Steel sheet with excellent formability and work hardening rate

Abstract

The present invention relates to a steel sheet suitable for use in automobile structural parts and the like, and more particularly to a steel sheet having high strength, excellent formability and work hardening rate, and a method for manufacturing the steel sheet.

Description

Technical Field

[0001] The present invention relates to a steel sheet suitable for use in automotive structural components and the like. More specifically, the present invention relates to a steel sheet having high strength, excellent formability, and work hardening rate, and a method for manufacturing the same. Background Art

[0002] In recent years, environmental and safety regulations in the automotive industry have become increasingly strict. Emission regulations for carbon dioxide (CO 2 ) have also become gradually strict, and thus regulations on fuel efficiency are being strengthened.

[0003] The Insurance Institute for Highway Safety in the United States has gradually strengthened collision safety regulations for protecting passengers and has required severe collision performance such as 25% small overlap since 2013.

[0004] The only solution to address such environmental and safety issues is to achieve vehicle lightweighting. To lightweight a vehicle, high strength of steel is required, and to apply high-strength steel, high formability is also needed.

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

[0006] Among them, in terms of solid solution strengthening and strengthening by grain refinement, there are limitations in manufacturing high-strength steel with a tensile strength of 490 MPa or more.

[0007] In addition, precipitation hardening type high-strength steel is a technology in which carbonitrides are precipitated by adding carbonitride forming elements such as Cu, Nb, Ti, V, etc. to strengthen the steel sheet, or the growth of grains is suppressed by fine precipitates to refine the grains, thereby ensuring strength. Compared with low manufacturing costs, this precipitation hardening technology has the advantage of easily obtaining high strength, but since the fine precipitates cause a sharp increase in the recrystallization temperature, there is a disadvantage that high-temperature annealing is required to achieve sufficient recrystallization to ensure ductility.

[0008] Furthermore, precipitation hardening steel that is strengthened by precipitating carbonitrides on a ferrite matrix has limitations in obtaining high-strength steel of 600 MPa or more.

[0009] Regarding transformation-strengthened high-strength steels, various steels have been developed, such as dual-phase (DP) steels that form a hard martensite phase in a ferrite matrix, transformation-induced plasticity (TRIP) steels that utilize the transformation-induced plasticity of retained austenite, or complexed-phase (CP) steels composed of ferrite and hard bainite or martensite structures.

[0010] In recent years, regarding steel sheets for automobiles, in order to improve fuel efficiency or durability, etc., steel sheets with higher strength are required. In terms of collision safety and protecting passengers, high-strength steel sheets with a tensile strength of 490 MPa or more are increasingly being used as body structures or reinforcements.

[0011] However, as the strength of the material gradually becomes higher, defects such as cracks or wrinkles are generated during the stamping process of automotive parts, so there is a limit in manufacturing complex parts.

[0012] Therefore, in terms of improving the workability of high-strength steels, if the uniform elongation (UE) of DP steels, which are currently the most widely used in transformation-strengthened high-strength steels, and the work hardening rate in the strain range of 10% or more can be increased, it is expected that the application of high-strength steels in complex parts can be expanded by preventing processing defects such as cracks or wrinkles that occur during stamping.

[0013] In addition, as an existing technology for improving the workability of high-tensile steel sheets, Patent Document 1 discloses a steel sheet composed of a composite structure with a martensite phase as the main phase, and discloses a method of dispersing fine precipitated copper particles with a particle size of 1 - 100 nm inside the structure to improve the workability of such a steel sheet.

[0014] However, in order to precipitate fine Cu particles, Cu needs to be added in a high content of 2 - 5 wt%, and in this case, red-hot brittleness caused by Cu may occur. In addition, the manufacturing cost will increase excessively.

[0015] As another example, Patent Document 2 discloses a steel sheet having a fine structure with ferrite as the matrix structure and containing 2 - 10 area% of pearlite phase, and adding elements such as Ti as precipitation-strengthening elements to improve the strength through precipitation strengthening and grain refinement. In this case, although the hole expansion property of the steel sheet is good, there are limitations in increasing the tensile strength, and the yield strength is high and the ductility is low, so there are problems such as cracks occurring during stamping.

[0016] As another example, Patent Document 3 discloses a method for manufacturing a cold-rolled steel sheet. In the cold-rolled steel sheet, high strength and high ductility are obtained simultaneously by using a tempered martensite phase, and the sheet shape after continuous annealing is also excellent. However, this technology has the following problems: the carbon content in the steel is as high as 0.2% or more, so the weldability is poor, and the furnace dent defect is caused due to the large amount of Si contained.

[0017] (Patent Document 1) Japanese Unexamined Patent Publication No. 2005-264176

[0018] (Patent Document 2) Korean Unexamined Patent Publication No. 2015-0073844

[0019] (Patent Document 3) Japanese Unexamined Patent Publication No. 2010-090432 Summary of the Invention

[0020] Technical Problem to be Solved

[0021] An object of one aspect of the present invention is to provide a steel sheet having a tensile strength of 590 MPa grade, excellent formability and work hardening rate (Nu), which is suitable for use in automotive structural components and the like.

[0022] The technical problem of the present invention is not limited to the above. The technical problem of the present invention can be understood from the entire content of this specification, and those skilled in the art should not have difficulty in understanding the additional technical problems of the present invention.

[0023] Technical Solution

[0024] One aspect of the present invention provides a steel sheet having excellent formability and work hardening rate. By weight%, the steel sheet contains: carbon (C): 0.10 - 0.16%, silicon (Si): 1.0% or less (except 0%), manganese (Mn): 1.4 - 2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less (except 0%), sulfur (S): 0.01% or less (except 0%), aluminum (sol.Al): 1.0% or less (except 0%), nitrogen (N): 0.01% or less (except 0%), antimony (Sb): 0.05% or less (except 0%), and the balance of Fe and other inevitable impurities.

