High-strength hot-rolled steel sheet with excellent formability and manufacturing method thereof

Abstract

The present invention relates to a hot-rolled steel sheet suitable for use in automobile chassis structural members and the like, and more particularly to a high-strength hot-rolled steel sheet excellent in formability and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet suitable for use in automobile chassis structural members and the like, and more particularly to a high-strength hot-rolled steel sheet with excellent formability and a method for producing the same. [Background technology]

[0002] Recently, in order to reduce global warming, there has been a rapid shift in the automobile market from internal combustion engine vehicles, which are the mainstream, to environmentally friendly vehicles such as electric vehicles.

[0003] As the transition from internal combustion engine vehicles to electric vehicles occurs, the types of components that make up the vehicle change, and the weight of the vehicle also changes. For example, when comparing the weight of an internal combustion engine vehicle and an electric vehicle released in the same model, it is known that the weight of the electric vehicle increases by almost the weight of the battery compared to the internal combustion engine vehicle.

[0004] Meanwhile, chassis parts of automobiles play a role in supporting the vehicle body and are important parts in ensuring ride comfort and driving stability by absorbing road vibrations and shocks while driving. As the weight of an automobile increases, the fatigue load applied to the chassis parts increases, so steel materials used in chassis parts of electric vehicles and the like are required to have excellent fatigue strength.

[0005] Since the fatigue strength of steel is proportional to its tensile strength and yield strength, steel used in chassis parts for electric vehicles and the like must have improved tensile strength and yield strength.

[0006] Furthermore, because chassis parts are manufactured by press molding, in addition to improving tensile strength and yield strength to improve fatigue strength, it is also necessary to ensure formability, such as elongation and hole expandability, suitable for press molding.

[0007] Various techniques have been proposed to improve the strength and formability of hot-rolled steel sheets.

[0008] As an example, Patent Document 1 discloses a method of forming a steel microstructure containing 90% or more bainitic ferrite and controlling the fractions of martensite and bainite to 5% or less each in order to improve hole expandability. While Patent Document 1 discloses that a tensile strength of 980 MPa or more and a hole expandability of 70% or more can be ensured for a hot-rolled steel sheet, it does not disclose any improvement in elongation required for press forming.

[0009] Therefore, in order to ensure the running stability of chassis parts in environmentally friendly vehicles such as electric vehicles, it is necessary to develop steel materials that not only have high tensile strength and yield strength and excellent fatigue life, but also have excellent formability such as elongation and hole expansibility so that they can be easily press-formed. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 2008-255484 Summary of the Invention [Problem to be solved by the invention]

[0011] An object of the present invention is to provide a hot-rolled steel sheet that not only has high strength and excellent fatigue performance but also has excellent formability, and a manufacturing method thereof.

[0012] However, the object of the present invention is not limited to the above-mentioned content, and the object of the present invention can be understood from the entire content of this specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention. [Means for solving the problem]

[0013] One embodiment of the present invention comprises, in weight percent, 0.05 to 0.17% carbon (C), 0.01 to 1.5% silicon (Si), 1.5 to 3.0% manganese (Mn), 0.01 to 0.1% aluminum (Al), 2.0% or less (including 0%) chromium (Cr), 2.0% or less (including 0%) molybdenum (Mo), 0.01 to 0.15% titanium (Ti), 0.001 to 0.05% phosphorus (P), 0.0001 to 0.05% sulfur (S), 0.0001 to 0.02% nitrogen (N), the balance being Fe and other unavoidable impurities, and satisfies the following relational formula 1: The present invention provides a high-strength hot-rolled steel sheet with excellent formability, whose microstructure is composed of a matrix structure of acicular ferrite or bainitic ferrite with an area fraction of 70 to 90% and one or more secondary phases selected from low-temperature bainite, tempered martensite, and MA phases.

[0014] [Equation 1] (1.5×[Si]+1.2×[Cr]+0.7×[Mo]+8.0×[Ti]) / [C]>20 (In Relational Formula 1, each element represents its weight content.)

[0015] Another embodiment of the present invention includes the steps of reheating a steel slab satisfying the above-described alloy composition and Relational Formula 1 in a temperature range of 1100 to 1350°C, hot rolling the reheated steel slab to produce a hot-rolled steel sheet, primarily cooling the hot-rolled steel sheet to a temperature of Bs or less at a cooling rate of 70°C / s or more, after the primary cooling, secondary cooling to a temperature of (Bs+Ms) / 2 or more at a cooling rate of 20°C / s or less, after the secondary cooling, tertiary cooling to a temperature range of Ms-20°C to 500°C at a cooling rate of 30°C / s or more, and coiling in the tertiary cooled temperature range, The present invention provides a method for producing a high-strength hot-rolled steel sheet with excellent formability, characterized in that during the hot rolling, finish hot rolling is performed within a temperature range of 750 to 1150°C so as to satisfy the following relational expression 2, and the total reduction in the final two passes is 10 to 40%.

