KR20240051961A - High toughness high carbon steel sheet and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a high-strength, high-toughness steel sheet and a manufacturing method thereof, and more specifically, to a high-strength, high-toughness steel sheet that can be used for automobile seat belt springs, etc., and a manufacturing method thereof.

Description

High-strength, high-toughness steel plate and manufacturing method thereof

The present invention relates to a high-strength, high-toughness steel sheet and a manufacturing method thereof, and more specifically, to a high-strength, high-toughness steel sheet that can be used for automobile seat belt springs, etc., and a manufacturing method thereof.

Typically, the material used for automobile seat belt springs is thin, with a final material thickness of about 0.1 to 0.3 mm, and is used in the form of a mainspring with a width of about 3 to 25 mm, so it requires high toughness. In addition, since rewinding performance, which is an important characteristic of the spring, must be excellent, the tensile strength of the final cold rolled steel sheet must be high in order to secure the target restoring force and torque for each product.

In order to secure the above-mentioned properties of thin material and high strength, the most widely used is high carbon steel, which contains more carbon than eutectoid steel. By utilizing the pearlite structure of hypereutectoid high carbon steel, high toughness and strength can be secured by controlling the shape of the elongated pearlite structure obtained after cold rolling. This is more economical than using expensive alloy elements or utilizing low-temperature transformation structures such as bainite or tempered martensite through an additional heat treatment process.

In order to use the spring for more than 300,000 rewinds without fracture or breakage during use, the uniform pearlite (fibrous pearlite) fraction in the microstructure of the final 0.2 tons of cold rolled material must be high.

Korean Patent Publication No. 10-2018-0034885 (published on April 5, 2018)

According to one aspect of the present invention, it is intended to provide a high-strength, high-toughness steel plate and a method of manufacturing the same.

The object of the present invention is not limited to the above-described content. A person skilled in the art will have no difficulty in understanding the additional problems of the present invention from the overall content of the present specification.

One aspect of the present invention is, in weight percent, carbon (C): 0.70 to 1.20%, manganese (Mn): 0.2 to 0.6%, silicon (Si): 0.01 to 0.4%, phosphorus (P): 0.005 to 0.02%. , Sulfur (S): 0.01% or less, Aluminum (Al): 0.01 to 0.1%, Chromium (Cr): 0.1 to 0.8%, Vanadium (V): 0.02 to 0.25%, Cobalt (Co): 0.01 to 0.2%, Contains residual iron (Fe) and other inevitable impurities,

It has a microstructure containing a pearlite structure as the main phase and less than 4% by area of remaining intergranular proeutectoid cementite,

The pearlite structure can provide a steel sheet composed of 40% or more of fibrous pearlite, 50% or less of bent pearlite, and 10% or less of heterogeneous pearlite in terms of area %.

When observing the microstructure cross section of the steel plate in the thickness direction, the average thickness of the uniform pearlite may be 2.5 μm or less.

The steel plate may have an A value of 1.2 or less in the following relational equation 1.

[Relationship 1]

A = [Mn]+[Cr]+[V]

(Here, [Mn], [Cr], and [V] are the weight percent of each element.)

The steel sheet has a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R/t) of 3.0 or less (R is the bending radius at which bending cracks do not occur after a 180° bending test, and t is the steel sheet thickness. It can be.)

The steel plate may have a tensile strength of 2200 to 2350 MPa.

The thickness of the steel plate may be 0.1 to 0.6 mm.

Another aspect of the present invention is, in weight percent, carbon (C): 0.70 to 1.20%, manganese (Mn): 0.2 to 0.6%, silicon (Si): 0.01 to 0.4%, phosphorus (P): 0.005 to 0.02. %, Sulfur (S): 0.01% or less, Aluminum (Al): 0.01~0.1%, Chromium (Cr): 0.1~0.8%, Vanadium (V): 0.02~0.25%, Cobalt (Co): 0.01~0.2% , reheating the steel slab containing residual iron (Fe) and other unavoidable impurities;

Rough rolling the reheated steel slab;

Obtaining a hot-rolled steel sheet by finishing rolling the rough-rolled steel sheet;

Cooling the hot-rolled steel sheet to a temperature range of 540-660°C at a cooling rate of 5-50°C/s and then winding it;

The cooled and wound steel sheet is heated to a temperature range of 850 to 1050°C and maintained for 5 to 20 minutes, then cooled to a temperature range of 520 to 590°C at a cooling rate of 50 to 150°C/s, and then heated to a temperature range of 30 to 120°C. heat treatment to maintain seconds; and

A method of manufacturing a steel sheet may be provided including the step of cold rolling the heat-treated steel sheet at a cumulative reduction rate of 80 to 96%.

The steel slab may have an A value of 1.2 or less in relational equation 1 below.

