KR20230048710A - High carbon steel sheet and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high-carbon steel sheet used in automobile seat belt springs, etc., and a method for manufacturing the same. The high-carbon steel sheet according to one aspect of the present invention comprises 0.75 to 0.85 wt% of C, 0.1 to 0.3 wt% of Si, 0.2 to 0.5 wt% of Mn, 0.1 to 0.3 wt% of Cr, 0.05 wt% or less (excluding 0 wt%) of Al, 0.03 wt% or less of P, 0.005 wt% or less of S, and the balance Fe and other inevitable impurities, the microstructure is based on pearlite, the average grain size of the pearlite is 5 to 14 ㎛ or less, and the number of unconsolidated cementite per 1 mm^2 is 100 or less. A spring with excellent durability can be manufactured using the high-carbon steel sheet according to the present invention.

Description

High carbon steel sheet and its manufacturing method {HIGH CARBON STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high carbon steel sheet used in springs of automobile seat belts and a method for manufacturing the same.

Springs used in seat belts require a restoring force to return to their original state when pulled by a user, and should not be degraded or broken even when used repeatedly. Recently, as safety standards for automobiles have been strengthened, durability standards of springs constituting seat belts have also been strengthened. The most suitable material for this is known as a cold-worked high-carbon thin steel sheet, the microstructure of which is composed of severely deformed pearlite.

In this regard, in the prior art, in order to secure durability, it has been developed in the direction of increasing the strength of the thin steel sheet, and for this purpose, a technique for making work hardening and pearlite layer spacing fine by cold working the steel sheet has been developed.

Patent Document 1 discloses a method of increasing the strength of the final thin steel sheet after cold working by reheating and constant temperature transformation of the steel sheet and then making the initial pearlite structure fine before cold working, and Patent Document 2 discloses an upper bay through constant temperature transformation A method of actively using the night is disclosed.

The above methods are capable of manufacturing high-strength plate strips. However, since the durability of the spring varies greatly compared to the strength, there is a limit to manufacturing a high-quality seat belt spring that requires strict durability.

Japanese Unexamined Patent Publication No. 8-302428 (published on November 19, 1996) Korean Patent Publication No. 10-2008-0060619 (published on July 2, 2008)

One aspect of the present invention is to provide a high carbon steel sheet capable of manufacturing a spring with excellent durability and a method for manufacturing the same.

The object of the present invention is not limited to the above. Additional tasks of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional tasks of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention, in weight %, C: 0.75 ~ 0.85%, Si: 0.1 ~ 0.3%, Mn: 0.2 ~ 0.5%, Cr: 0.1 ~ 0.3%, Al: 0.05% or less (excluding 0%), P : 0.03% or less, S: 0.005% or less, including the remaining Fe and other unavoidable impurities,

The microstructure relates to a high-carbon steel sheet having pearlite as a base structure, an average grain size of pearlite of 5 to 14 μm or less, and the number of unresolved cementite per 1 mm 2 of 100 or less.

Another aspect of the present invention, by weight %, C: 0.75 ~ 0.85%, Si: 0.1 ~ 0.3%, Mn: 0.2 ~ 0.5%, Cr: 0.1 ~ 0.3%, Al: 0.05% or less (excluding 0%), P: 0.03% or less, S: 0.005% or less, heating a steel slab containing Fe and other unavoidable impurities, hot rolling and winding to prepare a hot-rolled steel sheet;

manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 50%;

Reheating the cold-rolled steel sheet at 820 to 950° C., and maintaining the condition that satisfies Equation (1) below; and

After the maintenance, cooling to 530 ~ 600 ℃, constant temperature transformation treatment;

It relates to a method for manufacturing a high carbon steel sheet comprising a.

[Relationship 1]

T*(log t + 0.07) ≥ 124*[C] + 510*[C] ² + 928*[Cr] - 815*r ^0.5 + 2915

(Where T is the maximum reheating temperature (K), t is the time (sec), [C] is the carbon weight%, [Cr] is the chromium weight%, and r is the cold reduction.)

