KR100627475B1 - Method for manufacturing a high-strength hot rolled steel sheet havigng superior surface properties by using mini mill process - Google Patents

Abstract

Provided is a method for manufacturing a resistive high ratio high strength hot rolled steel sheet having excellent surface characteristics.

In the manufacturing method of the present invention, by weight% C: 0.03 to 0.08%, Mn: 0.5 to 2.0%, Si: 0.3% to 0.8%, P: 0.07% or less, S: 0.02% or less, balance Fe and other unavoidable impurities Continuous casting of molten steel made of a thin slab; Maintaining the thin slab in a range of 20 minutes or less after heating to a temperature range of 1050 ° C to 1180 ° C; After the hot thin slab finish hot rolling in the temperature range of Ar ₃ ~ Ar ₃ +60 ℃ and the first cooling at a cooling rate of 30 ℃ / sec or more in the temperature range of 650 ~ 730 ℃ and air-cooled; And winding the air-cooled hot rolled sheet at a temperature of 250 ° C. or less by secondary cooling at a cooling rate of 30 ° C./sec or more.

The present invention can provide a low-strength-type high-strength hot-rolled steel sheet having excellent surface properties having a tensile strength of 540 MPa or more, a yield ratio of 0.7 or less, and an elongation of 24% or more through a mini mill process.

Mini Mill Process, Composite Steel, Red Scale, Resistance Ratio

Description

METHOD FOR MANUFACTURING A HIGH-STRENGTH HOT ROLLED STEEL SHEET HAVIGNG SUPERIOR SURFACE PROPERTIES BY USING MINI MILL PROCESS}

The present invention relates to a method for manufacturing a resistance-ratio high tensile hot rolled steel sheet having excellent surface properties using a mini mill process, and more specifically, a tensile strength of at least 540 MPa, a yield ratio of 0.7 or less, and an elongation of 24% using a mini mill process. The present invention relates to a method for manufacturing a resistance-ratio-type high strength hot-rolled steel sheet having surface characteristics that is superior to conventional Si-added resistance-ratio type high tensile strength hot rolled steel sheets.

Recently, in order to improve the fuel efficiency of automobiles and increase the strength of automobile structural materials in order to protect the driver's safety in the event of a collision, the strength of automobiles is rapidly increasing. However, when high strength steel is adopted, it is difficult to form during normal processing, and thus, development of high strength steel that is easier to form is required. Therefore, ferrite and martensitic composite steel having excellent strength and elongation while having a resistivity ratio have been developed and used conventionally.

As a manufacturing method of such a resistive composite composite structure hot rolled steel sheet, the invention proposed by Unexamined-Japanese-Patent No. 81-54371, 83-18970, 86-11291, etc. is mentioned. In the present invention, a slab is manufactured by continuously casting molten steel of C-Si-Mn or C-Si-Mn-Cr-based component system, and then reheated, and then a composite tissue steel is manufactured through the proposed rolling and cooling conditions. Presenting.

However, in the case of such a resistive complex composite hot-rolled steel sheet, a large amount of Si is added to facilitate the production of the composite steel by promoting C concentration in ferrite transformation and austenite. However, when the Si-added steel is heated to a high temperature in a reheating furnace for a long time, fayalite (Fe ₂ SiO ₄ ) is formed. When the composite oxide is heated to 1180 ° C. or more, the complex oxide becomes liquefied and enters the grain boundary. And the invading fayalite acts as an anchor, degrading descalability. Accordingly, the undropped scale generates red scale by additional oxidation during the rolling process. However, since the red scale has a different pickling speed from the normal scale, it remains in the form of a surface stain after pickling, causing a problem of deterioration of surface quality and transfer of spots even after electrodeposition coating of molded automotive parts. In addition, there is a problem that the surface roughness of the steel sheet may be rough, leading to deterioration of the fatigue characteristics.

