KR100480003B1 - A method for manufacturing tmcp thick steel sheet for construction structure with superior yeild ratio - Google Patents

Abstract

The present invention relates to a method for manufacturing a TMCP thick steel sheet for building structure, after adding Cu, Ni, V alloy elements in steel and performing controlled rolling, quenching the hot rolled steel sheet to produce a low-temperature bainite structure, strength and toughness The purpose of the present invention is to provide a high and high yield ratio YP355 grade (N / mm 2) thick steel sheet manufacturing method.

In the present invention, C: 0.136 to 0.164%, Si: 0.40 to 0.50%, Mn: 1.00 to 1.25%, P: 0.012% or less, Nb: 0.025 to 0.035%, Ti: 0.015% or less, Cu: 0.30% or less, Ni: 0.30% or less, V: 0.020% or less, Ceq .: 0.42% or less, the balance Fe and other unavoidable impurities the steel slab containing, by re-heating and rolling in the temperature history of the non-recrystallized station immediately above, immediately above Ar ₃ Method of manufacturing a thick steel sheet for excellent yield ratio TMCP building structure, characterized in that the cooling temperature of 5 ~ 6 ℃ / sec at a temperature of 5 to 6 ~ 550 ℃ after the final temperature of the steel sheet is cooled by air Shall be.

Description

Manufacturing method of TMC thick steel sheet for building structure with excellent yield ratio {A METHOD FOR MANUFACTURING TMCP THICK STEEL SHEET FOR CONSTRUCTION STRUCTURE WITH SUPERIOR YEILD RATIO}

The present invention relates to a method for manufacturing a high-tension thick steel sheet for manufacturing BEAMs used in steel structures, and more specifically to the addition of Cu, Ni, V alloy elements in the steel, and to increasing the uncrystallized area reduction rate to control rolling The present invention relates to a method for producing a TMCP thick steel sheet for building structures having high strength and excellent yield ratio at the same time by quenching the hot rolled steel sheet to produce a low temperature structure.

In recent years, due to the increase in size and height of buildings, the use thickness of steel sheet for building steel is also increasing in structural members, and thus, the importance of securing strength, toughness, and anti-tearness of steel materials is increasing.

In the conventional manufacturing technology of the steel plate for building steel, in order to ensure the weldability, to minimize the strengthening mechanism by the alloy component, and to improve the strength by using the control rolling and accelerated cooling.

However, such a conventional technology has a limitation in improving strength, and in order to realize ultra-high-rise large structures, not only securing strength but also manufacturing that can share advantages of toughness and weldability using conventional control rolling and accelerated cooling. Method is required.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the above problems and propose the present invention based on the results. The present invention adds alloy elements of Cu, Ni, and V in steel and controls rolling. After performing, the hot rolled steel sheet is quenched to produce bainite structure, which is a low temperature structure, to provide a method for producing a YP355 grade (N / mm 2) thick steel sheet having high strength and toughness and excellent yield ratio.

In the present invention, C: 0.136 to 0.164%, Si: 0.40 to 0.50%, Mn: 1.00 to 1.25%, P: 0.012% or less, Nb: 0.025 to 0.035%, Ti: 0.015% or less, Cu: 0.30% or less, Ni: 0.30% or less, V: 0.020% or less, Ceq .: 0.42% or less, the balance Fe and other unavoidable impurities the steel slab containing, by re-heating and rolling in the temperature history of the non-recrystallized station immediately above, immediately above Ar ₃ Cooling at a temperature of 5 ~ 6 ℃ / sec at the temperature of the final temperature of the steel sheet to 600 ~ 550 ℃ after the cooling method and the production method of the TMCP thick steel sheet for excellent structural structure characterized in that the air cooling.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention seeks to improve the strength and toughness by adding an appropriate alloying element to steel, increasing the unrecrystallized zone reduction rate, and simultaneously cooling the hot rolled steel sheet, but at this time, the yield ratio must be considered in the steel structure for construction. do.

The yield ratio refers to the strength (YIELD STRENTH, YP) and the tensile strength (TNESILE STRENGTH, hereinafter TS) of the properties of the steel sheet, YP / TS,%. When the external force is received, it refers to the ability to respond to the external force inside the steel sheet. Normally, the limit line is within about 80% in steel sheet steel for construction, which is not simply a yield ratio of 80%, but refers to a state in which the steel sheet itself has a predetermined strength. That is, since the yield strength (YP) of steel is increased proportionally as the strength (TS) increases, the yield ratio of high strength high tensile strength steel generally exceeds 80%.

