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CN110073026B - High-strength cold-rolled steel sheet and hot-dip galvanized steel sheet having excellent yield strength, ductility and hole expansibility, and methods for producing same - Google Patents

High-strength cold-rolled steel sheet and hot-dip galvanized steel sheet having excellent yield strength, ductility and hole expansibility, and methods for producing same Download PDF

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CN110073026B
CN110073026B CN201780077454.0A CN201780077454A CN110073026B CN 110073026 B CN110073026 B CN 110073026B CN 201780077454 A CN201780077454 A CN 201780077454A CN 110073026 B CN110073026 B CN 110073026B
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steel sheet
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rolled steel
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CN110073026A (en
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郭在贤
曺恒植
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Posco Holdings Inc
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    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

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  • Chemical & Material Sciences (AREA)
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Abstract

The present invention provides a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet and a method for manufacturing the same, the high-strength cold-rolled steel sheet, the hot-dip galvanized steel sheet and the method being excellent in yield strength, ductility and hole expansibility, the cold-rolled steel sheet of the present invention includes, in weight%: carbon (C): 0.06-0.2%, 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 to 3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01 to 0.05%, antimony (Sb): 0.02 to 0.05%, boron (B): 0.0005 to 0.003%, nitrogen (N): 0.01% or less (excluding 0%), and the balance Fe and unavoidable impurities, and the microstructure thereof contains, 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 retained austenite, and 5% or less of ferrite.

Description

High-strength cold-rolled steel sheet and hot-dip galvanized steel sheet having excellent yield strength, ductility and hole expansibility, and methods for producing same
Technical Field
The present invention relates to a high-strength steel sheet for a vehicle body, and more particularly, to a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a method for manufacturing the same, which have high strength, and at the same time, excellent yield strength and formability, and thus excellent press formability.
Background
In order to reduce the weight by reducing the thickness of a steel sheet used as a structural member of a vehicle such as a building material, a vehicle, a train, or the like, many attempts have been made to improve the strength of a conventional steel material. However, when the strength is increased as described above, the yield strength is relatively low, and the ductility and hole expansibility are relatively lowered.
Therefore, a great deal of research has been conducted to improve the relationship between strength and ductility, and as a result, in recent years, transformation structure steels utilizing martensite, bainite, and retained austenite phases, which are low-temperature structures, have been developed and applied.
The Phase Transformation structure steel is classified into so-called Dual Phase (DP) steel, Transformation Induced Plasticity (TRIP) steel, Complex Phase (Complex Phase) steel, etc., and mechanical properties of each steel, i.e., levels of tensile strength and elongation, vary according to the kind and fraction of a parent Phase and a second Phase, and particularly, the balance (TS × El) of tensile strength and elongation of the TRIP steel containing retained austenite shows the highest value.
Among the above-mentioned phase transformation structure steels, CP steels have a low elongation compared to other steels, so that they are limited to simple processing such as roll forming, and DP steels and TRIP steels having high ductility are applied to cold press forming, etc.
Therefore, in recent years, a technique has been proposed in which the ductility of DP steel and TRIP steel, which are the transformation-structure steels, is higher, and the hole-expanding performance is improved to suppress deep workability and flange portion cracks. For example, patent document 2 discloses Quenching and Partitioning (Q & P) as a method for forming retained austenite and martensite as a main structure, and according to a report (non-patent document 1) applying this method, when carbon is as low as a level of 0.2%, yield strength is as low as about 400MPa, and it is confirmed that elongation obtained in a final product is only similar to that of conventional TRIP steel. The core of the Q & P process is quenching between the martensite start temperature (Ms) and the finish rolling temperature (Mf), and then reheating to cause carbon diffusion in the martensite and austenite interfaces to stabilize the austenite, thereby securing ductility. However, depending on the quenching and partition temperatures, a large amount of unstable austenite exists, so that Fresh Martensite (FM) is formed in the final cooling step, and the carbon content of the fresh martensite is high, thereby hindering the hole expansibility (patent document 3).
