KR101228746B1 - Cold rolled steel sheet having excellent workability for deep drawing and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet for deep processing and a method of manufacturing the same, having excellent plating properties, having an elongation of 50% or more and an r value of 1.4 or more.

In the present invention, by weight%, C: 0.0005 to 0.0035%, Si: 0.01 to 0.05%, Mn: 0.01 to 0.2%, P: 0.001 to 0.02%, S: 0.001 to 0.020%, Sol.Al: 0.02 to 0.05% , N: 0.004% or less, Ti: 0.005% to 0.007%, and Nb: 0.001% to 0.02%, including one or two, the remainder is composed of Fe and inevitable impurities,

In the microstructure, ferrite having an average particle diameter of 5 to 15 μm occupies 90% or more by area ratio, and single or complex precipitates of Nb (C, N) and Ti (C, N) are contained in the ferrite particles. Precipitation Free Zone (PFZ: Precipitation Free Zone) with 3 ~ 7 × 10 ⁶ holes / ㎛ ² , width 0.1 ~ 2.5㎛ depending on ferrite center and ferrite grain boundary, average area density of precipitate It relates to a cold-rolled steel sheet for deep processing excellent in workability formed of a ratio of 70 to 130% and a method of manufacturing the same.

For deep processing, workability, elongation, r value, precipitate

Description

COLD ROLLED STEEL SHEET HAVING EXCELLENT WORKABILITY FOR DEEP DRAWING AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a cold-rolled steel sheet for deep processing mainly used for automobiles, exterior plates, and a method for manufacturing the same, more specifically, having an elongation of 50% or more and an r value of 1.4 or more, which is superior to conventional cold-rolled steel sheets for deep processing. The present invention relates to a cold rolled steel sheet for deep working and a method of manufacturing the same.

In recent years, automotive steel sheet has been required to have a higher level of formability due to the complexity and integration of automobile molded products, as well as a steel plate with a beautiful plating surface in terms of the automotive environment.

In order to improve the formability of the steel sheet, so-called IF (Interstitial Free) steel which adds Ti, Nb and the like can be used based on a high purity steel having almost no impurity elements such as C, N, and S in the steel, In addition, in order to secure the target strength, it is common to add substituted solid solution strengthening elements such as Si, Mn, and P.

However, in this case, elements such as Ti, Mn, and Si are surface-concentrated during annealing, thereby deteriorating the hot-dip galvanizing characteristics. That is, there is carried out a recrystallization annealing heat treatment at more than 760 ℃ temperature in order to soften the processed hardened structure after cold rolling, the added elements are MnO, SiO _2, Al ₂ O of so annealing process oxygen affinity element than the most Fe _It grows to the surface thickener which has single or complex form, such as ₃ and TiO, and worsens the plating characteristic of a steel plate.

In addition, since the high purity IF steel has almost no impurity elements, the risk of grain brittleness is greatly increased. In order to prevent this, B, commonly known as grain boundary strengthening element, is added. However, in this case, it is well known that the surface thickening of B also occurs.

As the amount of these surface concentrates increases, the wettability of the plating bath is decreased during the hot dip plating, and the alloying reaction is inhibited, so that surface defects such as unplated are likely to be caused. When the surface thickener is coarsened, it is adsorbed by the Hearth Roll of the continuous annealing furnace, causing micro dents on the surface of the plated steel sheet, which adversely affects the surface quality.

On the other hand, in order to manufacture deep steel sheet, in order to secure good formability, the amount of invasive solid elements such as C and N is usually lowered to 750 ppm or less in the steelmaking process, and the carbon and nitride forming elements Ti, Nb, etc. are separately It is common to manufacture using so-called ultra-low carbon IF (Interstitial Free) steel, alone or in combination.

