WO2018117552A1 - Ultra-high strength hot-rolled steel sheet having excellent bending processability and method for manufacturing same - Google Patents

Abstract

An aspect of the present invention relates to an ultra-high strength hot-rolled steel sheet having excellent bending processability, the steel sheet comprising, in terms of weight%, 0.18-0.28% of C, 1.2-2.2% of Mn, 0.1-0.5% of Si, 0.005-0.05% of P, 0.01% or less of S, 0.01-0.05% of Al, 0.01-0.10% of Ti, 0.001-0.0045% of B, 0.001-0.01% of N, and the remainder Fe and unavoidable impurities, wherein a microstructure comprises, by area fraction, 90% or more of martensite, 4-10% of auto-tempered martensite, and 5% or less of residual austenite.

Description

Super high strength hot rolled steel sheet with excellent bending workability and manufacturing method

The present invention relates to an ultra high strength hot rolled steel sheet excellent in bending workability and a method of manufacturing the same.

Due to strong demands such as vehicle passenger collision stability and CO ₂ environmental regulations, it is necessary to realize super high strength and ultra light weight, and development of ultra high strength steel sheet of 1.0 GPa or more is actively underway.

Ultra-high strength hot rolled steel sheet used in bumper reinforcement, door impact beam, etc., which are reinforcement materials of most automobile bodies, is required to have high strength and excellent bending workability and shape quality for roll forming.

In order to satisfy these properties, the steel sheet for automobile structural members is basically composed of a combination of Ferrite, Baintie, Martensite, and Tempered Martensite phases. Therefore, it is classified and applied to DP (Dual Phase) steel, Transformation Induced Plasticity (TRIP) steel, Complex Phase steel, MART steel and so on.

These steels are mainly applied to parts that require high energy absorption in car collisions such as members, pillars, bumper stiffeners, sealsides, etc., and they must have a high elongation with a tensile strength of 1.0 GPa or more because they are processed using roll forming.

On the other hand, a lot of research and development has been carried out in order to provide ultra-high strength steel of 1.2GPa or more tensile strength used as a vehicle body reinforcement parts, the representative examples are Patent Documents 1 to 5.

In Patent Literature 1, C: 0.15 to 0.20%, Si 0.3 to 0.8%, Mn 1.8 to 2.5%, Al 0.02 to 0.06%, Mo 0.1 to 0.4%, Nb 0.03 to 0.06%, and S 0.02% or less by chemical component weight ratio After adding, P 0.02% or less, N 0.005% or less and homogenizing the aluminum-kilted steel containing elements inevitably contained in the manufacture of steel at 1050 ~ 1300 ℃, finishing hot rolling at 850 ~ 950 ℃ immediately above Ar3 transformation point, and then 550 Hot rolling at ˜650 ° C .; Cold rolling the steel sheet at a cold reduction rate of 30 to 80%, followed by continuous annealing at an A3 temperature or higher; And firstly slow cooling the steel sheet to 600 to 700 ° C., and rapidly quench the steel sheet to 350 to 300 ° C. at a cooling rate of -10 to 50 ° C./sec, and then slowly cool it to 350 to 250 ° C. for at least 1 minute. Disclosed is a method for producing a 1.2GPa grade ultra high strength cold rolled steel sheet for automobile bumper reinforcement comprising the step of maintaining.

In Patent Literature 2, C: 0.05 to 0.20%, Si: 2.5% or less, Mn: 3.0% or less, Cr: 0.3% or less, Mo: 0.3% or less, in a steel containing impurities and a small amount of alloying elements in weight%; Ni: The method of manufacturing the cold rolled steel sheet which has the favorable shape which adds 1 or 2 or more types of 0.3% or less, has 1180-1400 MPa strength, and the bending / twist of a steel plate is 10 mm or less is disclosed. In addition, after quenching the steel sheet at a high temperature using a continuous annealing heat treatment equipment, the plate shape defects due to tempering after normal water cooling (quench width) by the normal aging treatment in the temperature range of 150 ~ 200 ℃ Direction deformation).

In Patent Literature 3, a cold rolled steel sheet containing C: 0.1 to 0.6%, Si: 1.0 to 3.0%, Mn: 1.0 to 3.5%, Al: 1.5% or less, and Cr: 0.003 to 2.0% by weight% is obtained by Ac3-Ac3. After heating to a temperature of + 50 ° C and cooling at a cooling rate of 3 ° C / s or more, and maintaining a constant temperature in the range of (Ms-100 ° C) to Bs (bainite starting temperature), the percentage of retained austenite before processing is 10%. In the above, the method of producing a tensile strength 1470MPa grade ultra-high strength cold rolled steel sheet having hydrogen withdrawal characteristics having an austenite grain length of 1 micron or more and an average axial ratio (long axis / short axis) of 5 or more was introduced.

In Patent Literature 4, weight% C: 0.10 to 0.27%, Si: 0.001 to 1.0%, Mn: 2.3 to 3.5%, Al: 1.0% or less (excluding 0%), Cr: 2% or less (excluding 0%), P: 0.02% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti: 0.004 to 0.03% , Mo: 0.2% or less (except 0%), Nb: 0.05% or less (except 0%), cold rolled strips containing residual Fe and other unavoidable impurities at 1-5 ° C / s heating rate [(Ac3- 90 ° C.) to (Ac 3 ± 15 ° C.)], followed by primary cooling to a temperature range of 500 to 750 ° C. at a cooling rate of 1 to 3 ° C./s, and cooling of 3 to 50 ° C./s. Cool down to the temperature range of [(Ms-120) ~ 460 ℃] at a rate and then maintain constant transformation for 6 ~ 500sec, or slow cooling at a cooling rate of 1 ℃ / s or less. A cold rolled steel sheet manufacturing method was introduced.

