KR20040059579A - Method for manufacturing the ultra-fine ferrite by dynamic transformation - Google Patents

Abstract

PURPOSE: A method for manufacturing ultra-fine grained steel formed of ultra-fine ferrite structure having an average grain size of 3 μm or less by controlling austenite grain size to 15 μm or less by dynamic recrystallization and optimizing hot working conditions for strain induced dynamic transformation of ferrite is provided. CONSTITUTION: The method comprises a step of preparing a steel slab comprising 0.02 to 0.3 wt.% of C, 1.5 wt.% or less of Si, 2.0 wt.% or less of Mn, 0.005 to 0.1 wt.% of Ti, 0.003 to 0.03 wt.% of N and a balance of Fe and inevitable impurities, wherein the Ti and N satisfy the following relational expression: 1.2<=Ti/N<=3.4; a step of controlling a grain size of the steel slab to 15 μm or less by rolling the reheated steel slab so that the total strain amount is 10% or more in the temperature range of Thr-50 deg.C to Thr+50 deg.C after reheating the steel slab in such a manner that the steel slab has an austenite grain size of 50 μm or less; and a step of hot rolling the steel slab having controlled austenite grain size in such a way that the total reduction ratio is maintained to 40% or more as maintaining a reduction ratio per one pass of the steel slab to 40% or less in the temperature range of Ar3 to Ar3+50 deg.C, wherein the steel slab further comprises one or more of elements selected from 0.1 wt.% or less of Al, 0.005 to 0.1 wt.% of Nb and 0.1 wt.% or less of V, wherein an available accumulated reduction ratio of the steel slab during the dynamic recrystallization rolling is 30% or more, and wherein dynamic ferrite fraction of the steel slab is 40% or more, and ferrite grain size of the steel slab is 3 μm or less at a hot working completing time point.

Description

Method for manufacturing the ultra-fine ferrite by dynamic transformation

The present invention relates to a method for producing a low carbon structural steel containing a large amount of ultra-fine ferrite structure, more specifically, to make the grain size of the austenite to 15㎛ or less by dynamic recrystallization, using the strain organic deformation It relates to a method for producing low carbon structural steel for the production of thick plates, hot rolled steel, section steel, wire rods and bars made of ultra-fine ferrite structure of less than 3㎛ particle size.

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, a so-called TMCP (Thermo-Mechanical Controlled Process) method has been recently developed, in which a steel sheet is rolled in a non-recrystallized zone to generate austenite strain bands, and then accelerated cooling to increase the rate of nucleation of ferrite and to refine grains. This has provided breakthrough electricity for grain refinement technology.

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.

The most influential factor among the requirements for ultrafine ferrite is the grain size of austenite. When ferrite is transformed from austenite, the more sites for nucleation, the faster the ferrite nucleation rate and the finer the size. There are methods to refine the grains of austenite, of which static recrystallization is common. For example, Korean Patent Publication No. 1995-3290, Korean Patent Publication No. 1997-7158, Japanese Patent Application Laid-Open No. 9-296253, Japanese Patent Application Laid-Open No. 7-34125, Japanese Patent Application Laid-Open No. 9-316534, and the like. It is a ferrite grain refining technique by static recrystallization or unrecrystallization of knight, but its size is about 5 mu m or more.

Typically, austenite grains have a size of about 20 μm at 900 ° C., which is a stable region in which ferrite is transformed into austenite and has no relation to alloy addition. However, in the field working conditions produced by steel or cast slab to which alloying elements are added for a specific purpose, the normal heating temperature is over 1000 ° C., so that the recrystallized austenite grains have a size of 20 μm.

When austenite is regenerated dynamically, the grain size is significantly reduced, resulting in a grain size of 20 μm or less. Dynamic recrystallization refers to a phenomenon in which crystals are recrystallized during rolling when steel is reheated with austenite and deformed above a critical amount of deformation. In general, it is known that the critical strain is better when the size of the initial austenite grain is smaller and the Zener-Hollomon parameter is smaller. It is known that techniques such as ferrite, martensite or bainite can be refined by miniaturizing austenite using such dynamic recrystallization.

