KR20030053799A - Method for manufacturing high strength cold rolled steel sheet having Bake Hardening and superior press - Google Patents

Abstract

PURPOSE: A method for manufacturing high strength cold rolled steel sheet having bake hardenability of greater than 3 kgf/mm¬2, tensile strength of 30 to 40 kgf/mm¬2 and r value of greater than 2.0 is provided. CONSTITUTION: The method includes the steps of reheating a steel slab comprising 0.01 wt.% or less of C, 0.005 wt.% or less of N, 0.015 wt.% or less of S, Mn 0.3 to 0.8 wt.%, P 0.03 to 0.09 wt.%, sol.Al 0.1 to 0.7 wt.%, 0.15 wt.% or less of Ti, 0.015 wt.% of one or more than two elements selected from the group consisting of Nb, Mo and B, a balance of Fe and incidental impurities; hot rolling the steel slab, wherein finishing delivery temperature (FDT) satisfies below relation: A1*-55deg.C<=FDT<=A1*+35°C(A1* is 130xAl£%|+400xP+860°C); coiling the hot rolled steel sheet in the temperature range of 530 to 700°C, followed by pickling and cold rolling; and continuous annealing the cold rolled steel sheet in the range from recrystallization temperature to Ac3 point.

Description

Method for manufacturing high strength cold rolled steel sheet having excellent press formability with hardening hardenability {Method for manufacturing high strength cold rolled steel sheet having Bake Hardening and superior press}

The present invention relates to a method for manufacturing a high strength cold rolled steel sheet which is press-molded and coated with parts such as an inner plate and an outer plate of an automobile body, and more specifically, a press hardenability of 3 kgf / mm ² or more and an r value of 2.0 or more. It relates to a method for producing a high strength cold rolled steel sheet of excellent tensile strength 30 ~ 40kgf / mm2 class.

Most of the high strength steel sheets for deep processing with a tensile strength of 30 to 40 kgf / mm ² have been added with P or Mn and titanium (Ti) or niobium (Nb) as carbonitride forming elements [Japanese Patent Laid-Open No. 57-35662 , Pyung 5- 339641 ~ 3, Korean Patent 1998-53898, 1999-35104. However, since these steels add a large amount of Ti, precipitates such as FeTiP are formed, which causes a problem of deterioration in workability.

In order to solve this problem, Nb is added alone, but the addition of a small amount of Nb does not facilitate NbC precipitation, and when a large amount is added, solid solution Nb has a problem of degrading workability. As a result, in order to manufacture high-strength cold rolled steel sheet for super-core processing, a technique for increasing the content of solid solution P and reducing solid solution nitrogen, sulfur, and carbon as a precipitate is required. It is known that solid solution sulfur is not a problem because Mn added to improve strength forms MnS. However, Ti added to reduce solid solution nitrogen and carbon forms a precipitate called FeTiP in addition to TiN and TiC, but rather reduces workability. If nitrogen is precipitated with other elements together with Ti and the solid solution carbon is precipitated as TiC by a small amount of Ti, it will be effective in improving the workability of high strength steel because it will be able to suppress the formation of FeTiP with the reduction of solid solution nitrogen and carbon. In addition, since the amount of Ti can be reduced, it will be advantageous to suppress the clogging of the playing nozzle caused by the crystallization of Ti oxide during the manufacture of slabs.

The present invention finds that when aluminum is appropriately added, the reduction of solid solution nitrogen by AlN precipitation and the formation of FeTiP are suppressed. To provide, the purpose is.

Method for producing a high strength cold rolled steel sheet of the present invention for achieving the above object,

By weight%, C: 0.01% or less N: 0.005% or less, S: 0.015% or less, Mn: 0.3-0.8%, P: 0.03-0.09%, acid value Al: 0.1-0.7%, Ti: less than 0.15%, One or two or more selected from the group of Nb, Mo, and B: 0.015% or less, the sum of Ti and Nb content: 0.005% or more, and finishing by reheating the slab composed of the remaining Fe and other inevitable impurities The rolling temperature (FDT) is hot rolled under the conditions satisfying the following relationship A1 * -55 ° C≤FDT≤A1 * + 35 ° C (where A1 * is 130xAl [%] + 400xP + 860 ° C), After winding up in the range of 700 degreeC, it washes, cold-rolls, and continuously anneals in the temperature range of recrystallization temperature-Ac3.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

This decay reveals a new fact that Al acts to increase the Ar1 temperature as a ferrite expansion element by increasing the solid solution together with P and Mn, and increases the spacing between iron atoms, thereby employing P more. The present invention is completed by setting the manufacturing conditions together with the alloy design. The reason for limiting the composition range of the steel component system of the present invention will be described first.