[0025] By area fraction, the fine structure contains 5 - 25% bainite, 3% or more retained austenite, and the balance of ferrite and martensite, and satisfies the following relational expression 1.

[0026] [Relational Expression 1]

[0027] {(C + Si + Al) / ((10×(C + Ti + Nb))+(2×Si)+Mn + Cr) / (TS)}×1000 ≥ 0.28

[0028] (In formula (1), each element represents the weight content, and TS represents the tensile strength (MPa).)

[0029] Another aspect of the present invention provides a method for manufacturing a steel sheet with excellent formability and work hardening rate, which is characterized by including the following steps: preparing a steel billet that meets the above alloy composition; heating the steel billet within a temperature range of 1050 - 1300 °C; performing hot finish rolling on the heated steel billet above the Ar3 transformation point to manufacture a hot rolled steel sheet; coiling the hot rolled steel sheet within a temperature range of 450 - 700 °C; after coiling, cooling to room temperature at a cooling rate of 0.1 °C / second or less; after cooling, performing cold rolling on the cold rolled steel sheet at a cold rolling reduction rate of 40% or more to manufacture a cold rolled steel sheet; continuously annealing the cold rolled steel sheet within a temperature range of Ac1 + 30 °C to Ac3 - 30 °C; after continuous annealing, performing stepwise cooling; and after stepwise cooling, holding for 30 seconds or more,

[0030] wherein the cumulative reduction rate of the 1st stand to the 2nd stand during cold rolling is 25% or more, and the stepwise cooling includes the following steps: performing primary cooling at a cooling rate of 10 °C / second or less (except 0 °C / second) until cooling to 630 - 690 °C; and after primary cooling, performing secondary cooling at a cooling rate of 5 °C / second or more until cooling to 350 - 450 °C, and satisfying formula (1).

[0031] Advantageous Effects

[0032] According to the present invention, by optimizing the alloy composition system and manufacturing conditions of the steel, a steel sheet with high strength and improved formability can be provided.

[0033] As described above, the steel sheet of the present invention with improved formability can prevent processing defects such as cracks or wrinkles generated during stamping forming, and thus has the effect of being applicable to automotive structural components with complex shapes that require high processability. Brief Description of the Drawings

[0034] Figure 1 The figure shows the relationship between the work hardening index (N1, N4), elongation (TE, UE), and tensile strength (TS) corresponding to formula (2) according to the relationship between specific elements in the steel and the tensile strength in an embodiment of the present invention.

[0035] Best Embodiment

[0036] The inventors of the present invention have conducted in-depth research to develop a material having formability at a level applicable to components that need to be processed into complex shapes in automotive materials and the like.

[0037] As a result, it has been confirmed that by optimizing the alloy composition and manufacturing conditions, a high-strength steel sheet having a structure favorable for ensuring desired physical properties can be provided, and thus the present invention has been completed.

[0038] In particular, the present invention is characterized in that there is provided a steel sheet which, by controlling the content of specific elements in the alloy composition and optimizing the process conditions of the steel sheet manufactured through a series of processes, obtains a composite structure in which a soft phase and a hard phase are appropriately dispersed, and at this time, fine retained austenite phases are uniformly distributed around the bainite phase.

[0039] This steel sheet of the present invention has a high work hardening index in the initial stage of plastic deformation, so that work hardening of the entire material can be carried out uniformly, and thus an effect of increasing the work hardening index can also be obtained even in the later stage of plastic deformation. As described above, increasing the work hardening index in the entire strain rate range relieves stress and deformation so that stress and deformation do not concentrate on any part of the material, thereby simultaneously increasing the uniform elongation rate (UE) and the total elongation rate (TE), and thus having technical significance.

[0040] Hereinafter, the present invention will be described in detail.

[0041] In the steel sheet excellent in formability and work hardening rate according to one aspect of the present invention, it may contain, by weight%: carbon (C): 0.10 - 0.16%, silicon (Si): 1.0% or less (except 0%), manganese (Mn): 1.4 - 2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less (except 0%), sulfur (S): 0.01% or less (except 0%), aluminum (sol.Al): 1.0% or less (except 0%), nitrogen (N): 0.01% or less (except 0%), antimony (Sb): 0.05% or less (except 0%).

[0042] Next, the reasons for restricting the alloy composition of the steel sheet provided in the present invention will be described in detail.

[0043] In addition, unless otherwise specifically stated, the content of each element in the present invention is based on weight, and the proportion of the structure is based on area.

[0044] Carbon (C): 0.10 - 0.16%

[0045] Carbon (C) is an important element added to enhance the phase transformation structure of steel. This C promotes the high strength of steel and promotes the formation of martensite in duplex steel. As the content of the C increases, the amount of martensite in the steel also increases.

[0046] However, when the content of this C exceeds 0.16%, due to the increase in the amount of martensite in the steel, the strength becomes high, but the strength difference from ferrite with a relatively low carbon concentration increases. The problem with this strength difference is that cracking easily occurs at the interface between phases when stress is applied, so the ductility and work hardening rate decrease. In addition, due to poor weldability, welding defects occur when processing customer parts. On the other hand, when the content of the C is less than 0.10%, it is difficult to ensure the desired strength, and there is a problem that it is difficult to ensure a small amount of retained austenite phase that is beneficial for obtaining a high uniform elongation rate.

[0047] Therefore, the content of the C can be 0.10 - 0.16%, and more preferably, the content of the C can be 0.11% or more.

