WO2008062985A1

WO2008062985A1 - Cold rolled steel sheet having excellent in-plane anisotropy and workability and method of manufacturing the same

Info

Publication number: WO2008062985A1
Application number: PCT/KR2007/005825
Authority: WO
Inventors: Sang Ho Han
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2006-11-21
Filing date: 2007-11-20
Publication date: 2008-05-29
Anticipated expiration: 2009-05-21
Also published as: KR100775339B1

Abstract

There are provided a cold rolled steel sheet having excellent in-plane anisotropy and workability, which is used as an impact-resistance material for an automobile, and a method for manufacturing the cold rolled steel sheet. The cold rolled steel sheet has a composition including, in weight%, C: 0.005 or less, Mn: 1.5% or less, P:p. l5% or less, S: 0.01% or less, N: 0.006 to 0.008%, acid soluble Al: 0.11 to 0.16%, Ti: 0.03 to 0.05%, Nb: 0.01 to 0.03%, Mo: 0.04 to 0.08%, Sb: 0.2% or less, B: 0.0005 to 0.0015%, Fe, and other impurities, in which lxl06/mm2 or more of AlN and MoC precipitates having an average size of 20 nm or less are distributed. The cold rolled steel sheet may have in-plane anisotropy of a value (Δr) 0.2 or less, plastic anisotropy (r) of 1.7 or more, and excellent yield ratio.

Description

COLD ROLLED STEEL SHEET HAVING EXCELLENT IN- PLANE ANISOTROPY AND WORKABILITY AND METHOD OF MANUFACTURING THE SAME

Technical Field

[1] The present invention relates to a cold rolled steel sheet generally used as an impact resistance material such as inner or outer panels and some parts for an automobile, and more particularly, to a cold rolled steel sheet, which is an extremely low carbon steel sheet obtained by mixing Ti-Nb and has excellent in-plane anisotropy and workability due to fine AlN and MoC precipitates and a method of manufacturing the same.

[2]

Background Art

[3] Recently, precipitation hardened high strength steel sheets have been used in various ways such as members, beams, pillars, and inner panels to improve impact resistance of a car body as controls on impact safety of automobiles have been tightened. It is required to use steel sheets having excellent shock absorption ability when a car accident occurs. Since being designed to absorb collision energies of an automobile, precipitation hardened high strength steel sheets have high percentages of yield strength in tensile strength, that is, high yield ratio (YS/TS).

[4] Such precipitation hardened steels are generally manufactured using conventional low carbon steels. However, in this case, conventional low carbon steels are unsuitable for parts requiring high workability due to inferior formability thereof. As representative well-known art, Korean Patent Application No. 2003-0091535 discloses a method of manufacturing a low carbon precipitation hardened high strength steel, in which a low carbon steel containing 0.02 to 0.04 wt% of C is a base composition and yield strength thereof is increased. However, though providing strength, workability is not enough since formability, that is, a value of r stays at a level of 1.4.

[5] On the other hand, when manufacturing precipitation hardened steels by using extremely low carbon steels, there is a limitation on employing as impact resistance materials due to low yield strength. As conventional art, there are Japanese Patent Publication Nos. hei 4-280943, hei 5-263184, and hei 10-096051. The conventional art, due to low percentages of yield strength in tensile strength, is very advantageous when being applied to parts of an automobile, requiring workability, but is unsuitable for parts requiring impact resistance.

[6]

Disclosure of Invention Technical Problem

[7] An aspect of the present invention provides a cold rolled steel sheet having excellent in-plane anisotropy and workability while having excellent yield strength, by controlling alloy elements and fine precipitates in a basic composition of extremely low carbon steel, and a method of manufacturing the cold rolled steel sheet.

[8]

Technical Solution

[9] According to an aspect of the present invention, there is provided a cold rolled steel sheet having excellent in-plane anisotropy and workability, the cold rolled steel sheet having a composition including, in weight %, C: 0.005 or less, Mn: 1.5% or less, P: 0.15% or less, S: 0.01% or less, N: from 0.006 to 0.008%, acid soluble Al: from 0.11 to 0.16%, Ti: from 0.03 to 0.05%, Nb: from 0.01 to 0.03%, Mo: from 0.04 to 0.08%, Sb: 0.2% or less, B: from 0.0005 to 0.0015%, Fe, and other impurities, in which IxIO⁶ / mm or more of AlN and MoC precipitates having an average size of 20 nm or less are distributed.

