KR100376523B1 - Manufacturing method of tensile strength 60 kg/mm¬2 class hot rolled steel sheet having superior stretch flangeability - Google Patents

Abstract

PURPOSE: A manufacturing method of tensile strength 60 kg/mm¬2 class hot rolled steel sheet is provided to obtain superior stretch flangeability by properly controlling cooling patterns in the cooling section in front of down coiler and miniaturizing shape of grain boundary cementite. CONSTITUTION: In a 60 kg/mm¬2 class hot rolled steel sheet(4) manufacturing method comprising the processes of hot rolling a steel slab comprising 0.07 to 0.09 wt.% of C, 0.25 wt.% or less of Si, 1.40 to 1.60 wt.% of Mn, 0.005 to 0.06 wt.% of t-Al, 0.03 to 0.05 wt.% of Nb, 0.003 wt.% or less of S, 0.02 wt.% or less of P and a balance of Fe and other inevitably contained impurities; and coiling the water cooled steel slab after water cooling the hot rolled steel slab to a target coiling temperature in the cooling section in front of down coiler(3) equipped with a plurality of water cooling banks(2), the manufacturing method of tensile strength 60 kg/mm¬2 class hot rolled steel sheet(4) having superior stretch flangeability comprises the processes of water cooling the hot rolled steel slab to a temperature directly under Ar3 transformation point at a cooling rate of 40 to 90 deg.C on run-out table(8) just after the hot rolling; air cooling the water cooled steel slab to a temperature in which 70 to 80% of ferrite transformation is proceeded; and water cooling the air cooled steel slab to an intermediate temperature of bainite transformation initiation temperature Bs and martensite transformation initiation temperature Ms at a cooling rate of 40 to 90 deg.C so that final structure of the steel slab comprises 70 to 80% of equiaxed ferrite (F) and 20 to 30% of bainite (B).

Description

Manufacturing method of 60kg (kg / mm²) hot rolled steel sheet with excellent tensile strength

The present invention relates to a method for manufacturing a hot rolled steel sheet (hereinafter referred to as a 'hot rolled steel sheet') that is press-processed and used as a part of a wheel, a member, etc. of a vehicle, and more particularly, a tensile strength of 60 kg / It relates to a method of manufacturing a mm 2 grade hot rolled steel sheet.

Hot rolled steel sheets, which are relatively thicker than cold rolled steel sheets, are used for members and wheels of passenger cars. In applying the hot rolled steel sheet to the above-mentioned uses, efforts have been made to develop and apply high-strength hot rolled steel sheets having excellent workability in order to reduce the weight and safety of the vehicle body for the purpose of reducing fuel consumption of passenger cars.

Since most parts of wheels, members, etc. of automobiles are manufactured by press working, hot rolled steel sheets used for the above-mentioned applications must have sufficient press formability in addition to the required strength. The formability indexes include elongation and stretch elongation of the steel sheet. Among them, the stretch elongation (burring property) shows the ultimate deformation of the end portion generated when the hole is punched out, and the better it is that the generation and propagation of microcracks, which are the starting point of fracture during burring, are less likely to occur.

Therefore, in order to improve stretch elongation, hot rolling and post-rolling cooling processes are performed in order to form a grain boundary carbide which is a nucleation site of a crack, or to form a phase having a small difference in plastic deformation between constituent phases in order to suppress formation and propagation of cracks. It is important to control.

In general, the material of a hot rolled steel sheet decreases its ductility and stretch elongation as strength increases. Therefore, in order to manufacture a hot rolled steel sheet having excellent press formability, it is important to appropriately select a steel reinforcement mechanism corresponding to the strength level and combine strength and formability. High-strength hot-rolled steel sheets of 60kg / mm2 class are divided into precipitation-reinforced hot-rolled steel sheets and transformed structure-reinforced hot-rolled steel sheets properly controlled by the C-Mn-Si system.