[0016] [Equation 2] 800≦Du≦1106 (In Relational Formula 2, Du is an index showing the effective grain size of austenite immediately before the first cooling after hot rolling, and is expressed as Du = FDT + (7.35 × [C]) - (24.7 × [Si]) - (4.7 × [Mn]) - (3.9 × [Cr]) - (5.2 × [Mo]) - (560 × [Ti]) - (1110 × [Nb]), where FDT means the rolling finish temperature (°C) and each element means its weight content.) [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a steel sheet that has high strength and excellent formability. Such a steel material of the present invention is suitable for use in automobile chassis structural members and the like. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a graph showing the classification of second phase types according to the size of the second phase in a hot-rolled steel sheet according to an embodiment of the present invention. [Figure 2] 1 shows photographs of microstructures of an example (a) according to an embodiment of the present invention and a comparative example (b). DETAILED DESCRIPTION OF THE INVENTION

[0019] Conventional high-strength hot-rolled steel sheets used in chassis parts and other applications are made by adding large amounts of carbonitride-forming elements such as Ti, Nb, and V, and coiling them at high temperatures around 600°C to induce the precipitation of fine carbonitrides within the ferrite matrix. This precipitation-hardened steel simultaneously offers excellent yield strength, elongation, and hole expandability. However, when grain boundary carbides are formed at ferrite grain boundaries during high-temperature coiling, not only does this reduce resistance to crack propagation, but it also poses the problem of microcracks easily propagating into the steel during forming into parts, resulting in forming cracks, in processes where the formation of localized microdefects is unavoidable, such as shear forming.

[0020] Since the shear stress that induces crack propagation increases in proportion to the strength of the material, the risk of cracks occurring during shear forming is expected to increase sharply for materials with a tensile strength of 980 MPa.

[0021] Therefore, the inventors of the present invention have conducted extensive research to develop a dual-phase hot-rolled steel sheet that utilizes a low-temperature transformed structure and has high strength even in a coiling process at a low temperature.

[0022] Generally, the term "low-temperature transformed structure" refers to a microstructure produced by displacive phase transformation, and typical phases include bainite and martensite.

[0023] Bainite can be defined as a composite structure composed of bainitic ferrite, which is formed by diffusion-free shear transformation, and secondary products formed by the subsequent diffusion of interstitial alloying elements such as carbon. During the shear transformation of bainitic ferrite, numerous dislocations are generated within the structure to absorb the shear deformation. The dislocations generated within bainitic ferrite gradually decrease in density due to a recovery phenomenon while maintained at temperatures above room temperature, where bainite transformation progresses. Because the rate of recovery is significantly affected by temperature, the dislocation density within bainitic ferrite varies depending on the bainite formation and holding temperatures. Therefore, by adjusting the bainitic ferrite base structure, the phase fraction and internal dislocation density can be controlled by adjusting the bainite formation temperature and transformation time, thereby controlling the elongation and yield strength of hot-rolled steel.

[0024] After bainitic ferrite is formed, carbon diffuses from the bainitic ferrite, which has low solid solubility, into untransformed austenite, which then undergoes further bainite transformation or is converted into secondary products. Depending on the temperature of bainitic formation and the type of alloying elements, secondary products can exist as carbides, pearlite, or a martensite-austenite composite phase (MA phase), and the strength and formability of steel vary depending on the type of secondary product. Finely dispersed iron carbides are known to increase the strength of steel without degrading its hole expandability, whereas pearlite simultaneously reduces both the strength and hole expandability of steel. Furthermore, while the MA phase is effective in improving the strength of steel, excessive presence in steel can degrade hole expandability.

[0025] Therefore, the inventors have found that it is important to control the type and fraction of the microstructure to an appropriate level by adjusting the bainite formation temperature and transformation time. They have also confirmed that a steel sheet having high strength as well as excellent formability can be provided by optimizing the alloy composition range of the steel sheet and process conditions such as hot rolling and cooling to control the type and fraction of the matrix structure and secondary phases of the microstructure, and have completed the present invention.

[0026] The present invention will be described in detail below.

[0027] A high-strength hot-rolled steel sheet with excellent formability according to one embodiment of the present invention can contain, by weight, carbon (C): 0.05 to 0.17%, silicon (Si): 0.01 to 1.5%, manganese (Mn): 1.5 to 3.0%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 2.0% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.15%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.05%, and nitrogen (N): 0.0001 to 0.02%.

[0028] The reasons for limiting the alloy composition of the hot-rolled steel sheet provided in the present invention as described above will be explained in detail below. Meanwhile, unless otherwise specified in the present invention, the content of each element is based on weight, and the proportion of the structure is based on area.

[0029] Carbon (C): 0.05~0.17% Carbon (C) is the most economical yet effective element for strengthening steel, and the higher the C content, the more it suppresses the formation of ferrite during cooling.

[0030] In the present invention, the above-mentioned C diffuses into austenite during bainite transformation to stabilize the austenite, and then transforms into a second phase, such as low-temperature bainite, tempered martensite, or a martensite-austenite composite phase (MA phase), in the subsequent cooling process, and is effective in improving the tensile strength and yield strength of the steel.

[0031] If the C content is less than 0.05%, the fraction of the second phase described above will be low, making it difficult to ensure high strength. On the other hand, if the C content exceeds 0.17%, the formation of pearlite will be promoted, making it impossible to ensure strength, resulting in problems such as poor formability and weldability.

[0032] Therefore, in the present invention, the C content may be 0.05 to 0.17%, and more advantageously may be 0.06% or more and 0.15% or less.

[0033] Silicon (Si): 0.01 to 1.5% Silicon (Si) is an element that improves the hardenability of steel and plays a role in improving strength through the solid solution strengthening effect. It also delays the formation of carbides and prevents the formation of pearlite, thereby improving strength by allowing the secondary phase to form as low-temperature bainite, tempered martensite, and MA phase.