[10] According to another aspect of the present invention, there is provided a method for manufacturing a cold rolled steel sheet having excellent in-plane anisotropy and workability, the method including: reheating a steel slab having a composition comprising, in weight %, C: 0.005 or less, Mn: 1.5% or less, P: 0.15% or less, S: 0.01% or less, N: from 0.006 to 0.008%, acid soluble Al: from 0.11 to 0.16%, Ti: from 0.03 to 0.05%, Nb: from 0.01 to 0.03%, Mo: from 0.04 to 0.08%, Sb: 0.2% or less, B: from 0.0005 to 0.0015%, Fe, and other impurities at a temperature of 1100⁰C or more; hot finishing rolling the steel slab at a transformation point of Ar ; winding the steel slab at a temperature within a range from 550 to 63O⁰C; cold rolling the steel slab; and continuously annealing the steel slab at a temperature within a range from 760 to 81O⁰C.

[H]

Best Mode for Carrying Out the Invention

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

[13] While the present inventor has tried to find a method for improving in-plane anisotropy and workability simultaneously with providing excellent yield strength, the present invention has been completed based on a search result where yield strength may be increased by distributing and precipitating fine AlN and MoC in a grain boundary and plane, workability may be greatly improved by decreasing a content of C to a level of an extremely low carbon steel, and overall in-plane anisotropy (hereinafter, referred to as Δr) may be greatly improved by improving a value in an r45 direction among r values, which are workability estimation values by adding Nb. [14] Workability of a steel may be greatly improved by precipitating and fixing residual C by adding Ti or Nb in an extremely low carbon steel. According to experimental results, when separately adding Ti, workability is improved but in-plane anisotropy is not greatly improved. On the other hand, adding Ti with Nb, there is present an effect of improving a directional workability value of the steel.

[15] Also, according to an exemplary embodiment of the present invention, surface quality of a steel may be greatly improved by controlling a surface elution of inclusions while annealing, by adding a small amount of Sb. Composition ranges of elements of the steel will be described.

[16] A content of C may be 0.005 wt% or less.

[17] When allowing C to be present as precipitates by adding Ti to an extremely low carbon steel instead of a solute solution state, { 111 } texture advantageous for workability is developed while annealing, thereby greatly improving workability. When the content of the C is more than 0.005%, aging property and formability are not greatly improved. Therefore, the content of C may be 0.005% or less.

[18] A content of Mn may be 1.5wt% or less.

[19] Mn not only allows strength to be increased as solution strengthenig element but also precipitates S in the steel as MnS, thereby controlling strip breakage and bromination at high temperature due to S in hot rolling. When manufacturing the steel using an extremely low carbon steel, there is a limitation on increase of yield strength. In the present invention, yield strength may be increased as possible by using fine precipitates of AlN and MoC, and more excellent yield strength may be provided by increasing an amount of Mn. As the Mn content increases, an effect of increasing strength increases. However, when the content of the Mn is more than 1.5wt%, since workability, that is, an r value rapidly decreases, the content of Mn may be limited to 1.5wt%.

[20] A content of P may be 0.15wt% or less.

[21] P is not harmful to formability and is most advantageous to provide strength.

However, excessive addition of P notably increases a possibility of embrittlement occurrence, thereby not only increasing strip breakage of a slab during hot rolling but also deteriorating plating surface properties. Accordingly, the content of P may be limited to 0.15 wt%.

[22] A content of S may be 0.01wt% or less.

[23] Since S is an element unavoidably added as impurities, it is important to control the content of S to be as low as possible. Also, to provide excellent welding properties, the content thereof may be managed to be as low as possible. However, costs for refining the steel increases. Accordingly, the content of S may be limited to 0.01wt% or less in a range where an operation condition is possible. [24] A content of N may be in a range from 0.006 to 0.008wt%.

[25] N is important to increase strength in the present invention. When the content of N is less than 0.006wt%, since an effect of fine AlN precipitates is not enough, it is difficult to be accompanied with increase of yield strength. When the content of N is more than 0.008wt%, it may be difficult to provide aging properties due to solute nitrogen. Accordingly, the content of N may be limited to be in a range from 0.006 to 0.008wt%.

[26] A content of acid soluble Al may be in a range from 0.11 to 0.16wt%.

[27] The acid soluble Al is generally added to fine particles and dioxide the steel.

However, in the present invention, the acid soluble Al s used to increase yield strength of the steel by using the effect of fine AlN precipitates. When the content of the acid soluble AlN is less than 0.1 lwt%, extremely fine AlN precipitates are not provided enough and strength of the steel is not greatly increased. When the content of the acid soluble is more than 0.16wt%, ductility may be greatly decreased due to a large amount of Al in a solute state. Accordingly, the content of the acid soluble Al may be limited to be in a range from 0.11 to 0.16wt%.

[28] A content of Ti may be in a range from 0.03 to 0.05 wt%.

