 0.15 ~ 0.30%, silicon (Si) 0.3 ~ 2.0%, manganese (Mn) 1.5 ~ 4.0%, sulfur (S) 0.01% or less, phosphorus (P) 0.03% or less, titanium (Ti) 0.03 ~ 0.06%, boron (B) 0.001 ~ 0.003%, and the remaining content inevitably included in iron (Fe) and iron It is characterized by having a composition made of impurities.

Preferably, 0.02 to 0.06% by weight of niobium (Nb) is further provided in the composition.

In addition, it is preferable that molybdenum (Mo) 0.02-0.06 weight% is further provided in the said composition.

Meanwhile, another characteristic element of the present invention is a method for producing a superstructure steel super high strength hot rolled steel sheet in weight%, carbon (C) of 0.15 to 0.30%, silicon (Si) of 0.3 to 2.0%, and manganese (Mn) of 1.5 to 4.0%. , Sulfur (S) 0.01% or less, phosphorus (P) 0.03% or less, titanium (Ti) 0.03 ~ 0.06%, boron (B) 0.001 ~ 0.003%, the remaining content is inevitable to iron (Fe) and iron It is dissolved in a converter so as to have a composition consisting of impurities, including a continuous cast to form a slab,

Reheating the slab and hot rolling in the form of a steel sheet,

The hot rolled steel sheet is cooled to have martensite and ferrite abnormalities.

In the hot rolling step, the finish rolling temperature is maintained at Ar3 + 30 ~ 100 ℃,

In the step of cooling to have the abnormal tissue, it is quenched to have a rapid cooling rate of 30 ~ 60 ℃ per second and wound in a coil form,

The winding temperature is to maintain 200 ~ 400 ℃.

The present invention relates to one or more tissue steel ultra-high strength hot rolled steel sheet and a method for manufacturing the same, so that the invention can produce a steel sheet hot rolled steel with a significant improvement in strength, according to the present invention, the present invention relates to rigidity against impact energy in automotive parts. The strength is greatly improved so that it can be applied to the required area, has a useful effect that can achieve the purpose of safety and light weight of the vehicle.

Hereinafter, preferred embodiments of the present invention will be described in detail.

First, the ideal structure steel ultra-high strength hot rolled steel sheet according to the present invention in weight%, carbon (C) 0.15 ~ 0.30%, silicon (Si) 0.3 ~ 2.0%, manganese (Mn) 1.5 ~ 4.0%, sulfur (S) 0.01 Or less, 0.03% or less of phosphorus (P), 0.03 to 0.06% of titanium (Ti), 0.001 to 0.003% of boron (B), and consisting of iron (Fe) and its impurities inevitably Has a composition.

That is, the present invention is to increase the content of carbon, silicon, manganese and the like, and greatly increased the rigidity by adding elements such as titanium, boron.

Carbon (C): 0.15 to 0.30 weight%

Carbon (C) is the main element to increase the strength by making the martensite structure to increase the strength during hot rolling after hot rolling, it is preferable to add 0.15% by weight or more in order to satisfy the strength, in order to prevent the weldability and formability of the steel sheet to prevent degradation It is preferably included at 0.30% by weight or less.

manganese( Mn ): 0.15 ~ 4.0 weight%

Manganese (Mn) is a substitutional solid solution added to prevent the harmfulness of sulfur present in the steel, and is included in an amount of 0.15% by weight or more for the effect of increasing the strength of the steel sheet. 4.0 weight% or less is preferable since there exists a possibility that a moldability may fall and a hot rolling rate may fall.

Phosphorus (P): 0.03 wt% or less

Phosphorus (P) is an inevitably contained component, and when present in steel, it is limited to 0.03% by weight or less because it decreases moldability, precipitates at grain boundaries, increases soft brittle transition temperature, and lowers impact strength.

Sulfur (S): 0.01 wt% or less

Sulfur (S) is brittle when present in steel, so it is preferable to limit it to 0.01% or less.

titanium( Ti ): 0.03 ~ 0.06 wt%

Titanium (Ti) is precipitated in ferrite to form a precipitate to increase the strength of the ferrite, thereby increasing the strength of the steel sheet and also by adding a 0.03% or more to improve the formability and fatigue strength by reducing the hardness difference between the ferrite and martensite structure It is preferable to limit the content to 0.06% or less because excessively present the moldability and the steel sheet manufacturing cost increases.

Boron (B): 0.001 to 0.003 weight%

Boron (B) is the most economically essential ingredient to improve the hardenability, if the amount of boron in the material is added less than 0.001% by weight can not obtain a sufficient effect of improving the hardenability, when the boron compound exceeds 0.003% by weight Since hot brittleness due to formation appears, 0.001 to 0.003% by weight is preferable.

In addition to the above elements, the remainder may be composed of iron (Fe) and impurities inevitably contained. More preferably, niobium and molybdenum are selectively added as follows.

Niobium ( Nb ): 0.02 ~ 0.06 wt%

Niobium (Nb) is an element that refines crystal grains by dissolving in austenite or forming a precipitated phase during hot rolling, and is preferably added at least 0.02% for solid solution and precipitation. It is lowered to 0.06% or less.

molybdenum( Mo ): 0.02 ~ 0.06 wt%

Molybdenum is an element having a function of depositing and reinforcing carbide in ferrite, and it is preferable to add 0.02% or more for this purpose. However, molybdenum is limited to 0.06% or less because excessively increasing the production cost and lowering formability.

The method for producing a hot rolled steel sheet having such a composition ratio is as follows.

After dissolving the composition in the converter and continuous casting, the slab is heated in a heating furnace at 1100-1300 ℃ for 60-180 minutes to dissolve the coarse precipitate.

