KR20040059577A - Manufacturing of high-strength ultrafine grained steels with high toughness using Strain Induced Dynamic Transformations - Google Patents

Abstract

PURPOSE: A method for manufacturing high strength high toughness steel formed of composite structure of ultra-fine ferrite having an average grain size of 5 μm or less and pearlite having 40% or less of volume fraction by optimizing austenite grain size and hot working conditions is provided. CONSTITUTION: The method for manufacturing ultra-fine grained steel using strain induced dynamic transformation comprises a step of preparing a steel slab comprising 0.3 wt.% or less of C, 1.5 wt.% or less of Si, 2.0 wt.% or less of Mn, 0.01 to 0.08 wt.% of Nb and a balance of Fe and inevitable impurities; a step of controlling an average grain size of austenite structure of the steel slab to 50 μm or less before finally rolling the steel slab; a step of hot rolling the steel slab having austenite structure in multistage in such a way that the total reduction ratio is maintained to 60% or more as maintaining a reduction ratio per one pass of the steel slab to 30% or less in the temperature range of Ar3 to Ar3+100 deg.C; and a step of slowly cooling the hot rolled steel sheet so that fraction of pearlite in the steel structure becomes 40% or less, wherein the hot rolled steel sheet is slowly cooled in a cooling rate of 10 deg.C/sec or less.

Description

Manufacturing method of high strength, high toughness ultra fine steel using strain organic dynamic transformation {Manufacturing of high-strength ultrafine grained steels with high toughness using Strain Induced Dynamic Transformations}

The present invention relates to a method for producing low carbon structural steel containing a large amount of fine ferrite structure, and more specifically, using a modified organic dynamic transformation, the plate, hot rolled, made of fine ferrite and pearlite composite structure having a particle diameter of 5 μm or less It relates to a method for producing low carbon structural steel for the production of section steel, wire rods and bars.

As a method of improving the strength of the steel, various reinforcing methods such as precipitation strengthening, solid solution strengthening, martensite strengthening, and fine pearlite strengthening may be mentioned. However, these steel reinforcement methods improve strength while accompanied with deterioration of toughness. However, in the case of reinforcing the strength of steel by miniaturizing grains, not only can the toughness degradation problem accompanying high strength be solved, but also the reduction of the impact transition temperature can be expected. . In particular, low carbon structural steel, which is mainly used when many welded joints are required in the fabrication of structures, or when the impact toughness of steel is required as an important characteristic, is made of most of the microstructure except for the case of quenching (quenching). (Hereinafter, referred to as "ferrite steel"), in recent years, the technology of grain refinement of ferrite steel has advanced dramatically.

Among them, the so-called TMCP (Thermo-Mechanical Controlled Process) method, in which the steel is finished in the austenite non-crystallization zone and accelerated cooling while the austenite is elongated for a long time, increases the nucleation speed of phase transformation and refines the grains. It was developed in Korea to provide breakthrough electricity for grain refinement technology. In recent years, low-temperature stepping techniques have been developed that can further refine the grains than the TMCP method.

The TMCP method was groundbreaking at the time of development, but recently, it has been evaluated as a generalized fine grain steel manufacturing technology, and when applied to low carbon ferrite steel, it is known that the ferrite grains can be reduced to about 5 μm. However, when increasing the strength of the steel through grain refinement, the strength increases depending on the inverse of the grain size, so that the rate of increase in strength due to grain refinement becomes drastically sharp when the grain size of the ferrite is 5 µm or less. Therefore, in recent years, ultrafine grain refining technology has been developed in which the ferrite grain size is 5 μm or less.

Conventional techniques related to ultrafine grains include Korean Patent Applications, Publication Nos. 1999-029986, 1999-029987, 1999-58126, 1999-63186, US Patent Invention Nos. 4466842, 5200005, 6027587, and the like. Can be mentioned.

The above-mentioned patent application 1999-029986 proposes a method of compressing at least 30% of the reduction ratio in the austenite unrecrystallized zone temperature range during heating and cooling of low carbon steel, and miniaturizing ferrite through accelerated cooling. . In the above-mentioned Patent Application No. 1999-029987, after general carbon steel is first heat-treated with martensitic structure, the steel is heated to a ferrite stable temperature range (500 ^o C to Ac ₁ ) to be processed at a reduction ratio of 50% or more per pass. The present invention suggests a method for miniaturization to less than 5 μm through ferrite recovery and recrystallization.

In addition, Korean Patent Application Laid-Open No. 1999-58126 discloses a method of refining a ferrite grain size through heating down and cooling a low carbon steel at a pressure drop of 80% or more near Ar ₃ . In the rolling process after heating, finish rolling is incubated at a rate of reduction of 20% or more per pass within a temperature range of Ar ₃ ± 20 ^° C.