Therefore, the present invention has been made in order to solve the problems of the prior art, instead of the conventional re-heating-hot rolling process, by setting the conditions of the play-rolling direct connection process and the intermediate heating and heating apparatus of the mini-mill process, containing the conventional Si It is an object of the present invention to provide a method for manufacturing a high-resistance high-strength hot-rolled steel sheet having excellent surface properties that can effectively solve the problem of deterioration of surface quality due to the red scale occurring in the production of a high-resistance high-strength steel sheet.

The present invention for achieving the above object,

By weight casting, continuous casting of molten steel consisting of C: 0.03 ~ 0.08%, Mn: 0.5 ~ 2.0%, Si: 0.3% ~ 0.8%, P: 0.07% or less, S: 0.02% or less, balance Fe and other unavoidable impurities Preparing a thin slab; Maintaining the thin slab in a range of 20 minutes or less after heating to a temperature range of 1050 ° C to 1180 ° C; After the hot thin slab finish hot rolling in the temperature range of Ar ₃ ~ Ar ₃ +60 ℃ and the first cooling at a cooling rate of 30 ℃ / sec or more in the temperature range of 650 ~ 730 ℃ and air-cooled; And a secondary cooling of the air-cooled hot rolled sheet at a cooling rate of 30 ° C./sec or more, and winding up at a temperature of 250 ° C. or less. will be.

Hereinafter, the present invention will be described.

As described above, in the process of manufacturing a high-strength composite tissue steel using a conventional play-reheating-hot rolling process, etc., there was a practical problem of red scale generation. Therefore, the present inventors conducted research and experiments to develop a resistive ratio high-tension hot-rolled steel sheet manufacturing technology using a mini mill process instead of the conventional conventional process.

Unlike the existing mill, the mini mill process is directly connected to the continuous casting and hot rolling processes, and thus it is possible to manufacture the steel sheet continuously without the long time high temperature reheating process. Therefore, it is found that the generation of fayalite is not easy as in the conventional process, and the intermediate heating and the holding temperature are also low, so that no dropout scale is generated by the liquefaction of the fayalite, and the present invention is proposed.

In other words, the present invention can provide a method for producing a high-strength, high-strength hot-rolled steel sheet having excellent surface properties without the generation of scale while using a C-Si-Mn-based component system using this advantage of the mini-mill process.

Hereinafter, the emphasis component of the present invention and the reason for limitation thereof will be described.

Steel C is added to secure strength and is an essential element for producing martensite, which is a reinforcement phase of composite steel. In the present invention, it is preferable to limit the content of C to 0.03 to 0.08% by weight (hereinafter referred to as "%"). If the content is less than 0.03%, the effect of the addition cannot be expected, and if it exceeds 0.08%, the peritetic Continuous casting may be difficult due to the large amount of coagulation shrinkage in the component region where the reaction occurs. Particularly, in the case of a thin slab casting machine, the casting speed is generally 4.0 m / min or more, which is very fast compared to the casting speed of a general player, and breakout occurs frequently, so that the productivity is reduced, so the content thereof may be limited to 0.08% or less.

Si in steel is a useful element that has high solid-solution strengthening ability and can raise strength without impairing the balance of yield ratio, strength and elongation. Si also activates the transformation from austenite to ferrite to promote C concentration in austenite and contributes to the formation of a complex structure of ferrite and martensite. In addition, Si is an element useful for cleaning steel as a deoxidation element in steelmaking. However, if the amount of Si added is too small, the effective effect is not obtained, and if excessively, a scale having poor peelability is formed on the surface, and thus red scale may occur after rolling, resulting in deterioration of the surface quality.

Therefore, in the present invention, it is preferable to limit the amount of Si added to 0.3 ~ 0.8%.

Mn in the steel not only contributes to the strength improvement but also improves the quenching property, thereby facilitating the formation of the second phase martensite. In addition, the solid solution S, which is a cause of brittle cracks during hot working, is precipitated with MnS to have an effect of making harmless.

In the present invention, it is preferable to limit the Mn content to 0.5 to 2.0%. If the added amount is less than 0.5%, it is difficult to expect the effect of the addition, and if it exceeds 2.0%, the elongation decreases with increasing strength and the weldability may deteriorate.

