KR100643354B1 - Method for manufacturing high strength hot rolled steel sheet having excellent flangeability and ductility - Google Patents

Abstract

The present invention relates to a method of manufacturing a 55kg / mm2 high strength hot rolled steel sheet suitable for use in lower arms, suspensions and wheel disks of automobiles.

The present invention is, in mass%, C: 0.05 to 0.1%, Mn: 0.1 to 2.2%, Si: 0.05 to 0.5%, P: 0.03% or less, S: 0.003% or less, Al: 0.005 to 0.05%, and Ti : Finishing hot-rolling steel slab containing 0.005 to 0.05% and Nb: 0.005 to 0.05%, the remainder being composed of Fe and other unavoidable impurities at 800 to 900 ° C; Then quenching the hot rolled steel sheet at a cooling rate of 80 to 1000 ° C./sec to 650 to 720 ° C. and then air cooling; And cooling and then winding the air-cooled steel sheet at a cooling rate of 80 to 1000 ° C./sec to 400 to 550 ° C .;

The present invention can provide a high-strength hot-rolled steel sheet having excellent elongation flangeability and ductility having an optimal balance of elongation and hole expansion equipment.

Elongation, hole expansion equipment, extension flange, ductility, hot rolled steel, HER, high strength, automotive

Description

Method for manufacturing high strength hot rolled steel sheet having excellent flangeability and ductility}             

1 is a graph showing the correlation between the elongation and hole expansion equipment in the present invention.

Figure 2 is a graph comparing the elongation and hole expansion equipment of the invention and the comparative material.

3 is a graph showing the finish hot rolling temperature and the winding temperature of the present invention.

The present invention relates to a high-strength hot-rolled steel sheet of 55kg / ㎜ class, suitable for use in lower arms, suspensions, wheel disks, etc. of automobiles, and more specifically, steel components, finish hot rolling temperature and winding The present invention relates to a method for producing a high strength hot rolled steel sheet having excellent stretch flangeability and ductility, by controlling the temperature appropriately and having an optimum balance between the elongation flangeability (hole expansion equipment) and the elongation.

In order to cope with increasingly stringent environmental regulations, the automobile industry needs to increase fuel consumption when driving a car. In order to achieve the weight reduction of the vehicle body, it is effective to increase the strength of the hot rolled steel sheet used in the automobile chassis in the related art, and it is important to suppress the workability deterioration caused by the high strength as much as possible.

In the case of ferritic precipitation hardening steel which is used for general purpose, the elongation and extension flange property are deteriorated as the strength is increased. Therefore, improvement of the ferrite precipitation hardened steel is necessary to apply the high strength steel to the automobile chassis parts.

In order to solve the above problems, there is proposed a method of increasing the elongation flangeability without deterioration of ductility by forming a mixed structure of polygonal ferrite and bainite metal, a representative example is Japanese Patent Laid-Open No. 61-130454 , 6-293910 and Korea Patent Publication No. 2003-55339.

Japanese Patent Laid-Open Nos. 61-130454 and 6-293910 of the prior arts are ferrite-cooled by using three-stage cooling by cooling to a temperature of around 700 ° C. after hot rolling and then cooling by air for a predetermined time and then cooling again. It proposes a method of forming the bainite tissue to improve the kidney flangeability. In addition, Republic of Korea Patent Publication No. 2003-55339 relates to a hot-rolled steel sheet having a strength of more than 69kg / ㎠ and excellent elongation and elongation flange at the same time, mainly with a ferrite-bainite structure, the ferrite ratio is 80% or more It is proposed to control the ratio of the crystal grains having a ratio of the short diameter (ds) and the long diameter (dl) of the crystal grains to 0.1% or more.

However, the related arts lack an understanding of the correlation between the elongation and the elongation flange, and thus do not suggest a method of simultaneously controlling the two characteristic values.

The present invention is to solve the problems of the prior art, by optimizing the steel components and by optimizing the finish hot rolling temperature and the winding temperature, the elongation flange and the high strength excellent in ductility with an optimum balance between the elongation and hole expansion equipment To provide a method for producing a hot rolled steel sheet, the object is.