[Relationship 1]

A = [Mn]+[Cr]+[V]

(Here, [Mn], [Cr], and [V] are the weight percent of each element.)

The reheating is performed in a temperature range of 1100 to 1300°C,

The rough rolling is performed in a temperature range of 1000 to 1100°C,

The finish rolling can be performed at a temperature range of 860 to 940°C.

After the coiling, the step of pickling the steel sheet at a temperature range of 200° C. or lower may be further included.

After the heat treatment, the step of air cooling the steel sheet may be further included.

The microstructure of the heat-treated steel sheet may include a pearlite structure as the main phase and a remaining 4 area% or less of proeutectoid cementite.

The thickness of the hot rolled steel sheet after the finish rolling may be 1.5 to 2.6 mm.

The thickness of the cold rolled steel sheet after the cold rolling may be 0.1 to 0.6 mm.

According to one aspect of the present invention, a high-strength, high-toughness steel plate and a manufacturing method thereof can be provided.

According to one aspect of the present invention, it relates to a high-strength, high-toughness steel sheet that can be used for high-end industry/tools, automobile seat belt springs, etc., and a method of manufacturing the same.

Figure 1 is a photograph of the shape of uniform pearlite (fibrous pearlite) observed with a scanning electron microscope (x20,000).
Figure 2 is a photograph showing the method for calculating the uniform pearlite (fibrous pearlite) fraction of Inventive Example 2.

Best mode for carrying out the invention

Below, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

However, as described above, the reel spring materials currently being manufactured lack a uniform pearlite (fibrous pearlite) fraction, resulting in reduced durability and quality differences between materials, making it difficult to secure a stable, uniform fibrous pearlite structure. In addition, due to the composition and process characteristics of producing by cold rolling the pearlite single-phase structure of eutectoid steel or higher, quality deterioration due to proeutectoid cementite exists, so improvement is necessary.

The present inventor conducted in-depth research to manufacture cold-rolled steel sheets with excellent strength and toughness by controlling the steel composition and manufacturing process.

As a result, it was confirmed that the above physical properties can be secured by optimizing the alloy composition and manufacturing conditions to control the intergranular proeutectoid cementite of the steel sheet before cold rolling and by strictly controlling the uniform pearlite (fibrous pearlite) structure of the final steel sheet, The present invention has been completed.

Hereinafter, the present invention will be described in detail.

Below, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, the % indicating the content of each element is based on weight.

The steel sheet according to one aspect of the present invention has, in weight percent, carbon (C): 0.70 to 1.20%, manganese (Mn): 0.2 to 0.6%, silicon (Si): 0.01 to 0.4%, phosphorus (P): 0.005 to 0.005%. 0.02%, Sulfur (S): 0.01% or less, Aluminum (Al): 0.01 to 0.1%, Chromium (Cr): 0.1 to 0.8%, Vanadium (V): 0.02 to 0.25%, Cobalt (Co): 0.01 to 0.2 %, may include residual iron (Fe) and other unavoidable impurities.

Carbon (C): 0.70∼1.20%

Carbon (C) is an element that greatly affects the strength and toughness of the pearlite structure, and it is desirable to add 0.70% or more to secure more than 40% of uniform pearlite (fibrous pearlite) after cold rolling. However, if the carbon (C) content exceeds 1.20%, the grain boundary proeutectite cementite fraction increases after heat treatment, resulting in inferior toughness. The lower limit of the carbon (C) content is more preferably 0.75%, more preferably 0.76%, even more preferably 0.77%, and more preferably 0.78%. The upper limit of the carbon (C) content is more preferably 0.90%, more preferably 0.88%, even more preferably 0.87%, and more preferably 0.85%.

Manganese (Mn): 0.2∼0.6%

Manganese (Mn) can be added in an amount of 0.2% or more to improve strength due to solid solution strengthening. However, if added excessively, there is a risk of deterioration of toughness due to carbide formation, and there is a risk of brittleness due to low-temperature structure of the segregated area due to central segregation, so the upper limit of the content can be limited to 0.6%. The lower limit of the manganese (Mn) content is more preferably 0.22%, more preferably 0.24%, and more preferably 0.25%. The upper limit of the manganese (Mn) content is more preferably 0.5%, more preferably 0.48%, more preferably 0.46%, and more preferably 0.45%.

Silicon (Si): 0.01~0.4%

Silicon (Si) can be added in an amount of 0.01% or more to strengthen the solid solution of the ferrite structure in pearlite. However, if added excessively, the primary scale generated in the heating furnace is excessively formed, causing red scale defects, impeding heat treatment and machinability, and there is a risk of brittleness caused by residual cementite, so reduce the content to 0.4%. It can be limited to the following. The lower limit of the silicon (Si) content is more preferably 0.05%, more preferably 0.06%, more preferably 0.08%, and more preferably 0.1%. The upper limit of the silicon (Si) content is more preferably 0.3%, more preferably 0.28%, more preferably 0.26%, and more preferably 0.25%.

