Steel sheet and method for manufacturing same

Abstract

A steel sheet and a method for manufacturing the same are provided. The steel plate of the present invention has excellent ductility, formability, and weldability, and thus can be used for automobile structural components.

Description

Technical Field

[0001] The present invention relates to the manufacture of a steel sheet for automotive structural components, and more particularly, to a steel sheet having excellent ductility, formability, and weldability and a method for manufacturing the same. Background Art

[0002] In recent years, regulations on environmental protection in the automotive industry have become increasingly strict. Therefore, carbon emission reduction and fuel efficiency regulations have been gradually strengthened, and in order to ensure the stability of passengers in accidents such as collisions, the application of high-strength steel has been continuously increasing. In addition, in order to improve the impact resistance of the vehicle body, high-strength steel with excellent yield strength is used in structural components such as members, seat rails, and pillars. However, the increase in the strength of the steel sheet may lead to a decrease in ductility and formability. To solve this problem, it is necessary to develop a material that satisfies both high strength and high formability. Generally, as the strength of the steel sheet increases, the elongation rate decreases, resulting in a problem of reduced workability. Therefore, it is necessary to develop a material that can alleviate this defect. Generally, as a method of strengthening steel, solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening have been studied. However, among the above methods, steels using solid solution strengthening and grain refinement have a problem of being very difficult to manufacture high-strength steels of 490 MPa or more.

[0003] Precipitation-strengthened high-strength steel is a technique for ensuring strength by adding carbon and nitride-forming elements such as Nb, Ti, and V to precipitate carbon and nitrides and refining the grains by suppressing grain growth caused by fine precipitates. Although the technique has the advantage of easily ensuring high strength with respect to low manufacturing costs, the recrystallization temperature rises sharply due to the fine precipitates. Therefore, in order to generate sufficient recrystallization to ensure ductility, it has the disadvantage of requiring high-temperature annealing. In addition, precipitation-strengthened steel strengthened by precipitating carbon and nitrides in a ferrite matrix has a problem of being difficult to obtain high-strength steels of 600 MPa or more.

[0004] In addition, as transformation-strengthened high-strength steels, there are various steels such as Dual Phase (DP) steels composed of a soft ferrite matrix and a hard martensite two-phase, Transformation Induced Plasticity (TRIP) steels that ensure high ductility by utilizing the transformation-induced plasticity of retained austenite, or Complexed Phase (CP) steels composed of a composite structure of ferrite and a hard bainite or martensite. In recent years, for automotive steel sheets to improve fuel efficiency or durability, higher-strength steel sheets are required. Considering collision safety and passenger protection, the demand for high-strength steel sheets with a tensile strength of 780 MPa or more as body structures or reinforcement parts is also increasing. Among them, DP steels have excellent ductility and are the most widely used automotive steel sheets, but they have the disadvantages of a low yield ratio (YR), poor formability, and workability. In addition, as the strength of the steel sheet gradually increases, cracks and wrinkles are generated during the stamping process of automotive parts, making it difficult to manufacture complex parts. Among high-strength steels, in the case of TRIP steels and XF steels, compared with existing DP steels, they have high ductility and an excellent yield ratio, resulting in good workability. However, in order to ensure a high elongation rate, a large amount of Si and Al are added, so they have the disadvantage of poor weldability. To overcome the disadvantages of existing high-strength steels, by manufacturing a composition system with reduced Si and Al components to construct a composition system with good weldability while meeting the formability and ductility above the specified level, high-strength steels can be widely applied to more complex parts. This can be achieved by using the latest heat treatment technology, quenching and partitioning (Q&P) heat treatment, which can ensure retained austenite.

[0005] As an existing technology for simultaneously ensuring the ductility and workability of the high-tensile steel sheet, the invention disclosed in Patent Document 1 can be cited. In the invention disclosed in Patent Document 1, it is necessary to control the heat treatment temperature so that retained austenite is formed by precisely controlling the slow cooling and rapid cooling temperatures to ensure a high elongation rate.

[0006] As another existing technology, the invention disclosed in Patent Document 2 can be cited. The invention disclosed in Patent Document 2 is characterized by manufacturing a steel sheet with a high yield ratio through the Q&P process and painting treatment.

[0007] As another existing technology, the invention disclosed in Patent Document 3 can be cited. The invention disclosed in Patent Document 3 provides a method for manufacturing a high-strength cold-rolled steel sheet with a high bainite fraction by cooling to the bainite region. Compared with the Q&P process, the carbon partitioning effect is poor, resulting in a problem of poor elongation rate.

[0008] [Prior Art Documents]

[0009] [Patent Documents]

[0010] (Patent Document 1) Korean Patent Publication No. KR2018 - 0165176

[0011] (Patent Document 2) Japanese Patent Publication No. JP2010 - 090432

[0012] (Patent Document 3) Japanese Patent Publication No. JP2015 - 106696 Summary of the Invention

[0013] (1) Technical Problems to be Solved

[0014] The main object of the present invention is to provide a steel sheet that can be used for automotive structural components and a manufacturing method thereof.

[0015] In addition, the technical problems to be achieved by the present invention are not limited to the above - mentioned technical problems. Those skilled in the technical field to which the present invention pertains can clearly understand other technical problems not mentioned through the following description.

