JP7585328B2 - Cold-rolled steel sheet and plated steel sheet having excellent bake hardenability and room temperature aging resistance, and manufacturing method thereof - Google Patents

Description

The present invention relates to a steel sheet that has excellent bake hardenability and room temperature aging resistance, and has physical properties that are particularly suitable for use as a material for automotive exterior panels, and a method for manufacturing the same.

Materials for automotive exterior panels are required to have a certain level of bake hardening and aging resistance. Bake hardening refers to the phenomenon in which solute carbon and nitrogen activated during paint baking are fixed to the potential formed during processing of the steel sheet, increasing the yield strength of the steel sheet. Steel sheets with excellent bake hardening properties are easy to form before paint baking, and have the property of improved dent resistance in the final product, making them considered extremely ideal materials for automotive exterior panels.

However, when the bake hardenability of a steel sheet increases, the anti-aging resistance of the steel sheet tends to deteriorate. Therefore, even if the bake hardenability of the steel sheet is ensured, aging will occur over a certain period of time, and as a result, there is a high possibility that surface defects will occur during part processing. Therefore, materials for the exterior panels of automobiles are required to have at least an appropriate level of bake hardenability and at least an appropriate level of anti-aging resistance.

Patent Document 1 proposes a technique for improving bake hardenability by adding Sn, but does not provide a fundamental solution to the problem of deterioration of aging resistance due to increased bake hardenability.

Therefore, there is a need to supply steel sheets that simultaneously have above-optimal levels of bake hardenability and room-temperature aging resistance, and that have physical properties that are particularly suitable for use as materials for the exterior panels of automobiles.

Japanese Patent Publication No. 1994-306531 (published on November 1, 1994)

According to one aspect of the present invention, it is possible to provide cold-rolled steel sheets and plated steel sheets with excellent bake hardenability and room temperature aging resistance, and methods for manufacturing these.

The object of the present invention is not limited to the above. A person skilled in the art would have no difficulty in understanding the further object of the present invention from the general contents of this specification.

According to one aspect of the present invention, a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance contains, by weight, C: 0.002-0.015%, Mn: 1.5-3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol. Al: 0.02-0.06%, Cr: 1.2% or less (excluding 0%), the balance being Fe and unavoidable impurities, and contains ferrite as a base structure and hard structure as the balance as a microstructure, and the hard structure occupancy ratio (V) of the grain boundary triple junction defined by the following relational formula 1 can be 70% or more.

[Relationship 1]
V (%) = {Vtp/(Vgb+Vtp)}×100
In the above Relational Formula 1, Vgb means the number of hard structures observed at the ferrite grain boundaries in the observation area, and Vtp means the number of hard structures observed at the ferrite grain boundary triple junctions in the observation area.

The ferrite fraction is 95% by area or more, and the hard structure can contain martensite. The cold-rolled steel sheet can satisfy the range of Hel defined by the following relational expression 2 of 1.2 to 2.5.

[Relationship 2]
Hel=[C]+0.5×[Mn]+0.75×[Cr]
In the above Relational Formula 2, [C], [Mn] and [Cr] respectively mean the contents (wt%) of C, Mn and Cr.

The cold-rolled steel sheet may further contain, by weight, 0.1% or less (including 0%) of silicon (Si). The cold-rolled steel sheet may have a bake hardening amount (BH, tensile test after heat treatment at 170°C for 20 minutes) of 30 MPa or more and a yield point elongation (YP-El, tensile test after heat treatment at 100°C for 1 hour) of 0.2% or less.

According to one aspect of the present invention, a plated steel sheet having excellent bake hardenability and room temperature aging resistance can include the above-mentioned cold-rolled steel sheet and a plating layer or an alloyed plating layer formed on at least one side of the above-mentioned cold-rolled steel sheet.

According to one aspect of the present invention, a method for producing cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance has the following composition, in weight percent: C: 0.002-0.015%, Mn: 1.5-3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol. The method includes the steps of heating a slab containing Al: 0.02-0.06%, Cr: 1.2% or less (excluding 0%), the balance Fe, and inevitable impurities, hot rolling the slab to obtain a hot-rolled steel sheet, coiling the hot-rolled steel sheet, cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, and continuous annealing the cold-rolled steel sheet. The continuous annealing can be performed by heating the steel sheet to a temperature range of (Ac1+5°C) to (Ac3-20°C) at a heating rate of 1-10°C/s, and then holding the steel sheet for 30-240 seconds.