As a technique for manufacturing a thin steel sheet for deep processing using such IF steel, a technique using Ti-added steel disclosed in Japanese Patent No. 56385, a technique using Nb-added steel manufactured by Armco of the United States, and disclosed in Japanese Patent No. 1278670 And a technique using Ti-Nb composite additive steel of Japan Kawasaki Steel Mill (KSC). In addition to the above-described prior art, it is well known that many related patents have been filed around the world in terms of the components, composition, and manufacturing conditions of these prior arts, which are slightly different in their limitations. In order to secure the processability, it is generally manufactured by adding about 0.01 to 0.07% of carbonitride-forming elements such as Ti or Nb.

However, in this case, the problem of secondary processing brittleness is difficult to avoid because there is no invasive employment element that serves to strengthen the grain boundary in the river. This problem is more problematic in deep processing high-strength steel to which solid-solution strengthening elements such as P and Mn are added, so adding grain boundary element such as B or limiting carbon content in steel to 60 ppm or more, In addition to the inevitable, there is a problem that the plating characteristics are deteriorated when making a GA plated product.

Another technique is to control NbC precipitates in the Precipitation Free Zone (PFZ) near the ferrite grain boundary to 60% or less than the central part through cooling control to secure strength, internal dwarfness and elongation. 2005-187939. However, the above technique is advantageous in securing strength, but its effect is halved in securing a uniform elongation.

Therefore, there is a demand for a technique for deep-rolled cold rolled steel sheet capable of satisfying excellent moldability and plating characteristics at the same time.

The present invention is to provide a cold-rolled steel sheet for deep processing, and a method of manufacturing the same, having excellent plating properties, having an elongation of 50% or more and an r-value of 1.4 or more, and showing superior moldability than conventional cold-rolled steel sheets for deep processing.

In the present invention, by weight%, C: 0.0005 to 0.0035%, Si: 0.01 to 0.05%, Mn: 0.01 to 0.2%, P: 0.001 to 0.02%, S: 0.001 to 0.020%, Sol.Al: 0.02 to 0.05% , N: 0.004% or less, Ti: 0.005% to 0.007%, and Nb: 0.001% to 0.02%, including one or two, the remainder is composed of Fe and inevitable impurities,

In the microstructure, ferrite having an average particle diameter of 5 to 15 μm occupies 90% or more by area ratio, and single or complex precipitates of Nb (C, N) and Ti (C, N) are contained in the ferrite particles. Precipitation Free Zone (PFZ: Precipitation Free Zone) with 3 ~ 7 × 10 ⁶ holes / ㎛ ² , width 0.1 ~ 2.5㎛ depending on ferrite center and ferrite grain boundary, average area density of precipitate It provides a cold rolled steel sheet for deep workability, which is excellent in machinability with a ratio of 70 to 130%.

In addition, the present invention comprises heating the steel slab that satisfies the composition at 1050 ~ 1300 ℃, and finish rolling in an austenite single phase so that the strip cooling rate of the final three pass during the finish rolling is 30 ℃ / sec or more;

Winding the hot rolled steel sheet and cold rolling the wound hot rolled steel sheet at a cold reduction rate of 60% or more; And

Continuously annealing the cold rolled steel sheet at a temperature range of 780 ° C. to 860 ° C.

It provides a method for producing a cold rolled steel sheet for deep processing, including excellent workability.

According to the present invention, by providing a cold-rolled steel sheet for deep processing having an elongation of 50% or more and an r value of 1.4 or more and having excellent workability and excellent ridging property, parts such as an automobile side outer It can be usefully used.

Hereinafter, the present invention will be described in detail.

First, the composition range of the present invention will be described in detail (hereinafter,% by weight).

Cold rolled steel sheet of the present invention in weight%, C: 0.0005 ~ 0.0035%, Si: 0.01 ~ 0.05%, Mn: 0.01 ~ 0.2%, P: 0.001 ~ 0.02%, S: 0.001 ~ 0.020%, Sol.Al: 0.02 -0.05%, N: 0.004% or less, Ti: 0.005-0.007% and Nb: 0.001-0.02% of one or two, further B: 0.0002-0.003%, Sb: 0.005-0.1 and Sn: It contains one or two or more of 0.005 to 0.1% and is composed of the remaining Fe and other unavoidable impurities. The reason for limitation of the composition is explained below.

