KR100360091B1 - A METHOD FOR MANUFACTURING 60kg/㎟ GRADE STEEL WITHOUT QUENCHING AND TEMPERING - Google Patents

Abstract

The present invention relates to the production of high toughness high strength steel sheet used as a material for industrial machinery or heavy equipment parts; The purpose is to provide a manufacturing method of 60kg / mm grade ² high tensile strength steel sheet that does not require subsequent heat treatment by giving conditions for the thickness of the final product.

The present invention for achieving the above object by weight, C: 0.09 ~ 0.12%, Si: 0.25 ~ 0.35%, Mn: 1.45 ~ 1.60%, P: 0.023% or less, S: 0.005% or less, Al: 0.030 ~ 0.050 %, Ti: 0.010 ~ 0.020%, Nb: 0.040 ~ 0.060%, V: 0.050 ~ 0.080%, N: 0.0070% or less, roughly rolled steel slab containing other Fe and impurities in the range of 1150 ~ 1200 ℃ When the thickness of final product is less than 12mm, it is hot-rolled at room temperature of 850 ~ 690 ℃ and air-cooled to room temperature. If the thickness of final product is more than 12mm and less than 20mm, residual pressure drop rate is 70 ~ 80 at temperature of 980 ~ 850 ℃. After performing primary finishing rolling so as to be in the range of%, start secondary finishing rolling at a temperature of 890 to 800 ° C. to bring the residual reduction rate to 40 to 70%, and finish rolling at a temperature of 740 to 710 ° C. to normal temperature. Air-cooled to the end of the product, if the thickness of the final product is more than 18mm and less than 20mm, rolling is carried out as the rolling conditions of more than 12mm and less than 18mm, then accelerated cooling at a rate of 5 ~ 10 ℃ / Sir, the technical gist of the method of manufacturing a non-heat treatment type 60kg / mm ² high tensile steel sheet.

Description

Manufacturing method of non-heat treatment type 60㎏ / ㎡ class high tensile steel sheet {A METHOD FOR MANUFACTURING 60kg / ㎡ GRADE STEEL WITHOUT QUENCHING AND TEMPERING}

The present invention relates to the manufacture of high toughness high tensile strength steel sheet used as a material for industrial machinery or heavy equipment parts, and more specifically to the production of 60kg / mm grade ² high tensile strength steel sheet is not required to be subjected to subsequent heat treatment conditions by thickness of the final product It is about a method.

Recently, economic and stability of materials are required in the field of manufacturing industrial machinery and heavy equipment, and the size of equipment is inevitably required in terms of efficiency of work and energy saving, so that high strength, high toughness, high employment resistance, high workability The demand for low alloy high tensile strength steels is also increasing rapidly. In particular, in the case of 60kg / mm class ² high-tensile strength steel sheet is rapidly increasing demand in the field of manufacturing industrial machinery or heavy equipment has been a big problem supply. That is, in the case of the 60kg / mm class ² high tensile strength steel sheet conventionally after the slab is rolled to adjust the target physical properties by quench-tempering to meet the product characteristics, the production process according to the capacity of the heat treatment equipment This situation is greatly influenced and the shortage of products and delay in delivery are frequently caused.

Therefore, the present invention is different from the conventional hardening-thinning method, in the case of a thin material having a thickness of 20mm or less in the final product, by designing a new rolling method according to a small amount of alloying elements and product thickness. It is an object of the present invention to provide a method for producing a 60kg / mm ² high tensile steel sheet having physical properties.

The present invention for achieving the above object by weight, C: 0.09 ~ 0.12%, Si: 0.25 ~ 0.35%, Mn: 1.45 ~ 1.60%, P: 0.023% or less, S: 0.005% or less, Al: 0.030 ~ 0.050 %, Ti: 0.010 ~ 0.020%, Nb: 0.040 ~ 0.060%, V: 0.050 ~ 0.080%, N: 0.0070% or less, roughly rolled steel slab containing other Fe and impurities in the range of 1150 ~ 1200 ℃ ,

If the thickness of the final product is less than 12mm, rolling is carried out at a temperature of 850 ~ 690 ℃ and air-cooled to room temperature,

If the thickness of the final product is more than 12mm and less than 20mm, the first finishing rolling is carried out at the temperature of 980 ~ 850 ℃ so that the residual reduction rate is in the range of 70 ~ 80%, and then the second finishing rolling is performed at the temperature of 890 ~ 800 ℃. Start and finish reduction rolling at a temperature of 740 ~ 710 ℃ with air residual reduction rate of 40 ~ 70%, and then air-cool to room temperature, and