For example, Korean Patent Publication No. 1998-067680 introduced a convenient process to reduce the rolling force and refine the grain size in the hot rolling process by generating the dynamic recrystallization in the specific pass by controlling the deformation amount of the rolling pass. However, dynamic recrystallization is unlikely when the initial grains of austenite are too coarse. In addition, even if dynamic recrystallization occurs, the growth rate is very fast and the growth must be suppressed, but the present invention does not describe a solution for this.

Another technique is Korean Patent Application No. 2000-71889, which describes the calculation of critical strain for dynamic recrystallization and the establishment of quantitative evaluation of cumulative pressure reduction. The conditions of dynamic recrystallization are described in detail, but the degree of ferrite grain refinement is not described. In Korea Patent Application No. 2000-0081073, the ferrite becomes fine due to the dynamic recrystallization of austenite and the mechanical properties are improved, but the elements of titanium (Ti) and nitrogen (N) are too limited.

On the other hand, Japanese Patent Application Laid-Open No. 2000-290748 uses ferromagnetic recrystallization of austenite to refine the size of ferrite grains to 4 µm or less by continuous filament rolling. However, the conditions for dynamic recrystallization should be less than 1150 ℃ and the Ti content is 0.03 ~ 0.3wt%, which contains much more than the commonly used alloying additives. This can be caused, and there is also an economical weakness due to the large use of expensive alloys.

As a conventional technique that aims to achieve ultrafine grain size using dynamic transformation, Korean Patent Application, Publication No. 1999-029986, 1999-029987, 1999-58126, 1999-63186, US Patent No.4466842, 5200005 6027587 etc. can be mentioned.

The patent application 1999-029986 proposes a method of compressing a reduction ratio of 30% or more 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 1999-029987, the general carbon steel is first heat-treated with martensitic structure, and then the steel is heated to a ferrite stable temperature range (500 ° C to Ac1) to be processed at a reduction ratio of 50% or more per pass to recover ferrite. And a method of miniaturizing to 5 μm or less through recrystallization.

In addition, Korean Patent Application Publication No. 1999-58126 discloses a method of refining the ferrite grain size through heating and cooling a low carbon steel at a pressure drop of 80% or more near Ar ₃ , and in Patent Publication 1999-63186. The method of refining ferrite is performed by constant temperature rolling at a rolling rate of 20% or more per pass within a temperature range of Ar ₃ ± 20 ° C. in the rolling process after heating.

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 performing a hot rolling in a range of less than Ar1, which is a ferrite stable temperature, during heating and rolling of ultra low carbon steel. , U.S. Patent No. 6027587 discloses ultra-fine ferrites of 5 µm or less on steel surface layers by rolling unmodified austenite retained to 50 µm or more in the temperature range of 700 to 950 ^o C during heating and heating of low carbon steel. It proposes a method of obtaining.

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 that the ferrite tissue formed easily due to heat.

Therefore, the present invention is to solve the above-mentioned problems of the prior art, by controlling the austenite grain size to 15㎛ or less by dynamic recrystallization, by optimizing the hot processing conditions for deformation organic dynamic transformation of ferrite, average determination It is an object of the present invention to provide a method for producing ultrafine steel composed of an ultrafine ferrite structure having a particle size of 3 μm or less.

The present invention for achieving the above object,

By weight, C: 0.02 to 0.3%, Si: 1.5% or less, Mn: 2.0% or less, Ti: 0.005 to 0.1%, N: 0.003 to 0.03%, residual iron and added impurities, and 1.2≤Ti / Preparing a steel material satisfying N ≦ 3.4; The steel is reheated to have an austenite particle size of 50 μm or less, and then dynamically recrystallized and rolled to have a total strain of 10% or more in a temperature range of Thr −50 ° C. to Thr + 50 ° C. to a particle size of 15 μm or less. Controlling; Hot-rolling the steel having the austenitic particle size controlled as described above to maintain a total reduction ratio of less than 40% per pass in a temperature range of Ar ₃ to Ar ₃ + 50 ° C. such that the total reduction ratio is 40% or more; It relates to a super fine steel manufacturing method comprising a.

Hereinafter, the present invention will be described.