Another method is a method of performing heat treatment again on the martensite structure to perform heat treatment in the dual phase region to secure ductility and hole expansibility, but this method is uneconomical because it performs heat treatment twice (patent document 4).
Finally, a method of performing heat treatment in a conventional annealing method and maintaining a constant temperature for a long time after rapid cooling to a bainite formation interval to obtain a bainite structure has been developed, but the time for maintaining the constant temperature is very long, and bainite that is not sufficiently transformed forms martensite at the time of final cooling, and thus the hole expansibility is poor.
Documents of the prior art
Patent document
(patent document 1) Korean laid-open patent publication No. 1994-0002370
(patent document 2) U.S. publication No. 2006-
(patent document 3) Japanese patent publication JP2002-177278
(patent document 4) Japanese patent publication JP2001-300503
(patent document 5) Japanese patent publication JP2014-018431
Non-patent document
(non-patent document 1) ISIJ International, Vol.51,2011, p.137-144
Disclosure of Invention
Technical problem to be solved
Accordingly, the present invention has been made to solve the above-mentioned limitations of the prior art, and an object of the present invention is to provide a cold-rolled steel sheet having a bainite main phase, which uses less alloy cost than conventional goldenrain induced plasticity (TWIP) steel and has more excellent ductility and hole expansibility than a case of using a conventional transformation induced plasticity Bainitic Ferrite (TBF) Quenching and Partitioning (Q & P) heat treatment process, a hot-dip galvanized steel sheet manufactured using the cold-rolled steel sheet, an alloyed hot-dip galvanized steel sheet, and a manufacturing method thereof.
In addition, the technical problems to be solved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned may be clearly understood by those of ordinary skill in the art to which the present invention pertains through the following descriptions.
(II) technical scheme
The present invention for achieving the above object relates to a high-strength cold-rolled steel sheet excellent in yield strength, ductility, and hole expansibility, the cold-rolled steel sheet comprising, in wt%: carbon (C): 0.06-0.2%, 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 to 3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01 to 0.05%, antimony (Sb): 0.02 to 0.05%, boron (B): 0.0005 to 0.003%, nitrogen (N): 0.01% or less (excluding 0%), and the balance Fe and unavoidable impurities, and the fine structure of the cold-rolled steel sheet 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 retained austenite, and 5% or less of ferrite.
Preferably, the TM/FM ratio exceeds 2.
The present invention also relates to a hot-dip galvanized steel sheet obtained by hot-dip galvanizing the surface of the cold-rolled steel sheet, and an alloyed hot-dip galvanized steel sheet obtained by alloyed hot-dip galvanizing the surface of the cold-rolled steel sheet.
The present invention also relates to a method for manufacturing a high-strength cold-rolled steel sheet having excellent yield strength, ductility, and hole expansibility, the method comprising: reheating, followed by hot rolling and then coiling a steel slab comprising, in weight%: carbon (C): 0.06-0.2%, 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 to 3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01 to 0.05%, antimony (Sb): 0.02 to 0.05%, boron (B): 0.0005 to 0.003%, nitrogen (N): 0.01% or less (excluding 0%), and the balance Fe and inevitable impurities; and cold rolling the rolled hot rolled steel plate, and then performing Q & P continuous annealing, wherein the Q & P continuous annealing process comprises the following processes: soaking the cold-rolled steel sheet manufactured at a temperature of Ac3 or more for 30 seconds or more, and then cooling the cold-rolled steel sheet at a cooling rate of 5 to 20 ℃/second to a Quenching Temperature (QT) ± 10 ℃ as defined by the following relational expression 1; reheating the cooled steel sheet to a bainite temperature (PT). + -. 10 ℃ defined by the following relational expression 2, followed by holding at a temperature range of PT ≥ or ≥ PT-100 ℃ for 100 seconds or longer, and then cooling,
[ relational expression 1]
QT＝493.497+36.2874×Al-394.0×C-45.0×Mn-11.4332×Mo-20.8772×Ni-13.0438×Si-12.8×Cr
[ relational expression 2]
PT＝599.088+11.5214×Al-225.2×C-35.0×Mn-19.9474×Ni-24.9385×Si-56.718×Mo-22.1×Cr。
The microstructure of the steel sheet to which the Q & P continuous annealing is completed may include 50% or more of bainite, 10% or more of Tempered Martensite (TM), 10% or less of Fresh Martensite (FM), 20% or less of retained austenite, and 5% or less of ferrite, in terms of area fraction.
