KR100815747B1 - A method for manufacturing a high strength TMCP steel plate for hot forming - Google Patents

Abstract

Provided is a method for manufacturing a 60kg / mm2 thermal processing control (TMCP) structural plate for hot forming.

In the method of the present invention, by weight%, C: 0.06 to 0.20%, Si: 0.02 to 0.55%, Mn: 0.60 to 1.70%, P: 0.025% or less, S: 0.020% or less, Sol-Al: 0.005 to 0.060 %, Nb: 0.015 ~ 0.050%, V: 0.01 ~ 0.090, N: 0.007 ~ 0.030%, Cu: 1.0% or less, Cr: 1.0% or less, Mo: 0.6% or less, balance Fe and other inevitable impurities Preparing a slab; Heating the steel slab to a temperature range of 1130 ° C. to 1240 ° C., and then performing primary control rolling at a temperature of 990 ° C. to 900 ° C., which is a recrystallization temperature range; Final finishing control rolling of the first rolled hot rolled sheet at an unrecrystallized temperature range of 800 ° C. to 700 ° C .; And water-cooling at a cooling rate of 3 to 10 ° C. such that the final temperature of the rolled steel sheet becomes 560 to 450 ° C ..

Hot Forming, Control Rolling, TMCP, Recrystallization Area

Description

Method for manufacturing a high strength TMCP steel plate for hot forming

The present invention relates to a method for manufacturing a structural high tensile strength steel sheet used for axle housing of a large truck with a loading weight of 23 tons or more, more specifically, strength loss after hot hot forming after heating for 30 minutes at a high temperature of 800 ℃. Manufacture of high tensile plate for 60kg / mm2 thermal processing control (TMCP) structure for hot forming, which yields more than 100J of impact absorption energy at yield strength (46kgf / ㎣ <), tensile strength (60kgf / ㎣ <) and -5 ℃ It is about a method.

Since high-strength steel sheets for welded structures are inferior in formability, deep processing materials such as axle housings of automobiles may be cracked during cold forming, so they are hot formed after heating for 30 minutes at 800 ℃ high temperature. Since the strength at the time of manufacturing is lost in the heating process, the manufacturing technique of high strength steel sheet for large trucks of yield strength (46kgf / ㎣ <) and tensile strength (60kgf / ㎣ <) after hot forming has not been developed in Korea. It was.

Accordingly, the inventors of the present invention have conducted research and experiments to achieve the technical problem as described above, and proposed the present invention based on the results. By controlling the cooling process properly, it is to provide a method of manufacturing a high tensile plate that can provide high strength and impact toughness even after hot hot forming.

The present invention for achieving the above object

By weight%, C: 0.06-0.20%, Si: 0.02-0.55%, Mn: 0.60-1.70%, P: 0.025% or less, S: 0.020% or less, Sol-Al: 0.005-0.060%, Nb: 0.015- Preparing a steel slab composed of 0.050%, V: 0.01 to 0.090, N: 0.007 to 0.0130%, Cu: 1.0% or less, Cr: 1.0% or less, Mo: 0.6% or less, balance Fe and other unavoidable impurities; Heating the steel slab to a temperature range of 1130 ° C. to 1240 ° C., and then performing primary control rolling at a temperature of 990 ° C. to 900 ° C., which is a recrystallization temperature range; Final finishing control rolling of the first rolled hot rolled sheet at an unrecrystallized temperature range of 800 ° C. to 700 ° C .; And a step of water cooling at a cooling rate of 3 to 10 ° C. such that the final temperature of the rolled steel sheet becomes 560 to 450 ° C .; and a method for manufacturing a 60 kg / mm 2 grade hot work control (TMCP) structural plate for hot forming will be.

Hereinafter, the steel component of the present invention and the reason for limitation thereof will be described.

C is an element added to secure the strength of the steel, if the content is low, it is difficult to secure the strength, and if it is too high, the weldability and toughness deteriorate, so the amount is limited to 0.06 to 0.20%.

Si is effective in increasing strength due to solid solution strengthening and is an element that serves as a deoxidizer for removing oxygen components present in steel, but if the content is too high, low temperature toughness deteriorates, so it is reduced to 0.02 to 0.50%.

Mn is an element that is beneficial for strength improvement due to the Ar ₃ transformation delay effect and has an effect of suppressing the formation of sulfide (FeS), which is a harmful component in steel. However, if the content is too high, excessive generation of phase martensite degrades impact toughness, so the content is limited to 0.60-1.70%.

Sol.Al is a deoxidation element in steel, and if its content is less than 0.005%, deoxidation is insufficient, which may cause bubbles during performance.If the content exceeds 0.060%, it is combined with N in steel to precipitate and play with AlN. Causes surface cracks.

