KR20030052208A - High strength steel sheet having superior workability and method for manufacturing there of - Google Patents

Abstract

PURPOSE: A steel sheet having tensile strength of greater than 40 kg/mm¬2 and superior workability suitable for use in outer panel for automobile and a method for manufacturing the same are provided. CONSTITUTION: The method includes the steps of soaking an aluminum killed steel comprising 0.01 wt.% or less of C, 0.7 wt.% or less of Mn, P 0.04 to 0.10 wt.%, 0.015 wt.% or less of S, 0.06 wt.% or less of Sol.Al, 0.005 wt.% or less of N, Cu 0.5 to 1.0 wt.%, Ti 0.01 to 0.04 wt.%, a balance of Fe and incidental impurities in the temperature range of 1200 to 1250 deg.C, wherein Nb content satisfies below: 6.63£C|<=Nb<=6.63£C|+0.02 wt.%; finish hot rolling the aluminum killed steel after soaking at higher than Ar3 transformation temperature; coiling the hot rolled aluminum killed steel sheet at 550 to 700 deg.C, followed by cold rolling; continuous annealing at higher than 800 deg.C, followed by temper rolling.

Description

High strength steel sheet having superior workability and method for manufacturing there of}

The present invention relates to a high tensile steel sheet with a tensile strength of 40kg / ㎠ or more used in the interior and exterior boards of automobiles, and more specifically, [Ti] and workability, which can be fixed with carbon, quality, and sulfides while reducing the alloying ability by reducing the amount of alloy addition. It is related to a high tensile strength steel sheet and a method of manufacturing the same to improve the processability while securing a tensile strength of 40Kg / ㎜ or more by adding a compound of [Nb] to develop a structure to improve the structure according to a certain formula.

In general, when producing automobiles with high tensile steel with a tensile strength of 35kg / mm2 or more, the thickness of the steel sheet can be reduced, which can save energy and have advantages in terms of safety of the automobile. have.

Accordingly, high strength cold rolled steel sheets having excellent formability as Si-Mn-P composite additive steels have been proposed in Japanese Patent Laid-Open Publications Nos. 55-24952 and 59-159936, and Korean Patent Publication No. 1998-54117, however, Not only is it high, but its machinability (r _m value) is not very good.

The present invention is to solve the above-mentioned problems, and the high tensile strength steel sheet that can secure a tensile strength of 40Kg / mm2 or more while excellent workability and r _m is 2.1 or more while reducing the alloying operation by reducing the addition amount of the alloy It is an object to provide a manufacturing method thereof.

In order to achieve the above object, the high-strength steel sheet of the present invention has a weight% of C: 0.01% or less, Mn: 0.7% or less, P: 0.04 to 0.10%, S: 0.015% or less, Sol-Al: 0.06% or less, N : 0.005% or less, Cu 0.5 to 1.0%, Ti: 0.01 to 0.04%, Nb satisfies the following relationship, 6.63 [C] ≤ Nb ≤ 6.63 [C] + 0.02%, and impurity contained inevitably with the remaining Fe It is made up of.

In addition, the high tensile steel sheet manufacturing method of the present invention, by weight% C: 0.01% or less, Mn: 0.7% or less, P: 0.04 to 0.10%, S: 0.015% or less, Sol-Al: 0.06% or less, N: 0.005 % Or less, Cu 0.5 ~ 1.0%, Ti: 0.01-0.04%, Nb satisfies the following relationship, 6.63 [C] ≤ Nb ≤ 6.63 [C] + 0.02%, and is composed of impurities inevitably contained with the remaining Fe After the homogenization treatment of the aluminum-kilted steel at a temperature range of 1200 to 1250 ° C., hot rolling at a finish rolling temperature of Ar ₃ or more, winding at 550 -700 ° C., followed by cold rolling, and continuous annealing at 800 ° C. or higher, It comprises a temper rolling.

Hereinafter, the present invention will be described in detail.

The present invention is to reduce the amount of alloying and to increase the tensile strength of 40Kg / ㎜ or more and improve the workability, [Ti] which can be fixed with carbon, quality, sulfide and [Nb] to develop a good texture for workability It is a complex addition based on a certain formula. The composition range of the steel component of the present invention will be described.

· C: 0.01% or less

When the carbon content exceeds 0.01%, the yield strength is increased to reduce the forming defect.

In addition to the increase in dissolved carbon, which leads to the extension of the yield point after the final annealing.

It is desirable to limit the carbon content to 0.01% or less because it produces stretcher strain.

· Mn: 0.7% or less

When the content of Mn exceeds 0.70%, it is preferable to limit the content to 0.70% or less since the material is hardened or moldability deteriorates due to solid solution strengthening beyond the effect of Mn.