Preferably, the TM/FM ratio exceeds 2.
In addition, the present invention relates to a method of manufacturing a hot-dip galvanized steel sheet, further comprising the steps of: the surface of the cold rolled steel sheet subjected to Q & P continuous annealing is subjected to a hot-dip galvanizing treatment, and the present invention relates to a method of manufacturing an alloyed hot-dip galvanized steel sheet, further comprising the steps of: and performing alloying hot-dip galvanizing treatment on the surface of the cold-rolled steel plate subjected to Q & P continuous annealing.
(III) advantageous effects
The present invention having the above-described structure can effectively provide a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, which have excellent yield strength, ductility, and hole expansibility, and a tensile strength of 980MPa or more, by securing an accurate amount of Tempered Martensite (TM) and bainite, as compared to high-ductility phase-change structure steels such as conventional DP steels or TRIP steels, and Q & P steels subjected to a conventional Quenching and Partitioning (Q & P) heat treatment.
Therefore, the cold-rolled steel sheet and the like of the present invention have an advantage of high possibility of application to industrial fields such as building parts and steel sheets for vehicles.
Drawings
Fig. 1 is a graph showing an example of an annealing process according to the present invention (a dotted line in the heat treatment line in fig. 1 represents a thermal history at the time of hot-dip alloying plating).
Fig. 2 is a graph showing the low temperature transformation behavior of the transformation induced plastic bainitic ferrite (TBF) method and the method of the invention.
FIG. 3 is a photograph showing the microstructure of the steel of invention example (F) produced by the present invention.
Fig. 4 is a result of observing carbides in tempered martensite of a cold-rolled steel sheet manufactured by the present invention.
FIG. 5 is a photograph showing the microstructure of the steel of comparative example (E).
Best mode for carrying out the invention
The present inventors have conducted intensive studies on a scheme for improving low ductility of high strength steel manufactured by a conventional Quenching & Partitioning (Q & P) heat treatment, and as a result, found heat treatment conditions for promoting bainite transformation at a specific temperature interval more precise than the prior art at the time of Q & P heat treatment to significantly reduce Fresh Martensite (FM). The present inventors have confirmed that the structure after the final Q & P heat treatment can be refined and the physical properties of the final product can be improved by controlling the Quenching Temperature (QT) and the bainite temperature (PT) in the interval in which the formation amount of martensite and the bainite transformation by quenching are promoted, and have thus proposed the present invention.
The present invention will be described in detail below.
First, the composition of alloy components of the cold-rolled steel sheet and the like and the reason for controlling the content thereof provided in the present invention will be described in detail. At this time, the content of each component represents weight% unless otherwise specified.
C：0.06～0.2％
Carbon (C) is an effective element for strengthening steel, and in the present invention C is an important element added to ensure the stabilization and strength of retained austenite. In order to obtain the effect, it is preferable to add 0.06% or more of C, but when the C content is less than 0.06%, the temperature of the austenite single phase is excessively high, so that high-temperature annealing is inevitably performed, and it is difficult to secure strength and ductility. In addition, when the C content exceeds 0.2%, Ms decreases and the quenching temperature decreases, so that it is difficult to perform precise heat treatment. In addition, there is a problem that weldability is significantly reduced. Therefore, in the present invention, the content of C is preferably controlled to 0.06 to 0.2%.
Mn：1.5～3.0％
Manganese (Mn) is an element that controls transformation of ferrite and is effective for formation and stabilization of retained austenite. When the content of Mn is less than 1.5%, a large amount of ferrite transformation occurs, so that it is difficult to secure desired strength, and on the other hand, when the content of Mn exceeds 3.0%, phase transformation in the second annealing heat treatment step of the present invention is excessively delayed, and a large amount of martensite is formed, so that it is difficult to secure desired ductility. Therefore, in the present invention, the content of Mn is preferably controlled to 1.5 to 3.0%.
