KR100368241B1 - A method for manufacturing hot rolled trip steels with excellent flange formability - Google Patents

Abstract

The present invention relates to a method for producing a hot rolled metamorphic organic plastic steel having excellent flange workability, and its purpose is to control the composition and hot rolling conditions appropriately, so that it can be produced without other heat treatment processes, and has excellent tensile strength and local ductility. The present invention provides a method for producing a transformed organic plastic steel having excellent flange workability.

The present invention for achieving the above object by weight, contains 0.16-0.20% C, 1.5-1.9% Si, 1.4-1.6% Mn, 0.02-0.10% Al, to which Cu is added at least 0.5% , Ni is added 0.5% or more, while satisfying the range of 0.17≤% C +% Cu / 15≤0.21, and the rest is made of Fe and unavoidable impurity elements. Next, the present invention relates to a method for producing a hot rolled metamorphic organic plastic steel having excellent flange workability, which is characterized by starting water cooling at 680-720 ° C., stopping water cooling at a temperature range of 420-500 ° C., and then performing slow cooling. Shall be.

Description

Method for manufacturing hot rolled metamorphic organic plastic steel with excellent flange workability {A METHOD FOR MANUFACTURING HOT ROLLED TRIP STEELS WITH EXCELLENT FLANGE FORMABILITY}

Method of this invention for producing relates to the transformation of organic plastic steel production method that is used for various purposes such as automobile parts, and more particularly, the tensile strength of 80kg / mm ² or more, excellent in elongation of 25% or more flange formability hot rolled transformation organic plastic steel It is about.

Metamorphic organic plastic steel (hereinafter also referred to as "TRIP steel", abbreviation for TRansforamtion Induced Plasticity steel) is basically Si-Mn type steel, which is environmentally friendly because it has no influence of trace elements during scrap recycling. Also, since it shows much higher strength and high ductility than general rolled steel, it is attracting attention as a next-generation steel to be widely used as a high tensile steel for processing. In particular, the use of hot rolled TRIP steel is expected to increase significantly, especially for parts that require high strength while undergoing a lot of processing. Therefore, one of the important uses is a wheel disk of an automobile. In order to manufacture such a part, flange workability should be good, but since flange workability is not necessarily improved by good elongation, it is very necessary to improve flange workability of a material. Such a flange is excellent steel workability is good local processing characteristics to be tolerated when manufactured as a part through the actual machining, it is a steel suitable for practical use in various machining applications.

High-strength, high-ductility hot-rolled steel sheet has been developed and manufactured in a substantial number of manufacturing methods have been incorporated into actual commercialization. The development of high strength high ductility steels in the 80's was due to the dual phase steel (composite structure of ferrite and martensite), triphase steel (composite structure of ferrite, bainite and martensite), and ferrite-bainite (ferrite). -bainite) mainly for composite tissue steels. These steels have a tensile strength of about 60 kg / mm 2, and have an elongation of about 30%, showing high ductility and high ductility. TRIP steel has been developed in the 90's and is currently working for commercialization. The characteristic of this steel is that its tensile strength is about 80kg / mm2, and its elongation is excellent, indicating high strength and high ductility. A precipitation-reinforced dual-faced steel sheet that effectively strengthens soft ferrite in dual-face steel by precipitation strengthening using Ti has also been developed (December 4, 1993). Such product development has been mainly performed in developed countries mainly in Japan, and summarizes the known manufacturing techniques developed below.

In the case of Japan Nippon Steel Co., Ltd., it can be manufactured as dual-face steel by winding 0.06-0.10% C, 0.25-1.3% Si, 1.1-1.5% Mn steel at about 300 ℃ or less. (鐵 と 鋼, Vol. 68 (1982) No. 9, p. 1306), and in Kobe Steel, 0.04-0.06% C, 0.5-1.0% Si, 1.5% Mn steel -1.5% Cr is added and the steel is finished at around 850 ℃ and cold winding (about 250 ℃) is used to form ferrite as a base structure and 10-20% of bainite and 3-5 It has been reported that triphase steels containing% martensite can be produced (鐵 と 鋼, Vol. 68 (1982) No. 9, p. 1185). In addition, in Kobe Steel, the tensile strength containing 10-20% of bainite in the ferritic matrix is increased by 60kg / mm2 by adding less than 0.04% of Nb to 0.05-0.07% C, Si≤0.5% and 1.1-1.5% Mn steel. A grade ferrite-bainite composite steel can be manufactured (Trans, ISIJ, Vol. 23 (1983), p.303), and Sumitomo Metal Co., Ltd. has similar Nb and Ti in similar basic components, respectively. A ferrite-bainite composite tissue steel with a tensile strength of 70 kg / mm 2 was added by adding 0.04% and 0.06% (CAMP-ISIJ, Vol. 1 (1998, p.881).