The present invention for achieving the above object, in mass%, C: 0.05 ~ 0.1%, Mn: 0.1 ~ 2.2%, Si: 0.05 ~ 0.5%, P: 0.03% or less, S: 0.003% or less, Al: 0.005 ~ 0.05%, Ti: 0.005 ~ 0.05%, and Nb: 0.005 ~ 0.05% of one or two kinds, and the remainder is hot-rolled at 800-900 ° C. for steel slabs composed of Fe and other unavoidable impurities step;

Then quenching the hot rolled steel sheet at a cooling rate of 80 to 1000 ° C./sec to 650 to 720 ° C. and then air cooling; And

It relates to a method of manufacturing a high strength hot rolled steel sheet having excellent elongation flange property and ductility comprising the step of cooling the air-cooled steel sheet to 400 ~ 550 ℃ at a cooling rate of 80 ~ 1000 ℃ / sec and then winding.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention is characterized in providing a hot rolled steel sheet having a tensile strength of 55kg / mm 2 or more, an elongation of 21% or more, and a hole expanding device of 80% or more, and firstly, the steel component limitation reason for this will be described.

C: 0.05 to 0.1% by weight (hereinafter referred to as '%' only)

The carbon (C) is a basic component for obtaining the strength of the hot rolled steel sheet, and if the content of C is less than 0.05%, the desired strength cannot be obtained. It is preferable to limit the content to 0.05 to 0.1%.

Mn: 0.1-2.2%

The manganese (Mn) is an effective element for solid solution strengthening. If the Mn content is less than 0.1%, the effect of the addition cannot be obtained. If the content exceeds 2.2%, the strength is obtained, but the elongation decreases rapidly. It is preferable to limit to 0.1-2.2%.

Si: 0.05-0.5%

The silicon (Si) is a solid solution element that increases strength without deterioration of ductility, but not only causes defects due to oxidation scale on the surface of hot rolled steel sheet, but also lowers weldability. It is preferable to limit the content to 0.05 to 0.5% in order to obtain.

P: 0.03% or less

Phosphorus (P) has an effect of strengthening the solid solution, but it is easy to cause grain boundary segregation during the winding process, thereby inhibiting the kidney flangeability. Therefore, in the present invention, it is preferable to limit the content of P to 0.03% or less.

S: 0.003% or less

The sulfur (S) precipitates fine MnS in the course of finishing rolling. Since brittleness is greater than that of the metal base material, cracks are easily propagated during metal destruction, which adversely affects the extension flange. Therefore, in the present invention, it is preferable to limit the content of S to 0.003% or less.

Al: 0.005-0.05%

The aluminum (Al) is a deoxidizer and a carbide forming element. If the content of Al is less than 0.005%, the effect of the addition cannot be obtained. If the content of Al exceeds 0.05%, the workability is inhibited. Therefore, the content is preferably limited to 0.005 to 0.05%.

The present invention contains one or two of Ti and Nb in the above components.

Ti: 0.005-0.05%

In the present invention, the titanium (Ti) is precipitated as Ti carbide or Nb-Ti composite carbide in the ferrite structure together with the transformation during austenite / ferrite transformation after hot rolling, thereby contributing to the ferrite strengthening. If the content of Ti is less than 0.005%, the effect of the addition cannot be obtained. If the content is more than 0.05%, the effect is saturated, and therefore, the content is preferably limited to 0.005 to 0.05%.

Nb: 0.005-0.05%

In the present invention, niobium (Nb) is an element which is precipitated during hot rolling or is present in solid solution and contributes to grain refinement of austenite and is effective for strengthening the precipitation of ferrite phase. If the content of Nb is less than 0.005%, the above effect cannot be obtained by addition. If the content of Nb is more than 0.05%, it is highly likely to generate slag cracks in the manufacturing process of the continuous casting slab, so that the content is limited to 0.005 to 0.05%. It is preferable.

In addition to the above components, the remainder is composed of Fe and other unavoidable impurities.