[0048] Silicon (Si): 1.0% or less (except 0%)

[0049] Silicon (Si) is a ferrite stabilizing element that promotes ferrite phase transformation and promotes C enrichment in untransformed austenite, thus promoting the formation of martensite. In addition, silicon has excellent solid solution strengthening ability, thus effectively reducing the hardness difference between phases by increasing the strength of ferrite, and silicon is an effective element for ensuring strength without reducing the ductility of the steel plate.

[0050] When the content of this Si exceeds 1.0%, it causes surface scale defects, so the plating surface quality is poor, and the chemical conversion treatment property is impaired.

[0051] Therefore, in the present invention, it is preferable to control the content of the Si to 1.0% or less and except 0%. More preferably, the content of the Si can be 0.2 - 1.0%.

[0052] Manganese (Mn): 1.4 - 2.2%

[0053] Manganese (Mn) has the following effects: it makes the particles finer without reducing the ductility, and precipitates sulfur (S) in the steel as MnS, thus preventing hot brittleness caused by the formation of FeS. In addition, the Mn is an element that strengthens the steel and plays a role in reducing the critical cooling rate for obtaining a martensite phase in duplex steel, so it is effective for more easily forming martensite.

[0054] When the content of Mn is less than 1.4%, not only the above effects cannot be obtained, but also it is difficult to ensure the desired level of strength. On the other hand, when the content of Mn exceeds 2.2%, the possibility of problems such as weldability and hot rollability is high, and excessive martensite is formed, resulting in unstable material properties. Moreover, Mn bands (Mn oxide bands) are formed in the structure, increasing the risk of processing cracks and plate fracture. In addition, Mn oxides are dissolved on the surface during annealing, greatly damaging the plating properties.

[0055] Therefore, in the present invention, it is preferable to control the content of Mn to 1.4 - 2.2%. More preferably, the content of Mn can be 1.5 - 2.1%.

[0056] Chromium (Cr): 1.0% or less

[0057] Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength. This Cr is effective for the formation of martensite, and minimizes the reduction of elongation rate relative to the increase in strength, thus facilitating the manufacture of dual-phase steel with high ductility. In particular, during the hot rolling process, Cr-based carbides such as Cr 23 C 6 are formed. Part of these carbides is dissolved and part remains undissolved during annealing. After cooling, the amount of C dissolved in martensite can be controlled below an appropriate level, thereby suppressing the occurrence of yield point elongation (YP-El) and having the effect of facilitating the manufacture of dual-phase steel with a low yield ratio.

[0058] However, when the content of Cr exceeds 1.0%, not only its effect will saturate, but also the hot rolling strength will increase excessively, resulting in poor cold rollability. In addition, the fraction of Cr-based carbides increases and coarsens, so the size of martensite coarsens after annealing, leading to a reduction in elongation rate.

[0059] Therefore, in the present invention, it is preferable to control the content of Cr to 1.0% or less, and even if the content of Cr is 0%, there is no problem in ensuring the desired physical properties.

[0060] Phosphorus (P): 0.1% or less (except 0%)

[0061] Phosphorus (P) is a substitutional element with the largest solution strengthening effect, and is an element that improves planar anisotropy and is conducive to ensuring strength without significantly reducing formability. However, when too much of this P is added, the possibility of brittle fracture is greatly increased, the possibility of plate fracture in the slab during the hot rolling process becomes high, and the plating surface characteristics are damaged.

[0062] Therefore, in the present invention, it is preferable to control the content of P to 0.1% or less, except 0% considering the inevitably added level.

[0063] Sulfur (S): 0.01% or less (except 0%)

[0064] Sulfur (S) is an impurity element in steel and is an element that is inevitably added. It impairs ductility and weldability. Therefore, it is preferable to control the content of said S as low as possible. In particular, said S has the problem of increasing the possibility of red hot brittleness. Therefore, it is preferable to control its content to 0.01% or less. However, 0% is excluded considering the level inevitably added during the manufacturing process.

[0065] Aluminum (sol.Al): 1.0% or less (except 0%)

[0066] Aluminum (sol.Al) is an element added for grain refinement and deoxidation of steel. In addition, similar to Si, aluminum is a ferrite stabilizing element. It distributes carbon in the ferrite into the austenite, so it is an effective component for improving the hardenability of martensite. And when held in the bainite region, aluminum effectively inhibits the precipitation of carbides in the bainite, so it is an effective element for improving the ductility of the steel plate.

[0067] When the content of such Al exceeds 1.0%, it is beneficial to improve the strength through the grain refinement effect. However, during the steelmaking continuous casting operation, excessive inclusions are formed, so the possibility of surface defects occurring on the coated steel plate becomes high. In addition, it leads to an increase in manufacturing costs.

[0068] Therefore, in the present invention, it is preferable to control the content of said Al to 1.0% or less, and 0% is excluded. More preferably, the content of said Al can be 0.7% or less. Aluminum in the present invention refers to acid-soluble aluminum (Sol.Al).

[0069] Nitrogen (N): 0.01% or less (except 0%)

[0070] Nitrogen (N) is an effective element for stabilizing austenite. However, when the content of N exceeds 0.01%, the refining cost of the steel will increase sharply, and due to the formation of AlN precipitates, the risk of cracks occurring during continuous casting will increase significantly.

[0071] Therefore, in the present invention, it is preferable to control the content of said N to 0.01% or less, but 0% is excluded considering the inevitably added level.