In consideration of this, in the present invention, the amount of Sol.Al added is limited to 0.005 to 0.060%.

P is a harmful element that causes segregation and causes UT defects, so the content range is limited to 0.025% or less.                     

S is a harmful element that forms low melting sulfide (FeS) in molten steel to cause internal cracks during continuous casting, and also forms MnS inclusions to cause lamination.

Therefore, in the present invention, the content is limited to 0.020% or less.

Nb is an element that is useful for structure refinement and strength increase by precipitation during rolling and cooling. To show the effect, Nb should be added at 0.015% or more. However, if the content is high, Nb causes excessive upper bainite structure to deteriorate impact toughness. Therefore, the upper limit is limited to 0.050%.

V is an element that combines with C and N to form V (C, N) precipitates, and contributes greatly to the strength increase with increasing content. However, at the time of excessive addition, the toughness of the base metal and the weld part is reduced, so the amount of addition is limited to 0.01 to 0.090%.

N is an element that forms a single or complex compound with C, Al, Nb, V, and especially when the steel sheet is reheated to 800 ℃ high temperature for hot forming after steel sheet manufacturing, these nitrogen compounds precipitate at grain boundaries, resulting in excessive growth of the structure. As a result, it is an element that prevents loss of strength even after heating, and should be added at least 0.007% in order to exhibit such an effect, but if the content is high, the impact toughness deteriorates, so the upper limit of the content is 0.0130%. Restrict.

Cu is a solid solution strengthening element, which is useful for strength increase, but deteriorates the weldability when excessively added, so the amount of Cu is limited to 1.0% or less.

Cr is limited to 1.0% or less because it reduces the toughness of the weld by forming hardened structure on the weld heat affected zone when it is an element that is effective in increasing the strength of the base metal or when it is over-added.

Mo is a useful element that increases the high temperature strength and room temperature strength, but when excessively added, it deteriorates moldability and deteriorates weld toughness, so the amount of Mo is limited to 0.6% or less.

Hereinafter, the manufacturing method of this invention is demonstrated.

First, in the present invention to produce a steel slab (Slab) having the composition as described above and then heating it. At this time, the heating temperature is limited to 1130 ~ 1240 ℃. If the heating temperature is less than 1130 ° C., the solid solution of Nb is insufficient and the carbonitride stones such as NbC, NbC, N, etc. during rolling

This is because the effect of particle fineness due to the drawing is reduced, and the deformation resistance during rolling is increased, thereby deteriorating the rolling workability. If the heating temperature exceeds 1240 ℃ because the surface scale is excessively generated.

The steel slabs heated in the furnace are subsequently controlled rolled.

That is, in the present invention, after roughly rolling the gal slabs, the first controlled rolling is performed in the recrystallization area of the reinforced fabric, wherein the rolling temperature is preferably controlled to 990 to 900 ° C. This is because the reinforcement particles are made fine by repeated recrystallization during repeated rolling in this temperature range. In the present invention, when the repetitive rolling temperature is higher than this, the grains grow rapidly immediately after rolling, which is not preferable for microstructure.

It is preferable on the repetitive rolling schedule that this primary control rolling starts at 70% of the remaining reduction rate and ends at 40% of the residual reduction rate.

After performing the primary control rolling in this way, the secondary control rolling is performed again at 800 ° C. to 700 ° C., which is the unrecrystallized temperature region of the hardened fabric.

800 ° C is a temperature at which recrystallization of the reinforcement fibers is considerably suppressed. Rolling below this temperature increases the ferrite nucleation site due to the stretching and dislocation of the austenitic reinforcement particles and the formation of new sub-grains. It is advantageous to improve the strength. However, when the rolling is too low temperature ferrite and austenite abnormal reverse rolling is excessive, causing material deviation due to the generation of abnormal tissue, it is preferable to finish the rolling at a temperature of 700 ℃ or more.

The control rolled steel sheet as described above is then water-cooled so that the temperature of the final steel sheet is 560 ~ 450 ℃, when cooled in this way, the stress growth of the rolled plate is transformed into ferrite, the grain growth is suppressed, and rapid cooling from residual austenite By generating the low-temperature transformation structure bainite can effectively secure the strength. The water-cooled steel sheet is then air cooled to room temperature.                     

In the present invention, when the final cooling end temperature is higher than 560 ℃, the diffusion of carbon is faster to decrease the fine dispersion effect of carbides and cause a drop in strength is necessary because the restriction, lower than 450 ℃ it is difficult to secure the flatness is not preferable not.

In the present invention, it is also preferable to cool the water at a cooling rate of 3 ~ 10 ℃ / sec up to the cooling end temperature. If the cooling rate is less than 3 ° C / sec, bainite transformation is suppressed, which is disadvantageous in increasing the strength, and if it exceeds 10 ° C / sec, the flatness of the steel sheet is poor, which is not preferable.