In the case of the precipitation-reinforced hot rolled steel sheet, Nb, Ti, and the like are added to the C-Mn component system as precipitation-reinforcement elements, and have ferrite and pearlite structures. The strength of the precipitation-reinforced hot-rolled steel sheet by the addition of precipitation-reinforcing elements is superior to other reinforcement methods, but the degradation of workability due to the increase in strength is unavoidable, and is formed by high temperature winding of 580-600 ° C for precipitation strengthening. Ferrite-perlite tissue is known to have poor elongation compared to low temperature metamorphic reinforcement.

In the case of transformed tissue-reinforced hot-rolled steel sheets using low-temperature transformation tissues, (A) C-Si-Mn system was used to control the cooling process after hot rolling to create martensite structure as a second phase on the equiaxed ferrite matrix. Dual phase steel (NSC; CAMP-ISIJ, Vol. 4 (91), p. 980), (b) C-Si-Mn system (KOBE: JP90306762, KSC: JP91358007), C-Mn-Nb- Ti-based (SMI: CAMP-ISIJ, Vol. 1 (88), p. 881, KOBE; CAMP-ISIJ, Vol. 5 (92), p. 1859), C-Si-Mn-Nb system (KOBE: Trans Ferritic-bainite composite steel, in which bainite was formed as a second phase using ISIJ, Vo. 23 (83), p303), (C) C-Si-Mn-Cr system (KOBE: JP903704l, JP9037040, JP9037039) and C-Si-Mn-based (KOBE: JP817598) -based three-phase composite steels in which bainite and martensite are mixed as a second phase.

The ferrite-martensitic composite steels of the metamorphic structure-reinforced steels using the low temperature transformation tissue have excellent strength and elongation, while the elongation of stretching is inferior to other reinforcement methods. In addition, the strength-elongation balance of ferritic-bainite composite tissue type 60kg / mm2 hot rolled steel sheet is disadvantageous compared to ferritic-martensitic steel structure, but its elongation is excellent compared to other reinforcement methods. It is known to be the most suitable.

The transformation structure of high-strength hot-rolled steel sheet of ferrite-bainite is wound at low temperature of 300-500 ℃ after hot rolling based on C-Si-Mn system, and Si is added to promote ferrite transformation during cooling. Or, in the cooling process after hot rolling, set an air cooling section in the middle of the water cooling zone and wind up. However, when manufacturing a high-strength hot-rolled steel sheet with a component system added with Si, there is a problem that the surface quality deteriorates.

Therefore, the present inventors have a problem that the surface is degraded when manufacturing high-strength hot-rolled steel sheet, which is a disadvantage of ferritic-bainite-based composite tissue steel, which has poor draw elongation property, which is a disadvantage of precipitation-reinforced steel, and an alloy system containing Si, which improves draw elongation. In order to solve the research and experiments, and based on the results suggest the present invention,

The present invention controls the cooling pattern in the water cooling zone in front of the winder immediately after hot rolling a steel containing Nb, a precipitation strengthening element, to make the final structure into equiaxed ferrite and bainite structure, and fine grain shape of grain boundary cementite In order to provide a method for manufacturing a hot rolled steel sheet having a tensile strength of 60 kg / mm 2, which has excellent elongation and elongation, it is an object of the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is in weight%, C: 0.07-0.09%, Si: 0.3% or less, Mn: 1.40-1.60%, t-Al: 0.005-0.06%, Nb: 0.03-0.05%, S: 0.003% or less, P : 0.02% or less, hot-rolled steel slab composed of remaining Fe and other unavoidable impurities, followed by water cooling to the target winding temperature in the water cooler in front of the winder equipped with a plurality of water cooling banks, followed by winding up to 60 kg. In the method of manufacturing a / mm2 hot rolled steel sheet,

Immediately after the hot rolling, water was cooled on the runout table at a cooling rate of 40-90 ° C / s to a temperature directly below the Ar ₃ transformation point, followed by air cooling to a temperature at which ferrite transformation proceeded 70 to 80%, and then bainite transformation start temperature. Tensile strength excellent in stretch elongation, including water cooling at a cooling rate of 40-90 ° c / s to the intermediate temperature between B _s and martensitic transformation start temperature M _s will be.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

In general, when the steel of the alloy system containing Ti or Nb, which is a precipitation-reinforced element, is hot rolled and continuously cooled on a run-out table, the temperature change of the strip is produced when the tensile strength is 60 kg / mm2 hot rolled steel sheet. As shown in the figure.