The presence of retained austenite in steel tends to increase the uniform elongation because the work hardening becomes very large as the residual austenite is transformed into martensite during processing of the material. By using this phenomenon, steel containing residual austenite is often used in the production of high strength, high workability steel. Such residual austenite-containing steel shows high ductility at about 80 kg / mm2 or more when the manufacturing conditions are optimized, and many manufacturers have filed patents for various manufacturing conditions.

In Nippon Steel, 0.03-0.30% Mo is added to steel containing 0.06-0.22% C, 0.05-1.0% Si, 0.5-2.0% Mn and 0.25-1.5% Al as necessary to add residual austenite 3-20. It contains 50kg / mm2 or more of high strength and elongation of more than 35% with the addition of%, and has filed a patent for steel with excellent press workability, deep workability and bendability (Japanese Patent Laid-Open Publication No. 6-6). 145892). The amount of Al in this steel is adjusted in the range of 0.6% Si ≤% Al ≤ 3-12.5 x% C, and the steel is heat treated in a two-phase region (350-600 after holding for 10 seconds to 3 minutes at 650-900 ° C). After cooling to 4-200 ℃ / sec to the temperature range of ℃, it is maintained here for 5 seconds to 10 minutes and then cooled to a temperature below 250 ℃ at a cooling rate of 5 ℃ / sec or more). Patents are also submitted for the manufacturing method (Japanese Patent Laid-Open No. Hei 6-145788).

In Kawasaki Steel, 0.08% or less of C, 0.5-2.5% Si, 0.5-2.5% Mn, 0.05% or less P, 0.02% or less S, 0.01-0.1% Al steel, 0.02-0.5% Ti and 0.03-1.0% Nb are added alone or in combination (wherein C, Nb and Ti are added in the range of% C> (% Ti / 4) + (% Nb / 8)), at this time filament rolling After finishing the temperature above 820 ℃, the patent has been filed for the case of maintaining at least 10 seconds in the temperature range of 820-720 ℃, then cooling with the cooling rate of 10 ℃ / sec or higher and winding at the temperature below 500 ℃. . This steel has a high strength of 70kg / mm2 or more at a tensile strength of less than 0.18% C, and a low C content, which is excellent in spot weldability and fatigue properties (Japanese Laid-Open Patent Publication, 5-179396). In addition, because the steel shows a resistance yield ratio, the conventional precipitation-reinforced steel has a high yield ratio, which solves the problem of a large number of spring backs after pressing, and exhibits high strength, high ductility, and excellent stretch flangeability. do.

In Sumitomo Metal, the steel containing 0.05-0.25% C, 0.05-1.0% Si, 0.8-2.5% Mn, 0.8-2.5% Al and the precipitation-reinforcing elements such as Nb, Ti and V were added to the steel containing 780-840 ℃. After rolling in the temperature range of and then cooled to a temperature of 600-700 ℃ at a cooling rate of 10 ℃ / sec or more, and then subjected to air cooling of 2-10 seconds and at a temperature of 300-450 ℃ at a cooling rate of 220 ℃ / sec A patent for a method for producing steel with 5% or more of retained austenite as a result of accelerated cooling was also submitted (Japanese Patent Laid-Open No. 5-112846). In the case of a large amount of Si, the amount of hard martensite increases and the stretch flange is deteriorated, so that Si is limited to 1.0% or less, which is the amount required for solid solution strengthening in precipitated polygonal ferrite.