Phosphorus (P): 0.005~0.02%

If phosphorus (P) exceeds 0.02%, there may be a risk of brittleness due to segregation. Therefore, it is preferable that the phosphorus (P) content is 0.02% or less. The upper limit of the phosphorus (P) content is more preferably 0.015%, more preferably 0.014%, even more preferably 0.013%, and more preferably 0.012%. Meanwhile, considering the case where it is inevitably included during the manufacturing process, the lower limit can be limited to 0.005%.

Sulfur (S): 0.01% or less

Sulfur (S) is an element that forms non-metallic inclusions and reduces toughness, so it is necessary to keep its content as low as possible. Therefore, it is preferable that the sulfur (S) content is 0.01% or less. Meanwhile, in the present invention, the lower the sulfur (S) content, the lower the risk of brittleness due to segregation/inclusions, which is advantageous in securing toughness, so the lower limit is not particularly limited. The sulfur (S) content is more preferably 0.008% or less, more preferably 0.006% or less, and more preferably 0.005% or less.

Aluminum (Al): 0.01~0.1%

Aluminum (Al) can be added to refine the pearlite structure by refining austenite grains through the formation of AlN. If the aluminum (Al) content is less than 0.01%, it may be difficult to sufficiently obtain the above effect. On the other hand, if the content exceeds 0.1%, there may be a risk of brittleness due to inclusions due to oxide formation. The lower limit of the aluminum (Al) content is more preferably 0.012%, more preferably 0.014%, and more preferably 0.015%. The upper limit of the aluminum (Al) content is more preferably 0.06%, more preferably 0.05%, more preferably 0.04%, and more preferably 0.03%.

Chromium (Cr): 0.1~0.8%

Chromium (Cr) is preferably added in an amount of 0.1% or more to ensure strength and refine the pearlite layer spacing. On the other hand, if the content exceeds 0.8%, there is a risk of toughness deterioration due to excessive carbide formation. The lower limit of the chromium (Cr) content is more preferably 0.12%, more preferably 0.14%, and more preferably 0.15%. The upper limit of the chromium (Cr) content is more preferably 0.4%, more preferably 0.35%, even more preferably 0.33%, and more preferably 0.30%.

Vanadium (V): 0.02~0.25%

Vanadium (V) is an element necessary to refine pearlite crystal grains and secure strength through work hardening after cold rolling. In order to ensure the above effect, in the present invention, 0.02% or more of vanadium (V) may be added. On the other hand, if the content is excessive, coarse carbon/nitride may be formed and there may be a risk of brittleness, so the upper limit may be limited to 0.25%. The lower limit of vanadium (V) is more preferably 0.03%, more preferably 0.04%, and more preferably 0.05%. The upper limit of vanadium (V) is more preferably 0.22%, more preferably 0.20%, and more preferably 0.18%.

Cobalt (Co): 0.01~0.2%

Cobalt (Co) is an element that promotes the formation of uniform pearlite, increases the degree of orientation of pearlite, and is necessary to secure uniform pearlite (fibrous pearlite) after cold rolling. It can be contained in an amount of 0.01% or more. On the other hand, if the content is excessive, there is a risk of reducing heat treatability as it reduces hardenability and requires a faster cooling rate. Therefore, the upper limit of cobalt (Co) content can be limited to 0.2%. The lower limit of cobalt (Co) is more preferably 0.02%, more preferably 0.03%, and more preferably 0.05%. The upper limit of cobalt (Co) is more preferably 0.18%, more preferably 0.16%, and more preferably 0.15%.

The steel sheet of the present invention may contain remaining iron (Fe) and inevitable impurities in addition to the composition described above. Since unavoidable impurities may be unintentionally introduced during the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing field, all of them are not specifically mentioned in this specification.

The steel plate according to one aspect of the present invention may have an A value of 1.2 or less in relational equation 1 below.

In the present invention, it is intended to prevent poor bendability and excessive carbide formation due to segregation through the following relational equation 1. When Mn, Cr, and V are added excessively, they cause macro and micro segregation during the casting process, and form large amounts of carbides during the heat treatment process, which can reduce the toughness and bendability of the final product. Therefore, in the present invention, in order to prevent the above problem, the A value can be controlled to 1.2 or less. The lower limit of the A value can be the sum of the lower limits of the contents of each element Mn, Cr, and V.

[Relationship 1]

A = [Mn]+[Cr]+[V]

(Here, [Mn], [Cr], and [V] are the weight percent of each element.)

Below, the steel microstructure of the present invention will be described in detail.

