KR100504369B1 - Low carbon cold rolled steel sheets and its manufacturing method having low plastic deformation and anisotropy index - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet having a low plastic strain ratio anisotropy coefficient and uniform thickness reduction in each direction during deformation and excellent stretchability. The weight ratio is C: 0.010 to 0.050%, Mn: 0.10% or less, and Ti: 0.002 to 0.030%. , P: 0.020% or less, S: 0.015% or less, Si: 0.015% or less, Sol. Al: 0.04% or less, N: 0.004% or less, the remainder is composed of Fe and other impurities, wherein the Ti content range is

48/12 × N [%] <Ti [%] <48/32 × S [%] + 48/12 × N [%],

After reheating the steel of the component at 1050 ~ 1300 ℃ hot rolling finish temperature is 890 ~ 940 ℃, coiling temperature is 660 ~ 750 ℃ hot rolling step and the hot rolled plate of 65 ~ 90% of the cold rolling process Cold rolling step at a reduction rate, the heat treatment step of continuously annealing the cold rolled steel sheet at 840 ~ 870 ℃, the continuous annealing cold rolled steel sheet is a temper rolling step of 0.5 ~ 2.0%, and the tempered rolled steel sheet 20 ~ 100 It provides a method for producing a low carbon cold rolled steel sheet comprising a step of quenching at a cooling rate ℃ / s, and 350 ~ 500 ℃ overaging heat treatment step.

Description

Low carbon cold rolled steel sheets and its manufacturing method having low plastic deformation and anisotropy index

The present invention relates to a cold rolled steel sheet having a low plastic strain ratio anisotropy coefficient, and having a uniform thickness reduction rate in each direction during stretching, and excellent stretchability, and a method of manufacturing the same. It relates to a manufacturing method.

In general, automotive inner and outer plate materials manufactured by press molding require cold rolled steel sheets having excellent workability in order to smoothly manufacture parts of a desired shape without defects until molding is completed.

Here, the plastic strain ratio r value representing the degree of workability is a value representing the strain in the tensile direction with respect to the strain in the thickness direction, and is a value representing the rate of change in thickness when the sheet is tensioned by a certain amount. The large plastic strain ratio means that the thickness decreases when the sheet is tensioned, so that the necking does not occur until the large deformation. After all, in order to improve the workability of the plate, the elongation and plastic deformation ratio should be increased.

Along with these properties, the plastic strain ratio anisotropy is an important factor that indicates the degree of workability. The plastic strain ratio is a value representing the ratio of the strain in the width direction to the strain in the thickness direction when the material is uniaxially pulled. When this value is large, the thickness decreases even after large molding, so that the workability is improved. This value depends on which direction the uniaxial tension is taken, and the plastic strain ratio anisotropy coefficient indicates the degree of plastic strain ratio that varies in each direction. In general, the plastic strain ratio is measured by measuring r ₀ , r ₄₅ , and r ₉₀ of each sheet and substituting the following equation to obtain the anisotropy coefficient (Δr) of the plastic strain ratio.

_{Δr = (r 0 + r 90} - 2r 45) / 2

Here, r ₀ , r ₄₅ , and r ₉₀ represent the plastic strain ratio values when the tensile direction is 0 °, 45 °, and 90 ° with respect to the rolling direction of the sheet, respectively. The small plastic strain ratio anisotropy coefficient Δr means that the reduction in thickness is constant according to the deformation in each direction, and this means that the steel sheet has a uniform distribution in each direction. Since the occurrence of necking is suppressed when it is uniformly deformed in each direction, the stretchability is improved.