The slab can satisfy the range of He1 defined by the following Relation 2 of 1.25 to 2.42.
[Relationship 2]
Hel=[C]+0.5×[Mn]+0.75×[Cr]

The slab may further contain silicon (Si) in a weight percentage of 0.1% or less (including 0%). In the above Relational Formula 2, [C], [Mn] and [Cr] respectively mean the contents (weight percentages) of C, Mn and Cr. The slab heating temperature may be 1100 to 1300°C, the finish rolling temperature of the hot rolling may be 880°C or more, the coiling temperature may be 400 to 700°C, and the reduction ratio of the cold rolling may be 50 to 90%.

A method for producing a plated steel sheet having excellent bake hardenability and room temperature aging resistance according to one aspect of the present invention may further include a step of immersing the cold-rolled steel sheet produced by the above-mentioned method in a hot-dip galvanizing bath at 440 to 480°C to perform hot-dip galvanizing, and optionally a step of holding the hot-dip galvanizing bath at a temperature in the range of 460 to 610°C for 20 seconds or more to perform an alloying treatment.

The above solutions to the problems described above are not intended to be exhaustive, and the various features of the present invention and their associated advantages and benefits can be understood in more detail by referring to the specific description below.

According to a preferred aspect of the present invention, it is possible to provide a steel sheet having excellent bake hardenability and room temperature aging resistance, and physical properties particularly suitable for use as a material for automotive exterior panels, and a method for manufacturing the same.

1 is a photograph showing the microstructure of Sample 1-1.

The present invention relates to a cold-rolled steel sheet and a plated steel sheet having excellent bake hardenability and room temperature aging resistance, and a manufacturing method thereof. Preferred embodiments of the present invention will be described below. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to explain the present invention in more detail to those having ordinary skill in the art to which the present invention pertains.

The cold-rolled steel sheet and plated steel sheet with excellent bake hardenability and room temperature aging resistance according to one aspect of the present invention will be described in more detail below.

According to one aspect of the present invention, a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance contains, by weight, C: 0.002-0.015%, Mn: 1.5-3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol. Al: 0.02-0.06%, Cr: 1.2% or less (excluding 0%), the balance being Fe and unavoidable impurities, and contains ferrite as a base structure and hard structure as the balance as a microstructure, and the hard structure occupancy ratio (V) of the grain boundary triple junction defined by the following relational formula 1 can be 70% or more.

[Relationship 1]
V (%) = {Vtp/(Vgb+Vtp)}×100
In the above Relational Formula 1, Vgb means the number of hard structures observed at the ferrite grain boundaries in the observation area, and Vtp means the number of hard structures observed at the ferrite grain boundary triple junctions in the observation area.

The alloy composition of the present invention will be described in more detail below. Unless otherwise specified, percentages and ppm related to the contents of the alloy composition are based on weight.

According to one aspect of the present invention, the cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance can contain, by weight, C: 0.002-0.015%, Mn: 1.5-3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol. Al: 0.02-0.06%, Cr: 1.2% or less (excluding 0%), the balance being Fe and inevitable impurities.

Carbon (C): 0.002-0.015%
Carbon (C) is an element that effectively contributes to the formation of martensite, and in order to produce the dual phase steel of the present invention, a certain level of carbon (C) must be added. Therefore, in the present invention, from the viewpoint of ensuring bake hardenability and room temperature aging resistance by realizing a dual phase steel, the lower limit of the carbon (C) content can be limited to 0.002%. The lower limit of the carbon (C) content may be 0.003%, and more preferably the lower limit of the carbon (C) content may be 0.004%. However, if carbon (C) is added in excess, the composite structure may be deteriorated. While this is beneficial for steel forming, it increases the strength of the material and reduces its elongation, which increases the possibility of bending defects on the surface of the product when the client processes the part. Therefore, the present invention can limit the upper limit of the carbon (C) content to 0.015%. The preferred upper limit of the carbon (C) content may be 0.013%, and more preferred The upper limit of the carbon (C) content may be 0.01%.

Manganese (Mn): 1.5-3.0%
Manganese (Mn) is not only a component that contributes to improving hardenability, but also effectively contributes to the formation of martensite like carbon (C). Therefore, in the present invention, from the viewpoint of ensuring bake hardenability and room temperature aging resistance by realizing a dual phase steel, the lower limit of the manganese (Mn) content can be limited to 1.5%. A preferable lower limit of the manganese (Mn) content may be 1.6%, and a more preferable lower limit of the manganese (Mn) content may be 1.8%. On the other hand, when manganese (Mn) is added in excess, there are problems that the elongation rate decreases, the workability is deteriorated, and a band-shaped manganese (Mn) oxide band is formed in the structure, increasing the risk of processing cracks and sheet breakage. In addition, when manganese (Mn) is added in excess, there is a problem that manganese (Mn) oxide dissolves on the surface of the steel sheet during annealing, greatly impairing the galvanization. Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 3.0%. A preferred upper limit for the manganese (Mn) content may be 2.6%, and a more preferred upper limit for the manganese (Mn) content may be 2.3%.