C: 0.0005 ~ 0.0035%

C is an important element for controlling NbC and TiC because it combines with Nb and Ti. C acts as an invasive solid solution and inhibits the formation of {111} texture, which is advantageous for processability during the formation of the texture of the steel sheet during cold rolling and annealing, so the lower the content, the better, but 0.0005 to 0.0035% in consideration of the cost of extreme control. It is limited to.

Si: 0.01 ~ 0.05%

Si in steel is an element that can be usefully used for improving strength, but it is not only causing surface scale defects in relation to surface properties, but also known as an element that causes temper color (oxidation discoloration) during annealing and unplating during plating. have. However, due to recent advances in plating technology, the steel content can be manufactured without plating, even up to about 0.05%, so the content is limited to 0.01 to 0.05%.

Mn: 0.01 ~ 0.2%

Mn is added as a substituted solid solution strengthening element to secure the strength, but when the content exceeds 0.2%, the content is limited to 0.01 to 0.2% by inhibiting the plating as well as the ductility.

P: 0.001-0.02%

The excessive addition of P causes secondary workability brittleness, ductility deterioration, increase in YS value, and inhibits plating adhesion, but the lower the better, the content is limited to 0.001 to 0.02% for economic control.

S: 0.001-0.020%

S is present in the river as an emulsion. Therefore, if the amount is excessive, it causes ductility deterioration, so the content is limited to 0.001 to 0.020%.

N: 0.004% or less

The content of N is preferably as low as possible, but limited to 0.004% or less in consideration of steelmaking refining technology.

Sol.Al: 0.02 ~ 0.05%

Sol.Al is an element that improves the strength. If it is less than 0.02%, the strength improvement effect is insignificant. If it is more than 0.05%, it forms a precipitate free zone that inhibits the uniformity of the precipitate. do.

One or two of Ti: 0.005% to 0.007% and Nb: 0.001% to 0.02% are added.

Ti and Nb are very important elements in terms of processability, the minimum to ensure the synergistic effect of workability (especially r value). In consideration of the optimum amount, the content of Ti is preferably limited to 0.005 to 0.007% and the content of Nb is limited to 0.001 to 0.02%.

Additionally, one or two or more selected from the group consisting of B: 0.0002 to 0.003%, Sb: 0.005 to 0.1 and Sn: 0.005 to 0.1% are added.

The content of B is preferably 0.0002-0.0030%. B in steel is a grain boundary strengthening element that improves the fatigue characteristics of the molten metal at the time of welding and prevents grain boundary brittleness, and in order to secure the above effects, the content of B is preferably added at least 0.0002%. On the other hand, if the content exceeds 0.0030%, it is preferable to limit the upper limit to 0.0030% because the workability is sharply lowered and the surface properties of the plated steel sheet are degraded.

The content of Sb is preferably 0.005 to 0.10%. When Sb is present in steel, it is mainly present at grain boundaries, and thus, Mn, Si, Al, and B elements in the steel are diffused through grain boundaries during recrystallization annealing, thereby suppressing a phenomenon in which oxides are concentrated on the surface. If the surface thickener is large in size, it causes a dent defect in the furnace. Therefore, it is necessary to keep the average size at 1.0 mu m or less as much as possible. Therefore, it is necessary to add at least 0.005% or more in order to obtain the above-described surface thickening inhibitory effect by adding Sb in steel, but if it is added above a certain limit, a predetermined effect cannot be obtained, and the upper limit is 0.10% because workability is greatly deteriorated. It is preferable to limit to.

