KR101505270B1 - Cold-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

(TS): 637 ~ 670 Mpa, yield strength (YP), and tensile strength (YP) were obtained by further adding silicon (Si) and manganese : 350 to 387 MPa, elongation (El): 29.3 to 32.9%, and yield ratio (YR): 60% or more, and a method for producing the same.
The method for manufacturing a steel sheet according to the present invention comprises 0.055 to 0.08 wt% of carbon (C), 0.6 to 0.8 wt% of silicon (Si), 1.9 to 2.1 wt% of manganese (Mn) (Al): more than 0 wt% to 0.03 wt%; molybdenum (Mo): 0.04 wt% to 0.07 wt%; chromium (Cr) ): 0.25 to 0.45% by weight and the balance iron (Fe) and other unavoidable impurities to a slab reheating temperature (SRT) of 1180 to 1220 캜; Hot-rolling the reheated slab to a finishing delivery temperature (FDT) of 840 to 900 占 폚; Cooling the hot-rolled plate material, and winding the hot-rolled plate material; Subjecting the rolled sheet to a pickling treatment and cold rolling at a reduction ratio of 40 to 80%; And annealing the cold-rolled sheet material to a temperature of 790 to 830 ° C.

Description

Technical Field [0001] The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same,

More particularly, the present invention relates to a high-strength cold-rolled composite steel sheet for use in automobiles, which can improve the ferrite-martensite fraction of ferrite by using an existing production line having a significantly longer inter- And a manufacturing method thereof.

2. Description of the Related Art In recent years, in the field of automobiles, a high-strength steel plate has been applied in order to ensure both a function of protecting a passenger at the time of a collision and a lighter weight for the purpose of improving fuel economy. Particularly, in terms of ensuring the safety of collision, in addition to the increase in safety consciousness, there is a demand to apply a high strength steel sheet to components having complicated shapes which have been used only in low strength steel sheet have.

In particular, 590Mpa grade cold-rolled high-strength steel sheet has been increasing for the purpose of improving fuel efficiency for structural members for automotive interior panels. Therefore, when a high-strength steel sheet is manufactured using an existing production line in order to secure an increased production amount, a manufacturing condition with a significantly longer overhang length is formed.

Accordingly, a high-strength steel sheet and a method for manufacturing the same are needed for securing a tensile strength of 590 MPa or more as a cold-rolled sheet meeting the manufacturing conditions with a longer overhang length.

A related prior art is Korean Patent Laid-Open Publication No. 10-2012-0092704 (published on Aug. 21, 2012), which discloses a high-strength cold-rolled steel sheet and a manufacturing method thereof.

It is an object of the present invention to provide a method of controlling the content of ferrite and martensite by controlling the content of silicon (Si) and manganese (Mn) in a conventional production line, To provide a high-strength cold-rolled steel sheet for a plate material structural member in an automobile of 590Mpa class, and a manufacturing method thereof.

Another object of the present invention is to provide a process for producing an alloy having a tensile strength (TS) of 637 to 670 MPa, a yield strength (YP) of 350 to 387 MPa and an elongation (El) of 29.3 to 32.9% A steel sheet and a manufacturing method thereof.

A steel sheet manufacturing method for achieving the above object is characterized by comprising: 0.055 to 0.08 weight% of carbon (C), 0.6 to 0.8 weight% of silicon (Si), 1.9 to 2.1 weight% of manganese (Mn) (Al): more than 0 wt% to 0.03 wt%; molybdenum (Mo): 0.04 wt% to 0.07 wt%; chromium (Cr) (Slab reheating temperature) of 1180 to 1220 占 폚, wherein the steel slab is composed of 0.25 to 0.45% by weight of Cr and the balance of Fe and other unavoidable impurities; Hot-rolling the reheated slab to a finishing delivery temperature (FDT) of 840 to 900 占 폚; Cooling the hot-rolled plate material, and winding the hot-rolled plate material; Subjecting the rolled sheet to a pickling treatment and cold rolling at a reduction ratio of 40 to 80%; And annealing the cold-rolled sheet material to a temperature of 790 to 830 ° C.