However, according to Patent Documents 1 to 4, since the cold rolling and annealing heat treatment (CAL, Continuous Annealing Line) process after hot rolling has a disadvantage in that the manufacturing cost rises sharply, and the bumper or reinforcement for automobiles that are currently commercially used There is a problem in that the tensile strength is relatively low to apply to.

Moreover, in patent document 5, Ti of the quantity which satisfy | fills C: 0.26-0.45%, Mn + Cr: 0.5-3.0%, Nb: 0.02-1.0%, 3.42N + 0.001 <Ti <3.42N + 0.5 by weight%, Furthermore, Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less, 1 type or 2 or more types, B: 0.01% or less, Nb: 1.0% in some cases Hereinafter, a cold rolled steel sheet containing one or two or more Mo: 1.0% or less and Ca: 0.001 to 0.005% is disclosed for a super high strength manufacturing method having a tensile strength of 1.8 GPa through hot press molding.

According to Patent Document 5, ultra high strength of 1.8 GPa can be secured, but there is a problem in that the manufacturing cost increases further because the hot rolled steel sheet has to undergo additional hot press forming step.

Therefore, it is not only possible to replace the existing super high strength cold rolled steel sheet and hot formed steel, but also to develop a super high strength hot rolled steel sheet and a method for manufacturing the same, which can secure higher tensile strength and significantly lower the manufacturing cost. It is true.

(Prior art document)

(Patent Document 1) Korean Unexamined Patent Publication No. 2004-0057777

(Patent Document 2) Japanese Unexamined Patent Publication No. 2007-100114

(Patent Document 3) Korean Unexamined Patent Publication No. 2008-73763

(Patent Document 4) Korean Unexamined Patent Publication No. 2013-0069699

(Patent Document 5) Japanese Unexamined Patent Publication No. 2008-0111549

One aspect of the present invention is to provide a 1.8GPa-class ultra-high strength hot rolled steel sheet excellent in bending workability only by the hot rolling process using the continuous continuous rolling mode in the play-rolling direct connection process and its manufacturing method.

In addition, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the present invention is by weight, C: 0.18-0.08%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01-0.05% , Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001-0.01%, remaining Fe and inevitable impurities,

The microstructure relates to an ultra-high strength hot rolled steel sheet having excellent bendability, including not less than 90% martensite, 4-10% auto-tempered martensite, and 5% or less residual austenite.

In addition, another aspect of the present invention is a weight%, C: 0.18 ~ 0.28%, Mn: 1.2 ~ 2.2%, Si: 0.1 ~ 0.5%, P: 0.005 ~ 0.05%, S: 0.01% or less, Al: 0.01 Continuous casting of molten steel containing ~ 0.05%, Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001-0.01%, remaining Fe and unavoidable impurities with a thin slab of 60-120 mm;

Roughly rolling the heated thin slab so that the edge portion temperature of the bar plate may be 850 to 1000 ° C. at the rough rolling exit side to obtain a bar plate;

Finishing rolling the bar plate in a temperature range of Ar 3 + 10 ° C. to Ar 3 + 100 ° C. to obtain a hot rolled steel sheet;

Air-cooling the hot rolled steel sheet for 1 to 3 seconds, then cooling at a cooling rate of 200 ° C./sec or more, and winding the sheet at Mf-50 ° C. or less; And

And placing the wound hot rolled steel sheet between two other wound hot rolled steel sheets; each step relates to a method of manufacturing a super high strength hot rolled steel sheet having excellent bending workability.

In addition, the solution of the said subject does not enumerate all the characteristics of this invention. Various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a 1.8GPa grade super high strength hot rolled steel sheet and a method of manufacturing the same having excellent bending workability only by the hot rolling process using the continuous continuous rolling mode in the play-rolling direct connection process, and ultra high strength cold rolled steel sheet and hot formed steel As well as replaceable, it is possible to secure a higher tensile strength and significantly lower the manufacturing cost.

1 is a photograph of a strip after winding of Inventive Example 3. FIG.

2 is a photograph of the surface of the PO material of Inventive Example 3. FIG.

3 is a transmission electron microscope (TEM) tissue photograph of Example 3, (a) is 20,000 times, (b) is a magnification of 100,000 times magnification of the [X] portion in (a).

4 is a schematic diagram of a process using a continuous continuous rolling mode in the play-rolling direct connection process.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

The inventors of the present invention can replace the existing cold rolled steel sheet and hot press steel sheet if the ultra-high strength steel of 1.5GPa or more tensile strength used as the automobile body reinforcement parts can be manufactured only by the hot rolling process, thereby significantly reducing the manufacturing cost. In order to prepare 1.8GPa grade hot rolled steel sheet by only hot rolling process, it was studied in depth.

As a result, it was confirmed that the high strength hot rolled steel sheet having a tensile strength of 1.8 GPa can be manufactured by using the continuous continuous rolling mode in the performance-rolling direct connection process, and precisely controlling the components and the manufacturing process. Reached.

Super high strength hot rolled steel with excellent bending workability

Hereinafter, a super high strength hot rolled steel sheet excellent in bendability according to an aspect of the present invention will be described in detail.