Si：0.3～2.5％
Silicon (Si) is an element that suppresses precipitation of carbides in ferrite and promotes diffusion of carbon in ferrite into austenite, thereby finally contributing to formation of bainite and stabilization of residual austenite. In order to obtain the above effect, it is preferable to add 0.3% or more of Si, but when the Si content exceeds 2.5%, hot rolling property and cold rolling property become very poor, and oxide is formed on the steel surface, thereby inhibiting plating property. Therefore, in the present invention, the content of Si is preferably controlled to 0.3 to 2.5%.
Al：0.01～0.2％
Aluminum (Al) is an element that bonds with oxygen in the steel and plays a role of deoxidation, and for this reason, the Al content is preferably kept at 0.01% or more. In addition, like Si, Al contributes to stabilization of residual austenite by suppressing the formation of carbides in ferrite, and raises the bainite formation temperature. However, when the Al content exceeds 0.2%, the a3 temperature increases, so that high-temperature annealing is inevitably performed, and reacts with mold flux (mold flux) at the time of casting, so that it is difficult to manufacture a good slab, and plating property is hindered due to formation of surface oxide. Therefore, in the present invention, the content of Al is preferably controlled to 0.01 to 0.2%.
Nickel (Ni): 0.01 to 3.0%
Nickel is an element that secures strength by solid-solution strengthening and stabilizes austenite, and preferably holds 0.01% or more of Ni. However, since the effect of delaying bainite transformation is too large, when too much Ni is added, bainite is not completely transformed to form FM, and therefore, the upper limit of Ni is preferably controlled to 3%.
Molybdenum (Mo): less than 0.2%
Mo is added to improve strength by solid solution strengthening and to form a TiMo carbide to refine the bainite structure, but since the alloy iron is expensive and the cost increases, the upper limit of Mo is preferably controlled to 0.2%.
Titanium (Ti): 0.01 to 0.05 percent
Titanium preferentially forms TiN, so Ti must be added to improve hardenability by adding solid-solution boron. In the present invention, the lower limit of Ti is controlled to 0.01% to preferentially form TiN compared to BN, and when the Ti content is excessive, TiN is crystallized to cause nozzle clogging at the time of continuous casting, so that the upper limit of Ti is preferably controlled to 0.05%.
Antimony (Sb): 0.02-0.05%
Antimony is a grain boundary segregation substance, and antimony forms a grain boundary oxide, and 0.02% or more of Sb is preferably added as a method for suppressing decarburization through a grain boundary and suppressing a decrease in galvanizability due to surface enrichment of Mn, Si, and the like. However, when the Sb content is too large, grain boundary segregation increases and causes brittleness of steel, and therefore, the upper limit of Sb is controlled to 0.05%.
Boron (B): 0.0005 to 0.003%
Boron is a low-priced alloy element that easily secures strength by Quenching (Quenching), has an effect of reducing the total amount of the alloy, and is controlled to 0.005% in the lower limit as an element advantageous to weldability or suppression of high-temperature brittleness. However, when the content of B is too large, the BN forming temperature is higher than that of TiN, thereby causing high temperature brittleness of the steel, and therefore, the upper limit of B is preferably controlled to 0.003%.
Nitrogen (N): less than 0.01%
Since nitrogen reduces the alloying efficiency of the alloying element by forming BN or TiN, the content of B is preferably controlled to be 0.01% or less, which is a generally controllable range.
The remaining component of the present invention is iron (Fe). In other general steelmaking processes, unexpected impurities may inevitably be mixed from the raw materials or the surrounding environment. These impurities are well known to those skilled in the art of steel making in general, and therefore, the entire contents thereof are not particularly mentioned in the present specification.
On the other hand, the cold-rolled steel sheet of the present invention satisfying the above steel composition has a microstructure including, in 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 retained austenite, and 5% or less of ferrite. Among them, the bainite has high strength next to martensite and has intermediate characteristics of ferrite and martensite, and further, when fine residual austenite is distributed inside the bainite phase, the balance of strength and ductility of the steel is very high.