Namely, by weight%, C: 0.07-0.09%, Si: 0.3% or less, Mn: 1.3-1.5%, t-Al: 0.005-0.06%, Nb: 0.03-0.05%, S: 0.003% or less, P: The hot rolled steel sheet 4, which has exited the finishing mill 1 after hot rolling a steel slab composed of 0.02% or less and the remaining Fe and other unavoidable impurities, may be discharged to the catcher 70 while reaching the winding machine 3. Forced water cooling while going through, and the temperature drop up to the target winding temperature is water cooling is carried out for the remaining temperature drop except the temperature drop due to cooling. At this time, the water is made from the front bank of the water-cooled bank (2).

Such a cooling method is called a shear cooling method, and when the coiling temperature is 530 ° C. or more, the final structure becomes a ferrite-perlite structure, and at a coiling temperature below that, it becomes a needle-like ferrite and bainite structure or bainite single phase structure. When the ferrite shape changes to needle shape or the entire phase becomes bainite single phase, the strength increases, but the ductility required for press working is lowered.

Therefore, in the present invention, when the steel formed by hot rolling is formed on the runout table 8 after hot rolling as described above, at a temperature below B _s which is the bainite transformation start temperature as shown in FIG. The above object can be attained by controlling the cooling to form the tissue into a ferrite bainite composite.

That is, in the present invention, by weight%, C: 0.07-0.09%, Si: 0.3% or less, Mn: 1.3-1.5%, t-Al: 0.005-0.06%, Nb: 0.03-0.05%, S: 0.003% or less , P: 0.02% or less, the steel slab composed of the remaining Fe and other unavoidable impurities, from the temperature (T ₁ ) at which the first water cooling is started after hot rolling to 40 2-90 to the temperature T ₂ directly below the Ar ₃ transformation point. Water-cooled at a cooling rate of ° C / s, and then air-cooled to a temperature T ₃ where the ferrite transformation proceeds about 70-80% to promote the formation of fine ferrite and the increase of the ferrite fraction, and then the bainite transformation start temperature. And martensite transformation start temperature Secondary water cooling is carried out at a cooling rate of 40-90 ° c / s to a temperature T ₄ , which is in the middle, so that unaffected austenite becomes bainite structure at T ₃ , and the final structure of the wound hot rolled steel sheet is soft It is desirable to have a bainite formed in the ferrite matrix because of the following reasons.

When control cooling by the method of the present invention, the first water cooling end temperature T ₂ is more than the Ar ₃ transformation point, the passage time in the ferrite transformation region of the steel sheet is shortened, the second phase fraction increases, and conversely, it is too much than the Ar ₃ transformation point. When the low ferrite transformation is suppressed and the growth of crystal grains is because the ferrite shape of the final tissue is a needle-like decrease in ductility of the hot rolled steel sheet of the water-cooling end temperature T ₂ is preferably a primary managing in a temperature range of Ar ₃ transformation point immediately below.

And when the cooling rate in the first water cooling is 40 ° c / s or less, the air cooling time from the first water cooling end temperature T ₂ to the second water cooling start temperature T ₃ is shortened, so that the equalization of the ferrite is not smooth and the upper limit is 90 ° c / s. s considers the minimum speed of hot rolling line.

After the first water cooling to the temperature just below the transformation point with Ar ₃ , it is preferable to perform air cooling such that the ferrite transformation proceeds about 70-80% to secure the elongation, because the strength increases when the ferrite transformation degree is 70% or less. On the other hand, the elongation is lowered, and if it is 80% or more, it is because the tensile strength of the present invention can not obtain the strength characteristics of 60kg / mm2 class.

Ferrite transformation is advanced after 70-80% of the non-transformed austenite is bainite structure is such that T ₃ and in the embodiment the secondary water-cooled bainite transformation starting temperature T ₃ in the And martensite transformation start temperature It is preferable to cool the water in the cooling speed range of 40-90 ° c / s up to the intermediate temperature of. The reason is that when the cooling speed is 40 ° c / s or less, the pearlite transformation is promoted and the elongation is inferior. . And the upper limit is set to 90 ° c / s is the same as in the case of the primary water cooling.