The elongation and elongation flangeability of the steel sheet will greatly depend on the manufacturing process for controlling the structure of the metal, and the manufacturing process of the present invention will be described in detail below.

In the present invention, the steel slab formed as described above is reheated by a conventional method and then hot rolled.

The finish hot rolling temperature at the time of hot rolling affects the operability and the ferrite grain size after transformation in manufacturing steel sheet. In the present invention, when the finishing hot rolling temperature is less than 800 ° C., not only the rolling load is excessive, but also adversely affects the rolling sheet property. When the finishing hot rolling temperature is higher than 900 ° C., the structure becomes coarse, leading to a decrease in strength and ductility. Therefore, the finishing hot rolling temperature is preferably controlled to 800 ~ 900 ℃.

As described above, the hot rolled hot rolled plate is then quenched at a cooling rate of 80 ~ 1000 ℃ / sec and then air cooled. At this time, if the quenching end temperature is less than 650 ℃, the ferrite ratio is reduced and the form is changed to the non-polygonal type of cyclic, there is a fear that the ductility is lowered, if it exceeds 720 ℃, the ratio of ferrite decreases and the ductility is reduced Therefore, the quench end temperature is preferably controlled to 650 ~ 720 ℃. In addition, when the cooling rate during the quenching is less than 80 ℃ / sec, there is a fear that the pearlite is generated and workability is lowered, and if the cooling rate exceeds 1000 ℃ / second, it is difficult to precisely control the temperature, the operation is impossible, It is preferable to limit the speed to 80-1000 degreeC / sec.

In addition, the air cooling is to allow the ferrite transformation occurs, the air cooling time determines the fraction of ferrite and bainite in the final product tissue. When the air cooling time is less than 4 seconds, the bainite structure may be excessively generated to decrease the ductility, and when the air cooling time exceeds 6 seconds, most of the tissue may become ferrite and thus it may be difficult to exhibit the properties of the ferrite-bainite complex tissue. The air cooling time during air cooling is preferably controlled to 4 to 6 seconds.

The air-cooled steel sheet is then cooled to the winding temperature at a cooling rate of 80 ~ 1000 ℃ / second and then wound. At this time, the untransformed austenite phase is transformed into bainite during air cooling. If the cooling rate up to the coiling temperature is less than 80 ℃ / sec, there is a fear that the pearlite is generated and workability is lowered, and if the cooling rate exceeds 1000 ℃ / second, precise temperature control becomes difficult and operation is impossible, Cooling rate is preferably limited to 80 ~ 1000 ℃ / second.

If the coiling temperature is less than 400 ° C. during the winding, hard ferrite is easily generated and harmful to the stretch flangeability. If the coiling temperature is higher than 550 ° C., pearlite and grain boundary cementite harmful to the stretch flange property are easily generated. It is preferable to control it at -550 degreeC.

On the other hand, the present inventors while studying the relationship between the elongation and hole expansion equipment, and the finish hot rolling temperature and the coiling temperature, when the finish hot rolling temperature is 820 ~ 845 ℃, the winding temperature is 400 ~ 450 ℃, and the finish hot When the rolling temperature is 845 ~ 900 ℃ it was confirmed that the better hole expansion equipment can be obtained in the section (winding area 2 of Figure 3) the winding temperature is 400 ~ 475 ℃.

In addition, it was confirmed that a better elongation can be secured in the section (region 3 of FIG. 3) where the finish hot rolling temperature is 800 to 845 ° C. and the winding temperature is 475 to 550 ° C. FIG.

In addition, the elongation and hole expansion equipment are intended for the purpose of the present invention (elongation: 21 to 29%) in the section where the finish hot rolling temperature is 820 to 845 ° C. and the winding temperature is 450 to 475 ° C. (area 1 in FIG. 3). , Hole expansion equipment: 80 ~ 150%), it was confirmed that it is possible to maintain the optimal balance between elongation and hole expansion equipment.