The cold-rolled steel sheet of the present invention satisfying the above-described microstructure has a tensile strength of 980Mpa or more, and can provide a high-strength steel sheet of giga-gauge grade having excellent yield strength, press formability, ductility and hole expansibility, compared to a steel sheet produced by conventional Q & P heat treatment.
The present invention can also provide a hot-dip galvanized steel sheet in which a hot-dip galvanizing treatment is performed on the surface of the cold-rolled steel sheet, and an alloyed hot-dip galvanized steel sheet in which an alloying heat treatment is performed on the hot-dip galvanized steel sheet.
Next, a method for producing a cold-rolled steel sheet or the like of the present invention will be described in detail.
The cold rolled steel sheet according to the present invention may be manufactured by subjecting a steel slab satisfying the above-described steel composition to a reheating-hot rolling-cold rolling-annealing process, and is described in detail as follows.
(billet reheating process)
Preferably, the present invention is subjected to a process of reheating a steel slab for homogenization treatment before hot rolling is performed, and more preferably, the reheating process is performed at a temperature range of 1000 to 1300 ℃.
When the temperature at the time of reheating is less than 1000 ℃, the rolling load increases sharply, on the other hand, when the temperature at the time of reheating exceeds 1300 ℃, the energy cost increases, and the amount of surface scale becomes excessive. Therefore, in the present invention, the reheating process is preferably performed at 1000 to 1300 ℃.
(Hot Rolling Process)
The reheated slab is hot-rolled to manufacture a hot-rolled steel sheet, and at this time, hot finish rolling is preferably performed at 800 to 950 ℃.
When the rolling temperature at the time of hot finish rolling is less than 800 ℃, the rolling load is greatly increased, and rolling becomes difficult, while when the hot finish rolling temperature exceeds 950 ℃, thermal fatigue of the roll is greatly increased, and the life is shortened. Therefore, in the present invention, the hot finishing temperature during hot rolling is preferably controlled to 800 to 950 ℃.
(Rolling process)
Next, the hot-rolled steel sheet produced as described above is wound, and at this time, the winding temperature is preferably 750 ℃ or less.
When the coiling temperature at coiling is too high, excessive scale is formed on the surface of the hot-rolled steel sheet, causing surface defects and deteriorating the plating property. Therefore, the winding process is preferably performed below 750 ℃. At this time, the lower limit of the rolling temperature is not particularly limited, and rolling is more preferably performed at Ms (martensite start temperature) to 750 ℃ in consideration of the formation of martensite so that the strength of the hot-rolled steel sheet becomes excessively high to make it difficult to perform subsequent cold rolling.
(Cold rolling process)
The rolled hot rolled steel sheet is subjected to an acid pickling process to remove an oxide layer, and then, cold rolling is performed to adjust the shape and thickness of the steel sheet, thereby manufacturing a cold rolled steel sheet.
In general, cold rolling is performed in order to secure a thickness required by a customer, and at this time, a reduction ratio is not limited, but in order to suppress generation of coarse ferrite grains at the time of recrystallization in a subsequent annealing process, cold rolling is preferably performed at a cold rolling reduction ratio of 30% or more.
(Q & P continuous annealing Process)
In the present invention, it is important to control the subsequent annealing process in order to manufacture a cold-rolled steel sheet having a final microstructure containing 50% or more of bainite, 10% or more of Tempered Martensite (TM), 10% or less of Fresh Martensite (FM), 20% or less of retained austenite, and 5% or less of ferrite. In particular, the present invention is characterized in that, in order to secure a desired fine structure in redistribution (partiton) of elements such as carbon, manganese, etc. at the time of annealing, a Q & P continuous annealing process is employed after conventional cold rolling, and QT, PT are controlled according to alloy elements as described later.
Soaking and rapid cooling
First, the cold-rolled steel sheet produced is preferably soaked at a temperature of Ac3 or higher for 30 seconds or longer, and then cooled to a Quenching Temperature (QT) ± 10 ℃ (see fig. 1) defined by the following relational expression 1 at a cooling rate of 5 to 20 ℃/second.