· P: 0.04 ~ 0.10%

The P is an element that is excellent in solid solution strengthening and precipitation strengthening effect due to FeTiP precipitation even by the addition of a small amount, but should be added at least 0.04% for this purpose. However, when 0.1% or more is added, it is preferable to limit it to 0.04 to 0.10%. Do.

· S: 0.015% or less

S is a fragile element that causes hot brittleness, and the lower the component range, the better. Further, the lower limit of Mn is preferably limited to 0.015% in order to precipitate Mn-based sulfides.

· Al: 0.06% or less

Al is a component added for deoxidation of steel, and if the amount is more than 0.06%, it causes hardening of the material. Therefore, the Al content is preferably limited to 0.06% or less.

· N: 0.005% or less

N is an invasive element, which inhibits {111} texture, impairs workability, impedes grain growth, and lowers elongation. Therefore, the lower the N, the better the processability, and the upper limit is 0.005% to minimize the aging phenomenon as an aging element. It is desirable to limit to

· Cu: 0.5 ~ 1.0%

The Cu is usually added for precipitation strengthening, but it is difficult to obtain the precipitation strengthening effect by a short heat treatment, so that Cu is added to increase the strength without inhibiting workability. In the case of excess, it is disadvantageous in terms of steelmaking operability and steelmaking unit, it is preferable to limit to 0.5 ~ 1.0%.

· Ti: 0.01 ~ 0.04%

The Ti serves to suppress the generation of stretch strain by lowering the yield strength and removing the yield point by depositing solid solution elements (nitrogen, sulfur) with TiN and TiS. In general, the yield strength after annealing and the stretcher strain generated during forming are increased in proportion to the amount of solid solution present in the steel. In the present invention has a function to lower the yield strength and remove the strainer strain by the effect of fixing nitrogen and sulfur by the addition of Ti. However, if the amount of Ti is less than 0.01%, the solid solution cannot be effectively precipitated. If the amount of Ti is more than 0.04%, the amount of Ti is preferably 0.01 to 0.04% because it is meaningless in terms of its effect.

· Nb: Nb is added employed to satisfy more than 0.02%

The Nb is an element that combines with the solid solution C in the steel to form fine NbC precipitates, thereby reinforcing particle size, precipitation strengthening, and dispersion strengthening at the same time by miniaturizing the particle size due to the fine NbC precipitate distribution in the micro carbon steel. It is an element that develops the {111} texture at the time of annealing. Therefore, Nb is employed. When Nb exceeds 0.02%, Nb is less than 0.02% because strength is too high to obtain the target material.

Therefore, in order to precipitate NbC by reacting with solid solution C while the solid solution Nb becomes 0.02% or less, it is preferable to add Nb so as to satisfy the following Equation 1.

[Relationship 1]

6.63 [C] <Nb ≤ 6.63 [C] + 0.02%

When Nb is added below 6.63 [C]%, stable NbC content may not be secured, resulting in insufficient strength, insufficient workability, and stretcher-strain, and when exceeding 6.63 [C] + 0.02%, excess strength and elongation decrease. May result.

The slabs formed as described above are obtained through molten steel through a steelmaking process and then through a ingot or continuous casting process. The slab is produced by hot rolling, winding, cold rolling, and annealing to produce high tensile steel having a desired mechanical property. The manufacturing conditions for each process will be described in detail.

· Hot rolling

The slab is heated at 1200 to 1250 ° C., where the austenite structure can be sufficiently homogenized before hot rolling, followed by finishing hot rolling directly above the Ar ₃ temperature. If the hot finish rolling temperature is less than Ar ₃ transformation point, it is rolled in the ferrite + pearlite two-phase structure, which causes abnormal coarse grains, which in turn causes defects in product processing. Therefore, the hot finishing rolling temperature is preferably performed at an Ar ₃ transformation point or more.

· Winding process

The hot rolled hot rolled sheet is wound, but the winding temperature is preferably limited to 550 ~ 700 ℃. If the coiling temperature is higher than 700 ℃, a large amount of scale may occur due to the high temperature work, which may cause surface quality problems, and the fraction of the {111} direction of the tissue is lowered, which causes deterioration of workability. Although the {111} direction fraction of is increased, the workability is improved, but there is a possibility that a large temperature deviation occurs inside and outside the coil, causing variations in the material in the coil.

- cold rolling

The wound hot rolled sheet is cold rolled, in which case the cold rolling may be appropriately set to a rolling condition so as to obtain a cold rolled sheet at a desired final thickness. Therefore, since cold rolling conditions are not specifically limited, normal conditions are possible.