When controlled cooling of the precipitation-reinforced steel to which Nb is added so as to satisfy the above conditions, it is possible to produce a high strength hot rolled steel sheet excellent in stretch elongation.

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

Example 1

A slab composed of 0.08% by weight C-1.4% by weight Mn-0.04% by weight Nb was heated at 1250 ° C. for 3 hours, and then hot-rolled to finish at 870 ° C. above Ar ₃ temperature to 3.5 mm thick. A hot rolled plate was prepared.

In order to check the effect of the cooling pattern and the coiling temperature on the elongation elongation after performing the hot rolling as described above, in the conventional examples (1, 2) of Table 1, the shear cooling, that is, the coiling temperature is 620 ℃ and 580 In the case of Comparative Examples (1, 2) and Inventive Example (1), control cooling was carried out in a temperature range of 480-580 ° C. in order to control the structure of the second phase. At this time, the coiling temperatures of Comparative Examples (1) and (2) are 580 ° C and 530 ° C, respectively, and the coiling temperature of Inventive Example (1) is 480 ° C, which is below the bainite transformation start temperature.

In the controlled cooling, the first water cooling end temperature (T ₂ ) is 700 ° C., the second water cooling start temperature (T ₃ ) is 670 ° C., and the first water cooling end temperature (T ₂ ) to the second water cooling start temperature (T ₃ ). Air cooling time is 4-6 seconds in the temperature range of. And in the water cooling from the primary water cooling start temperature (T ₁ ) to the primary water cooling end temperature (T ₂ ) and from the secondary water cooling start temperature (T ₃ ) to the secondary water cooling end temperature (T ₄ ), the cooling rate Is determined by the thickness of the hot rolled strip and the sheet speed, in which case both have a cooling rate of 40–90 ° c / s.

As described above, the mechanical properties and the elongation of the hot rolled steel sheet having a thickness of 3.5 mm manufactured by applying the shear cooling and the control cooling method were shown in Table 1 below.

At this time, the stretch elongation was punched in the hole of the initial diameter (do) of the hole 18mm in the raw material plate, and then the hole was expanded using a shoe-shaped punch having a radius of 25mm. The hole diameter (df) at the time of raising a punch until a crack penetrates the edge part of the expanded hole was measured, and it evaluated by the following formula.

Elongation at Stretch (%) = (df-do) / do% 100

Table 1

As can be seen from Table 1, in the conventional example (1, 2) of the ferrite-perlite structure produced by the shear cooling method, which is a conventional method, the elongation elongation is about 77%.

In comparison between Comparative Examples (1) and (2) and Inventive Example (1), there are almost no changes in mechanical properties due to the decrease in winding temperature, but the extension elongation is increased as the second phase changes from pearlite to bainite. Bainite transformation start temperature In the invention example (1) wound up at 480 degreeC or less, the hot-rolled steel sheet of 60 kg / mm <2> or more of tensile strength and 90% or more of draw elongation is manufactured. And when comparing the conventional example (2) and the invention example (1) to which the shear cooling is applied, there is no significant change in the mechanical properties, but it can be seen that the elongation is increased by more than 15% compared to the conventional example (2).

Example 2

0.08% by weight C-1.4% by weight Mn-0.04% by weight Nb, the slab formed by heating the slab formed by heating at 1250 ℃ for 3 hours and then hot-rolled to finish at 870 ℃ above the Ar ₃ temperature 3.2mm and A 3.0 mm hot rolled sheet was prepared.

In order to check the effect of the cooling pattern and the winding temperature on the stretch elongation after performing the hot rolling as described above, in the case of the following conventional example (3), it was subjected to shear cooling and wound up at 580 ° C. In the case of 3), controlled cooling was carried out and wound up at 480 ° C. which was a temperature below the bainite transformation start temperature.

