WO2018110867A1 - High strength cold rolled steel plate having excellent yield strength, ductility, and hole expandability, hot dip galvanized steel plate, and method for producing same - Google Patents

Abstract

Provided are a high strength cold rolled steel plate having excellent yield strength, ductility, and hole expandability, a hot dip galvanized steel plate, and a method for producing the same. The cold rolled steel plate of the present invention comprises 0.06-0.2 wt% of carbon (C), 1.5-3.0 wt% of manganese (Mn), 0.3-2.5 wt% of silicon (Si), 0.01-0.2 wt% of aluminum (Al), 0.01-3.0 wt% of nickel (Ni), 0.2 wt% or less of molybdenum (Mo), 0.01-0.05 wt% of titanium (Ti), 0.02-0.05 wt% of antimony (Sb), 0.0005-0.003 wt% of boron (B), 0.01 wt% or less (but not 0%) of nitrogen (N), with the remainder comprising Fe and unavoidable impurities, and the microstructure thereof comprises, in terms of area fraction, 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), 20% or less of residual austenite, and 5% or less of ferrite.

Description

High strength cold rolled steel, hot dip galvanized steel with excellent yield strength, ductility and hole expansion, and method of manufacturing the same

The present invention relates to a high strength steel sheet used in an automobile body, and more particularly, to a high strength cold rolled steel sheet, a hot dip galvanized steel sheet, and a method of manufacturing the same, which have high strength and excellent yield strength and formability. .

Many attempts have been made to improve the strength of existing steels in order to achieve a lighter weight by lowering the thickness of steel sheets applied as structural members of vehicles such as building materials, automobiles and trains. However, in the case of increasing the strength, it was found that the yield strength was relatively low and the ductility and hole expandability were lowered.

Accordingly, many studies have been made to improve the relationship between strength and ductility, and as a result, transformation tissue steels utilizing residual austenite phases together with martensite and bainite, which are low-temperature structures, have been developed and applied.

The metamorphic steel is classified into so-called DP (Dual Phase) steel, Transformation Induced Plasticity (TRIP) steel, and Complex Phase (CP) steel. Each of these steels has mechanical properties, that is, according to the type and fraction of the parent phase and the second phase. The level of tensile strength and elongation is different, especially in the case of TRIP steel containing residual austenite, the balance of tensile strength and elongation (TS x El) shows the highest value.

CP steel of the metamorphic structure steel as described above is lower than the other steels, and is limited to simple processing such as roll forming, and high ductility DP steel and TRIP steel are applied to cold press forming.

Therefore, in recent years, there has been proposed a technique for suppressing deep workability and flange crack by increasing ductility and improving hole expansion performance than the transformation steels DP steel and TRIP steel. For example, Patent Document 2 discloses a method of forming residual austenite and martensite as a main structure (Quenching and Partitioning Process (Q & P)). According to a report using this (non-patent document 1), the carbon level is 0.2%. In the case of low, the yield strength has a disadvantage of low around 400MPa, and it can be confirmed that the elongation obtained in the final product is only similar to the existing TRIP steel. The essence of the Q & P method is to secure ductility by quenching between the martensite transformation start temperature (Ms) and the finish temperature (Mf) and then reheating to stabilize the austenite by carbon diffusion at the martensite and austenite interface. However, due to the presence of a significant amount of austenite that cannot be stabilized by quenching and partitioning temperature, fresh martensite (FM) is formed in the final cooling step, and the fresh martensite has a high carbon content and inhibits pore expansion (Patent Document 3).

As another method, there is a method of securing the ductility and hole expandability by heat-treating the martensite structure again in an abnormal region, but this is not economical by performing two heat treatments (Patent Document 4).

Finally, a method of obtaining a bainite structure by heat treatment by a conventional annealing method but rapidly cooling to bainite forming section and then keeping constant temperature for a long time has been developed. Since it forms a hole expansion is not excellent.

