Steel sheet excellent in formability and work hardening rate

Abstract

The present invention relates to a steel sheet suitable for use in structural components of automobiles, and more particularly to a steel sheet having high strength yet excellent formability and work hardening rate, and a method for producing the same.

Description

Technical Field

[0001] The present invention relates to a steel sheet suitably used for automobile structural members and the like, and more particularly to a steel sheet having high strength and excellent formability and work hardening rate, and a method for manufacturing the same.

Background Art

[0002] Recently, regulations on the environment and safety in the automobile industry have become increasingly strict, and emission regulations for carbon dioxide (CO 2 ) have also become increasingly severe, and accordingly, fuel consumption regulations have been strengthened.

[0003] The Insurance Institute for Highway Safety in the United States has been gradually strengthening regulations on collision stability for protecting passengers, and since 2013, it has required strict collision performance of 25% small overlap.

[0004] The only solution to solve such environmental and safety issues is to achieve weight reduction of automobiles. To reduce the weight of automobiles, it is necessary to increase the strength of steel materials, and high formability is also required for applying high-strength steel materials.

[0005] Generally, methods for strengthening steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, transformation strengthening, and the like.

[0006] Among these, solid solution strengthening and strengthening by grain refinement have limitations in the production of high-strength steel with a tensile strength of 490 MPa or more.

[0007] On the one hand, precipitation-strengthened high-strength steel is a technology that strengthens the steel sheet by precipitating carbonitrides by adding carbon and nitride forming elements such as Cu, Nb, Ti, V, etc., or secures strength by suppressing the growth of crystal grains by fine precipitates. Such precipitation strengthening technology has the advantage that high strength can be easily obtained compared to low manufacturing costs, but since the recrystallization temperature rises rapidly due to fine precipitates, there is a drawback that high-temperature annealing is necessary to cause sufficient recrystallization to secure ductility.

[0008] In addition, precipitation-strengthened steel that strengthens by precipitating carbonitrides in a ferrite matrix has limitations in obtaining high-strength steel of 600 MPa or more.

[0009] Transformation-strengthened high-strength steel includes various steels such as ferrite-martensite two-phase (Dual Phase, DP) steel in which a hard martensite phase is formed in a ferrite matrix, TRIP (Tranformation Induced Plasticity) steel using transformation-induced plasticity of retained austenite, or CP (Complexed Phase) steel composed of a ferrite and a hard bainite or martensite structure.

[0010] Recently, for steel sheets for automobiles, steel sheets with even higher strength have been demanded for improving fuel efficiency and durability. High-strength steel sheets with a tensile strength of 490 MPa or more are used as body structures and reinforcements in terms of collision safety and passenger protection, and their usage is increasing.

[0011] However, as the strength of the material gradually increases to higher strength, defects such as cracks or wrinkles occur during the process of press-forming automotive parts, and there are limitations in manufacturing complex parts.

[0012] Therefore, from the perspective of improving the workability of high-strength steel, if it is possible to improve the uniform elongation rate (UE) of dual-phase (DP) steel, which is currently the most widely used among transformation-induced plasticity (TRIP)-type high-strength steels, and the work hardening rate in the deformation range of 10% or more, it is predicted that by preventing processing defects such as cracks or wrinkles generated during press forming, the application of high-strength steel to complex parts can be expanded.

[0013] On the other hand, as a conventional technique for improving the workability of high-tensile steel sheets, Patent Document 1 discloses a steel sheet composed of a composite structure mainly composed of a martensite phase, and discloses a method of dispersing fine precipitated copper particles with a particle size of 1 to 100 nm inside the structure in order to improve the workability of such a steel sheet.

[0014] However, in order to precipitate fine Cu particles, it is necessary to add Cu at a high content of 2 to 5% by weight. In this case, there is a risk of red-hot brittleness due to Cu. In addition, there is a problem that the manufacturing cost increases excessively.

[0015] As another example, Patent Document 2 discloses a steel sheet having a microstructure containing 2 to 10% by area of a pearlite phase with ferrite as a matrix structure, and adding elements such as Ti, which is a precipitation strengthening type element, to improve the strength by precipitation strengthening and grain refinement. In this case, the hole expansion property of the steel sheet is good, but there is a limit to increasing the tensile strength, and there is a problem that defects such as cracks occur during press forming because the yield strength is high and the ductility is low.

[0016] Furthermore, as still another example, Patent Document 3 discloses a method for manufacturing a cold-rolled steel sheet that utilizes a tempered martensite phase to simultaneously obtain high strength and high ductility and is excellent in the plate shape after continuous annealing. However, this technique has problems such as a high carbon content in the steel of 0.2% or more, resulting in poor weldability, and dent defects in the furnace caused by a large amount of Si contained.

Prior Art Documents

Patent Documents

[0017] [Patent Document 1] Japanese Patent Laid-Open Publication No. 2005-264176 [Patent Document 2] Korean Patent Laid-Open Publication No. 2015-0073844 [Patent Document 3] Japanese Patent Laid-Open Publication No. 2010-090432 [Summary of the Invention] [Problems to be Solved by the Invention]

[0018] One aspect of the present invention is to provide a steel sheet suitable for use in automotive structural members and the like, which has a high strength of 590 MPa class in tensile strength and excellent formability and work hardening rate (Nu).