The objective is to obtain a ferrite structure which is not favorable for hole expansibility and is within 5%, and the cooling rate for not forming ferrite is set to be 5-20 ℃. There is no problem even if the cooling rate is higher than this, but the slower the cooling rate is, the more excellent the plate shape is and no distortion occurs, so a faster cooling rate is not required.
QT is the temperature at which 20-50% of the martensite is formed. When martensite formed at the time of quenching is reheated to PT and subjected to partitioning treatment in Q & P, tempering is caused, whereby the strength is further lowered, and also a function of promoting the formation of bainite is exerted. As shown in fig. 2, it is understood that even if the distribution treatment is performed at the same temperature, the TBF rapidly cooled to the bainite region temperature and kept at a constant temperature does not completely precipitate bainite over 600 seconds to form FM, and if martensite is sufficiently formed, bainite can completely transform in a short time to prevent FM formation. As described above, the object of minimizing FM as much as possible in the present invention is that since elements such as carbon and manganese are enriched in the austenite remaining during the bainite transformation, it cannot remain as austenite, and FM transformed at the time of final cooling is very strong due to martensite having a very high amount of alloying elements, causing interfacial separation and easily causing cracks at the time of hole expansion, thus significantly reducing hole expandability.
In the present invention, such new characteristics are found, and by this, a new high formability high strength steel sheet having a bainite main phase is developed, and QT that promotes the formation of bainite and maximizes the bainite area fraction is calculated by the following experiment.
[ relational expression 1]
QT＝493.497+36.2874×Al-394.0×C-45.0×Mn-11.4332×Mo-20.8772×Ni-13.0438×Si-12.8×Cr
Partition heat treatment
Then, in the present invention, the cooled steel sheet is reheated to a bainite temperature (PT). + -. 10 ℃ defined by the following relational expression 2, and then, is maintained in a temperature range of PT ≧ or ≧ PT-100 ℃ for 100 seconds or more, and then is cooled.
After the quenching, when reheated to the bainite temperature (PT) and kept at a constant temperature, the temperature at which bainite is most rapidly formed is calculated through experiments. When the temperature is higher than this temperature, the formation amount of bainite is small, the stabilization of the residual austenite is insufficient, and the formation of FM increases instead, and therefore PT must be heated to PT ± 10 ℃.
[ relational expression 2]
PT＝599.088+11.5214×Al-225.2×C-35.0×Mn-19.9474×Ni-24.9385×Si-56.718×Mo-22.1×Cr
Unlike the prior art, the present invention does not require constant temperature maintenance at a constant temperature. The constant temperature is maintained for more than 100 seconds within the temperature range of PT ≥ or ≥ PT-100 deg.C, and then cooling is performed, so that the constant temperature furnace has the advantage of being easily applied to equipment with a constant temperature furnace without a heating and maintaining device.
When the Q & P heat treatment is performed as described above, steel including 50% or more bainite, 10% or more Tempered Martensite (TM), 10% or less Fresh Martensite (FM), 20% or less retained austenite, and 5% or less ferrite can be manufactured, and ferrite and FM having a large difference in strength are minimized.
(plating)
The cold-rolled steel sheet subjected to the first and second annealing heat treatments is subjected to a plating treatment to enable the production of a plated steel sheet. At this time, preferably, the plating treatment is performed by a hot-dip plating method or an alloying hot-dip plating method, and preferably, the plating layer thus formed is a zinc plating layer.
In the case of using the hot dip plating method, a hot dip plated steel sheet may be manufactured by being immersed in a galvanizing bath, and in the case of using the alloying hot dip plating method, an alloying hot dip plated steel sheet may also be manufactured by performing a general alloying hot dip plating process.
Detailed Description
The present invention will be described in detail below with reference to examples.