[Preceding technical literature]

{Patent Literature}

(Patent Document 1) Korean Unexamined Patent Publication No. 1994-0002370

(Patent Document 2) US Publication No. 2006-0011274

(Patent Document 3) Japanese Patent Application JP2002-177278

(Patent Document 4) Japanese Patent Publication JP2001-300503

(Patent Document 5) Japanese Patent Publication JP2014-018431

{Non Patent Literature}

(Non-Patent Document 1) ISIJ International, Vol. 51, 2011, p. 137-144

Therefore, the present invention has been made to solve the above-mentioned limitations of the prior art, and implements a lower alloy cost compared to the existing TWIP steel, compared to the case of applying the conventional TBF (Trip aided Bainitic Ferrite) Q & P (Quenching and Partitioning) heat treatment process. It is an object of the present invention to provide a cold rolled steel sheet of bainite columnar having better ductility and hole expandability, a hot-dip galvanized steel sheet produced using the same, an alloyed hot-dip galvanized steel sheet, and a method of manufacturing the same.

In addition, the technical problems to be achieved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above are clearly understood by those skilled in the art from the following description. Could be.

The present invention for achieving the above object,

By weight%, carbon (C): 0.06 to 0.2%, manganese (Mn): 1.5 to 3.0%, silicon (Si): 0.3 to 2.5%, aluminum (Al): 0.01 to 0.2%, nickel (Ni): 0.01 -3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01-0.05, antimony (Sb): 0.02-0.05, boron (B): 0.0005-0.003, nitrogen (N): 0.01% or less (Excluding 0%), balance Fe and inevitable impurities,

The microstructure yields 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), 20% or less of residual austenite and 5% or less of ferrite. It relates to a high strength cold rolled steel sheet excellent in strength, ductility and hole expansion properties.

Preferably, the TM / FM ratio is greater than two.

The present invention also relates to a hot-dip galvanized steel sheet hot-dip galvanized on the surface of the cold-rolled steel sheet, and an alloyed hot-dip galvanized steel sheet which has been alloyed hot-dip galvanized.

In addition, the present invention,

By weight%, carbon (C): 0.06 to 0.2%, manganese (Mn): 1.5 to 3.0%, silicon (Si): 0.3 to 2.5%, aluminum (Al): 0.01 to 0.2%, nickel (Ni): 0.01 -3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01-0.05, antimony (Sb): 0.02-0.05, boron (B): 0.0005-0.003, nitrogen (N): 0.01% or less (Excluding 0%), reheating the steel slab containing the remaining Fe and unavoidable impurities, followed by hot rolling and winding up; And

And cold rolling the wound hot rolled steel sheet, followed by Q & P continuous annealing.

The Q & P continuous annealing process,

 Cracking the produced cold rolled steel sheet at a temperature of Ac 3 or more for 30 seconds or more, and then cooling to a quenching temperature (QT) ± 10 ° C. defined by the following Equation 1 at a cooling rate of 5 to 20 ° C./sec;

Reheating the cooled steel sheet to bainite temperature (PT) ± 10 ° C. as defined by the following relation 2, and then maintaining it for 100 seconds or more within a temperature range of QT ≧ or ≧ QT-100 ° C., followed by cooling It relates to a high strength cold rolled steel sheet manufacturing method excellent in yield strength, ductility and hole expansion characterized in that it comprises a.

[Relationship 1]

QT = 493.497 + 36.2874 x Al-394.0 x C-45.0 x Mn-11.4332 x Mo-20.8772 x Ni-13.0438 x Si-12.8 x Cr

[Relationship 2]

PT = 599.088 + 11.5214 x Al-225.2 x C-35.0 x Mn-19.9474 x Ni-24.9385 x Si-56.718 x Mo-22.1 x Cr

The steel sheet after the continuous Q & P continuous annealing has a microstructure of 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), and 20% or less of residual austenite. And 5% or less of ferrite.

Preferably, the TM / FM ratio is greater than two.

The present invention also includes a process of hot-dip galvanizing the surface of the cold-rolled steel sheet continuously annealed Q &P; and a process of hot-dip galvanizing of the surface of the cold-rolled steel sheet continuously annealed Q &P; It relates to a method for producing an alloyed hot-dip galvanized steel sheet further comprising.

According to the present invention of the above-described configuration, compared with the conventional high-ductile transformation tissue steel such as DP steel or TRIP steel and Q & P steel subjected to conventional Q & P (Quenching & Partitioning) heat treatment, it is possible to ensure accurate TM amount and bainite It can effectively provide high strength cold rolled steel sheet, hot dip galvanized steel sheet and alloyed hot dip galvanized steel sheet with yield strength and ductility and hole extension property excellent in tensile strength of 980MPa or more.

Therefore, the cold rolled steel sheet of the present invention has the advantage of high availability in the industrial field, such as building member, automotive steel sheet.