Therefore, in the present invention, as shown in Table 1, YP355 grade (N / ㎜) that can have excellent to intrinsic by controlling the yield ratio to 80% or less while improving the weldability to a low carbon equivalent without increasing a large amount of chemical components ) It becomes TMCP thick steel plate for building steel structure.

division Quality characteristic thickness Mechanical property Yield strength The tensile strength Toughness (0 ℃) Z-RA Steel structure -High yield strength requirement-80% yield ratio-Low carbon equivalent (welding) 6 ~ 80mm ≥36kg / ㎡ 53 ~ 65kg / ㎡ ≥2.8kgm ≥25%

Hereinafter, the steel component and manufacturing conditions of the present invention will be described.

The C is an element that increases the hardness and strength by increasing the hardenability during heat treatment of steel, but if the content is too high, it is harmful to toughness and weldability, and if the content is small, the hardness cannot be guaranteed because the hardness is low. It is preferable to set the range at 0.136% to 0.164%.

The Si is an element which forms carbide and is solid-solution in Fe to increase the elastic limit tensile strength, but if the content is more than 0.5%, the ferrite structure is lowered and the non-metallic inclusion (SILICATE) is formed to damage the toughness, so the content is 0.40 to 0.50%. It is preferable to set to.

The Mn may improve the hardness when heat-treated as an element for improving the hardenability, but when it is added, the weldability is impaired, and when the small amount is added, the hardness is unstable due to the decrease in the hardenability. Therefore, the content is preferably limited to 1.00 to 1.25%. Do.

P is the largest impurity that inhibits the low temperature impact toughness of the steel sheet, and deteriorates the internal quality, so the content is preferably limited to 0.012%.

S is the same harmful element as P, which causes deterioration of low-temperature impact toughness and impairs weldability in thick plates, so the content thereof is preferably limited to 0.003% or less.

The Nb is solid-dissolved in austenite phase in order to increase the hardenability of the austenite to lower the ferrite transformation temperature, thereby having an effect of making the ferrite grain size fine. In addition, the precipitation strengthening not only increases tensile strength and yield strength, but also has an effect of increasing yield strength. However, if the content is excessive, it may cause brittle brittleness and is detrimental to the deterioration of weld toughness, so the content is preferably set at 0.025 to 0.035%.

The Ti is finely dispersed at the grain boundaries as TiN precipitates at high temperatures such as slab reheating processes and welds to suppress initial austenite grain growth, thereby improving toughness with partial increase in strength. However, if the content is excessive, the oxide-based inclusions are formed or coarse precipitates are formed to reduce toughness. Therefore, the content is preferably limited to 0.015% or less.

The Cu has a reinforcing effect due to the formation of precipitates in the steel sheet and an effect of improving the corrosion resistance of the steel, but when the content exceeds 0.30%, an oxide film is formed heavily on the surface of the steel sheet, so the upper limit thereof is set to 0.30%. It is preferable.

Ni is an element that suppresses the oxide film on the surface of the steel sheet, which is a problem when Cu is added, but has a good effect on improving strength and toughness. However, Ni is an expensive element, and when added in large amounts, the quenchability increases to increase the strength. Its content is preferably limited to 0.30%.

The V is an element that makes carbides fine to make fine carbides by making fine carbides, and greatly improves the high temperature strength. However, vanadium pentoxide (V ₂ 0 ₅ ), which is an oxide, tends to evaporate at a high temperature, so it is preferably limited to 0.020% or less. Do.

In the molten steel formed as described above, it is preferable that the impurity S minimizes its content. In addition, Ca-Si is introduced into the molten steel to spheroidize nonmetallic inclusions.

The ingot thus obtained is subjected to filamentous rolling at a rolling reduction ratio of 40 to 50% at 800 to 900 ° C, which is directly above the unrecrystallized temperature range, to improve anisotropy of the steel sheet material, and to promote microstructure of the structure to secure toughness. If the filament rolling temperature is higher than 900 ° C., which is a partial recrystallization zone, a mixed structure is formed and impact toughness is lowered. If the filament rolling temperature is lower than 800 ° C., a band structure is formed on the structure during abnormal phase rolling (2 PHASE ROLLED). Is generated so that the impact value is lowered as in the partial recrystallization rolling and the thickness direction physical properties are poor. In addition, when the reduction ratio is low, austenite recrystallized grains formed after rolling become large, and thus physical properties are difficult to obtain, which is undesirable.

Therefore, it is preferable to perform the finishing rolling temperature at 900-800 degreeC and the residual reduction rate at 45-50%.