As described above, in the past, attempts were made to secure ultrafine steel by adding as much a reduction ratio per pass as possible, but as a result, the amount of heat generated by processing was so great that the temperature of the material increased and grain growth occurred. there was. Therefore, to solve this problem in actual operation, it was necessary to install an extraordinary cooling device immediately after the rolling mill to prevent the temperature rise.

Therefore, the present inventors have repeatedly studied and experimented to overcome the limitations of the prior art, and based on the results, the present invention proposes the present invention, and the present invention does not cause grain growth problems due to the rise of the material temperature during the hot working process. It is characterized by optimizing the manufacturing process conditions so that it can be made ultra-fine in the range.

That is, the present inventors have found that the following three process conditions must be satisfied for the ultrafineness of steel materials, and the present invention is proposed.

First, the present invention dynamically recrystallizes steel of austenite tissue having stable carbonitride at high temperature under predetermined conditions to refine the structure to 15 µm or less.

Secondly, the present invention manufactures ultra-fine ferritic steel by using the modified organic dynamic transformation phenomenon by hot working the steel of the dynamic recrystallized austenitic structure, wherein the hot processing conditions are optimally controlled.

Third, the present invention distributes fine carbonitrides by containing Ti, Nb, V, and Al in steel so that ferrite grains formed by dynamic transformation do not occur.

First, the steel composition component of the present invention.

The content of carbon (C) is limited to 0.02 to 0.3% by weight (hereinafter referred to as only%). C is an element that needs to be contained in an appropriate amount for effective reinforcing steel, but if the content is less than 0.02%, it may be difficult to form carbonitride for grain growth inhibition of austenite or ferrite, which is used for the purpose of the present invention. In addition, if C exceeds 0.3%, the percentage of ferrite in the final microstructure is less than about 60%, which cannot be classified as a low carbon steel, and the toughness of the heat affected zone at the time of welding may be a big problem.

The content of silicon (Si) is limited to 1.5% or less.

Si is a component element that needs to be added for deoxidation in the steelmaking process with 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.

The content of manganese (Mn) is limited to 2.0% or less.

Mn lowers Ar ₃ and contributes to ferrite graining. If the amount is more than 2.0%, the hardenability is unnecessarily increased to lower the transformation rate of ferrite during rolling, and the possibility of low temperature structure during welding may increase.

Aluminum (Al) is an optional addition element and its content is preferably limited to 0.1% or less. Al plays a role of deoxidation in molten steel and forms fine AlN precipitates, thereby suppressing grain growth inhibition or ferrite grain growth of austenite. However, when the added amount exceeds 0.1%, the hardenability is increased to inhibit the dynamic transformation.

Niobium (Nb) is also an optional addition element, and its content is preferably limited to 0.005 to 0.1%. Nb is a very important element 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.

In the present invention, fine precipitates capable of suppressing grain growth of austenite after dynamic recrystallization or ferrite growth generated by dynamic transformation are required. The most effective method is to use niobium carbonitride. However, if the content of niobium is less than 0.005%, the above-described effect cannot be expected. If the content of niobium exceeds 0.1%, the effect of the addition is not only saturated, but the critical strain amount for dynamic recrystallization tends to be too large.

Vanadium (V) is an optional addition element that may be contained in the range of 0.1% or less.

V forms carbonitrides to promote ferrite nucleation and inhibits grain growth of ferrite. However, when the V content exceeds 0.1%, the hardenability is increased to inhibit the ferrite dynamic transformation.

Preferably, the steel of the present invention contains one or two or more of Al, Nb and V.

The content of titanium (Ti) is limited to 0.005 ~ 0.1%.

Ti combines with N to form fine TiN precipitates that are stable at high temperatures to inhibit grain growth of austenite upon reheating. In order to obtain such fine TiN, the content of Ti should be 0.005% or more. However, if the content exceeds 0.03%, coarse precipitates are formed in molten steel, which is not preferable because it does not inhibit austenite grain growth.

The content of nitrogen (N) is limited to 0.003 ~ 0.03%.

N is an indispensable element for forming TiN, AlN, Nb (CN), V (CN) and the like. However, if the content is less than 0.003%, it is difficult to form the required carbonitride, and if the content is more than 0.02%, the effect may be saturated and the dynamic recrystallization may be inhibited.

The ratio of Ti / N is preferably limited to 1.2 to 3.4.