[29] Ti forms TiC precipitates by acting with solute C during hot rolling to perfectly precipitating the solute C, thereby scavenging and fixing the steel to improve formability thereof. When the content of Ti is less than 0.03 wt%, since the solute C is not perfectly precipitated, it is disadvantageous in an aspect of the formability of the steel. When the content of the solute C is more than 0.05 wt%, not only Ti, which is not precipitated, remains but also an effect of fining particles is decreased due to large TiC precipitates, thereby greatly deteriorating yield strength and plating properties. Accordingly, the content of Ti may be limited to be in a range from 0.03 to 0.05 wt%.

[30] A content of Nb may be in a range from 0.01 to 0.03 wt%.

[31] Nb is important to improve in-plane anisotropy in the present invention. That is, the in-plane anisotropy is greatly improved when precipitating and fixing C by using Ti- Nb composite rather than using only Ti. This is due to a difference in development of texture in an r45 direction during annealing. Workability for each direction becomes different by adding Nb, and formability finally becomes different. When the content of Nb is less than 0.01wt%, the texture in the r45 direction is not greatly developed, thereby decreasing an effect of improving the in-plane anisotropy. When the content of Nb is more than 0.03wt%, due to excessive solute Nb, workability and surface quality may be deteriorated. Therefore, the content of Nb may be limited to be in a range from 0.01 to 0.03 wt%.

[32] A content of Mo may be in a range from 0.04 to 0.08wt%.

[33] Mo not only may allow the steel to be fine to increase strength of the steel but also may increase strength, that is, yield strength by finely distributing MoC precipitates. When the content of Mo is less than 0.04wt%, MoC is not precipitated to be effectively fine, thereby deteriorating the strength of the steel. When the content of Mo is more than 0.08wt%, the precipitates become rough, thereby deteriorating the strength of the steel. The content of Mo may be limited to be in a range from 0.04 to 0.08wt%.

[34] A content of Sb may be 0.2wt% or less.

[35] Sb prevents an occurrence of a defect on a surface of the steel, such as a dent generated due to an elution of inclusions such as TiO to the surface of the steel. That is, Sb in steel is present in a grain boundary and prevents a transfer of oxides present in the steel. However, when the content of Sb is more than 0.2wt%, it is disadvantageous in an aspect of cost and an effect thereof is not great. Accordingly, the content of Sb may be limited to be 0.2wt% or less.

[36] A content of B may be in a range from 0.0005 to 0.0015.

[37] B is effective in improving surface properties of a plated steel sheet together with

Mo. When the content of B is less than 0.0005 wt%, the effect is unsatisfactory. When the content of B is more than 0.0015wt%, a recrystallization temperature is rapidly increased to cause a defect on the plated steel sheet due to high temperature annealing, such as a stripe and a being unalloyed. Accordingly, the content of B may be limited to be in a range from 0.0005 to 0.0015wt%.

[38] In the steel according to an exemplary embodiment of the present invention, there are distributed 1x10 /mm or more of AlN and Moc precipitates having an average size of 20 nm or less. In a composition according to an exemplary embodiment of the present invention, it becomes more advantageous as the precipitates are finely distributed. According to a result of the present invention, when the average size of the precipitates is more than 20 nm, particularly, strength becomes decreased and yield strength is not greatly improved.

[39] In addition, to maximize a strength increase effect in the composition, a number of distributed AlN and MoC of 20 nm or less may be 1x10 per 1 mm . In this case, the strength is greatly improved, and a value of r, which is a plastic anisotropy value, may be improved. Accordingly, the number of the AlN and MoC precipitates having an average size of 20 nm or less may be limited to 1x10 /mm .

[40] Hereinafter, a method of manufacturing a cold rolled steel sheet having a composition as described above will be described in detail.

[41] A steel slab having the composition is reheated at a temperature of 1100⁰C or more and hot finishing rolled at a transformation point of Ar3 or more.

[42] When the reheating temperature is less than 1100⁰C, rough precipitates generated in continuous casting remain in a state of being imperfectly solved and there is present a large amount of the rough precipitates after hot rolling, thereby being ineffective to increase strength. Accordingly, the reheating temperature may be limited to be 1100⁰C or more.

[43] Also, when a temperature of the hot finishing rolling is less than the transformation point of Ar , not only workability is decreased but also the strength is decreased due to generation of rolled grains. Accordingly, the temperature of the hot finishing rolling may be limited to be the transformation point of Ar or more.