Figure 1 shows an example of the annealing process according to the present invention (in Fig. 1 the dotted line of the heat treatment line shows the thermal history during the molten alloy plating).

Figure 2 shows the low temperature transformation behavior of the TBF method and the present invention method.

Figure 3 is a photograph observing the microstructure of the inventive example (F) steel produced by the present invention.

Figure 4 is a result of observing the carbide in the tempered martensite of the cold rolled steel sheet produced by the present invention.

5 is a photograph observing the microstructure of the steel of Comparative Example (E).

The present inventors have studied in depth how to improve the low ductility of high strength steel manufactured by conventional Q & P (Quenching & Partitioning) heat treatment, so that the bainite transformation is promoted in a specific temperature range more sophisticated than the prior art during Q & P heat treatment. A heat treatment condition was found to decrease significantly. By controlling the QT and PT in the amount of martensite formation and bainite transformation by quenching, it is confirmed that the microstructure of the tissue after the final Q & P heat treatment and the improvement of physical properties of the final product are possible, and the present invention is proposed.

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

First, the composition of the alloy components, such as the cold-rolled steel sheet provided by the present invention and the reason for the content limitation will be described in detail. At this time, the content of each component means weight% unless otherwise specified.

C: 0.06-0.2%,

Carbon (C) is an effective element for reinforcing steel, and is an important element added in the present invention for stabilizing residual austenite and securing strength. In order to obtain the above-mentioned effect, it is preferable to add at 0.06% or more. If the content is less than 0.06%, the austenite single-phase temperature is very high, and high temperature annealing is inevitable and strength and ductility are difficult to secure. In addition, when the content exceeds 0.2%, Ms is lowered and the quenching temperature is lowered, so that precise heat treatment is difficult. In addition, there is a problem that the weldability is also greatly reduced. Therefore, the content of C in the present invention is preferably limited to 0.06 ~ 0.2%.

Mn: 1.5 ~ 3.0%

Manganese (Mn) is an element effective in forming and stabilizing residual austenite while controlling the transformation of ferrite. If the Mn content is less than 1.5%, a large amount of ferrite transformation occurs, thereby making it difficult to secure the target strength. On the other hand, when the Mn content exceeds 3.0%, the phase transformation in the second annealing heat treatment step of the present invention is too delayed. As a large amount of martensite is formed, there is a problem that it is difficult to secure the intended ductility. Therefore, the content of Mn in the present invention is preferably limited to 1.5 ~ 3.0%.

Si: 0.3 ~ 2.5%

Silicon (Si) is an element that suppresses the precipitation of carbides in ferrite and promotes diffusion of carbon in the ferrite into austenite and consequently contributes to the formation of bainite and stabilization of residual austenite. In order to obtain the above-mentioned effect, it is preferable to add 0.3% or more, but when the content exceeds 2.5%, hot and cold rolling properties are very poor, and there is a problem of inhibiting plating property by forming an oxide on the steel surface. have. Therefore, the content of Si in the present invention is preferably limited to 0.3 ~ 2.5%.

Al: 0.01 ~ 0.2%

Aluminum (Al) is an element that deoxidizes by combining with oxygen in the steel, for this purpose it is preferable to maintain the content of 0.01% or more. In addition, Al contributes to stabilization of residual austenite and increases bainite formation temperature through suppression of carbide formation in ferrite as in Si. However, when the Al content exceeds 0.2%, the A3 temperature increases, so that high temperature annealing is inevitable, and it becomes difficult to manufacture a healthy slab through reaction with the mold plus during casting, and also form a surface oxide to form a plating property. There is a problem that inhibits. Therefore, the content of Al in the present invention is preferably limited to 0.01 ~ 0.2%.

Nickel (Ni): 0.01 ~ 3.0%

Nickel is preferably an element that stabilizes strength by solid solution strengthening and stabilizes austenite. However, since the effect of delaying bainite transformation is too large, the addition of too much bainite transformation is not achieved, so that the upper limit is preferably limited to 3%.

Molybdenum (Mo): 0.2% or less

Mo is also added to strengthen the strength by strengthening the solid solution to form TiMo carbide to refine the bainite structure, but it is preferable to limit the upper limit to 0.2% due to the high cost of ferroalloy.