, Carbon (C): 0.01% or less

As the carbon content increases, the strength of the (110) and (100) aggregates is strengthened, and the (111) aggregates, which are advantageous for workability, are weakened, thereby reducing the press formability and increasing the density of the TiC precipitates. Therefore, it is desirable to limit it to 0.01% or less.

And nitrogen (N): 0.005% or less

Nitrogen will precipitate as TiN or AlN. However, since AlN is very fine and causes workability deterioration, it is preferable to lower the content of nitrogen as much as possible, so that the nitrogen is 0.005% or less in the present invention. The conventional nitrogen content range that can be reduced through the converter-vacuum degassing process is 0.005%, and the upper limit of the nitrogen content of the present invention is higher than 0.003% of some known technologies according to the Al addition effect, so that steel can be easily manufactured. There is this.

, Sulfur (S): 0.015% or less

Sulfur is known as an element that causes high temperature brittleness of steel and deteriorates the surface quality of steel sheets such as edge cracks when the Mn content is not sufficient. In the present invention, Mn is sufficiently added to prevent this by MnS precipitation, but it is not preferable because the strengthening action of the solid solution Mn is reduced, and the coarse MnS precipitate lowers the elongation, so the upper limit is preferably 0.015%.

, Manganese (Mn): 0.3 ~ 0.8%

Manganese is a solid solution element as described above. In order to strengthen the solid solution Mn, a predetermined amount or more is required. When the S content is 0.015%, the Mn / S ratio must be 20 or more, so that the high temperature brittleness of the steel is suppressed, so the lower limit should be 0.3%. In the prior art, a large amount of Mn is added to secure the strength of the steel. However, since Mn is an element that increases the austenite-ferrite transformation temperature, the upper limit is preferably 0.8% or less in the present invention. As will be described in detail later, in the present invention, an important means for securing the workability is hot rolling near the Ar1 temperature to refine the hot rolled grains, thereby using a technique for securing the strength and workability of the cold rolled steel sheet. The Ar1 temperature is a temperature at which steel is completely transformed from austenite to ferrite during cooling, and decreases when the Mn content increases, and increases when the P or Al content increases. Therefore, when the Mn content increases and the Ar1 temperature decreases, the above-mentioned hot-rolled grain refining effect cannot be obtained at a normal hot finish rolling temperature. However, the upper limit should be 0.8% even though Mn is an effective element for securing strength.

, The (P): 0.03 ~ 0.09%

Phosphorus is the element with the largest solid solution strengthening effect, and the lower limit is 0.03 where the strengthening effect occurs. However, when too much P is added, the formation of FeTiP is easy, brittle fracture by grain boundary segregation P occurs, while the Ar1 temperature is extremely raised, leaving the strain structure on the hot-rolled sheet, resulting in poor workability. The upper limit is 0.09%. It is preferable to set it as.

, Acid soluble aluminum (Al): 0.1 ~ 0.7%

Acid-soluble Al refers to the Al content in the steel that limits acid-insoluble Al compounds such as Al2O3, including AlN and two types of solid aluminum. Aluminum is a key element in the present invention, unlike the prior art, and newly discovered in the study of the present invention are three following. Together with P and Mn, it causes a solid-solution strengthening, and increases the ferrite station expansion element, that is, the temperature of Ar1, and increases the spacing between the iron atoms, thereby suppressing the formation of FeTiP because it increases the amount of P in between. In the prior art, Al is merely a deoxidizer and has an upper limit of 0.1% or less. However, in the present invention, Al is added in excess of 0.1% to obtain all three effects described above. However, when the Al content is too high, the Ar1 temperature becomes extremely high, and thus, fine grains cannot be obtained at a normal hot finishing rolling temperature. Rather, the upper limit is 0.7% because a strained structure is formed and the workability of steel is impaired.

, Titanium (Ti): less than 0.15%

In the present invention, the lower limit of the Ti content is not determined because there is Al other than Ti as the element to precipitate solid solution nitrogen. However, the upper limit is preferably less than 0.015% because there is a problem that workability deteriorates due to FeTiP formation.