Thereafter, for a yield ratio within 80%, cooling is performed at a cooling rate of 5 to 6 ° C./sec at 750 ° C. to 850 ° C., which is directly above Ar ₃ , so that the final temperature of the steel sheet is 600 to 550 ° C., and air-cooled. do.

At this time, if the cooling rate is slow, the ferrite grain size is coarse to produce the secondary phase, so that it is difficult to secure proper strength. On the contrary, if the cooling rate is fast, the amount of the secondary phase is increased to increase the strength rapidly. By raising the ratio and lowering the plastic deformation capacity, the predetermined property value for use in the construction steel structure, that is, the yield ratio exceeds 80%.

On the other hand, when the temperature of the steel sheet after the end of cooling is less than 550 ° C, the internal stress is excessively generated due to the difference in heat transfer coefficient inside and outside the steel sheet, resulting in poor steel sheet shape. In addition, when the steel sheet temperature is higher than 600 ℃, the structure is transformed into pearlite, a soft structure, the strength does not satisfy the properties.

Therefore, the final temperature of the steel sheet is preferably controlled in the range of 600 ~ 550 ℃.

As described above, bainite, which is a low temperature structure, is formed in the final structure of the manufactured thick steel sheet, thereby providing high strength and high toughness.

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

(Example)

For the steel slab composed as shown in Table 2, the rolling and cooling conditions of Table 3 were applied.

Thereafter, the mechanical properties of the specimens were measured, and the results are shown in Table 3 below.

division C Si Mn P S Nb Cu Ni V Ceq Comparative Steel 1 0.150 0.41 1.20 0.014 0.002 0.015 0.03 0.01 0.001 0.3674 Comparative Steel 2 0.145 0.45 1.22 0.018 0.002 0.013 0.01 0.01 0.007 0.3678 Inventive Steel 1 0.160 0.45 1.19 0.009 0.002 0.026 0.23 0.23 0.014 0.3838 Inventive Steel 2 0.158 0.46 1.21 0.009 0.001 0.033 0.25 0.23 0.015 0.3857 * Ceq. = C + Mn / 6 + Si / 24 + Cr / 5 + V / 14 + Ni / 40 + Mo / 4 * The content of each element is in weight percent

division Finish rolling temperature (℃) Residual Pressure Reduction Rate (%) Cooling end temperature (℃) Cooling rate (℃ / s) YP (kg / ㎡) TS (kg / ㎡) YR [%] Impact Toughness (0 ℃, kgm) Comparative Material 1 870 30 607 5.0 34.7 52.5 68 19.8 Comparative Material 2 890 30 647 3.6 33.6 50.9 66 13.5 Invention 1 812 45 572 5.1 38.4 57.9 66 24.7 Invention 2 804 45 578 5.4 41.1 59.3 69 18.8

As shown in Table 3, the invention materials (1), (2) is added to the Cu, Ni, V components, and more strictly manage the rolling temperature, rolling rate, and cooling conditions to improve the strength without reducing the impact toughness I could make it.

On the other hand, Figure 1 shows a tissue picture of the invention material (1) and the comparative material (1), the present invention material (1) can be seen that a significant portion of the bainite transformation tissue is observed compared to the comparative material (1) This is the factor that increases the strength.

As described above, the present invention can be used as a structural material in accordance with the trend of recent high-rise and large-scale construction of buildings by appropriately controlling the steel components, rolling conditions and subsequent cooling conditions to improve strength, toughness and anti-vibration without changing weldability. In addition, there is an effect that can reduce the amount of use by 10% to ensure high strength compared to conventional materials.

1 is a photograph showing the structure of the present invention and the comparative material

Claims (1)

By weight%, C: 0.136 ~ 0.164%, Si: 0.40 ~ 0.50%, Mn: 1.00 ~ 1.25%, P: 0.012% or less, Nb: 0.025 ~ 0.035%, Ti: 0.015% or less, Cu: 0.30% or less, Ni : 0.30% or less, V: 0.020% or less, Ceq .: 0.42% or less, steel slab containing remainder Fe and other unavoidable impurities is reheated, followed by finishing rolling at a temperature directly above the unrecrystallized region (800 to 900 ° C), Cooling at a cooling rate of 5-6 ℃ / sec at the temperature directly above Ar ₃ (750 ~ 850 ℃) to make the final temperature of the steel sheet 600 ~ 550 ℃ and then air-cooled after excellent yield ratio of the building structure TMCP Method of manufacturing steel sheet.