The content of Sn is preferably 0.005 to 0.10%. Sn in steel, like Sb, has an effect similar to Sb as an element having a tendency of grain boundary segregation. Therefore, at least 0.005% or more is required to obtain the above effect, but if it is added over a certain limit, not only a predetermined effect is obtained but also the workability deteriorates, so the upper limit of Sn is preferably limited to 0.10%.

The remainder is composed of Fe and other unavoidable impurities.

Hereinafter, the microstructure and precipitates of the present invention will be described in detail.

The cold rolled steel sheet of the present invention has a structure of ferrite particles having an average particle diameter of 5 to 15 µm, and has a ferrite columnar shape and occupies 90% or more by area ratio. Most of the microstructure of the present invention is a ferrite structure, when the average particle diameter is less than 5㎛ there is an improvement in strength due to the microstructure, but r value disadvantageous in terms of processability, and when the diameter is 15㎛ or more, the ridging property by coarse ferrite Deteriorates.

In addition, single or complex precipitates of Nb (C, N) and Ti (C, N) are present in the ferrite particles, 3-7x10 ⁶ particles / μm ² , and the ferrite center and the ferrite grain boundaries are present. According to the present invention, a precipitate prezone (PFZ: Precipitation Free Zone) having a width of 0.1 to 2.5 μm is present and the average area density ratio of the precipitate is 70 to 130%.

The average area density ratio of the precipitates means a percentage of the average density of ferrite center precipitates relative to the average density of precipitates in the precipitate free zone (PFZ).

The inventors have recognized that if the characteristics of the structure and the dispersion density of the precipitates are constant, a cold rolled steel sheet having excellent elongation resistance and ridging resistance can be obtained while having elongation of 50% or more and r value of 1.4 or more. It became.

Hereinafter, the manufacturing method of the cold rolled steel sheet of the present invention will be described in detail.

The steel slab that satisfies the above composition is heated at 1050 to 1300 ° C., and finish rolling is completed in the austenitic single phase so that the strip cooling rate of the final three passes is 30 ° C./sec or more during the finish rolling.

The finish-rolled steel sheet is wound and subjected to cold rolling at a cold rolling rate of 60% or more.

The cold rolled steel sheet is continuously annealed at a temperature range of 780 ° C to 860 ° C.

The cold rolled steel sheet manufactured as described above may then be treated by a conventional plating process as necessary. Examples of the plating include zinc plating and alloyed hot dip galvanizing.

Hereinafter, embodiments of the present invention will be described in detail.

(Example)

The slab is heated to a composition as shown in Table 1 at 1050 ~ 1300 ℃, the finish rolling in the austenitic single-phase zone so that the strip cooling rate of the final three pass during the finish rolling 30 ℃ / sec, then wound up, then wound hot rolled sheet Was cold rolled at a cold reduction rate of 60%, and the cold rolled steel sheet was Continuous annealing at a temperature range of 780 ~ 860 ℃ to produce a cold rolled steel sheet for deep processing.

The mechanical properties, the number and area of precipitates, density, etc. of the inventive steel satisfying the composition of the present invention and the comparative steel outside the scope of the present invention were measured and the results are shown in Table 2.

In Table 2, the average number and area density of precipitates, such as Nb (C, N) or Ti (C, N), were measured using a transmission electron microscope with an acceleration voltage of 300 kv. It is the result of measuring the average value measured in several places.