[0019] The problems of the present invention are not limited to the above-described content. The problems of the present invention can be understood from the entire content of this specification, and there is no difficulty in understanding additional problems of the present invention for those having ordinary knowledge in the technical field to which the present invention pertains. [Means for Solving the Problems]

[0020] One aspect of the present invention provides a steel sheet excellent in formability and work hardening rate, which contains, by weight%, carbon (C): 0.10 to 0.16%, silicon (Si): 1.0% or less (excluding 0%), manganese (Mn): 1.4 to 2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), aluminum (sol.Al): 1.0% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), antimony (Sb): 0.05% or less (excluding 0%), and the balance being Fe and other inevitable impurities, and as a microstructure, contains bainite with an area fraction of 5 to 25%, retained austenite of 3% or more, and the balance being ferrite and martensite, and satisfies the following relational expression 1.

[0021] [Relational Expression 1] {(C + Si + Al) / ((10×(C + Ti + Nb))+(2×Si)+Mn + Cr) / (TS)}×1000 ≥ 0.28 (In relational expression 1, each element means the weight content, and TS means the tensile strength (MPa).)

[0022] Another aspect of the present invention includes the steps of preparing a steel slab satisfying the above-described alloy composition, heating the steel slab in a temperature range of 1050 to 1300 °C, finish hot rolling the heated steel slab above the Ar3 transformation point to produce a hot-rolled steel sheet, coiling the hot-rolled steel sheet in a temperature range of 450 to 700 °C, after coiling, cooling at a cooling rate of 0.1 °C / s or less to room temperature, cold rolling the cooled steel sheet at a cold rolling reduction rate of 40% or more to produce a cold-rolled steel sheet, continuously annealing the cold-rolled steel sheet in a temperature range of Ac1 + 30 °C to Ac3 - 30 °C, performing stepwise cooling after the continuous annealing, and holding for 30 seconds or more after the stepwise cooling. In the cold rolling, the cumulative rolling reduction rate of stands 1 to 2 is 25% or more. The stepwise cooling includes a first cooling step at a cooling rate of 10 °C / s or less (excluding 0 °C / s) to 630 to 690 °C and a second cooling step at a cooling rate of 5 °C / s or more to 350 to 450 °C after the first cooling. A method for manufacturing a steel sheet excellent in formability and work hardening rate satisfying the above relational expression 1 is provided.

Effects of the Invention

[0023] According to the present invention, by optimizing the alloy component system and manufacturing conditions of steel, it is possible to provide a steel sheet having high strength and improved formability.

[0024] Thus, the steel sheet of the present invention with improved formability can prevent processing defects such as cracks or wrinkles generated during press forming, and has the effect of being suitably applicable to automotive structural parts with complex shapes that require high workability.

Brief Description of the Drawings

[0025]

Figure 1

Mode for Carrying Out the Invention

[0026] The inventors of the present invention have intensively studied to develop a material having formability at a level suitable for parts that require processing into complex shapes among automotive materials.

[0027] As a result, it was confirmed that by optimizing the alloy composition and manufacturing conditions, a high-strength steel sheet having a structure advantageous for ensuring the target physical properties can be provided, and the present invention has been completed.

[0028] In particular, the present invention controls the content of specific elements among alloy components and optimizes the process conditions of the steel sheet manufactured through a series of processes to obtain a composite structure in which a soft phase and a hard phase are appropriately dispersed. At this time, there is a feature of providing a steel sheet in which a fine retained austenite phase is uniformly distributed around the bainite phase.

[0029] Such a steel sheet of the present invention has a high work hardening index in the initial stage of plastic deformation, can uniformly advance work hardening throughout the material, and can obtain an effect of increasing the work hardening index even in the later stage of plastic deformation. In this way, by increasing the work hardening index in the entire range of the deformation rate, the stress and deformation are relaxed so as not to concentrate on any part of the material, and it can be said that there is a technical significance in that both the uniform elongation rate (UE) and the total elongation rate (TE) are improved.

[0030] Hereinafter, the present invention will be described in detail.

[0031] The steel sheet excellent in formability and work hardening rate according to one aspect of the present invention contains, by weight%, carbon (C): 0.10 to 0.16%, silicon (Si): 1.0% or less (excluding 0%), manganese (Mn): 1.4 to 2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), aluminum (sol.Al): 1.0% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and antimony (Sb): 0.05% or less (excluding 0%).

[0032] Hereinafter, the reasons for restricting the alloy composition of the steel sheet provided by the present invention as described above will be described in detail.

[0033] On the other hand, unless otherwise specified in the present invention, the content of each element is based on weight, and the ratio of the structure is based on area.

[0034] Carbon (C): 0.10 to 0.16% Carbon (C) is an important element added to strengthen the transformation structure of steel. Such C increases the strength of the steel and promotes the formation of martensite in the duplex steel. As the above C content increases, the amount of martensite in the steel increases.

[0035] However, when the content of such C exceeds 0.16%, although the strength increases due to the increase in the amount of martensite in the steel, the difference in strength from ferrite with a relatively low carbon concentration increases. Such a strength difference has a problem that ductility and work hardening rate decrease because fracture easily occurs at the interface between phases when stress is applied. In addition, there is a problem that the weldability is inferior and welding defects occur during the processing of parts by customer companies. On the other hand, when the content of the above C is less than 0.10%, it is difficult to ensure the target strength, and there is a problem that it becomes difficult to ensure a small amount of retained austenite phase advantageous for obtaining a high uniform elongation rate.

[0036] Therefore, the above C can be contained in an amount of 0.10 to 0.16%, and more preferably 0.11% or more.

