KR100957976B1 - Cr-Nb Added Bake Hardenable Steel Sheet with Excellent Strain Aging Resistance and Manufacturing Method Thereof - Google Patents

Abstract

The present invention, in weight percent, C: 0.003% or less (excluding 0), Si: 0.01% to 0.1%, Al: 0.01% to 0.08%, P: 0.01% to 0.1%, S: 0.02% or less, other impurities and residues Fe, including Mn: 0.05-0.2%, Cu: 0.005-0.2% or Mn: 0.05-0.2% and Cu: 0.005-0.2%, including Cr: 0.05-3.0%, Nb: 30-80 ppm Or Cr: 0.05-3.0% and Nb: 30-80 ppm to provide MnS precipitated hardened steel, CuS precipitated hardened steel and CuMnS precipitated hardened steel. In this case, 0.58 * Mn / S is 10 or less relationship between Mn and S in MnS precipitation type hardened steel, and 0.5 * Cu / S is 1-10 between Cu and S in CuS precipitation type hardened steel. It is preferable that 0.5 * (Mn + Cu) / S satisfy | fills 2-20 relationship between phosphorus relationship and Cu, Mn, and S in CuMnS precipitation type hardened steel.

As in the present invention, when an appropriate amount of Cr and Nb is added to the calcined hardened steel to produce a calcined hardened steel having stable solid solution carbides, aging degradation of the calcined hardened steel can be suppressed by efficient solid solution stabilization of carbides.

Baking Hardened Steel, Aging Degradation, Carbide, Plated Steel Sheet, Hot Dip Galvanized

Description

Cr-Nb Added Bake Hardenable Steel Sheet with Excellent Strain Aging Resistance and Manufacturing Method Thereof}

The present invention relates to a hardening type MAFE high strength alloyed hot-dip galvanized steel sheet that can be used as an inner and outer shell of an automobile. More particularly, the present invention relates to strength over time due to solid solution carbon and solid solution nitrogen present in a steel sheet during steel sheet manufacture. The present invention relates to a method for manufacturing a steel sheet stabilized with solid solution C in order to suppress processing defects due to aging that increases hardness.

Recently, as environmental issues have emerged as social issues, research on technologies using high-strength hot dip galvanized steel sheets has been increasing in the automobile field in terms of automobile stability, light weight, and low fuel consumption. In particular, research and development on hardened hardened steel has been increasing as a steel grade that can satisfy this purpose, and particularly in the case of very low carbon hardened hardened steel (Bake Haredening Steel), aging defects that may occur when the coil is transported at a long distance ( Various efforts are underway to prevent Aging Defects.

The generation of aging defects in ultra low carbon steel has been studied in the past. Particularly, in case of hardened steel that does not precipitate solid solution C, when the amount of invasive solid solution elements such as carbon or nitrogen increases, it shows uneven yield behavior during tensile test, and a stretcher strain occurs during press molding. It can be difficult to use in the car body shell that the surface appearance is important is a problem. Therefore, there has conventionally been a technique to solve the above-mentioned problems by generating many movable potentials on the surface of the steel sheet by adding a special component capable of controlling the solid solution C or by performing appropriate temper rolling. However, even with this method, there is still a problem in that a deformation aging phenomenon occurs in which [C], [N], etc. move to a potential and the potential is fixed when time passes again after temper rolling.

Until the 1970s, aging defects were solved by batch-annealing low-carbon Al-Killed steels.However, in the 1980s, due to the introduction of CAL (Continuous Annealing Line) and the development of steelmaking technology, 2) The aging and formability problems were simultaneously solved by the development of IF steel, which alone or in combination, added Nb as either carbide or nitride.

In recent years, because of the rapid development of steelmaking field, the hardening steel has been developed due to the technology that can control the working carbon below 15ppm in the steelmaking sector, and this hardening steel is carbide by controlling the solid solution carbon below 30ppm in steelmaking. It is not necessary to add alloy elements such as Ti or Nb, and it is possible to produce plating products superior to IF steel when plating by reducing the amount of alloying elements such as Mn and Si which cause strip thickening. Therefore, such hardened hardened steel is easier to control aging than IF steel, and has the advantages of excellent surface products and less material deviation.