(examples)
A molten metal having a composition shown in table 1 below was manufactured into a steel ingot having a thickness of 90mm and a width of 175mm by vacuum dissolution. Subsequently, the steel sheet is reheated at 1200 ℃ for 1 hour to be homogenized, and then hot finish rolled at a temperature of not less than Ar3, i.e., not less than 900 ℃ to produce a hot-rolled steel sheet. Then, after cooling the hot rolled steel sheet, it was put into a furnace heated to 600 ℃ in advance and held for 1 hour, and then furnace cooling was performed, thereby simulating hot rolling. The plate hot-rolled as above was cold-rolled at a cold reduction ratio of 50 to 60%, and then annealing heat treatment was performed under the conditions of table 2 below to manufacture a final cold-rolled steel sheet.
The structure fraction, yield strength, tensile strength, elongation and Hole Expansion Ratio (HER) of each of the cold rolled steel sheets manufactured as above were measured, and the results thereof are shown in table 2 below.
[ Table 1]
C Mn Si Al Cr Ni Mo Ti Sb B N
Invention steel A 0.1 2.2 1.4 0.02 0.6 0.02 0.04 0.017 0.027 0.0021 0.005
Invention steel B 0.14 1.9 1.4 0.03 0.5 0.02 0.04 0.021 0.04 0.0017 0.003
Invention steel C 0.18 2.5 0.6 0.02 1.1 0.02 0.05 0.014 0.021 0.0012 0.004
Invention steel D 0.16 2.1 1.9 0.04 0.3 0.02 0.05 0.022 0.035 0.0023 0.006
Invention steel E 0.09 2.2 1.4 0.15 0.6 0.02 0.06 0.031 0.027 0.0018 0.005
Invention 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 1 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
[ Table 2]
Figure GDA0002792719900000131
In table 2, B represents bainite, TM represents tempered martensite, FM represents fresh martensite, a represents retained austenite, and F represents ferrite.
As shown in table 1 above, it is understood that the inventive examples (a-G) in which the steel composition and the manufacturing process satisfy the scope of the present invention all exhibit excellent yield strength, ductility, and hole expansibility.
FIG. 3 is a photograph showing the microstructure of the steel of invention example (F) produced by the present invention. As shown in table 2, it is understood that the steel of invention example (F) can be produced as a bainite steel having 75% bainite as a main phase, 14% TM and 5% FM, respectively, TM/FM exceeding 2, and F being less than 5%. This is a technical feature of the present invention, and conventionally TRIP steels for ferrite base materials or tempered martensite steels have been produced mainly by Q & P heat treatment, but when the steel composition, QT and PT, are specified as in the present invention, the bainite base structure can be produced more easily than by TBF heat treatment.
On the other hand, fig. 4 is a view for observing TM in the tissue of fig. 3 by an Atom Probe Tomography (APT). The tempered martensite was known to be a mixture of migrating carbides and coarse iron carbides.
On the other hand, it is found that the comparative example (H-L, B, E, G) in which the steel composition and the manufacturing process are out of the range of the present invention is inferior to the inventive example in all of yield strength, ductility and hole expansibility.
In particular, as shown in table 2, it was found that the steel composition did not satisfy the range of the present invention, or the desired physical properties were not obtained in the comparative example (B, E, G) in which the manufacturing process was not performed according to the present invention.
From fig. 5, it was confirmed that, even though the structure of the steel of comparative example (E) was the same composition as that of the present invention, ferrite and FM were formed due to the dual-phase annealing and the TBF heat treatment, and the strength and HER were low.
From the results, it is understood that the cold-rolled steel sheet manufactured according to the present invention can secure a yield strength of 980MPa or more and excellent elongation and HER, and thus has an advantage in that it can easily perform cold forming applied to a structural member, compared to a steel material manufactured through a conventional Q & P heat treatment process.
As described above, although the preferred embodiments of the present invention have been described in detail, those skilled in the art to which the present invention pertains can make various modifications without departing from the scope of the present invention. Thus, the scope of the claims of the present invention should not be limited to the illustrated embodiments, but should be determined by the appended claims and their equivalents.