[0034] If the Si content is less than 0.01%, carbides are formed, the proportion of MA phase is relatively low, and it becomes difficult to ensure tensile strength. On the other hand, if the Si content exceeds 1.5%, Fe-Si composite oxides are formed on the surface of the slab during slab reheating, which not only deteriorates the surface quality of the steel sheet but also reduces weldability.

[0035] Therefore, in the present invention, the Si content may be 0.01 to 1.5%, more preferably 0.1% or more, and even more preferably 0.3% or more. It is also effective that the Si content is 1.3% or less.

[0036] Manganese (Mn): 1.5-3.0% Manganese (Mn) is an element that improves the hardenability of steel, and prevents the formation of ferrite during cooling after finish rolling, facilitating the formation of a low-temperature transformation structure.

[0037] If the Mn content is less than 1.5%, there are problems such as insufficient hardenability and an excessive increase in the ferrite fraction, whereas if the Mn content exceeds 3.0%, the hardenability increases significantly, preventing smooth bainite transformation in the cooling zone, resulting in an excessive increase in the holding time required to fully form the acicular ferrite or bainitic ferrite that is the matrix structure of the present invention, and a decrease in elongation.

[0038] Therefore, in the present invention, the Mn content may be 1.5 to 3.0%, and more advantageously may be 1.8% or more and 2.4% or less.

[0039] Aluminum (Al): 0.01 to 0.1% Aluminum (Al) is an element added to deoxidize molten steel, and some of it remains in the steel after deoxidation. If the Al content exceeds 0.1%, it increases oxide and nitride inclusions in the steel, deteriorating the formability of the steel sheet. On the other hand, if the Al content is excessively reduced to less than 0.01%, it increases unnecessary refining costs, which is economically disadvantageous.

[0040] Therefore, in the present invention, the Al content may be 0.01 to 0.1%.

[0041] Chromium (Cr): 2.0% or less (including 0%) Chromium (Cr) is an element that improves the hardening ability of steel and suppresses the formation of ferrite during cooling after finish rolling. Chromium also has a strong affinity for carbon, slowing the diffusion rate of carbon and preventing excessive carbon concentration in untransformed austenite after coiling, thereby suppressing the formation of pearlite and inducing the second phase to become a low-temperature transformation phase, thereby contributing to improved yield strength and tensile strength.

[0042] If the Cr content exceeds 2.0%, the hardenability increases significantly, the bainite transformation does not occur smoothly in the cooling zone, and the holding time for securing the fraction of acicular ferrite or bainitic ferrite, which is the base structure, increases excessively, resulting in a deterioration in elongation.

[0043] Therefore, in the present invention, the Cr content may be 2.0% or less, and more advantageously 1.5% or less.

[0044] On the other hand, in the present invention, it is not difficult to ensure the intended physical properties without including the above-mentioned Cr, but when adding the above-mentioned Cr, it is effective to add a minimum of 0.01%.

[0045] Molybdenum (Mo): 2.0% or less (including 0%) Molybdenum (Mo) is an element that improves the hardenability of steel, plays a role in improving strength through the solid solution strengthening effect, and suppresses the formation of ferrite during cooling after finish rolling.Mo also slows the diffusion rate of carbon, preventing excessive carbon concentration in untransformed austenite after coiling, thereby suppressing the formation of pearlite and allowing the second phase to become a low-temperature transformation phase, thereby improving yield strength and tensile strength.

[0046] If the Mo content exceeds 2.0%, the hardening ability increases significantly and the bainite transformation does not occur smoothly in the cooling zone, which results in an excessive increase in the holding time required to secure the fraction of acicular ferrite or bainitic ferrite, which is the matrix structure, resulting in a decrease in elongation.

[0047] Therefore, in the present invention, the Mo content may be 2.0% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.

[0048] On the other hand, in the present invention, it is not difficult to ensure the intended physical properties without including the above-mentioned Mo, but when adding the above-mentioned Mo, it is effective to add a minimum of 0.01%.

[0049] Titanium (Ti): 0.01 to 0.15% Titanium (Ti) is an element that forms carbonitrides in steel and is widely used in applications that ensure the strength of steel by inducing the formation of such precipitates. In the present invention, Ti is added to obtain the effect of slowing down the diffusion rate of carbon and preventing the formation of pearlite.

[0050] To fully achieve the effects of the present invention, the Ti content is preferably 0.01% or more. However, if the Ti content exceeds 0.15%, the proportion of the MA phase constituting the second phase becomes excessive, resulting in poor hole expandability.

[0051] Therefore, in the present invention, the Ti content may be 0.01 to 0.15%, and more advantageously may be 0.05% or more and 0.10% or less.

[0052] Phosphorus (P): 0.001 to 0.05% Phosphorus (P) is an impurity that is inevitably contained in steel and is the element that is the main cause of impairing the workability of steel due to segregation, so it is preferable to control its content as low as possible.

[0053] Theoretically, it is advantageous to limit the P content to 0%, but controlling the P content to less than 0.001% would require excessive manufacturing costs, so the lower limit can be set to 0.001%. However, if the P content exceeds 0.05%, there is a risk of reduced workability, so the upper limit can be set to 0.05%.

[0054] Sulfur (S): 0.0001 to 0.05% Sulfur (S) is an impurity that is inevitably contained in steel and combines with Mn and other elements to form non-metallic inclusions, which can reduce the workability of the steel. Therefore, it is preferable to control the S content as low as possible.

[0055] Theoretically, it is advantageous to limit the S content to 0%, but controlling the S content to less than 0.0001% would require excessive manufacturing costs, so the lower limit can be set to 0.0001%. However, if the S content exceeds 0.05%, there is a risk of reduced workability, so the upper limit can be set to 0.05%.