Steel grade P is added in an appropriate amount for high strength and improved corrosion resistance, which also acts as a ferrite forming element, thereby facilitating formation of composite tissue steel. However, if the content exceeds 0.07%, the ductility and toughness of the material may be lowered, and segregation may also be severe. Therefore, the present invention is preferably limited to 0.07% or less.

S in steel is an element that inhibits hot workability, toughness and weldability, and especially when the content exceeds 0.02%, deterioration of these properties may be severe. In addition, a large amount of S increases coarse inclusions and may also degrade fatigue characteristics.

Therefore, in the present invention, it is preferable to limit the content of P to 0.02% or less.

In the present invention, Cu, Sn and the like can be contained as impurities.

In the present invention, after preparing the molten steel having the composition as described above, to produce a thin slab through the continuous casting. In the present invention, the thickness of the thin slab is preferably limited to 50 ~ 100mm. In the present invention, after the thin slab is manufactured as described above, rough rolling may be performed immediately.

Subsequently, in the present invention, the thin slab is heated through an intermediate heating facility and a heating facility, and then maintained for a predetermined time. That is, in the present invention, the thin slab is manufactured by continuous casting, and is immediately heated and maintained in an intermediate heating facility and a heat keeping facility without cooling to room temperature. This process is a process of the present invention that is largely distinguished from the conventional process of roughly rolling the slab and then performing rough rolling.

In order to secure a good material of steel, it is necessary to maintain a certain range of finish rolling temperature. In order to secure this temperature, the mini mill has a heating and heating device between the roughing mill and the finish rolling mill. However, the present invention is not limited to the specific heating equipment and the thermal insulation equipment, and any of them satisfy the above-mentioned conditions, and any of them fall within the scope of the present invention.

On the other hand, in order to manufacture a resistive ratio high strength steel sheet having excellent surface characteristics, it is necessary to set the heating and heat-resistance temperature ranges and the setting of the holding time, which can suppress formation of a scale containing Si oxide to the maximum.

Therefore, in the present invention, the heating and heating temperature range is limited to 1050 ℃ ~ 1180 ℃. If the temperature of the thin slab (or bar) is less than 1050 ℃, it is difficult to secure the finish rolling temperature, it is difficult to secure the material, if the temperature exceeds 1180 ℃, the amount of non-falling scale generated by the liquefaction of fayalite may increase. .

In addition, in the present invention, the holding time after heating is limited to 20 minutes or less, because if it exceeds 20 minutes, the scale grows and adheres well, and the remaining scale increases after descaling, causing red scale after finishing rolling. . More preferably, the holding time is limited to 10 to 20 minutes.

As described above, when the Si-added steel is heated in a reheating furnace in a conventional manner, fayalite (Fe ₂ SiO ₄ ) is formed. When the oxide is heated to 1180 ° C. or more, the oxide becomes liquefied and enters the grain boundaries. In addition, the invading fayalite acts as an anchor, degrading descalability. Accordingly, the undropped scale has a problem of generating red scale on the surface through additional oxidation during the rolling process.

However, in the present invention, by adopting the mini mill process, since the rough rolling can be continuously performed without reheating for a long time at a high temperature after continuous casting, the scale generation amount is also very small.

Next, in the present invention, the heated thin slab (or roughly rolled bar) is finished hot rolled at a temperature range of Ar3 to Ar3 + 60 ° C. If the finishing temperature is less than the Ar3 transformation point, the elongation may be lowered by the abnormal reverse rolling and the workability may also be very poor. When Ar3 + 60 ° C is exceeded, the introduced strain is removed by recovery, and the austenite grains also grow to decrease the ferrite nucleation site, thereby minimizing the ferrite grains, resulting in a decrease in balance between strength and elongation. have.

