KR100530057B1 - Method for Manufacturing Cold Rolled Steel Sheet with Superior Workability and Secondary Working Embrittlement Resistance - Google Patents

Abstract

The present invention relates to a method for manufacturing a cold rolled steel sheet used in automobile panels and structural parts, etc., in the method of manufacturing a high strength cold rolled steel sheet using Ti or Nb-added ultra low carbon steel, Mn, Al, B, etc. It is to provide a high strength cold rolled steel sheet excellent in workability and secondary processing brittleness by addition, the purpose is.

The present invention contains C: 0.0005-0.005%, Mn: 0.5-2.0%, S: 0.005% or less, N: 0.003% or less, acid soluble Al: 0.1-0.2%, B: 0.0005-0.003% by weight. One or two of Ti: 0.01-0.10% and Nb: 0.001-0.10% are added to the steel, and hot-rolling is performed on the condition that hot-rolling finishes at a temperature of ₃ or more Ar in the steel composed of the balance Fe and unavoidable impurities. After winding, it was wound at 620-720 ° C, cold rolled at 60-90% reduction rate, and recrystallized annealing at a temperature of 780-880 ° C for 10 seconds to 180 seconds by a continuous annealing method to obtain workability and secondary resistance. A method of producing a cold rolled steel sheet excellent in workability and brittleness is the main point.

Description

Method for Manufacturing Cold Rolled Steel Sheet with Superior Workability and Secondary Working Embrittlement Resistance}

The present invention relates to a method for manufacturing a cold rolled steel sheet used in automobile panels, structural parts, and the like, and more particularly, to a method for producing a tensile strength 32-40kgf / mm grade ² high strength cold rolled steel sheet having excellent workability and secondary workability. will be.

Conventionally, the development of a high strength cold rolled steel sheet to increase the strength by adding P, Si, Mn, Cr, etc. as a basic composition of the steel to which the Ti, Nb, etc. are added to the ultra low carbon steel.

For example, Japanese Special Office 57-57945 suggests a cold rolled steel sheet containing a large amount of P added to Ti, and Japanese Special Office 60-47328 shows a high strength cold rolled steel added P to Ti and Nb added ultra low carbon steel. A steel sheet is presented, and Japanese Patent Application Laid-Open No. 63-100158 presents a high strength cold rolled steel sheet mainly containing Si in Ti-added ultra low carbon steel.

However, when P is added to ultra-low carbon steel to which carbide forming elements such as Ti or Nb are added, as in the cold rolled steel sheets described above, secondary processing brittleness due to grain boundary segregation becomes a problem.

Therefore, it is common when P is not added or B is added.

For example, Mn is mainly used as a solid solution strengthening element, as in Japanese Patent Application Laid-Open No. 58-29129, Japanese Patent Laid-Open No. 5-78784, or B is added as in Japanese Patent Application Laid-Open No. 658-48633.

However, in the case of these methods, the improvement of the secondary work brittleness problem depends on the content of P and the effect of improvement is high in the high P content or hot dip galvanized steel sheet.

Recently, in order to reduce the brittleness caused by P as in Japanese Patent Laid-Open No. 6-172871 and Japanese Patent Laid-Open No. 10-265898, instead of lowering the P content, the carbon content was increased to 0.005-0.015% to compensate for the decrease in strength. A large amount of precipitated strength is produced,

However, this steel has to deposit all the carbon in the steel with TiC in order to obtain drawing, but because the carbon content is high, the amount of Ti required increases, which leads to problems of nozzle blockage and manufacturing cost in slab production. have.

The present inventors have conducted research and experiments to solve the above problems of the prior art, and based on the results, the present invention proposes a high strength cold rolled steel sheet using Ti or Nb-added ultra low carbon steel. In order to provide a high-strength cold-rolled steel sheet excellent in workability and secondary processing brittleness by adding Mn, Al, B, etc. instead of the conventional P addition, the purpose is to.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention contains C: 0.0005-0.005%, Mn: 0.5-2.0%, S: 0.005% or less, N: 0.003% or less, acid soluble Al: 0.1-0.2%, B: 0.0005-0.003% by weight. One or two of Ti: 0.01-0.10% and Nb: 0.001-0.10% are added to the steel, and hot-rolling is performed on the condition that hot-rolling finishes at a temperature of ₃ or more Ar in the steel composed of the balance Fe and unavoidable impurities. After winding, it was wound at 620-720 ° C, cold rolled at 60-90% reduction rate, and recrystallized annealing at a temperature of 780-880 ° C for 10 seconds to 180 seconds by a continuous annealing method to obtain workability and secondary resistance. It relates to a method for producing a cold rolled steel sheet excellent in workability brittleness.