At this time, it is difficult to dissolve the precipitate at 1100 ° C. or lower, and the temperature of hot rolling is not sufficiently secured. When the temperature is higher than 1300 ° C., the structure becomes coarse and the energy cost due to heating is limited to less than that. It is preferable.

Subsequently, in the process of hot rolling after reheating in the furnace, the finish rolling temperature is maintained at Ar3 + 30 to 100 ° C.

This means that when Ar3 + 30 ℃ or less, the load of the roll during hot rolling increases greatly, resulting in increased energy consumption and slowing down of work speed.At the temperature below Ar3, cornerstone ferrite is generated and the required amount of martensite can be obtained after cooling. Because it is not.

In addition, when the finish rolling at a temperature of Ar3 + 100 ℃ or more, the grains are coarse and high strength cannot be obtained, and in order to obtain sufficient martensite, the cooling rate must be further increased.

Subsequently, after hot rolling, rapid cooling is performed at a cooling rate of 30 ° C. to 60 ° C. per second on the R.O.T (Run out Table) to prepare the structure of the steel sheet as an ideal structure of martensite and ferrite to improve strength.

At this time, when the cooling rate is lower than 30 ℃ per second it is difficult to obtain martensite and accordingly the strength is lowered, the cooling rate for obtaining a sufficient amount of martensite is preferably a cooling rate of 50 ℃ per second.

Subsequently, after being manufactured as a hot rolled steel sheet, it has a step of winding in the form of a coil, and the winding temperature is maintained at 200 to 400 ° C.

This is because the amount of martensite to increase the strength when the temperature is above 400 ° C. cannot be obtained sufficiently, and when the temperature is 200 ° C. or below, the amount of ferrite cannot be obtained sufficiently, so that the toughness of the steel sheet is lowered.

Hereinafter, the present invention will be described in more detail with reference to Examples.

These examples are intended to illustrate the present invention in more detail, and the scope of the present invention is not limited by these examples, which will be apparent to those of ordinary skill in the art.

The molten steel was prepared using the composition and content as shown in Table 1, and then the yield strength, tensile strength, and elongation of the steel sheet prepared by dissolving the molten steel in a converter, followed by continuous casting, followed by reheating, hot rolling, and cooling. The results were summarized in Table 1 below.

At this time, in the test method, the steel sheet prepared above was first made a tensile test piece of KS5 No. and a tensile test at a rate of 20 mm / min using a 25 ton Zivic tensile tester to measure the yield strength, tensile strength, elongation.

division Example 1 (% by weight) Example 2 (% by weight) Example 3 (% by weight) Comparative Example 1 (% by weight) Comparative Example 2 (wt%) Furtherance carbon 0.22 0.24 0.23 0.09 0.10 silicon 0.85 0.68 0.74 0.28 0.35 manganese 2.85 3.13 3.09 1.52 1.58 sign 0.012 0.008 0.006 0.02 0.02 sulfur 0.003 0.002 0.003 0.012 0.013 titanium 0.045 0.042 0.038 - - Boron 0.0021 0.0024 0.0022 - - Niobium - - 0.031 molybdenum - - 0.043 iron Remaining amount Remaining amount Remaining amount Remaining amount Remaining amount Yield strength (MPa) 1064 1112 1168 454 506 Tensile Strength (MPa) 1256 1326 1384 612 650 % Elongation 8.4 8.2 8.1 23.5 22.4

As shown in the above table, it can be seen that Examples 1, 2, and 3, which were tested by using the test pieces manufactured by the manufacturing method of the present invention, have significantly improved tensile strength as compared with Comparative Examples 1 and 2.

In addition, in particular, Example 3 containing niobium and molybdenum shows a larger physical property change than Examples 1,2.

Claims (5)

By weight%, carbon (C) 0.15 to 0.30%, silicon (Si) 0.3 to 2.0%, manganese (Mn) 1.5 to 4.0%, sulfur (S) 0.01% or less, phosphorus (P) 0.03% or less, titanium (Ti) ) 0.03% to 0.06%, boron (B) 0.001% to 0.003% of boron (B), the ultrastructure steel super high strength hot rolled steel sheet, characterized in that it has a composition consisting of iron (Fe) and impurities inevitably included in the iron. The method according to claim 1, Niobium (Nb) 0.02 ~ 0.06% by weight of the composition is an ideal tissue steel ultra high strength hot rolled steel sheet characterized in that it is further provided. The method according to claim 1 or 2, Molybdenum (Mo) 0.02 ~ 0.06% by weight of the composition is characterized in that the ultra-high strength hot rolled steel sheet. By weight%, carbon (C) 0.15 to 0.30%, silicon (Si) 0.3 to 2.0%, manganese (Mn) 1.5 to 4.0%, sulfur (S) 0.01% or less, phosphorus (P) 0.03% or less, titanium (Ti) ) 0.03 ~ 0.06%, boron (B) 0.001 ~ 0.003%, and the remaining content is dissolved in the converter so as to have a composition consisting of iron (Fe) and impurities inevitably contained in the iron and continuously cast to form a slab. , Reheating the slab and hot rolling in the form of a steel sheet, The hot rolled steel sheet is cooled to have martensite and ferrite abnormality structure. The method according to claim 4, In the hot rolling step, the finish rolling temperature is maintained at Ar3 + 30 ~ 100 ℃, In the step of cooling to have the abnormal tissue, it is quenched to have a rapid cooling rate of 30 ~ 60 ℃ per second and wound in a coil form, Method for producing a superstructure steel super high strength hot rolled steel sheet, characterized in that to maintain the winding temperature 200 ~ 400 ℃.