According to the present invention, it can be applied to the manufacture of high-quality seat belt springs by improving the durability of spring thin steel sheets manufactured through cold working.

Various advantageous advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

1 is an EBSD photograph of the microstructure of Inventive Example 1 of Examples of the present invention observed with a scanning electron microscope.
Figure 2 is an EBSD photograph of the microstructure of Comparative Example 1 of Examples of the present invention observed with a scanning electron microscope.

The terms used herein are intended to describe the present invention and are not intended to limit the present invention. Also, the singular forms used herein include the plural forms unless the related definition clearly dictates the contrary.

The meaning of "comprising" as used in the specification specifies a component, and does not exclude the presence or addition of other components.

Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. The terms defined in the dictionary are interpreted to have a meaning consistent with the related technical literature and the currently disclosed content.

The inventors of the present invention confirmed the microstructure of each process step in order to find the cause of the low durability and large deviation of the spring, and as a result, it was possible to confirm that the microstructure of the steel sheet before cold working was coarse or that unresolved cementite existed in common. . Accordingly, it was recognized that the durability of the thin steel sheet for springs made through cold working is improved when the formation of cementite for unsolidified use is dramatically suppressed while the structure is fine, and the present invention has been reached.

In particular, it was confirmed that the decomposition of the plate-like lamellar cementite structure is promoted when cold rolling of a hot-rolled steel sheet having a predetermined alloy composition without annealing, and after performing cold rolling without annealing using this, a short Even if the time is maintained, it was confirmed that all cementite was employed and the growth of austenite was suppressed, thereby securing refinement of pearlite and applied to the present invention.

Hereinafter, an example of the high carbon steel sheet of the present invention will be described in detail.

In the steel sheet of the present invention, by weight %, C: 0.75 to 0.85%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, Al: 0.05% or less (excluding 0%), P: 0.03% or less, S: 0.005% or less, including the remainder Fe and other unavoidable impurity elements. Hereinafter, each alloy composition will be described in detail.

Carbon (C): 0.75 to 0.85% (hereinafter, the content of each alloy composition means % by weight)

The C is an element for securing strength of steel. If the C is less than 0.75%, it is difficult to secure sufficient strength, whereas if it exceeds 0.85%, it may be easy to remain unresolved cementite. Also cold worked The possibility of cracking is high, and the toughness and fatigue properties of the steel sheet may be deteriorated. Therefore, the content of C is preferably 0.75 to 0.85%.

Silicon (Si): 0.1 to 0.3%

Since Si is an element having a solid-solution strengthening effect in the pearlite matrix, strength and permanent set resistance increase as the content of Si increases. For this purpose, it is preferable to include 0.1% or more. However, if it exceeds 0.3%, not only the cold rolling property is inferior, but also the possibility of decarburization increases during heat treatment, and it may cause an increase in scale defects on the steel surface. Therefore, the Si content is preferably 0.1 to 0.3%.

Manganese (Mn): 0.2 to 0.5%

The Mn is a hardenability improving element with a solid solution strengthening effect, and solid solution sulfur in steel is precipitated as manganese sulfide to prevent red heat embrittlement caused by sulfur. For this purpose, it is preferable to include 0.2% or more. However, when it exceeds 0.5%, not only is cold workability inferior, but there is a concern about deterioration of workability due to center segregation, so the content of Mn is preferably 0.2 to 0.5%.

Chromium (Cr): 0.1~0.3%

Cr, like Mn, improves the hardenability of steel and is an element that has a large effect of increasing the strength by cold working of a pearlite structure. For this purpose, it is preferable to include 0.1% or more. However, if it exceeds 0.3%, the decomposition of cementite by reheating is too slow, so unresolved cementite is formed, so the Cr content is preferably 0.1 to 0.3%.

Aluminum (Al): 0.05% or less (excluding 0%)

Al in steel is an alloying element added for deoxidation, and when added in large amounts, it affects the size of the microstructure due to the formation of AlN. When the Al is excessively added, an aluminum-based oxide is formed inside the steel material, which may deteriorate the fatigue properties of the material. Therefore, the Al content is preferably not more than 0.05%.