Titanium (Ti): 0.01 ~ 0.05

Since titanium forms TiN first, it is essential to improve the hardenability by addition of solid solution boron. In the present invention, it is preferable to limit the upper limit to 0.01% because the lower limit is 0.01% so that TiN can be formed in preference to BN, and if too much TiN is crystallized to cause nozzle clogging during playing.

Antimony (Sb): 0.02-0.05

Since antimony forms a grain boundary oxide as a grain boundary segregation material, it is preferable to add 0.02% or more as a means for suppressing decarburization through grain boundaries and a decrease in zinc plating property due to surface concentration of Mn and Si. However, if too much, the grain boundary segregation increases, causing brittleness of the steel, so the upper limit was limited to 0.05%.

Boron (B): 0.0005-0.003

Boron is an inexpensive alloy element that is easy to secure strength due to quenching, and has an effect of reducing the total amount of alloys, and has a lower limit of 0.005% as an element advantageous for suppressing weldability and high temperature brittleness. However, if too much, the BN forming temperature is higher than TiN, causing high temperature brittleness of the steel, so it is preferable to limit the upper limit to 0.003%.

Nitrogen (N): 0.01% or less

Since nitrogen reduces the alloying efficiency of alloying elements by forming BN and TiN, it is preferable to limit it to 0.01% or less, which is usually in a controllable range.

The remaining component of the present invention is iron (Fe). In other conventional steelmaking processes, undesired impurities from raw materials or the surrounding environment can be inevitably incorporated. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

On the other hand, the cold-rolled steel sheet of the present invention that satisfies the above-mentioned steel composition components has an area fraction of 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), and residual austenite 20 It has a microstructure comprising less than% and less than 5% ferrite. Herein, the bainite has the highest strength after martensite and has intermediate properties between ferrite and martensite, and when the fine residual austenite is distributed inside the bainite phase, the strength and ductility of the steel are very high.

The cold rolled steel sheet of the present invention that satisfies the above-described microstructure has a tensile strength of 980 MPa or more, and has high yield strength and press formability, ductility, and hole expansion properties compared to steel sheets manufactured through conventional Q & P heat treatment. High strength steel sheet can be provided.

The present invention may also provide a hot dip galvanized steel sheet subjected to hot dip galvanizing on the surface of the cold rolled steel sheet, and an alloy hot dip galvanized steel sheet obtained by alloying and heat-treating the hot dip galvanized steel sheet.

Next, the manufacturing method of the cold rolled steel plate etc. of this invention is demonstrated in detail.

The cold rolled steel sheet according to the present invention may be manufactured by reheating, hot rolling, winding, cold rolling, and annealing a steel slab satisfying the above-described steel composition, which will be described below.

Steel slab reheating process

In the present invention, it is preferable to go through the process of re-heating and homogenizing the steel slab prior to performing the hot rolling, which is more preferably carried out in the temperature range of 1000 ~ 1300 ℃.

If the temperature is less than 1000 ° C during reheating, the rolling load may increase rapidly. On the other hand, if the temperature exceeds 1300 ° C, not only energy costs increase, but the amount of surface scale may be excessive. Therefore, in the present invention, the reheating step is preferably carried out at 1000 ~ 1300 ℃.

(Hot rolling process)

The reheated steel slab is hot rolled to produce a hot rolled steel sheet, wherein hot finish rolling is preferably performed at 800 to 950 ° C.

If the rolling temperature is less than 800 ℃ during the hot finish rolling, there is a problem that the rolling load is increased to increase the difficulty of rolling, while if the hot finishing rolling temperature exceeds 950 ℃, the thermal fatigue of the rolling roll increases a lot of water shortening Cause. Therefore, in the present invention, it is preferable to limit the hot finish rolling temperature to 800 to 950 ° C during hot rolling.

(Winding process)

Subsequently, the hot rolled steel sheet manufactured according to the above is wound, wherein the winding temperature is preferably 750 ° C. or less.

If the winding temperature is too high during winding, the scale is invented excessively on the surface of the hot-rolled steel sheet, causing surface defects and deteriorating the plating property. Therefore, it is preferable to perform a winding process at 750 degreeC or less. At this time, the lower limit of the coiling temperature is not particularly limited, but considering the difficulty of subsequent cold rolling due to excessively high strength of the hot rolled sheet due to the formation of martensite, it is carried out at Ms (Martensite transformation start temperature) to 750 ° C. More preferred.