In the present invention, unless specifically stated otherwise, the % indicating the fraction of microstructure is based on area.

The steel sheet according to one aspect of the present invention may have a microstructure including a pearlite structure as a main phase and a remaining 4 area% or less of proeutectoid cementite. In addition, the pearlite is composed of 40% or more of fibrous pearlite, 50% or less of bent pearlite, and 10% or less of heterogeneous pearlite in terms of area %, and the average thickness of the uniform pearlite is 2.5 μm. It may be below.

A sheet having a pearlite structure before cold rolling finally has three types of pearlite structures through compression deformation in the thickness direction through cold rolling. Fibrous pearlite is stretched with the layered structure parallel to the rolling direction, and has the same shape as the center of Figure 1. Bent pearlite is one that is bent more than once in the vertical direction of rolling, resulting in a zig-zag layered structure of pearlite. Non-uniform pearlite is one in which the layered structure of pearlite bends, bends, and bends after cold rolling. It is broken at μm intervals, showing a form in which fibrous or bent pearlite is difficult to clearly observe. The form of the final pearlite structure after such cold rolling may vary in proportion depending on the composition and manufacturing conditions.

Meanwhile, in the overall microstructure, if the grain boundary proeutectoid cementite fraction exceeds 4 area%, there may be a problem of brittle fracture due to grain boundary proeutectoid cementite.

In the present invention, in order to secure high strength and high toughness, when the pearlite structure before cold rolling is formed into uniform pearlite (fibrous pearlite), zigzag pearlite (bent pearlite), and heterogeneous pearlite by cold rolling, the fraction of each pearlite formed is controlled. It is characterized by . Specifically, in the present invention, the pearlite, after cold rolling, is characterized in that it is composed of 40% or more of uniform pearlite (fibrous pearlite), 50% or less of bent pearlite, and 10% or less of heterogeneous pearlite in terms of area %. do. In order to secure bending properties for high toughness, it is desirable to contain more than 40% of fibrous pearlite in terms of area, and bent pearlite and heterogeneous pearlite should contain less than 50% and less than 10%, respectively, to secure the desired properties in the present invention. It is desirable to limit it. More preferably, uniform pearlite may be included in an amount of 50% or more. In the present invention, the fibrous pearlite fraction may contain 100%, and the bent pearlite and heterogeneous pearlite fractions may each contain 0%. Meanwhile, in the present invention, the fraction of the pearlite phase can be expressed by calculating the average of the microstructure fraction measured when observing 10 to 15 arbitrary points in the thickness direction cross section of the entire steel plate, and the thickness of the fibrous pearlite is also It can be expressed by calculating the average.

If the average thickness of uniform pearlite exceeds 2.5 μm, the strength decreases as the crystal grains become larger, and the desired level of strength cannot be secured due to the formation of coarse uniform pearlite, and bendability is also secured due to increased brittleness. Can not.

Below, the steel sheet manufacturing method of the present invention will be described in detail.

The steel sheet according to one aspect of the present invention can be manufactured by reheating, rolling, cooling, coiling, heat treating, and cold rolling a steel slab satisfying the above-described alloy composition.

reheat

Steel slabs satisfying the alloy composition of the present invention can be reheated to a temperature range of 1100 to 1300°C.

If the reheating temperature is less than 1100°C, it may be difficult to sufficiently secure the temperature of the slab required for whole plate. On the other hand, if the temperature exceeds 1300°C, abnormal austenite growth and surface defects due to excessive scale may occur.

Rough rolling

The reheated steel slab can be crudely rolled at a temperature range of 1000 to 1100°C.

If the rough rolling temperature is less than 1000°C, there may be a disadvantage in that the rolling load increases and the sheet-through performance is deteriorated. On the other hand, if the temperature exceeds 1100°C, excessive scale may be formed, resulting in very poor surface quality.

finishing rolling

A hot rolled steel sheet can be obtained by finishing rolling the rough rolled steel sheet at a temperature range of 860 to 940°C.

If the finish rolling temperature is less than 860°C, hot rolling properties may be greatly reduced due to excessive rolling load. On the other hand, if the temperature exceeds 940°C, the austenite grain size becomes very coarse and there is a risk of brittleness. In the present invention, the thickness of the hot rolled steel sheet after finish rolling may be 1.5 to 2.6 mm. A more preferable upper limit of the thickness of the hot rolled steel sheet may be 2.5 mm, and a more preferable lower limit of the thickness may be 1.6 mm.

Cooling and Winding

The hot-rolled steel sheet can be cooled to a temperature range of 540-680°C at a cooling rate of 5-50°C/s and then coiled.

During the cooling, if the cooling rate is less than 5°C/s, the pearlite structure becomes coarse and there is a risk of brittleness. On the other hand, if the cooling rate exceeds 50°C/s, the shape may be deteriorated due to material deviation in the width direction due to supercooling of the edge portion in the width direction, making winding difficult.