In the controlled cooling of the inventive examples (2, 3), the primary water cooling end temperature (T ₂ ) is 700 ℃, the secondary water cooling start temperature (T ₃ ) is 670 ℃, 2 at the primary water cooling end temperature (T ₂ ) The air cooling time is 3-5 seconds in the temperature range up to the starting water cooling temperature (T ₃ ). In the water cooling from the first water cooling start temperature (T ₁ ) to the first water cooling end temperature (T ₂ ) and from the second water cooling start temperature (T ₃ ) to the second water cooling end temperature (T ₄ ), the cooling rate is It is determined by the thickness of the hot rolled strip and the sheet speed, in which case both have cooling rates of 40-90 ° c / s.

As described above, the mechanical properties and the elongation of the steel sheet having a thickness of 3.0 mm and 3.2 mm manufactured by applying the cutting cooling and the control cooling method were shown in Table 2 below.

At this time, stretch elongation was performed by punching a hole having an initial diameter (do) of 18 mm in the raw material disc, and then using a shoe punch having a shoulder radius of 25 mm to expand the hole. The hole diameter (df) at the time of raising a punch until a crack penetrates the edge part of the expanded hole was measured, and it evaluated by the following formula.

Elongation at Stretch (%) = (df-do) / do% 100

Table 2

As can be seen in Table 2, the conventional example (3) is a ferrite-perlite structure in comparison with the conventional example (3) and the invention example (2), where the thickness of the hot rolled sheet is equal to 3.2 mm, 79%. On the other hand, Inventive Example (2) is an equiaxed ferrite-bainite composite tissue steel, which has the same mechanical properties as those of the prior art, but has a stretch elongation of about 90%, resulting in an increase of 10% or more compared with the conventional example. Inventive Example (3) is a result of 3.0 mm hot-rolled steel sheet, which is thinner than Inventive Example (2), and shows that stretch elongation of 90% is obtained.

As described above, the present invention uses a controlled cooling method to control the cooling pattern of the hot-rolled steel sheet on the run-out table after hot rolling using the component system of the precipitation-reinforced steel added with Nb and to wind it in the temperature range where bainite is generated. It is possible to produce 60kg / mm2 high strength hot rolled steel sheet with excellent ductility and elongation property in comparison with hot rolled steel sheet of ferrite-fine pearlite structure obtained by high temperature winding or bainite single phase structure obtained by low temperature winding. It works.

1 is a cooling curve showing a cooling pattern when manufacturing a hot rolled steel sheet using a conventional method

2 is a cooling curve showing a cooling pattern when manufacturing a hot rolled steel sheet by applying the method of the present invention

Explanation of symbols on the main parts of the drawings

1 .... hot rolling mill 2 .... water cooling bank 3 .... winder

4 .... Hot rolled steel sheet 5 .... Hot rolled finish thermometer 6 .... Winding thermometer

7 .... water cooler 8 .... run out table F: ferrite

P: pearlite B: bainite A: austenite M: martensite

T ₁ : 1st water cooling start temperature T ₂ : 1st water cooling end temperature (air cooling start temperature)

T ₃ : 2nd water cooling start temperature (air cooling end temperature) T ₄ : 2nd water cooling end temperature

Claims (1)

By weight% C: 0.07-0.09%, Si: 0.25% or less, Mn: 1.40-1.60%, t-Al: 0.005-0.06%, Nb: 0.03-0.05%, S: 0.003% or less, P: 0.02% or less , Hot-rolled steel slab composed of the remaining Fe and other unavoidable impurities, followed by water cooling to the target winding temperature in the front of the winder equipped with a plurality of water cooling banks, followed by winding up to 60kg / mm2 hot In the method of manufacturing a rolled steel sheet, Immediately after the hot rolling, the water was cooled at a cooling rate of 40-90 ° C./s to a temperature directly below the Ar ₃ transformation point on the run-out table, followed by air cooling to a temperature at which ferrite transformation proceeds to 70-80%, and then bainite transformation start temperature Bs. And martensitic transformation start temperature Ms to water at a cooling rate of 40-90 ℃ / s at a cooling rate of 40-90 ℃ / s so that the final structure is composed of 70-80% equiaxed ferrite and 20-30% bainite A method for producing hot rolled steel sheets having a tensile strength of 60 kg / mm2 having excellent elongation property.