In addition, the present invention relates to a method for producing a cold rolled steel sheet excellent in workability and secondary workability resistance to the addition of 0.02-0.2% by weight of Mo to the cold rolled steel sheet described above.

Hereinafter, the chemical composition and the manufacturing conditions of the steel according to the present invention will be described in detail. The present invention adds manganese [Mn] to ultra-low carbon aluminum-kilted (Al-killed) steel to secure ductility and tensile strength at the same time. Ti and Niobium [Nb] were added to improve the drawability in steel and segregation of aluminum [Al] and boron [B] at the grain boundaries to improve secondary workability.

In addition, in the present invention, excellent hot dip galvanizing properties can be ensured.

The carbon in steel [C] is preferably added as little as possible because it suppresses the development of the (111) aggregate structure of the cold rolled steel sheet and reduces the drawing property. If the lower limit is too low, the manufacturing cost increases, so the lower limit is It is preferable to limit the content to 0.0005%. If the content is more than 0.005%, carbides are excessively increased to increase the recrystallization temperature during the annealing process, thereby inhibiting the growth of grains having the (111) aggregate structure, thereby degrading the drawing property. It is preferable to limit the upper limit to 0.005%.

The manganese [Mn] is an element that improves strength by solid solution strengthening, the content is less than 0.5%, the strength increase effect is small, when more than 2.0% by making the steel structure of the needle-like ferrite to reduce the workability, the manganese The content of is preferably limited to 0.5-2.0%.

When the sulfur [S] content is more than 0.005%, coarse TiS and MnS are formed in the hot rolled sheet and fine Ti ₄ C ₂ S ₂ decreases to decrease workability, so that the content is limited to 0.005% or less. desirable.

Since the nitrogen [N] is combined with Ti in the steel to reduce the ductility and drawability, the content is preferably limited to 0.003% or less.

The acid soluble aluminum [Al] is segregated at the grain boundary in the high temperature zone to refine the hot-rolled sheet grains, and after the annealing recrystallization, it promotes grain growth and improves workability, so that it is added in an amount larger than the amount to be used for deoxidation purposes. In the case of 0.1% or less, the addition effect is small, and in the case of 0.2% or more, the elongation is sharply lowered and the alloy cost is excessively increased. Therefore, the content is preferably limited to 0.1-0.2%.

The boron [B] refines the grains, increases the grain strength, improves secondary work brittleness, increases the recrystallization temperature, increases the strength, but when excessively added, the recrystallization temperature is excessively increased, so the content is 0.0005-0.003. It is desirable to limit to%.

Titanium [Ti] is added to the steel in accordance with the amount of nitrogen, carbon or Mn in the steel to improve the processability, especially when excess titanium is present because it increases the drawability, when the content is less than 0.01% Insufficient to combine with these elements, if the content is more than 0.10%, excessive amount of titanium oxide is formed in the slab manufacturing, clogging the nozzle, which reduces productivity and excessively increases the input cost, so the content is 0.01-0.10%. It is desirable to limit.

The niobium [Nb] is delayed recrystallization in the hot rolling process and precipitated in the form of [Ti, Nb] C in the winding process to refine the grains of the hot rolled sheet to increase the drawability in the rolling direction and 45 degree direction However, if the addition amount is large, since the elongation is lowered, the content is preferably limited to 0.001-0.1%.

The molybdenum [Mo] is a composite precipitate with TiC or NbC in the cooling process after hot rolling to refine the grain size of the hot rolled sheet, and some solid solution Mo is added to increase the strength and to improve the secondary workability due to grain boundary segregation of P as If the content is less than 0.02%, the effect of addition is less. If the content is more than 0.2%, the workability is lowered and the cost is increased. Therefore, the content is preferably limited to 0.02-0.2%.

According to the present invention, when the hot finish rolling temperature during the hot rolling is lower than Ar ₃ , the drawing property is degraded due to surface coarse grains caused by strain annealing, and thus the hot finish rolling temperature is set to Ar ₃ or more. It is desirable to.

When the coiling temperature is less than 620 ℃, needle-like ferrite is generated according to the addition of Mn, B, the workability is lowered, and if the temperature is 720 ℃ or more, drawing ability in the rolling direction and 45 degrees direction decreases due to excessive grain growth and strength Since the lowering temperature, the winding temperature is preferably limited to 620-720 ℃.