Ultra-high strength hot rolled steel sheet having excellent bending workability according to an aspect of the present invention is a weight%, C: 0.18 ~ 0.28%, Mn: 1.2 ~ 2.2%, Si: 0.1 ~ 0.5%, P: 0.005 ~ 0.05%, S: 0.01% or less, Al: 0.01% to 0.05%, Ti: 0.01% to 0.10%, B: 0.001% to 0.0045%, N: 0.001% to 0.01%, remaining Fe and inevitable impurities, and

The microstructure comprises at least 90% martensite, 4-10% autotempered martensite and 5% or less residual austenite.

First, the alloy composition of the present invention will be described in detail. The unit of each element content hereafter means weight% unless there is particular notice.

C: 0.18 ~ 0.28%

Carbon (C) is a very important element to increase the strength by making the martensite structure during quenching after hot rolling.

When the C content is less than 0.18%, the strength of martensite itself may be difficult to secure the strength targeted by the present invention. On the other hand, when the C content is more than 0.28%, there is a problem in that bending workability is lowered due to weldability and excessive strength increase. Therefore, it is preferable that C content is 0.18 to 0.28%.

Also, the lower limit of the C content may be 0.20%, and the lower limit may be 0.21%. In addition, the more preferable upper limit of the C content may be 0.27%, and even more preferred upper limit may be 0.26%.

Mn: 1.2-2.2%

Manganese (Mn) suppresses the formation of ferrite and increases the strength by increasing the austenite stability to facilitate the formation of low-temperature transformation phase.

If the Mn content is less than 1.2%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the Mn content is higher than 2.2%, segregation zones are formed on the inside and / or outside of the performance slab and hot rolled steel sheet, causing cracking and propagation, resulting in deterioration of final quality of the steel sheet and weldability and / or bending workability. Can be inferior. Therefore, it is preferable that Mn content is 1.2 to 2.2%.

In addition, the lower limit of the Mn content may be 1.30%, and the lower limit may be 1.4%. In addition, the more preferred upper limit of the Mn content may be 2.1%, and the even more preferred upper limit may be 2.0%.

Si: 0.1 ~ 0.5%

Silicon (Si) is a useful element that can secure strength without lowering the ductility of the steel sheet. It is also an element that promotes ferrite formation and promotes martensite formation by encouraging C concentration into unmodified austenite.

When the Si content is less than 0.1%, it is difficult to secure the above-described effects sufficiently. On the other hand, when the Si content is more than 0.5%, red scale is generated on the surface of the steel sheet and traces may remain on the surface of the steel sheet after pickling, thereby degrading the surface quality. Therefore, it is preferable that Si content is 0.1 to 0.5%.

P: 0.005 to 0.05%

Phosphorus (P) is an element showing the effect of strengthening the steel sheet.

If the P content is less than 0.005%, it is difficult to secure the effect. On the other hand, when the P content is greater than 0.05%, segregation may occur at grain boundaries and / or interphase boundaries, causing brittleness. Therefore, the content of P is preferably limited to 0.005 to 0.05%.

S: 0.01% or less

Sulfur (S) as impurities may segregate during MnS non-metallic inclusions and performance solidification in steel, causing hot cracks. Therefore, the content should be controlled as low as possible, preferably at 0.01% or less. However, controlling the S content to 0% may exclude 0% because excessive cost is consumed.

Al: 0.01 ~ 0.05%

Aluminum (Al) serves to increase carbide ductility by inhibiting carbide formation.

When the Al content is less than 0.01%, the above effects are insufficient. On the other hand, when the Al content is greater than 0.05%, a large amount of AlN precipitates may be formed to reduce the edge quality of the cast or bar plate due to the low temperature ductility, and may be concentrated on the surface of the steel sheet to degrade the plating property. Therefore, it is preferable that Al content is 0.01 to 0.05%.

Ti: 0.01 ~ 0.10%

Titanium (Ti) is an element that increases the strength of steel as a precipitate and nitride forming element. In addition, Ti is an element that reduces the amount of precipitates such as AlN by reducing the amount of precipitates such as AlN by removing the solid solution N through the formation of TiN near the solidification temperature, thereby preventing the reduction of high temperature ductility.

If the Ti content is less than 0.01%, the ductility of the cast slab is reduced due to excessive precipitation of fine AlN and / or BN precipitates, thereby degrading slab quality. On the other hand, when the Ti content is more than 0.10%, it is difficult to expect grain refinement effect due to the formation of coarse TiN precipitates, and the manufacturing cost increases. Therefore, the Ti content is preferably 0.01 to 0.10%.

B: 0.001-0.0045%

Boron (B) is an element which increases the hardenability of steel.

If the B content is less than 0.001%, the above-described effects are insufficient, and if it exceeds 0.0045%, the austenite recrystallization temperature is increased and the weldability is bad. In addition, precipitates such as BN may be excessively precipitated and the edge quality of the cast and / or bar plates may be inferior due to high temperature ductility deterioration. Therefore, the B content is preferably 0.001 to 0.0045%.

In addition, the lower limit of the B content may be more preferably 0.0015%, and more preferably 0.004%.

N: 0.001-0.01%

Nitrogen (N) is an austenite stabilizing and nitride forming element.