In US Pat. No. 4,466,842, when the reheated low carbon steel is finish rolled near the Ar ₃ temperature, the total reduction ratio is 75% or more through a single pass or a multistage pass, and the holding time between passes is less than 1 second. A technique for miniaturizing ferrite grains to 4 µm or less by accelerated cooling has been proposed. In addition, U.S. Patent No. 5200005 proposes a method for producing ultrafine steel having a ferrite grain size of 5 µm or less by heating the ultra-low carbon steel after heating and rolling it in a range of less than Ar ₁ , which is a ferrite stable temperature. In the US Patent 6027587, the ultra-fine grain of 5㎛ or less in the surface layer of the steel material by rolling the low-carbon steel in the temperature range of 700 ~ 950 ^o C of the unmodified austenite maintained at a size of 50㎛ or more during the rolling process It proposes a manufacturing method for obtaining ferrite.

That is, the inventions described in the above-described prior art presuppose the concept that ultrafine ferrite can be obtained only by applying a large pressure in a hot or warm processing process, which is a main process for manufacturing steel, and accordingly, according to the patent Although there is a difference, as a prerequisite for refining ferrite, the minimum reduction rate per pass or the maximum holding time between passes is specified. However, in order to impart a large pressure during hot processing as in the prior art, it is almost impossible to achieve it with a conventional facility because it requires a hot processing equipment such as a rolling mill with a huge capacity. There was a limit to the formation of ultra-fine ferrate tissue, such as the easy growth of ferrite tissue formed by heat.

Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art, by optimizing the austenite grain size and hot processing conditions, such as ultrafine ferrite having an average grain size of 5 µm or less and a pearlite composite having a volume fraction of 40% or less. It is an object of the present invention to provide a method of manufacturing a high strength, high toughness steel made of tissue.

The present invention for achieving the above object, in the weight%, C: 0.3% or less, Si: 2.0% or less, Mn: 3.0% or less, Nb: 0.01 to 0.08%, balance iron and inevitable impurities Providing a; Controlling the average grain size of the austenite structure to be 50 µm or less before finishing rolling the steel; Thermally processing the steel of the structure so that the total reduction ratio is 60% or more while maintaining a reduction ratio per pass in a temperature range of Ar ₃ to Ar ₃ + 100 ° C. or less; And slow cooling the hot-processed steel so that the pearlite fraction of the steel structure is 40% or less.

Hereinafter, the present invention will be described.

The present inventors have proposed a method of manufacturing ultrafine steel that can overcome the above limitations of the prior art as Korean Patent Application No. 2002-63394. This patent application proposes a technique for forming an ultra-fine ferrite structure by using a strained organic dynamic transformation by hot working steel of an austenitic structure. Specifically, in the patent application, in order to manufacture ultra-fine high strength steel, the austenitic grain size should be controlled to 50 μm or less so that such deformation organic dynamic transformation may occur even at a small reduction rate, and the cumulative reduction rate at the time of hot working This suggests that the processing temperature, etc. should be optimally controlled.

However, according to the results of the present inventors, in addition to the above conditions, if the hot-treated steel is cooled to a predetermined condition, after the final cooling, it consists of ultra-fine ferrite of 5 µm or less and pearlite composite structure of 40% or less by volume. It is to present the invention that the discovery that a high strength, high toughness steel can be produced.

Hereinafter, the steel composition component of this invention is demonstrated.

Carbon (C) is an element that needs to be contained in an appropriate amount for effective reinforcing steel. However, if the content exceeds 0.3% by weight (hereinafter referred to simply as%), the percentage of ferrite in the final microstructure becomes about 60% or less (percent of pearlite is 40% or more) and cannot be classified as a low carbon steel. The toughness reduction of the heat affected zone at the time of welding may be a big problem.

Therefore, in the present invention, the carbon content is limited to 0.3% or less.

Silicon (Si) is a component element that needs to be added for deoxidation in the steelmaking process with a solid solution strengthening effect. However, if the content exceeds 1.5%, the weldability is lowered, and an oxide film that is difficult to remove is likely to be formed on the surface of the steel sheet, and particularly, coarsening of ferrite grains can be promoted.

Therefore, in consideration of this, the content is limited to 1.5% or less.

Manganese (Mn) needs to be added for deoxidation, but if the amount exceeds 2.0%, the hardenability is unnecessarily increased, thereby reducing the transformation rate of ferrite during rolling and increasing the possibility of low temperature structure during welding. have.

Therefore, in this invention, content of Mn is restrict | limited to 2.0% or less.

Niobium (Nb) is very important in the present invention as a component that combines with carbon or nitrogen in steel during reheating or hot rolling to form ultra-fine precipitates of several tens of nanometers in size.