[0056] Nitrogen (N): 0.0001-0.02% Nitrogen (N) is an impurity that is inevitably contained in steel, and has the problem of combining with Al etc. to form nitrides that impair the workability of steel. Therefore, it is preferable to control the N content as low as possible.

[0057] Theoretically, it is advantageous to limit the N content to 0%, but controlling the N content to less than 0.0001% would require excessive manufacturing costs, so the lower limit can be set to 0.0001%. However, if the N content exceeds 0.02%, there is a risk of reduced workability, so the upper limit can be set to 0.02%.

[0058] The hot-rolled steel sheet of the present invention may further contain at least one of niobium (Nb) and boron (B) in addition to the alloy composition described above.

[0059] Niobium (Nb): 0.01 to 0.1% Niobium (Nb) has the effect of slowing down the diffusion rate of carbon and preventing the formation of pearlite, similar to Ti. However, compared to Ti, it delays recrystallization during hot rolling, which has a greater effect of refining austenite grains. If its content exceeds 0.1%, it excessively forms the MA phase, which is a second phase, and this deteriorates hole expandability.

[0060] On the other hand, when adding the above-mentioned Nb, it is advantageous to include 0.01% or more in order to obtain the effect.

[0061] Boron (B): 0.0005 to 0.005% Boron (B) is an element that significantly improves the hardenability of steel by segregating at austenite grain boundaries and delaying the nucleation of ferrite. The addition of B is highly effective in suppressing the formation of ferrite during cooling after hot rolling.

[0062] The present inventors have found that in addition to the well-known effect of adding B, the transformation rate of bainite is also retarded when B is added. That is, the addition of B affects the fraction of acicular ferrite or bainitic ferrite that is generated during cooling (preferably during secondary cooling) after hot rolling, and therefore, in the present invention, the addition of B makes it possible to easily adjust the secondary cooling conditions.

[0063] When adding the above-mentioned B, if the content is less than 0.0005%, the effect of delaying the phase transformation of not only ferrite but also bainite cannot be sufficiently obtained. On the other hand, if the content exceeds 0.005%, the above-mentioned effect saturates, so it is advantageous to include 0.005% or less of B.

[0064] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the entire contents of these impurities will not be specifically mentioned in this specification.

[0065] In the hot-rolled steel sheet of the present invention having the above-mentioned alloy composition, it is preferable that the content relationship of the specific elements satisfies the following relational expression 1.

[0066] [Equation 1] (1.5×[Si]+1.2×[Cr]+0.7×[Mo]+8.0×[Ti]) / [C]>20 (In Relational Formula 1, each element represents its weight content.)

[0067] In order to ensure the target strength in the present invention, it is necessary to form the low-temperature transformation phase as intended, and therefore it is necessary to suppress the formation of pearlite after coiling of the hot-rolled sheet. Since the driving force for pearlite formation increases as the content of carbon concentrated in untransformed austenite increases, it is necessary to add an element that slows the diffusion rate of carbon to prevent excessive carbon concentration.

[0068] In the present invention, Cr or Mo is added together with Ti as an element that retards the diffusion rate of carbon, and it has been confirmed that this prevents the untransformed austenite from becoming excessively concentrated and retards the formation of pearlite.

[0069] On the other hand, Si has a low solid solubility in the iron carbides that constitute pearlite, and therefore plays a role in preventing the formation of carbides, and as a result, prevents the formation of pearlite in the same way as the above elements.

[0070] In particular, in the present invention, by controlling the ratio of the sum of Si, Cr, Mo, and Ti that prevent pearlite formation and the ratio of C that promotes pearlite formation as shown in the above relational expression 1, it is possible to prevent pearlite formation while securing the second phase as a low-temperature transformation phase, thereby improving the yield strength and tensile strength.

[0071] The hot-rolled steel sheet of the present invention, which satisfies the above-mentioned alloy composition and component relational formula (Relational Formula 1), can have a microstructure containing acicular ferrite or bainitic ferrite as a matrix structure, and the matrix structure is preferably contained at an area fraction of 70 to 90%.

[0072] The steel of the present invention is cooled to a temperature below Bs (the bainite transformation start temperature) during the primary cooling after hot rolling to avoid ferrite phase transformation, and then slowly cooled during the subsequent secondary cooling to cause bainite transformation. Because the bainite transformation occurs in the high-temperature bainite transformation region, bainitic ferrite is formed and carbon diffuses into untransformed austenite, resulting in the absence of carbides within the bainitic ferrite. While numerous dislocations exist within the bainitic ferrite formed by shear transformation, the dislocation density is reduced to an appropriate level by the secondary cooling and subsequent recovery after coiling, resulting in improved elongation of the steel sheet.

[0073] The bainitic ferrite formed at or below the Bs temperature is similar in shape and properties to the acicular ferrite formed when ultra-low carbon steel is supercooled. Therefore, the present invention clarifies that the total fraction of bainitic ferrite and acicular ferrite is used for management.

[0074] If the total fraction of acicular ferrite or bainitic ferrite, which is the base structure, is less than 70%, there is a problem in that it becomes difficult to ensure elongation, while if the fraction exceeds 90%, there is a problem in that it becomes difficult to ensure a low-temperature transformation structure that plays a role in improving strength.