[0016] (2) Technical Solutions

[0017] Therefore, one aspect of the present invention relates to a steel sheet which, by weight %, comprises: carbon (C): 0.06 - 0.16%, silicon (Si): 0.8% or less (except 0%), manganese (Mn): 1.6 - 2.6%, molybdenum (Mo): 0.40% or less (except 0%), chromium (Cr): 1.0% or less (except 0%), phosphorus (P): 0.10% or less (except 0%), sulfur (S): 0.020% or less (except 0%), aluminum (sol.Al): 0.60% or less (except 0%), titanium (Ti): 0.001 - 0.04%, niobium (Nb): 0.001 - 0.04%, nitrogen (N): 0.01% or less (except 0%), boron (B): 0.010% or less (including 0%), the balance being Fe and unavoidable impurities, wherein C, Si, Mn, Cr, and Mo satisfy the following relational expression 1. By area %, the microstructure comprises 20% or more and 40% or less of ferrite, 30% or more and 60% or less of the sum of bainite and tempered martensite, the balance being fresh martensite and retained austenite, the area fraction of the retained austenite being less than 5% (except 0%), and the hole expansion ratio (HER) being 30% or more, the value according to the following relational expression 2 for the hole expansion ratio (HER), yield strength (YS), tensile strength (TS), and elongation (EL) being 500 or more, and the yield ratio (YS / TS) being 0.65 or more.

[0018] [Relational Expression 1]

[0019] C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≥ 0.300

[0020] Among them, C, Si, Mn, Cr, and Mo represent the steel component contents (%) in the matrix structure at the position of t / 4 of the base steel plate thickness.

[0021] [Relationship 2]

[0022] HER EL YS / TS ≥ 500

[0023] In addition, another aspect of the present invention relates to a method for manufacturing a steel plate, which includes the following processes: preparing a steel billet, and then reheating the steel billet, the steel billet satisfying the above steel composition components and Relationship 1; hot-rolling the reheated steel billet so that the temperature on the finishing mill exit side becomes Ar3 to Ar3 + 50 °C, then, coiling at 400 - 650 °C, and then cooling to room temperature at an average cooling rate of 0.1 °C / second (s) or less; cold-rolling the cooled hot-rolled steel plate at a reduction rate of 40 - 70% to manufacture a cold-rolled steel plate; continuously annealing the cold-rolled steel plate at a temperature of Ac3 - 50 °C to Ac3 - 20 °C; cooling the continuously annealed cold-rolled steel plate at an average cooling rate of 10 °C / second or less for the first cooling to a temperature range of 650 - 680 °C, and then cooling at an average cooling rate of 5 °C / second or more for the second cooling to a temperature of 300 - 340 °C; reheating the cold-rolled steel plate after the second cooling to a temperature above Ms, and then holding for 60 seconds or more; and cooling the held cold-rolled steel plate at an average cooling rate of 5 °C / second or more to a temperature below 150 °C.

[0024] (III) Beneficial Effects

[0025] The present invention configured as described above has beneficial effects in manufacturing a high-strength steel plate that satisfies high ductility as a characteristic of DP steel and has an excellent yield ratio (YS / TS) and hole expansion rate compared with existing DP steels. Therefore, by preventing processing defects such as cracks generated during stamping forming, it can be widely applied to automotive structural components with complex shapes that require high formability. In addition, by reducing Si and Al, it has the advantage of good weldability compared with existing TRIP steels. Description of the Drawings

[0026] Figure 1 shows the hole expansion rate (HER), yield strength (YS), tensile strength (TS), and elongation (EL) relationships HER according to Relationship 1 of the inventive steel and the comparative steel in the embodiments of the present invention EL Graph of the change in YS / TS.

[0027] Figure 2 It is a graph showing the change in the hole expansion ratio (HER) according to Relational Expression 1 for the inventive steel and the comparative steel in the embodiments of the present invention.

[0028] Figure 3 It is a graph showing the change in the yield strength ratio according to Relational Expression 1 for the inventive steel and the comparative steel in the embodiments of the present invention. Best Mode

[0029] The technical terms used herein are only used to refer to specific embodiments and are not intended to limit the present invention. Unless explicitly stated to the contrary in the text, the singular forms used therein also include the plural forms. The meanings of "comprising" or "including" used in the specification are to specify specific features, regions, integers, steps, operations, elements, and / or components, and do not exclude the existence or addition of other specific features, regions, integers, steps, operations, elements, components, and / or groups.

[0030] Unless otherwise defined, all terms, including technical terms and scientific terms, used herein have the same meaning as commonly understood by those skilled in the art to which the present invention pertains. Terms defined in commonly used dictionaries should be further interpreted as having meanings consistent with the relevant technical literature and the currently disclosed content, and should not be interpreted as having ideal or overly formal meanings in cases where they are not defined.

[0031] The present inventors confirmed through experiments that, by optimizing the alloy composition and manufacturing process, when introducing 20% or more and 40% or less of ferrite, 30% or more and 60% or less of the sum of bainite and tempered martensite, and the balance of fresh martensite and retained austenite in the final fine structure, compared with existing DP steels, the yield strength ratio can be increased to improve workability, and less than 5% of retained austenite is formed during final cooling, thereby further improving ductility, and the present invention was completed based on this result.