In the present invention, when the ratio of Ti / N is less than 1.2, the amount of N becomes too large compared to Ti of the base metal, so that the high solute nitrogen increases and the dynamic transformation is suppressed, and when the ratio exceeds 3.4, the coarse TiN in the steelmaking process This is because coarse TiN is formed and it is difficult to expect effective grain growth inhibition effect of austenite.

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 material prepared as described above is manufactured and reheated, it is required to control the reheating temperature so that the average grain size of the austenite structure of the steel is 50 µm or less. In the subsequent dynamic recrystallization rolling, the critical strain for the dynamic recrystallization becomes larger as the initial grains become smaller, and thus, fine precipitates which are stable even at high temperatures are required. Therefore, in the present invention, the presence of TiN precipitates that maintain a stable precipitate up to 1250 ℃ is essential.

Preferably the reheating temperature is limited to 1000 ~ 1250 ℃.

As such, the reheated steel is then dynamically recrystallized for dynamic recrystallization, whereby the austenite grain size can be controlled to 15 μm or less.

In the present invention, at this time, it is preferable that the dynamic recrystallization rolling in the temperature range of Thr-50 ℃ ~ Thr + 50 ℃.

Dynamic recrystallization zone rolling temperature is generally suitable near the known non-recrystallization zone temperature (hereinafter referred to as "Tnr"), in the present invention a temperature range of Thr-50 ℃ ~ Thr + 50 ℃. Here, the unrecrystallized inverse temperature refers to a region in which recrystallization does not occur between passes under normal rolling working conditions. If recrystallization occurs at an excessively high temperature, the micronized effect of the austenite grain is weak and the critical strain is too low if the temperature is too low. This becomes so large that dynamic recrystallization does not occur.

The critical strain for dynamic recrystallization varies from steel grade to effective cumulative pressure drop above the threshold. The effective cumulative reduction amount is not an arithmetic sum that adds the sum of the reduction amounts for each pass, but the accumulation amount considering the deformation amount remaining in the next pass after removing the deformation amount that disappears due to recovery between the passes.

In the present invention, it is preferable to limit the effective accumulated pressure drop to 30% or more during the dynamic recrystallization rolling, because the dynamic recrystallization hardly occurs at less than 30%.

In the present invention, the total strain in the final rolling pass should be 10% or more. If less than 10% of the deformation amount is added, even if the dynamic recrystallized austenite occurs strain organic abnormal growth phenomenon, the recrystallized crystals do not become fine grain growth.

After this dynamic recrystallization is completed, the cooling rate until finishing rolling requires about 1 to 10 ° C / sec in the present invention. If the cooling rate is too slow, austenite growth may be caused, and if too fast, Ar ₃ may be too low, which may limit processing during finishing rolling.

Thereafter, the cooled steel is subjected to thermal end processing, wherein the finishing hot rolling start temperature is limited to Ar ₃ to Ar ₃ + 50 ° C. If the finish hot rolling start temperature is lower than Ar ₃ , coarse saltpeter ferrite is formed along the austenite grains before hot rolling, and elongated during rolling, thereby deteriorating various properties, causing Ar ₃ + 50 ° C. 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 finishing hot rolling, it is required to perform hot rolling in such a manner that the cumulative rolling rate is 40% or more while maintaining the rolling reduction per pass of 40% or less. If the reduction rate per pass of the hot rolled rolling exceeds 40%, the temperature of the hot worked material is excessively increased due to the processing heat, thereby causing a problem of halving the micronized effect of grains. In addition, if the cumulative reduction ratio of hot rolling is less than 40%, it is difficult to obtain an 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.

As such, in the present invention, by maintaining the average austenite grain size below a certain level and thermally cutting the steel, a large amount of strain-organic dynamic transformation ferrite can be effectively and uniformly formed at high speed. In addition, while maintaining the rolling reduction rate for each pass during a thermal cutting process, the cumulative reduction ratio is limited to a predetermined value or more, minimizing the amount of heat generated. By controlling the growth of ultrafine ferrite using precipitates such as Nb, V, and Al, the average size of the ferrite grains after final cooling can be controlled to 3 µm or less.

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

(Example)

To prepare a steel composition as shown in Table 1 below.