As discussed above, many types of hot rolled high tensile strength steels are summarized in terms of component systems and manufacturing conditions which can improve the strength and elongation as main points. However, from a practical application point of view, most of the real part machining processes include flange machining, and the parts where machining defects occur during deep machining are often placed on these flange machining sites. Therefore, in order to improve such a flange workability, it is calculated | required that the local stretching characteristic of a raw material is excellent.

In order to meet the above demands, the inventors have repeatedly conducted research and experiments and proposed the present invention based on the results. The present invention can be produced without any heat treatment process by appropriately controlling the composition and hot rolling conditions. The present invention also provides a method for producing a modified organic plastic steel with improved flange workability due to excellent tensile strength and excellent local ductility.

1 is a graph showing the control range of the% C +% Cu / 15 amount and the water cooling stop temperature to secure the target material

The present invention for achieving the above object by weight, contains 0.16-0.20% C, 1.5-1.9% Si, 1.4-1.6% Mn, 0.02-0.10% Al, to which Cu is added at least 0.5% , Ni is added 0.5% or more, while satisfying the range of 0.17≤% C +% Cu / 15≤0.21, and the rest is made of Fe and unavoidable impurity elements. Next, after the start of water cooling at 680-720 ° C, after stopping the water cooling in the temperature range of 420-500 ° C, the present invention relates to a method for producing hot rolled metamorphic organic plastic steel excellent in flange workability.

Hereinafter, the present invention will be described in detail. First, the composition of the composition and the reason for limitation thereof will be described.

The C is an element that increases the hardenability of the steel, it is difficult to obtain the target strength at less than 0.16%, and do not cause sufficient C concentration due to the unmodified austenite, thereby preventing the remaining austenite from stabilizing at room temperature. In addition, in the case of exceeding 0.20%, the strength is increased but correspondingly, the ductility decreases and the flange workability decreases, so it is limited to the range of 0.16-0.20%.

The Si is an element mainly used for deoxidation to increase the activity of C, thereby facilitating the formation of ferrite at a high temperature. In addition, since the concentration of austenite remaining in the formation of ferrite at a high temperature promotes the concentration of C, the addition of Si is essential in the metamorphic organic steel. Excessive addition of Si produces a red scale on the surface, which not only deteriorates the surface but also easily forms oxides during welding, so that defects in the weld portion (penetration, etc.) are likely to occur. Therefore, in the present invention, it was limited to 1.5-1.9%, which is an amount necessary to sufficiently form residual austenite.

The Mn is an element that increases the strength and toughness of the steel and stabilizes austenite, thereby lowering the Ms temperature and increasing the hardenability of the steel. Increasing Mn increases the amount of low temperature transformation products, which is advantageous in strength but difficult to secure ductility. In addition, excessive addition of Mn is disadvantageous by increasing the amount of nonmetallic inclusions and increasing segregation degree. However, when the amount of Mn is low, the unmodified austenite remaining after ferrite transformation is difficult to form bainite effectively, and thus it is difficult to form residual austenite, and it is easy to form pearlite at high temperature, thus making it easy to form pearlite. It is easy to fall. Therefore, in the present invention, in order to effectively form the residual austenite, it was limited to the range of 1.4-1.6%.

Al is an element mainly used for deoxidation, and thus is advantageous in terms of processability improvement since it helps to form ferrite. When Al is added to improve workability, a decrease in tensile strength occurs. Therefore, in order to obtain a target material, it is preferable to limit Al to a range in which Al is added for deoxidation. In other words, at least 0.02% is required for deoxidation, and if excessively contained, the strength is lowered and the upper limit is limited to 0.10% because it is easy to form an oxide during welding for use as a structural material.

The Cu has the advantage of achieving high strength due to precipitation hardening by forming the ε-Cu precipitate, which is Cu alone, and the decrease in elongation and the decrease in flange formability are not so large as the strength is increased. In the case of general corrosion resistant steels, the amount of Cu added up to about 0.3-0.4% is generally used. In the present invention, addition of 0.5% or more is used to actively utilize the precipitation of ε-Cu in addition to improving the corrosion resistance. Conduct.

In general, in the case of Cu-added steel, hot ductility may be reduced, so that cracks may occur during hot working, and thus, Ni is effectively added to prevent this, and the present invention prevents deterioration of hot workability by Cu. In order to achieve this, 0.5% or more of Ni is added, but it is preferably added below the upper limit of Cu that satisfies 0.17 ≦% C +% Cu ≦ 0.21. At this time, the amount of Cu and Ni is more preferably similar.