[0072] Antimony (Sb): 0.05% or less (except 0%)

[0073] Antimony (Sb) is distributed at the grain boundaries and serves to delay the diffusion of oxidizing elements such as Mn, Si, and Al through the grain boundaries. Therefore, antimony has the following effects, namely, suppressing the surface enrichment of oxides and being conducive to suppressing the coarsening of surface enrichment substances caused by the rise in temperature and changes in the hot rolling process.

[0074] When the content of this Sb exceeds 0.05%, not only will its effect saturate, but also the manufacturing cost will increase and the workability will deteriorate.

[0075] Therefore, in the present invention, it is preferable to control the content of the Sb to be 0.05% or less, excluding 0%. More preferably, the content of the Sb can be 0.005% or more.

[0076] The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, it is inevitable to mix in unwanted impurities from the raw materials or the surrounding environment, so it is impossible to exclude them. These impurities are well known to those skilled in the normal manufacturing process, so no special description is given to all of its content in this specification.

[0077] In addition, the steel plate of the present invention does not contain titanium (Ti) and niobium (Nb). When Ti and Nb are contained in the steel, the strength of ferrite is increased significantly, so when stress is applied from the outside, the deformation of effective ferrite is restricted, and as a result, the work hardening rate and uniform elongation rate may be significantly impaired.

[0078] Therefore, in the present invention, the Ti and the Nb are not contained. However, there is a possibility that Ti and Nb are added at the impurity level during the process of manufacturing steel. In this case, the physical properties of the present invention are not impaired. Specifically, when the respective contents of Ti and Nb are 0.010% or less, it indicates the impurity level. More preferably, the content of each element can be 0.008% or less.

[0079] The relationship between the contents of C, Si, Al, Mn, Cr, Nb, Ti in the steel of the steel plate of the present invention having the above alloy composition and the tensile strength (TS) of the steel plate preferably satisfies the following relational expression 1. Herein, in the steel (inside the steel) refers to the position at 1 / 4t in the thickness direction of the steel plate (t represents the thickness (mm) of the steel plate).

[0080] The main object of the present invention is to achieve high strength and improve formability and work hardening rate. For this purpose, it is necessary to form a structure conducive to ensuring the desired physical properties by optimizing the alloy composition and manufacturing conditions of the steel.

[0081] As will be specifically described below, the present inventors have found that when a soft phase and a hard phase are uniformly distributed as the steel structure, it is possible to promote the improvement of formability and work hardening rate.

[0082] Therefore, preferably, the contents of elements Ti and Nb that may impair the uniform elongation of steel are reduced as much as possible, and while increasing the contents of elements (C, Si, Al) that are beneficial to the formation of bainite phase and fine retained austenite phase, the ratio of Mn to Cr that is beneficial to improving hardenability is controlled.

[0083] More specifically, by ensuring that the value of the compositional relationship represented by the following relational expression 1 is 0.28 or more, the desired tissue composition and physical properties in the present invention can be advantageously obtained.

[0084] When the value of the following relational expression 1 is less than 0.28, the desired tissue composition cannot be ensured.

[0085] [Relational expression 1]

[0086] {(C + Si + Al) / ((10×(C + Ti + Nb))+(2×Si)+Mn+Cr) / (TS)}×1000≥0.28

[0087] (In relational expression 1, each element represents the weight content, and TS represents the tensile strength (MPa).)

[0088] The fine structure of the steel plate of the present invention having the above alloy composition uniformly contains a soft phase and a hard phase, and in terms of area fraction, it can specifically be composed of 5 - 25% bainite, 3% or more retained austenite, and the balance ferrite and martensite.

[0089] The steel plate of the present invention contains a specified amount of Si and Al in the steel, whereby the precipitation of carbides is delayed during bainite transformation, so that carbon (C) is enriched in the untransformed austenite around the bainite. Therefore, the martensite transformation temperature is reduced below room temperature, and a retained austenite phase can be ensured at room temperature.

[0090] The bainite phase helps to ensure the strength of the steel and affects the ensuring of a retained austenite phase of a specified fraction or more. Therefore, it is preferably composed of 5 area% or more of the bainite phase. That is, when the fraction of the bainite phase is 5 area% or more, the enrichment of C in the untransformed austenite is promoted, and thus a retained austenite phase that contributes to ductility at a target level can be ensured. More advantageously, 10 area% or more of the bainite phase can be included. However, when the fraction of the bainite phase exceeds 25%, the ductility of the steel decreases, and thus there is a problem that it is difficult to promote the improvement of uniform elongation.

[0091] Moreover, the steel sheet of the present invention contains the retained austenite phase with an area fraction of 3% or more, and transformation-induced plasticity is caused during the forming of the steel sheet, so it has the effect of being beneficial to ensuring ductility. When the fraction of this retained austenite phase is too large, it tends to be vulnerable to liquid metal embrittlement (LME) during spot welding for assembling automotive components. Therefore, considering this situation, it is preferable to contain 10% or less of the retained austenite phase.

[0092] In particular, in the present invention, by mainly distributing the retained austenite phase around the bainite phase, the work hardening rate of the steel is increased.

[0093] Specifically, in the present invention, the fine retained austenite phase adjacent to the bainite phase preferably has an average grain size of 2 μm or less, and the number of retained austenite phases is preferably distributed at 80% or more of the total number of all retained austenite phases. That is, in the present invention, by mainly distributing a specified fraction of the retained austenite phase around the bainite phase, the effect of uniformly performing work hardening during plastic deformation can be obtained.

[0094] Here, being adjacent to the bainite phase means the region up to about 10 μm based on the grain boundary of the bainite phase. At this time, the inside of the grains of the bainite phase is not excluded.