When the N content is less than 0.001%, the above effects are insufficient. On the other hand, when the N content is greater than 0.01%, the nitride may be excessively precipitated, resulting in inferior edge quality of the cast and / or bar plate due to high temperature ductility deterioration. In addition, the precipitate size is coarse, which may cause the strength to decrease. Therefore, N content is preferably 0.001 to 0.01%.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

At this time, in addition to the above-described alloy composition, by weight%, Nb: 0.001 ~ 0.05%, Cr: 0.5 ~ 1.0%, Mo: 0.001 ~ 0.05% and Sb: 0.005 ~ 0.02% may further include one or more.

Nb: 0.001-0.05%

Niobium (Nb) is an element effective for increasing the strength of steel sheet and miniaturizing the grain size.

If the Nb content is less than 0.001%, it is difficult to secure the above-described effects. If the Nb content is more than 0.05%, excessive NbC, (Ti, Nb) CN, or the like may be formed to cause low temperature brittleness of the playing slab. Therefore, the Nb content is preferably 0.001 to 0.05%.

Cr: 0.5-1.0%

Chromium (Cr) solidifies the steel and delays the bainite phase transformation during cooling to help form martensite.

When the Cr content is less than 0.5%, the above effects are insufficient. On the other hand, when the Cr content is more than 1.0%, there is a problem that the manufacturing cost rises and the ductility of the steel sheet is lowered. Therefore, it is preferable that Cr content is 0.5 to 1.0%.

Mo: 0.001-0.05%

Molybdenum (Mo) is a useful component to improve the impact toughness and bending workability by strengthening the yield strength and grain boundary due to solid solution strengthening and fine precipitate formation.

If the Mo content is less than 0.001%, it is difficult to obtain the above-described effects. If the Mo content is more than 0.05%, the effect may not only be saturated, but ductility may be reduced. Therefore, it is preferable that Mo content is 0.001 to 0.05%.

Sb: 0.005-0.02%

Antimony (Sb) is an element that serves to suppress the formation of hot-rolled scale defects.

If the Sb content is less than 0.005%, it is difficult to secure the above-described effects. If the Sb content is more than 0.02%, Sb may cause problems such as edge cracking as a low melting point element as well as an increase in manufacturing cost and workability deterioration. Therefore, the Sb content is preferably 0.005 to 0.02%.

In addition to the alloy composition described above, one or more of Cu, Ni, Sn, and Pb may be included as a tramp element, and the sum thereof may be 0.2% by weight or less. The tramp element is an impurity element derived from scrap used as a raw material in the steelmaking process, and when the sum is more than 0.2%, the surface crack of the thin slab and the surface quality of the hot rolled steel sheet may be reduced.

In addition, the C, Si, Mn, P and S may be a Ceq is 0.25 ~ 0.45 defined by the following relational formula 1.

Relationship 1: Ceq = C + Si / 30 + Mn / 20 + 2P + 3S

(In the above relation 1, each element symbol is a value indicating the weight of each element.)

The relational formula 1 is a component relational formula for securing the weldability of the steel sheet. In the present invention, by managing the Ceq (carbon equivalent) value of 0.25 to 0.45, it is possible to secure excellent resistance spot weldability, and to provide excellent mechanical properties to the welded portion. can do.

When Ceq is less than 0.25, there is a problem in that the hardenability is low and it is difficult to secure a target tensile strength. On the other hand, when Ceq is more than 0.45, the weldability decreases and the properties of the healthy welds cannot be obtained. Therefore, it is preferable to control the components so that Ceq satisfies the range of 0.25 to 0.45.

Hereinafter, the microstructure of the present invention will be described in detail.

The microstructure of the present invention comprises at least 90% martensite, 4-10% autotempered martensite and 5% or less residual austenite in area fraction.

Martensite is to secure ultra high strength, it may be difficult to secure the target tensile strength when the martensite fraction is less than 90%.

If the tempering martensite is less than 4%, bending workability may be inferior, and if it exceeds 10%, it may be difficult to obtain the target toughness.

On the other hand, residual austenite may play a role of improving the strength and workability by transforming to martensite when processing into an unstable phase at room temperature, but when the residual austenite is more than 5%, the amount of transformation of martensite into martensite is large. Volume expansion causes deformation of the product and can cause the cause of hardness imbalance. On the other hand, even when the residual austenite is 0%, the ultra high strength and the bending workability, which are targeted in the present invention, can be ensured, so the lower limit thereof is not particularly limited.

In this case, a lattice shortening interval of the martensite may be 5 μm or less.

The lattice shortening interval of martensite may affect the strength and bending workability, and when it is more than 5 µm, it may be difficult to secure the target strength and bending workability.

In addition, the microstructure of the present invention may further comprise less than 4.5% ferrite in the area fraction.

If the area fraction of ferrite is more than 4.5%, it may be difficult to secure the tensile strength, so the area fraction of ferrite is preferably 4.5% or less. On the other hand, even when the ferrite is 0%, the ultra-high strength and bending workability targeted in the present invention can be ensured, so the lower limit thereof is not particularly limited.

Meanwhile, the hot rolled steel sheet according to the present invention may have a tensile strength of 1.8 GPa or more, and a bending workability of 3 or less.

Bending workability (R / t) refers to a value obtained by dividing the minimum bending radius (R) without cracking by a steel plate thickness (t) after a 90 ° bending test, and when the bending workability is 3 or less, a bumper reinforcement that is an automobile body reinforcement and It can be preferably applied to the manufacture of the door impact beam and the like. More preferably, the bendability may be 2.5 or less.

In addition, the hot rolled steel sheet according to the present invention may have a thickness of 2.0 mm or less.