On the other hand, Nb is an element effective for high strength by causing precipitation strengthening by forming carbonitrides in a matrix. In addition, Nb effectively inhibits the recrystallization of austenite at high temperature, thereby miniaturizing the austenite grains before transformation, and the carbon pushed out by the formation of ferrite during transformation is easily concentrated into unmodified austenite, thereby increasing the austenite stability. . However, the precipitation of carbonitrides causes a large reduction in elongation and greatly deteriorates flange workability. Therefore, the present invention restricts the addition of carbide forming elements.

In addition, P and S are elements that promote the formation of ferrite and can increase the ductility without harming the strength of the steel, but in general, when the steel is manufactured, the segregation is extremely severe and deteriorates the material due to the formation of the center segregation. In addition, S forms a nonmetallic inclusion represented by MnS, greatly degrading the workability of the steel. These nonmetallic inclusions are elongated during rolling, and are prone to fatal defects such as cracking during processing. Therefore, it is desirable to manage P and S as low as possible. For this purpose, it is also necessary to manage the low S by adding Ca, the addition amount is preferably 50ppm or less in the steel.

Next, a method of hot rolling using a steel material composed of the composition as described above will be described in detail.

In the present invention, in order to secure excellent strength and ductility of the hot rolled material, it is necessary to control the microstructure, and therefore, it is necessary to control the rolling finish temperature, the water cooling start, and the water cooling end temperature.

If the rolling finish temperature is too high, the austenite grains after rolling are increased to increase the hardenability, so that the amount of low-temperature transformation structure such as martensite or bainite is greatly increased after rolling, thereby strengthening the material of the steel after rolling and decreasing ductility, If this temperature is too low, the microstructure and material deviation are greatly generated due to the locally generated material temperature deviation during hot rolling. In the present invention, the lower limit thereof is set to 780 ° C to prevent this.

The start of the water cooling is preferably carried out after forming sufficient ferrite through air cooling before, if the water starting temperature is too high enough ferrite is not formed, so the fraction of the second phase after cooling greatly increases the strength Ductility is lowered, and if this temperature is too low, pearlite is produced, so in the present invention, it was limited to 680-720 ° C. which is experimentally confirmed. In addition, the water cooling stop temperature is the most important factor in determining the material. As mentioned above, the upper limit is set to 500 ° C. so that pearlite is not produced by the slow cooling and the strength is not significantly lowered. It is necessary to set it to 420 degreeC so that it may not generate | occur | produce. The water cooling stop temperature should be changed as the component system of steel is changed. Actually, the control of the amount of C and Cu, which are elements that greatly affect the formation of residual austenite, and the control of the water cooling stop temperature are the target materials. It is very important to get. The amount of C and Cu is controlled by% C +% Cu / 15 since the influence of Cu on the formation of residual austenite is 1/15 of C.

In order to secure the target material level as shown in Fig. 1, it is preferable to keep the% C +% Cu / 15 between 0.17 and 0.21 as the component system even if the water cooling stop temperature is maintained within 420-500 ° C. When the value of% C +% Cu / 15 is less than 0.11 and less than 0.21, the target material of the present invention cannot be obtained as shown in Table 2 and FIG. 1.

That is, the target material of the present invention can be obtained by controlling the water cooling stop temperature and% C +% Cu / 15 at the same time to secure appropriate residual austenite.

When manufacturing a hot rolled steel sheet according to the composition and manufacturing conditions of the steel composition of the present invention as described above, the tensile strength is 80kg / mm ² or more and the elongation is 25% or more can be produced a steel material having excellent flange workability, processing It can be used to manufacture high strength high ductility hot rolled transformation organic plastic steel which can be used for

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

Example 1

To prepare a slab (slab) by dissolving a steel having a composition as shown in Table 1. The slab was reheated at 1200 ° C. and hot rolled to produce a hot rolled steel sheet having a final thickness of 3.0 mm.