Steels of such a composition were reheated under the conditions shown in Table 2 to be austenitic, and then rough-rolled under the conditions shown in Table 2 and then cooled. At this time, the austenite grain size and the precipitate fraction in the steel was also measured and shown in Table 2.

The steel cooled as described above was hot-rolled and rolled under the conditions as shown in Table 3, and the dynamic and static deformations according to the hot working were measured from the difference in the coefficient of linear expansion during cooling after deformation in the hot deformation tester. The results are also shown in Table 3. And the hot-rolled steel was cooled to 10 ℃ to measure the average grain size of the ferrite. In addition, after cooling by varying the cooling conditions as shown in Table 3, measured mechanical properties of the steel is shown in Table 4. On the other hand, the microstructure obtained after cooling was analyzed using an optical microscope and an image analyzer.

Steel grade used C Si Mn Al Ti Nb V N A 0.10 0.25 1.5 - 0.010 0.04 - 0.005 B 0.15 0.30 1.0 0.03 0.013 - - 0.007 C 0.12 0.25 1.4 - 0.015 - - 0.010 D 0.12 0.20 1.5 - 0.02 - 0.05 0.008 E 0.08 0.35 1.0 0.03 0.01 0.03 0.03 0.015 F 0.10 0.2 1.2 - 0.3 - - 0.005 G 0.15 0.2 1.2 - - 0.04 - 0.003

Steel grade used Steel plate number Heating temperature (℃) Initial AGS (μm) Dynamic recrystallization rolling start temperature (℃) Effective accumulated pressure drop (%) Final rolling load (%) AGS (㎛) just before finishing rolling A Invention 1 1200 50 950 40 12 15 Invention 2 1000 25 900 50 30 7 Invention 3 1050 30 900 50 20 11 Comparative Material 1 1100 35 950 20 10 40 Comparative Material 2 1300 150 1000 20 10 80 B Invention 4 1150 30 950 40 15 13 C Invention 5 1150 23 900 50 25 10 Comparative Material 3 1150 23 900 50 5 35 D Invention 6 1000 21 870 50 30 9 E Invention 7 1100 25 920 45 20 12 Invention Material 8 1100 25 900 50 20 8 Comparative Material 4 1150 35 900 45 15 15 F Comparative Material 5 1150 25 900 40 15 20 G Comparative Material 6 1150 100 950 40 10 75

Steel grade AGS (㎛) just before finishing rolling Cooling rate after finishing rolling (℃ / s) Ar3 (℃) Rolling amount per pass during rolling (%) Finished rolling total rolling load (%) Finish rolling temperature (℃) Dynamic transformation ferrite fraction (%) Static Ferrite Fraction (%) Average FGS (μm) A Invention 1 15 2 750 20 60 760 40 11 3.0 Invention 2 7 8 735 25 65 740 49 10 1.8 Invention 3 11 3 753 40 40 760 49 10 2.6 Comparative Material 2 40 2 740 20 50 760 35 12 4.2 Comparative Material 2 80 One 725 20 60 750 25 20 6.5 B Invention 4 13 3 750 30 50 760 50 15 2.7 C Invention 5 10 5 750 25 60 760 40 16 2.8 Comparative Material 3 35 One 745 20 60 750 25 20 4.8 D Invention 6 9 10 730 20 50 740 55 10 2.0 E Invention 7 12 2 805 15 50 810 42 23 3.0 Invention Material 8 8 4 795 25 70 800 45 15 2.5 Comparative Material 4 15 One 800 10 30 810 30 15 5.8 F Comparative Material 5 20 2 783 10 30 800 35 20 4.5 G Comparative Material 6 75 2 740 20 30 750 30 25 5.2

As shown in Table 2 and Table 3, in the case of steel A, as in the invention materials (1 to 3), the lower the dynamic recrystallization reverse rolling temperature, the smaller the initial austenite crystal, the more the austenite crystal after dynamic recrystallization Was small. The ferrite refinement by filamentary rolling showed that the smaller the austenite crystals, the more dynamic transformation occurred, the smaller the size of the ferrite grains and the smaller the size was. In particular, 7 µm of austenite obtained by dynamic recrystallization of the inventive material (2) was subjected to a reduction ratio of 25% and a total reduction of 65% per pass at 760 ° C to produce ultrafine grains of 1.8 µm of ferrite grains.