According to another aspect of the present invention, there is provided a steel sheet comprising 0.055 to 0.08 weight% of carbon (C), 0.6 to 0.8 weight% of silicon (Si), 1.9 to 2.1 weight% of manganese (Mn) (Al): more than 0 wt% to 0.03 wt%; molybdenum (Mo): 0.04 wt% or less; (TS): 637 to 670 Mpa, yield strength (YP): 350 to 387 Mpa, and the balance of iron (Fe) and other inevitable impurities. And elongation (El): 29.3 to 32.9%.

The steel sheet according to the present invention and the method for manufacturing the steel sheet according to the present invention can be obtained by securing the ferrite and martensite fractions by adding silicon (Si) and manganese (Mn) using an existing production line, TS: 637 to 670 Mpa, yield strength (YP): 350 to 387 Mpa and elongation (El): 29.3 to 32.9%.

1 is a flowchart illustrating a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.
FIG. 2 shows a cold-rolled steel sheet structure in which a conventional ingot carbide is formed.
3 illustrates a cold rolled steel sheet structure in which grain boundary martensite is formed according to an embodiment of the present invention.
FIG. 4 is a graph showing changes in austenite volume ratio with temperature of specimens prepared by differently producing silicon (Si) and manganese (Mn) contents.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, a cold-rolled steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Cold rolled steel plate

The cold-rolled steel sheet according to the present invention is intended to have a tensile strength (TS) of 637 to 670 MPa, a yield strength (YP) of 350 to 387 MPa and an elongation (El) of 29.3 to 32.9%.

The cold rolled steel sheet according to the present invention comprises 0.055 to 0.08 wt% of carbon (C), 0.6 to 0.8 wt% of silicon (Si), 1.9 to 2.1 wt% of manganese (Mn) (Al): more than 0 wt% to 0.03 wt%; molybdenum (Mo): 0.04 wt% to 0.07 wt%; chromium (Cr) (Cr): 0.25 to 0.45% by weight, and the balance of iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component contained in the cold-rolled steel sheet according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added for securing strength and controlling microstructure.

The carbon (C) is preferably added in a content ratio of 0.055 to 0.08% by weight based on the total weight of the cold-rolled steel sheet according to the present invention. When the content of carbon (C) is less than 0.055 wt%, it may be difficult to secure strength. On the contrary, when the content of carbon (C) exceeds 0.08% by weight, the strength of the steel increases, but the impact resistance and weldability at low temperatures are deteriorated.

Silicon (Si)

In the present invention, silicon (Si) can improve the elongation and the yield strength by pushing the carbide formed in the ferrite grain into the austenite region according to the decrease of the primary cooling end temperature according to the present invention and the increase in the overhanging section length .

However, when the content of silicon (Si) is less than 0.6% by weight, the effect due to hardening of the solid solution can not be expected. On the contrary, when the content of silicon (Si) exceeds 0.8% by weight, the retained austenite is excessively increased and the hole expandability and stretch flangeability after punching and cutting are deteriorated.

Therefore, silicon (Si) is preferably added at a content ratio of 0.6 to 0.8% by weight based on the total weight of the cold-rolled steel sheet according to the present invention.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element and improves the amount of carbon concentration in austenite, thereby improving tensile strength and inhibiting the formation of transformed ferrite.

Manganese (Mn) is preferably added at a content ratio of 1.9 to 2.1 wt% of the total weight of the cold-rolled steel sheet according to the present invention. When the content of manganese (Mn) is less than 1.9% by weight, the pearlite transformation can not be suppressed and sufficient tensile strength can not be secured. On the contrary, when the content of manganese (Mn) exceeds 2.1% by weight, the amount of MnS-based nonmetallic inclusions increases, which may cause defects such as cracking during welding.

Phosphorus (P), sulfur (S)

Phosphorus (P) contributes partly to strength improvement, but it is an element that lowers corrosion resistance by slab center segregation. The lower the content, the better. Therefore, in the present invention, the content of phosphorus (P) is limited to more than 0 wt% and not more than 0.02 wt% of the total weight of the steel material.