In addition, the hot rolled steel sheet according to the present invention may have a thickness variation (Crown) of 40 μm or less. Here, the width thickness variation (Crown) means the difference between the thickness at the edge 25mm and the center thickness.

Manufacturing method of super high strength hot rolled steel sheet with excellent bending workability

Hereinafter, another aspect of the present invention will be described in detail a method for producing a super high strength hot rolled steel sheet excellent in bending workability.

According to another aspect of the present invention, a method of manufacturing a super high strength hot rolled steel sheet having excellent bending workability includes continuously casting molten steel satisfying the alloy composition described above with a thin slab of 60 to 120 mm; Roughly rolling the heated thin slab so that the edge portion temperature of the bar plate may be 850 to 1000 ° C. at the rough rolling exit side to obtain a bar plate; Finishing rolling the bar plate in a temperature range of Ar 3 + 10 ° C. to Ar 3 + 100 ° C. to obtain a hot rolled steel sheet; Air-cooling the hot rolled steel sheet for 1 to 3 seconds, then cooling at a cooling rate of 200 ° C./sec or more, and winding the sheet at Mf-50 ° C. or less; And placing the wound hot rolled steel sheet between two other wound hot rolled steel sheets, wherein each step is performed continuously.

When the above steps are performed continuously means that the continuous rolling mode is used in the play-rolling direct connection process.

The new steel manufacturing process, which is attracting attention recently, is a process of using the so-called thin slab (mini mill process), which is a direct rolling-rolling process, and has a good material deviation due to the small temperature deviation in the width and length directions of the strip. It is attracting attention as a process with the potential to manufacture tissue steel.

In such a performance-rolling direct connection process, there is a conventional batch mode and a newly developed continuous continuous rolling mode.

In case of batch mode, in order to compensate for the difference between the playing speed and rolling speed, the coil is rolled up in the coil box before the finishing mill and finish rolling is performed. Problems such as breakage may occur.

In the continuous continuous rolling mode, unlike the batch mode, there is no winding process before finishing rolling, so the problem of the batch mode is solved, but more precise control is required to compensate for the difference between the playing speed and the rolling speed.

Figure 4 shows an example of a process using the continuous continuous rolling mode in the play-rolling direct connection process. In the continuous casting machine 100, a thin slab (a) having a thickness of 50 to 150 mm is manufactured, and there is no coil box between the roughing mill 400 and the finishing mill 600, so that the steel sheet can be continuously rolled to obtain a good flowability. The risk of plate breakage is very low, making it possible to produce thin films with a thickness of less than 3.0mm. Rough mill scale breaker (RSB) 300 in front of rough mill 400 and finishing mill scale breaker 500 in front of finishing mill 600 (FSB) for easy surface scale removal. When pickling hot rolled steel plate in post process, it is possible to produce PO (Pickled & Oiled) material with excellent surface quality. In addition, in the finish rolling step, isothermal isothermal rolling is possible with a difference in rolling speed of 5% or less in one steel sheet, so that the steel plate width and longitudinal temperature variation are remarkably low and precisely cooled in the run out table (ROT). The controllable steel sheet can be manufactured with excellent material variation.

Hereinafter, each step will be described in detail.

Continuous casting step

The molten steel that satisfies the alloy composition described above is continuously cast with a thin slab of 60 to 120 mm.

When the thickness of the thin slab exceeds 120mm, not only the high speed casting is difficult, but also the rolling load increases during rough rolling, and when the thickness of the slab is less than 60mm, a temperature drop of the cast slab rapidly occurs to form a uniform structure. In order to solve this problem, it is possible to additionally install a heating device, but this is a factor to improve the production cost, it is desirable to exclude as possible. Therefore, it is preferable that the thickness of a thin slab is 60-120 mm.

In this case, the continuous casting may be performed using a mold flux having a basicity of 1.0 or more. Basicity here means the CaO (%) / SiO 2 (%) ratio.

In general, high-strength steel has a lot of components added to ensure high strength, the linear crack sensitivity is very high. Therefore, when a mold flux having a basicity of less than 1.0 is used, it is preferable to use a mold flux having a basicity of 1.0 or more because the heat transfer amount is high and the sensitivity of linear crack generation is increased by the slab surface cooling.

In addition, the casting speed of the continuous casting may be 4 ~ 8mpm.

The reason why the casting speed is more than 4mpm is because the high-speed casting and the rolling process are connected, and a certain casting speed is required to secure a target rolling temperature. In addition, if the circumferential speed is low, there is a risk of segregation from the cast, and if this segregation is not only difficult to secure strength and workability, but also increases the risk of material deviation in the width direction or the longitudinal direction. If the casting speed exceeds 8mpm, there is a fear that the operation success rate may be reduced due to the instability of the molten steel.

Rough rolling stage

The heated thin slab is roughly rolled at the rough rolling exit side such that the edge temperature of the bar plate is 850 to 1000 ° C. to obtain a bar plate.

When the temperature of the edge portion of the bar plate is less than 850 ° C. at the rough rolling side, a large amount of AlN precipitates are generated, and thus there is a possibility that the edge crack generation sensitivity is very high due to the low temperature ductility. On the other hand, if the edge temperature of the bar plate on the rough rolling side is more than 1000 ℃, the center temperature of the thin slab is too high, a large number of arithmetic scale may occur, the surface quality may be inferior after pickling.