Conventionally, in order to improve the formability of automotive steel sheets, IF steel has been developed by adding components capable of depositing solid solution C and N in ultra low carbon steel. Cryogenic carbon steel refers to steel grades with low carbon content. In order to manufacture ultra-low carbon steel, RH work for a long time should be performed in steelmaking process. Since Ti or Nb is added to steel with minimal carbon content to precipitate solid solution C and N, there is no solid solution in the steel, so workability is improved, but secondary workability is inferior and there is no BH (Bake Hardening). . When the primary processed steel sheet is processed again in the second process, fracture occurs as secondary processing brittle fracture, which occurs when the grain boundary is weak due to the small amount of solid solution in the steel. In addition, the BH property is to increase the yield strength by suppressing the movement of dislocations while the solid element in the steel is combined with the dislocations in the steel generated during molding when the steel sheet is heated during the automobile coating process. In order to be used as an automotive part, the steel sheet requires excellent secondary brittleness and BH. However, since there are few employment elements in the ultra low carbon steel, secondary processing brittleness is inferior and BH property does not appear. On the other hand, since the solid carbon exists in the low carbon steel containing more than the amount of solid solution C which Fe ₃ C carbide can precipitate, it shows excellent secondary brittle brittleness and BH property. However, the conventional low carbon steel is inferior in workability because the plastic deformation ratio is small and the plastic strain ratio anisotropy coefficient is large.

In order to solve this problem, an isotropic steel sheet having low plastic strain ratio anisotropy coefficient was developed by using an annealing method. This steel sheet was able to improve the stretchability by lowering the plastic strain ratio anisotropy coefficient. However, since the average plastic deformation ratio is very low as 1, there is a problem in that the deep drawing property is poor, and the productivity of low productivity and high production cost must be used.

The present invention has been made to solve the above problems, low plastic strain ratio anisotropy coefficient, excellent secondary processing brittleness, low carbon cold rolled steel sheet having good BH properties and a production method capable of producing by continuous continuous annealing method The purpose is to provide.

In order to achieve the above object, the present invention provides a low carbon cold rolled steel sheet having a low plastic deformation ratio anisotropy coefficient and a method of manufacturing the same.

C: 0.010 to 0.050% by weight ratio, Mn: 0.10% or less, Ti: 0.002 to 0.030%, P: 0.020% or less, S: 0.015% or less, Si: 0.015% or less, Sol. Al: 0.04% or less, N: 0.004% or less, the remainder is composed of Fe and other impurities, wherein the Ti content range is

48/12 × N [%] <Ti [%] <48/32 × S [%] + 48/12 × N [%],

After reheating the steel of the component at 1050 ~ 1300 ℃ hot rolling finish temperature is 890 ~ 940 ℃, coiling temperature is 660 ~ 750 ℃ hot rolling step and the hot rolled plate of 65 ~ 90% of the cold rolling process Cold rolling step at a reduction rate, the heat treatment step of continuously annealing the cold rolled steel sheet at 840 ~ 870 ℃, the continuous annealing cold rolled steel sheet is a temper rolling step of 0.5 ~ 2.0%, and the tempered rolled steel sheet 20 ~ 100 It provides a method for producing a low carbon cold rolled steel sheet comprising a step of quenching at a cooling rate ℃ / s, and 350 ~ 500 ℃ overaging heat treatment step.

Hereinafter, the present invention will be described in detail.

In order to secure the formability of low carbon steel sheet, solid solution carbon in steel should be minimized. In order to minimize the solid solution carbon of the low carbon steel sheet, Fe ₃ C should be formed as much as possible.

If carbon (C) is less than 0.010%, Fe ₃ C is difficult to form in steel, so C should be added at least 0.010%. If the content of C exceeds 0.050%, since a large amount of pearlite is present, the strength is greatly increased and the elongation is lowered. Therefore, the content of C is made 0.050% or less.

In low carbon steels, Mn is an element added for the precipitation of S. The presence of solid solution S in the steel causes surface cracks due to red brittleness during hot rolling of the steel. For this purpose, in the conventional low carbon steel sheet, Mn was added in an amount of 0.10% to 0.25% to precipitate solid solution S as MnS. However, when a large amount of Mn is added, Mn-C dipole, which is a solid solution of Mn and solid solution C in steel, forms. When the Mn-C dipole is formed, the movement of the solid solution C becomes difficult, so that the precipitation of the solid solution C to Fe ₃ C is suppressed, and the {111} aggregate structure is suppressed when the rolled plate is recrystallized in the annealing process, thereby reducing the plastic strain ratio. Anisotropy coefficient increases This is because the value of r ₄₅ is greatly decreased due to the development of other tissues other than the {111} tissue.