Claims (9)

1. A high-strength cold-rolled steel sheet excellent in yield strength, ductility and hole expansibility, comprising in wt%: carbon (C): 0.06-0.2%, 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 to 3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01 to 0.05%, antimony (Sb): 0.02 to 0.05%, boron (B): 0.0005 to 0.003%, nitrogen (N): 0.01% or less except 0%, and the balance of Fe and inevitable impurities,
the fine structure of the cold-rolled steel sheet comprises 62% or more of bainite, 10% or more of Tempered Martensite (TM), 10% or less of Fresh Martensite (FM), 20% or less of retained austenite, and 5% or less of ferrite in terms of area fraction.
2. The high-strength cold-rolled steel sheet excellent in yield strength, ductility and hole expansibility as recited in claim 1,
the TM/FM ratio exceeds 2.
3. A hot-dip galvanized steel sheet, which is obtained by subjecting the surface of the cold-rolled steel sheet according to claim 1 to a hot-dip galvanizing treatment.
4. An alloyed hot-dip galvanized steel sheet, which is obtained by subjecting the surface of the cold-rolled steel sheet according to claim 1 to an alloying hot-dip galvanizing treatment.
5. A method for manufacturing a high-strength cold-rolled steel sheet having excellent yield strength, ductility and hole expansibility, comprising the steps of:
reheating, followed by hot rolling and then coiling a steel slab comprising, in weight%: carbon (C): 0.06-0.2%, 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 to 3.0%, molybdenum (Mo): 0.2% or less, titanium (Ti): 0.01 to 0.05%, antimony (Sb): 0.02 to 0.05%, boron (B): 0.0005 to 0.003%, nitrogen (N): 0.01% or less except 0%, and the balance of Fe and inevitable impurities; and
cold rolling the rolled hot rolled steel plate, then carrying out quenching-partitioning continuous annealing,
the quenching-partitioning continuous annealing process comprises the following processes:
soaking the cold-rolled steel sheet manufactured at a temperature of Ac3 or more for 30 seconds or more, and then cooling the cold-rolled steel sheet at a cooling rate of 5 to 20 ℃/second to a Quenching Temperature (QT) ± 10 ℃ as defined by the following relational expression 1;
reheating the cooled steel sheet to a bainite temperature (PT). + -. 10 ℃ defined by the following relational expression 2, followed by holding at a temperature range of PT ≥ or ≥ PT-100 ℃ for 100 seconds or longer, and then cooling,
[ relational expression 1]
QT＝493.497+36.2874×Al-394.0×C-45.0×Mn-11.4332×Mo-20.8772×Ni-13.0438×Si-12.8×Cr
[ relational expression 2]
PT＝599.088+11.5214×Al-225.2×C-35.0×Mn-19.9474×Ni-24.9385×Si-56.718×Mo-22.1×Cr。
6. The method for manufacturing a high-strength cold-rolled steel sheet excellent in yield strength, ductility and hole expansibility as recited in claim 5,
the fine structure of the steel sheet to which the quenching-partitioning continuous annealing is completed contains 62% or more of bainite, 10% or more of Tempered Martensite (TM), 10% or less of Fresh Martensite (FM), 20% or less of retained austenite, and 5% or less of ferrite in terms of area fraction.
7. The method for manufacturing a high-strength cold-rolled steel sheet excellent in yield strength, ductility and hole expansibility as recited in claim 6,
the TM/FM ratio exceeds 2.
8. A method for manufacturing a hot-dip galvanized steel sheet having excellent yield strength, ductility, and hole expansibility, further comprising the steps of:
the method for manufacturing a high-strength cold-rolled steel sheet excellent in yield strength, ductility and hole expansibility according to claim 5, wherein the surface of the continuously annealed cold-rolled steel sheet is subjected to hot-dip galvanizing.
9. A method for manufacturing an alloyed hot-dip galvanized steel sheet having excellent yield strength, ductility, and hole expansibility, further comprising the steps of:
the method of manufacturing a high-strength cold-rolled steel sheet excellent in yield strength, ductility and hole expansibility according to claim 5, wherein the surface of the continuously annealed cold-rolled steel sheet is subjected to an alloying hot-dip galvanizing treatment.
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