As described above, the dynamic metamorphic ferrite structure is very fine in its properties, so it easily grows during hot rolling, particularly during holding time between passes. Therefore, it is essential to suppress such growth. As a suitable technique, fine niobium precipitates are used in the present invention. That is, in order to effectively prevent the grain growth of the fine ferrite, fine precipitates are required, and thus the most effective niobium carbonitride is used.

However, if the content of niobium is less than 0.01%, the number of niobium precipitates is too small to effectively suppress the grain growth of the entire ultrafine ferrite, and if it exceeds 0.08%, the effect of the addition is saturated and the steel is too hardened. It is not easy to obtain sufficient dynamic transformation ferrite structure.

Therefore, in the present invention, the niobium content is limited to 0.01 to 0.08%.

Next, a method of manufacturing ultrafine ferrite steel using the steel material prepared as described above will be described.

In the present invention, after the steel is prepared as described above, it is reheated and subjected to a rough rolling process, if necessary, at this time, the reheating temperature, pressure drop rate so that the average grain size of the austenite structure of the steel is less than 50㎛ Control is required. If the size of the austenite grain size of the steel immediately before finishing rolling exceeds 50 µm, the formation rate of the strain organic dynamic transformation ferrite becomes significantly uneven during the subsequent hot working, and the formation position of the deformation organic dynamic transformation ferrite becomes very uneven. This is because it is very difficult to obtain ultrafine grained ferritic steel because the mixed ferrite is very likely to be formed.

Therefore, in order to effectively obtain fine ferrite grains in the final product state, it is necessary to maintain the subcooled austenite grain size in the following finish hot rolling to an average of 50 μm or less. If the austenite is supercooled in the unrecrystallized region, the average length of the major and minor axes of the austenite grains deformed elliptical in the austenite cross section should be kept below 50 µm.

As described above, the steel in which the average grain size of the austenitic structure is controlled is cooled to a predetermined cooling rate and then subjected to finishing hot rolling in a supercooled state, wherein the starting temperature of finishing hot rolling is set to Ar ₃ to Ar ₃ + 100 ° C. Restrict. Ten thousand and one, and the problem to which the finish hot rolling start temperature low, the pro-eutectoid ferrite coarse before hot rolling are formed along the austenite grain lowering various physical properties by being long stretch of the rolling process than the Ar ₃ occurs, Ar ₃ + 100 ℃, If exceeded, the fraction of dynamic transformation ferrite may not be sufficiently secured, and the tissue refinement itself may be impossible.

On the other hand, in the present invention, in performing the final hot rolling, it is required to perform hot rolling in such a manner that the cumulative rolling rate becomes 60% or more while maintaining the rolling reduction per pass at 30% or less. If the reduction ratio per pass of the hot rolled rolling is set to 30% or more, the temperature of the hot work material is excessively increased due to the processing heat, thereby causing a problem of halving the effect of refining grains. Therefore, in the present invention, it is essential to keep the rolling reduction per pass of the hot cut processing at 30% or less.

In addition, when the cumulative reduction ratio of the hot rolled rolling becomes less than 60%, it is difficult to obtain ultrafine grain structure because the amount of formation of dynamic transformation ferrite effective for miniaturization is insufficient.

Such hot rolling is preferably carried out so that the strain organic dynamic transformation ferrite fraction at the end of rolling is 40% or more in terms of securing the final fine ferrite microstructure. If the fraction is less than 40%, the size of the static ferrite formed during the cooling after processing becomes coarse, so that a structure of sufficiently fine and uniform final product cannot be secured.

Subsequently, in the present invention, the hot-processed steel is slowly cooled so that the pearlite fraction in the steel structure is 40% or less, because it cannot be used as structural steel only by miniaturization of soft ferrite. However, if the pearlite fraction exceeds 40%, there is a problem that the strength of the steel is deteriorated, but the toughness of the steel is deteriorated.

Preferably, the hot rolled steel is slowly cooled at a cooling rate of 10 ° C./sec or less.

As described above, in the present invention, by appropriately controlling the particle size, finish hot rolling, and cooling conditions of the austenite structure before finishing hot rolling, steel materials having a ferrite + pearlite composite structure having an average grain size of 5 µm or less can be effectively produced. have.

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

(Example)

Steel grade Chemical composition (% by weight) C Si Mn Nb N Fe and impurities A 0.08 0.25 1.51 0.04 0.005 Balance B 0.15 0.42 1.10 0.05 0.005 Balance C 0.20 0.24 2.21 0.04 0.003 Balance D 0.28 1.53 1.08 0.06 0.004 Balance E 0.35 0.23 1.49 - 0.005 Balance

As shown in Table 1, a steel material having different compositional components was prepared. The steel thus prepared was reheated and then rough-rolled to control the austenite grain size (AGS) as shown in Table 2 below. Subsequently, the steel was air cooled to hot end rolling and then controlled to a supercooled state, followed by hot rolling. The specific conditions are as shown in Table 2 below. On the other hand, the pause time between passes in the hot rolling stage was 10-15 seconds.