[0032] First, the present inventors hot-rolled a slab with the matrix structure at the position of t / 4 of the base steel plate thickness controlled such that C, Si, Mn, Cr, and Mo satisfied the relational expression 1, so that the temperature on the finish rolling exit side became Ar3 to Ar3 + 50 °C, and coiled at 400 - 650 °C. Then, it was cooled to room temperature at an average cooling rate of 0.1 °C / second or less, thereby manufacturing a hot-rolled steel plate in which carbides that become austenite nucleation sites during annealing were finely dispersed. In addition, the hot-rolled steel plate was cold-rolled with a reduction ratio of 40 - 70%, thereby manufacturing a cold-rolled steel plate. Then, after continuous annealing at a temperature of Ac3 - 50 °C to Ac3 - 20 °C, the continuously annealed steel plate was first cooled at an average cooling rate of 10 °C / second or less to a temperature range of 650 - 680 °C, and then secondarily cooled at an average cooling rate of 5 °C / second or more to a temperature of 300 - 340 °C, thereby introducing fresh martensite. Then, the steel plate was reheated to a temperature above Ms and held for 60 seconds or more to form tempered martensite and bainite, enriching carbon into the untransformed austenite around it. Then, it was cooled at an average cooling rate of 5 °C / second or more to a temperature below 150 °C, introducing fine fresh martensite in the remainder.

[0033] By controlling the above composition and manufacturing process, compared with the existing DP steel, the fractions of ferrite and fresh martensite can be reduced, and tempered martensite, bainite, and retained austenite can be introduced. Therefore, compared with the existing DP steel, the yield ratio is increased, thereby ensuring workability. In addition, during plastic deformation, a large number of mobile dislocations are formed around the retained austenite, which helps to improve ductility. Through this precisely controlled composite structure steel, compared with the existing DP steel, ductility can be ensured while maintaining a high yield ratio. Therefore, a high-strength steel plate with excellent ductility, formability, and weldability can be obtained.

[0034] Hereinafter, with reference to various embodiments and drawings, the technical configuration according to the present invention will be described in more detail.

[0035] First, the steel plate of the present invention, in weight %, comprises: carbon (C): 0.06 - 0.16%, silicon (Si): not more than 0.8% (except 0%), manganese (Mn): 1.6 - 2.6%, molybdenum (Mo): not more than 0.40% (except 0%), chromium (Cr): not more than 1.0% (except 0%), phosphorus (P): not more than 0.10% (except 0%), sulfur (S): not more than 0.020% (except 0%), aluminum (sol.Al): not more than 0.60% (except 0%), titanium (Ti): 0.001 - 0.04%, niobium (Nb): 0.001 - 0.04%, nitrogen (N): not more than 0.01% (except 0%), boron (B): not more than 0.010% (including 0%), the balance being Fe and inevitable impurities, and satisfying Relationship 1.

[0036] Hereinafter, the characteristics of each alloying element and the critical significance of the composition range will be briefly described. At this time, unless otherwise specifically stated, the content of each component represents weight %.

[0037] C: 0.06 - 0.16%

[0038] Carbon (C) is a very important element added to strengthen the phase transformation structure. Carbon promotes the formation of hard martensite in dual-phase steel, thereby increasing the strength. When the carbon content increases, the amount of martensite also increases. However, when the carbon content exceeds 0.16%, although the strength of martensite increases, the strength difference from ferrite with a low carbon concentration becomes larger. This strength difference causes easy fracture at the interphase interface during plastic deformation, thereby reducing ductility and the work hardening rate. In addition, the weldability deteriorates, and welding defects occur during the processing of components in the customer company. On the other hand, when the carbon content decreases and is less than 0.06%, it is difficult to ensure the required strength. Therefore, it is preferable to limit the carbon content to 0.06 - 0.16%. More preferably, the carbon content can be limited to the range of 0.07 - 0.15%.

[0039] Si: not more than 0.8% (except 0%)

[0040] Silicon (Si) is a ferrite stabilizing element that promotes ferrite phase transformation and promotes carbon enrichment into untransformed austenite in the Q&P process, thus contributing to the formation of retained austenite. In addition, it increases the strength of ferrite through solid solution strengthening, effectively reducing the hardness difference between phases, and the silicon is a useful element that can ensure strength without reducing the ductility of the steel plate. However, when the silicon content exceeds 0.8%, it will cause surface scale defects, have an adverse effect on the surface quality, and reduce the weldability and chemical conversion treatability. Therefore, it is preferable to limit the addition amount of silicon to not more than 0.8%, and more preferably, it can be limited to not more than 0.7%.

[0041] Mn: 1.6 - 2.6%

[0042] Manganese (Mn) is an element that refines grains slightly without reducing ductility and completely precipitates sulfur (S) in steel as MnS, thereby preventing hot brittleness caused by the formation of FeS while strengthening the steel. At the same time, in duplex-structured steel, manganese plays a role in reducing the critical cooling rate for obtaining a martensite phase, making it easier to form martensite. When the content of manganese is less than 1.6%, it is difficult to ensure the strength desired in the present invention. On the other hand, when the content of manganese exceeds 2.6%, there is a high possibility of problems such as weldability and hot-rolling properties, excessive martensite is formed, which will make the material unstable, and Mn-bands (bands of Mn oxides) are formed in the structure, thus having the problem of increasing the risk of processing cracks and plate fracture. In addition, during annealing, Mn oxides dissolve on the surface, thus significantly hindering the surface quality. Therefore, in the present invention, it is preferable to limit the content of Mn to the range of 1.6 - 2.6%, and more preferably, it can be limited to the range of 1.8 - 2.4%.