[0037] Silicon (Si): 1.0% or less (excluding 0%) Silicon (Si) is a ferrite stabilizing element that promotes the formation of martensite by promoting ferrite transformation and facilitating C enrichment into untransformed austenite. It also has good solid solution strengthening ability, is effective in increasing the strength of ferrite and reducing the hardness difference between phases, and is a useful element for ensuring strength without reducing the ductility of the steel sheet.

[0038] When the content of Si exceeds 1.0%, there is a problem of inducing surface scale defects, resulting in poor surface quality of plating and inhibiting the chemical conversion treatment property.

[0039] Therefore, in the present invention, it is preferable to control the content of the above Si to 1.0% or less, excluding 0%. More preferably, it can contain 0.2 to 1.0%.

[0040] Manganese (Mn): 1.4 - 2.2% Manganese (Mn) has the effect of refining particles without reducing ductility, precipitating sulfur (S) in steel as MnS, and preventing hot brittleness caused by the formation of FeS. In addition, the above Mn is an element that strengthens the steel and plays a role in lowering the critical cooling rate at which a martensite phase can be obtained in duplex steel, so it is useful for more easily forming martensite.

[0041] When the content of such Mn is less than 1.4%, not only the above-described effects cannot be obtained, but it is also difficult to ensure the target level of strength. On the other hand, when its content exceeds 2.2%, problems such as weldability and hot rolling properties are likely to occur, excessive martensite is formed, the material is unstable, Mn-Band (Mn oxide band) is formed in the structure, and the risk of occurrence of processing cracks and plate fracture increases. In addition, there is a problem that Mn oxide elutes on the surface during annealing, greatly inhibiting the plating property.

[0042] Therefore, in the present invention, it is preferable to control the content of Mn to 1.4 to 2.2%. More preferably, it can contain 1.5 to 2.1%.

[0043] Chromium (Cr): 1.0% or less Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength. Such Cr is effective for the formation of martensite, minimizes the decrease in elongation rate with respect to the increase in strength, and is advantageous for the production of a composite structure steel having high ductility. In particular, during the hot rolling process, Cr 23 C 6 forms Cr-based carbides such as this. Since this carbide partially dissolves and partially remains undissolved during the annealing process, and the amount of solid solution C in the martensite after cooling can be controlled below an appropriate level, the occurrence of yield point elongation (YP-El) is suppressed, which has an advantageous effect on the production of a composite structure steel with a low yield ratio.

[0044] However, when the content of Cr exceeds 1.0%, not only does its effect saturate, but there is also a problem that the hot rolling strength excessively increases and the cold rolling property deteriorates. In addition, the fraction of Cr-based carbides increases and coarsens, and the size of martensite coarsens after annealing, leading to a problem of a decrease in elongation rate.

[0045] Therefore, in the present invention, it is preferable to control the content of Cr to 1.0% or less, and it is clarified that even if the content is 0%, there is no problem in ensuring the target physical properties.

[0046] Phosphorus (P): 0.1% or less (excluding 0%) Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect. It is an element that improves in-plane anisotropy and is advantageous for ensuring strength without significantly reducing formability. However, when such P is added in excess, the possibility of brittle fracture significantly increases, the possibility of slab fracture during hot rolling increases, and there is a problem of inhibiting the properties of the plating surface.

[0047] Therefore, in the present invention, it is preferable to control the content of P to 0.1% or less, excluding 0% considering the inevitably added level.

[0048] Sulfur (S): 0.01% or less (excluding 0%) Sulfur (S) is an impurity element in steel and an element that is inevitably added. Since it inhibits ductility and weldability, it is preferable to control its content as low as possible. In particular, since the above S has a problem of increasing the possibility of generating red hot brittleness, it is preferable to control its content to 0.01% or less. However, 0% is excluded considering the inevitably added level during the manufacturing process.

[0049] Aluminum (sol.Al): 1.0% or less (excluding 0%) Aluminum (sol.Al) is an element added for grain refinement and deoxidation of steel. Also, like Si, it is a ferrite stabilizing element and is an effective component for distributing carbon in ferrite to austenite to improve the martensite hardening ability. When held in the bainite region, it is an element useful for improving the ductility of the steel sheet by effectively suppressing the precipitation of carbides in bainite.

[0050] When the content of such Al exceeds 1.0%, it is advantageous for the strength increase due to the grain refinement effect. However, the formation of inclusions becomes excessive during the steelmaking continuous casting operation, and the possibility of surface defects occurring in the plated steel sheet increases. Also, there is a problem of increasing the manufacturing cost.

[0051] Therefore, in the present invention, it is preferable to control the content of the above Al to 1.0% or less, excluding 0%. More preferably, it can contain 0.7% or less. In the present invention, aluminum means acid-soluble aluminum (Sol.Al).

[0052] Nitrogen (N): 0.01% or less (excluding 0%) Nitrogen (N) is an element effective in stabilizing austenite. However, when its content exceeds 0.01%, the refining cost of steel increases sharply, and the risk of crack generation during continuous casting due to the formation of AlN precipitates increases significantly.

[0053] Therefore, in the present invention, it is preferable to control the content of the above N to 0.01% or less, excluding 0% considering the inevitably added level.

[0054] Antimony (Sb): 0.05% or less (excluding 0%) Antimony (Sb) is distributed at the grain boundaries and plays a role in retarding the diffusion of oxidizing elements such as Mn, Si, and Al along the grain boundaries. Thereby, it has an advantageous effect in suppressing the surface enrichment of oxides and suppressing the coarsening of surface enrichments due to temperature rise and changes in the hot rolling process.

[0055] When the content of such Sb exceeds 0.05%, not only does its effect saturate, but there are also problems such as an increase in manufacturing cost and inferior workability.