However, in the case of such hardened hardened steels, sulfide-based reinforcing mechanisms such as CuS and MnS are used instead of carbides, and thus have excellent surface quality unlike IF steels. However, the conventional method for suppressing aging is effective. It is a bad thing to reality. This is because, unlike the IF steel, in the hardened steel, the hardened carbon is controlled to 30ppm at the steelmaking stage and does not add an alloying element that precipitates carbides. In operations such as cold rolling, there is a need to suppress aging in new ways.

Accordingly, the present invention is to solve the above-described problems and to provide a steel material and a method of manufacturing the same which suppresses the aging deterioration due to the presence of the solid solution C in the hardened hardened steel surface surface is even better than IF steel.

In order to solve this problem, the present invention was intended to solve the aging deterioration by forming a carbide using an appropriate addition of Cr and Nb, by weight, C: 0.003% or less (excluding 0), Si: 0.01 ~ 0.1% , Mn: 0.05 to 0.2%, Al: 0.01 to 0.08%, P: 0.01 to 0.1%, S: 0.02% or less, containing other impurities and balance Fe, Cr: 0.05 to 3.0% and Nb: 30 to 80 ppm It provides an MnS precipitation type hardened steel characterized in that it further comprises one or two selected. In this case, it is preferable that a relationship of 0.58 * Mn / S is 10 or less between Mn and S.

Furthermore, in the present invention, by weight%, C: 0.003% or less (excluding 0), Si: 0.01% to 0.1%, Cu: 0.005% to 0.2%, Al: 0.01% to 0.08%, P: 0.01% to 0.1%, and S: 0.02% or less, other impurities and the balance Fe, and provides a CuS precipitation type hardened hardened steel, characterized in that it further comprises one or two selected from Cr: 0.05 ~ 3.0% and Nb: 30 ~ 80ppm. In this case, it is preferable that the relationship of 0.5 * Cu / S is 1-10 between said Cu and S.

Furthermore, in the present invention, by weight%, C: 0.003% or less (excluding 0), Si: 0.01 to 0.1%, Mn: 0.05 to 0.2%, Cu: 0.005 to 0.2%, Al: 0.01 to 0.1%, P: 0.01-0.1%, S: 0.02% or less, CuMnS precipitate type, characterized in that it contains other impurities and the balance Fe, and additionally comprises one or two selected from Cr: 0.05 ~ 3.0% and Nb: 30 ~ 80ppm In this case, it is preferable that 0.5 * (Mn + Cu) / S satisfy | fills the relationship of 2-20.

Further, the present invention, the steel slab consisting of the above components, the re-heating at 1100 ℃ or more and the hot rolling step of the finish rolling temperature is more than the Ar ₃ transformation temperature, the winding step wound up to 700 ℃ or less, with a reduction ratio of 50 ~ 90% It provides a method for producing a hardened hardened steel, characterized in that it comprises a cold rolling step of cold rolling and a continuous annealing step of continuous annealing at 500 ~ 900 ℃.

Furthermore, the present invention is a steel slab consisting of the above components, the re-heating at 1100 ℃ or more and the hot rolling step of the finish rolling temperature is more than the Ar ₃ transformation temperature, the winding step wound up to 700 ℃ or less, cold rolling at a reduction ratio of 50 ~ 90% Cold rolling step of rolling Provides a method for producing a hardened plated hardened steel sheet comprising a continuous annealing step of continuous annealing at 500 ~ 900 ℃ and a plating step of hot dip galvanizing in a hot dip plating line.

As described above, when the calcined hardened steel of the present invention is manufactured by adding an appropriate amount of Cr and Nb, aging deterioration of the calcined hardened steel can be suppressed by efficient solid solution stabilization of carbide.