[0095] In addition, in the steel sheet of the present invention, as the hard phase, in addition to the above-mentioned bainite phase, a martensite phase may also be contained, and preferably, a martensite phase with an area fraction of 10 - 30% may be contained.

[0096] 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 of the martensite phase exceeds 30%, the ductility of the steel decreases, so the improvement of the uniform elongation rate cannot be promoted.

[0097] As described above, the steel sheet of the present invention has a high work hardening index in the initial stage of plastic deformation (4 - 6%) by uniformly dispersing fine retained austenite phases around the bainite phase and forming a composite structure with ferrite phase and martensite phase in appropriate fractions, and uniformly performs work hardening of the entire material. Therefore, the effect of increasing the work hardening index can also be obtained in the later stage of plastic deformation (10% to Uniform Elongation %).

[0098] In particular, the relationship between the work hardening index (N1) measured in the strain range of 4 - 6%, the work hardening index (N4) measured in the strain range of 10% to Uniform Elongation (%), the total elongation rate (TE), the uniform elongation rate (UE), and the tensile strength (TS) of the steel sheet of the present invention can satisfy the following relational expression 2.

[0099] Moreover, the steel sheet of the present invention may have a high strength with a tensile strength of 590 MPa or more.

[0100] [Relationship 2]

[0101] (TS × TE × UE × N1 × N4) ≥ 14000

[0102] (where the unit is MPa%).

[0103] The high-strength steel sheet of the present invention may include a zinc-based coating on at least one surface.

[0104] At this time, the zinc-based coating is not particularly limited, but the zinc-based coating may be a zinc coating mainly containing zinc, or a zinc alloy coating containing aluminum and / or magnesium in addition to zinc.

[0105] Hereinafter, a method for manufacturing the steel sheet with excellent formability and work hardening rate provided by the present invention according to another aspect of the present invention will be described in detail.

[0106] In short, the present invention can manufacture the desired steel sheet through [reheating of the steel billet - hot rolling - coiling - cold rolling - continuous annealing - cooling], and then the process of [hot dip galvanizing - (final) cooling] can also be performed.

[0107] The conditions of each step will be described in detail below.

[0108] [Heating of the steel billet]

[0109] First, a steel billet satisfying the above alloy composition is prepared, and then the steel billet can be heated.

[0110] This process is carried out to smoothly perform the subsequent hot rolling process and fully obtain the physical properties of the desired steel sheet. In the present invention, the process conditions of this heating process are not particularly limited as long as they are conventional conditions. As an example, the heating process can be carried out in the temperature range of 1050 - 1300 °C.

[0111] [Hot rolling]

[0112] The heated steel billet as described above can be hot finish rolled above the Ar3 transformation point to manufacture a hot rolled steel sheet, and the outlet side temperature at this time preferably satisfies Ar3 to Ar3 + 50 °C.

[0113] When the outlet side temperature during the hot finish rolling is lower than Ar3, two-phase zone rolling of ferrite and austenite is carried out, which may lead to non-uniform material quality. On the other hand, when the outlet side temperature exceeds Ar3 + 50 °C, abnormally large grains are formed due to hot rolling at high temperature, which may lead to non-uniform material quality, and thus the phenomenon of coil deformation will occur during subsequent cooling.

[0114] More specifically, the hot finish rolling can be carried out in a temperature range of 800 - 1000 °C.

[0115] [Coiling]

[0116] Preferably, the hot rolled steel sheet manufactured as described above is coiled.

[0117] The coiling is preferably carried out in a temperature range of 450 - 700 °C. If the coiling temperature is lower than 450 °C, too much martensite phase or bainite phase is formed, resulting in an excessive increase in the strength of the hot rolled steel sheet, and thus problems such as poor shape due to load may occur during subsequent cold rolling. On the other hand, when the coiling temperature exceeds 700 °C, the surface enrichment and internal oxidation of elements such as Si and Mn in the steel that reduce the wettability of hot dip galvanizing may become serious.

[0118] [Cooling]

[0119] Preferably, the coiled hot rolled steel sheet is cooled to room temperature at an average cooling rate of 0.1 °C / second or less (except 0 °C / second). More preferably, it can be carried out at an average cooling rate of 0.05 °C / second or less, and even more preferably, it can be carried out at an average cooling rate of 0.015 °C / second or less. Herein, the cooling refers to the average cooling rate.

[0120] As described above, the coiled hot rolled steel sheet is cooled at a specified rate, so that a hot rolled steel sheet in which carbides serving as nucleation sites of austenite are finely dispersed can be obtained. That is, during hot rolling, fine carbides are uniformly dispersed in the steel, and during subsequent annealing, the carbides dissolve, and an austenite phase can be finely dispersed and formed in the steel. Therefore, a uniformly dispersed fine martensite phase can be obtained after the annealing is completed.

[0121] [Cold Rolling]

[0122] The hot rolled steel sheet coiled as described above can be cold rolled to manufacture a cold rolled steel sheet, and at this time, a cold rolling reduction rate (cumulative reduction rate) of 40% or more can be used.

[0123] In particular, in the present invention, by controlling the cumulative reduction rate of the initial stands during cold rolling, preferably the cumulative reduction rate of stands 1 to 2, to 25% or more, the stored energy inside the steel is increased, and thus the effect of driving force for promoting the recrystallization of ferrite during subsequent annealing can be obtained. Thereby, the effect of reducing the fraction of unrecrystallized ferrite in the steel can be imparted.

[0124] When unrecrystallized ferrite exists in steel, due to local concentration of deformation and stress, the ductility of the steel deteriorates. On the other hand, recrystallized ferrite relieves the concentration of deformation and stress, thus contributing to improving ductility.