[0056] Therefore, in the present invention, it is preferable to control the content of the above Sb to 0.05% or less, excluding 0%. More preferably, it is clarified that it can contain 0.005% or more.

[0057] The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, it is inevitable that unintended impurities may be mixed in from the raw materials or the surrounding environment, and thus it cannot be excluded. Since these impurities are understandable to any technician in the normal manufacturing process, all of their contents are not particularly mentioned in this specification.

[0058] On the other hand, the steel sheet of the present invention does not contain titanium (Ti) and niobium (Nb). When Ti and Nb are contained in the steel, since the strength of ferrite is greatly increased, effective deformation of ferrite is restricted when stress is applied from the outside, and as a result, there is a risk of greatly inhibiting the work hardening rate and the uniform elongation rate.

[0059] Therefore, in the present invention, Ti and Nb are not included. However, they may be added at impurity levels during the steel manufacturing process, but in this case, the physical properties of the present invention are not impaired. Specifically, it is clarified that if the content of each is 0.010% or less, it is at the impurity level. More preferably, the content of each of the above elements may be 0.008% or less.

[0060] It is preferable that the relationship between the contents of C, Si, Al, Mn, Cr, Nb, and Ti in the steel and the tensile strength (YS) of the steel sheet having the above alloy composition satisfies the following relational expression 1. Here, "inside the steel" (inside the steel plate) means the 1 / 4t point in the thickness direction of the steel sheet (t means the thickness (mm) of the steel sheet).

[0061] The main object of the present invention is to improve formability and work hardening rate together with high strength. For this purpose, it is necessary to optimize the alloy composition and manufacturing conditions of the steel to form a structure advantageous for ensuring the intended physical properties.

[0062] As will be specifically described later, the present inventors have found that when the soft phase and the hard phase are uniformly distributed as the steel structure, the formability and the work hardening rate can be improved.

[0063] For this purpose, Ti and Nb, which are elements that may inhibit the uniform elongation rate of the steel, should be minimized in content, while increasing the contents of elements (C, Si, Al) that are advantageous for the formation of the bainite phase and the fine retained austenite phase, and controlling the ratio with Mn and Cr that are advantageous for improving hardenability is preferable.

[0064] More specifically, by ensuring that the value of the component relational expression represented by the following relational expression 1 is 0.28 or more, the tissue structure and physical properties intended in the present invention can be advantageously obtained.

[0065] If the value of the following relational expression 1 is less than 0.28, the target tissue structure cannot be ensured.

[0066] [Relational Expression 1] {(C + Si + Al) / ((10 × (C + Ti + Nb)) + (2 × Si) + Mn + Cr) / (TS)} × 1000 ≥ 0.28 (In relational expression 1, each element means the weight content, and TS means the tensile strength (MPa).)

[0067] The steel sheet of the present invention having the above alloy composition homogeneously contains a soft phase and a hard phase as a microstructure. Specifically, it can be composed of bainite with an area fraction of 5 to 25%, retained austenite of 3% or more, and the balance ferrite and martensite.

[0068] The steel sheet of the present invention contains a certain amount of Si and Al in the steel. As a result, the precipitation of carbides is delayed during bainite transformation, and carbon (C) is accumulated in the untransformed austenite around the bainite, so that the martensite transformation temperature becomes lower than normal temperature, and a retained austenite phase can be ensured at normal temperature.

[0069] The above bainite phase contributes to ensuring the strength of the steel and affects the ensuring of a retained austenite phase with a certain fraction or more. Therefore, it is preferably contained in an amount of 5 area% or more. That is, when the fraction of the above bainite phase is 5 area% or more, the enrichment of C in the untransformed austenite is promoted, and a retained austenite phase contributing to ductility can be ensured at a target fraction. More preferably, the above bainite phase can be contained in an amount of 10 area% or more. However, when the fraction exceeds 25%, there is a problem that the ductility of the steel decreases and it becomes difficult to improve the uniform elongation rate.

[0070] Furthermore, the steel sheet of the present invention has an effect that by containing the above retained austenite phase in an area fraction of 3% or more, transformation-induced plasticity occurs during the forming of the steel sheet, which is advantageous for ensuring ductility. When the fraction of such a retained austenite phase becomes excessive, it tends to be vulnerable to liquid metal embrittlement (LME) during spot welding for the assembly of automotive parts. Therefore, in consideration of this, the above retained austenite phase is preferably contained in an amount of 10% or less.

[0071] In particular, the present invention has the effect of increasing the work hardening rate of the steel material by mainly distributing the above-mentioned retained austenite phase around the bainite phase.

[0072] Specifically, in the present invention, the number of fine retained austenite phases existing adjacent to the bainite phase, preferably, the number of retained austenite phases with an average grain size of 2 μm or less, is preferably distributed at 80% or more of the total number of all retained austenite. That is, the present invention can obtain the effect of uniformly advancing work hardening during plastic deformation by mainly distributing a certain fraction of the retained austenite phase around the bainite phase.

[0073] Here, existing adjacent to the bainite phase means a region up to about 10 μm based on the grain boundary of the bainite phase. At this time, it is clarified that the inside of the grains of the bainite phase is not excluded.

[0074] On the other hand, the steel sheet of the present invention can further contain a martensite phase in addition to the bainite phase described above as a hard phase, and preferably, it can be contained at an area fraction of 10 to 30%.

[0075] When the fraction of the martensite phase is less than 10%, the target level of strength cannot be ensured. On the other hand, when the fraction exceeds 30%, the ductility of the steel decreases, and it becomes impossible to improve the uniform elongation rate.