If the thickness of the final product is more than 18mm or less than 20mm, the primary sequential rolling is carried out at the temperature of 980 ~ 850 ℃ so that the residual pressure reduction rate is in the range of 70 ~ 80%, and then the secondary sequential rolling is performed at the temperature of 890 ~ 800 ℃. Non-heat treatment type 60kg / mm ² , characterized in that accelerated cooling at a rate of 5 ~ 10 ℃ / second after finishing rolling at a temperature of 740 ~ 710 ℃ to ensure that the residual pressure reduction rate is in the range of 40 ~ 70% It relates to a method for producing a high tensile steel sheet.

Hereinafter, the present invention will be described in detail.

First of all, the carbon is an essential element for securing strength, which leads to an increase in strength due to an increase in pearlite friction in the steel structure during thick plate rolling, but generates carbide when a large amount is added, causing severe deformation during transformation of the steel. It is preferable to limit the content to 0.09 to 0.12% because the impact toughness is lowered and the hardened structure is formed in the heat affected zone (HAZ) during welding to lower the weldability due to embrittlement.

The silicon forms carbide and forms a ferrite phase to increase hardness, elastic limit, and tensile strength. However, the addition of more than 0.35% of silicon is not preferable because the toughness decreases due to the reduction of the ferrite structure and the formation of non-metallic inclusions, and the plastic workability is also impaired. In order to keep the river tensile strength 60kg / mm ² or above must contain not less than 0.25%.

The manganese acts as a major component to improve the strength and toughness during rolling by forming carbide (Mn ₃ C) by ferrite reinforcement and particle refining effect, but in the case of high tensile steel, it is difficult to form two phase when added below 1.45% When the excessive amount of 1.60% or more is added, rather than binding to S in the steel to form inclusions such as MnS to reduce the impact toughness of the weld.

The phosphorus reduces the impact resistance by forming Fe ₃ P in the steel, thereby causing room temperature brittleness, thereby impairing the impact toughness and not easily spreading even at high temperatures, and thus extending the band structure formed in the longitudinal direction during rolling. It is preferable to set the content to 0.023% or less since it deteriorates the quality.

The sulfur forms a segregation zone by inclusions such as MnS, Fe ₃ P in the steel to embrittle the steel and lower the weldability, so it is preferable to manage the content below 0.005%.

The aluminum precipitates AlN as a strong deoxidizer, and exhibits a grain-fine effect, but when it is added in a large amount, dendritic carbides make the steel brittle and inhibit the surface cracks and impact toughness of the slab. Limited to.

The titanium forms fine carbides and nitrides during the solidification process of steel, thereby inhibiting grain growth of austenite, thereby contributing to the refinement of ferrite. However, when the content is high in the steel, it is difficult to suppress grain growth of austenite due to coarsening of nitride (TiN) precipitated particles and precipitation of carbide (TiC), which is coarse precipitated compared to nitride, and due to carbide precipitated at grain boundaries. Excessive tissue embrittlement occurs, and it is preferable to limit the content to 0.010 to 0.020% because it is difficult to precipitate carbides when a small amount is added.

The niobium increases the grain coarsening temperature at a high temperature to prevent grain coarsening and lowers the hardenability at room temperature and high temperature to inhibit age hardening. In other words, it promotes grain refinement and improves the ductility and toughness of steel. In general, when 0.01% is added, there is an effect of increasing strength of 1.5 ~ 3.0kg / mm ² , but when it is added more than 0.06%, there is almost no increase effect. Since it affects the content is preferably limited to the range of 0.04 ~ 0.06%.

The vanadium forms the carbonitride to show the grain boundary and intragranular strengthening effect of the ferrite by precipitation strengthening, and the vanadium acts as a starting point for nucleation of ferrite, and is useful for miniaturization of ferrite, but the impact toughness due to coarsening of carbonitride when excessively added Since it deteriorates, it is preferable to limit the content to 0.05 to 0.08% of range.

Nitrogen generates various inclusions in the steel, impairing the internal quality of the steel and the workability during welding, and thus extremely low management is advantageous. However, the management cost within 70 ppm is economical because the management cost is excessive and the management difficulties are incurred. It is preferable to secure.

Hereinafter, a method of manufacturing high tensile strength steel that satisfies the material properties and requirements of existing steel even when the slab is manufactured by applying a conventional continuous casting method to the slab to omit the heat treatment will be described.