At this time, the rough rolling may be performed so that the surface temperature of the thin slab is 1000 ~ 1200 ℃ at the entrance side of the rough rolling.

If the surface temperature of the thin slab at the rough rolling side is less than 1000 ° C., there is a possibility that cracks may occur at the edge of the bar plate during the rough rolling load increase and the rough rolling process, and this may cause edge defects of the hot rolled steel sheet. On the other hand, when the surface temperature is more than 1200 ° C, problems such as deterioration of the hot rolled surface quality due to the remaining hot rolled scale may occur.

At this time, before the rough rolling step of removing the scale by spraying the cooling water to the heated thin slab at a pressure of 100 ~ 200bar or more;

After the rough rolling step of removing the scale by sequentially passing through the bar plate to the first row of spraying the cooling water at a pressure of 50 ~ 250bar and the second row of spraying the cooling water at a pressure of 100 ~ 300bar; .

For example, before rough rolling, spray the cooling water below 40 ℃ from the roughing mill scale breaker (RSB) nozzle to the heated thin slab at a pressure of 100 ~ 200bar to make the surface scale thickness 200 It can be removed to a micrometer or less, and the surface scale thickness is reduced by using the first row and second row nozzles of the Finishing Mill Scale Breaker (FSB) after bar rolling. It can remove to micrometer or less.

If the pressure of the coolant is less than 100 bar before rough rolling, a large number of arithmetic scales may remain on the surface of the thin slab, resulting in inferior surface quality after pickling.In the case of more than 200 bar, the bar plate edge temperature is drastically reduced, resulting in an edge crack. High risk of occurrence.

If the pressure of the first and second row nozzles after rough rolling is less than 50 and 100, respectively, the scale is insufficient to be removed, and a large amount of fusiform and scale scales may be generated on the surface of the steel sheet after finishing rolling, resulting in inferior surface quality after pickling. . On the other hand, when the pressure of the first row nozzle is greater than 250 bar or the pressure of the second row nozzle is more than 300 bar, the finishing rolling temperature is too low to obtain an effective austenite fraction, it may be difficult to secure the target tensile strength. .

In addition, since it is difficult to remove the scale sufficiently with only one row of nozzles, and a fusiform scale, which is a fatal defect in the product, may occur during finishing rolling, it is preferable to remove the scale by using both the first row and the second row nozzles as described above.

Hot rolling stage

The bar plate is finish rolled at a temperature range of Ar3 + 10 ° C to Ar3 + 100 ° C to obtain a hot rolled steel sheet.

If the finish rolling temperature is less than Ar3 + 10 ℃, the load of the roll increases greatly during hot rolling, which increases energy consumption and slows down the work speed. You may not get enough of the required martensite.

In addition, when the finish rolling at a temperature of more than Ar3 + 100 ° C the grains are coarse to obtain a high strength, there is a disadvantage that the cooling rate must be further increased to obtain sufficient martensite.

Cooling and winding stage

After cooling the hot-rolled steel sheet for 1 to 3 seconds, it is cooled at a cooling rate of 200 ° C / sec or more and wound up to Mf-50 ° C or less.

The reason why the strip is air-cooled for 1 to 3 seconds after finishing rolling is to remove residual internal stresses in the strip and to refine the martensite class, and to cool to the temperature directly above Ar3. If the air cooling time is less than 1 second, the residual stress inside the strip generated during finish rolling cannot be removed, and the shape may be inferior during winding. On the other hand, when the air cooling time is more than 3 seconds, the cornerstone ferrite may be formed, and there is a fear that sufficient martensite may not be secured after the end of cooling.

If the cooling rate after the air cooling is less than 200 ℃ / sec because there is a ferrite and bainite nose (Nose) there is a fear that a sufficient martensite structure can not be secured. In addition, when the winding temperature is higher than Mf-50 ° C, not only martensite structure is hard to be obtained, but also martensite structure obtained by cooling can be auto tempered to form a large amount of auto tempered martensite. It may be difficult to achieve the desired tensile strength.

Accumulation stage

The wound hot rolled steel sheet is placed between two or more other wound hot rolled steel sheets.

The cold rolled hot rolled steel sheet has a large amount of residual water on the surface and inside, and when the skin pass is performed in this state, the hot rolled steel sheet may be in close contact with each other between the residual water and the scale, which may cause a fatal indentation defect on the surface of the strip.

When the hot rolled steel sheet wound between two or more wound hot rolled steel sheets is loaded, not only the residual water can be removed, but also some martensite is auto tempered to secure the auto tempered martensite.

At this time, the step of skin pass rolling the stacked hot-rolled steel sheet at 50 ~ 150 ℃; may further include.

The reason why the skin pass rolling temperature is limited to 50 to 150 ° C. is because shape correction of the hot rolled steel sheet is easy for the warm rolling effect.

On the other hand, pickling the stacked hot-rolled steel sheet to obtain a pickled PO (Pickled & Oiled) material; may further include. In general, if the treatment method used in the thermal calculation washing step is applicable it is not particularly limited.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

The molten steel having the composition of composition shown in Table 1 was applied to the manufacturing conditions shown in Table 2 to obtain a 1.4 mm thick hot rolled steel sheet in a continuous continuous rolling mode in a performance-rolling direct connection process (slab thickness: 93 mm, circumferential speed: 5.8 mpm). Then, skin pass rolling was performed at 100 ° C. to produce a hot rolled steel sheet.