[44] After this, the steel slab is wound at a temperature in a range from 550 to 63O⁰C and cold rolled. When the winding temperature is less than 55O⁰C, solute C is imperfectly precipitated, thereby not only decreasing an effect of precipitates but also causing a defect in a shape of a panel while winding. When the winding temperature is more than 63O⁰C, since precipitates notably become rough, an effect of the precipitates are small and yield ratios may be increased. Accordingly, the winding temperature may be limited to be in a range from 550 to 63O⁰C.

[45] Also, though the cold rolling after winding is advantageous in an aspect of workability as a reduction ratio is high, a desired level of formability may be provided in a general condition caused by a field application limit. Accordingly, it is not necessary to define a range of a reduction ratio.

[46] The cold rolled steel sheet is continuously annealed at a temperature in a range from

760 to 81O⁰C. Though the continuous annealing temperature is very important, which determines a material of a product, but may be in the range from 760 to 81O⁰C, which is a general working condition. When the annealing temperature is less than 76O⁰C, though recrystallization of steel is provided, growth of grains is unsatisfactorily performed, thereby generating a defect in providing formability. When the annealing temperature is more than 81O⁰C, inclusions may elute to a surface of the steel sheet and generate a large amount of fine dents, thereby deteriorating surface quality. Accordingly, the continuous annealing temperature may be limited to be in a range from 760 to 81O⁰C.

[47] Hereinafter, an embodiment of the present invention will be described in detail.

[48] [Embodiment]

[49] A steel slab having a composition as shown in following Table 1 was reheated at a temperature of 1100⁰C or more and hot finishing rolled at a temperature of a transformation point of Ar or more. The steel slab was wound at a manufacturing condition as shown in following Table 2, was rolled to 78% that is a general reduction ratio, and was annealed in a continuous annealing condition as shown in following Table 2.

[50] Mechanical properties of an obtained annealed panel, such as yield strength, tensile strength, an elongation rate, a value of r, and an in-plane anisotropy Δr value, were measured. In this case, JIS fifth test specimen is used for a tensile test. Table 2 shows mechanical properties of inventive steels and comparative steel.

[51] [52] Table 1 [Table 1] [Table ]

[53] [54] Table 2 [Table 2] [Table ]

[55]

[56] As shown in Tables 1 and 2, in the case of inventive steels 1 to 4 manufactured according to the element ranges and the manufacturing method according to an exemplary embodiment of the present invention, 1x10 or more of fine precipitates having a size of 20 nm or less are formed, thereby providing excellent yield strengths of 235 MPa or more, 34% or more of elongation rates, 1.7 or more of plastic anisotropy values (value of r), and 0.2 or less of in-plane anisotropy values (Δr).

[57] However, in the case of comparative steel 1, which does not satisfy the element range according to an exemplary embodiment of the present invention, since distribution and size of precipitates is unsatisfactory, there is provided inferior yield strength. Also, workability and in-plane anisotropy properties are inferior such as 1.46 of plastic anisotropy value (value of r) and 0.38 of in-plane anisotropy value (Δr).

[58] As described above, according to an exemplary embodiment of the present invention, there is provided a cold rolled steel sheet with excellent yield strength, in-plane anisotropy, and workability. Also, an elution of inclusions to a surface is controlled, thereby providing greatly improved surface quality.

[59]

Claims

[1] A cold rolled steel sheet having excellent in-plane anisotropy and workability, the cold rolled steel sheet having a composition comprising, in weight %, C: 0.005 or less, Mn: 1.5% or less, P: 0.15% or less, S: 0.01% or less, N: from 0.006 to 0.008%, acid soluble Al: 0.11 to 0.16%, Ti: from 0.03 to 0.05%, Nb: from 0.01 to 0.03%, Mo: from 0.04 to 0.08%, Sb: 0.2% or less, B: from 0.0005 to 0.0015%, Fe, and other impurities, in which 1x10 /mm or more of AlN and MoC precipitates having an average size of 20 nm or less are distributed.

[2] A method for manufacturing a cold rolled steel sheet having excellent in-plane anisotropy and workability, the method comprising: reheating a steel slab having a composition comprising, in weight %, C: 0.005 or less, Mn: 1.5% or less, P: 0.15% or less, S: 0.01% or less, N: from 0.006 to 0.008%, acid soluble Al: from 0.11 to 0.16%, Ti: from 0.03 to 0.05%, Nb: from 0.01 to 0.03%, Mo: from 0.04 to 0.08%, Sb: 0.2% or less, B: from 0.0005 to 0.0015%, Fe, and other impurities at a temperature of 1100⁰C or more; hot finishing rolling the steel slab at a transformation point of Ar ; winding the steel slab at a temperature in a range from 550 to 63O⁰C; cold rolling the steel slab; and continuously annealing the steel slab at a temperature within a range from 760 to 81O⁰C.