Phosphorus (P): 0.03% or less Phosphorus (P) in steel is the most advantageous element for ensuring strength without significantly impairing formability. However, if phosphorus (P) is added in excess, the possibility of brittle fracture increases, which may not only induce sheet breakage of the slab during hot rolling, but may also significantly deteriorate the surface properties of the plated steel sheet. Therefore, the present invention may limit the upper limit of phosphorus (P) content to 0.03%. However, taking into account the level of unavoidable inflow, the present invention may exclude 0% from the lower limit of phosphorus (P) content.

Sulfur (S): 0.01% or less Sulfur (S) is an impurity element that inevitably flows into steel, and it is preferable to control its content as low as possible. In particular, since sulfur (S) in steel may induce red shortness, the present invention may limit the upper limit of sulfur (S) content to 0.01%. However, taking into consideration the level of unavoidable flow, the present invention may exclude 0% from the lower limit of sulfur (S) content.

Nitrogen (N): 0.01% or less Nitrogen (N) is also an impurity element that inevitably flows into steel. Therefore, it is preferable to control its content as low as possible, but in consideration of the steelmaking load and operating conditions, the present invention can limit the upper limit of the nitrogen (N) content to 0.01%. However, in consideration of the level of unavoidable inflow, the present invention can exclude 0% from the lower limit of the nitrogen (N) content.

Acid-soluble aluminum (sol. Al): 0.02 to 0.06%
Aluminum (Al) is an ingredient added for grain refinement and deoxidation of steel. In order to produce a stable aluminum-killed steel, the present invention may limit the lower limit of the acid-soluble aluminum (sol. Al) content to 0.02%. A preferred lower limit of the acid-soluble aluminum (sol. Al) content may be 0.025%. On the other hand, if aluminum (Al) is added excessively, the strength increases due to grain refinement, but inclusions are excessively formed during continuous steel casting operation, which may result in poor surface quality of the steel sheet and increased manufacturing costs. Therefore, the present invention may limit the upper limit of the acid-soluble aluminum (sol. Al) content to 0.06%, and a more preferred upper limit of the acid-soluble aluminum (sol. Al) content may be 0.07%.

Chromium (Cr): 1.2% or less (excluding 0%)
Chromium (Cr) is an element that not only improves the hardening ability of steel but also effectively contributes to the formation of martensite because it has similar properties to manganese (Mn) described above. When chromium (Cr) is added to steel, it forms coarse chromium (Cr ₎ -based carbides such as _Cr23C6 during hot rolling, and the amount of solute carbon (C) in the steel is controlled to an appropriate level or less to suppress the occurrence of yield point elongation (YP-El), so that a dual-phase steel with a low yield ratio can be provided. In addition, chromium (Cr) is also an element that effectively contributes to ensuring the elongation of dual-phase steel by minimizing the decrease in elongation with respect to the increase in strength. Therefore, in the present invention, chromium (Cr) can be added as an essential element to achieve such effects. On the other hand, when chromium (Cr) is added excessively, the ratio of martensite formation is excessively increased, so that not only the elongation is deteriorated but also the corrosion resistance may be reduced. Therefore, the present invention can limit the upper limit of the chromium (Cr) content to 1.2%, and more preferably the upper limit of the chromium (Cr) content can be 0.95%.

In addition, the cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance according to one aspect of the present invention may further contain, by weight percent, 0.1% or less of silicon (Si).

Silicon (Si): 0.1% or less Silicon (Si) is a component that contributes to increasing the strength of steel through solid solution strengthening, but in the present invention, silicon is not intentionally added. In the present invention, the desired physical properties can be secured without adding silicon (Si). On the other hand, if the silicon (Si) content exceeds a certain level, there is a problem that the surface characteristics of the final plated product are deteriorated due to Si oxide formed during the hot rolling stage, so in the present invention, the upper limit of the silicon (Si) content can be limited to 0.1%. A preferred upper limit of the silicon (Si) content may be 0.08%. On the other hand, in consideration of the level of unavoidable inflow, the present invention can exclude 0% from the lower limit of the silicon (Si) content.

The cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance according to one aspect of the present invention may contain the balance Fe and other inevitable impurities in addition to the above-mentioned components. However, in normal manufacturing processes, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and therefore, it is not possible to completely eliminate these impurities. Since these impurities are known to anyone with ordinary knowledge in this technical field, the contents of all of them will not be specifically mentioned in this specification. However, the addition of effective components other than the above composition is not excluded.