[0076] In this way, the steel sheet of the present invention forms a composite structure in which the ferrite phase and the martensite phase are formed in appropriate fractions while uniformly dispersing fine retained austenite phases around the bainite phase, so that the work hardening index in the initial stage of plastic deformation (4 to 6%) is high, and by uniformly advancing work hardening to the whole material, the effect of increasing the work hardening index can also be obtained in the later stage of plastic deformation (10% to Uniform Elongation%).

[0077] In particular, for the steel sheet of the present invention, the relationships among the work hardening index (N1) measured in the deformation range of 4 to 6%, the work hardening index (N4) measured in the deformation range of 10% to Uniform Elongation (%), the total elongation rate (TE), the uniform elongation rate (UE), and the tensile strength (TS) can satisfy the following relational expression 2. Furthermore, the steel sheet of the present invention can have a high strength with a tensile strength of 590 MPa or more.

[0078] [Relational Expression 2] (TS × TE × UE × N1 × N4) ≥ 14000 (Here, the unit is MPa%).

[0079] The high-strength steel sheet of the present invention can include a zinc-based plating layer on at least one surface. At this time, the zinc-based plating layer is not particularly limited, and it may be a zinc plating layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0080] Hereinafter, a method for manufacturing a steel sheet excellent in formability and work hardening rate provided by the present invention, which is another aspect of the present invention, will be described in detail.

[0081] Briefly speaking, the present invention can manufacture the target steel sheet through [reheating of steel slab - hot rolling - coiling - cold rolling - continuous annealing - cooling], and then, the process of [hot-dip galvanizing - (final) cooling] can be further performed.

[0082] The conditions for each stage will be described in detail below.

[0083] [Heating of Steel Slab] First, after preparing a steel slab satisfying the above-described alloy composition, it can be heated.

[0084] This process is carried out to smoothly perform the subsequent hot rolling process and fully obtain the physical properties of the target steel plate. In the present invention, there are no particular restrictions on the process conditions of such a heating process, and normal conditions may be used. As an example, the heating process can be carried out in the temperature range of 1050 to 1300 °C.

[0085] [Hot rolling] The hot-rolled steel slab heated as described above can be finish hot-rolled at a temperature above the Ar3 transformation point to produce a hot-rolled steel plate. At this time, it is preferable that the outlet-side temperature satisfies Ar3 to Ar3 + 50 °C.

[0086] During the above finish hot rolling, if the outlet-side temperature is less than Ar3, two-phase region rolling of ferrite and austenite is performed, which may cause material variations. On the other hand, if the temperature exceeds Ar3 + 50 °C, material variations may occur due to the formation of abnormally coarse grains by hot rolling at high temperatures, resulting in problems such as coil distortion during subsequent cooling.

[0087] More specifically, the above finish hot rolling can be carried out in the temperature range of 800 to 1000 °C.

[0088] [Coiling] It is preferable to coil the hot-rolled steel plate manufactured as described above. The above coiling is preferably carried out in the temperature range of 450 to 700 °C. However, if the coiling temperature is less than 450 °C, martensite or bainite phases are excessively formed, leading to an excessive increase in the strength of the hot-rolled steel plate, and problems such as shape defects due to load may occur during subsequent cold rolling. On the other hand, when the coiling temperature exceeds 700 °C, surface enrichment and internal oxidation of elements such as Si and Mn in the steel, which reduce the wettability of hot-dip galvanizing, may become intense.

[0089] [Cooling] It is preferable to cool the wound hot-rolled steel sheet to room temperature at an average cooling rate of 0.1 °C / s or less (excluding 0 °C / s). More preferably, it can be carried out at an average cooling rate of 0.05 °C / s or less, and even more preferably 0.015 °C / s or less. Here, cooling means the average cooling rate.

[0090] In this way, by cooling the wound hot-rolled steel sheet at a constant speed, a hot-rolled steel sheet with carbides that become nucleation sites of austenite finely dispersed can be obtained. That is, fine carbides are uniformly dispersed in the steel during the hot rolling process, and during subsequent annealing, this carbide can dissolve while finely dispersing and forming an austenite phase in the steel. As a result, a uniformly dispersed fine martensite phase can be obtained after annealing is completed.

[0091] [Cold Rolling] The hot-rolled steel sheet wound as described above can be cold-rolled to produce a cold-rolled steel sheet, and at this time, it can be carried out with a cold rolling reduction rate (cumulative reduction rate) of 40% or more.

[0092] In particular, in the present invention, during the above cold rolling, by controlling the cumulative reduction rate of the initial stand, preferably the 1st to 2nd stands, to 25% or more, the energy stored in the steel is increased, and in the subsequent annealing process, an effect that acts as a driving force to promote the recrystallization of ferrite can be obtained. As a result, an effect of reducing the fraction of unrecrystallized ferrite in the steel can be imparted.

[0093] When unrecrystallized ferrite exists in the steel, deformation and stress are locally concentrated and the ductility of the steel is inferior, whereas recrystallized ferrite contributes to the improvement of ductility by relaxing deformation and stress concentration.

[0094] During the cold rolling, if the cumulative reduction ratio of the initial 1st to 2nd stands is less than 25%, or the cold rolling reduction ratio up to the final stand is less than 40%, there is a problem that not only is it difficult to ensure the target thickness, but also the shape correction of the steel plate becomes difficult. When the cold rolling reduction ratio up to the final stand exceeds 90% during the cold rolling, there is a high possibility of cracks occurring at the edge part of the steel plate, resulting in a problem of causing the load of cold rolling.