After pickling the hot rolled steel sheet to obtain a PO material, the microstructure, mechanical properties, crack occurrence, width direction thickness variation (Crown), PO material indentation defect occurrence and PO material surface quality were measured and evaluated. It is described in.

The microstructure was observed by scanning electron microscope (SEM) and transmission electron microscope (TEM) to determine the area ratio of martensite (M), haute tempered martensite (AT) and ferrite (F). In the case of residual austenite (RA), the area ratio was measured using an Electron BackScatter Diffraction (EBSD) instrument. In addition, the lattice spacing shown in Table 3 below describes the average value obtained by measuring the lattice shortening spacing of martensite using a photograph taken by a transmission electron microscope.

Yield strength (YS), tensile strength (TS), and yield ratio (YR), which are mechanical properties, are measured by taking JIS No. 5 specimens based on a 90 ° direction with respect to the rolling direction at a width W / 4 point.

The bending workability was evaluated by determining whether cracks occurred after the 90 ° bending test by setting a value obtained by dividing the bending radius (R) by the steel plate thickness (t) to be 2.5, and indicating the occurrence of cracks in Table 3 below.

Linear and edge cracks were visually checked first at the bar plate and coil, and secondly, using a Surface Defect Detector (SDD) device, which is a surface defect detector.

Width direction thickness variation (Crown) was described by measuring the difference between the thickness at the edge 25mm and the center thickness. Here, the thickness deviation acceptance criterion may be 40 μm or less.

Determination of the occurrence of PO material indentation defects was visually confirmed first at the coil and secondly confirmed using the SDD.

Evaluation criteria for the surface quality of the PO material is as follows.

○: glossiness width direction average deviation is 10% or less

△: 10-20% glossiness width direction average deviation

X: glossiness width direction average deviation exceeds 20%

On the other hand, in Table 2, Ar3 is the temperature at which the ferrite transformation starts, Mf means the temperature at which the martensite transformation is terminated, calculated using a commercial thermodynamic software JmatPro V-9.

In Table 1, Ceq = C + Si / 30 + Mn / 20 + 2P + 3S, and each element symbol in the relation is a value representing each element content in weight%.

In Table 2, RSB (Roughing Mill Scale Breaker, rough rolling scale brake) is the coolant injection pressure before the rough rolling, FSB (Finishing Mill Scale Breaker, finish rolling scale brake) is the cooling water injection pressure after rough rolling.

In addition, whether or not loading is indicated whether the wound hot rolled steel sheet was stacked between the other two wound hot rolled steel sheet.

In Table 3, M is Martensite, AT is Auto Tempered Martensite, RA is Retained Austeniste, and F is Ferrite.

Inventive Examples 1 to 6 satisfying all the conditions presented in the present invention satisfied the tensile strength and bending workability of 2.5 GPa or more.

In addition, when comparing the invention examples 1 to 3 and the invention examples 4 to 6, in the case of the invention examples 1 to 3 satisfying the cooling water injection pressure before and after rough rolling further proposed in the present invention, as well as excellent bending workability and ultra-high strength Surface quality was also available.

FIG. 1 is a coil shape after winding of Inventive Example 3, and FIG. 2 is a picture of the surface of PO material pickled from the hot rolled steel sheet of Inventive Example 3, and it can be confirmed that the wound shape and the surface quality are excellent.

3 is a transmission electron microscope (TEM) micrograph of Inventive Example 3, (a) is 20,000 times, and (b) is a magnification of 100,000 times magnification of [X] in (a). As can be seen from this result, the martensite class of less than 1 μm is developed finely, and the fine tempered cementite of less than 100 nm is present inside the coarse class (automatic tempered martensite is present). It can be seen.

Comparative Example 1 did not satisfy the basicity conditions presented in the present invention, a linear crack occurred due to the surface cooling.

Comparative Examples 2 to 4 did not satisfy the cooling conditions presented in the present invention and did not satisfy the target material.

Comparative Example 5 did not satisfy the finish rolling temperature presented in the present invention did not satisfy the target material.

In Comparative Example 6, the bending crack occurred due to the small amount of auto-tempered martensite as an example of the implementation of the present invention, and the surface quality was inferior due to the presence of residual water in the strip.

In Comparative Examples 7 and 8, since the bar plate edge temperature was low, edge cracks occurred due to the decrease in high-temperature ductility due to excessive precipitation of AlN and BN.

Comparative Example 9 was inferior in bending workability because the carbon content exceeded 0.28%.

Comparative Examples 10 and 11 did not satisfy the components presented in the present invention, the tensile strength was inferior.

Comparative Examples 12 to 15 did not satisfy the Al, Ti, B and N content for controlling the edge cracks in the present invention, the edge cracks were inferior to the surface quality.

In addition, in the case of Comparative Examples 2 to 5, 10 to 12 and 15 containing ferrite in excess of 4.5 area%, it can be confirmed that the tensile strength is inferior.