division C Si Mn Sol.Al P S Nb Ti N Etc Inventive Steel 1 0.0005 0.016 0.020 0.025 0.0090 0.003 0.011 0.005 0.0026 B: 0.002, Sn: 0.01 Invention river 2 0.0020 0.029 0.130 0.040 0.0030 0.001 0.009 0.006 0.0023 Invention steel 3 0.0018 0.025 0.150 0.030 0.0110 0.004 0.017 0.007 0.0022 Inventive Steel 4 0.0009 0.023 0.070 0.032 0.0100 0.005 0.007 0.005 0.0026 B: 0.001, Sb: 0.01 Invention steel 5 0.0035 0.027 0.030 0.024 0.0060 0.009 0.015 0.006 0.0025 Inventive Steel 6 0.0021 0.030 0.050 0.047 0.0070 0.005 0.011 0.006 0.0037 Invention steel 7 0.0015 0.050 0.070 0.026 0.0090 0.004 0.013 0.006 0.0022 Inventive Steel 8 0.0035 0.040 0.090 0.031 0.0080 0.008 0.016 0.006 0.0025 Sb: 0.03 Invention river 9 0.0025 0.020 0.120 0.036 0.0050 0.005 0.005 0.007 0.0038 Invented Steel 10 0.0012 0.039 0.180 0.028 0.0040 0.011 0.004 0.005 0.0018 Comparative Steel 1 0.0065 0.170 1.700 0.120 0.0520 0.003 0.095 - 0.0066 - Comparative River 2 0.0067 0.170 1.600 0.280 0.0500 0.005 0.101 - 0.0053 - Comparative Steel 3 0.0055 0.050 0.850 0.210 0.0450 0.004 0.075 - 0.0056 - Comparative Steel 4 0.0097 0.006 1.900 0.750 0.0350 0.003 0.130 - 0.0055 - Comparative Steel 5 0.0071 0.180 1.300 0.300 0.0470 0.001 0.102 - 0.0047 - Comparative Steel 6 0.0056 1.700 0.300 0.150 0.0050 0.002 0.070 - 0.0032 - Comparative Steel 7 0.0050 0.010 1,000 0.150 0.0100 0.004 0.070 - 0.0055 - Comparative Steel 8 0.0040 0.170 0.750 0.060 0.0110 0.008 0.103 - 0.0041 - Comparative Steel 9 0.0045 0.130 0.650 0.047 0.0090 0.005 0.095 - 0.0048 -

Psalter Average number of precipitates
(Pcs / nm2)
(PFZ) Average number of precipitates
(Pcs / nm2)
(Ferrite center) Precipitate
Average area
(nm2)
(PFZ) Precipitate
Average area
(nm2)
(Ferriet center) Precipitation Average Area Density (%)
(PFZ precipitate density /
Ferrite center precipitate density) Ridging resistance Hand
(%) r value Inventive Steel 1 6.14 389 58366 63143 92 ○ 57 1.7 Invention river 2 3.87 514 72569 56513 128 × 49 1.4 Invention steel 3 4.12 390 74189 65888 113 ○ 55 1.7 Inventive Steel 4 3.02 370 104733 80436 130 ○ 57 1.8 Invention steel 5 4.50 331 113897 96534 118 × 50 1.3 Inventive Steel 6 6.50 389 74150 62611 118 ○ 52 1.4 Invention steel 7 4.92 572 55943 52883 106 ○ 54 1.7 Inventive Steel 8 4.06 490 50329 46993 107 ○ 53 1.6 Invention river 9 4.30 567 49146 70616 70 ○ 52 1.6 Invented Steel 10 4.09 531 63351 85502 74 × 49 1.4 Comparative Steel 1 5.41 375 34302 91214 38 × 50 1.4 Comparative River 2 4.47 393 98489 70495 140 × 50 1.4 Comparative Steel 3 4.65 473 83258 49526 168 × 51 1.5 Comparative Steel 4 6.98 423 47775 69126 69 × 49 1.4 Comparative Steel 5 6.50 533 69895 42020 166 ○ 49 1.3 Comparative Steel 6 5.11 532 64558 44192 146 × 47 1.3 Comparative Steel 7 6.25 381 44957 70916 63 × 48 1.4 Comparative Steel 8 5.94 527 41067 59623 69 × 51 1.5 Comparative Steel 9 3.91 551 58255 38348 152 × 51 1.5

As shown in Table 2, in the case of the invention steel that satisfies the component range and manufacturing method of the present invention, the average number of precipitates is 3 ~ 7 × 10 ⁶ / ㎛ ² , the ferrite center and the ferrite (Ferrite) Depending on the grain boundary, the average area density ratio of the PFZ Precipitation Free Zone (PFZ) with a width of 0.1 ~ 2.5㎛ is 70 ~ 130%, and the dispersion density of the precipitate is constant, and the processability and elongation is over 50% and the r value is 1.4 It was found that this has excellent properties and excellent ridging resistance.