[0043] Mo: 0.40% or less (except 0%)

[0044] Molybdenum (Mo) is an element that delays the transformation of austenite to pearlite, refines ferrite slightly, and increases strength. This Mo improves the hardenability of the steel and has the advantage of forming martensite finely at grain boundaries to control the yield ratio. However, as a high-valence element, the higher its content, the higher the manufacturing cost, and there is a problem of being disadvantageous in terms of cost. Therefore, it is preferable to appropriately control the content of Mo. To obtain the above effects, it is preferable to add up to 0.40% of Mo. When the content of Mo exceeds 0.40%, it will cause a sharp increase in alloy cost and reduce economy, and due to excessive grain refinement effect and solid-solution strengthening effect, there is instead a problem of reducing the ductility of the steel. Therefore, in the present invention, it is preferable to limit the content of Mo to 0.40% or less, and more preferably, it can be limited to 0.20% or less. In addition, in the present invention, considering the amount of Mo that will inevitably be added during manufacturing, except 0%.

[0045] Cr: 1.0% or less (except 0%)

[0046] Chromium (Cr) is an ingredient added to improve the hardenability of the steel and ensure high strength. In addition, chromium (Cr) is an element that plays a very important role in the formation of martensite, minimizing the decrease in elongation rate relative to the increase in strength, and thus is also beneficial for manufacturing duplex-structured steel with high ductility. In particular, during the hot-rolling process, chromium forms such as Cr 23The Cr-based carbides of C6, some of these carbides are dissolved during annealing, and some remain undissolved, so that the amount of solid-solved C in martensite after cooling can be controlled below an appropriate level. Therefore, the occurrence of yield point elongation is suppressed, and it is an element beneficial to the manufacture of a dual-phase steel with a low yield ratio. However, when the chromium content exceeds 1.0%, not only will its effect saturate, but also due to the excessive increase in hot rolling strength, there is a problem of poor cold rolling property, and since the fraction of Cr-based carbides increases and becomes coarse, the size of martensite after annealing becomes coarse, resulting in a problem of reduced elongation. Therefore, in the present invention, it is preferable to limit the chromium content to 1.0% or less, and more preferably, it can be limited to 0.8% or less. In addition, in the present invention, considering the amount of Cr that is inevitably added during manufacturing, 0% is excluded.

[0047] P: 0.10% or less (excluding 0%)

[0048] Phosphorus (P) is a substitutional element with the largest solid solution strengthening effect. It is the most favorable element for improving in-plane anisotropy and ensuring strength without significantly impairing formability. However, when an excessive amount of phosphorus above a certain level is added, the possibility of brittle fracture increases significantly, so that slab breakage may occur during hot rolling and there is a problem of acting as an element that hinders plating surface characteristics. Therefore, in the present invention, the phosphorus content is limited to a maximum of 0.10%, but 0% is excluded considering the inevitably added level.

[0049] S: 0.020% or less (excluding 0%)

[0050] Sulfur (S) is an impurity element that is inevitably added to steel and is an element that hinders ductility and weldability. Therefore, it is important to control it at as low a level as possible. In particular, there is a problem of increasing the possibility of red hot brittleness, so it is preferable to control the sulfur content to 0.020% or less. However, 0% is excluded considering the inevitably added level during the manufacturing process.

[0051] Acid-soluble aluminum (sol.Al): 0.60% or less (excluding 0%)

[0052] Soluble aluminum (sol.Al) is an element added for grain refinement and deoxidation of steel. Similar to Si, it is a ferrite-stabilizing element. In addition, soluble aluminum (sol.Al) distributes carbon in ferrite into austenite, thereby improving the hardenability of martensite and forming an effective component of retained austenite. Moreover, during annealing, when held in the bainite region, it is a useful element that can effectively inhibit the precipitation of carbides in bainite and improve the ductility of the steel sheet. However, when the content of soluble aluminum (sol.Al) exceeds 0.60%, although it is beneficial for strength improvement due to the grain refinement effect, excessive formation of inclusions during the steelmaking continuous casting operation increases the possibility of surface defects in the coated steel sheet and causes problems of increased manufacturing costs. Therefore, in the present invention, it is preferably to control the content of soluble aluminum (sol.Al) to 0.60% or less, and more preferably, it can be limited to 0.40% or less.

[0053] Ti, Nb: 0.001 - 0.04% respectively

[0054] Titanium (Ti) and niobium (Nb) are effective elements for improving the strength of the steel sheet and refining grains by forming nano-precipitates. Adding these elements combines with carbon to form very fine nano-precipitates, which play a role in strengthening the matrix structure and reducing the hardness difference between phases. When the contents of Ti and Nb are less than 0.001% respectively, it is difficult to ensure the above effects. When the contents of Ti and Nb exceed 0.04% respectively, the manufacturing cost increases, and due to excessive precipitates, the ductility may be significantly reduced. Therefore, in the present invention, the contents of Ti and Nb can be limited to 0.001 - 0.04% respectively.

[0055] N: 0.01% or less (except 0%)

[0056] Nitrogen (N) is a component that effectively stabilizes austenite. However, when the nitrogen content exceeds 0.01%, there are problems of a sharp increase in the refining cost of steel and a significant increase in the risk of crack formation during continuous casting due to the formation of AlN, etc. Therefore, it is preferably to limit the upper limit of the nitrogen content to 0.01%. However, considering the inevitably added level, 0% is excluded.

[0057] B: 0.010% or less (including 0%)

[0058] Boron (B) is a component that delays the transformation of austenite to pearlite during the cooling process of annealing and is a hardenability element that inhibits the formation of ferrite and promotes the formation of martensite. However, when the boron content exceeds 0.01%, excessive B is enriched on the surface, which may cause deterioration of coating adhesion. Therefore, the boron content is controlled to 0.010% or less, and more preferably, 0.005% or less is added.