Phosphorus (P): 0.03% or less

The P is an element that is inevitably included in the steel manufacturing process, but is also the most advantageous element for securing strength without significantly impairing formability. However, when P is excessively added, the possibility of brittle fracture increases, which can cause plate breakage of the slab during hot rolling, and can also significantly degrade the surface properties of the coated steel sheet. Therefore, the P is preferably 0.03% or less.

Sulfur (S): 0.005% or less

S is an impurity element that is inevitably introduced into steel, and it is desirable to keep its content as low as possible. In particular, since S in steel may cause red-hot brittleness, the upper limit of the S content is preferably 0.005%.

The rest includes iron (Fe), and since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art during the manufacturing process, not all of them are specifically mentioned in the present specification.

The microstructure of the steel sheet is based on pearlite, in which ferrite and cementite form a lamellar layered structure, and is preferably 96 area% or more.

In addition to the base structure, the ferrite structure may include less than 3 area %, and other structures such as bainite, martensite, and retained austenite may contain less than 1 area %. In addition, a small amount of proeutectoid cementite may be produced depending on the manufacturing process.

The average grain size of the pearlite is preferably 5 to 14 μm. Since the effect of the present invention increases as the size of the crystal grains is finer, the average grain size of the pearlite is preferably 14 μm or less. However, in order to control the grain size to less than 5 μm, additional processes and costs are incurred, which is undesirable.

The steel sheet preferably contains 100 or less unsolidified cementite per unit area of 1 mm 2 . Unresolved cementite provides a path for generating and propagating cracks during use of the final product, thereby impairing durability, so it is preferable to form as little unresolved cementite as possible. Here, unresolved cementite refers to cementite constituting pearlite in the step of reverse transformation of pearlite structure to austenite structure by holding at 820 to 950 ° C. for a short time to produce the fine pearlite. Rather, it means remaining as carbide particles. In the present invention, a preferred example as a method for measuring the number of cementite for unresolved use is as follows. With respect to the point t/4 in the thickness direction of the steel plate, it is determined that the size of the carbide of the unresolved cementite is 1㎛ ² or more on the area basis and the long to short axis ratio is 2.0 or less. It was measured by irradiating for an area of 30,000 μm ² or more.

Next, an example of the steel sheet manufacturing method of the present invention will be described in detail.

In the steel material of the present invention, a steel slab satisfying the above-described composition is heated, hot-rolled, coiled, and cold-rolled. Thereafter, the cold-rolled steel sheet is reheated, maintained, cooled, and subjected to constant temperature transformation treatment to manufacture a steel sheet. Hereinafter, each process will be described in detail.

First, a steel slab satisfying the above-described composition is heated, and hot-rolled and coiled. The hot rolling and winding are not particularly limited in the present invention, and any method performed by a person of ordinary skill in the art pertaining to the present invention is possible. As a preferred example, a hot-rolled steel sheet may be prepared by finishing hot rolling at a temperature range of 850 to 1100° C., and winding may be performed at 600 to 680° C.

Cold rolling is performed at a reduction ratio of 20 to 50% without performing separate annealing (spheroidizing heat treatment) on the hot-rolled steel sheet. Through the cold-rolled steel sheet, it is possible to manufacture a steel sheet having a structure in which cementite is mechanically segmented. Since separate annealing is not performed before the cold rolling, the re-dissolution of cementite is promoted in the austenite reverse transformation process during the subsequent heat treatment, and the decomposition of cementite is completed in a short time even at low temperature, thereby reducing the residual of unresolved cementite. It can be prevented.

If the reduction ratio during the cold rolling is less than 20%, the decomposition rate of lamellar cementite cannot be sufficiently secured, and thus unresolved cementite may increase. On the other hand, when the rolling reduction ratio exceeds 50%, cold rolling is not preferable because the rolling load is high, which not only puts strain on the equipment, but also poses a risk of plate breakage of the material. An pickling process may be added before the cold rolling.

