JP5393459B2 - High manganese type high strength steel plate with excellent impact characteristics - Google Patents

Description

  The present invention relates to a high manganese steel used for automobile steel plates and a method for producing the same. More specifically, the present invention relates to a high workability high-strength steel sheet having a high draw ratio, excellent workability, high yield strength, and excellent impact characteristics, and a method for producing the same.

  Automakers are expanding the use of lightweight materials and high-strength materials for the purpose of preventing environmental pollution, improving fuel efficiency, and improving stability. This is a characteristic that materials applied to many structural members other than automotive parts should have. But there is. However, when the strength of the raw material increases, the stretch ratio decreases, and in order to overcome this, duplex stainless steel and transformation-induced plastic steel with excellent formability are used.

  However, high strength steel for machining applied to automotive structural members and inner plate materials developed to date does not satisfy the workability required for automotive parts, so it is difficult to produce parts having complicated shapes. . In order to solve this problem, automakers use a process of simplifying the shape of the parts, dividing the parts into a plurality of parts, and then welding them again. When welding, the strength of the welded part is different from the base metal part, which not only imposes many restrictions on the design of the car body, but also reduces the part characteristics due to the inferiority of the welded part. Greatly increases. Therefore, in order to increase the degree of freedom in designing a vehicle body by applying it to a component having a complicated shape, an automobile manufacturer continues to demand a material having high strength but high workability.

  In the field of automotive steel sheets, recently, there is a demand for high-strength steel sheets with excellent formability that can reduce the weight of automobiles in order to improve fuel efficiency and reduce air pollution. As a conventional automobile steel sheet, a low-carbon steel series high-strength steel whose base structure is ferrite is used in consideration of formability. However, when a high-strength steel of the low carbon steel series is used as a steel plate for automobiles, it is difficult to ensure a stretch ratio of 30% or more commercially when the tensile strength is 800 MPa or higher. Therefore, since it is difficult to apply high strength steel of 800 MPa class or higher to a component having a complicated shape, it is difficult to freely design a component such as simplifying the shape of the component. In order to solve the above problems, austenitic high manganese steel (Patent Document 1, Patent Document 2) excellent in ductility and strength has been proposed. However, in Patent Document 1, ductility is ensured by addition of high manganese, but there is a phenomenon that the strained part is severely strain-hardened and the steel sheet is easily broken after processing. Further, although the above-mentioned Patent Document 2 also ensures ductility, there is a disadvantage that electroplating properties and hot dipping properties are disadvantageous by adding a large amount of silicon. Further, the above steel sheet is excellent in workability, but has a disadvantage that the impact property is inferior due to low yield strength.

  A material used as an automobile material is advantageous when the yield strength is high in order to absorb collision energy and prevent deformation in the event of a collision. However, in the case of high manganese steel, since it has a low yield strength due to having an austenite structure, it is necessary to overcome this.

Japanese Patent Publication No. 1992-259325 International Publication WO02 / 101109 Pamphlet

  It is an object of the present invention to provide a high workability high strength steel sheet having excellent stretch ratio, high workability, high yield strength and excellent impact characteristics, and a method for producing the same.

  In order to achieve the above object, the steel sheet of the present invention is, by weight%, carbon (C) 0.2 to 1.5%, manganese (Mn) 10 to 25%, aluminum (Al) 0.01 to 3.0. %, Phosphorus (P) 0.03% or less, sulfur (S) 0.03% or less, nitrogen (N) 0.040% or less, silicon (Si) 0.02-2.5%, titanium It is composed of at least one selected from the group of (Ti) 0.01 to 0.10% and niobium (Nb) 0.01 to 0.10%, the remaining Fe and other impurities inevitably contained. . The steel sheet of the present invention has an austenite single phase structure.

  The steel sheet of the present invention has a yield strength of 400 to 600 MPa or more and a tensile strength of 900 to 1000 MPa or more, and has a yield strength of 750 MPa or more and a tensile strength of 1000 MPa or more by strain hardening by cold rolling. .