· Annealing Process

Next, the cold rolled sheet is annealed, and the annealing at this time is preferably continuous annealing. The continuous annealing temperature is below the recrystallization completion temperature. Therefore, it is recommended to do it above the recrystallization completion temperature because there is a possibility of material deviation and processing cracks during processing. More preferably, the continuous annealing at 800 ° C. or higher in consideration of the short-term continuous annealing point is preferable.

The cold rolled steel sheet obtained by annealing as mentioned above is temper-rolled on normal conditions.

The high tensile strength steel sheet produced according to the present invention has a characteristic of cold rolled steel sheet having a tensile strength of 40 Kg / mm 2 or more and an r _m value (average value of vertical anisotropy, r _m = (r 0 +2 r 45 + r 90) / 4) of 2.1 or more. In addition, the high tensile strength steel sheet provided according to the present invention may be applied with conventional electroplating or hot-dip plating in accordance with the required characteristics of the product.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

To prepare a steel slab by dissolving the ultra-low carbon aluminum-kilted steel in a converter so as to have a composition as shown in Table 1, after the external refining treatment. At this time, both the invention steel (1-3) and the comparative steel (4-6) shown in Table 1 are the final steel components after the out-of-furnace refining. Inventive steel (1-3) was added within the range of 6.63 [C]% (denoted as α) and less than 6.63 [C] + 0.02% (denoted β) so that Nb contained in solid solution was less than 0.02%. In addition, the comparative steel 4 is a steel in which Nb is added below α, and in the comparative steel 5, Nb is added beyond the β range.

It is a river. Comparative steel (6) is a steel in which Nb is added less than α, which exceeds the limit of C degree.

The P component is also a steel added beyond the limit and Ti is an unadded steel.

The steel slab having the composition shown in Table 1 was homogenized at a temperature of 1250 ℃

After finishing hot rolling to a thickness of 2.3mm near 910 ℃, which is directly above Ar3, Table 2

It was wound up at the hot rolled winding temperature indicated in, and pickling was carried out by the usual method. Pickled

Hot rolled steel sheet is applied by continuous annealing after cold rolling, and the final cold rolled steel sheet has a thickness of 0.70mm

A cold rolled steel sheet was obtained. Table 2 shows the mechanical properties and workability indices of r _m of the inventive and comparative steels.

Indicates.

As shown in Tables 1 and 2, in the case of comparative steel (4), Nb is in the range of securing Nb.

Since the addition is insufficient, the TS is less than 40Kg and the strength is insufficient and the workability is poor. In the case of the comparative steel (5), since Nb was added in excess of the solid solution Nb securing range, the TS strength exceeded the target value and the workability was poor. In the case of the comparative steel (6), Ti was not added, and [P] was added beyond the control range to secure strength, but [Nb] was added below the solid solution Nb securing range, resulting in poor workability.

However, in the invention steel (1-3), the addition of P was limited to 0.04 to 0.10%, and Nb and Ti were added, and Nb was added in the solid solution Nb securing range, so that the target strength and workability were good.

As described above, according to the present invention, a conventional core value for adding a solid solution strengthening element [P]

In order to improve the workability of common cold-rolled high tensile steel, the addition of [P] is limited to 0.04 ~ 0.10% and [Ti] and [Nb] are added. In particular, [Nb] is used to secure [Nb] and Addition based on the correlation has a useful effect of providing a cold rolled 40Kg high tensile strength steel sheet having excellent workability.

Claims (3)

By weight% C: 0.01% or less, Mn: 0.7% or less, P: 0.04 ~ 0.10%, S: 0.015% or less, Sol-Al: 0.06% or less, N: 0.005% or less, Cu 0.5 ~ 1.0%, Ti: 0.01-0.04%, Nb satisfies the following relationship, 6.63 [C] ≤ Nb ≤ 6.63 [C] + 0.02%, high tensile steel sheet with excellent workability composed of the remaining Fe and inevitable impurities. The high tensile strength steel sheet according to claim 1, wherein the high tensile strength steel sheet has a tensile strength of 40 Kg / mm 2 or more and an r _m value (average value of vertical anisotropy of 2.1 or more). By weight% C: 0.01% or less, Mn: 0.7% or less, P: 0.04 ~ 0.10%, S: 0.015% or less, Sol-Al: 0.06% or less, N: 0.005% or less, Cu 0.5 ~ 1.0%, Ti: 0.01-0.04%, Nb satisfies the following relationship, 6.63 [C] ≤ Nb ≤ 6.63 [C] + 0.02%, and the temperature of 1200-1250 ° C for aluminum-kilted steel composed of the remaining Fe and inevitable impurities High-strength steel with excellent workability including hot-rolling under the condition of finish rolling temperature Ar ₃ or more after homogenization in the range, winding at 550 -700 ° C, followed by cold rolling, continuous annealing at 800 ° C or higher, and temper rolling. Method of manufacturing plate.