The cold-rolled cold-rolled steel sheet is reheated to 820 to 950 ° C, and maintained to satisfy the condition of [Relationship 1] below in the range of T-20 ° C to T relative to the maximum temperature (T).

[Relationship 1]

T*(log t + 0.07) ≥ 124*[C] + 510*[C] ² + 928*[Cr] - 815*r ^0.5 + 2915

(Where, T is the maximum temperature during reheating (K), t is the holding time (sec), [C] is the carbon weight%, [Cr] is the chromium weight%, and r is the cold rolling reduction.)

It is preferable to perform a process of reheating the cold-rolled steel sheet at 820 to 950° C. and maintaining the condition to satisfy the condition of [Relationship 1]. When the reheating temperature exceeds 950° C., the transformation into the austenite phase is easy, but an additional austenite structure is grown, which is undesirable because durability of the final product is reduced. On the other hand, when the reheating temperature is less than 820 ° C or the [Relational Expression 1] is not satisfied, not only does it take a long time to transform into an austenite structure, but also the decomposition of cementite is not sufficiently performed, resulting in unresolved cementite. It is undesirable because it remains and reduces the durability of the final product. Therefore, in order to minimize the formation of undissolved cementite while suppressing the coarsening of the austenite structure, a reheating and holding treatment that satisfies the above [Relational Expression 1] is preferable.

When the holding time t managed through [Relationship 1] is short, all of the cementite is not dissolved in the austenite matrix, and some remains, so that a large number of unresolved cementite may be formed in the subsequent cooling process. On the other hand, it is preferable not to perform the maintaining process for an excessively long time so that the crystal grain size of the pearlite does not exceed 14 μm. More preferably, it is preferable not to exceed twice the holding time (t) corresponding to the equal sign condition of [Relational Expression 1].

It is preferable to perform cooling after the holding, and to perform constant temperature transformation treatment at 530 to 600 ° C.

The cooling is preferably performed at a high cooling rate of 50° C./s or more at an average cooling rate to the target temperature. There are various methods for securing a sufficient cooling rate, but it is not particularly limited in the present invention. As a preferred example, the reheated and maintained material may be immersed in a salt bath. If the cooling rate is less than 50 °C / s, the pearlite phase transformation starts at a high temperature before reaching the target cooling stop temperature, and the microstructure becomes coarse and non-uniform. Therefore, the average cooling rate is preferably 50 ℃ / s or more.

After cooling, constant temperature transformation treatment may be performed at a target temperature of 530 to 600 ° C for 1 minute or more. When the constant temperature transformation temperature exceeds 600 ° C., the interval between cementite impregnations in pearlite becomes wide, which is not preferable because not only strength decreases, but also crack propagation resistance deteriorates and durability decreases. On the other hand, if the constant temperature transformation temperature is less than 530 ° C., bainite phase transformation occurs before pearlite phase transformation, and crack propagation resistance deteriorates, which is not preferable because durability is reduced.

The constant temperature transformation holding time is preferably performed for 30 seconds or more for appropriate pearlite phase transformation, and more preferably sufficiently maintained beyond the point at which the pearlite phase transformation is completed by 95% or more. However, it is preferable that the constant temperature transformation holding time does not exceed 5 minutes in consideration of productivity when heat treatment is performed by passing the plate material through the continuous production line.

Next, examples of the present invention will be described.

Of course, the following examples can be modified in various ways without departing from the scope of the present invention to those skilled in the art. The following examples are for understanding of the present invention, and the scope of the present invention should not be limited to the following examples and should not be defined, but should be defined by the claims described later as well as those equivalent thereto.

(Example 1)

After dissolving the steel having the composition of Table 1 (the rest being Fe and unavoidable impurities), hot-rolled at 850 to 1100 ° C. to prepare a hot-rolled steel sheet having a thickness of 2.0 to 2.3 mm. Thereafter, cold rolling was performed according to the process of Table 2, and then reheating, maintenance, and constant temperature transformation treatment were performed to manufacture a steel sheet.