By the above manufacturing process, the present invention enables the production of hot rolled steel sheet having an optimal balance of elongation and hole expansion equipment (elongation flange), that is, 21 to 29% elongation, 80 to 150% elongation. In addition, the inventors of the present invention while studying the correlation between the elongation and hole expansion equipment, it was found that the elongation and hole expansion equipment is inverse relationship with each other, as shown in the graph of FIG.

In addition, the inventors pay attention to the fact that the elongation EL and the hole expanding equipment HER change according to the finish hot rolling temperature FDT and the coiling temperature CT. It was. By controlling the finish hot rolling temperature and the winding temperature so as to satisfy the following Equations 1 and 2, the desired elongation EL and the hole expanding equipment HER can be obtained.

[Equation 1]

EL (%) = 296.1-0.338 × FDT (° C) -0.48 × CT (° C) + 0.000605 × FDT (° C) × CT (° C)

[Equation 2]

HER (%) = 2204.7-6.78 × FDT (℃) + 3.26 × CT (℃) + 0.00508 × FDT (℃) ² -0.000706 × CT (℃) ² -0.00344 × FDT (℃) × CT (℃)

(However, FDT is finishing hot rolling temperature and CT is winding temperature)

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

Example 1

The specimen prepared as shown in Table 1 was hot-rolled finish under the conditions of Table 2 to prepare a thickness of 3.2mm. It was then cooled and wound up under the conditions of Table 2 below.

The mechanical properties of the specimens prepared as described above were evaluated, and the results are shown in Table 2 below. At this time, the tensile test was made by fabricating a JIS No. 5 tensile test specimen in a direction perpendicular to the rolling direction, and evaluated by pulling at a rate of 10 mm / min at room temperature. Ductility was evaluated by elongation. In addition, the extension flange is punched by a 10mm diameter hole in the center of the specimen of 120mmｘ120mm size according to ISO TS16630, and then expanded into a 60-degree cone punch to measure the hole expansion ratio (HER) defined by the following equation. Evaluated.

HER (%) = (D _f -D _o ) / D _o × 100

In the above formula, D _o is the initial diameter of the punched hole, and D _f is the hole diameter measured by stopping the rise of the punch when the crack penetrates the thickness during expansion.

Ingredient Content (wt%) C Mn Si P S Al Nb 0.08 1.4 0.15 0.016 0.002 0.025 0.02

division FDT (℃) Primary Quenching Speed (℃ / sec) Primary quench end temperature (℃) Air Cooling Time (sec) Second quench rate (℃ / sec) Coiling temperature (℃) Yield strength (kg / ㎡) Tensile strength (kg / ㎡) Elongation (%) Bore Expansion Ratio (%) Invention 1 840 110.0 708 5.1 140.0 540 48.2 55.5 28.9 96.8 Invention 2 803 82.5 704 5.1 154.2 519 48.6 56.5 26.9 98.6 Invention 3 886 165.0 688 5.1 140.8 519 46.5 56.7 25.4 99.2 Invention 4 834 108.3 704 5.1 195.0 470 49.3 56.7 24.6 121 Invention 5 834 116.7 694 5.1 186.7 470 50.7 56.2 26.9 114 Invention 6 841 114.2 704 5.1 195.0 470 51.8 57.3 25.9 105 Invention 7 900 174.2 691 5.1 184.2 470 48.3 56.3 22.4 143 Invention Material 8 898 175.0 688 5.1 222.5 421 47.8 57.0 21.1 146 Invention 9 835 116.7 695 5.1 245.8 400 49.7 57.1 23.5 133 Invention Material 10 854 145.0 685 4.5 154.17 495 49.1 58.3 25.3 103 Invention Material 11 864 132.5 705 6.0 212.5 450 48.4 55.5 24 122 Comparative Material 1 775 105 710 6.0 133 470 49.8 57.6 28.9 77.5 Comparative Material 2 803 105 710 6.0 133 590 48.4 58.3 30.1 78.0

In the above Table 2, the invention material (1 ~ 11) satisfying the scope of the present invention is a tensile strength of 55kg / ㎜ or more grade, 80-150% of the hole expansion equipment and 21-29% elongation of excellent properties Able to know.