Bevel hardened steel can improve workability by precipitating / removing to maintain an appropriate amount of solid solution C and N, but unlike IF steel whose surface quality is somewhat degraded due to the addition of a large amount of alloying elements such as Ti and Nb, The content of C at the time of tapping is controlled to 15 ppm or less without removing, and sulfide-based precipitation can be used to maintain proper strength.

In order to maintain the properties of the hardened hardened steel and to suppress aging defects, the present inventors have found that the amount of solid solution C can be stabilized by using a small amount of Cr and Nb elements without adding a large amount of alloying elements. And the present invention was completed according to this principle.

Hereinafter, the component system constituting the steel of the present invention will be described in detail.

C: 0.003% or less (excluding 0)

In the present invention, C is kept as low as possible below 0.003%. If C exceeds 0.003%, aging deterioration may occur and the BH property may deteriorate.

Si: 0.01 ~ 0.1%

Si is an element that has a good solid solution effect and low elongation, and thus, when used in a steel for controlling precipitates as in the present invention, high strength can be ensured, so that 0.01% or more is added. However, when the content of Si exceeds 0.1%, the plating quality may be drastically lowered. The upper limit is 0.1%.

Mn: 0.02-0.2%

Mn precipitates solid solution S in steel as MnS and functions as an element preventing hot shortness due to solid solution S. In particular, in the present invention, Mn is fine MnS and / or (Mn, Cu) S is precipitated when the ratio of S and / or Cu and cooling rate are appropriate. These fine precipitates segregate carbon around the grain boundaries or precipitates, rather than in the grains, to impart hardening hardening during the coating baking process. In the present invention, in order to exhibit such an effect, Mn is added at least 0.02%. However, if the content of Mn is excessively excessive and exceeds 0.2%, coarse precipitates may be generated and the hardening hardening characteristic may be lowered, so the Mn content is limited to 0.02 to 0.2%. In addition, in the present invention, when Mn is added alone without addition of Cu, it is more preferable to control the content of Mn to 0.05 to 0.2%.

Al: 0.01% to 0.1%

Al is an element to be added as a deoxidizer, but in the present invention, Al is added in an amount of 0.01% or more because precipitation of N in steel serves to prevent formability deterioration due to solid solution N. If the Al content is less than 0.01%, the amount of solid solution nitrogen will increase and the moldability will decrease. On the contrary, if the Al content exceeds 0.1% and is excessively excessive, the amount of Al present in the solid state will increase. Ductility may be reduced. However, in the CuS precipitated steel and MnCu precipitated steel without Mn addition, preferable Al content is about 0.01 to 0.08%. In addition, when the content of N in the present invention is high as 0.005 ~ 0.02% can obtain a strengthening effect by the AlN precipitates, it is possible to manufacture a high strength steel sheet.

P: 0.01 ~ 0.1%

P is an element having a high solid solution strengthening effect and a small decrease in the r (plasticity anisotropy index) value, which guarantees high strength in steels that control precipitates. Therefore, in order to secure the high strength by P, it is preferable to add P content 0.01% or more. However, when the content of P is excessively excessively more than 0.1%, the ductility may be lowered.

S: 0.02% or less, 0.58 * Mn / S: 10 or less, 0.5 * Cu / S: 1-10

The content of S is preferably added to 0.02% or less for precipitation strengthening, but basically needs to vary depending on the type of precipitation steel.

First, in the case of MnS precipitated steel, it is preferable to add S so that the weight ratio of Mn and S satisfies 0.58 * Mn / S ≦ 10 (wherein Mn and S are by weight%). Mn is combined with S and precipitated as MnS, and the precipitated MnS precipitates vary depending on the amount of Mn and S added, thereby affecting the hardening hardening properties, yield strength, and in-plane anisotropy index. When the value of 0.58 * Mn / S exceeds 10, MnS precipitates are coarsened, so that the hardening hardening characteristic is lowered, which may lower the in-plane anisotropy index.