In the case where the solid carbon in the steel is completely precipitated as a precipitate, the higher the cold reduction rate, the higher the drawing property. When the cold reduction rate is less than 60%, the drawing effect is small, and when the cold reduction rate is more than 90%, the required thickness of the hot rolled sheet is greater than 90%. Since is too thick to increase the rolling load, the reduction ratio is preferably set to 60-90%.

The annealing temperature is an important variable in recovering ductility through recrystallization and grain growth and improving drawing performance by developing (111) aggregated tissue. When the annealing temperature is 780 ° C. or less, the (111) aggregated tissue cannot be sufficiently developed. If it exceeds C, the austenite is formed to reduce the drawing property, so the annealing temperature is preferably limited to 780 ~ 880 ℃.

In addition, the annealing time is set in terms of securing the thermal energy required for recrystallization. When the annealing time is less than 10 seconds, the (111) aggregate structure development due to the recrystallization grain growth is small, and when it is more than 180 seconds, the heavy carbide Since this redissolves and the intensity | strength falls, it is preferable to select the annealing time for 10 to 180 second.

As described above, the main feature of the present invention is the addition of Mn, B, which is a solid solution element, to ultra-low carbon steel to which Ti and Nb are added to ensure ductility and strength, and to promote grain growth upon annealing by adding Al. To improve, and to produce a high-strength cold-rolled steel sheet with the addition of B, Mo and improved secondary work brittleness.

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

(Example)

Inventive steel (steel 1-12 steel) and comparative steel (steel 13-17 steel) slabs having the composition shown in Table 1 were heated at 1200 ° C. for 1 hour, hot rolled at 910 ° C., and then wound at 680 ° C. for 1 hour. After holding, the furnace was cooled.

The hot rolled sheet was pickled, cold rolled to 77%, and then recrystallized annealed at 840 ° C. for 45 seconds.

Mechanical properties and secondary workability of the cold rolled steel sheets annealed as described above were measured, and the results are shown in Table 2 below.

The secondary work brittleness is measured by using a drawing ratio of 2.16.

Meanwhile, Table 2 below also shows specimens (Comparative Materials 18-21) prepared for the invention steel 2 under the condition that the hot rolling temperature, cold rolling rate, and annealing temperature are outside the scope of the present invention.

Steel grade Chemical composition (% by weight) C Mn S Sol.Al N Ti Nb Mo B Invention steel One 0.0012 0.88 0.003 0.12 0.0017 0.022 0.015 - 0.001 2 0.0020 0.87 0.003 0.13 0.0017 0.025 0.010 - 0.001 3 0.0035 0.87 0.003 0.12 0.0021 0.035 0.005 - 0.001 4 0.0019 1.02 0.003 0.15 0.0026 0.027 0.005 - 0.001 5 0.0019 1.56 0.004 0.16 0.0021 0.036 0.003 - 0.001 6 0.0022 1.83 0.004 0.16 0.0029 0.035 0.003 - 0.001 7 0.0020 0.85 0.003 0.12 0.0016 0.045 - - 0.001 8 0.0021 0.86 0.003 0.13 0.0015 0.068 - - 0.001 9 0.0018 0.90 0.003 0.15 0.0016 0.025 0.035 - 0.001 10 0.0017 0.83 0.003 0.14 0.0019 0.026 0.056 - 0.001 11 0.0019 0.89 0.003 0.13 0.0020 0.042 - 0.04 0.001 12 0.0020 0.90 0.003 0.13 0.0019 0.035 0.005 0.05 0.001 Comparative steel 13 0.0065 0.90 0.003 0.15 0.0020 0.052 0.010 - 0.001 14 0.0020 2.12 0.003 0.12 0.0019 0.030 0.010 - 0.001 15 0.0019 0.89 0.012 0.13 0.0021 0.040 0.010 - 0.001 16 0.0022 0.91 0.004 0.04 0.0020 0.029 0.010 - 0.001 17 0.0024 0.85 0.004 0.15 0.0022 0.030 0.010 - -