Subsequently, in the present invention, the finish-rolled hot rolled plate is first cooled in a temperature range of 650 to 730 ° C. at a cooling rate of 30 ° C./sec or more, and then air cooled. The resistive-ratio high strength steel is composed of ferrite, martensite and some bainite, so the strength, elongation and yield ratio of the steel sheet may vary depending on the cooling conditions in the cooling process after finishing rolling. Therefore, in order to properly deposit the fine ferrite after the finish rolling in accordance with the strength, cooling in the temperature range of 650 ~ 750 ℃ at a cooling rate of 30 ℃ / sec or more is required. If it is out of the temperature range, the ferrite transformation rate is lowered from the ferrite transformation nose, and thus, the air cooling time required to secure a portion of the ferrite becomes longer, and if the length of the cooling stand is insufficient, the lowering of the mail speed May cause a decrease in productivity.

In addition, in the present invention, it is preferable to maintain air cooling for at least 3 seconds after such cooling. In order to manufacture a high strength steel having a high resistivity ratio with excellent strength and elongation, it is necessary to secure an appropriate fraction of ferrite, and for this purpose, it is preferable to maintain an air cooling time of at least 3 seconds. More preferably, the air is maintained in the range of 3 to 100 seconds after the cooling.

In the present invention, the air-cooled hot rolled sheet is secondarily cooled at a cooling rate of 30 ° C./sec or more, and wound up at a temperature of 250 ° C. or less. If the cooling rate is less than 30 ° C / sec or the winding temperature exceeds 250 ° C the bainite fraction is increased may lead to a deterioration of the balance between strength and elongation.

Hereinafter, the present invention will be described through preferred embodiments.

(Example)

division              Chemical composition (% by weight)  Remarks C Mn Si P S A 0.061 1.40 0.33 0.03 0.005 Invention steel B 0.062 1.39 0.65 0.02 0.006 C 0.055 1.35 0.87 0.03 0.004 Comparative steel D 0.025 1.37 0.61 0.03 0.005 E 0.058 1.44 0.02 0.03 0.005

After preparing a molten steel having a composition as shown in Table 1, to produce a thin slab of 60mm thickness through continuous casting. Subsequently, the thin slab was roughly rolled to have a thickness of 30 mm, and then hot-rolled composite steel was manufactured by varying the process conditions as shown in Table 2 below.

On the other hand, for comparison, the molten steel having a composition of Table 1 was cast slab having a thickness of 230 mm, and then cooled to room temperature. Then, after re-heating the cooled slab again at 1200 ° C. for 180 minutes in a reheating furnace, hot-rolled composite steel was prepared by varying the process conditions as shown in Table 2 below.

Yield strength, tensile strength and elongation of the hot rolled steel sheets having a thickness of 3 mm thus prepared were measured and the results are shown in Table 3 below. In addition, these hot rolled steel sheets were visually observed and their surface grades are shown in Table 3 below. In Table 3, O denotes a case where there is no red scale, Δ denotes a case where the red scale is partially observed, and X denotes a case where the red scale is severe.

Steel grade Slab thickness (mm) Heating condition Finish rolling temperature (℃) Primary cooling rate (℃ / s) Medium air cooling temperature (℃)                                              2nd cooling rate (℃ / s) Winding temperature (℃) Remarks Temperature (℃) Time (min) One A 60 1100 14 820 44 670 37 220 Inventive Example 1 2 60 1170 11 835 38 691 33 210 Inventive Example 2 3 230 - - 830 50 680 45 180 Conventional Example 1 4 B 60 1080 18 800 45 667 44 200 Inventive Example 3 5 60 1120 12 700 41 623 37 150 Comparative Example 1 6 60 1105 15 815 15 756 55 130 Comparative Example 2 7 60 1089 16 833 46 689 15 221 Comparative Example 3 8 60 1205 26 860 51 687 49 180 Comparative Example 4 9 60 1120 18 805 31 678 56 178 Comparative Example 5 10 230 - - 826 55 671 54 160 Conventional Example 2 11 C 60 1140 18 832 46 682 41 220 Comparative Example 6 12 60 1190 21 840 39 691 43 201 Comparative Example 7 13 230 - - 838 51 662 48 153 Conventional Example 3 14 D 60 1126 13 829 38 688 42 238 Comparative Example 8 15 230 - - 845 51 662 46 192 Conventional Example 4 16 E 60 1103 19 805 39 682 39 244 Comparative Example 9 17 230 - - 834 47 681 47 207 Conventional Example 5