[0059] [Relationship 1]

[0060] C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≥ 0.300

[0061] Among them, C, Si, Mn, Cr, and Mo represent the content percentage of steel components in the matrix structure at the position of t / 4 of the base steel plate thickness, where t is the thickness of the base steel plate.

[0062] In the present invention, it is important to control the contents of C, Si, Al, Cr, and Mo in the alloy components in the matrix structure at the position of t / 4 of the base steel plate thickness to satisfy the above-mentioned Relationship 1.

[0063] Si and Al are ferrite stabilizing elements that promote ferrite phase transformation and promote the enrichment of C into the untransformed austenite, thus contributing to the formation of retained austenite and martensite. C is also an element that contributes to the formation and adjustment of the fraction of martensite by promoting the enrichment of C into the untransformed austenite. However, C, Si, and Al have an adverse effect on weldability and cause cracks on the surface and inside of the welded part.

[0064] On the other hand, Mn, Cr, and Mo are elements that contribute to improving hardenability, but compared with C, Si, and Al, their effect of enriching C into austenite is relatively low. Therefore, it is very important to well adjust the ratio of C, Si, Al to other hardenability elements Mn, Cr, and Mo.

[0065] When the value defined according to the above-mentioned Relationship 1 is 0.300 or more, the relationship HER of the hole expansion rate (HER), yield strength (YS), tensile strength (TS), and elongation (EL) can be ensured. EL The value of YS / TS is 500 or more. In addition, by ensuring that the sum of bainite and tempered martensite is 30% or more and 60% or less while reducing the interphase hardness difference to ensure a hole expansion rate of 30% or more, the required ductility can be ensured. Furthermore, a yield ratio (YS / TS) of 0.65 or more can also be ensured. However, when the value defined according to the above-mentioned Relationship 1 is less than 0.300, the above-mentioned effects cannot be expected.

[0066] In addition to the above components, the present invention can also be composed of the balance of Fe and other inevitable impurities.

[0067] In addition, the steel plate of the present invention is a composite structure steel plate. Compared with the existing DP steel, while improving the yield ratio and workability, a certain degree of ductility can be ensured. For this purpose, in addition to the above alloy composition, the following fine microstructure and phase fraction control conditions also need to be satisfied.

[0068] Specifically, the steel sheet of the present invention has the following fine microstructure. By area percentage, the fine microstructure contains 20% or more and 40% or less of ferrite, 30% or more and 60% or less of the sum of bainite and tempered martensite, the balance of fresh martensite and retained austenite, and the area fraction of the retained austenite is less than 5% (except 0%).

[0069] In order to satisfy the low-alloy (Lean) composition system and high ductility as the characteristics of DP steel while increasing the yield strength ratio (YS / TS) compared with the existing DP steel, it is necessary to control the microstructure and composition and perform precise heat treatment. First of all, it is important to introduce a small amount of retained austenite. The retained austenite contributes to improving the ductility of the steel sheet by inducing transformation-induced plasticity. For the introduction of this retained austenite, a Q&P process is carried out in which it is cooled to a temperature below Ms during rapid cooling to form fresh martensite, and then reheated to a temperature above Ms. At this time, a large amount of tempered martensite and bainite are formed, and C is stably distributed and helps to form a small amount of retained austenite in the final microstructure. In order to ensure a high hole expansion ratio and ductility, it is also important to precisely control the fraction of tempered martensite and bainite. By performing duplex annealing close to the single-phase region, the fraction of ferrite during annealing is adjusted, and by the Q&P process of rapid cooling to a temperature below Ms and reheating to a temperature above Ms, the formation of fresh martensite is suppressed to the maximum extent. In addition, the difference in hardness between phases is further reduced by precipitating fine nano-precipitates in the ferrite, thereby improving the workability. Finally, the required strength can be ensured by introducing a small amount of fresh martensite during the final cooling.

[0070] Therefore, in the fine microstructure constituting the steel sheet of the present invention, the sum of bainite and tempered martensite is controlled to be 30% or more and 60% or less. When the sum of bainite and tempered martensite is less than 30%, there is a problem that the required yield strength ratio and hole expansion ratio cannot be ensured. More preferably, the sum of bainite and tempered martensite is limited to 40% or more.

[0071] In addition, in the present invention, in the fine microstructure constituting the steel sheet, the fraction of ferrite is preferably limited to 20% or more and 40% or less. When the fraction of ferrite exceeds 40%, there is a problem that the required yield strength ratio and hole expansion ratio cannot be ensured. More preferably, the fraction of the ferrite microstructure is limited to 30% or less.

[0072] In addition, the steel sheet of the present invention has a fine microstructure containing the balance of fresh martensite and retained austenite, and the area fraction of the retained austenite is less than 5% (except 0%). In order to make the fraction of the retained austenite 5% or more, it is necessary to increase the Si content, and at this time, an LME problem may occur. More preferably, the fraction of the retained austenite is limited to the range of 1-4 area%.

[0073] The steel sheet of the present invention having the microstructure as described above has an excellent yield ratio compared with the existing DP steels due to the effect of tissue homogenization, thereby improving workability and formability. Specifically, a steel sheet having excellent ductility, formability, and weldability can be provided, in which the value of relation 2 for the hole expansion ratio (HER), yield strength (YS), tensile strength (TS), and elongation (EL) is 500 or more, and the yield ratio (YS / TS) is 0.65 or more, and no LME cracks are generated on the surface and inside of the welded part.