[0125] When the cumulative reduction ratio of the initial No. 1 stand to No. 2 stand during cold rolling is less than 25% or the cold rolling reduction ratio until the final stand is less than 40%, it is difficult to ensure the desired thickness and correct the shape of the steel sheet. When the cold rolling reduction ratio until the final stand during cold rolling exceeds 90%, there is a high possibility of cracks occurring in the edge part of the steel sheet, leading to cold rolling load.

[0126] In the present invention, the cold rolling can be carried out using a rolling mill composed of 5 or 6 stands, but is not limited thereto.

[0127] [Continuous annealing]

[0128] Preferably, the cold-rolled steel sheet manufactured as described above is subjected to a continuous annealing treatment. As an example, the continuous annealing treatment can be carried out in a continuous alloying hot-dip galvanizing furnace.

[0129] The continuous annealing step is a process for recrystallizing while forming a ferrite phase and an austenite phase and decomposing carbon.

[0130] The continuous annealing treatment is preferably carried out in a temperature range of Ac1 + 30°C to Ac3 - 30°C, and more preferably, it can be carried out in a temperature range of 770 - 830°C.

[0131] When the temperature during the continuous annealing is lower than Ac3 - 30°C, sufficient recrystallization cannot be achieved, and it is difficult to form sufficient austenite, so that the fractions of martensite phase and bainite phase at the desired level cannot be ensured after annealing. On the other hand, when the temperature during the continuous annealing exceeds Ac1 + 30°C, productivity decreases, and excessive austenite phase is formed, greatly increasing the fractions of martensite phase and bainite phase after cooling, thus increasing the yield strength and decreasing the ductility, so it is difficult to ensure a low yield ratio and high ductility. In addition, the surface enrichment of elements such as Si and Mn that damage the wettability of hot-dip galvanizing becomes serious, which may reduce the quality of the plating surface.

[0132] [Stepwise cooling]

[0133] Preferably, the cold-rolled steel sheet subjected to the continuous annealing treatment as described above is subjected to stepwise cooling.

[0134] Specifically, the cooling is preferably carried out at an average cooling rate of 10°C / second or less (excluding 0°C / second) (the cooling at this time is called primary cooling) to 630 - 690°C, and then at an average cooling rate of 5°C / second or more (the cooling at this time is called secondary cooling) to 350 - 450°C.

[0135] Primary cooling

[0136] When the termination temperature during the primary cooling is lower than 630°C, due to the too low temperature, the diffusion activity of carbon is low, and the carbon concentration in the ferrite increases, thereby increasing the yield ratio and the tendency to generate cracks during processing. On the other hand, when the termination temperature during the primary cooling exceeds 690°C, it is favorable for the diffusion of carbon, but it has the disadvantage of requiring an excessively high cooling rate during the subsequent cooling (secondary cooling). In addition, when the average cooling rate during the primary cooling exceeds 10°C / second, sufficient diffusion of carbon cannot be achieved.

[0137] In addition, there is no special limitation on the lower limit of the average cooling rate, but considering productivity, it can be carried out at 1°C / second or more.

[0138] Secondary cooling

[0139] After completing the primary cooling under the above conditions, secondary cooling is preferably carried out. At this time, the desired fine microstructure can be induced by controlling the cooling termination temperature and the cooling rate.

[0140] When the termination temperature during the secondary cooling is lower than 350°C or exceeds 450°C, the bainite phase cannot be fully formed, so the fine retained austenite phase distributed around the bainite phase cannot be fully ensured. As a result, a uniform dispersion effect of each phase in the steel cannot be obtained, and it is difficult to improve the workability.

[0141] In addition, when the average cooling rate during the secondary cooling is less than 5°C / second, the pearlite phase is formed, so the desired level of bainite phase may not be formed. In addition, there is no special limitation on the upper limit of the average cooling rate, and a person of ordinary skill in the art can make an appropriate selection considering the specifications of the cooling equipment. As an example, it can be carried out at 100°C / second or less.

[0142] In addition, the secondary cooling can use a hydrogen cooling device using hydrogen (H 2 gas). As described above, by cooling using a hydrogen cooling device, the effect of suppressing surface oxidation that may occur during the secondary cooling can be obtained.

[0143] In addition, when performing segmented cooling as described above, the cooling rate during the secondary cooling can be faster than the cooling rate during the primary cooling.

[0144] [Retention]

[0145] After the stepwise cooling is completed as described above, it is preferable to retain for 30 seconds or more within the cooling temperature range.

[0146] After the above secondary cooling, a retention process is carried out to form a bainite phase, and carbon can be enriched in the untransformed austenite phase adjacent to the formed bainite phase. This aims to form a fine retained austenite phase in the region adjacent to the bainite after all subsequent processes are completed.

[0147] At this time, when the retention time is less than 30 seconds, the amount of carbon enriched in the untransformed austenite phase is insufficient, thus failing to ensure the desired fine microstructure. Additionally, during the retention process, when the retention time exceeds 200 seconds, the bainite fraction is too large, so a specified fraction of martensite phase may not be ensured as the final microstructure.

[0148] [Hot-dip galvanizing]

[0149] After the stepwise cooling and retention processes as described above, it is preferable to immerse the steel sheet in a hot-dip galvanizing bath to manufacture a hot-dip galvanized steel sheet.

[0150] At this time, the hot-dip galvanizing can be carried out under conventional conditions. As an example, it can be carried out within the temperature range of 430 - 490 °C. In addition, the composition of the hot-dip galvanizing bath during the hot-dip galvanizing is not particularly limited and can be a pure zinc bath or a zinc-based alloy bath containing Si, Al, Mg, etc.