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

(Example)

Steel slabs each having the composition shown in Table 1 were prepared. The steel slabs were heated to 1150 ° C. and roughly rolled, and then steel sheets were manufactured by varying specific rolling and cooling conditions as shown in Table 2.

In this experiment, first, the strength of the material of the hot pressing process was shown in Table 2, and the material properties were measured after heating the manufactured steel sheet at 800 ° C. for 30 minutes, and the results are also shown in Table 2.

                Ingredients (% by weight) C Si Mn P S S.Al Nb V N Cu Cr Mo Inventive Steel 1 0.16 0.45 1.46 0.008 0.019 0.031 0.030 0.054 0.0098 0.02 0.04 0 Inventive Steel 2 0.17 0.44 1.55 0.012 0.017 0.032 0.026 0.063 0.0101 0.01 0.03 0.01 Invention Steel 3 0.16 0.46 1.52 0.010 0.018 0.035 0.028 0.062 0.0094 0.02 0.03 0.01 Conventional Steel 1 0.16 0.45 1.25 0.019 0.005 0.030 0.001 0.004 0.0045 0.02 0.02 0.01 Conventional Steel 2 0.17 0.40 1.45 0.18 0.003 0.033 0.019 0.060 0.0041 0.02 0.01 0.01

Heating temperature (℃) Primary controlled rolling start temperature (℃) Finish rolling temperature (℃) Cooling end temperature (℃) Cooling rate (℃ / sec) Tensile Strength Before Hot Processing Material test result after heating at 800 ℃ for 30 minutes YP (kgf / mm ² ) TS (kgf / mm ² ) YP (kgf / mm ² ) TS (kgf / mm ² ) EL (%) Shock Absorption Energy (-5 ℃, J) Invention 1 1220 950 785 550 5.5 59.7 68.5 48.9 63.9 32 220 Invention 2 1210 980 741 540 4.8 62.4 70.5 50.1 64.8 33 191 Invention 3 1200 970 739 520 5.3 64.4 71.5 49.1 65.1 31 183 Comparative Material 1 1200 970 825 615 2.6 65.5 65.5 44.4 58.1 33 198 Comparative Material 2 1120 985 830 580 3.2 64.1 64.1 42.9 57.8 34 212 Conventional Materials 1 1120 1050 815 620 2.3 64.2 64.2 40.9 53.2 33 188 Conventional material 2 1180 970 820 570 3.5 64.5 64.5 43.3 60.1 32 242

As shown in Table 1 and Table 2, the present invention materials (1 to 3), in which not only the steel components but also the manufacturing conditions are properly controlled, are all required by the customer's required material for a large truck axle steel with a loading weight of 23 tons or more. It can be seen that the result satisfies. For reference, customer material requirements are YP 46kgf / ㎣ <, TS 60kgf / ㎣ <. Impact absorption energy (-5 ° C.) 48J <.

On the other hand, the comparative materials (1 ~ 2) whose manufacturing conditions are outside the scope of the present invention, and the conventional materials whose stress and manufacturing conditions are outside the invention range are the material test results after the 800 ° C X 30 minutes heating, yield strength It can be seen that it does not meet the needs of customers.

As described above, the present invention is to control the steel components and hot processing conditions, the steel sheet produced there is a high strength and impact toughness even after heating for 30 minutes at 800 ℃, 23 tons are used for axle housing of heavy trucks It can be effectively applied to hot work control high tension plate for hot forming.

Claims (2)

By weight%, C: 0.06-0.20%, Si: 0.02-0.55%, Mn: 0.60-1.70%, P: 0.025% or less, S: 0.020% or less, Sol-Al: 0.005-0.060%, Nb: 0.015- Preparing a steel slab composed of 0.050%, V: 0.01 to 0.090, N: 0.007 to 0.0130%, Cu: 1.0% or less, Cr: 1.0% or less, Mo: 0.6% or less, balance Fe and other unavoidable impurities; Heating the steel slab to a temperature range of 1130 ° C. to 1240 ° C., and then performing primary control rolling at a temperature of 990 ° C. to 900 ° C., which is a recrystallization temperature range; Final finishing control rolling of the first rolled hot rolled sheet at an unrecrystallized temperature range of 800 ° C. to 700 ° C .; And 60 kg / mm2 grade hot working control (TMCP) structural plate manufacturing method for hot forming comprising; step of water cooling at a cooling rate of 3 ~ 10 ℃ so that the final temperature of the rolled steel sheet is 560 ~ 450 ℃ The method of claim 1, wherein the primary control rolling starts at 70% of residual pressure reduction rate and ends at 40% of residual pressure reduction ratio.