[0095] In the present invention, the cold rolling can be carried out using a rolling mill composed of 5 or 6 stands. However, it is clarified that it is not limited thereto.

[0096] [Continuous annealing] It is preferable to subject the cold-rolled steel sheet manufactured as described above to a continuous annealing treatment. As an example, the continuous annealing treatment can be carried out in a continuous alloying molten plating furnace. The continuous annealing stage is a process for forming ferrite and austenite phases simultaneously with recrystallization and decomposing carbon.

[0097] The continuous annealing treatment is preferably carried out in a temperature range of Ac1 + 30°C to Ac3 - 30°C, and more preferably in a temperature range of 770 - 830°C.

[0098] During the continuous annealing, if the temperature is less than Ac3 - 30°C, not only is sufficient recrystallization not carried out, but also sufficient formation of austenite is difficult, and it is impossible to ensure the fraction of the martensite phase and bainite phase at the target level after annealing. On the other hand, if the temperature exceeds Ac1 + 30°C, the productivity decreases, an excessive amount of austenite phase is formed, the fraction of the martensite phase and bainite phase increases significantly after cooling, the yield strength increases, and the ductility decreases. Therefore, there is a problem that it becomes difficult to ensure a low yield ratio and high ductility. In addition, surface enrichment by elements that inhibit the wettability of hot-dip galvanizing such as Si and Mn becomes intense, and there is a possibility that the surface quality of the plating deteriorates.

[0099] [Stepwise cooling] As described above, it is preferable to cool the continuously annealed cold-rolled steel sheet step by step.

[0100] Specifically, the above cooling is preferably carried out by cooling at an average cooling rate of 10°C / s or less (excluding 0°C / s) to 630 - 690°C (this cooling is referred to as primary cooling), and then cooling at an average cooling rate of 5°C / s or more to 350 - 450°C (this cooling is referred to as secondary cooling).

[0101] "Primary cooling" When the end temperature during the above primary cooling is less than 630°C, the diffusion activity of carbon is too low due to the too low temperature, the carbon concentration in ferrite becomes high, the yield ratio increases, and the tendency of crack generation during processing becomes high. On the other hand, when the end temperature exceeds 690°C, although it is advantageous from the viewpoint of carbon diffusion, there is a drawback that an excessively high cooling rate is required during subsequent cooling (secondary cooling). Also, when the average cooling rate during the above primary cooling exceeds 10°C / s, sufficient carbon diffusion does not occur.

[0102] Note that the lower limit of the above average cooling rate is not particularly limited, but it can be carried out at 1°C / s or more in consideration of productivity.

[0103] "Secondary cooling" After completing the primary cooling under the above-mentioned conditions, it is preferable to carry out secondary cooling. At this time, by controlling the cooling end temperature and the cooling rate, it is possible to induce the formation of a target fine structure.

[0104] When the end temperature during the above secondary cooling is less than 350°C or exceeds 450°C, the bainite phase cannot be sufficiently formed, and the fine retained austenite phase distributed around the bainite phase cannot be sufficiently ensured. As a result, a uniform dispersion effect of each phase in the steel cannot be obtained, and it becomes difficult to improve workability.

[0105] On the other hand, if the average cooling rate during the secondary cooling is less than 5°C / s, a pearlite phase is formed, and there is a risk that the bainite phase may not be formed at the target level. On the other hand, the upper limit of the average cooling rate is not particularly limited and can be appropriately selected by an ordinary technician in consideration of the specifications of the cooling equipment. As an example, it can be carried out at 100°C / s or less.

[0106] Furthermore, for the secondary cooling, a hydrogen cooling facility using hydrogen gas (H 2 gas) can be used. In this way, by performing cooling using a hydrogen cooling facility, an effect of suppressing surface oxidation that may occur during the secondary cooling can be obtained.

[0107] On the other hand, when performing stepwise cooling as described above, the cooling rate during the secondary cooling can be made faster than the cooling rate during the primary cooling.

[0108] [Holding] After completing the stepwise cooling as described above, it is preferable to hold for 30 seconds or more in the cooled temperature range.

[0109] By performing a holding step after the secondary cooling described above, a bainite phase can be formed, and carbon can be concentrated in the untransformed austenite phase adjacent to the formed bainite phase. This is to form a fine retained austenite phase in the region adjacent to the bainite after all subsequent steps are completed.

[0110] At this time, if the holding time is less than 30 seconds, the amount of carbon concentrated in the untransformed austenite phase is insufficient, and the target microstructure cannot be ensured. On the other hand, if the time during the holding step exceeds 200 seconds, the bainite fraction becomes excessive, and there is a risk that a certain fraction of the martensite phase cannot be ensured as the final microstructure.

[0111] [Hot-dip galvanizing] After undergoing stepwise cooling and the holding step as described above, it is preferable to immerse the steel sheet in a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet.

[0112] At this time, hot dip galvanizing can be carried out under normal conditions. For example, it can be carried out in a temperature range of 430 to 490°C. Further, the composition of the hot dip zinc-based plating bath during the above hot dip galvanizing is not particularly limited, and it may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0113] [Final Cooling] After the above hot dip galvanizing is completed, it is preferable to cool at a cooling rate of 5°C / s or more to Ms (martensite transformation start temperature) - 100°C or lower. In this process, a fine retained austenite phase can be sufficiently formed in the region adjacent to the bainite phase of the steel sheet (here, the steel sheet corresponds to the base material below the plating layer).