On the other hand, for the steel sheet sample prepared as described above, the microstructure was analyzed to measure the average grain size of pearlite and the number of unresolved cementite, and are shown in Table 3. The grain size of the pearlite was measured using a scanning electron microscope and EBSD, and the grain boundary was classified based on a 15 degree orientation difference. Unresolved cementite was analyzed by taking more than 10 pictures at x2,000 magnification using a scanning electron microscope, ^{30,000㎛ 2} based on area. As the criterion for discrimination, the number per unit area (mm 2 ) was obtained by measuring and summing the numbers in the photographed area.

On the other hand, durability evaluation was performed by cold-working the steel sheet sample to prepare a thin material having a thickness of 0.2 to 0.25 mm, then slitting to a width of 10 mm to prepare a steel strip, and winding it into a circular spring to prepare and evaluate. The cold working was performed by cold rolling at a reduction ratio of 83 to 85%, heat treatment at 240 ° C. for 30 minutes after preparing a spring, inserting it into a frame to prepare a tapered spring, and evaluating durability. Durability was evaluated by applying an external force to the end of the manufactured ring spring, pulling it to a length of 2m, then removing the external force and rolling it in once, measuring the number of repetitions until breakage occurred, and the results are shown in Table 3 below showed up

division Chemical composition (% by weight) C Si Mn Cr P S Al invention steel 1 0.76 0.15 0.25 0.11 0.0113 0.0027 0.012 invention steel 2 0.84 0.28 0.49 0.28 0.0109 0.0031 0.021 comparative steel 1 1.02 0.21 0.41 0.14 0.0102 0.0036 0.018

division Jaemyung Kang hot rolled thickness
(mm) annealing heat treatment cold reduction
(%) Reheat and hold constant temperature transformation [Relationship 1] Satisfied highest temperature
(℃) hour
(s) temperature
(℃) hour
(s) Invention example 1 invention steel 1 2.0 X 22 947 300 591 120 OK Invention example 2 invention steel 1 2.3 X 48 830 420 583 168 OK Invention Example 3 invention steel 2 2.0 X 22 947 480 542 192 OK Invention example 4 invention steel 2 2.3 X 48 880 420 538 168 OK Comparative Example 1 invention steel 1 2.0 X 16 940 300 580 120 OK Comparative Example 2 invention steel 1 2.0 X 32 880 240 542 96 NG Comparative Example 3 invention steel 1 2.0 X 32 970 400 577 160 OK Comparative Example 4 invention steel 2 2.3 X 40 800 480 555 192 OK Comparative Example 5 invention steel 2 2.0 O 22 948 480 551 120 OK Comparative Example 6 comparative steel 1 2.0 X 22 945 480 550 192 OK Comparative Example 7 comparative steel 1 2.3 X 48 892 420 543 68 OK

The [Relationship 1] is T * (log t + 0.07) ≥ 124 * [C] + 510 * [C] ² + 928 * [Cr] - 815 * r ^0.5 + 2915 (where T is the maximum reheating temperature (K ), t is time (sec), [C] is carbon weight%, [Cr] is chromium weight%, and r is reduction ratio.)

division Jaemyung Kang Perlite size (μm) The number of cementite for unrefined use (pcs/㎟) Cold working reduction (%) cold rolled thickness
(mm) Durability (times) Invention example 1 Invention Steel 1 13.9 0 85 0.23 198,774 Invention example 2 Invention Steel 1 9.2 33 83 0.20 214,362 Inventive example 3 Invention Steel 2 13.1 0 85 0.23 199,233 Inventive example 4 Invention Steel 2 10.6 0 83 0.20 221,558 Comparative Example 1 Invention Steel 1 18.4 299 85 0.25 132,270 Comparative Example 2 Invention Steel 1 13.2 12,221 85 0.20 98,563 Comparative Example 3 Invention Steel 1 22.0 0 85 0.20 143,275 Comparative Example 4 Invention Steel 2 9.5 16,916 85 0.21 76,354 Comparative Example 5 Invention Steel 2 13.7 16,617 85 0.23 81,674 Comparative Example 6 Comparative Lecture 1 12.9 133 85 0.23 92,579 Comparative Example 7 Comparative Lecture 1 11.5 67 83 0.20 90,487

Inventive examples 1 to 4 in Table 3 satisfy the conditions of the present invention, and by securing fine crystal grains and minimizing the number of cementanite for unresolved use in the microstructure, the springs manufactured after cold working have durability of 190,000 or more cycles. was able to secure

1 and 2 are EBSD photographs of the microstructures of Inventive Example 1 and Comparative Example 1, respectively, observed with a scanning electron microscope. Accordingly, in Inventive Example 1 according to the present invention, fine pearlite average particles were confirmed, but in Comparative Example 1, a coarse pearlite structure was confirmed.