After cooling the finished hot-rolled steel at different cooling rates as shown in Table 2, the microstructure was examined to determine the average ferrite grain size (FGS), and the results are shown in Table 3 below. In addition, the hot-break stage machining process was simulated in a hot simulation tester to measure the dynamic transformation ferrite fraction. The fraction of dynamic transformation ferrite was calculated by measuring the change of the length of specimen after milling by Dillatometry.

On the other hand, the mechanical properties such as yield strength, tensile strength and elongation of each test steel manufactured as described above were measured, and the impact transition temperature is shown in Table 3 to evaluate the impact transition temperature.

Steel grade AGS (μm) Ar ₃ temperature (℃) Rolling Start Temperature (℃) Rolling end temperature (℃) Pressure drop per pass (%) Total Pressure Drop (%) Cooling rate (℃ / s) Cooling temperature (℃) Invention One A 50 740 800 760 20 67 Air cooling - 2 B 45 755 800 770 25 75 8 550 3 C 43 710 750 720 20 76 Air cooling - 4 D 48 700 730 710 28 76 Air cooling - Comparative material One B 45 755 800 770 25 75 15 260 2 C 43 710 750 720 20 76 11 560 3 D 48 700 730 710 28 76 12 580 4 E 47 680 710 700 25 76 Air cooling -

Steel grade Dynamic transformation ferrite fraction (%) Final Ferrite Fraction (%) Perlite fraction (%) Average FGS (μm) Yield strength (kgf / ㎛ ² ) Tensile strength (kgf / ㎛2) Elongation (%) Impact Transition Temperature (℃) Invention One A 44 72 28 3.7 43.8 61.8 33 -108.8 2 B 47 74 26 3.2 45.2 62.5 21 -119.3 3 C 43 66 34 3.3 43.1 65.5 35 -116.9 4 D 41 62 38 3.1 44.2 68.3 32 -121.5 Comparative material One B 47 58 0 3.2 40.6 69.3 28 -35.2 2 C 43 55 45 3.3 43.5 69.1 23 -75.5 3 D 41 52 48 3.1 45.6 70.3 21 -65.3 4 E 35 48 52 3.4 41.6 71.2 18 -45.1

As shown in Table 3 and Table 4, the steels with appropriately controlled contents of Si, Mn, and Nb were hot-processed to meet the conditions of the present invention and then cooled to control the pearlite fraction in the final structure to 40% or less. In the case of the invention materials (1 to 4) it can be seen that not only the yield strength and tensile strength are excellent but also the impact characteristics thereof.

On the contrary, in the comparative material (1) cooled to a cooling rate of 15 ° C./s or more after hot working at 260 ° C., the retained austenite was immediately transformed into martensite to obtain a pearlite structure. Was not good.

In addition, after hot working, the steel was cooled to a high temperature of 500 ° C or higher, but the comparative materials (2 to 3) having the cooling rate of 10 ° C / s or higher had a desired impact characteristic value with a pearlite fraction exceeding 40% in the final structure. Could not get

On the other hand, the comparative material 4 is a case where the carbon component of the steel material is contained more than the present invention, the impact properties of the steel material was not very good.

As described above, the present invention effectively refines ferrite grains by performing continuous multi-stage processing under conditions that promote so-called "strain organic deformation" directly on the Ar ₃ temperature, without adding a large amount of alloys or heat treatment in low carbon steels. By mixing and complexing the second phase pearlite, there is an effect useful in the production of excellent structural steels that can improve the physical properties of the steel while maintaining excellent weldability.

Claims (2)

Preparing a steel comprising, by weight, C: 0.3% or less, Si: 1.5% or less, Mn: 2.0% or less, Nb: 0.01% to 0.08%, balance iron and inevitable impurities; Controlling the average grain size of the austenite structure to be 50 µm or less before finishing rolling the steel; Thermally processing the steel of the structure so that the total reduction ratio is 60% or more while maintaining the reduction ratio per pass in a temperature range of Ar ₃ to Ar ₃ + 100 ° C. or less; And Slow cooling the hot-processed steel so that the pearlite fraction of the steel structure is 40% or less; Ultrafine steel production method using a strained organic dynamic transformation comprising a. The method of claim 1, wherein the hot rolled steel is slowly cooled at a cooling rate of 10 ° C / sec or less.