[0074] [Relation 2]

[0075] HER EL YS / TS≥500

[0076] Next, the manufacturing method of the steel sheet of the present invention will be described in detail.

[0077] The manufacturing method of the steel sheet of the present invention includes the following processes: preparing a steel slab, then reheating the steel slab, the steel slab satisfying the above steel composition and relation 1; hot-rolling the reheated steel slab so that the finishing mill exit side temperature becomes Ar3 to Ar3 + 50 °C, then winding at 400 - 650 °C, and then cooling to room temperature at an average cooling rate of 0.1 °C or less; cooling the continuously annealed cold-rolled steel sheet at an average cooling rate of 10 °C / second or less for the first cooling to a temperature range of 650 - 680 °C, and then cooling at an average cooling rate of 5 °C / second or more for the second cooling to a temperature of 300 - 340 °C; reheating the cold-rolled steel sheet after the second cooling to a temperature above Ms, and then holding for 60 seconds or more; and cooling the held cold-rolled steel sheet at an average cooling rate of 5 °C / second or more to a temperature below 150 °C.

[0078] First, in the present invention, the steel slab having the above composition is reheated under conventional conditions. The reheating process of the slab is a process of heating the steel slab in order to smoothly carry out the subsequent rolling process and fully obtain the desired physical properties of the steel sheet. The present invention does not particularly limit such reheating conditions, as long as they are conventional reheating conditions. As an example, reheating is carried out in the temperature range of 1100 - 1300 °C.

[0079] Next, in the present invention, the reheated steel slab is hot-rolled so that the finishing mill exit side temperature becomes Ar3 to Ar3 + 50 °C, then wound at 400 - 650 °C, and then cooled to room temperature at an average cooling rate of 0.1 °C / second or less.

[0080] The reheated steel billet is hot finish rolled under conventional conditions above the Ar3 transformation point. At this time, the present invention is not restricted by specific hot rolling conditions and conventional hot rolling temperatures can be used. As an example, hot finish rolling can be carried out in the temperature range of 800 - 1000 °C.

[0081] After that, in the present invention, the hot rolled steel plate after the above hot finish rolling is coiled in the temperature range of 400 - 650 °C, and then cooled to room temperature at an average cooling rate of 0.1 °C / second or less, thereby manufacturing a hot rolled steel plate in which carbides serving as austenite nucleation sites are finely dispersed. By uniformly dispersing fine carbides during such a hot rolling process, carbides are dissolved during the subsequent annealing process while austenite is finely dispersed and formed, and finally fine martensite can be uniformly dispersed after annealing.

[0082] In addition, in the present invention, the hot rolled steel plate after the above cooling is cold rolled at a reduction ratio of 40 - 70%, thereby manufacturing a cold rolled steel plate. When the cold rolling reduction ratio is less than 40%, it is difficult to ensure the desired thickness and it is difficult to correct the shape of the steel plate. On the other hand, when the cold rolling reduction ratio exceeds 70%, there is a high possibility of cracks occurring in the edge portion of the steel plate, and there is a problem of bringing cold rolling load. Therefore, in the present invention, it is preferable to limit the cold rolling reduction ratio to 40 - 70%.

[0083] Then, in the present invention, the cold rolled steel plate is continuously annealed in the temperature range of Ac3 - 50 °C to Ac3 - 20 °C. The continuous annealing step is to adjust the fractions of ferrite and austenite to achieve the optimal fine structure fraction. When the continuous annealing temperature is lower than Ac3 - 50 °C, it is difficult to ensure a sufficient austenite fraction, so the ferrite fraction increases after annealing, and thus the desired formability and strength cannot be ensured. On the other hand, when the continuous annealing temperature exceeds Ac3 - 20 °C, excessive austenite is formed and the grain size becomes too coarse, making it difficult to ensure the desired ductility. In addition, the surface enrichment caused by elements such as Si, Mn, and B that deteriorate the surface quality is aggravated, which may reduce the surface quality. Considering this, in the present invention, it is preferable to limit the continuous annealing temperature to Ac3 - 50 °C to Ac3 - 20 °C. More preferably, continuous annealing can be carried out in the temperature range of 810 - 850 °C.

[0084] In addition, in the present invention, the cold rolled steel plate after the above continuous annealing is first cooled at an average cooling rate of 10 °C / second or less to a temperature range of 650 - 680 °C, and then secondarily cooled at an average cooling rate of 5 °C / second or more to a temperature of 300 - 340 °C, thereby introducing fresh martensite. At this time, in the present invention, hydrogen from a hydrogen rapid cooling device can also be used to cool the steel plate during the secondary cooling.

[0085] In the present invention, it is very important to control the rapid cooling temperature during secondary cooling to 300 - 340 °C below the martensite formation temperature, i.e., Ms. When the rapid cooling temperature during secondary cooling exceeds 340 °C, the fraction of martensite formed initially is very small or it is difficult to form martensite. Therefore, it is difficult to form the required fraction of tempered martensite and bainite during final cooling, and the required hole expansion rate cannot be obtained. In addition, when the rapid cooling temperature during secondary cooling is reduced to below 300 °C, the fractions of tempered martensite and bainite are too high, and it is difficult to form the required fraction of newly formed martensite during final cooling, thus the required strength cannot be obtained.