  In the present invention, the steel sheet having the above component system of the present invention is cold-rolled at a rolling reduction of 10 to 80%. Here, the rolling can be any one of temper rolling, double rolling, and hot final rolling. The steel plate may be selected from any one of a hot rolled steel plate, a cold rolled steel plate, and a plated steel plate.

  In the present invention, when producing a cold-rolled steel sheet, the steel having the above component system is homogenized at 1050 to 1300 ° C. and then hot-rolled at a finish rolling temperature condition of 850 to 1000 ° C. After winding in the temperature range, it can be cold rolled at a rolling reduction of 30 to 80% and continuously annealed at a temperature of 600 ° C. or higher.

  According to the present invention, a steel sheet suitable not only as a structural member of a vehicle body but also as an inner plate material having a complicated shape is provided because the stretch ratio is high and the strength is high. This steel plate has a useful effect that can be used for parts such as a front side member of an automobile because of its excellent shock absorbing ability among the properties of the steel plate.

It is a photograph of a fine structure. It is the graph which showed the change of the tension curve by the increase amount of a cold rolled steel plate, and the change of a strength-stretching rate.

The present invention will be described in detail below.
The present invention seeks to increase the yield strength of the steel sheet by adding an appropriate amount of at least one of silicon, titanium, and niobium. By using the produced hot rolling, cold rolling, and cold rolling using the plated steel sheet, steel having high yield strength and excellent impact characteristics can be produced.

  In the present invention, in order to obtain an austenite single phase and improve workability by twinning, in order to increase the yield strength by controlling the microstructure while appropriately adjusting the addition amount of manganese, carbon, aluminum, silicon, There is a feature in optimizing the addition amount of titanium and niobium. Moreover, when the strain hardening process is performed by twins, the stretch ratio is very excellent, and therefore, based on the experimental results that the formability necessary for automobile parts can be secured even if the stretch ratio is slightly reduced by cold working. Thus, the yield strength is increased through cold working.

  In the present invention, in order to ensure a complete austenite phase at room temperature, the amounts of manganese and carbon, which are austenite stabilizing elements, are optimized, and twins are formed by deformation of these components. Also, the tensile properties are improved by controlling the rate at which twins are formed by adjusting the amount of aluminum. It is important to minimize the amount of manganese (Mn) added in order to reduce manufacturing costs, and in order to reduce the amount of manganese, it is important to add some carbon. In order to promote twin deformation during the processing of the steel, the addition amount of carbon and aluminum is appropriately adjusted. On the other hand, in order to increase the yield strength, it is preferable to reduce the crystal grain size. For this reason, at least one of silicon, titanium, niobium and the like is added.

  In the present invention, the steel sheet corresponds to a hot rolled steel sheet, a cold rolled steel sheet, and a plated steel sheet.

  Hereinafter, selection of the above steel components and reasons for limiting the component range will be described.

  The carbon (C) content is preferably 0.2 to 1.5%.

  Since carbon contributes to the stabilization of the austenite phase, it is more advantageous as the amount added increases. If the amount of carbon added is less than 0.2%, an α'-martensite phase is generated during phase deformation, so that cracks are generated during processing and ductility may be reduced. When the amount of carbon added exceeds 1.5%, the stability of the austenite phase is greatly increased, and the workability may be lowered due to the transition of deformation behavior due to slip deformation.

  The content of manganese (Mn) is 10 to 30%, more preferably 10 to 25%.

  Manganese is also an essential element for stabilizing the austenite phase, but if it is less than 10%, an α′-martensite phase that impairs the formability is generated, the strength increases, but the ductility decreases rapidly. If the amount of manganese added exceeds 30%, twin formation is suppressed and the strength increases but the ductility decreases. And as the addition amount of manganese increases, cracks are likely to occur during hot rolling, and the manufacturing cost of the steel sheet increases due to the large amount of addition of manganese, which has a high raw material cost. Therefore, the manganese content is preferably 25% or less. .

  The content of aluminum (Al) is preferably 0.01 to 3.0%.