(Cold rolling process)

The wound hot rolled steel sheet is pickled to remove an oxide layer, and then cold rolled to produce a cold rolled steel sheet to match the shape and thickness of the steel sheet.

In general, cold rolling is carried out to secure the thickness required by the customer, and there is no limitation on the reduction ratio, but cold rolling reduction is performed at 30% or more to suppress the formation of coarse ferrite grains during recrystallization in a subsequent annealing process. It is preferable.

(Q & P Continuous Annealing Process)

In the present invention, the final microstructure is 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), 20% or less of residual austenite and 5% or less of ferrite In order to prepare the control of the subsequent annealing process is important. Particularly, in the present invention, in order to secure a desired microstructure from redistribution of elements such as carbon and manganese during annealing, conventional Q & P continuous annealing process is adopted after cold rolling, but QT and PT are alloy elements as described below. Control according to the.

Crack and quench

First, the produced cold-rolled steel sheet is cracked for more than 30 seconds at a temperature of Ac3 or more, and then cooled to a quenching temperature (QT) ± 10 ° C defined by the following relation 1 at a cooling rate of 5 to 20 ° C / sec. (See Figure 1).

This is to obtain a ferrite structure that is detrimental to pore expandability within 5%, the ferrite unformed cooling rate of the present invention was designed to be 5 ~ 20 ℃. There is no problem even if the cooling rate is higher than this, but the slower the cooling rate, the better the plate shape without distortion, and does not need to be increased.

QT cools down to 20-50% martensite. Martensite formed during quenching at Q & P, when reheated and partitioned to PT, causes tempering and lowers strength, and promotes bainite formation. As shown in FIG. 2, even when partitioning is performed at the same temperature, the TBF rapidly cooled to the bainite reverse temperature and maintained at a constant temperature does not completely bainite precipitation after 600 seconds, whereas FM is formed, but sufficiently forms martensite. In this case, it can be seen that even in a short time, the bainite transformation is completely performed, and thus no FM is formed. Thus, to minimize the FM in the present invention, the elements such as carbon and manganese are concentrated in the austenite remaining in the bainite transformation process, so that the FM does not remain austenite and is transformed during final cooling. This is because the strength is very high, and the interfacial separation occurs during the hole expansion, so that the cracks are easily broken, thereby greatly reducing the hole expandability.

In the present invention, these properties are newly discovered and newly developed high-strength high-strength steel having bainite columnar, and the QT which promotes bainite formation and the bainite area ratio is obtained through experiments as follows.

[Relationship 1]

QT = 493.497 + 36.2874 x Al-394.0 x C-45.0 x Mn-11.4332 x Mo-20.8772 x Ni-13.0438 x Si-12.8 x Cr

Partitioning Heat Treatment

Subsequently, in the present invention, the cooled steel sheet is reheated to a bainite temperature (PT) ± 10 ° C. defined by Equation 2 below, and then maintained for 100 seconds or more within a temperature range of QT≥ or ≥ QT-100 ° C. After that, it is cooled.

After the above-mentioned quenching, the temperature at which bainite is formed fastest in the reheating and constant temperature holding at bainite temperature PT was determined by experiment. If the temperature is higher than this, the amount of bainite formation is small, the stabilization of residual austenite is insufficient and the FM formation is rather increased. Therefore, PT must be heated to PT ± 10 ℃.

[Relationship 2]

PT = 599.088 + 11.5214 x Al-225.2 x C-35.0 x Mn-19.9474 x Ni-24.9385 x Si-56.718 x Mo-22.1 x Cr

Unlike the prior art, the present invention does not need to maintain constant temperature at a constant temperature. Constant temperature maintenance has the advantage that it is easy to apply to a facility having a constant temperature furnace without a heating maintenance device because it only needs to cool after maintaining for more than 100 seconds within the temperature range of QT≥ or ≥ QT-100 ℃.

In this way, Q & P heat treatment produces steel containing 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), 20% or less of residual austenite and 5% or less of ferrite. By minimizing ferrite and FM with a large difference in strength, it is possible to manufacture a high-strength giga-grade high-strength steel sheet having excellent yield strength, ductility, and hole expansion properties compared to steel sheets manufactured through conventional Q & P heat treatment.

(Plated)

The first and second annealing heat-treated cold rolled steel plate may be plated to produce a plated steel sheet. At this time, the plating treatment is preferably carried out by a hot dip plating method or an alloyed hot dip plating method, and the plating layer formed from them is preferably zinc-based.