[0114] During the above cooling, if the end temperature exceeds Ms - 100°C, a fine martensite phase and an appropriate fraction of retained austenite phase cannot be sufficiently ensured. If the average cooling rate is less than 5°C / s, the martensite fraction becomes low due to the too slow cooling rate, and the target level of strength cannot be ensured. Although the upper limit of the cooling rate during the above final cooling is not particularly limited, it should be clarified that it can be carried out at 100°C / s or lower in consideration of the specifications of the cooling equipment.

[0115] During the above cooling, there is no problem in ensuring the target structure even when cooled to room temperature. Here, room temperature can indicate about 10 to 35°C.

[0116] If necessary, before the final cooling, by subjecting the hot dip zinc-based plated steel sheet to an alloying heat treatment, an alloyed hot dip zinc-based plated steel sheet can be obtained. In the present invention, there are no particular restrictions on the conditions of the alloying heat treatment process, and normal conditions may be used. For example, the alloying heat treatment process can be carried out in a temperature range of 480 to 600°C.

[0117] Furthermore, if necessary, by performing temper rolling on the finally cooled hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet, a large amount of dislocations can be formed in the ferrite in the steel, thereby further improving the bake hardenability.

[0118] At this time, the reduction ratio is preferably less than 1% (excluding 0%). If the reduction ratio is 1% or more, although it is advantageous from the viewpoint of dislocation formation, side effects such as occurrence of sheet breakage may occur due to the limit of equipment capacity.

[0119] The steel sheet of the present invention manufactured as described above can include, as a microstructure, bainite with an area fraction of 5 to 25%, retained austenite of 3% or more, and the balance ferrite and martensite. At this time, the number of retained austenite grains with an average grain size of 2 μm or less formed around the bainite phase can be 80% or more of the total number of all retained austenite grains.

[0120] Such a steel sheet of the present invention not only satisfies the relational expression 1 described above regarding the relationship between specific alloy elements in the steel and the tensile strength, but also achieves an improvement in formability and work hardening rate by satisfying the relational expression 2 for mechanical properties.

[0121] Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are for explaining the present invention in more detail by way of illustration, and do not limit the scope of rights of the present invention. The scope of rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

Examples

[0122] (Example) After producing a steel slab having the alloy composition shown in Table 1 below, the steel slab was heated in the temperature range of 1050 to 1250 °C, and then finish hot-rolled in the temperature range of Ar3 + 50 °C to 950 °C. Then, each hot-rolled steel sheet was coiled at 450 to 700 °C, and then cooled to room temperature at a cooling rate of 0.1 °C / s or less to produce a hot-rolled steel sheet.

[0123] Thereafter, each hot-rolled steel sheet was cold-rolled under the rolling conditions shown in Table 2 below to produce a cold-rolled steel sheet, and then subjected to continuous annealing treatment under the conditions shown in Table 2 below, followed by stepwise cooling (primary and secondary cooling). After the secondary cooling was completed, it was held at that temperature for 30 to 200 seconds.

[0124] Thereafter, it was subjected to hot-dip galvanizing treatment in a hot-dip galvanizing bath at 430 to 490 °C, finally cooled to room temperature, and then subjected to temper rolling to less than 1% to produce a hot-dip galvanized steel sheet.

[0125] For each steel sheet manufactured as described above, the microstructure was observed and the mechanical properties were evaluated, and the results are shown in Table 3 below.

[0126] At this time, the tensile test for each test piece was carried out in the L direction using the DIN standard, and the work hardening rate (n) was measured for the work hardening rate values in the strain range of 4 to 6% and the strain range of 10 to UE%.

[0127] In addition, the microstructure fraction was analyzed for the matrix structure at the 1 / 4t point of the thickness of the continuously annealed steel sheet. Specifically, after Nital etching, the fractions of ferrite (F), bainite (B), martensite (M), and retained austenite (R-A) were measured using FE-SEM, an image analyzer, EBSD, and XRD (X-ray diffractor), and the occupancy ratio of the fine-sized (average grain size of 2 μm or less) retained austenite present within 10 μm of the bainite grain boundaries was calculated. The number of retained austenite for calculating the occupancy ratio of the fine retained austenite (occupancy ratio of R-A, %) was carried out by the point count method.

[0128]

Table 1

[0129]

Table 2

[0130]

Table 3

[0131] (In Table 3, for Invention Steels 1 to 6, among the microstructures, ferrite is included as the remaining structure excluding B, M, and R-A. On the other hand, for Comparative Steels 1 to 7, the remaining structure consists only of ferrite or includes some pearlite in ferrite.) Also, in Table 3, the occupancy ratio of R-A is represented as a percentage by calculating the ratio (R-A* / R-At) of the number (R-A*) of fine retained austenite with an average grain size of 2 μm or less existing within 10 μm based on the bainite grain boundary to the number (R-At) of all retained austenite grains.) (And in Table 3, YS represents the yield strength, TS represents the tensile strength, UE represents the uniform elongation rate, TE represents the total elongation rate, N1 and N4 represent the work hardening index at the corresponding strain rate, and the unit of Relational Expression 2 is MPa%.)

[0132] As shown in Tables 1 to 3, Invention Steels 1 to 6, whose alloy component systems and manufacturing conditions all satisfy the ranges proposed in the present invention, form the intended fine microstructure, and thus have a high strength with a tensile strength of 590 MPa or more. At the same time, the relationship between the tensile strength, elongation rates (UE, TE), and work hardening indices (N1, N4) (corresponding to Relational Expression 2) is ensured to be 14,000 or more, and the target formability and work hardening rate can be ensured.)