[0086] Next, in the present invention, the cold-rolled steel sheet after the secondary cooling is reheated to a temperature above Ms and then held for 60 seconds or more.

[0087] In the Q&P process of the present invention, after the above-mentioned primary slow cooling process and secondary rapid cooling process to below Ms and then reheating to above Ms, it is important to control the rapid cooling temperature and the reheating temperature to form the required fine microstructure. By holding the reheated cold-rolled steel sheet for 60 seconds or more, tempered martensite and bainite are formed, and C is enriched into the surrounding austenite that has not undergone a phase change.

[0088] After that, in the present invention, the cold-rolled steel sheet after the holding is cooled at an average cooling rate of 5 °C / second or more to a temperature below 150 °C, thereby introducing fine newly formed martensite.

[0089] In addition, if necessary, the cooled steel sheet can be subjected to skin pass rolling of less than 1%.

[0090] In addition, if necessary, the cooled steel sheet is subjected to one of hot-dip galvanizing and alloyed hot-dip galvanizing, thereby a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet can be manufactured. Detailed Embodiments

[0091] Hereinafter, the present invention will be described in detail by preferred embodiments. However, it should be noted that the following embodiments are only used to illustrate the present invention for more detailed description and are not used to limit the scope of the rights of the present invention.

[0092] (Embodiment)

[0093] Prepare a steel billet having the composition shown in Table 1 below, then reheat it in the temperature range of 1050 - 1250 °C, and perform hot finish rolling in the temperature range of Ar3 + 50 °C above the Ar3 transformation point temperature. Coil the hot-rolled steel plate as described above at 400 - 650 °C, and then cool it at a cooling rate of 0.1 °C per second or less to manufacture a hot-rolled steel plate. Pickle the hot-rolled steel plate, then perform cold rolling with a reduction rate of 40 - 70%, then perform continuous annealing under the conditions shown in Table 2 below, and perform Q&P heat treatment. After that, cool it at an average cooling rate of 5 °C per second or more to a temperature below 150 °C, and then perform skin pass rolling with a reduction of less than 1% to manufacture the final steel plate. In addition, in this experiment, when performing Q&P heat treatment, the following conditions are applied: the cooling rate during the first cooling is 6 °C per second, the cooling rate during the second cooling is 15 °C per second, and the holding time after reheating is 200 seconds.

[0094] Evaluate the mechanical properties and microstructure properties of each of the steel plates manufactured as described above, and show the results in Table 3. At this time, the tensile test of each test piece is carried out along the C direction using the JIS standard to evaluate the tensile properties. The microstructure fraction is analyzed based on the matrix structure at the t / 4 position of the annealed steel plate, and the results are used. Specifically, after etching with nitric acid ethanol (Nital), use FE-SEM and an image analyzer to measure the fractions of ferrite, bainite, martensite, and austenite. In addition, measure the hole expansion rate using a hole expansion testing machine. In addition, evaluate whether spot welding LME cracks occur.

[0095] [Table 1]

[0096]

[0097] In Table 1, the balance components are Fe and inevitable impurities. In addition, Relationship 1 is C + Si / 30 + Mn / 20 + (Cr + Mo) / 5

[0098] In Table 1, the content of N is uniformly at the level of 30 - 50 ppm.

[0099] [Table 2]

[0100]

[0101] [Table 3]

[0102]

[0103] In Table 3, F represents ferrite, B+TM represents bainite+tempered martensite, FM represents fresh martensite, and RA represents retained austenite. In addition, S is the relational expression HER EL YS / TS

[0104] As shown in Tables 1 to 3 above, in the case of Invention Examples 1 to 4 where the steel composition range, manufacturing conditions, and steel microstructure satisfy the scope of the present invention, the hole expansion ratio (HER) is 45% or more, and the values of the hole expansion ratio (HER), yield strength (YS), tensile strength (TS), and elongation (EL) according to Relational Expression 2 are all 500 or more. In addition, while no surface cracks in the welded part and internal LME cracks are generated, the yield ratio (YS / TS) is 0.65 or more. Therefore, it can be seen that the material quality, hole expansion ratio, and weldability of the steel plate desired by the present invention can be ensured.

[0105] On the other hand, in the case of Comparative Examples 1 to 8 where the steel composition (Relational Expression 1) and manufacturing process deviate from the scope of the present invention or the fraction of the steel microstructure deviates from the category of the present invention, it can be seen that the values of the hole expansion ratio (HER), yield strength (YS), tensile strength (TS), and elongation (EL) according to Relational Expression 2 are all less than 500, or the yield ratio (YS / TS) is less than 0.65, and the hole expansion ratio is less than 30%. Therefore, it can be seen that the strength, ductility, hole expansion ratio, and weldability of the steel plate desired by the present invention cannot be ensured simultaneously.

[0106] Specifically, in Comparative Example 1, the annealing temperature is too high, resulting in a low elongation rate and failure to ensure the hole expansion ratio. In Comparative Example 2, the annealing temperature is too low, forming too much duplex zone ferrite, thus failing to obtain the desired strength.

[0107] In Comparative Example 3, the secondary cooling termination temperature is too high, thus failing to sufficiently ensure tempered martensite, and therefore failing to ensure the hole expansion ratio.

[0108] In Comparative Example 4, the secondary cooling termination temperature is too low, resulting in excessive formation of tempered martensite and bainite, and thus the strength deteriorates.