  Aluminum is usually added for deoxidation of steel. In the present invention, aluminum is added for improving ductility. That is, aluminum is a ferrite phase stabilizing element, but increases the stacking fault energy on the slip surface of the steel, suppresses the formation of the ε-martensite phase, and improves the ductility. In addition, since aluminum suppresses the formation of the ε-martensite phase even at a low manganese addition amount, it is very useful for minimizing the addition amount of manganese and improving workability. Therefore, when the added amount of aluminum is less than 0.01%, ε-martensite is generated and the strength is increased, but the ductility is rapidly decreased. And when the amount of aluminum added exceeds 3.0%, twinning is suppressed and ductility is reduced, castability is deteriorated during continuous casting, surface oxidation is severe during hot rolling, and surface quality of the product. Will come down.

  The contents of phosphorus (P) and sulfur (S) are each preferably 0.03% or less.

  Phosphorus and sulfur are elements inevitably contained during the production of the steel sheet, so the range of addition is limited to 0.03% or less. In particular, phosphorus causes segregation to reduce the workability of steel, and sulfur forms coarse manganese sulfide (MnS) to generate defects such as flange cracks, thereby reducing the hole expandability of the steel sheet. It is preferable to suppress the addition amount to a minimum.

  The content of nitrogen (N) is preferably 0.04% or less.

  Nitrogen acts with aluminum during the solidification process to precipitate fine nitrides in the austenite crystal grains and promote the generation of twins, thus improving the strength and ductility when forming the steel sheet. However, when the addition amount of nitrogen exceeds 0.04%, nitrides are excessively precipitated and the hot workability and the stretching ratio are lowered.

  The steel composed as described above includes at least one selected from the group of silicon, titanium, and niobium.

  The content of silicon (Si) is preferably 0.02 to 2.5%.

  Silicon is an element that is solid solution strengthened and is an element that increases the yield strength by reducing the crystal grain size due to the solid solution strengthening effect. Usually, when silicon is added excessively, it is known that a silicon oxide layer is formed on the surface of the steel sheet to deteriorate the hot dipping property. However, in steels with a large amount of manganese added, when an appropriate amount of silicon is added, a thin silicon oxide layer is formed on the surface and suppresses oxidation of manganese. Can prevent the formation of a manganese oxide layer, can prevent corrosion that proceeds in the cold-rolled steel sheet after annealing, improve the surface quality, and maintain excellent surface quality as a base steel sheet for electroplating it can.

  However, when the amount of silicon added is increased, silicon oxide is formed on the surface of the steel sheet when the steel sheet is hot-rolled, thereby deteriorating the pickling property and the surface quality of the hot-rolled steel sheet. Silicon is concentrated on the surface of the steel sheet during high-temperature annealing in the continuous annealing process and the continuous hot-dip plating process, and reduces the wettability of molten zinc on the steel sheet surface when hot-plating the steel sheet, thus reducing the plating characteristics. Further, the addition of a large amount of silicon greatly reduces the weldability of steel. Therefore, the upper limit addition amount of silicon is preferably 2.5%. The impact characteristics are different from the corrosiveness of the plated layer and are related to the mechanical characteristics of the inner metal matrix layer, and the heat treatment conditions for plating do not affect the mechanical characteristics of the high manganese steel sheet with austenite single phase structure. This product includes the impact characteristics of plated products.

  The content of titanium (Ti) is preferably 0.01 to 0.1%.

  Titanium is a strong carbide-forming element that forms a carbide by combining with carbon. The carbide formed at this time is an element that prevents the growth of crystal grains and is effective in reducing the crystal grain size. However, when a very small amount of titanium is added at less than 0.005%, no effect appears, and when it exceeds 0.10%, an excessive amount of titanium segregates at the grain boundaries to cause grain boundary embrittlement or a precipitation phase. May be excessively coarsened to deteriorate the crystal grain growth effect.

  The content of niobium (Nb) is 0.005 to 0.1%, more preferably 0.01 to 0.1%.