When the hot dip plating method is used, the hot dip galvanizing bath may be manufactured as a hot dip galvanized steel sheet, and in the case of the hot dip galvanizing method, an alloy may be manufactured by performing a conventional alloy hot dip plating process.

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

(Example)

The molten metal having the composition shown in Table 1 was prepared in a 90 mm thick, 175 mm wide ingot through vacuum melting. Subsequently, it was reheated for 1 hour at 1200 ° C. for homogenization treatment, and hot-rolled and rolled at 900 ° C. or higher, which is a temperature of Ar 3 or higher, to prepare a hot rolled steel sheet. Thereafter, the hot rolled steel sheet was cooled, charged into a furnace preheated to 600 ° C., held for 1 hour, and then cold rolled to simulate hot rolled winding. The hot rolled sheet material as described above was cold rolled at a cold reduction rate of 50 to 60%, and then subjected to annealing heat treatment under the conditions of Table 2 to produce a final cold rolled steel sheet.

For each cold rolled steel sheet prepared as described above, the tissue fraction, yield strength, tensile strength, elongation, and HER were measured, and the results are also shown in Table 2 below.

	C	Mn	Si	Al	Cr	Ni	Mo	Ti	Sb	B	N
Inventive Steel A	0.1	2.2	1.4	0.02	0.6	0.02	0.04	0.017	0.027	0.0021	0.005
Inventive Steel B	0.14	1.9	1.4	0.03	0.5	0.02	0.04	0.021	0.04	0.0017	0.003
Inventive Steel C	0.18	2.5	0.6	0.02	1.1	0.02	0.05	0.014	0.021	0.0012	0.004
Inventive Steel D	0.16	2.1	1.9	0.04	0.3	0.02	0.05	0.022	0.035	0.0023	0.006
Inventive Steel E	0.09	2.2	1.4	0.15	0.6	0.02	0.06	0.031	0.027	0.0018	0.005
Inventive Steel F	0.13	1.8	1.3	0.02	0.4	2	0.02	0.027	0.032	0.0017	0.004
Invention Steel G	0.08	2.5	1.3	0.06	0.3	0.03	0.18	0.019	0.041	0.0024	0.005
Comparative Steel H	0.05	2.1	1.6	0.02	0.45	0.06	0.05	0.015	0.03	0.0017	0.003
Comparative Steel I	0.18	1.8	0.21	0.02	0.28	0.02	0.04	-	0.04	-	0.005
Comparative Steel J	0.08	One	1.3	0.04	0.9	0.02	0.02	0.012	0.02	0.0011	0.004
Comparative Steel K	0.17	2.2	1.3	0.25	0.3	0.02	0.05	0.027	0.04	0.0018	0.004
Comparative Steel L	0.18	1.6	1.5	0.02	1.6	3.5	0.05	0.032	0.021	0.0013	0.005

In Table 2, B represents bainite, TM represents tempered martensite, FM represents fresh martensite, A represents residual austenite, and F represents ferrite.

As shown in Table 1, it can be seen that not only the steel composition but also the production process satisfying the scope of the present invention (A-G) all show excellent yield strength, ductility and hole expandability.

Figure 3 is a photograph observing the microstructure of the inventive example (F) steel produced by the present invention. As shown in Table 2, inventive steel (F) steel has a bainite of 75%, a TM, FM of 14%, and 5% of TM / FM of more than 2, and F of less than 5% of bays. It can be seen that the night steel can be produced. This is a technical feature of the present invention, but conventionally focused on making ferritic TRIP steel or tempered martensitic steel through Q & P heat treatment, but if the steel composition and QT, PT are specified, the bainite matrix structure Can be made easier than the TBF heat treatment method.

Meanwhile, FIG. 4 is an APT observation of TM in the tissue of FIG. 3. The transition carbide and coarse cementite are mixed to show tempered martensite.

On the contrary, it can be seen that the comparative examples (H-L, B, E, G) in which the steel composition or the manufacturing process are out of the scope of the present invention are all poor in yield strength, ductility, and hole expandability compared to the present invention.

 In particular, as shown in Table 2, Comparative Examples (B, E, G) in which the steel composition component satisfies the scope of the present invention but the manufacturing process does not depend on the present invention can be seen that the desired physical properties are not obtained. have.