As a result, when Mn is large in low carbon steel, workability worsens. In the present invention, it was intended to precipitate S while reducing as much as possible the addition of Mn to inhibit workability in low carbon steel sheet. Minimization of Mn to 0.10% or less ensures workability and the residual S present in a small amount of Mn is intended to precipitate as Ti. The added Ti element serves to precipitate a part of S, and S which is not precipitated by Ti is precipitated by Mn, so that red brittleness due to FeS formation does not occur. When FeS is formed, melting at high temperature causes cracking and brittleness to cause cracking, so the content of S in the present invention is minimized to 0.015% or less.

Another major reason for adding Ti in the present invention is to reduce the anisotropy coefficient by TiC precipitation. Low carbon cold rolled steel sheet is recrystallized in the heating zone of the continuous annealing process. At this time, many crystal grains of the? Fibrous texture having a <110> direction in the rolling direction are formed, and after recrystallization, the temperature increases further to grow the grains. In the steel of the present invention, TiC precipitates having a size of 2 to 3 nm or less exist in the steel, and the precipitates inhibit the movement of grain boundaries, thereby slowing the growth of grains. In addition, when the annealing temperature is increased, growth of other grains is suppressed, and only grains having a {111} plane with a rapid grain boundary moving speed grow to develop γ fibrous aggregates. Finally, in the steel sheet in which Ti is added to precipitate TiC, γ fibrous aggregates develop simultaneously with α fibrous aggregates. The steel sheet having γ-fiber aggregate has a high r value, but the texture of the actual annealed steel sheet is {554} <225>. Steel plate with this texture has high r value of 0 ° and 90 °, but low r value of 45 °. On the other hand, steel sheets having α-fibrous texture have low r values at 0 ° and 90 ° directions and high r values at 45 ° directions. For this reason, steel sheets in which α-fiber aggregates and γ-fiber aggregates simultaneously develop may exhibit low plastic strain ratio anisotropy coefficients in each direction.

In the present invention, Ti is added to precipitate an appropriate amount of TiC to control grain growth and texture, and the texture simultaneously develops α-fiber aggregate and γ-fiber aggregate, resulting in low plastic strain anisotropy. Steel plate was manufactured. However, too much TiC precipitate increases the recrystallization temperature and suppresses grain growth so that the elongation and r value decrease. For this reason, the content of Ti was limited to 0.002 to 0.030%.

Therefore, if the amount of TiC is too small, the effect of suppressing grain growth is small, and if the amount is too large, it is important to control the number of TiC precipitates because grains having the {111} plane are not grown. Since the precipitate of TiC depends on the content of N, S and Ti, the content of Ti capable of depositing appropriate TiC according to each component in the present invention is determined by the following equation.

48/12 × N [%] <Ti [%] <48/32 × S [%] + 48/12 × N [%]

The derivation process of this equation is as follows. Since the precipitation temperature of TiN is 1200 degreeC or more, it precipitates in the steelmaking process and the reheating process of hot rolling. Precipitating N and remaining Ti reacts with S and C in the hot rolling process to form a precipitate. For this reason, in order to precipitate TiC, Ti must be more than the amount which can precipitate N. In addition, if more Ti is added than the amount capable of depositing S, all the remaining Ti is precipitated by TiC and the number of TiCs is too large. Therefore, it is necessary to add less than the amount of Ti that can precipitate N and S. As such, when Ti was added, the total Ti was precipitated, and the Ti content was determined as shown in the above equation so that the remaining Ti was combined with S and C, respectively, to precipitate an appropriate amount of TiC.

If P is large, yield strength increases and springback phenomenon increases, so P should be kept below 0.020%.

If the Si content is too high, the yield strength is increased, so it is managed to 0.015% or less.

If N exists in solid solution, it has the effect of inhibiting the development of {111} aggregated structure and greatly increases the yield strength, thereby degrading the workability. Lower contents are advantageous. Particularly, when the content of N in the present invention is high, the content of Ti increases and the precipitation of TiN increases. If a large amount of precipitate is present, the yield strength increases and the elongation decreases. In the case of the steel of the present invention, when the content of N is 0.004% or less, a steel sheet having a stable material may be manufactured.