[0075] The hot-rolled steel sheet of the present invention contains a second phase in addition to the above-mentioned matrix structure, and the second phase is a low-temperature transformation structure, preferably one or more of low-temperature bainite, tempered martensite, and MA phase, and can be contained in an area fraction of 10 to 30%.

[0076] In the present invention, when cooling is performed after hot rolling, bainitic ferrite is generated during secondary cooling, and carbon diffuses into untransformed austenite. However, the untransformed austenite is transformed into second phases, namely, low-temperature bainite, tempered martensite, and the MA phase, during the additional cooling process after the secondary cooling (for example, the cooling process after coiling).

[0077] Thus, when bainitic ferrite is formed during secondary cooling, the untransformed austenite distributed within the structure varies in size depending on the position, and the type of second phase also varies. Relatively large untransformed austenite has a low carbon content and can transform into low-temperature bainite during subsequent cooling to the coiling temperature, while smaller austenite transforms into martensite at a lower temperature. Because the martensite transforms in a relatively high temperature range, tempering occurs after the martensite transformation, resulting in the final structure being tempered martensite.

[0078] In the present invention, since the low-temperature bainite and tempered martensite both contain iron carbides at the grain boundaries and within the grains in a lath structure, it is important to note that the total fraction is used for management.

[0079] Meanwhile, during secondary cooling, small-sized austenite has the highest carbon concentration and therefore does not transform into low-temperature bainite or martensite immediately after coiling, but transforms into martensite in the final cooling stage, or if it does not transform into martensite, it can remain as austenite. In this case, martensite with a high carbon content is characterized by having a plate-type martensite rather than a lath-type, and the internal twin structure cannot be clearly observed during nital etching, so it can be clearly distinguished from low-temperature bainite and tempered martensite.

[0080] Such an MA phase is effective in improving yield strength and tensile strength, but there is a large difference in hardness between the bainitic ferrite (or acicular ferrite) that is the base structure and the phase (pgase), which deteriorates hole expandability.

[0081] Therefore, in the present invention, from the viewpoint of ensuring yield strength and tensile strength, it is preferable that the second phase is contained in an area fraction of 10% or more, and in order to simultaneously ensure elongation, it is preferable to limit it to 30% or less.

[0082] Furthermore, in the present invention, the proportion of the MA phase in the second phase is controlled for the purpose of improving the hole expandability of the steel sheet, and it is preferable that the MA phase accounts for less than 30% of the total area ratio of the second phase.

[0083] The hot-rolled steel sheet of the present invention may contain, in addition to the above-mentioned matrix structure and second phase, one or more of ferrite and carbides as other structures, but these are preferably controlled to an area fraction of less than 5%. Here, ferrite means granular ferrite.

[0084] Ferrite formed during cooling after hot rolling is usually formed by diffusion transformation, and is therefore characterized by low strength. The present inventors have confirmed that when the amount of ferrite formed is less than 5%, the previously formed ferrite undergoes shear deformation to absorb the particle deformation that occurs when the retained austenite after ferrite formation transforms into bainite and martensite. This maintains a high dislocation density within the ferrite, preventing a significant decrease in the strength of the steel. However, if the ferrite fraction is 5% or more, the strength of the steel decreases, which is undesirable.

[0085] On the other hand, iron deoxidization products may be generated along with carbon diffusion into austenite during bainite transformation. Since the present invention aims to improve strength by utilizing a low-temperature transformation structure as a second phase, the generation of iron carbides may reduce the fraction of the second phase. In other words, excessive generation of iron carbides hinders the strengthening effect targeted by the present invention. However, when Ti and Nb are present in the steel, alloy carbonitrides may be formed. In this case, further strengthening effects can be expected due to grain refinement. However, since coarse carbides hinder the toughness of the steel, it is preferable that the carbides present in the hot-rolled steel sheet of the present invention be less than 5%.

[0086] The hot-rolled steel sheet of the present invention having the above-described alloy composition and microstructure has high strength, with a yield strength of 750 MPa or more and a tensile strength of 980 MPa or more, while also having excellent formability, with an elongation of 9% or more and a hole expansion ratio of 30% or more.

[0087] Hereinafter, a method for producing a high-strength hot-rolled steel sheet with excellent formability according to another embodiment of the present invention will be described in detail.

[0088] The hot-rolled steel sheet according to the present invention can be produced by subjecting a steel slab that satisfies the alloy composition and relational expression 1 proposed in the present invention to a series of steps of [reheating - hot rolling - cooling - coiling].

[0089] The conditions for each of the above steps will be described in detail below.

[0090] [Steel slab reheating] In the present invention, it is preferable to reheat the steel slab and subject it to a homogenization treatment before carrying out the rolling step, and this can be carried out in a temperature range of 1100 to 1350°C.

[0091] If the temperature during reheating of the steel slab is less than 1100°C, the homogenization of the alloying elements will be insufficient, whereas if the temperature exceeds 1350°C, excessive oxides will be formed on the slab surface, which may result in a deterioration in the surface quality of the steel sheet.

[0092] [Hot rolling] The reheated steel slab can be hot-rolled to produce a hot-rolled steel sheet. At this time, the hot-rolling is preferably carried out in a temperature range of 750 to 1150°C, and the total reduction in the final two passes is preferably controlled to 10 to 40%.

[0093] First, if hot rolling is started at a temperature above 1150°C, excessive oxides are formed on the surface of the rolled steel sheet, which cannot be effectively controlled even by a pickling process, resulting in poor surface quality.On the other hand, if hot rolling is performed at a temperature below 750°C, the rolling load increases excessively, reducing workability, and ferrite is generated during rolling, resulting in poor anisotropy.