In particular, in the case of the invention materials (4, 5, 6), it was found that the hole expansion equipment and the elongation have an optimal balance with the hole expansion equipment of 105 to 121% and the elongation of 24 to 27% (region 1 of FIG. 3). ).

In addition, in the case of the invention materials (7, 8, 9, 11), it was found that the hole expansion equipment of 122-146% and the elongation of 21-24% have very good hole expansion equipment with good elongation (Fig. 3). 2 areas of).

In addition, in the case of the invention materials (1, 2), it was found that the hole expansion equipment of 96-99% and the elongation of 26-29% have very good elongation together with the good hole expansion equipment (3 areas in FIG. 3).

However, in the case of the comparative materials (1 to 2) in which the finish hot rolling temperature or the coiling temperature did not satisfy the scope of the present invention, the hole expanding equipment intended for the present invention could not be obtained.

Example 2

The measured elongation (EL) and hole expansion equipment (HER) of the inventive material (1-11) of Example 1 was compared with the elongation and hole expansion equipment calculated by Equations 1 and 2 of the present invention. Is shown in Table 3 below. In this case, an error of (+) means that the measured value is larger than the calculated value, and (-) means that the measured value is smaller than the calculated value.

division Elongation Hole expansion equipment Measures(%) Calculated value (%) error(%) Measures(%) Calculated value (%) error(%) Invention 1 28.9 27.4 +5.19 96.8 88.1 +8.99 Invention 2 26.9 27.7 -2.97 98.6 104.1 -5.58 Invention 3 25.4 25.7 -1.18 99.2 105.3 -6.15 Invention 4 24.6 25.8 -4.88 121 111.4 +7.93 Invention 5 26.9 25.8 +4.09 114 111.4 +2.28 Invention 6 25.9 25.4 +1.93 105 112.2 -6.86 Invention 7 22.4 22.2 +0.89 143 138.6 +3.08 Invention Material 8 21.1 19.2 +9.00 146 159.6 -9.32 Invention 9 23.5 23.9 -1.70 133 127.4 +4.21 Invention 10 25.3 25.6 -1.19 103 106 -2.91 Invention 11 24 23.3 +2.92 122 125.5 -2.87

In Table 3, both the elongation and the hole expansion equipment was found to have a value of ± 10% of the error of the calculated value for the measured value.

As described above, according to the present invention can provide a high-strength hot-rolled steel sheet excellent in the elongation flange property and ductility having an optimal balance of elongation and hole expansion equipment.

Claims (7)

In mass%, C: 0.05-0.1%, Mn: 0.1-2.2%, Si: 0.05-0.5%, P: 0.03% or less, S: 0.003% or less, Al: 0.005-0.05%, Ti: 0.005-0.05 % And Nb: finishing hot-rolled steel slab containing one or two of 0.005 to 0.05%, the remainder is composed of Fe and other unavoidable impurities at 820 ~ 845 ℃; Then quenching the hot rolled steel sheet at a cooling rate of 80 to 1000 ° C./sec to 650 to 720 ° C. and then air cooling; And And cooling and then winding the air-cooled steel sheet at a cooling rate of 80 to 1000 ° C / sec to 450 to 475 ° C. Elongation (EL) and hole expansion equipment (HER) defined by the following equations (1) and (2) is a manufacturing method of high strength hot rolled steel sheet excellent in stretch flange and ductility balance of 24 to 27% and 105 to 121%, respectively. [Equation 1] EL (%) = 296.1-0.338 × FDT (° C) -0.48 × CT (° C) + 0.000605 × FDT (° C) × CT (° C) [Equation 2] HER (%) = 2204.7-6.78 × FDT (℃) + 3.26 × CT (℃) + 0.00508 × FDT (℃) ² -0.000706 × CT (℃) ² -0.00344 × FDT (℃) × CT (℃) (However, FDT is finishing hot rolling temperature and CT is winding temperature) delete delete delete delete The method of manufacturing a high strength hot rolled steel sheet having excellent extension flange and ductility balance according to claim 1, wherein the air cooling time during air cooling is controlled to 4 to 6 seconds. delete

Images