Steel grade Yield strength (MPa) Tensile Strength (MPa) Yield fee Elongation (%) Surface grade Remarks One A 379 566 0.67 29 O Inventive Example 1 2 384 569 0.67 29 O Inventive Example 2 3 377 571 0.66 28 △ Conventional Example 1 4 B 374 597 0.63 29 O Inventive Example 3 5 470 547 0.86 22 O Comparative Example 1 6 455 722 0.62 20 O Comparative Example 2 7 483 661 0.73 24 O Comparative Example 3 8 383 586 0.65 30 △ Comparative Example 4 9 437 691 0.63 22 O Comparative Example 5 10 389 614 0.63 28 X Conventional Example 2 11 C 392 616 0.64 31 △ Comparative Example 6 12 393 607 0.65 30 X Comparative Example 7 13 387 613 0.63 30 X Conventional Example 3 14 D 406 546 0.74 33 O Comparative Example 8 15 412 561 0.73 31 X Conventional Example 4 16 E 402 551 0.73 29 O Comparative Example 9 17 407 573 0.71 28 O Conventional Example 5

In the remarks column of Table 2 and Table 3, Conventional Examples 1 and 2 are cases where conventional operating conditions (not applied to the mini-mill method) were manufactured for Inventive Steels A and B, and Comparative Examples 1 to 5 were applied to Inventive Steels A and B. It applies to the mini-mill method but the conditions are manufactured under conditions outside the conditions of the present invention.
As can be seen from Table 2-3, in the case of the inventive example (1-3) where the emphasis component and the manufacturing process conditions satisfy the scope of the present invention, not only the mechanical properties of the manufactured hot rolled steel are excellent as well. It can be seen that the red scale is not observed.

On the contrary, in the case of the comparative example (1-5) in which the emphasis component is within the scope of the present invention but the manufacturing process is outside the scope of the present invention, mechanical properties of the steel are generally deteriorated. In addition, Comparative Example (6-9), whose emphasis was outside the scope of the present invention, could not secure the desired mechanical properties and surface grade.

On the other hand, also in the case of the conventional example (1-5) which does not use the mini-mill process like this invention, it turns out that red scale is observed overall and a desired characteristic cannot be secured.

Although the present invention has been described through the above embodiments, the present invention is not limited to the description of these embodiments. It is apparent that various improvements, modifications, and the like are possible within the scope of the appended claims of the present invention, and all of them belong to the technical scope of the present invention.

As described above, the present invention is not only excellent in surface properties, but also useful in the production of Si-added resistive high strength steels having a yield ratio of 0.7 or less and an elongation of 24% or more, compared to Si-added resistive high strength steels manufactured by conventional methods. There is.

Claims (4)

By weight casting, continuous casting of molten steel consisting of C: 0.03 ~ 0.08%, Mn: 0.5 ~ 2.0%, Si: 0.3% ~ 0.8%, P: 0.07% or less, S: 0.02% or less, balance Fe and other unavoidable impurities Preparing a thin slab; Maintaining the thin slab in a range of 20 minutes or less after heating to a temperature range of 1050 ° C to 1180 ° C; After the hot thin slab finish hot rolling in the temperature range of Ar ₃ ~ Ar ₃ +60 ℃ and the first cooling at a cooling rate of 30 ℃ / sec or more in the temperature range of 650 ~ 730 ℃ and air-cooled; And The air-cooled hot rolled sheet secondary cooling at a cooling rate of 30 ℃ / sec or more and wound at a temperature of 250 ℃ or less; resistance-complex type high tensile strength hot rolled steel sheet manufacturing method using a mini-mill process including a The method according to claim 1, wherein the thin slab obtained by the continuous casting has a thickness of 50 to 100 mm. The method according to claim 1, wherein rough rolling is performed immediately after the thin slab is manufactured by the continuous casting. The method according to claim 1, wherein the high-temperature resistance high-strength hot-rolled steel sheet having excellent surface characteristics using a mini mill process is characterized by air cooling for at least 3 seconds after the primary cooling.