At this time, the hot rolling finish temperature (FRT), the water cooling start temperature (CS) and the water cooling end temperature (CF, winding temperature at the time of hot rolling) performed for cooling control after hot rolling are as shown in Table 2 below. It was done. That is, after the end of hot rolling, after quenching by roll quenching, air cooling is performed for a predetermined time to change the water cooling start temperature, and then transferred to a water cooling simulator to cool the water cooling stop temperature by water cooling. Later, by performing the furnace cooling, a process of manufacturing the hot rolled coil and then slowly cooling was simulated.

The hot rolled steel sheet thus manufactured was subjected to a tensile test in a hot rolled state to investigate tensile properties such as yield strength (YS), tensile strength (TS) and elongation (EI), and NTE (Notch Tensile), which may be an index of flange formability. Elongation, HER (Hole Expansion Ratio) and bending characteristics were measured. The measured results are shown in Table 2 below.

Steel grade C Si Mn P S Al Nb Cu Ni Comparative Steel A 0.201 1.94 1.51 0.015 0.004 0.042 - - - Comparative Steel B 0.180 1.92 1.48 0.015 0.004 0.034 - - - Comparative Steel C 0.151 1.96 1.52 0.015 0.004 0.042 - - - Comparative Steel D 0.149 1.51 1.49 0.016 0.004 0.043 - - - Comparative Steel E 0.179 1.76 1.50 0.015 0.004 0.039 0.031 - - Comparative Steel F 0.160 1.52 1.50 0.016 0.004 0.037 0.031 - - Comparative Steel G 0.182 1.68 1.49 0.016 0.005 0.036 - 0.594 0.495 Inventive Steel H 0.160 1.54 1.47 0.015 0.004 0.037 - 0.617 0.500

Steel grade Rolling and cooling temperature Tensile Properties Machinability FRT CS CF YS TS EI NTE HER flex Comparative Example 1 Comparative Steel A 799 701 467 58.6 87.3 26.6 5.9 35.0 100 Comparative Example 2 Comparative Steel B 801 706 455 56.0 85.1 29.2 6.6 36.8 100 Comparative Example 3 Comparative Steel C 795 700 424 55.5 85.8 24.8 5.9 33.3 100 Comparative Example 4 Comparative Steel D 792 703 460 53.2 73.1 34.0 8.8 53.9 100 Comparative Example 5 Comparative Steel E 801 700 450 69.9 87.9 19.7 5.3 36.5 0 Comparative Example 6 Comparative Steel F 799 696 447 70.0 83.9 23.2 6.5 39.0 88 Comparative Example 7 Comparative Steel G 804 703 466 58.2 88.2 30.8 6.6 40.5 100 Inventive Example 8 Inventive Steel H 802 705 427 60.8 82.1 27.9 7.4 52.6 100

Table 2 shows tensile properties and flange workability for each steel type when the hot rolling finish temperature and the start and end temperature during water cooling were similarly maintained. The material properties of steels are different. Tensile strength is changing within the range of 70-90kg / mm ² , and elongation is changing in the range of 19-35%. Flange machinability is set at 7% or more with NTE, 50% or more with HER, and bending characteristics are based on the case where no crack occurs (100%) for 180 ° 2t bending. The level of formability required for steels.

In Table 2, Comparative Example 1 is a steel type containing a lot of C and Si, but high tensile strength, but poor flange workability. In Comparative Example 2 having a lower C content than this, the strength and ductility were high, but the flange workability was not sufficient. Even when the amount of C was further lowered (0.15% C, Comparative Example 3), sufficient flange formability was not obtained. When C and Si were lowered to 0.15% and 1.5%, respectively (Comparative Example 4), the ductility was excellent and the flange workability was higher than the standard value, but the strength was not sufficient, and the effect of precipitation hardening was added. Both Nb-added steels (Comparative Examples 5 and 6) show poor elongation and flange workability. In the case of adding the Cu precipitation strengthening effect, in the case of Comparative Example 7 having a high amount of C, sufficient workability was not secured, but in the case of a low amount of C (Invention Example 8), very excellent strength, elongation, and flange workability were shown.

Example 2

In Example 1, using the invention steel H, comparative steel B, comparative steel C, comparative steel D and comparative steel G steel grades, hot-rolled steel sheet was prepared by varying the hot rolling conditions for each steel type. Tensile properties and workability of the prepared steel sheet were measured in the same manner as in Example 1, and the results are shown in Table 3 below.