[0151] [Final cooling]

[0152] After the hot-dip galvanizing is completed, it is preferable to cool at a cooling rate of 5 °C / second or more to below Ms (martensite transformation start temperature) - 100 °C. During this process, a fine retained austenite phase can be fully formed in the region of the steel sheet (where the steel sheet corresponds to the base material under the coating) adjacent to the bainite phase.

[0153] When the termination temperature during the cooling exceeds Ms - 100 °C, a fine martensite phase and an appropriate fraction of retained austenite phase cannot be fully ensured. When the average cooling rate is less than 5 °C / second, due to the too slow cooling rate, the martensite fraction decreases, so the desired level of strength cannot be ensured. The upper limit of the cooling rate during the final cooling is not particularly limited, but considering the specifications of the cooling equipment, it can be carried out at 100 °C / second or less.

[0154] During the cooling, even when cooled to room temperature, it will not affect ensuring the desired microstructure, where room temperature can represent around 10 - 35 °C.

[0155] As needed, before final cooling, the hot-dip galvanized steel sheet can be subjected to an alloying heat treatment to obtain an alloyed hot-dip galvanized steel sheet. In the present invention, the process conditions for the alloying heat treatment are not particularly limited as long as they are normal conditions. As an example, the alloying heat treatment process can be carried out in the temperature range of 480 - 600°C.

[0156] Furthermore, as needed, the finally cooled hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet is subjected to skin pass rolling, so that a large number of dislocations are formed in the ferrite in the steel, and thus the bake hardenability can be further improved.

[0157] At this time, the reduction ratio is preferably less than 1% (except 0%). If the reduction ratio is 1% or more, although it is beneficial for the formation of dislocations, due to the limitations of equipment capabilities, side effects such as sheet breakage may occur.

[0158] In the steel sheet of the present invention manufactured as described above, in terms of area fraction, the microstructure can contain 5 - 25% bainite, more than 3% retained austenite, and the balance ferrite and martensite. At this time, it can be formed such that the number of retained austenite with an average grain size of 2 μm or less around the bainite phase is 80% or more of the total number of the entire retained austenite.

[0159] The relationship between the specific alloying elements and the tensile strength in the steel of this steel sheet of the present invention satisfies the above-mentioned relational expression 1, and the mechanical and physical properties satisfy relational expression 2, so that an improvement in formability and work hardening rate can be achieved.

[0160] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples 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. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom. Detailed Description of the Invention

[0161] (Examples)

[0162] Steel billets with the alloy compositions shown in Table 1 below are manufactured, and then the steel billets are heated in the temperature range of 1050 - 1250°C, and then hot finish rolled in the temperature range of Ar3 + 50°C to 950°C. After that, each hot-rolled steel sheet is coiled at 450 - 700°C, and then cooled to room temperature at a cooling rate of 0.1°C / second or less to manufacture hot-rolled steel sheets.

[0163] After that, 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 continuous annealing treatment was carried out under the conditions shown in Table 2 below, and then stepped cooling (primary cooling and secondary cooling) was carried out. After completion of the secondary cooling, it was held at this temperature for 30 - 200 seconds.

[0164] After that, galvanizing treatment was carried out in a hot-dip galvanizing bath at 430 - 490 °C, and then after final cooling to room temperature, temper rolling was carried out at a reduction rate of less than 1% to manufacture a hot-dip galvanized steel sheet.

[0165] The microstructure of each steel sheet manufactured as described above was observed, and the mechanical properties were evaluated, and the results are shown in Table 3 below.

[0166] At this time, the tensile test of each specimen was carried out in the L direction using the DIN standard, and the work hardening rate (n) was measured for the strain rate range of 4 - 6% and the strain rate range of 10% to UE%.

[0167] In addition, the fraction of the microstructure was analyzed for the matrix structure at the 1 / 4t position of the plate thickness of the steel sheet subjected to continuous annealing treatment. Specifically, after etching with nitric acid ethanol (Nital), the fractions of ferrite (F), bainite (B), martensite (M), and retained-austenite (R-A) were measured using FE-SEM, an image analyzer, EBSD, and an X-ray diffractometer (XRD), and the proportion of retained austenite with a fine size (average grain size of 2 μm or less) existing within 10 μm of the bainite grain boundary was calculated. The number of retained austenite used to calculate the proportion of fine retained austenite (R-A proportion, %) was determined by the point count method.

[0168] [Table 1]

[0169]

[0170] [Table 2]

[0171]

[0172]

[0173] [Table 3]

[0174]

[0175] In Table 3, in the microstructure of Invention Steels 1 to 6, ferrite is included as the balance structure other than B, M, and R-A. In addition, the balance structure of Comparative Steels 1 to 7 consists only of ferrite or includes ferrite and a part of pearlite.

[0176] In addition, in Table 3, the R-A ratio is calculated as a percentage and shown by 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 present within 10 μm based on the bainite grain boundary to the total number (R-At) of retained austenite.

[0177] And, in Table 3, YS represents the yield strength, TS represents the tensile strength, UE represents the uniform elongation, TE represents the total elongation, N1 and N4 represent the work hardening indices at the corresponding strain rates, and the unit of relational expression 2 is MPa%.

[0178] As shown in Tables 1 to 3, Invention Steels 1 to 6, in which the alloy composition system and manufacturing conditions of the steel both satisfy the conditions proposed in the present invention, form the desired 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 (UE, TE), and work hardening index (N1, N4) (corresponding to relational expression 2) is ensured to be 14,000 or more, thereby ensuring the desired formability and work hardening rate.