Usually 60kg / mm ² high-strength steel is subjected to general rolling, and then secured by quenching, so reheating is normally performed at a high temperature of 1250 ℃ or higher before general rolling, but in the present invention, the heating temperature is lowered by about 50 to 100 ℃. After reheating in the range of 1150 ~ 1200 ℃, by controlling the rolling conditions and cooling conditions according to the thickness of the final product to produce high tensile steel of tensile strength 60kg / mm ² grade without the heat treatment after rolling.

First, when the thickness of the final product is less than 12mm, it is subjected to finishing rolling at a temperature of 850 ~ 690 ℃ and air cooled to room temperature. That is, in the case of a thin material having a final thickness of 12 mm or less, the slabs are reheated at the reheating temperature, and the reheated slabs are subjected to low temperature rolling at a temperature of 850 to 690 ° C., followed by air cooling. The grain growth of austenite is caused by TiN nitride deposition. It is suppressed and the grain size of austenite becomes fine, and when a transformation from austenite to ferrite forms fine ferrite by precipitation of carbonitrides such as Nb and V, high tensile steel having high toughness can be obtained.

However, in order to obtain a high tensile strength steel having a target property in the case of a thick material of more than 12mm compared to the general rolling material, it is necessary to perform controlled controlled rolling and accelerated cooling, unlike the general rolling method. First, when the final thickness is greater than 12mm and 20mm or less in order to make a sufficient abnormal transformation during rolling in the present invention, in consideration of the alloy addition, in the unrecrystallized zone (980 ~ 850 ℃) austenite transformed only partly into ferrite Is done. Specifically, in the control rolling of the present invention, after performing primary finishing rolling at a temperature of 980 to 850 ° C. such that the residual reduction ratio is in a range of 70 to 80%, the secondary rolling is started at a temperature of 890 to 800 ° C. The reduction ratio is 40 to 70%, and finish rolling is carried out at a temperature of 740 to 710 ° C to air-cool to room temperature. If the residual reduction ratio during the control rolling is higher or lower than the above range, there is a problem that growth of austenite grains after rolling is difficult to secure toughness of the final product.

At this time, when the thickness of the final product is more than 18mm and less than 20mm, after performing primary sequential rolling at a temperature of 980 ~ 850 ℃ so that the residual pressure reduction rate is in the range of 70 ~ 80%, secondary frosting at a temperature of 890 ~ 800 ℃ After rolling is started and finish rolling is carried out at a temperature of 740 to 710 DEG C so that the residual reduction ratio is in the range of 40 to 70%, acceleration cooling must be performed at a rate of 5 to 10 DEG C / sec. In other words, if the thickness of the final product is more than 18mm and less than 20mm, it is difficult to secure the strength if only the control rolling is carried out. The low temperature transformation tissues include bainite tissue and martensite tissue. When accelerated cooling is terminated above Ms (martensite transformation start temperature), bainite tissue is obtained. Is obtained. Most preferably, the bainite structure is advantageous in strength and toughness, so that the accelerated cooling is finished before the martensite transformation start temperature. In the case of accelerated cooling, when the temperature is less than 5 ° C./sec, the low temperature transformation tissue is not sufficiently secured. If the temperature is 10 ° C./sec or more, the strength is increased, but the impact value is lowered.

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

EXAMPLE

Inventive steel having a composition as shown in Table 1 was produced in a steel sheet of 20mm or less according to the rolling conditions and cooling conditions as shown in Table 2 according to the thickness of each product. Table 2 compares the physical properties of each of the steel sheets produced and the properties of high tensile steel sheets (JS-SM570; conventional materials) manufactured by the conventional hardening-thinning method.

Unit: weight% division C Si Mn P S Nb V Ti Al N Ni Mo Conventional Steel 0.14 0.35 1.40 0.020 0.005 - 0.03 - - - 0.15 0.15 Invention steel 0.107 0.28 1.40 0.021 0.002 0.041 0.063 0.013 0.035 50 ppm - -