[0109] In addition, in Comparative Examples 5 to 8, which are cases where the steel composition (Relational Expression 1) deviates from the scope of the present invention, the desired mechanical properties cannot be satisfied, and surface cracks in the welded part and internal LME cracks are generated.

[0110] In addition, Figure 1 is a graph showing the change of Relational Expression 1 of the inventive steel and comparative steel in the examples of the present invention according to the values of the hole expansion ratio (HER), yield strength (YS), tensile strength (TS), and elongation (EL) according to Relational Expression 2, Figure 2It is a graph showing the change in the hole expansion ratio (HER) of the inventive steel and the comparative steel according to Equation 1 in the embodiments of the present invention, and Figure 3 It is a graph showing the change in the yield ratio of the inventive steel and the comparative steel according to Equation 1 in the embodiments of the present invention. Additionally, in the above Figures 1 to 3 the inventive steel refers to the steel corresponding to Invention Examples 1 to 4, and the comparative steel refers to the steel corresponding to Comparative Examples 5 to 8.

[0111] As described above, although the preferred embodiments of the present invention are described in the disclosure of the present invention, it is natural that those skilled in the art to which the present invention pertains can make various modifications without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but is determined by the claims and their equivalents.

Claims

1. A steel plate, by weight %, the steel plate comprises: carbon (C): 0.06 - 0.16%, silicon (Si): below 0.8% and excluding 0%, manganese (Mn): 1.6 - 2.6%, molybdenum (Mo): below 0.40% and excluding 0%, chromium (Cr): below 1.0% and excluding 0%, phosphorus (P): below 0.10% and excluding 0%, sulfur (S): below 0.020% and excluding 0%, aluminum (sol.Al): below 0.60% and excluding 0%, titanium (Ti): 0.001 - 0.04%, niobium (Nb): 0.001 - 0.04%, nitrogen (N): below 0.01% and excluding 0%, boron (B): below 0.010% and including 0%, the balance of Fe and inevitable impurities, and C, Si, Mn, Cr and Mo satisfy the following relational expression 1, By area %, the microstructure comprises ferrite of 20% or more and 40% or less, the sum of bainite and tempered martensite of 30% or more and 60% or less, the balance of fresh martensite and retained austenite, and the area fraction of the retained austenite is less than 5% and excluding 0%, And the hole expansion rate (HER) is 30% or more, the value according to the following relational expression 2 for the hole expansion rate (HER), yield strength (YS), tensile strength (TS) and elongation (EL) is 500 or more, and the yield ratio (YS / TS) is 0.65 or more, [Relational expression 1] C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≥ 0.300 Among them, C, Si, Mn, Cr and Mo represent the steel component content % in the matrix structure at the position of the base plate thickness t / 4, [Relational expression 2] HER EL YS / TS ≥ 500.

2. The steel plate according to claim 1, wherein, The fraction of the retained austenite satisfies 1 - 4% by area %.

3. The steel plate according to claim 1, wherein, One of a hot-dip galvanized layer and an alloyed hot-dip galvanized layer is formed on at least one surface.

4. A manufacturing method of a steel plate, which comprises the following processes: preparing a steel billet, and then reheating the steel billet. By weight %, the steel billet comprises: carbon (C): 0.06 - 0.16%, silicon (Si): below 0.8% and excluding 0%, manganese (Mn): 1.6 - 2.6%, molybdenum (Mo): below 0.40% and excluding 0%, chromium (Cr): below 1.0% and excluding 0%, phosphorus (P): below 0.10% and excluding 0%, sulfur (S): below 0.020% and excluding 0%, aluminum (sol.Al): below 0.60% and excluding 0%, titanium (Ti): 0.001 - 0.04%, niobium (Nb): 0.001 - 0.04%, nitrogen (N): below 0.01% and excluding 0%, boron (B): below 0.010% and including 0%, the balance of Fe and inevitable impurities, and C, Si, Mn, Cr and Mo satisfy the following relational expression 1; Hot-rolling the reheated steel billet to make the finish rolling exit side temperature be Ar3 to Ar3 + 50 °C, then, coiling at 400 - 650 °C, and then cooling to room temperature at an average cooling rate of 0.1 °C / second or less; The hot-rolled steel sheet cooled as described above is cold-rolled at a reduction ratio of 40 - 70% to manufacture a cold-rolled steel sheet; The cold-rolled steel sheet is continuously annealed at a temperature of Ac3 - 50°C to Ac3 - 20°C; The cold-rolled steel sheet after the continuous annealing is first cooled at an average cooling rate of 10°C / second or less to a temperature range of 650 - 680°C, and then secondarily cooled at an average cooling rate of 5°C / second or more to a temperature of 300 - 340°C; The cold-rolled steel sheet after the secondary cooling is reheated to a temperature above Ms and then held for 60 seconds or more; and The cold-rolled steel sheet after the holding is cooled at an average cooling rate of 5°C / second or more to a temperature below 150°C, [Relationship 1] C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≥ 0.300 Among them, 5. The manufacturing method of the steel plate according to claim 4, wherein, C, Si, Mn, Cr, and Mo represent the steel component contents % in the matrix structure at the position of t / 4 of the base steel sheet thickness.

6. The manufacturing method of the steel plate according to claim 4, wherein, Continuous annealing is carried out in the temperature range of 810 - 850°C.

7. The manufacturing method of the steel plate according to claim 4, wherein, The cooled steel sheet is further subjected to skin pass rolling of less than 1%. The cooled steel sheet is further subjected to one kind of plating among hot-dip galvanizing and alloyed hot-dip galvanizing.

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