Specifically, Comparative Example 1 was not rolled at a sufficient reduction ratio during cold rolling before reheating treatment, so both the pearlite crystal grain size and the number of undissolved cementite exceeded the scope of the present invention after reheating treatment. Comparative Example 2 did not satisfy the condition of [Relationship 1] for the reheat treatment and maintenance process, and it was confirmed that excess cementite was produced. In Comparative Examples 3 and 4, the reheating temperature was out of the range of the present invention, and it was confirmed that the microstructure requirements required in the present invention were not satisfied. For the above reasons, it was confirmed that the springs manufactured using Comparative Examples 1 to 4 had durability less than 150,000 cycles.

Comparative Example 5 satisfies the composition of the present invention, but spheroidization annealing heat treatment was performed before cold rolling, so that excess cementite was generated and the durability of the spring was not secured. Comparative Examples 6 and 7 met the manufacturing conditions of the present invention, but were out of the component range, and it was confirmed that it was difficult to secure the durability of the final spring.

Claims (10)

In % by weight, C: 0.75 to 0.85%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, Al: 0.05% or less (excluding 0%), P: 0.03% or less, S : 0.005% or less, including the remaining Fe and other unavoidable impurities,
The microstructure is a high-carbon steel sheet in which pearlite is used as a base structure, the pearlite average grain size is 5 to 14 μm or less, and the number of unresolved cementite per 1 mm 2 is 100 or less.
The method of claim 1,
The pearlite is a high carbon steel sheet having an area fraction of 96% or more.
The method of claim 1,
The microstructure includes a ferrite structure of 3 area% or less in addition to the base structure, and at least one of beanite, martensite, and retained austenite is included in 1 area% or less of the high-carbon steel sheet.
The method of claim 1,
The unresolved cementite is unresolved cementite present at the point t/4 in the thickness direction of the steel sheet, and has a size of 1 μm ² or more based on area and a long-short axis ratio of 2.0 or less.
In % by weight, C: 0.75 to 0.85%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, Al: 0.05% or less (excluding 0%), P: 0.03% or less, S : preparing a hot-rolled steel sheet by heating, hot-rolling and winding a steel slab containing 0.005% or less of Fe and other unavoidable impurities;
manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 50%;
Reheating the cold-rolled steel sheet at 820 to 950° C., and maintaining the condition that satisfies Equation (1) below; and
After the maintenance, cooling to 530 ~ 600 ℃, constant temperature transformation treatment;
Method for producing a high carbon steel sheet comprising a.
[Relationship 1]
T*(log t + 0.07) ≥ 124*[C] + 510*[C] ² + 928*[Cr] - 815*r ^0.5 + 2915
(Where T is the maximum reheating temperature (K), t is the time (sec), [C] is the carbon weight%, [Cr] is the chromium weight%, and r is the cold reduction.)
The method of claim 5,
The hot rolling is a method for producing a high carbon steel sheet that is hot-rolled in a temperature range of 850 to 1100 ° C.
The method of claim 5,
The winding is a method for producing a high carbon steel sheet performed at 600 ~ 680 ℃.
The method of claim 5,
A method for producing a high carbon steel sheet in which annealing is omitted before the cold rolling.
The method of claim 5,
The cooling is a method for producing a high carbon steel sheet performed at a cooling rate of 50 ° C / s or more.
The method of claim 5,
The holding time of the constant temperature transformation treatment is a method of manufacturing a high carbon steel sheet of 30 seconds or more.

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