[0094] The reason why hot rolling is usually performed in a multi-stage rolling mill is to reduce the rolling load and precisely control the thickness. When hot rolling is performed in such a multi-stage rolling mill, if the total reduction rate of the final two passes (the latter two passes) exceeds 40%, the rolling load of the final two passes becomes excessive, resulting in a problem of reduced workability. On the other hand, if the total reduction rate of the final two passes is less than 10%, the temperature of the steel sheet drops rapidly, which causes a problem of defective shape.

[0095] On the other hand, the austenite grain size after hot rolling is affected by the alloying elements, the rolling finish temperature, and the reduction, which affect the formation behavior of ferrite and bainite in the subsequent cooling process and the final microstructure. In addition, the fraction of the MA phase in the second phase, which is the main constituent phase in the present invention, is significantly affected by the austenite grain size after hot rolling.

[0096] As shown in Figure 1, the smaller the size (grain size) of the second phase, the more likely it is that the second phase will exist as an MA phase. Although the size (grain size) of this second phase may be affected by the nucleation behavior during bainite transformation, due to the characteristics of shear transformation, the size of the second phase will never be larger than the size of the austenite before transformation. Therefore, in order to control the size of the second phase, it is advantageous to control the austenite grain size after hot rolling.

[0097] Therefore, in the present invention, the effective grain size of austenite after hot rolling is derived as a relationship between the rolling finish temperature (FDT) and a specific alloy composition, and specifically, is defined by the following relational expression 2. When the value of Du according to the following relational expression 2 is 800 or more, the MA phase is appropriately formed and a hole expansion ratio of 30% or more can be ensured, whereas when the value exceeds 1106, the austenite grain size becomes too coarse, delaying the bainite transformation and resulting in a problem of poor elongation.

[0098] [Equation 2] 800≦Du≦1106 (In Relational Formula 2, Du is an index showing the effective grain size of austenite immediately before the first cooling after hot rolling, and is expressed as Du = FDT + (7.35 × [C]) - (24.7 × [Si]) - (4.7 × [Mn]) - (3.9 × [Cr]) - (5.2 × [Mo]) - (560 × [Ti]) - (1110 × [Nb]), where FDT means the rolling finish temperature (°C) and each element means its weight content.)

[0099] [Cooling and winding] The hot-rolled steel sheet produced as described above is cooled, and it is preferable to cool it stepwise depending on the temperature to be cooled.

[0100] Specifically, it is preferable to perform primary cooling of the above-mentioned hot-rolled steel sheet at a cooling rate of 70°C / s or more to a temperature below Bs, secondary cooling at a cooling rate of 20°C / s or less to a temperature above (Bs+Ms) / 2, and then tertiary cooling at a cooling rate of 30°C / s or more to a temperature range of Ms-20°C to 500°C.

[0101] The hot-rolled steel sheet manufactured as described above is rapidly cooled to below the temperature (Bs) at which bainite begins to form, thereby suppressing the formation of ferrite (granular ferrite), and then gradually cooled to an intermediate temperature between the bainite start temperature (Bs) and the martensite start temperature (Ms), or to a temperature higher than that, thereby ensuring acicular ferrite or bainitic ferrite as the base structure.

[0102] After the hot rolling is completed, if the cooling rate is less than 70°C / s, there is a problem that the ferrite phase is excessively formed during cooling. In this case, there is no particular upper limit to the primary cooling rate, but if the steel sheet is cooled excessively, the sheet shape may be distorted, so it can be limited to 200°C / s or less.

[0103] There is no particular lower limit for the cooling end temperature during the primary cooling. However, if the temperature is too low, the cooling time for the subsequent secondary cooling may be insufficient. Therefore, it should be noted that the temperature can be limited to Bs-100°C.

[0104] When the temperature of the hot-rolled steel sheet reaches Bs or below by the above-mentioned primary cooling, the intensive cooling is terminated, and secondary cooling can be carried out at a cooling rate of 20°C / s or less to a temperature of (Bs+Ms) / 2 or above.

[0105] During the cooling of the primarily cooled hot-rolled steel sheet from the primarily cooled temperature to the target temperature for secondary cooling, growth of bainitic ferrite and diffusion of carbon into untransformed austenite occur. In particular, in order to obtain the target fractions of the matrix structure and second phase in the present invention, it is preferable to maintain the secondary cooling for a time (ts, seconds (sec)) that satisfies the following Relation 3:

[0106] In Equation 3, k(T) is an index showing the growth rate of bainitic ferrite, and is affected not only by the alloying elements of the steel, but also by the phase transformation temperature and the grain size after hot rolling. Therefore, the value of Equation 3, that is, the relationship between k(T) and holding time (exp(-k(T)×(ts)) 2 If the value of ) is less than 0.1, the proportion of the matrix structure becomes excessive, and although the elongation is excellent, the target level of strength cannot be ensured. On the other hand, if the value exceeds 0.3, there is a problem in that the elongation deteriorates.

[0107] [Equation 3] 0.1≦exp(-k(T)×(ts) 2 )≦0.3 (The above k(T) is expressed by the following formula, where each element is the weight content. In the formula below, T1 represents the temperature at the end of the first cooling (°C), and T2 represents the temperature at the end of the second cooling (°C).)