· Mo, B, 1, or two or more selected from Nb: 0.015% or less

Mo, B, and Nb form carbides and nitrides, respectively, to improve workability. When the added amount is more than 0.015%, it is preferable to limit the heat distortion deformation structure to within 0.015%. In the case of Nb, it is precipitated with NbC to reduce solid solution carbon to improve workability. However, when Nb is more than 0.015%, solid solution Nb is increased to reduce workability by decreasing elongation. In addition, B and Mo are added to suppress grain boundary brittleness due to P, but, like Nb, B and Mo are used to retain the hot-rolled deformed structure, so that the content is 0.015% or less. In the present invention, by hot rolling at a temperature of Ar1 or less at which ferrite transformation is completed, recrystallization is performed to obtain a fine hot-rolled structure, thereby securing the strength and workability of the cold-rolled steel sheet. If not removed, there is a phenomenon that workability is very deteriorated, so one or two or more selected from the group of Nb, Mo, and B are added within 0.015%.

· Ti + Nb: 0.005% or more

If Ti or Nb is a carbide-forming element and none of them is formed, it is preferable that the total content of Ti and Nb is at least 0.005% because the workability is degraded by solid solution carbon.

The slab is produced by converter and continuous casting of steel containing the remainder Fe and other unavoidable elements that satisfy the above component range, and then cold rolled sheet through hot rolling, winding, cold rolling, pickling, and continuous annealing. To prepare, the manufacturing conditions at this time will be described.

· Hot rolling

The hot finish rolling temperature is characterized in that the conventional technique is carried out at a temperature of Ar3 or more, whereas in the present invention, it is carried out in the temperature range directly above or directly below the ferrite transformation for miniaturization of the hot rolled crystal grains.

In the present invention, Al plays a role of raising the temperature of Ar1, so the Ar1 temperature should be determined according to the amount of Al added. Until now, the fact that Al does not affect the Ar1 temperature has not been reported, and further studies are needed because the equation for obtaining Ar1 according to the amount of Al added is unknown.

To this end, the Ar1 transformation temperature was investigated according to Al and P contents, and the Ar1 transformation temperature was investigated according to Al and P contents in less than 0.01% of carbon steel containing 0.3 ~ 0.8% Mn. As a result, 130xAl [%] + 400xP + 860 It was found that there is a relationship of ℃, which is defined as A1 *.

According to the results of the present invention, when the hot finish rolling temperature is less than -55 ℃ from A1 *, the temperature is low, so that the deformed structure remains on the hot-rolled sheet and the workability is lowered. When the hot finish rolling temperature exceeds 35 ℃, hot rolling is made in the austenite region. Since enough crystals grow and then transform into ferrite, the hot rolled grain becomes coarse, making it difficult to secure the workability and strength of the cold rolled steel sheet. Therefore, the range of -55 ~ 35 ℃ at A1 * temperature of steel defined as 130xAl [%] + 400xP + 860 ℃ It is preferable to carry out hot finishing rolling by the furnace.

· Winding process

Hot rolling as described above, hot-rolled, the winding temperature at this time is preferably set to 530 ~ 700 ℃. If the coiling temperature is too low, it is not sufficient to form carbides such as TiC and NbC, so the lower limit is set to 530 ° C. If the coiling temperature is too high, the amount of grain boundary segregation and FeTiP formation increases, so the upper limit of the hot rolled coiling temperature is preferably 700 ° C. .

, Pickling, cold rolling

The hot oxide film formed on the hot rolled coil is completely removed by normal pickling conditions, cold rolled under normal rolling conditions, and when the cold rolling is completed, the rolled oil is removed through degreasing, followed by continuous annealing.

· Continuous Annealing Process

The continuous annealing of the cold rolled sheet is annealed at a temperature above Ac3 depending on the target strength level above the recrystallization temperature at which the deformed tissue formed by cold rolling is completely removed.

After continuous annealing, by temper rolling in a conventional manner to impart roughness and smoothness, it is possible to produce a cold rolled steel sheet of a high strength cold rolled steel sheet having extremely excellent press formability and hardening after coating heat treatment. The cold rolled steel sheet produced by the present invention has an excellent tensile strength of 30 to 40 kgf / mm2, which is excellent in strength and formability, and has a small part hardening ability of 3 kgf / mm ² or more and a r value of 2.0 or more. Since it has a hardenability, there is an advantage that it can be applied to deep processing parts such as inner and outer shells of automobile bodies.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

Inventive steels (a ~ d) and comparative steels (ma ~ ta) having the chemical components shown in Table 1 are melted in vacuum and then cast into ingots, or melted in converters and continuously cast, followed by temperatures of 1200 to 1230 ° C. After reheating and rolling at 1050 ℃, the rolling temperature was 870 ~ 930, the hot rolling temperature was 520 ~ 720 ℃. The thickness of the hot rolled sheet or coil was 2.8mm to 3.0mm, and the high temperature oxide film was removed by pickling and then rolled at a cold reduction rate of 75-80% so as to have a thickness of 0.7-0.8mm. After degreasing, continuous annealing was performed at 830 ° C. for 30 seconds, and rough rolling was performed at about 0.8% to prepare a cold rolled steel sheet.