Moreover, it is preferable that the value of 0.5 * Cu / S (Cu, S is weight%) is 1-10 about CuS precipitation steel. Cu binds to S and precipitates as CuS. The CuS precipitates also affect the baking hardening property, plastic anisotropy index, and in-plane anisotropy index because the precipitated state varies depending on the amount of Cu and S added. In the present invention, the effective CuS precipitates may be precipitated when 0.5 * Cu / S is 1 or more, but when it exceeds 10, the CuS precipitates may be coarse to deteriorate the hardening property, plastic anisotropy index, and in-plane anisotropy index. More preferably, as a value for stably securing CuS of 0.1 µm or less, the value of 0.5 * Cu / S satisfies the range of 1-3.

Mn + Cu: 0.3% or less, 0.5 * (Mn + Cu) / S: 2-20

When Mn and Cu are added simultaneously, the sum of Mn and Cu is limited to 0.3% or less. When the sum of Mn and Cu exceeds 0.3%, the size of the precipitate may be too large to secure the baking hardening characteristics. In addition, the value of 0.5 * (Mn + Cu) / S (Mn, Cu and S are weight%) is limited to 2-20. This is also to control the (Mn, Cu) S precipitates so that the precipitates can improve the baking hardening characteristics, plastic anisotropy index, in-plane anisotropy index and the like as described above. When 0.5 * (Mn + Cu) / S is 2 or more, an effective precipitate can be obtained. On the other hand, when it exceeds 20, the precipitate is coarse, which may cause degradation of the hardening property, plastic anisotropy index, and in-plane anisotropy index.

In particular, in the present invention, when the ratio of 0.5 * (Mn + Cu) / S satisfies the range of 2 ~ 20, the average size of the precipitate can be finened to 0.1 ㎛ or less. In this case, the precipitate is preferably distributed in 2x10 ⁶ or more. In particular, the ratio of 0.5 * (Mn + Cu) / S is significantly different from the starting point of 7, and the number of distributions thereof is significantly different. Single precipitates of MnS and CuS, which are very fine than the composite precipitates of, are uniformly distributed.On the other hand, when the ratio is larger than 7, the number of complex precipitates of (Mn, Cu) S increases, so that the number of distribution decreases even though the size difference of the precipitates is small. It is characteristic.

Cu: 0.005 ~ 0.2%

In the present invention, Cu forms fine precipitates when the content ratio with S and / or Mn and the cooling rate before winding in the hot rolling process are appropriate. These fine precipitates segregate carbon around the grain boundaries or precipitates rather than in the grains, thus providing hardening hardening during the coating baking process. In the present invention, the Cu content is added 0.005% or more in order to exert such effects. However, when the Cu content exceeds 0.2%, the precipitates are coarsened, which may degrade the baking hardening characteristic. In addition, in the present invention, when Cu is added alone without adding Mn, the content of Cu is preferably controlled to 0.01 to 0.2%.

Cr: 0.05-3.0%

Cr is an element that improves room temperature aging by Cr-based carbides while having a high solid-solution strengthening effect, and is an important element that can secure high strength and lower in-plane anisotropy index in steels controlling precipitates as in the present invention. In the present invention, the Cr content is more than 0.05% to obtain this effect, it can be seen in room temperature aging. However, if the amount of Cr added is too high to exceed 3.0%, the production cost may increase and the baking hardening characteristic may be deteriorated.

Nb: 30 ~ 80ppm

Nb is a strong alloying element that forms carbide together with Ti, so it should be added more than 30ppm because it functions as an important element to secure BH property in the present invention. The disadvantage of having to work up can arise. Therefore, by adding Nb at 30 to 80ppm, several ppm of solid solution carbon is precipitated as a carbide to improve aging resistance and attain satisfactory BH.

Hereinafter, a method of manufacturing the steel of the present invention will be described in detail.

Hot rolling condition: Reheat above 1100 ℃, finish rolling temperature is Ar ₃ Abnormal transformation temperature

In the present invention, the steel that satisfies the above-mentioned emphasis is reheated and hot rolled. The reheating temperature is controlled at 1100 ℃ or higher. If the reheating temperature is lower than 1100 ℃, the coarse precipitates produced during continuous casting may remain completely insoluble and many coarse precipitates remain after hot rolling. Can be.