Sulfur (S) is an element that is inevitably contained in the manufacture of steel together with phosphorus (P), and when added in large amounts, adversely affects weldability and manufacturability during casting and hot rolling. Therefore, in the present invention, the content of sulfur (S) is limited to not less than 0 wt% and not more than 0.05 wt% of the total weight of the steel sheet.

Aluminum (Al)

Aluminum (Al) has the effect of promoting ferrite formation to improve ductility, and can also be used as a deoxidizing agent.

However, when the content of aluminum (Al) exceeds 0.03% by weight, the number of coarse inclusions is increased, which may cause deterioration of hole expandability and surface damage.

Therefore, aluminum (Al) is preferably added in a content ratio of more than 0 wt% to 0.03 wt% or less of the total weight of the cold-rolled steel sheet according to the present invention.

Molybdenum (Mo)

Molybdenum (Mo) contributes to improving the strength and toughness of steel and securing stable strength at room temperature and high temperature.

Molybdenum (Mo) is preferably added in a content ratio of 0.04 to 0.07% by weight based on the total weight of the cold-rolled steel sheet according to the present invention. When the content of molybdenum (Mo) is less than 0.04% by weight, the effect of adding molybdenum is insufficient. On the contrary, when the content of molybdenum (Mo) exceeds 0.07% by weight, there is a problem that the weldability is lowered.

Chromium (Cr)

Chromium (Cr) is an element that suppresses center segregation by increasing diffusion of manganese (Mn) in the manufacture of slabs, and forms a low-temperature transformation phase upon accelerated cooling after hot rolling to provide excellent strength and corrosion resistance.

Cr (Cr) is preferably added at a content ratio of 0.25 to 0.45% by weight based on the total weight of the cold-rolled steel sheet according to the present invention. If the content of chromium (Cr) is less than 0.25% by weight, the effect of the addition can not be exhibited properly. On the contrary, when the content of chromium (Cr) exceeds 0.45% by weight, the weldability and the heat affected zone (HAZ) toughness are lowered.

Cold rolled steel sheet manufacturing method

1 is a flowchart illustrating a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, the method for manufacturing a cold-rolled steel sheet according to the present invention includes a slab reheating step S110, a hot rolling step S120, a cooling and winding step S130, a pickling step S140, a cold rolling step S150, Step S160 and cooling step S170. At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to perform the reheating step (S110) in order to obtain effects such as the reuse of the precipitate.

In the steel sheet manufacturing method according to the present invention, the semi-finished steel slab to be subjected to the hot rolling process is composed of 0.055 to 0.08% by weight of carbon (C), 0.6 to 0.8% by weight of silicon (Si) (S): more than 0 wt% to 0.05 wt%, aluminum (Al): 0 wt% to less than 0.03 wt%, phosphorus (P) Mo: 0.04 to 0.07% by weight, Cr: 0.25 to 0.45% by weight, and the balance of Fe and other unavoidable impurities.

Reheating slabs

In the slab reheating step S110, the slab having the above composition is reheated to a slab reheating temperature (SRT) of 1180 to 1220 ° C.

When the slab reheating temperature is lower than 1180 ° C, the segregated components are not sufficiently reused during casting. On the other hand, when the reheating temperature exceeds 1220 DEG C, the austenite grain size increases and it may be difficult to secure strength.

Hot rolling

In the hot rolling step (S120), the reheated slab plate is hot-rolled.

In the hot rolling step (S120), the finishing delivery temperature (FDT) is preferably 840 to 900 占 폚. If the finish rolling temperature (FDT) is lower than 840 DEG C, an abnormal reverse rolling may occur and the ductility may be lowered. On the other hand, when the finishing rolling temperature (FDT) exceeds 900 캜, the strength of the plate to be produced sharply decreases.

Cooling and winding

In the cooling and winding step (S130), the hot rolled plate is cooled at a cooling rate of 10 to 30 DEG C / sec and then wound in a temperature range of 500 to 600 DEG C (Coiling Temperature).