[0108]

number

[0109] During secondary cooling according to the above-mentioned conditions, the temperature of the steel sheet may rise due to transformation heat generated by the bainite phase transformation. At this time, excessive heat generation may cause an excessive decrease in dislocation density. Therefore, to minimize the temperature rise of the steel sheet due to transformation heat generation, the cooling rate during secondary cooling may be controlled to 20°C / s or less. If the cooling rate exceeds 20°C / s, the sheet shape may be distorted. It should be noted that in the present invention, the secondary cooling also includes an air cooling process.

[0110] The hot-rolled steel sheet that has undergone secondary cooling as described above is preferably tertiarily cooled to a temperature range of Ms-20°C to 500°C at a cooling rate of 30°C / s or more, and then coiled at that temperature.

[0111] During the secondary cooling, austenite is stabilized, further lowering the Ms temperature. Therefore, the end temperature of the tertiary cooling, in other words, the coiling temperature, can be set lower than Ms. When the bainitic ferrite fraction is 70% or more, cooling can be performed to Ms-20°C.

[0112] During the tertiary cooling, low-temperature bainite transforms, and depending on the carbon content in the austenite, some of it can transform to martensite even after coiling. Therefore, by setting the cooling rate during the tertiary cooling to 30°C / s or higher, it is possible to prevent further formation of high-temperature bainite during cooling. There is no particular upper limit to the cooling rate, but it can be set to 100°C / s or lower to prevent distortion of the sheet shape.

[0113] On the other hand, if the cooling end temperature, i.e., the coiling temperature, exceeds 500°C, pearlite is easily generated, and the dislocation density inside the acicular ferrite or bainitic ferrite that constitutes the base structure decreases excessively, which may result in a decrease in yield strength.

[0114] In the present invention, Bs and Ms can be calculated by the following formula, and each element represents a weight content.

[0115] Bs(℃)=830-(320×[C])-(90×[Mn])-(35×[Si])-(70×[Cr])-(120×[Mo]) Ms(℃)=550-(330×[C])-(41×[Mn])-(20×[Si])-(20×[Cr])-(10×[Mo])+(30×[Al])

[0116] [Final cooling] After the cooling and coiling steps are completed as described above, the target hot-rolled steel sheet can be obtained by final cooling. At this time, the final cooling can be completed by air-cooling to room temperature.

[0117] On the other hand, the hot-rolled steel sheet of the present invention obtained after the completion of final cooling as described above can be further pickled and oiled.

[0118] The pickled and oiled hot-rolled steel sheet can be heated in a temperature range of 450 to 740°C to carry out a hot-dip galvanizing process.

[0119] The hot dip galvanizing step can use a zinc-based plating bath, and the alloy composition in the zinc-based plating bath is not particularly limited.

[0120] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. [Example]

[0121] (Example) Steel slabs having the alloy compositions shown in Table 1 below were prepared. The residual components of each steel slab were Fe and unavoidable impurities.

[0122] Each prepared steel slab was reheated at 1200°C and then subjected to hot rolling, cooling, coiling, and final cooling (air cooling) processes under the conditions shown in Table 2 below to produce hot-rolled steel sheets with a thickness of 2.5 mm. The total reduction rate of the final two passes during hot rolling was 25%, and the cooling rate during the tertiary cooling was uniformly 35°C / s.

[0123] The mechanical properties of each hot-rolled steel sheet were measured and the microstructure was observed, and the results are shown in Table 3 below.

[0124] Among the mechanical properties, yield strength, tensile strength, and elongation were measured at room temperature using a universal tensile testing machine after JIS-5 standard test pieces were taken in a direction perpendicular to the rolling direction. At this time, yield strength, tensile strength, and elongation were expressed as 0.2% off-set yield strength, maximum tensile strength, and breaking elongation, respectively.

[0125] The hole expandability was measured based on the ISO TS16630 standard method using the same test pieces as those used in the tensile test.

[0126] The microstructure of each hot-rolled steel sheet was also determined by etching the same test specimens as those used in the tensile tests using a Nital etching method, observing them at 10,000 magnifications using a scanning electron microscope and an image analyzer, and calculating the fraction of each phase. The microstructure was observed on the cross section of the test specimen, i.e., the cross section perpendicular to the rolling direction.

[0127] [Table 1]

[0128] [Table 2]

[0129] [Table 3]

[0130] As shown in Tables 1 to 3 above, Examples 1 to 13 of the present invention, which satisfy all of the alloy compositions and manufacturing conditions proposed in the present invention, had sufficient acicular ferrite or bainitic ferrite formed as the base structure, and a low-temperature transformed structure (LB+TM+MA) was appropriately formed as the second phase, thereby ensuring the target strength and formability.

[0131] In contrast, Comparative Examples 1 to 3, which do not satisfy the alloy component system (relationship 1) proposed in the present invention, showed a tendency for a relatively high elongation rate due to the formation of a large amount of pearlite in the microstructure, making it impossible to ensure the target strength.

[0132] On the other hand, Comparative Examples 4 to 8 are cases in which the alloy compositions satisfy the present invention, but the manufacturing conditions deviate from the present invention.

[0133] Of these, in Comparative Examples 4 and 5, the rolling end temperature during hot rolling was too low, which deviated from Relational Formula 2, resulting in excessive formation of MA phases, and as a result, the hole expandability was poor.

[0134] In Comparative Examples 6 and 7, the cooling time during the secondary cooling process after hot rolling was insufficient, and the matrix structure was not sufficiently formed, resulting in poor elongation.

[0135] In Comparative Example 8, the coiling temperature was quite high, and the dislocation density in the matrix structure disappeared, resulting in poor yield strength.