According to one aspect of the present invention, a cold-rolled steel sheet with excellent bake hardenability and room temperature aging resistance can satisfy the range of Hel defined by the following relational expression 2 of 1.2 to 2.5.

[Relationship 2]
Hel=[C]+0.5×[Mn]+0.75×[Cr]
In the above Relational Formula 2, [C], [Mn] and [Cr] respectively mean the contents (wt%) of C, Mn and Cr.

In the present invention, the range of carbon (C) content is limited to the range of 0.002 to 0.015%, so that in order to realize the desired composite structure, appropriate addition of hardenability improving elements such as Mn and Cr is essential, and Relational Formula 2 specifies the optimal component contents of these hardenability improving elements C, Mn, and Cr. In the present invention, the lower limit of Hel specified by Relational Formula 2 can be limited to 1.2 in order to form the desired composite structure. If the Hel value of Relational Formula 2 is less than 1.2, martensite is not formed even by quenching after annealing due to low hardenability, and the desired composite structure cannot be formed. A preferred lower limit of the Hel value may be 1.25, and a more preferred lower limit of the Hel value may be 1.5. On the other hand, if the Hel value exceeds a certain level, a composite structure can be formed, but the addition of a large amount of alloying elements increases the yield strength and tensile strength, resulting in a decrease in elongation, so the present invention can limit the upper limit of the Hel value to 2.5. A preferred upper limit for the Hel value may be 2.42, and a more preferred upper limit for the Hel value may be 2.0.

According to one aspect of the present invention, a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance can have a composite structure in which ferrite is the matrix structure and hard structure is the remaining structure. The lower the fraction of ferrite, the higher the fraction of the hard phase is, which is somewhat advantageous for realizing a composite structure, but it inevitably leads to an increase in yield strength and yield ratio, which increases the likelihood of surface bending defects occurring during part processing. Therefore, the present invention can limit the fraction of ferrite to 95 area % or more based on the total thickness (t) of the steel sheet.

The hard structure contained as the remaining structure may be martensite, and may also contain some bainite and pearlite. However, it is preferable to minimize the amount of bainite and pearlite formed. The martensite of the present invention may be fine martensite with an average diameter of 1 μm or less. The finer the martensite, the more sites (mobile dislocations) where solute carbon (C) or nitrogen (N) is fixed are formed, so that the bake hardenability and aging resistance aimed at by the present invention can be more effectively ensured. On the other hand, if a large amount of martensite is formed, not only the elongation rate decreases, but there is also a possibility that surface bending may occur during part processing, so it is preferable to limit the martensite fraction to a certain level or less. Therefore, the martensite fraction of the present invention may be 2 area % or less (excluding 0%).

According to one aspect of the present invention, the cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance may have a hard structure occupancy ratio (V) of the grain boundary triple junction defined by the following relational expression 1 of 70% or more.

[Relationship 1]
V (%) = {Vtp/(Vgb+Vtp)}×100

In the above relational expression 1, Vgb means the number of hard structures observed at the ferrite grain boundaries in the observation area, and Vtp means the number of hard structures observed at the ferrite grain boundary triple junctions in the observation area.

As an example, when observing a microstructure using an optical or electron microscope, an observation region of 10,000 ^μm2 size is specified, and the microstructure within the observation region is observed. The total number of martensite particles observed at the ferrite grain boundaries within the observation region is defined as Vgb, and the number of martensite particles observed at the ferrite grain boundary triple junctions within the same observation region is defined as Vtp, and the hard structure occupancy ratio (V) of the grain boundary triple junctions can be calculated.

Here, the total number of martensites (Vgb) refers to the total number of martensites observable in all ferrite grain boundaries in the observation area using a microscope, and the number of martensites at grain boundary triple junctions (Vtp) refers to the number of martensites that occupy even a part of an area within a diameter of 50 nm set around a point where three or more ferrite grain boundaries meet in the observation area.

The inventors of the present invention conducted extensive research into simultaneously ensuring the bake hardenability and room temperature aging resistance of steel sheets, and discovered that not only the overall martensite fraction but also the distribution of martensite has a significant effect on bake hardenability. In other words, the inventors of the present invention confirmed that by controlling the distribution of martensite, it is possible to control the frequency of interaction between mobile dislocations around martensite and solute carbon (C), and derived the present invention from the focus on controlling the distribution of martensite to optimal conditions in order to simultaneously ensure bake hardenability and room temperature aging resistance.

Martensite is formed during cooling of the steel plate, and a large number of mobile dislocations are formed around the martensite due to volume expansion. One method for improving bake hardenability is to increase the proportion of martensite, but this inevitably results in poor room temperature aging resistance, making it very difficult to achieve the goal of simultaneously securing both bake hardenability and room temperature aging resistance.