Figure 5 is the same component as the structure of the steel (E), but the ferrite and FM are formed due to the annealing annealing and TBF heat treatment can be confirmed that the strength and HER is low.

As a result, the cold rolled steel sheet produced according to the present invention can secure a yield strength and excellent elongation and HER of 980MPa or more, the cold forming for applying to the structural member compared to the steel produced through the conventional Q & P heat treatment process There is an advantage that can be easily performed.

As described above, in the detailed description of the present invention has been described with respect to preferred embodiments of the present invention, those skilled in the art to which the present invention pertains various modifications without departing from the scope of the present invention Of course this is possible. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined by the equivalents as well as the claims to be described later.

Claims (9)

1. By weight%, carbon (C): 0.06 to 0.2%, manganese (Mn): 1.5 to 3.0%, silicon (Si): 0.3 to 2.5%, aluminum (Al): 0.01 to 0.2%, nickel (Ni): 0.01 -3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01-0.05, antimony (Sb): 0.02-0.05, boron (B): 0.0005-0.003, nitrogen (N): 0.01% or less (Excluding 0%), balance Fe and inevitable impurities,

   The microstructure yields 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), 20% or less of residual austenite and 5% or less of ferrite. High strength cold rolled steel with excellent strength, ductility and hole expansion.
2. The high strength cold rolled steel sheet having excellent yield strength, ductility, and hole expansion property according to claim 1, wherein the TM / FM ratio exceeds 2.
3. Hot-dip galvanized steel sheet is hot-dip galvanized on the surface of the cold-rolled steel sheet of claim 1.
4. An alloyed hot-dip galvanized steel sheet obtained by alloying hot-dip galvanized on the surface of the cold-rolled steel sheet of claim 1.
5. By weight%, carbon (C): 0.06 to 0.2%, manganese (Mn): 1.5 to 3.0%, silicon (Si): 0.3 to 2.5%, aluminum (Al): 0.01 to 0.2%, nickel (Ni): 0.01 -3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01-0.05, antimony (Sb): 0.02-0.05, boron (B): 0.0005-0.003, nitrogen (N): 0.01% or less (Excluding 0%), reheating the steel slab containing the remaining Fe and unavoidable impurities, followed by hot rolling and winding up; And

   And cold rolling the wound hot rolled steel sheet, followed by Q & P continuous annealing.

   The Q & P continuous annealing process,

    Cracking the produced cold rolled steel sheet at a temperature of Ac 3 or more for 30 seconds or more, and then cooling to a quenching temperature (QT) ± 10 ° C. defined by the following Equation 1 at a cooling rate of 5 to 20 ° C./sec;

   Reheating the cooled steel sheet to bainite temperature (PT) ± 10 ° C. as defined by the following relation 2, and then maintaining it for 100 seconds or more within a temperature range of QT ≧ or ≧ QT-100 ° C., followed by cooling Yield strength, ductility and hole expansion properties excellent cold rolled steel sheet manufacturing method comprising a.

   [Relationship 1]

   QT = 493.497 + 36.2874 x Al-394.0 x C-45.0 x Mn-11.4332 x Mo-20.8772 x Ni-13.0438 x Si-12.8 x Cr

   [Relationship 2]

   PT = 599.088 + 11.5214 x Al-225.2 x C-35.0 x Mn-19.9474 x Ni-24.9385 x Si-56.718 x Mo-22.1 x Cr
6. According to claim 5, The Q & P continuous annealing steel sheet, the microstructure of the area fraction 50% or more bainite, 10% or more tempered martensite (TM), 10% or less fresh martensite (FM), A method for producing cold rolled steel sheet having excellent yield strength, ductility, and hole expandability, comprising 20% or less of residual austenite and 5% or less of ferrite.
7. 7. The method of claim 6, wherein the TM / FM ratio is greater than two.
8. A method of manufacturing a hot-dip galvanized steel sheet excellent in yield strength, ductility, and hole expandability, further comprising the step of hot-dip galvanizing the surface of the Q & P continuous annealed cold-rolled steel sheet of claim 5.
9. A method of manufacturing an alloyed hot-dip galvanized steel sheet excellent in yield strength, ductility and hole expansion, comprising: a step of alloying hot-dip galvanized on the surface of the Q & P continuous annealed cold-rolled steel sheet of claim 5.

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