If the coiling temperature is less than 540°C, it may be difficult to obtain a uniform hot-rolled structure because bainite or martensite structures, which are low-temperature transformation structures, are formed. Meanwhile, the upper limit of the coiling temperature can be limited to 680°C. However, since surface defects may be caused by forming an internal oxidation layer and a decarburization layer on the surface, the temperature can be more preferably limited to 660°C or lower.

After the coiling, the present invention may further include a process of pickling the hot rolled steel sheet. The pickling may be performed after the coiled steel sheet is naturally cooled to 200° C. or lower, and scale formed on the surface of the steel sheet can be removed through the pickling.

heat treatment

The cooled and coiled steel sheet is heated to a temperature range of 850 to 1050°C and maintained for 5 to 20 minutes, then cooled to a temperature range of 500 to 650°C at a cooling rate of 50 to 250°C/s, and then heated to a temperature range of 30 to 180°C. Heat treatment can be performed for seconds. More preferably, the upper limit of the cooling rate may be 150°C/s, the lower limit of the more preferable cooling temperature range may be 520°C, and the upper limit may be 590°C. A more desirable upper limit of holding time may be 120 seconds.

If the heating temperature, that is, the austenizing heating temperature, is less than 850°C, undissolved carbides may remain due to insufficient austenizing, causing brittleness. On the other hand, if the temperature exceeds 1050°C, the austenite grains become coarse and there is a risk that toughness may decrease, and the work hardening ability of the pearlite structure may be reduced, making it difficult to secure a uniform pearlite (fibrous pearlite) structure later. In the present invention, the heating method is not particularly limited, but methods such as high-frequency induction heating or a BOX type heating furnace can be used.

If the holding time after heating is less than 5 minutes, complete austenizing may be difficult, and if the time exceeds 20 minutes, the grains may become excessively coarse.

When cooling after heating and holding, if the cooling rate is less than 50°C/s, the proportion of intergranular proeutectoid cementite may increase excessively, causing brittleness and making it difficult to form a uniform pearlite (fibrous pearlite) structure. Meanwhile, the upper limit of the cooling rate can be limited to 250°C/s. However, since the cooling rate is not easy to control, there may be a risk of forming low-temperature structures other than pearlite, so a more preferable upper limit of the cooling rate may be 150°C/s.

The lower limit of the cooling end temperature may be 500°C. However, in order to prevent the risk of forming low-temperature structures such as bainite other than pearlite, a more preferable lower limit may be 520°C. In addition, the upper limit of the cooling end temperature may be 650°C. However, considering that it may be difficult to form uniform pearlite (fibrous pearlite) after cold rolling due to coarse grains in the structure, a more preferable upper limit of the cooling end temperature may be 590°C.

If the holding time after cooling is less than 30 seconds, the pearlite structure may not be sufficiently formed, and the upper limit of the time can be limited to 180 seconds. On the other hand, since it may be difficult to secure sufficient strength through work hardening after cold rolling due to a decrease in strength, a more desirable upper limit of the holding time may be 120 seconds. In the present invention, the heat treatment method may use hydrogen gas, salt bath, lead bath, etc., and may not be particularly limited. In addition, in the present invention, the steel sheet can be air cooled after heat treatment.

In the present invention, it is preferable that the microstructure of the steel sheet after heat treatment includes a main phase pearlite structure and a remaining 4 area% or less of intergranular proeutectoid cementite. At this time, in the present invention, by appropriately controlling the grain boundary proeutectoid cementite fraction, the amount of cementite with very high strength is minimized, thereby facilitating stretching during cold rolling of pearlite and ensuring a uniform pearlite thickness of 2.5 μm or less. can do. On the other hand, if the intergranular proeutectoid cementite fraction exceeds 4 area%, there may be a problem of brittle fracture due to intergranular proeutectoid cementite during cold rolling.

cold rolling

The heat-treated steel sheet can be cold rolled at a cumulative reduction rate of 75 to 96%. A more preferable lower limit of the cumulative reduction rate may be 80%, and a more preferable upper limit of the cumulative reduction rate may be 95%.

In the present invention, cold rolling can be performed by applying a certain reduction ratio to manufacture a cold rolled steel sheet of a desired thickness. The lower limit of the reduction rate can be limited to 75%. However, since securing a uniform pearlite (fibrous pearlite) fraction may become difficult, a more desirable lower limit may be 80%. On the other hand, if the reduction ratio exceeds 96%, there may be a risk of cracking due to excessive work hardening. A more desirable lower limit of reduction ratio may be 95%. Detailed rolling pass schedules such as reduction rate, speed, and width size per individual pass vary depending on equipment and use, and are not specified in the present invention. In the present invention, the thickness of the cold rolled steel sheet may be more preferably 0.1 to 0.6 mm. More preferably, the thickness may be 0.3 mm or less.