[0133] On the other hand, Comparative Steels 1 to 7, in which one or more of the alloy component systems and manufacturing conditions do not satisfy what is proposed in the present invention, do not form the fine microstructure intended in the present invention. As a result, it can be confirmed that the value of Relational Expression 2 is ensured to be less than 14,000, and the formability and work hardening rate cannot be ensured.)

[0134] Figure 1 graphically shows the changes in the relationship between the specific alloying elements (C, Si, Al, Mn, Cr, Nb, Ti) of the inventive steel and the comparative steel and the work hardening index (N1, N4), elongation (TE, UE) and tensile strength (TS) according to the relationship (corresponding to relational expression 1).

[0135] As shown in Figure 1, it can be seen that when the relationship between C, Si, Al, Mn, Cr, Nb, Ti and the tensile strength satisfies 0.28 or more, the value of relational expression 2 can be ensured to be 14,000 or more.

Claims

1. By weight, carbon (C): 0.10 - 0.16%, silicon (Si): 0.2 - 1.0%, manganese (Mn): 1.4 - 2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol. Al): 0.025 - 1.0%, nitrogen (N): 0.01% or less, and antimony (Sb): 0.005 - 0.05%, the balance consisting of Fe and other inevitable impurities, as the microstructure, containing bainite with an area fraction of 5 - 25%, retained austenite of 3 - 10% and martensite with an area fraction of 10 - 30%, the balance consisting of ferrite, a steel sheet excellent in formability and work hardening rate, satisfying the following relational expression 1. [Relational Expression 1] {(C + Si + Al) / ((10×(C + Ti + Nb))+(2×Si)+Mn + Cr)}×1000 / (TS)≥0.28 (In Relational Expression 1, each element means the weight content, and TS means the tensile strength (MPa).)

2. The steel sheet excellent in formability and work hardening rate according to Claim 1, wherein the number of retained austenite grains with an average grain size of 2 μm or less existing adjacent to the bainite phase is 80% or more of the total number of all retained austenite grains.

3. The steel sheet excellent in formability and work hardening rate according to Claim 1, wherein the steel sheet includes a zinc-based plating layer on at least one surface.

4. The steel sheet has a tensile strength of 590 MPa or more, and the relationship among the work hardening index (N1) measured in the deformation range of 4 - 6%, the work hardening index (N4) measured in the deformation range of 10 - Uniform Elongation (%), the tensile strength (TS), the total elongation rate (TE) and the uniform elongation rate (UE) satisfies the following relational expression 2. The steel sheet excellent in formability and work hardening rate according to Claim 1. [Relational Expression 2] (TS×TE×UE×N1×N4)≥14000 (Here, the unit is MPa%).

5. preparing a steel slab containing, by weight, carbon (C): 0.10 - 0.16%, silicon (Si): 0.2 - 1.0%, manganese (Mn): 1.4 - 2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol. Al): 0.025 - 1.0%, nitrogen (N): 0.01% or less, and antimony (Sb): 0.005 - 0.05%, the balance consisting of Fe and other inevitable impurities, heating the steel slab in a temperature range of 1050 to 1300 °C; finish hot rolling the heated steel slab at a temperature above the Ar3 transformation point to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 450 to 700 °C; after coiling, cooling at a cooling rate of 0.1 °C / s or less to room temperature; after cooling, cold rolling at a cold rolling reduction rate of 40% or more to produce a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet in a temperature range of Ac1 + 30 °C to Ac3 - 30 °C; performing stepwise cooling after the continuous annealing; holding for 30 seconds or more after the stepwise cooling, including: in the cold rolling, the cumulative reduction rate of stands 1 to 2 is 25% or more; the stepwise cooling includes a primary cooling step at a cooling rate of 10 °C / s or less (excluding 0 °C / s) to 630 to 690 °C and a secondary cooling step at a cooling rate of 5 °C / s or more to 350 to 450 °C after the primary cooling; satisfying the following relational expression 1; a method for manufacturing a steel sheet excellent in formability and work hardening rate, including, as a microstructure, 5 to 25% by area fraction of bainite, 3 to 10% of retained austenite, and 10 to 30% by area fraction of martensite, with the balance being ferrite. [Relational Expression 1] {(C + Si + Al) / ((10×(C + Ti + Nb)) + (2×Si) + Mn + Cr)}×1000 / (TS) ≥ 0.28 (In Relational Expression 1, each element means a weight content, and TS means a tensile strength (MPa).)

6. The method for manufacturing a steel sheet excellent in formability and work hardening rate according to claim 5, wherein in the finish hot rolling, the outlet side temperature satisfies Ar3 to Ar3 + 50 °C.

7. The method for manufacturing a steel sheet excellent in formability and work hardening rate according to claim 5, wherein the secondary cooling is performed using a hydrogen cooling facility using hydrogen (H2) gas.

8. further including a step of hot-dip galvanizing after the holding; and after the hot-dip galvanizing, further including a step of final cooling at an average cooling rate of 5 °C / s or more to Ms - 100 °C or less. The method for manufacturing a steel sheet excellent in formability and work hardening rate according to claim 5.

9. The method for manufacturing a steel sheet excellent in formability and work hardening rate according to claim 8, further including a step of alloying heat treatment after the hot-dip galvanizing and before the final cooling.

10. The method for manufacturing a steel sheet excellent in formability and work hardening rate according to claim 8, further including a step of temper rolling at a reduction rate of less than 1% after the final cooling.

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