In addition, the hot rolling is a finish rolling temperature of Ar ₃ Carry out under the transformation temperature. Finish rolling temperature is Ar ₃ If the temperature is lower than the transformation temperature, the workability and ductility of the rolled grain may be greatly reduced.

Winding Condition: Below 700 ℃

As described above, after performing hot rolling, winding is performed, and the winding temperature is preferably 700 ° C. or lower. If the coiling temperature exceeds 700 ℃, precipitates may grow coarse to degrade the baking hardening characteristics.

Cold Rolling Condition: 50 ~ 90% Rolling Rate

Cold rolling is rolled to the required thickness, in the present invention is carried out at a reduction ratio of 50 ~ 90%. If the cold reduction rate is less than 50%, the amount of nucleation of the annealing material crystals may be reduced so that the grains may grow too large during annealing, thereby decreasing the strength and formability due to coarsening of the annealing recrystallized grains. On the other hand, if the cold reduction rate exceeds 90%, the moldability is improved, but the amount of nucleation is too large, so the annealing recrystallized grains become too fine and the ductility may be reduced, so that the reduction rate is controlled to 50 to 90%.

Continuous Annealing: 500 ~ 900 ℃

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 500-900 degreeC. If the continuous annealing temperature is lower than 500 ° C, the recrystallized grain becomes too fine to obtain a target ductility value. If the annealing temperature exceeds 900 ° C, the strength may decrease due to coarsening of the recrystallized grain. have. On the other hand, the continuous annealing time is maintained to complete the recrystallization, if the recrystallization is completed in about 10 seconds or more.

Plating: Hot dip galvanized

Hot-dip plating of the steel sheet is carried out by hot dip galvanizing in a plating solution containing Al. In this case, the amount of Al contained in the plating solution for GA alloying should be 0.1% or more. However, if the content exceeds 0.3%, foreign matter may be generated on the plating surface, so the upper limit thereof is limited to 0.3%.

The present invention will be described in more detail with reference to the following examples.

Cr: 0.5%, 1% and 3%, and Nb were added in 30 ppm and 60 ppm, respectively, to inventive steels 1 to 5 in the conventional CuMnS precipitated hardened steel. Cr and Nb were not added to the comparative steel which is a baking hardened steel. (Table 1) was then subjected to annealing, SPM (based on 1.4% of Tenm Roll) in the same operation method applied to the comparative steel (see Fig. 1). The aging evaluation method used a tensile test, AI (Aging index) and BH (Bake hardenability) test (Figs. 2, 3 and 4).

C Mn Si P S Al Cu N Cr Nb Remarks 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - - Comparative steel 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - 0.003 Inventive Steel 1 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - 0.006 Inventive Steel 2 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 0.5 0.003 Invention Steel 3 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 One 0.003 Inventive Steel 4 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 3 0.003 Inventive Steel 5

The AI (Aging index) test is to measure the existence of the yield point elongation through the tensile test after heat-treating the existing materials and inventions in 100 ℃ water for 2 hours, a condition that can guarantee about 6 months of aging at room temperature (Fig. 2). And 3). In addition, the BH (Bake hardenability) test was performed by prestraining 2% using a tensile tester on the comparative steel and each invention steel, and then heat-treating the oil at 170 ° C. for 20 minutes to measure the YP value difference before and after the heat treatment. It was done in a way (Fig. 4).

Tensile test and accelerated aging test of the hardened steel with a small amount of Cr and Nb added to stabilize the solid solution C showed that the yield point elongation did not occur in the YP-El curve. And the amount of addition) could be confirmed. These results are shown in Table 2 below.