1 is a photograph showing the average area density of precipitates in the ferrite phases of the inventive steels 1 and 9 in the ferrite phases and the central portion as (a) and (b), respectively. (A) of FIG. 1 shows that the average area density of precipitates is about 92%, and (b) shows that the average area density of precipitates is about 70%, satisfying the scope of the present invention.

2 is a photograph showing PFZ and ferrites in the ferritic phases of the inventive steels 1 to 4 as (a), (b), (c) and (d), respectively. It can be seen that the precipitate is evenly distributed in the center.

1 is a photograph showing the average area density of precipitates in the ferrite phases of the inventive steels 1 and 9 in the ferrite phases and the central portion as (a) and (b), respectively.

2 is a photograph showing the PFZ in the ferrite phase of the inventive steels 1 to 4 and the precipitates in the center portion as (a), (b), (c) and (d), respectively.

Claims (5)

By weight%, C: 0.0005-0.0035%, Si: 0.01-0.05%, Mn: 0.01-0.2%, P: 0.001-0.02%, S: 0.001-0.020%, Sol.Al: 0.02-0.05%, N: It contains 0.004% or less, Ti: 0.005% to 0.007% and Nb: one or two of 0.001% to 0.02%, the remainder is composed of Fe and unavoidable impurities, In the microstructure, ferrite having an average particle diameter of 5 to 15 μm occupies 90% or more by area ratio, and single or complex precipitates of Nb (C, N) and Ti (C, N) are contained in the ferrite particles. Precipitation Free Zone (PFZ: Precipitation Free Zone) with 3 ~ 7 × 10 ⁶ holes / ㎛ ² , width 0.1 ~ 2.5㎛ depending on ferrite center and ferrite grain boundary, average area density of precipitate A cold rolled steel sheet for deep processing having a ratio of 70 to 130%, an elongation of 50% or more, and an excellent r workability of 1.4 or more. According to claim 1, B: 0.0002 ~ 0.003%, Sb: 0.005 ~ 0.1 and Sn: 0.005 ~ 0.1% of the cold-rolled steel sheet for deep processing excellent excellent workability is further included one or more selected from the group consisting of. . delete By weight%, C: 0.0005 to 0.0035%, Si: 0.01 to 0.05%, Mn: 0.01 to 0.2%, P: 0.001 to 0.02%, S: 0.001 to 0.020%, Sol.Al: 0.02 to 0.05%, N: A steel slab comprising not more than 0.004%, comprising one or two of Ti: 0.005% to 0.007% and Nb: 0.001% to 0.02%, the remainder being composed of Fe and unavoidable impurities, heated at 1050-1300 ° C., and finished Finishing rolling in an austenite single phase so that the strip cooling rate of the last three passes during rolling is 30 ° C./sec or more; Winding the rolled steel sheet and cold rolling the wound hot rolled steel sheet at a cold reduction ratio of 60% or more; And Continuously annealing the cold rolled steel sheet at a temperature range of 780 ° C. to 860 ° C. Process for producing a cold rolled steel sheet for deep processing excellent claim 1 of the above. The cold rolled steel sheet having excellent workability according to claim 4, wherein the composition further includes one or two or more selected from the group consisting of B: 0.0002 to 0.003%, Sb: 0.005 to 0.1, and Sn: 0.005 to 0.1%. Manufacturing method.

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