As described above, through this cold rolling, the pearlite structure, which is the main phase forming the microstructure of the sheet material, can have three final types of pearlite structure due to compression deformation in the thickness direction.

Therefore, the steel sheet of the present invention may have a microstructure containing a pearlite structure as the main phase and a remaining 4 area% or less of cementite, a grain boundary proeutectoid, and through the above-described cold rolling, the pearlite structure, in terms of its own area%, becomes a uniform pearlite. It can be formed into a structure containing more than 40% of fibrous pearlite, less than 50% of zigzag pearlite (bent pearlite), and less than 10% of heterogeneous pearlite.

The steel sheet of the present invention manufactured in this way has a thickness of 0.1 to 0.6 mm, a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R/t) of 3.0 or less (R is the bending value after a 180° bending test). (is the bending radius at which minor cracks do not occur, and t is the thickness of the steel sheet), it can have high strength and excellent toughness characteristics. A more desirable upper limit of the steel plate thickness may be 0.3 mm. More preferably, the tensile strength value may be 2200 MPa or more, and the upper limit of the more preferable tensile strength value may be 2350 MPa.

Hereinafter, the present invention will be described in more detail through examples. However, it is important to note that the examples below are only for illustrating and explaining the present invention in more detail, and are not intended to limit the scope of the present invention.

Forms for practicing the invention

(Example)

A steel slab having the alloy composition shown in Table 1 below was heated to 1200°C for 2 hours, and then a cold rolled steel sheet was manufactured under the conditions shown in Table 2 below. At this time, the rough rolling temperature was 1080°C and the finish rolling temperature was 900°C. In addition, the cooling rate from hot rolling to coiling was 20°C/s, and coiling was performed under the coiling temperature conditions in Table 2. The manufactured hot rolled steel sheet was pickled, heated at 950°C for 10 minutes, cooled at a cooling rate of 70°C/s, and then cold rolled under the conditions in Table 2.

Steel grade Alloy composition (wt%) C Mn Cr Si Al P S V Co A 0.82 0.35 0.27 0.23 0.035 0.011 0.002 0.06 0.07 B 0.63 0.36 0.23 0.21 0.037 0.010 0.003 0.07 0.11 C 0.83 0.11 0.26 0.22 0.033 0.011 0.002 0.09 0.09 D 0.83 0.64 0.28 0.24 0.034 0.010 0.003 0.07 0.08

Psalter
number Steel grade winding heat treatment cold rolling Steel plate thickness (mm) temperature
(℃) cooling and
Maintaining temperature
(℃) hour
(candle) accumulate
Reduction rate
(%) After hot acting After cold rolling One A 600 560 70 90.5 2.1 0.20 2 A 620 550 60 93.2 2.2 0.15 3 A 620 545 80 86.1 1.8 0.25 4 A 500 560 60 90.0 2.2 0.18 5 A 720 560 60 90.0 2.2 0.18 6 A 600 490 70 89.0 2.0 0.22 7 A 600 630 70 89.0 2.0 0.22 8 A 620 550 16 91.7 1.8 0.15 9 A 620 550 150 91.7 1.8 0.15 10 A 600 560 65 75 1.6 0.40 11 A 600 560 65 96.9 3.5 0.11 12 B 620 550 70 91.0 2.0 0.18 13 C 620 550 70 91.0 2.0 0.18 14 D 620 550 70 91.0 2.0 0.18

Table 3 below shows the measured microstructure and physical properties of the manufactured steel sheets. The microstructure was observed and shown after heat treatment and cold rolling, respectively. First, the area fraction of grain boundary proeutectoid cementite was measured and shown using an electron microscope photograph of a steel sheet heat-treated before cold rolling at a magnification of x3000. In the microstructure of the steel sheet before cold rolling in Table 3 below, all fractions other than grain boundary proeutectoid cementite include pearlite. After cold rolling, the steel sheet was photographed multiple times using an electron microscope at x4300 magnification of about 10 to 15 cross-sections in the thickness direction of the steel sheet. After measuring the thickness length occupied by the microstructure, the thickness was expressed as a ratio and the average value was calculated as a microstructure. Expressed as tissue fraction. In addition, each thickness was measured for the uniform pearlite (fibrous pearlite) structure, and the average values are shown in Table 3 below. At this time, the fractions of uniform pearlite, zigzag pearlite, and heterogeneous pearlite represent the fraction with respect to the total pearlite fraction.