At the grain boundaries of ferrite, a large amount of carbon (C) is concentrated compared to within the crystal grains of ferrite, and the grain boundary triple junction of ferrite shows a high carbon (C) concentration even among the ferrite grain boundaries. When applying normal baking heat treatment conditions (170°C, 20 minutes) to steel sheets, carbon (C) diffusion from the grain boundary triple junction of ferrite occurs most actively, which means that carbon (C) can be more easily fixed to the mobile dislocations present at the grain boundary triple junction of ferrite. On the other hand, under artificial aging conditions (100°C, 1 hour), the temperature is relatively low and carbon (C) diffusion from the grain boundaries and martensite is limited, so no significant difference occurs due to the distribution degree of martensite. In other words, when a large amount of martensite is distributed at the grain boundary triple junction of ferrite, it means that the bake hardenability of the steel sheet can be further improved while maintaining the room temperature aging resistance.

Therefore, the present invention limits the hard structure occupancy ratio (V) of the grain boundary triple junction defined by Relational Formula 2 to 70% or more, and therefore effectively improves bake hardenability while maintaining a certain level of room temperature aging resistance.

According to one aspect of the present invention, a cold-rolled steel sheet with excellent bake hardenability and room temperature aging resistance has a bake hardening amount (BH, tensile test after heat treatment at 170°C for 20 minutes) of 30 MPa or more and a yield point elongation (YP-El, tensile test after heat treatment at 100°C for 1 hour) of 0.2% or less.

According to another aspect of the present invention, a plated steel sheet having excellent bake hardenability and room temperature aging resistance may include a plated layer or an alloyed plated layer formed on at least one side of the above-mentioned cold-rolled steel. The plated layer and the alloyed plated layer may be, but are not necessarily limited to, a hot-dip galvanized layer and an alloyed hot-dip galvanized layer, and may be interpreted as including all plated layers and alloyed plated layers suitable for use as materials for automotive exterior panels.

The manufacturing method of a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance according to one aspect of the present invention will be described in more detail below.

A method for producing a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance according to one aspect of the present invention includes the steps of heating a slab having a predetermined alloy composition, hot rolling the slab to obtain a hot-rolled steel sheet, coiling the hot-rolled steel sheet, cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, and continuously annealing the cold-rolled steel sheet, and the continuous annealing can be performed by heating the steel sheet to a temperature range of (Ac1+5°C) to (Ac3-20°C) at a heating rate of 1 to 10°C/s and then holding the steel sheet for 30 to 240 seconds.

Slab Heating After preparing a slab having a desired alloy composition, the slab may be reheated. Since the slab of the present invention has an alloy composition corresponding to the above-mentioned cold-rolled steel sheet, the description of the alloy composition of the slab is substituted for the description of the alloy composition of the above-mentioned cold-rolled steel sheet.

Since slab reheating is performed to facilitate the subsequent hot rolling and to fully obtain the target physical properties of the steel sheet, the present invention does not place any particular restrictions on the conditions of such a slab reheating process. Therefore, the slab reheating of the present invention can be performed under normal conditions, and as an example, the slab can be reheated in the temperature range of 1100 to 1300°C.

Hot Rolling and Coiling The reheated slab can be finish rolled in the temperature range of 880°C or higher and then coiled in the temperature range of 400-700°C.

Finish hot rolling is preferably performed in the austenite single phase region. When the finish hot rolling is performed in the austenite single phase region, pancake-shaped austenite and deformation bands are formed, which is advantageous for refining the final structure. Also, when the finish hot rolling is performed in the two-phase region of austenite and ferrite, it may induce material non-uniformity and cause excessive rolling load. Therefore, the present invention can limit the temperature range of the finish hot rolling to 880°C or more so that the finish hot rolling is performed in the austenite single phase region. The present invention does not particularly limit the upper limit of the finish rolling temperature. However, in order to prevent material imbalance due to the formation of abnormally coarse grains, the upper limit of the finish hot rolling temperature range can be limited to 950°C.

Then, the steel sheet after hot rolling can be wound into a hot rolled coil. If the coiling temperature does not reach a certain level, a large amount of hard phases such as martensite or bainite may be formed, which may result in an excessive increase in the strength of the steel sheet. Therefore, the present invention can limit the coiling temperature to 400°C or more from the viewpoint of reducing the rolling load in the subsequent cold rolling after coiling and preventing shape defects. On the other hand, if the coiling temperature exceeds a certain range, there is a problem that the surface concentration of oxidizing elements in the steel becomes severe. Therefore, the present invention can limit the upper limit of the coiling temperature to 700°C in order to ensure the surface quality and plating quality of the steel sheet.