In Table 1, the inventive steel satisfies the composition of the present invention, the comparative steel 'ma' is a steel C and N is outside the scope of the present invention, the comparative steel 'bar' is Mn, 'sa' is Ti and P, ' Ah 'is S,' car 'is Al,' ta 'is Nb + Mo + B contents of steels beyond the scope of the present invention. Meanwhile, 'za' is less than Al, and 'car' is less than Ti while Al is less than the present invention.

The steels of Table 1 were subjected to the same manufacturing conditions as in Table 2, and evaluated for tensile strength and plastic anisotropy. Here plastic anisotropy in the rolling direction (r _0), 45 degree direction relative to the rolling direction (r _45), obtained the r value by measuring the tension before and after the width variation of 15% with respect to the width direction (r ₉₀₎ and then (r ₀ The formula of + 2r ₄₅ + r ₉₀ ) / 4 was applied.

As shown in Table 2, the production method 1 to 5 according to the present invention has a r value of 2.0 or more, and the tensile strength is 32 to 38kgf / mm ^{2 It} can be seen that excellent workability and sufficient strength can also be obtained. All of these steels had a quench hardening capacity of 332 to 38 kgf / mm ² or more.

On the other hand, even the invention steel, if the production method is not satisfied, it is difficult to secure the tensile strength and r value, the manufacturing method 6 is a hot rolling temperature is too low, r value is significantly reduced. In addition, manufacturing method 7 having a high hot rolling temperature has low strength due to coarsening of hot rolled grains and decreases r feeling due to FeTiP formation, thereby failing to satisfy the reference value of 2.0.

When the steel sheet with low Mn and P content is hot-rolled at a high temperature as in Preparation Method 8, the r value is low and the strength is low due to the coarsening of crystal grains and the low content of solid solution. When steel and steel with sufficient P and Al contents are hot-rolled at a relatively low temperature as in Preparation Method 10 and hot-rolled at a temperature lower than the range of the present invention, the strength is sufficiently secured, but sufficient precipitation of carbide does not occur, so the r value is the target. It does not satisfy the range.

In the manufacturing method 11, like the manufacturing method 6, hot rolling is performed at a very low temperature, so that the deformed structure remains severely on the hot-rolled sheet, and even though the strength is quite high, the r value is extremely low, so that desired press workability cannot be obtained.

Manufacturing methods 12-19 apply the manufacturing method of this invention to a comparative steel. Even if the production method proposed in the present invention is applied, it is difficult to obtain a desired r value.

On the other hand, the steel grade 'car' has a relatively high P content and a very high Al content of 0.9%. In this case, since the temperature of Ar1 is very high, the hot rolling temperature is very low and the value of r is very low. In order to secure the steel grade r value, hot rolling must be made higher than 955 ° C, which is a temperature difficult to be secured in a normal hot rolling mill.

As described above, the present invention can produce a high strength cold rolled steel sheet having a tensile strength of 30 ~ 40kgf / mm2 excellent in press workability of 3kgf / mm ² or more, hardening hardening ability of 3kgf / mm ² or more, press-formed and painted heat treatment The present invention can be usefully applied to parts such as inner and outer panels of a vehicle body.

Claims (1)

By weight%, C: 0.01% or less, N: 0.005% or less, S: 0.015% or less, Mn: 0.3-0.8%, P: 0.03-0.09%, acid value Al: 0.1-0.7%, Ti: less than 0.15%, One or two or more selected from the group of Nb, Mo, and B: 0.015% or less, the sum of Ti and Nb content: 0.005% or more, and finishing by reheating the slab composed of the remaining Fe and other inevitable impurities The rolling temperature (FDT) is hot rolled under the conditions satisfying the following relationship A1 * -55 ° C≤FDT≤A1 * + 35 ° C (where A1 * is 130xAl [%] + 400xP + 860 ° C), A method for producing a high strength cold rolled steel sheet having excellent press formability, comprising winding in the range of 700 ° C., followed by pickling, cold rolling, and continuous annealing at a temperature range of recrystallization temperature to Ac 3.