Al reacts with the oxygen present in the steel to make slag and lower the oxygen content in the steel. Oxygen in the steel reduces formability and should be removed with slag. In this process, Al remains in the steel. The remaining Al is precipitated as AlN to reduce the content of N. However, in the steel of the present invention, Ti is precipitated mainly with N instead of Al, so the content of Al is at least safe. If the content of Al is too high, Al is in solid solution, increasing strength and decreasing elongation. In order to prevent this, it is necessary to control the content of Al, and in the case of the present invention steel, since it does not have a large effect on the material when Sol.Al that can be dissolved in acid is 0.04% or less, it is recommended to manage the Sol.Al content to 0.04% or less desirable.

The steel melted with the above components is hot rolled, and the hot rolling is rolled in the austenitic state of the steel structure above the Ar ₃ temperature of the steel. Rolling in the ferrite station results in the formation of many {110} <001> structures, resulting in deterioration of final workability. In order to prevent this phenomenon, the finish temperature of hot rolling is limited to 880 ~ 950 ℃.

In addition, the coiling temperature of the coil after hot rolling is maintained at a high temperature to smoothly form carbide in the wound state to minimize the solid solution carbon. The minimization of solid solution carbon in this process is to develop the {111} recrystallized texture when rolling and annealing the hot rolled steel sheet. This is because {111} collective organization develops only when there are few employment elements during annealing. If the {111} aggregate structure is well developed, the plastic deformation ratio is increased, which is advantageous for the press working. For this reason, the coiling temperature of the hot rolled steel sheet was set at 660 to 750 ° C.

The hot rolled steel sheet cooled to room temperature is pickled, cold rolled and then annealed. During annealing, it is rolled at a large reduction ratio so that recrystallization is easily formed, and annealing is performed in a temperature range where {111} recrystallized texture is well developed. In the cold rolling process, rolling is carried out at a reduction ratio of 65 to 90%. During continuous annealing conditions, the crack zone temperature is 840 ~ 870 ℃. After the cracking zone, the steel sheet is subjected to quenching and overaging, which is a process of forming carbides and leaving a certain amount of solid carbon in the steel. Excess dissolved carbon exists in the river cooled at 20 ~ 100 ℃ / s in the quench zone, and most of it is precipitated as Fe ₃ C in the overageing zone and remains in the river with a certain amount of dissolved carbon. As such, the remaining solid carbon forms the BH of the steel. The overaging zone must maintain a temperature that precipitates carbides stably, so limit the overaging temperature to 350 ~ 500 ℃.

The yield point phenomenon occurs due to the solid solution element in the cold rolled plate heat-treated as described above. When molding automotive parts from steel plate with yield point phenomenon, there is a possibility of stretcher strain defect. Therefore, yield point phenomenon should be removed by temper rolling. If the rolling reduction rate of the temper rolling is small, the yield point phenomenon cannot be completely eliminated. If the yield point phenomenon is too large, the yield strength increases and the springback increases, so that the temper rolling must be carried out at an appropriate rolling rate. In the present invention, the yield point phenomenon was removed by performing rough rolling of 0.5% to 2.0%.

The following describes an embodiment of the present invention.

Steels of the components shown in Table 1 were dissolved and hot rolled. At the time of hot rolling, the reheating temperature was 1250 ℃, the finishing temperature was 910 ℃, and the winding temperature was 720 ℃. After removing the surface oxide layer of the hot rolled plate by pickling, 75% cold rolling was performed. The cold rolled steel sheet was heat treated in a continuous annealing furnace. During the heat treatment, the crack stage temperature was 850 ° C. and the holding time at the crack stage was 28 seconds. After-age band temperature was 400 degreeC, and temper rolling was carried out with the rolling reduction rate of 1.4%.

Table 2 shows the results of measuring the material properties of the cold rolled steel sheet manufactured by the components of Table 1.

The inventive steels (B1 to B10) exhibited equivalent properties in tensile strength, elongation, glossiness, and BH compared to the comparative materials (A1 to A10), but the plastic strain ratio anisotropy coefficient was much lower than that of the comparative materials. It can be seen that. Therefore, the steel of the present invention having a low plastic strain ratio anisotropy coefficient has a characteristic of uniform deformation and excellent stretchability due to a decrease in thickness variation in each direction during deformation.