Example Steel grade Thickness of final product (mm) Rolling method Rolling condition Cooling condition Final properties T3 (℃) Residual Pressure Reduction Rate (%) T4 (℃) Residual Pressure Reduction Rate (%) T5 (℃) SCT (℃) FCT (℃) CS (° C / sec) YP (kg / mm ² ) TS (kg / mm ² ) vE-5 standard 6-20 ≥46 60-72 ≥4.8 Invention 1 Invention steel 6 ER - - - - 690 - - - 55.6 64.4 11.5 Invention 2 12 - - - - 850 - - - 53.7 65.6 17.0 Invention 3 15 CR 850 70 800 40 710 - - - 55.5 64.1 23.2 Invention 4 16 850 80 800 60 710 - - - 53.3 63.3 19.8 Invention 5 16 980 80 890 60 740 - - - 47.0 62.8 23.0 Comparative Material 1 16 CR + TMCP 980 80 890 60 740 730 540 5 55.0 64.3 29.1 Comparative Material 2 19 CR 980 80 890 70 740 - - - 52.6 60.0 27.0 Invention 6 19 CR + TMCP 980 80 890 70 740 730 510 10 55.1 64.6 26.5 Conventional Conventional Steel 20 Q: 910 ° C; T: 640 ℃ - - - 56.1 67.6 25.8 (Note) ER: General rolling CR: Control rolling TMCP: Acceleration control rolling T3: Primary finishing rolling temperature T4: Secondary finishing rolling start temperature T5: Secondary finishing rolling end temperature SCT: End rolling temperature during accelerated cooling FCT: Acceleration End temperature after cooling CS: Cooling rate YP: Yield strength TS: Tensile strength vE-5: Impact value at -5 ° C Q: Hardening temperature T: Consideration temperature

As shown in Table 2, in the case of the invention (1) (2) of the thickness of the final product is less than 12mm it can be seen that by the general rolling method to maintain the same impact value and strength as the existing steel grades.

On the other hand, when the thickness of the final product is 12mm to 20mm it was confirmed that 60kg / mm ² high-tensile strength steel sheet can be obtained only by performing the control rolling according to the present invention. However, even if the thickness of the final product is in the range of 12 to 20 mm, when the thickness is 16 mm, when the control rolling and accelerated cooling are performed together (in the case of the comparative material (1)), the physical properties of the final product meet the target values. Shape defects due to accelerated cooling were generated, and thus a corrective work process was added. That is, it was found that it is more advantageous to perform only control rolling without performing accelerated cooling when the thickness of the final product is 12-18 mm.

On the other hand, in the case of the comparative material 2 of which the thickness of the final product was 19 mm and only the control rolling was performed, sufficient strength was not secured, indicating that the tensile strength showed a threshold value, which was not preferable. In other words, if the final product thickness is 18 ~ 20mm only by performing the control rolling enough low temperature transformation structure is not secured, the strength decreases, it was difficult to commercialize.

As described above, the present invention eliminates heat treatment such as quenching and souring by appropriately setting general rolling, control rolling, and accelerated cooling for each thickness of the final product while using a single component system appropriately added with a small amount of alloying elements. It is possible to manufacture 60kg / mm class ² high tensile strength steel sheet having properties equal to or higher than that of high tensile strength steel produced by hardening-soaking. Thus, there is an effect of improving the productivity of the steel grade with heat treatment load and delayed delivery delay.

Claims (1)

By weight%, C: 0.09 to 0.12%, Si: 0.25 to 0.35%, Mn: 1.45 to 1.60%, P: 0.023% or less, S: 0.005% or less, Al: 0.030 to 0.050%, Ti: 0.010 to 0.020% , Nb: 0.040 ~ 0.060%, V: 0.050 ~ 0.080%, N: 0.0070% or less, The steel slab containing other Fe and impurities is rough-rolled in the range of 1150 ~ 1200 ℃, and the final product thickness is 12mm or less In the case of finishing rolling at a temperature of 850 ~ 690 ℃ and air-cooled to room temperature, When the thickness of the final product is more than 12mm and less than 18mm, the primary finishing rolling is carried out at the temperature of 980 ~ 850 ℃ so that the residual reduction rate is in the range of 70 ~ 80%, and then the secondary finishing rolling is performed at the temperature of 890 ~ 800 ℃. Start and finish rolling at a temperature of 740-710 ° C. to allow the residual reduction rate to be in the range of 40 to 70%, and then air-cool to room temperature, and If the thickness of the final product is more than 18mm or less than 20mm, the primary finishing rolling is carried out at the temperature of 980 ~ 850 ℃ so that the residual reduction rate is in the range of 70 ~ 80%, and then the secondary finishing rolling is performed at the temperature of 890 ~ 800 ℃. Non-heat treatment type 60kg / mm ² characterized in that after the finish rolling at a temperature of 740 ~ 710 ℃ to the residual pressure reduction rate in the range of 40 ~ 70%, accelerated cooling at a rate of 5 ~ 10 ℃ / ^second Manufacturing method of high tensile steel sheet