Psalm No. Hot rolled winding temperature (℃) Cold rolling reduction (%) Annealing Temperature (℃) Mechanical properties Secondary processing brittleness (BBTT, ℃)              Steel grade Yp (kg / mm ² ) TS (kg / mm ² ) El (%)                  r Invention One 680 77 840 17.7 33.2 45 2.36 -65 Inventive Steel 1 2 " " " 18.2 34.6 43 2.29 -70 Inventive Steel 2 3 " " " 20.1 36.1 42 1.95 -70 Inventive Steel 3 4 " " " 22.0 37.5 42 2.15 -65 Inventive Steel 4 5 " " " 23.4 40.5 40 2.07 -65 Inventive Steel 5 6 " " " 24.7 42.9 37 1.95 -60 Inventive Steel 6 7 " " " 17.5 33.5 46 2.31 -65 Inventive Steel 7 8 " " " 18.1 33.0 44 2.42 -65 Inventive Steel 8 9 " " " 21.5 37.0 41 2.04 -70 Inventive Steel 9 10 " " " 23.6 38.9 39 1.95 -75 Inventive Steel 10 11 " " " 19.4 34.5 45 2.18 -70 Inventive Steel 11 12 " " " 20.7 45.8 43 2.25 -75 Inventive Steel 12 Comparative material 13 " " " 23.5 37.6 38 1.78 -70 Comparative Steel 13 14 " " " 27.0 45.8 33 1.64 -50 Comparative Steel 14 15 " " " 18.6 34.2 40 1.88 -55 Comparative Steel 15 16 " " " 20.8 33.5 41 1.85 -55 Comparative Steel 16 17 " " " 16.5 31.7 45 2.07 -40 Comparative Steel 17 18 560 " " 20.7 35.0 40 1.83 -60 Inventive Steel 2 19 750 " " 16.7 32.1 43 1.95 -55 Inventive Steel 2 20 680 50 " 16.5 32.5 45 1.75 -50 Inventive Steel 2 21 " 77 900 21.8 36.8 39 1.66 -40 Inventive Steel 2

As shown in Table 2, in the case of the inventive material (1-12) in which the chemical composition and manufacturing conditions of the steel conform to the present invention, the elongation to strength, r value, and secondary processing brittleness are not compared to the range of the present invention. Compared with ash (13-21), it can be seen that it is very good. This is because Mn, Nb, B addition and Al addition effect on ductility, which are less deteriorated compared to reinforcement effect, and Mo, Nb, It is achieved by the effect of B.

For example, the comparative material 13 has a higher strength, a lower elongation and a lower r value, and a comparative material 14 compared to the inventive material 2 by TiC excessively precipitated due to excessive carbon content in the steel. ) Can be seen that the Mn content in the steel is a steel exceeding the scope of the present invention, the needle-like ferrite is formed in the structure, and thus the strength, processability, and secondary processing brittleness are lowered compared to the invention steel.

In addition, the comparative material 15 having an S content outside the scope of the present invention has a lower workability and secondary work brittle resistance than the steel of the present invention, and a high sol.Al comparative material 16 has high strength and workability and secondary work resistance. It is found that the brittleness is low, and the comparative material 17 without adding B has low strength and poor secondary work brittleness.

On the other hand, in the case of the comparative materials 18 and 19 wound up at a temperature outside the winding temperature of the present invention after hot rolling the inventive steel 2, the cold reduction rate is lower than the range of the present invention (comparative material) (20) And when the annealing temperature is higher than the annealing temperature range of the present invention (comparative material 21), it can be seen that the strength, workability or secondary workability resistance is lower than the invention materials.

As described above, the present invention is easy to manufacture a high-strength cold rolled steel sheet having a tensile strength of 32-40kg / mm ² excellent in workability and secondary workability resistance compared to the conventional method, for the light weight and safety of the automobile body The cold rolled steel sheet or hot-dip galvanized steel sheet for the effect that can be applied very effectively.

Claims (2)

Ti in steel containing C: 0.0005-0.005% by weight, Mn: 0.5-2.0%, S: 0.005% or less, N: 0.003% or less, acid value Al: 0.1-0.2%, B: 0.0005-0.003% One or two of: 0.01-0.10% and Nb: 0.001-0.10% are added, and hot-rolling is performed under the condition that hot-rolling finishes at a temperature of ₃ or more Ar in a steel composed of residual Fe and unavoidable impurities. Workability and secondary processing brittleness, which are wound at 620-720 ℃, cold rolled at 60-90% reduction rate, and recrystallized annealing at 780-880 ℃ for 10 seconds to 180 seconds by continuous annealing. Manufacturing method of excellent cold rolled steel sheet The method of claim 1, wherein 0.02-0.2% by weight of Mo is further added to the steel.