Cold rolling The coiled hot-rolled steel sheet may be pickled under normal conditions, and then cold-rolled to provide a cold-rolled steel sheet. The cold rolling of the present invention is preferably performed at a reduction of 50 to 90%. If the reduction of the cold rolling is less than a certain level, it is difficult to ensure the target thickness of the steel sheet and it is difficult to calibrate the shape of the steel sheet. Therefore, the present invention may limit the lower limit of the cold rolling reduction to 50%. On the other hand, if the reduction of the cold rolling exceeds a certain level, cracks are likely to occur at the edge of the steel sheet and excessive rolling load may become a problem, so the present invention may limit the upper limit of the cold rolling reduction to 90%.

Continuous annealing In order to control the microstructure targeted by the present invention, particularly the fraction of ferrite and martensite and the distribution degree of martensite, strict control of the continuous annealing conditions is essential. In order to secure the microstructure targeted by the present invention, the cold-rolled steel sheet after cold rolling can be heated to a temperature range of (Ac1+5°C) to (Ac3-20°C) at a heating rate of 1 to 10°C/s, and then held for 30 to 240 seconds to perform continuous annealing.

If the heating rate during continuous annealing is less than a certain level, the size non-uniformity between structures will deepen due to the heating being too slow, and the initial ferrite size will be formed unnecessarily coarse, which may lead to a decrease in the strength of the steel sheet. That is, as the grain size of ferrite increases, the proportion of the ferrite grain boundary triple junction in the ferrite grain boundary will decrease, and even if the desired martensite occupancy ratio (V) of the ferrite grain boundary triple junction is secured, the total content of martensite will be low, making it difficult to secure the target physical properties. Therefore, the present invention may limit the lower limit of the heating rate to 1°C/s, and the more preferable upper limit of the heating rate may be 2°C/s. Meanwhile, the present invention does not particularly specify the upper limit of the heating rate during continuous annealing. However, if the heating rate is excessively high, it may cause excessive burden on the on-site equipment, so the present invention may limit the upper limit of the heating rate to 10°C/s.

The annealing temperature is preferably in the range of (Ac1+5°C) to (Ac3-20°C). In order to control the fraction of ferrite and martensite and the distribution of martensite in the final steel sheet, the present invention can perform continuous annealing by holding the temperature range in the two-phase region for a certain period of time. If the annealing temperature is excessively low, the austenite fraction at the two-phase region temperature becomes excessively low, and the desired level of martensite fraction cannot be achieved in the final steel sheet. Therefore, in the present invention, the lower limit of the annealing temperature can be limited to (Ac1+5°C) in order to ensure the desired martensite fraction. The preferred lower limit of the annealing temperature may be (Ac1+10°C), and the more preferred lower limit of the annealing temperature may be (Ac1+15°C).

On the other hand, in the case of a typical 590 MPa class dual-phase steel (DP), when the annealing temperature is high, the austenite fraction at the dual-phase temperature increases, which can cause problems such as the formation of a large amount of coarse martensite in the final steel sheet. However, in the case of low-strength dual-phase steel and composite steel of 490 MPa class or less, when the annealing temperature is high, the austenite fraction at the dual-phase temperature increases, but this does not mean that the martensite fraction is high in the final steel sheet. The fact that the austenite fraction increases at the dual-phase temperature means that the hardening elements (typically C, Mn) present in the steel sheet diffuse into the austenite region where there is more of them, and that the concentration of the hardening elements in the austenite is low relative to the low dual-phase temperature (meaning a low dual-phase austenite fraction). In other words, when the annealing temperature is high, the stability of austenite is reduced and it becomes easier to transform into ferrite during cooling after annealing, so that the martensite content finally generated is reduced, making it difficult to secure the target martensite content. In other words, in the low-strength dual-phase steel of 490 MPa class or less that is the subject of this invention, if the annealing temperature is excessively high, the stability of the two-phase austenite becomes excessively low, resulting in a low final martensite fraction and the problem that the desired level of bake hardenability cannot be secured.

In addition, the continuous annealing of the present invention is aimed at being performed in a two-phase temperature range, but it is preferable to perform the continuous annealing in a temperature range favorable for the formation of ferrite. This is because when the continuous annealing is performed in a temperature range favorable for the formation of ferrite, the formation of initial ferrite can be promoted, providing a more favorable environment for grain growth. In addition, when the continuous annealing is performed in a temperature range favorable for the formation of ferrite, the martensite start temperature (Ms) can be lowered to increase the concentrations of carbon (C) and manganese (Mn) in the austenite, and fine and uniform martensite can be induced to be formed in a large amount distributed in the ferrite grains in the subsequent cooling process or the cooling process after plating. Therefore, in the present invention, the upper limit of the annealing temperature can be limited to (Ac3-20°C) in order to ensure the desired martensite occupancy ratio (V) of the ferrite grain boundary triple point. The preferred upper limit of the annealing temperature may be (Ac3-25°C), and the more preferred upper limit of the annealing temperature may be (Ac3-30°C).