  Niobium is a strong carbide-forming element that combines with carbon to form carbides in the same form as titanium. Similarly, the carbide formed at this time is an element that hinders the growth of crystal grains and is effective in refining the crystal grain size, and forms a precipitation phase at a temperature lower than that of normal titanium. It is an element that has a large effect of precipitation strengthening due to the formation of phases. However, when a small amount of niobium is added to less than 0.005%, no precipitation strengthening effect appears, and when it exceeds 0.10%, an excessive amount of niobium segregates at the crystal grain boundary, causing grain boundary embrittlement, The precipitated phase may be excessively coarsened to deteriorate the crystal grain growth effect. The preferred amount of niobium is 0.01 to 0.1%.

  Hereinafter, the manufacturing method of a high manganese steel plate is demonstrated.

  Generally, the production of a high manganese hot rolled steel sheet can utilize a continuous casting method in the same manner as a normal steel production process. The steel is homogenized with the above composition in the same manner as normal conditions, and then finish-rolled and wound to produce a hot-rolled steel sheet.

  In this invention, it is preferable that the continuous slab heating temperature of a high manganese steel plate shall be 1050-1300 degreeC at the time of hot rolling. The reason why the upper limit of the heating temperature is limited to 1300 ° C. is that the higher the temperature, the larger the crystal grain size, the surface oxidation occurs, the strength decreases, and the physical properties of the surface are degraded. . Moreover, since a liquid film will arise in the columnar grain boundary of a continuous casting slab when it heats exceeding 1300 degreeC, a crack may generate | occur | produce at the time of hot rolling. On the other hand, the reason why the lower limit of the heating temperature is limited to 1050 ° C. is that if the heating temperature is low, it becomes difficult to secure the necessary temperature at the time of finish rolling, the rolling load increases due to the decrease in temperature, and the rolling can be sufficiently rolled to a predetermined thickness. This is because it cannot be done. In other words, the normal finish rolling temperature is at least 850 ° C. or more, preferably about 900 ° C. in the hot rolling process, so that lowering the finish rolling temperature increases the rolling load and does not force the rolling mill. It will also adversely affect the quality of the product. And when making rolling finishing temperature too high, since surface oxidation generate | occur | produces at the time of rolling, rolling finishing temperature is restrict | limited to 1000 degreeC.

  The hot rolling coiling temperature is 700 ° C. or less. When the coiling temperature exceeds 700 ° C., a thick oxide film is formed on the surface of the hot rolled steel sheet, and internal oxidation occurs, so that the oxide layer is not easily removed during the pickling process. Therefore, it is preferable to lower the winding temperature of the hot-rolled steel sheet.

  The hot-rolled steel sheet obtained from the above is manufactured into a cold-rolled steel sheet as necessary.

  The cold-rolled steel sheet is obtained by cold rolling in order to match the shape and thickness of the steel sheet, but preferred cold rolling is performed at a reduction rate of 30 to 80%.

  Cold rolled steel sheets are continuously annealed at 600 ° C. or higher. At this time, if the annealing temperature is too low, it is difficult to ensure sufficient workability, and the transformation to austenite does not occur enough to maintain the austenite phase at a low temperature, so the annealing temperature should be 600 ° C. or higher. Is preferred. In the present invention, since austenitic steel that does not easily undergo phase transformation is used, workability can be ensured when heated to a temperature higher than the recrystallization temperature, and thus cold-rolled steel sheets are manufactured by annealing under normal annealing conditions. be able to.

  The annealed steel sheet obtained from the above is plated as necessary. For plating, a method such as hot dipping, electroplating, and vapor deposition can be selected, preferably hot dipping. The method for producing a plated steel sheet is configured to produce a hot-dip plated, electroplated, and vapor-deposited steel sheet by performing continuous annealing at 600 ° C. or higher using a cold-rolled steel sheet. Normal heat treatment conditions during electroplating and hot dipping processes have an effect on typical transformation-induced plastic steel sheets, but the steel of the present invention has an austenite single phase and has no transformation and mechanical properties. Since there is no significant difference, plating can be performed under normal conditions.