Alloy element addition amount YP-EL occurrence UP-BH value LO-BH value 0.2% Off-Set Mpa TS Cr (%) Nb (ppm) Mpa Inventive Steel 1 0 30 Not Occurred 44.78 36.20 233.02 360.13 Inventive Steel 2 0 60 Not Occurred 44.2 35.37 231.02 353.80 Invention Steel 3 0.5 30 Not Occurred 42.48 36.44 253.05 365.17 Inventive Steel 4 One 30 Not Occurred 38.34 33.65 255.34 374.91 Inventive Steel 5 3 30 Not Occurred 34.56 31.52 272.02 389.00

Looking at the Table 2, the maximum tensile strength was hardly changed as the Nb addition amount increased, but when looking at the invention material fixed Nb to 30ppm, the maximum tensile strength increases and the BH value decreases as the amount of Cr is increased. It can be seen that (Table 2 and Figure 5). The BH values of all the inventive materials were measured to be at least 30 Mpa as well as the UP-BH value (difference in the upper yield strength before and after the heat treatment) as well as the LO-BH value (difference in lower yield strength before and after the heat treatment). Furthermore, it can be seen from FIG. 5 that the yield strength is also increased when Cr is added in the invention having Nb fixed at 30 ppm. Therefore, it can be seen that due to the addition of Cr and Nb, both tensile strength, yield strength and BH value can be satisfied, thereby optimizing the suppression force against aging deformation.

1 is a view showing an annealing heat cycle of the hardened hardened steel

2 is a graph showing the yield point elongation after 100 ° C 2h accelerated aging

3 is a graph showing no yield point elongation after 100 ° C 2h accelerated aging

4 is a graph relating to 170 ° C. 20min BH measurement.

5 is a graph showing the change in yield strength and tensile strength according to Cr content

Claims (10)

delete delete By weight%, C: 0.003% or less (excluding 0), Si: 0.01 to 0.1%, Cu: 0.005 to 0.2%, Al: 0.01 to 0.08%, P: 0.01 to 0.1%, S: 0.02% or less, etc. Impurity and balance Fe, Sinter hardened steel, characterized in that it further comprises one or two selected from Cr: 0.05 ~ 3.0% and Nb: 30 ~ 80ppm. The bake hardened steel according to claim 3, wherein the Cu and S have 0.5 * Cu / S of 1 to 10. By weight%, C: 0.003% or less (excluding 0), Si: 0.01-0.1%, Mn: 0.05-0.2%, Cu: 0.005-0.2%, Al: 0.01-0.1%, P: 0.01-0.1%, S: 0.02% or less, containing other impurities and balance Fe, It further comprises one or two selected from Cr: 0.05 ~ 3.0% and Nb: 30 ~ 80ppm, and includes (Mn, Cu) S precipitates, the size of the (Mn, Cu) S precipitates are 0.1㎛ or less The hardened hardened steel characterized by the above-mentioned. The bake hardened steel according to claim 5, wherein Cu, Mn, and S have 0.5 * (Mn + Cu) / S of 2 to 20. The baking hardening steel according to claim 5 or 6, wherein the sum of Cu and Mn is 0.3% or less. For steel slabs consisting of the components of claim 3 or 5, Reheating at 1100 ° C. or higher and finishing rolling temperature of at least Ar ₃ transformation temperature; A winding step of winding up at 700 ° C. or lower; Cold rolling step of cold rolling at a reduction ratio of 50 to 90%; And Continuous annealing step of continuous annealing at 500 ~ 900 ℃; Method for producing a sinter hardened steel comprising a. For steel slabs consisting of the components of claim 3 or 5, Reheating at 1100 ° C. or higher and finishing rolling temperature of at least Ar ₃ transformation temperature; A winding step of winding up at 700 ° C. or lower; Cold rolling step of cold rolling at a reduction ratio of 50 to 90%; Continuous annealing step of continuous annealing at 500 ~ 900 ℃; And A plating step of hot dip galvanizing in a hot dip plating line; Method for producing a hardened plated hardened steel sheet comprising a. 10. The method of claim 9, wherein the hot dip galvanizing is performed in a plating solution containing 0.12 to 0.2% of Al.

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