In addition, tensile tests and bending tests were performed on the manufactured cold rolled steel sheets to show physical properties and whether cracks existed. The tensile test was performed at room temperature according to the JIS No. 5 standard and measured the tensile strength and elongation. The presence or absence of cracks was determined when R/t was 3.0 or less after the 180° bending test (R indicates that no cracks occur in the bending area after the 180° bending test). (is the bending radius, and t is the thickness of the steel plate), it is indicated as O, otherwise, it is indicated as X.

city
side
th
like river
bell microstructure Properties division perlite grain boundary cornerstone
cementite
Fraction (%) fraction
(%) (based on 100% total perlite) Fibrous Pearlite Bent Perlite Heterogeneous pearlite tensile strength
(MPa) elongation
(%) bend test
presence or absence of cracks
(O,X) average thickness
(μm) fraction
(%) fraction
(%) fraction
(%) One A 98 2.2 49 46 5 2 2287 3.5 X Invention Example 1 2 A 99 2.1 48 48 4 One 2276 3.4 X Invention example 2 3 A 97 2.3 51 45 4 3 2311 3.8 X Invention Example 3 4 A 96 2.0 31 50 19 4 1982 5.6 X Comparative Example 1 5 A 96 3.1 29 38 33 4 2139 4.2 O Comparative example 2 6 A 99 1.2 32 13 55 One 2614 0.8 X Comparative Example 3 7 A 94 3.2 25 39 36 6 1898 6.9 O Comparative Example 4 8 A 95 1.9 19 25 56 5 2013 4.8 O Comparative Example 5 9 A 96 1.8 28 64 8 4 1899 6.1 O Comparative Example 6 10 A 97 3.4 27 55 18 3 1985 6.4 O Comparative example 7 11 A 95 2.1 25 56 19 5 2529 1.1 X Comparative example 8 12 B 99 3.4 29 48 23 One 2013 6.7 O Comparative Example 9 13 C 98 3.2 35 44 21 2 1957 6.6 X Comparative Example 10 14 D 94 2.6 44 51 5 6 2493 1.2 X Comparative Example 11

As shown in Table 3, in the case of the invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties desired in the present invention were secured.

Figure 2 is a photograph showing the method for calculating the microstructure fraction and uniform pearlite (fibrous pearlite) thickness of Inventive Example 2. When microstructure photos are taken in the thickness direction of the steel sheet, uniform pearlite (fibrous pearlite) appears to be characterized by a layered structure without bends or segments, and can be indicated by a dotted line as shown in Figure 2. In addition, bent pearlite is characterized by a zigzag shape in which the layered structure is bent more than once, as shown by the solid line in Figure 2, and is distinguished from uniform pearlite (fibrous pearlite) by mixing zigzag and wave shapes. This allows you to measure the thickness. The portion excluding the solid and dotted lines in Figure 2 represents heterogeneous pearlite. After measuring the thickness of each microstructure, the sum of these can be calculated and expressed as a fraction, and the uniform pearlite (fibrous pearlite) thickness can be expressed as the average value of the measured thickness values.

On the other hand, Comparative Example 1 satisfies the alloy composition of the present invention, but the coiling temperature is too low and strength cannot be sufficiently secured by work hardening during cold rolling due to low-temperature structure formation, so the tensile strength satisfies the level desired in the present invention. Couldn't do it.

Comparative Example 2 satisfies the alloy composition of the present invention, but the coiling temperature was too high, so a coarse pearlite structure was formed. This coarse pearlite structure interferes with the formation of a uniform pearlite (fibrous pearlite) structure during cold rolling, resulting in a uniform pearlite (fibrous pearlite) structure. fibrous fraction) did not satisfy the level desired in the present invention. As a result, the desired strength could not be secured.

Comparative Example 3 satisfied the alloy composition of the present invention, but the heat treatment temperature was too low and some low-temperature structures were formed, so the tensile strength did not meet the level desired in the present invention.

Comparative Example 4 satisfied the alloy composition of the present invention, but the heat treatment temperature was too high to form a coarse pearlite structure, and the uniform pearlite (fibrous pearlite) fraction did not satisfy the level desired in the present invention. As a result, the strength was inferior.

Comparative Example 5 is a case in which the retention time after cooling during heat treatment does not fall within the scope of the present invention, and sufficient uniform pearlite (fibrous pearlite) was not formed due to insufficient time. As a result, the strength due to work hardening during cold rolling was reduced. The increase was insufficient.

Comparative Example 6 is a case in which the holding time after cooling during heat treatment exceeded the range of the present invention, and uniform pearlite (fibrous pearlite) was not sufficiently formed, and the pearlite softened after formation and the strength was lowered, thereby failing to achieve the desired effect of the present invention. The level of intensity was not met.

Comparative Example 7 is a case where the reduction rate during cold rolling is outside the range of the present invention, and the desired uniform pearlite (fibrous pearlite) fraction and tensile strength were not secured due to the low reduction rate.