On the contrary, in the comparative material 1, the effective cumulative reduction in pressure was not so small that the dynamic recrystallization was not performed. Accordingly, the austenite grain size was large as 40 mu m. In the comparative material 2, when the heating temperature was too high at the time of heating, the initial austenite grains were large and dynamic recrystallization did not occur.

In the case of B steel, the initial grain size of austenite was about 30 μm at a heating temperature of 1150 ° C., and the austenite grain was 13 μm by adding a dynamic cumulative rolling start temperature of 900 ° C. and an effective cumulative reduction of 40%. 50% multi-stage hot rolling with a 30% reduction per pass was obtained to obtain a ferrite steel having a grain size of 2.7 µm.

In the case of C steel, inventive material 4 had an initial crystal grain size of austenite at a heating temperature of 1150 ° C. of about 30 μm, and the grain size was 10 μm by dynamic recrystallization rolling. Then, this was hot rolled at a total reduction rate of 60% with a rolling reduction of 25% per pass to obtain a ferritic steel having a grain size of 2.8 μm.

On the contrary, in the comparative material 3, the final pressure drop in the dynamic recrystallization rolling was small, and the austenite grains were somewhat larger. This is due to the growth of austenite grains due to strain organic grain boundary migration under low pressure when the rolling reduction in the final rolling step is less than 10%.

In the case of D steel, inventive material (6) had an initial grain size of austenite after reheating of about 21 μm, and after completion of dynamic recrystallization, austenite grains of 9 μm were obtained. Subsequently, after cooling, 60% of the total rolls were rolled at 20% with a reduction of 20% per pass to obtain ferrite grains of 2.0 mu m.

In the case of the E steel, all of the inventive materials 7 to 8 had the reheating conditions, the rolling conditions, and the like satisfying the scope of the present invention, thereby obtaining a final ferrite structure having a grain size of less than 3 µm. However, in the comparative material (4) in which the cumulative reduction in the hot rolling conditions was outside the scope of the present invention, the final ferrite grain size was 5.8 µm.

On the other hand, in the case of the comparative material 5 of the F steel, the austenite grain size after the dynamic recrystallization was 20 µm, and a fine ferrite structure was not obtained after the final hot rolling, and the G steel did not contain Ti. As in Ash (6), the initial austenite grains were excessively large, and the austenite immediately before finishing rolling was very coarse to 75 µm, whereby ultrafine ferrite structures could not be obtained.

As described above, the present invention provides excellent weldability by effectively refining the ferrite grains by effectively refining the grains of austenite by dynamic recrystallization in low carbon steel, and then performing continuous multi-stage hot working to promote the production of dynamic transformation ferrite. It is possible to manufacture excellent structural steel that can improve the physical properties of the steel while maintaining.

Claims (4)

By weight, C: 0.02 to 0.3%, Si: 1.5% or less, Mn: 2.0% or less, Ti: 0.005 to 0.1%, N: 0.003 to 0.03%, residual iron and added impurities, and 1.2≤Ti / Preparing a steel material satisfying N ≦ 3.4; The steel is reheated to have an austenite particle size of 50 μm or less, and then dynamically recrystallized and rolled to have a total strain of 10% or more in a temperature range of Thr −50 ° C. to Thr + 50 ° C. to a particle size of 15 μm or less. Controlling; And Hot-rolling the steel having the austenitic particle size controlled as described above to maintain a total reduction ratio of less than 40% per pass in a temperature range of Ar ₃ to Ar ₃ + 50 ° C. such that the total reduction ratio is 40% or more; Ultrafine steel production method comprising a The method of claim 1, wherein the steel is made of ultrafine steel, characterized in that it further comprises one or more selected from Al: 0.1% or less, Nb: 0.005 ~ 0.1%, and V: 0.1% or less. Way The method of claim 1 or 2, wherein the effective cumulative reduction ratio during the dynamic recrystallization rolling is 30% or more. 2. The method of claim 1, wherein the ferrite fraction is 40% or more at the time of completion of hot working, and the grain size of the ferrite is 3 m or less.