The holding time after the temperature rise is also a major process variable in ensuring the fine structure targeted by the present invention. If the holding time after the temperature rise is less than a certain level, carbon (C) and manganese (Mn) do not sufficiently diffuse into the austenite formed in the two-phase region, reducing the stability of the austenite and increasing the possibility that the austenite will transform into other fine structures rather than the targeted martensite during cooling after annealing. Therefore, in the present invention, the lower limit of the holding time after the temperature rise is limited to 30 seconds, and the more preferable lower limit of the holding time after the temperature rise can be 60 seconds. On the other hand, if the holding time after the temperature rise exceeds a certain level, the ferrite formed initially is formed unnecessarily coarse, which may lead to a structure size imbalance with the ferrite formed after the final cooling and other structures. Such a structure size imbalance causes deterioration of tensile properties, bake hardenability, and aging resistance, so in the present invention, the upper limit of the holding time after the temperature rise can be limited to 240 seconds. A more preferable upper limit of the holding time after the temperature rise may be 180 seconds.

The cold-rolled steel sheet manufactured by the above-mentioned manufacturing process can contain 95% or more by area of ferrite as a microstructure, with the remainder being martensite, and can satisfy the hard structure occupancy ratio (V) of the grain boundary triple junction defined by the following relational expression 1 of 70% or more.

[Relationship 1]
V (%) = {Vtp/(Vgb+Vtp)}×100
In the above Relational Formula 1, Vgb means the number of hard structures observed at the ferrite grain boundaries in the observation area, and Vtp means the number of hard structures observed at the ferrite grain boundary triple junctions in the observation area.

In addition, the cold-rolled steel sheet manufactured by the above-mentioned manufacturing process can satisfy a bake hardening amount of 30 MPa or more (BH, tensile test after heat treatment at 170°C for 20 minutes) and a yield point elongation of 0.2% or less (YP-El, tensile test after heat treatment at 100°C for 1 hour).

According to one aspect of the present invention, a plated steel sheet having excellent bake hardenability and room temperature aging resistance can be provided by applying a plating process to a cold-rolled steel sheet manufactured by the above-mentioned manufacturing method. The plating process may be a hot-dip galvanizing process or a hot-dip galvannealing process, but is not necessarily limited thereto, and it may be interpreted that all plating processes applied to materials for ordinary automobile exterior panels are applicable.

As a non-limiting example of the plating process, a hot-dip galvanizing process can be applied in which the above-mentioned cold-rolled steel sheet is immersed in a hot-dip galvanizing bath (Pot) having a normal temperature range of 440 to 480°C. As another non-limiting example of the plating process, an alloying hot-dip galvanizing process can be applied in which the above-mentioned cold-rolled steel sheet is immersed in a hot-dip galvanizing bath (Pot) having a normal temperature range of 440 to 480°C, and then held at a temperature range of 460 to 610°C for 20 seconds or more to perform an alloying process.

The present invention will be described in more detail below through examples. However, it should be noted that the examples described below are intended to illustrate and further embody the present invention, and are not intended to limit the scope of the invention.

(Example)
After preparing a slab having the alloy composition shown in Table 1, hot-dip galvanized steel sheets were manufactured by applying the process conditions shown in Table 2. For each specimen, a slab reheating temperature condition of 1200°C and a cold rolling reduction of 70% were commonly applied. The microstructure observation results and physical property measurement results for each specimen are also shown in Table 2.

The hard structure occupation ratio (V) of the grain boundary triple junction was measured using a scanning electron microscope (SEM, JEOL JSN-7001F, resolution: 1 nm). Specifically, an observation area of 10,000 ^μm2 was designated at 1/4t point in the thickness direction of each specimen, and the number of martensites present in the grain boundaries of ferrite within the observation area was measured to calculate the hard structure occupation ratio (V) of the grain boundary triple junction. Here, the total number of martensites means the total number of martensites observable in all ferrite grain boundaries within the observation area using a scanning electron microscope. The number of martensites at the grain boundary triple junction means the number of martensites occupying even a part of an area set within a diameter of 50 nm from a point where three or more ferrite grain boundaries meet within the observation area.

Bake hardenability ( _BH2 ) was measured by pre-straining each specimen by 2% and measuring the flow-stress at 2%, then heat-treating the specimen at 170°C for 20 minutes and conducting a tensile test. Yield point elongation (YP-El) was measured by heat-treating the specimen at 100°C for 1 hour and conducting a tensile test. The tensile test conditions were in accordance with ASTM-e8/e8m-16a.