Although described with reference to the embodiments above, those skilled in the art will understand that the present invention can be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

(Explanation of the sign)

a: slab b: coil

100: continuous casting machine 200: heater

300: Roughing Mill Scale Breaker (RSB)

400: roughing mill

500: Finishing Mill Scale Breaker (FSB)

600: finish rolling mill 700: runout table

800: high speed shear 900: winder

Claims (19)

1. By weight%, C: 0.18-0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01-0.05%, Ti: 0.01-0.10 %, B: 0.001-0.0045%, N: 0.001-0.01%, containing the remaining Fe and inevitable impurities,

   Microstructure is an ultra-high strength hot rolled steel sheet having excellent bending workability including 90% or more of martensite, 4-10% of auto-tempered martensite, and 5% or less of retained austenite.
2. The method of claim 1,

   The hot rolled steel sheet is in weight percent, Nb: 0.001 ~ 0.05%, Cr: 0.5 ~ 1.0%, Mo: 0.001 ~ 0.05% and Sb: 0.005 ~ 0.02% of the ultra-high strength excellent excellent bending workability further comprising Hot rolled steel sheet.
3. The method of claim 1,

   The hot rolled steel sheet is an ultra high strength hot rolled steel sheet having excellent bending workability including at least one of Cu, Ni, Sn, and Pb as a tramp element, and the sum thereof is 0.2% by weight or less.
4. The method of claim 1,

   The hot rolled steel sheet is a super high strength hot rolled steel sheet having excellent bending workability Ceq is 0.25 ~ 0.45 defined by the following relational formula 1.

   Relationship 1: Ceq = C + Si / 30 + Mn / 20 + 2P + 3S

   (In the above relation 1, each element symbol is a value indicating the weight of each element.)
5. The method of claim 1,

   Ultra high strength hot rolled steel sheet having excellent bending workability of the lattice short axis spacing of the martensite is 5㎛ or less.
6. The method of claim 1,

   The microstructure is a super high strength hot rolled steel sheet having excellent bending workability further comprises a ferrite 4.5% or less in an area fraction.
7. The method of claim 1,

   The hot rolled steel sheet has a tensile strength of 1.8 GPa or more, and bendability of 3 or less, ultra high strength hot rolled steel sheet.

   (The bendability means the value obtained by dividing the minimum bending radius (R) by which the crack does not occur after the 90 ° bending test by the steel plate thickness (t).)
8. The method of claim 1,

   The hot rolled steel sheet is ultra-high strength hot rolled steel sheet having excellent bending workability of 2.0 mm or less in thickness.
9. The method of claim 1,

   The hot rolled steel sheet is ultra-high strength hot rolled steel sheet having excellent bending processability in which the width variation (Crown) is 40 μm or less.
10. By weight%, C: 0.18-0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01-0.05%, Ti: 0.01-0.10 Continuous casting of molten steel containing%, B: 0.001-0.0045%, N: 0.001-0.01%, remaining Fe and inevitable impurities with a thin slab of 60-120 mm;

    Roughly rolling the heated thin slab so that the edge portion temperature of the bar plate may be 850 to 1000 ° C. at the rough rolling exit side to obtain a bar plate;

    Finishing rolling the bar plate in a temperature range of Ar 3 + 10 ° C. to Ar 3 + 100 ° C. to obtain a hot rolled steel sheet;

    Air-cooling the hot rolled steel sheet for 1 to 3 seconds, then cooling at a cooling rate of 200 ° C./sec or more, and winding the sheet at Mf-50 ° C. or less; And

    And stacking the wound hot rolled steel sheet between two other wound hot rolled steel sheets, wherein the steps are successively performed, and the super high strength hot rolled steel sheet has excellent bending workability.
11. The method of claim 10,

    The continuous casting step is a method of producing a super high strength hot rolled steel sheet having excellent bending workability using a mold flux having a basicity of 1.0 or more.
12. The method of claim 10,

    Casting method of the continuous casting is 4 ~ 8mpm bending method of producing a super high strength hot rolled steel sheet excellent in workability.
13. The method of claim 10,

    The rough rolling is a method of producing a super high strength hot rolled steel sheet having excellent bending workability is performed so that the surface temperature of the thin slab at the rough rolling entrance side is 1000 ~ 1200 ℃.
14. The method of claim 10,

    Spraying cooling water at a pressure of 100 to 200 bar or more to the heated thin slab before the rough rolling; further comprising:

    After the rough rolling step of removing the scale by sequentially passing through the bar plate to the first row of spraying the coolant at a pressure of 50 ~ 250bar and the second row of spraying the coolant at a pressure of 100 ~ 300bar; Method for producing this excellent super high strength hot rolled steel sheet.
15. The method of claim 10,

    Skin pass rolling of the stacked hot-rolled steel sheet at 50 ~ 150 ℃; Superb high strength steel sheet manufacturing method further comprising a.
16. The method of claim 10,

    Pickling the stacked hot-rolled steel sheet to obtain a pickled (PO) material (Pickled & Oiled) material; The method of manufacturing a super high strength hot rolled steel sheet further comprising a.
17. The method of claim 10,

    The molten steel is by weight%, Nb: 0.001 ~ 0.05%, Cr: 0.5 ~ 1.0%, Mo: 0.001 ~ 0.05% and Sb: 0.005 ~ 0.02% of ultra-high strength hot rolled steel having excellent bending workability further comprising Method of manufacturing steel sheet.
18. The method of claim 10,

    The molten steel comprises at least one of Cu, Ni, Sn, and Pb as a tramp element, the total manufacturing method of the ultra-high strength hot rolled steel sheet excellent in bending workability of 0.2% by weight or less.
19. The method of claim 10,

    The molten steel is a method of producing a super high strength hot rolled steel sheet having excellent bending workability Ceq is 0.25 ~ 0.45 defined by the following relational formula 1.

    Relationship 1: Ceq = C + Si / 30 + Mn / 20 + 2P + 3S

    (In the above relation 1, each element symbol is a value indicating the weight of each element.)

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