Temperature during rolling and cooling Tensile Properties Machinability Remarks FRT CS CF YS TS EI NTE HER flex Comparative Steel B One 801 690 548 65.0 78.7 24.6 7.4 59.3 100 Comparative example 2 804 695 518 65.7 78.9 24.7 7.8 57.2 100 Comparative example 3 801 706 455 56.0 85.1 29.2 6.6 36.8 100 Comparative example 4 797 707 411 55.9 96.1 20.9 4.5 27.2 0 Comparative example Comparative Steel C One 804 682 531 63.6 77.5 25.2 7.7 57.5 100 Comparative example 2 805 692 519 51.4 80.5 22.7 7.1 46.1 100 Comparative example 3 795 700 424 55.5 85.8 24.8 5.9 33.3 100 Comparative example 4 806 702 384 88.2 93.1 18.7 4.0 26.7 0 Comparative example Comparative Steel D One 793 685 549 59.6 71.9 27.5 8.8 76.4 89 Comparative example 2 803 692 498 59.4 72.9 26.9 8.1 71.2 100 Comparative example 3 792 703 460 53.2 73.1 34.0 8.8 53.9 100 Comparative example 4 783 696 384 52.0 85.7 22.8 5.3 40.6 67 Comparative example Comparative Steel G One 795 685 554 66.3 80.1 22.2 6.6 55.5 100 Comparative example 2 800 696 503 53.9 95.5 23.1 5.0 32.7 55 Comparative example 3 804 703 466 58.2 88.2 30.8 6.6 40.5 100 Comparative example 4 800 703 347 59.9 96.2 26.0 4.6 3.2 11 Comparative example Inventive Steel H One 799 682 547 62.0 74.1 27.5 7.9 66.2 100 Comparative example 2 798 697 527 55.7 83.1 24.3 7.1 53.6 89 Comparative example 3 802 705 427 60.8 82.1 27.9 7.4 52.6 100 Inventive Example 4 802 697 340 62.0 96.1 19.3 4.5 43.3 100 Comparative example

As can be seen in Table 3, when the amount of alloying elements added in most steel grades increases, the strength increases, but the elongation and flange formability tend to deteriorate. When the water cooling stop temperature decreases, the strength increases but the elongation decreases. This tends to be lowered and the flange workability deteriorates. In particular, when the invention H steel grade is used, conditions for improving tensile properties and flange workability are obtained by maintaining the water cooling stop temperature appropriately.

If the water cooling stop temperature is too high, the microstructure becomes ferrite + pearlite, so that the residual austenite in the steel is lost, so that the strength and elongation are deteriorated in many cases, and the flange workability shows the best value. Residual austenite was formed at the appropriate water cooling stop temperature, resulting in good strength-elongation and good flange workability. When the water cooling stop temperature is too low, the strength is greatly increased, but the elongation and workability tend to be greatly decreased, so it was found that the proper water cooling stop temperature and the setting of the proper water cooling stop temperature are very important.

According to the present invention as described above, by controlling the alloy component system and hot rolling conditions, the hot rolled steel sheet exhibits a high ductility of 25% or more with a high tensile strength of at least 80kg / mm ^{2 and} high elongation after hot rolling, and at the same time excellent flange workability It can be obtained, which is a steel grade that can be used as a material for processing.

Claims (2)

By weight, 0.16-0.20% C, 1.5-1.9% Si, 1.4-1.6% Mn, 0.02-0.10% Al, wherein Cu is 0.17≤% C +% Cu / 15≤0.21 at 0.5% or more The upper limit obtained from the upper limit is added, the upper limit is added, Ni is 0.5% or more, the Cu is added below the upper limit, and the remainder is steel roll composed of Fe and unavoidable impurity elements, and hot rolled at a temperature range of 780 ° C or higher After cooling by air, starting water cooling at 680-720 ° C., stopping the water cooling in the temperature range of 420-500 ° C., and then performing slow cooling. The method of claim 1, The steel is a method for producing a hot rolled metamorphic organic plastic steel excellent in flange workability, characterized in that Ca is added below 50ppm for inclusion spheroidization.