*A _c1 =739-22×[C]-7×[Mn]+2×[Si]+14×[Cr]+13×[Mo]-13×[Ni]
**A _c3 =902-255×[C]-11×[Mn]+19×[Si]-5×[Cr]+13×[Mo]-20×[Ni]+55×[V]

It can be seen that specimens that satisfy both the alloy composition and process conditions restricted by the present invention satisfy both the bake hardenability and room temperature aging resistance that are the objectives of the present invention, whereas specimens that do not satisfy one or more of the alloy composition or process conditions restricted by the present invention do not simultaneously satisfy the bake hardenability and room temperature aging resistance that are the objectives of the present invention.

As described above, the present invention has been described in detail through the examples, but other embodiments are possible. Therefore, the technical ideas and scope of the claims described below are not limited to the examples.

Claims (9)

In weight percent, it contains C: 0.002 to 0.015%, Mn: 1.5 to 3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol. Al: 0.02 to 0.06%, and Cr: 1.2% or less (excluding 0%), with the balance being Fe and inevitable impurities;
The microstructure is composed of ferrite, which is the base structure, and the remaining hard structure.
The hard structure occupancy ratio (V) of the grain boundary triple junction defined by the following Relational Formula 1 is 70% or more,
A cold-rolled steel sheet having excellent bake hardenability and room-temperature aging resistance, in which Hel defined by the following relational formula 2 falls within the range of 1.2 to 2.5.
[Relationship 1]
V (%) = {Vtp/(Vgb+Vtp)}×100
In the above Relational Formula 1, Vgb means the number of hard structures having a minor axis of 1 nm or more observed at ferrite grain boundaries in the observation area, and Vtp means the number of hard structures having a minor axis of 1 nm or more observed at ferrite grain boundary triple junctions in the observation area.
[Relationship 2]
Hel=[C]+0.5×[Mn]+0.75×[Cr]
In the above Relational Formula 2, [C], [Mn] and [Cr] respectively mean the contents (wt%) of C, Mn and Cr. The ferrite fraction is 95 area % or more,
The cold-rolled steel sheet having excellent bake hardenability and room-temperature aging resistance according to claim 1, wherein the hard structure contains martensite. The cold-rolled steel sheet according to claim 1 further contains, by weight percent, 0.1% or less (including 0%) of silicon (Si), and has excellent bake hardenability and room temperature aging resistance. The cold-rolled steel sheet is
The bake hardening amount (BH, tensile test after heat treatment at 170°C for 20 minutes) is 30 MPa or more,
The cold-rolled steel sheet according to claim 1, having a yield point elongation (YP-El, tensile test after heat treatment at 100°C for 1 hour) of 0.2% or less. The cold-rolled steel sheet according to any one of claims 1 to 4,
and a plating layer or an alloyed plating layer formed on at least one side of the cold-rolled steel sheet. heating a slab containing, by weight percent, C: 0.002-0.015%, Mn: 1.5-3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol. Al: 0.02-0.06%, and Cr: 1.2% or less (excluding 0%), with the balance being Fe and inevitable impurities;
hot rolling the slab to provide a hot rolled steel sheet;
coiling the hot-rolled steel sheet;
cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;
and continuously annealing the cold-rolled steel sheet. The slab satisfies the range of He1 defined by the following Relation 2 of 1.2 to 2.5:
The continuous annealing is performed by heating the steel sheet to a temperature range of (Ac1 + 5 ° C.) to (Ac3 - 20 ° C.) at a heating rate of 1 to 10 ° C./s and then holding the steel sheet for 30 to 240 seconds.
[Relationship 2]
Hel=[C]+0.5×[Mn]+0.75×[Cr]
In the above Relational Formula 2, [C], [Mn] and [Cr] respectively mean the contents (wt%) of C, Mn and Cr. The method for producing a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance according to claim 6, wherein the slab further contains, by weight percent, 0.1% or less (including 0%) of silicon (Si). The slab heating temperature is 1100 to 1300°C,
The finish rolling temperature of the hot rolling is 880° C. or higher,
The winding temperature is 400 to 700° C.
The method for producing a cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance according to claim 6, wherein the cold rolling reduction is 50 to 90%. immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at 440 to 480°C to hot-dip galvanize the cold-rolled steel sheet;
The method for producing a plated steel sheet excellent in bake hardenability and room temperature aging resistance according to any one of claims 6 to 8, further comprising: selectively, after the hot-dip galvanizing, performing an alloying treatment by holding the steel sheet at a temperature in the range of 460 to 610°C for 20 seconds or more.

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