[0179] On the other hand, it can be confirmed that Comparative Steels 1 to 7, in which one or more of the alloy composition system and manufacturing conditions of the steel do not satisfy the conditions proposed in the present invention, do not form the desired fine microstructure in the present invention. Thus, the value of relational expression 2 is ensured to be less than 14,000, and therefore, the formability and work hardening rate cannot be ensured.

[0180] Figure 1 The figure shows the changes in the relationship between the work hardening index (N1, N4), elongation (TE, UE), and tensile strength (TS) (corresponding to relational expression 2) of Invention Steels and Comparative Steels according to the relationship between specific alloying elements (C, Si, Al, Mn, Cr, Nb, Ti) and tensile strength (corresponding to relational expression 1).

[0181] As Figure 1 shown, it can be seen that when the relationship between C, Si, Al, Mn, Cr, Nb, Ti, and tensile strength satisfies 0.28 or more, the value of relational expression 2 can be ensured to be 14,000 or more.

Claims

1. A steel sheet having excellent formability and work hardening rate, comprising, in terms of weight %, carbon (C): 0.10-0.16%, silicon (Si): 1.0% or less and 0% excluded, manganese (Mn): 1.4-2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less and 0% excluded, sulfur (S): 0.01% or less and 0% excluded, aluminum (sol.Al): 1.0% or less and 0% excluded, nitrogen (N): 0.01% or less and 0% excluded, antimony (Sb): 0.05% or less and 0% excluded, and the balance of Fe and other inevitable impurities, In terms of area fraction, the microstructure contains 5-25% bainite, more than 3% retained austenite, and the remainder ferrite and martensite. And the following relationship 1 is satisfied: And the following relationship 2 is satisfied, [Equation 1] {(C+Si+Al) / ((10×(C+Ti+Nb))+(2×Si)+Mn+Cr) / (TS)}×1000≥0.28 In Relational Formula 1, each element represents the weight content, and TS represents the tensile strength and the unit is MPa, [Equation 2] (TS×TE×UE×N1×N4)≥14000 in, N1 represents the work hardening index measured in the strain range of 4-6%, N4 represents the work hardening index measured in the strain range of 10% to uniform elongation%, TS represents tensile strength and the unit is MPa, TE represents total elongation and the unit is %, UE represents uniform elongation and the unit is %, and the unit of relation 2 is MPa·% 2 .

2. The steel sheet having excellent formability and work hardening rate according to claim 1, wherein The amount of retained austenite having an average grain size of 2 μm or less existing adjacent to the bainite phase accounts for more than 80% of the total amount of the entire retained austenite.

3. The steel sheet having excellent formability and work hardening rate according to claim 1, wherein The steel sheet contains a martensite phase with an area fraction of 10-30%.

4. The steel sheet having excellent formability and work hardening rate according to claim 1, wherein At least one side of the steel plate includes a zinc coating.

5. The steel sheet having excellent formability and work hardening rate according to claim 1, wherein The steel plate has a tensile strength of 590 MPa or more.

6. A method for manufacturing a steel sheet having excellent formability and work hardening rate, characterized in that: The following steps are involved: A steel billet is prepared, wherein the steel billet comprises, in terms of weight %, carbon (C): 0.10-0.16%, silicon (Si): 1.0% or less and excluding 0%, manganese (Mn): 1.4-2.2%, chromium (Cr): 1.0% or less, phosphorus (P): 0.1% or less and excluding 0%, sulfur (S): 0.01% or less and excluding 0%, aluminum (sol.Al): 1.0% or less and excluding 0%, nitrogen (N): 0.01% or less and excluding 0%, antimony (Sb): 0.05% or less and excluding 0%, and the balance of Fe and other inevitable impurities; Heating the steel billet at a temperature ranging from 1050° C. to 1300° C. The heated steel billet is hot finished rolled at a temperature above the Ar3 phase transformation point to produce a hot rolled steel sheet; Coiling the hot rolled steel sheet at a temperature ranging from 450°C to 700°C; After the winding, cooling to room temperature at a cooling rate of less than 0.1°C / second; After the cooling, cold rolling is performed 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; After the continuous annealing, performing segmented cooling; and After the segmented cooling, keep for more than 30 seconds, The cumulative reduction ratio of the No. 1 stand to the No. 2 stand during the cold rolling is more than 25%. The step cooling comprises the following steps: performing a primary cooling at a cooling rate of less than 10°C / second and excluding 0°C / second to cool to 630-690°C; and performing a secondary cooling at a cooling rate of more than 5°C / second to cool to 350-450°C after the primary cooling. And the following relationship 1 is satisfied: [Equation 1] {(C+Si+Al) / ((10×(C+Ti+Nb))+(2×Si)+Mn+Cr) / (TS)}×1000≥0.28 In Relational Formula 1, each element represents the weight content, and TS represents tensile strength and the unit is MPa.

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

8. The method for manufacturing a steel sheet having excellent formability and work hardening rate according to claim 6, wherein: The secondary cooling is performed in a hydrogen cooling device using hydrogen.

9. The method for manufacturing a steel sheet having excellent formability and work hardening rate according to claim 6, wherein: The following steps are also included: After the holding, hot-dip galvanizing is performed; as well as After the hot-dip galvanizing, final cooling is performed at an average cooling rate of 5°C / sec or more to a temperature of Ms-100°C or less.

10. The method for manufacturing a steel sheet having excellent formability and work hardening rate according to claim 9, wherein: After the hot-dip galvanizing and before the final cooling, the process further includes an alloying heat treatment step.

11. The method for manufacturing a steel sheet having excellent formability and work hardening rate according to claim 9, wherein: After the final cooling, the method further comprises performing temper rolling at a reduction rate of less than 1%.

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