[0136] FIG. 1 is a graph showing the classification of the types of second phases according to the size of the second phases in each hot-rolled steel sheet.

[0137] As shown in Figure 1, the smaller the size of the second phase, the more likely it is that the second phase will remain as an MA phase due to excessive carbon concentration.

[0138] FIG. 2 shows photographs of the microstructures of Example 4 and Comparative Example 3 observed with a scanning microscope.

[0139] As shown in FIG. 2, in Example 4 (a), the matrix structure and second phase intended to be realized by the present invention are appropriately formed as a microstructure, whereas in Comparative Example 3 (b), excessive pearlite, which is not intended by the present invention, is formed.

Claims

1. A steel sheet comprising, by mass%, carbon (C): 0.05 to 0.17%, silicon (Si): 0.01 to 1.5%, manganese (Mn): 1.5 to 3.0%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 2.0% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.15%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.05%, and nitrogen (N): 0.0001 to 0.02%, with the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1: A high-strength hot-rolled steel sheet with excellent formability, whose microstructure is composed of a base structure of acicular ferrite or bainitic ferrite with an area fraction of 70 to 90% and one or more secondary phases selected from low-temperature bainite, tempered martensite, and MA phase. [Relationship 1] (1.5×[Si]+1.2×[Cr]+0.7×[Mo]+8.0×[Ti]) / [C]>20 (In Relational Formula 1, each element represents its weight content.)

2. The high-strength hot-rolled steel sheet having excellent formability according to claim 1, wherein the hot-rolled steel sheet further contains one or more of niobium (Nb): 0.01 to 0.1% and boron (B): 0.0005 to 0.005%.

3. 2. The high strength hot rolled steel sheet having excellent formability according to claim 1, wherein a ratio of the MA phase to the total fraction of the second phase is less than 30%.

4. The high strength hot rolled steel sheet with excellent formability according to claim 1, wherein the hot rolled steel sheet further contains one or more of ferrite and carbide as other structures in an area fraction of less than 5%.

5. 2. The high-strength hot-rolled steel sheet with excellent formability according to claim 1, wherein the hot-rolled steel sheet has a yield strength of 750 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 9% or more.

6. The high-strength hot-rolled steel sheet with excellent formability according to claim 1, wherein the hot-rolled steel sheet has a hole expansion ratio of 30% or more.

7. A step of reheating a steel slab containing, by mass%, carbon (C): 0.05-0.17%, silicon (Si): 0.01-1.5%, manganese (Mn): 1.5-3.0%, aluminum (Al): 0.01-0.1%, chromium (Cr): 2.0% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01-0.15%, phosphorus (P): 0.001-0.05%, sulfur (S): 0.0001-0.05%, and nitrogen (N): 0.0001-0.02%, the balance being Fe and other unavoidable impurities, and satisfying the following relational expression 1, in a temperature range of 1100-1350°C; hot rolling the reheated steel slab to produce a hot rolled steel sheet; a step of primarily cooling the hot-rolled steel sheet to a temperature of Bs or less at a cooling rate of 70°C / s or more; After the primary cooling, secondary cooling is performed at a cooling rate of 20°C / s or less to a temperature of (Bs + Ms) / 2 or more; After the second cooling, a third cooling is performed at a cooling rate of 30°C / s or more to a temperature range of Ms-20°C to 500°C; and winding the film in the tertiary cooled temperature range.

2. The method for producing a high strength hot rolled steel sheet having excellent formability according to claim 1, wherein the hot rolling is performed at a temperature in the range of 750 to 1150°C so as to satisfy the following relational expression 2, and the total reduction in the final two passes is 10 to 40%. [Relationship 1] (1.5×[Si]+1.2×[Cr]+0.7×[Mo]+8.0×[Ti]) / [C]>20 (In Relational Formula 1, each element represents its weight content.) [Relationship 2] 800≦Du≦1106 (In Relational Formula 2, Du is an index showing the effective grain size of austenite immediately before the primary cooling after hot rolling, and is expressed as Du = FDT + (7.35 x [C]) - (24.7 x [Si]) - (4.7 x [Mn]) - (3.9 x [Cr]) - (5.2 x [Mo]) - (560 x [Ti]) - (1110 x [Nb]), where FDT means the rolling finish temperature (°C), and each element means the weight content.)

8. The method for producing a high-strength hot-rolled steel sheet having excellent formability according to claim 7, wherein the steel slab further contains one or more of niobium (Nb): 0.01 to 0.1% and boron (B): 0.0005 to 0.005%.

9. 8. The method of manufacturing a high strength hot rolled steel sheet having excellent formability according to claim 7, wherein the secondary cooling is performed for a time (ts) that satisfies the following Relation 3: [Relationship 3] 0.1≦exp(-k(T)×(ts) ２ )≦0.3 (The k(T) is expressed by the following formula, where each element is a weight content. In the formula, T1 represents the temperature at the end of the primary cooling (°C), and T2 represents the temperature at the end of the secondary cooling (°C).) [Equation 1]

10. The method for manufacturing a high strength hot rolled steel sheet having excellent formability according to claim 7, further comprising a step of final cooling to room temperature after the coiling.

11. The method for manufacturing a high strength hot rolled steel sheet with excellent formability according to claim 10, further comprising the steps of pickling and oiling after the final cooling.

12. The method for manufacturing a high strength hot rolled steel sheet with excellent formability according to claim 11, further comprising the step of hot dip galvanizing after the pickling and oil coating.

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