When the cooling rate is less than 10 DEG C / sec, the austenite is transformed into pearlite, so that it is difficult to secure the volume ratio of martensite necessary for securing strength. On the other hand, when the cooling rate exceeds 30 DEG C / sec, bainite and martensite structure are generated, and toughness and hydrogen organic cracking resistance are impaired.

If the coiling temperature (CT) is lower than 500 ° C, there is a problem that strength is lowered due to formation of coarse microstructure. On the other hand, when the coiling temperature (CT) exceeds 600 ° C, the bainite fraction increases and the strength increases, but toughness and corrosion resistance are difficult to secure.

Accordingly, the cold rolled steel sheet formed by the manufacturing method according to the present invention may have a yield strength (YP) of 342 to 392 MPa, a tensile strength (TS) of 590 MPa or more, and an elongation (El) of 27.9 to 32.3%.

Pickle

In the pickling step S140, acid pickling is performed to remove the scale of the rolled plate, that is, the hot-rolled coil manufactured through the hot rolling process. Although not shown in the drawing, after the pickling step S140, a step of applying oil may be further performed to prevent oxidation of the surface of the steel sheet.

Cold rolling

In the cold rolling step (S150), the pickled plate is cold-rolled.

In the cold rolling step (S150), the cold rolling reduction rate is preferably 40 to 80%. If the cold rolling reduction rate is less than 40%, there is a problem that the amount of annealed recrystallized nuclei is small, so that the crystal grains are excessively grown during the annealing heat treatment to be described later and the strength is rapidly lowered. On the other hand, when the cold rolling reduction is more than 80%, the amount of nucleation becomes too large, so that the annealing grains are rather too fine to reduce the ductility and deteriorate the formability.

Annealing heat treatment

In the annealing heat treatment step (S160), the cold-rolled sheet material is annealed to 790 to 830 ° C.

At this time, the annealing heat treatment is one of important process parameters for determining the material of the final product. This is to induce the finally formed structure to be very fine and the average particle size of NbC precipitate and AlN precipitate to be 0.05 탆 or less.

If the annealing heat treatment temperature is less than 790 占 폚 in this step, recrystallization is not sufficiently completed and it may be difficult to secure the desired ductility. On the other hand, when the annealing heat treatment temperature exceeds 830 캜, there is a problem that the strength is lowered due to coarsening of the recrystallized grains.

Cooling

In the cooling step (S170), the recrystallized sheet material is cooled to 400 to 500 ° C by annealing heat treatment. In this case, it is preferable to perform the cooling from 400 to 500 ° C. at a low temperature and the indirect water cooling or gas jet cooling (GJC) from 400 ° C. to a room temperature.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Specimen Manufacturing

The specimens according to Examples 1 to 3 and Comparative Examples 1 to 3 were produced under the composition shown in Table 1 and the process conditions shown in Table 2.

 [Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 to 3.

[Table 3]

Referring to Tables 1 to 3, the specimens prepared according to Examples 1 to 3 have tensile strengths (TS) of 637 to 670 MPa, yield strengths (YP) of 350 to 387 MPa and elongation (El): 29.3 to 32.9%.

On the other hand, in the case of the specimen prepared according to Comparative Example 1 in which silicon (Si) and manganese (Mn) were added in a content range lower than that of the present invention as compared with Example 1, the ferrite- It can be seen that the tensile strength TS, the yield strength YP and the elongation El fall below the target value.

In addition, in the case of the specimen prepared according to Comparative Example 2 in which manganese (Mn) was little added as compared with Example 1, the yield strength (YP) Value, but it can be seen that the tensile strength (TS) and elongation (EL) are below the target value.

In addition, in the case of the specimen produced according to Comparative Example 3 in which manganese (Mn) was little added and the annealing was performed at a temperature lower than the temperature range suggested in the present invention as compared with Example 1, the carbide formed during hot rolling was sufficiently dissolved The yield strength (YP) satisfies the target value, but the tensile strength (TS) and the elongation (El) are less than the target value.

FIG. 2 shows a structure of a cold-rolled steel sheet on which conventional grain carbonates have been formed, and FIG. 3 shows a structure of a cold-rolled steel sheet on which grain boundary martensite is formed according to an embodiment of the present invention.