1 and 2, the material change and texture of the steel (A1) without added special elements, the steel (A7) with B, the steel (A8) with Zr, and the inventive steel (B1) among the comparative materials. The change of was observed. In Figure 1 it is a result of observing the change in r value according to the annealing temperature of each comparative steel and the invention steel. When the annealing temperature is above 840 ° C, the Δr value, which is the plastic strain ratio anisotropy coefficient, of the invention steel is lowered to 0 or less, which is very low. This cause can be explained by the change in the texture of the steel sheet depending on the annealing temperature. On the other hand, the r value and the r value of the 45 degree azimuth can be seen that the invention steel is higher than the comparative steel.

FIG. 2 shows that the α-fibrous aggregates and the γ-fiber aggregates simultaneously develop in the inventive steel as a result of the change in the azimuth distribution function according to the annealing temperature of each steel sheet. Unlike the comparative steels, the development of α fibrous aggregates and γ fibrous aggregates in the inventive steel is due to the TiC precipitates of the inventive steel.

3 is a photograph showing the sizes of BN, MnS, (Zr, Mn) (N, S, O), TiS, TiN and TiC precipitates present in the comparative steel and the inventive steel. Compared with TiC, other precipitates are larger and smaller in number, but TiC is larger and smaller in size, which is effective in suppressing grain growth.

FIG. 4 is a photograph of TiC precipitates suppressing grain growth when the prepared invention steel (B1) is annealed at an annealing temperature of 800 ° C. FIG. TiC precipitate effectively inhibits grain movement when the annealing temperature is 800 ℃, but when the annealing temperature is higher than 840 ℃, the grains of {111} plane with high mobility are rapidly grown and γ-fibrous texture is developed. . On this principle, when TiC precipitates are precipitated under the conditions of the present invention, steel having low plastic strain ratio anisotropy coefficient can be produced.

According to the present invention as described above it is possible to manufacture a low-carbon steel sheet excellent in stretch properties due to a low plastic strain ratio anisotropy coefficient, and when processing the automotive parts using the steel of the present invention, due to the excellent workability, it is easy to process parts of complex shape can do. In addition, there is a feature that can improve the stability of automotive parts due to the excellent BH and secondary workability of low carbon steel sheet.

1 is a graph showing the change in plastic strain ratio according to the annealing temperature of the present invention.

2 is a view showing the azimuth distribution function of the steel annealed at each annealing temperature according to the present invention.

Figure 3 is a transmission electron micrograph showing the size of each precipitate in accordance with the present invention.

Figure 4 is a transmission electron micrograph showing the shape of the TiC precipitate according to the present invention inhibits the growth of grains.

Claims (3)

C: 0.010 to 0.050% by weight, Mn: 0.10% or less, Ti: 0.002 to 0.030%, P: 0.020% or less, S: 0.015% or less, Si: 0.015% or less, Sol. A low carbon cold rolled steel sheet having a low plastic deformation ratio anisotropy coefficient, wherein Al is 0.04% or less, N: 0.004% or less, and the remainder is made of Fe and other impurities. The method of claim 1, The content range of Ti is A low carbon cold rolled steel sheet having a low plastic strain ratio anisotropy coefficient determined by 48/12 × N [%] <Ti [%] <48/32 × S [%] + 48/12 × N [%]. C: 0.010 to 0.050% by weight, Mn: 0.10% or less, Ti: 0.002 to 0.030%, P: 0.020% or less, S: 0.015% or less, Si: 0.015% or less, Sol. Al: 0.04% or less, N: 0.004% or less, the remainder is composed of Fe and other impurities, and the content range of Ti is 48/12 × N [%] <Ti [%] <48/32 × S [%] Reheating the steel sheet determined by + 48/12 × N [%] at 1050-1300 ° C .; Hot rolling the heated steel sheet under conditions of a finishing temperature of 890 to 940 ° C and a winding temperature of 660 to 750 ° C; Cold rolling the hot rolled plate at a reduction ratio of 65 to 90%; Continuous annealing of the cold rolled steel sheet at 840 ~ 870 ℃; Tempering rolling the continuously annealed cold rolled steel sheet at a reduction ratio of 0.5 to 2.0%; Quenching the tempered rolled steel sheet at a cooling rate of 20 to 100 ° C./s; A low carbon cold rolled steel sheet having a low plastic strain ratio anisotropy coefficient, characterized in that it comprises an over-aging heat treatment step at 350 ~ 500 ℃.