In Comparative Example 8, the cold rolling reduction ratio was excessive, and the strength increased excessively, so the tensile strength did not satisfy the range targeted by the present invention.

Comparative Example 9 is a case where the C content is below the range of the present invention, the uniform pearlite (fibrous pearlite) fraction is below the desired range due to the formation of coarse pearlite, and the strength is also below the desired range of the present invention.

Comparative Example 10 was a case where the Mn content was below the range of the present invention, the uniform pearlite (fibrous pearlite) fraction did not satisfy the level desired in the present invention, and there was difficulty in securing the desired level of strength.

Comparative Example 11 is a case where the Mn content exceeds the range of the present invention, and the strength is excessively increased, exceeding the desired range of the present invention.

Although the present invention has been described in detail above through examples, other forms of embodiments are also possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (14)

By weight percent, carbon (C): 0.70 to 1.20%, manganese (Mn): 0.2 to 0.6%, silicon (Si): 0.01 to 0.4%, phosphorus (P): 0.005 to 0.02%, sulfur (S): 0.01. % or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.1 to 0.8%, vanadium (V): 0.02 to 0.25%, cobalt (Co): 0.01 to 0.2%, balance iron (Fe) and others. Contains inevitable impurities,
It has a microstructure containing a pearlite structure as the main phase and less than 4% by area of remaining intergranular proeutectoid cementite,
The pearlite structure is a steel sheet composed of 40% or more of fibrous pearlite, 50% or less of bent pearlite, and 10% or less of heterogeneous pearlite in terms of area %. According to paragraph 1,
When observing the microstructure cross section in the thickness direction, the steel plate has an average uniform pearlite thickness of 2.5 μm or less. According to paragraph 1,
The steel sheet has an A value of 1.2 or less in the following relational expression 1.
[Relationship 1]
A = [Mn]+[Cr]+[V]
(Here, [Mn], [Cr], and [V] are the weight percent of each element.) According to paragraph 1,
The steel sheet has a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R/t) of 3.0 or less (R is the bending radius at which bending cracks do not occur after a 180° bending test, and t is the steel sheet thickness. ) is a steel plate. According to paragraph 1,
The steel sheet has a tensile strength of 2200 to 2350 MPa. According to paragraph 1,
A steel plate having a thickness of 0.1 to 0.6 mm. By weight percent, carbon (C): 0.70 to 1.20%, manganese (Mn): 0.2 to 0.6%, silicon (Si): 0.01 to 0.4%, phosphorus (P): 0.005 to 0.02%, sulfur (S): 0.01. % or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.1 to 0.8%, vanadium (V): 0.02 to 0.25%, cobalt (Co): 0.01 to 0.2%, balance iron (Fe) and others. reheating the steel slabs containing unavoidable impurities;
Rough rolling the reheated steel slab;
Obtaining a hot-rolled steel sheet by finishing rolling the rough-rolled steel sheet;
Cooling the hot-rolled steel sheet to a temperature range of 540-660°C at a cooling rate of 5-50°C/s and then winding it;
The cooled and wound steel sheet is heated to a temperature range of 850 to 1050°C and maintained for 5 to 20 minutes, then cooled to a temperature range of 520 to 590°C at a cooling rate of 50 to 150°C/s, and then heated to a temperature range of 30 to 120°C. heat treatment to maintain seconds; and
A steel sheet manufacturing method comprising cold rolling the heat-treated steel sheet at a cumulative reduction rate of 80 to 96%. In clause 7,
The steel slab is a steel plate manufacturing method in which the A value of the following relational equation 1 is 1.2 or less.
[Relationship 1]
A = [Mn]+[Cr]+[V]
(Here, [Mn], [Cr], and [V] are the weight percent of each element.) In clause 7,
The reheating is performed in a temperature range of 1100 to 1300°C,
The rough rolling is performed in a temperature range of 1000 to 1100°C,
A steel sheet manufacturing method in which the finish rolling is performed in a temperature range of 860 to 940 ° C. In clause 7,
A method of manufacturing a steel sheet further comprising the step of pickling the steel sheet at a temperature range of 200° C. or lower after the coiling. In clause 7,
A method of manufacturing a steel sheet further comprising air cooling the steel sheet after the heat treatment. In clause 7,
A method of manufacturing a steel sheet in which the microstructure of the heat-treated steel sheet includes pearlite structure as the main phase and residual cementite, a grain boundary pro-eutectoid, of less than 4 area%. In clause 7,
A method of manufacturing a steel sheet in which the thickness of the hot rolled steel sheet after the finish rolling is 1.5 to 2.6 mm. In clause 7,
A method of manufacturing a steel sheet in which the thickness of the cold rolled steel sheet after the cold rolling is 0.1 to 0.6 mm.

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