2 and 3, a conventional cold-rolled steel sheet is manufactured using an existing production line without adjusting the alloy components. As a result, it can be seen that the ferrite grains are formed by decreasing the primary cooling end temperature and increasing the length of the overflow zone. Therefore, when a steel sheet is produced using a conventional production line, there is a problem that it is difficult to secure sufficient yield strength and tensile strength.

On the other hand, the present invention regulates the content of silicon (Si) and manganese (Mn), and a test piece is manufactured using a conventional production line. As the content of silicon (Si) is increased as compared with the conventional one, the formation of carbide in ferrite grains is suppressed and the elongation improvement and the yield strength can be ensured. Further, as the content of manganese (Mn) is increased as compared with the prior art, the amount of carbon concentration in austenite can be improved and the formation of metamorphic ferrite can be suppressed and the tensile strength can be improved.

FIG. 4 is a graph showing changes in austenite volume ratio with temperature of specimens prepared by differently producing silicon (Si) and manganese (Mn) contents.

Referring to FIG. 4, the changes in austenite volume ratio according to the temperature of the specimen prepared according to Comparative Example 1, Comparative Example 2 and Example 1 of the present invention can be seen.

In the case of Comparative Example 1 in which the content of silicon (Si) and manganese (Mn) is less than the range suggested by the present invention, while the austenite volume ratio is 50% at 800 ° C, The austenite volume ratio is 62%, and it is difficult to secure the elongation and yield strength because the formation of the ferrite ingot carbide is not inhibited.

In the case of Comparative Example 2 in which the content of manganese (Mn) is less than the range suggested by the present invention, the amount of carbon concentration in austenite can not be sufficiently improved due to the austenite volume ratio of 43% at 800 ° C, There is a possibility that a problem of degradation may occur.

As described above, the cold-rolled steel sheet and the manufacturing method thereof according to the present invention use an existing production line having a significantly longer overhanging section length, and further add silicon (Si) and manganese (Mn) (TS): 637 to 670 Mpa, a yield strength (YP): 350 to 387 Mpa and an elongation (El): 29.3 to 32.9% by securing a martensite fraction. Accordingly, it is possible to provide a cold-rolled steel sheet that secures economical efficiency by using an existing production line and satisfies conventional mechanical properties.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Reheating step
S120: Hot rolling step
S130: Cooling and winding step
S140: pickling step
S150: Cold rolling step
S160: annealing heat treatment step
S170: cooling step

Claims (4)

(a) from 0.055 to 0.08% by weight of carbon (C), from 0.6 to 0.8% by weight of silicon (Si), from 1.9 to 2.1% by weight of manganese (Mn) (Al): more than 0 wt% to 0.03 wt%, molybdenum (Mo): 0.04 to 0.07 wt%, chromium (Cr): 0.25 to 0.45 Reheating a steel slab composed of Fe and other unavoidable impurities to a slab reheating temperature (SRT) of 1180 to 1220 占 폚;
(b) hot-rolling the reheated slab to a finishing delivery temperature (FDT) of 840 to 900 占 폚;
(c) cooling the hot-rolled plate and winding it;
(d) subjecting the rolled sheet to a pickling treatment, followed by cold rolling at a reduction ratio of 40 to 80%; And
(e) subjecting the cold-rolled sheet to annealing at 790 to 830 ° C.
The method according to claim 1,
In the step (c)
The cooling
At a cooling rate of 10 to 30 占 폚 / sec.
The method according to claim 1,
In the step (c)
The cooling
And a CT (Coiling Temperature): 500 to 600 占 폚.
(P): not less than 0 wt% to not more than 0.02 wt%, sulfur (0.03 wt% or less), carbon (C): 0.055 to 0.08 wt% (Al): more than 0 wt% to 0.03 wt%; molybdenum (Mo): 0.04 to 0.07 wt%; chromium (Cr): 0.25 to 0.45 wt%; The remaining iron (Fe) and other unavoidable impurities,
, A tensile strength (TS) of 637 to 670 MPa, a yield strength (YP) of 350 to 387 MPa and an elongation (El) of 29.3 to 32.9%.

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