  In this invention, the example of the high manganese steel plate which satisfy | fills the said component of this invention is given, and any one is hot-rolled steel plate, a cold-rolled steel plate, and a plated steel plate again with the rolling reduction of 10 to 80% in cold. The yield strength can be increased by rolling. Here, the rolling can be performed using any one of temper rolling, double rolling, and hot coil treatment used in the steelworks.

  The present invention will be described in detail through the following examples.

  Table 1 shows the chemical composition of the steel of the present invention and the comparative steel. A steel ingot of molten steel was maintained in a heating furnace at 1200 ° C. for 1 hour, and then hot rolled. At this time, the finishing temperature of hot rolling was 900 ° C., and the winding temperature was 650 ° C. A part of the hot-rolled steel sheet was subjected to a tensile test using a universal tensile testing machine after processing a tensile specimen according to the standard of JIS5. And the steel plate which hot-rolled was utilized, pickling was implemented, and cold rolling was implemented by making the cold reduction rate 50%. The cold-rolled specimen was subjected to a continuous annealing pseudo heat treatment at an annealing temperature of 800 ° C. and an overaging temperature of 400 ° C. After the continuous annealing simulated heat treatment, a tensile test was performed using a universal tensile tester. On the other hand, a cold-rolled specimen was annealed at a temperature of 800 ° C. and a molten zinc bath at 460 ° C., and a continuous annealing pseudo heat treatment was performed.

  Table 2 shows changes in mechanical properties depending on the production conditions of the steel of the present invention and the comparative steel. The steel sheet of the present invention, which is indicated as “invention steel” in the remarks, shows a hot-rolled steel sheet, and after carrying out a continuous annealing pseudo heat treatment, when a tensile test is carried out, a tensile strength (TS) of 700 MPa or more, a draw ratio (T-El) ) A hot-rolled steel sheet having a yield strength (YS) of 500 MPa or more was obtained. Therefore, a material suitable for a structural member material such as an automobile member or filler was secured.

  Sample Nos. 1 and 2 have a small amount of manganese and cannot ensure sufficient strength and ductility.

  As steel plates of sample numbers 3-6, 8-9, 14-15, 19 are not suitable for the addition amount of carbon, manganese, silicon, and aluminum, the stretching ratio is low, and the yield strength is as low as 500 MPa or less, so that it is a structural member. Not suitable.

Steel plates of Sample Nos. 7, 10-13, and 16-18 have appropriate yields of carbon, manganese, and aluminum, and have preferable yield strength due to the addition of silicon, titanium, and niobium. Have.
[Example 2]

  The mechanical properties with respect to the case where the high workability high manganese type high strength steel sheet having an austenite single phase structure obtained from Example 1 was cold-rolled again were measured, and the results are shown in Table 3.

  As shown in Table 3, it shows that the yield strength increases. Usually, even when the deformation is 10%, the yield strength is greatly increased to 750 MPa or more, and the drawing ratio shows an excellent result of 44%, which indicates that the structural member has formability and impact characteristics.

  In the present invention, the above embodiment is an example, and the present invention is not limited to this. Anything that has substantially the same configuration as the technical idea described in the claims of the present invention and has the same function and effect is included in the technical scope of the present invention. Accordingly, a person having ordinary knowledge in the technical field can perform various forms of substitution, modification, and change within the scope of the technical idea of the present invention described in the claims of the present invention. Such are within the scope of the present invention.

Claims (2)

  % By weight, carbon 0.2-1.5%, manganese 10-25%, aluminum 0.01-3.0%, phosphorus 0.03% or less, sulfur 0.03% or less, nitrogen 0.040% or less At least one selected from the group consisting of 0.02-2.5% silicon, 0.01-0.10% titanium, and 0.01-0.10% niobium, the remaining Fe and others High manganese with excellent impact characteristics, produced by rolling steel plate composed of impurities inevitably contained at a rolling reduction of 10-80%, having yield strength of 600 MPa or more and tensile strength of 900 MPa or more Type high strength steel plate.   The high manganese-type high-strength steel sheet having excellent collision characteristics according to claim 1, wherein the steel sheet has an austenite single-phase structure.

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