JP2013545890A - Steel blank hot forming method and hot formed parts - Google Patents

Abstract

  According to a first aspect, the invention is a method of hot forming a steel blank into a product, d) during hot forming, starting the heated steel blank at a starting temperature T2 above Ar3; Cooling to an interruption temperature T3 in the range of 400 to 550 ° C. with a cooling rate V2 of at least 25 ° C./s and molding the product; e) the product further from the interruption temperature T3 to the ambient temperature of 0.2 to 10 ° C. / s cooling rate V3 immediately cooling step T3 and cooling rate V3, the product thus obtained is 55% to 90% bainite ferrite and 5% to 15% residual austenite in the volume fraction. Selected from having a multiphase microstructure comprising 5-30% martensite.

Description

  The present invention is used in a method for hot forming a steel blank into a hot formed product having very high mechanical properties, such as an automotive part, such a hot formed product, and a thermomechanical treatment process. In particular, it relates to a steel strip, sheet or blank for use in the hot forming method of the present invention.

  Recent developments in the development of advanced high-strength steel (AHSS) allow automotive component manufacturers to increase vehicle safety and automotive component crash resistance, and further reduce vehicle weight. Despite the high strength and light weight of AHSS, the inferior formability, poor ductility and toughness compared to conventional steel use AHSS for complex automotive parts such as A- or B-pillar reinforcement It has become a big hurdle against this.

  In order to solve the problem of formability, hot (press) forming (hot punching or press hardening) has been developed. Steel products by hot forming (pressing), heating the product to a temperature range in which an austenite phase is present and then rapidly cooling and by phase transformation from austenite to harder phases such as bainite and martensite. The strength of is improved.

  A steel composition suitable for the basis and use of hot forming technology was first described in British Patent No. 1490535. Since then, many carbon-manganese-boron steels have been developed for hot press forming.

  A typical steel used for hot pressing is based on the 22MnB5 system specified in EN10083, ie C0.22%, Mn1.2%, B max 50ppm. Hot pressing of 22MnB5 steel produces complex parts such as bumpers and pillars with ultra-high strength, minimal springback, and reduced steel thickness. The tensile strength of boron steel is up to 1600 MPa, which is much higher than that of conventional highest strength cold stamped steel. However, the ductility of the total elongation A5 is less than 6%.

  To improve the impact resistance of steel, it is necessary to further increase the ductility and toughness. Advanced steel strength steel with multiphase microstructure added by TRIP (transformation induced plasticity) is the solution. These steels are characterized by a ferrite-bainite microstructure with retained austenite, which undergoes martensitic transformation during product molding and further contributes to strengthening the finished part.

  US Patent Application Publication No. 2008/0286603 A1 describes a steel sheet that exhibits ultra-high strength by rapid cooling following hot press forming and has increased yield strength after painting. This steel sheet has a composition (expressed in% by weight) of C0.1 to 0.5, Si 0.01 to 1.0, Mn 0.5 to 4.0, P0.1 or less, S0.03 or less, soluble Al0.1, N0.01. ~ 0.1, W0.3 or less, and the balance Fe and other inevitable impurities. Furthermore, a hot-pressed part made of steel sheet and a method for manufacturing the hot-pressed part are also disclosed. Hot-pressed parts achieve a high increase in yield strength after heat treatment for coating, while ensuring ultra high tensile strength. The steel provided by this US patent application has an ultimate tensile strength (UTS) of 1500 MPa and a total elongation of less than 8%.

  From US 2008/0251160 A1, a high strength cold rolled steel sheet with a uniform elongation is known, C0.1-0.28 (% by weight), Si1.0-2.0 and Mn1. Includes 0.0-3.0. The structure has a porosity of 30-65% bainitic ferrite, 30-50% polygonal ferrite, and 5-20% retained austenite. A desired microstructure can be obtained by the following manufacturing method. After hot rolling and cold rolling, the steel sheet is heated so that it is soaked to a temperature equal to or higher than the A3 transformation point (A3), and then according to the formula A3-250 (° C) ≤ Tq ≤ A3-20 (° C) The temperature Tq is temporarily cooled at an average cooling rate of 1 to 10 ° C./s, and then rapidly cooled from this temperature to the bainite transformation temperature range at an average cooling rate of 11 ° C./s or more. The tensile properties of steel after such heat treatment are typically UTS = 1000 MPa and the total elongation is less than 15%.

  US Patent Application Publication No. 2008 / 0308194A1 discloses a method for manufacturing a steel part having a multiphase microstructure. The composition of this steel is% by weight, C0.01-0.50, Mn0.5-3.0, Si0.001-3.0, Al0.005-3.0, Mo ≦ 1.0, Cr ≦ 1.5, P ≦ 0.10, Ti ≦ 0.20, V ≦ 1.0, and optionally one or more elements, such as Ni ≦ 2.0, Cu ≦ 2.0, S ≦ 0.05 and Nb ≦ 0.15, the balance of the composition being iron and inevitable impurities. In this method, the steel sheet is heated to a soaking temperature Ts that is higher than Ac1, but lower than Ac3, and soaking temperature Ts so that the austenite content of the steel is 25% or more per area. Hold for time ts, transfer the steel blank into a forming tool for hot forming, cool the resulting part in the tool at a cooling rate V, containing ferrite, martensite or bainite and residual austenite A multiphase microstructure is formed, and the ferrite is homogeneous in each of these regions.

  Ryu H.-B et al., Published the paper `` Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel '' (METALLURGICAL & MATERIALS TRANSACTIONS A, Vol. 33A, No. 9, 2002 p.2811-2816), a method for hot forming steel samples into deformed products is reported. This known method comprises austenitizing at 1000 ° C. for 5 minutes and then compressively deforming at 920 ° C. Thereafter, the hot forming process is completed at this temperature, and the deformed sample is cooled at a rate of 10 to 35 ° C./s to a simulated coil winding temperature range of 420 to 480 ° C. At this temperature, the sample is gradually cooled to 420 ° C. over 2 minutes to complete the bainite transformation. Finally, the sample is air cooled to ambient temperature.

  From EP 1686195A1, a heat treatment procedure is known for a wire which has already been drawn, without deformation. This heat treatment procedure comprises an intermediate isothermal transformation period of 60-3600 seconds at a temperature Ms-50 ° C. to Bs.

  Japanese Unexamined Patent Publication No. 2003193193 discloses a steel sheet having a two-phase microstructure comprising bainite / bainite-based ferrite as a main phase and austenite as a second phase. The manufacturing method comprises isothermal heat treatment at 200 to 450 ° C. for 1 to 3000 seconds.

  There is a constant need for steel products with improved mechanical properties to meet the requirements of the automotive industry.

  The object of the present invention is to produce such steel products, as well as steel compositions, in particular suitable for use in this method, more particularly suitable for hot press forming as a boron steel replacement for automotive applications. Is to provide advanced high-strength TBF (TRIP-supported bainite ferrite) steel products.

  Another object of the present invention is to provide a multiphase microstructure steel with improved strength and ductility simultaneously.

  Yet another object is to provide a steel having an ultimate tensile strength in excess of 1400 MPa, for example about 1500 MPa. Yet another object is to provide steel with a total elongation of over 8%, for example about 12%.

According to a first aspect, the present invention provides a method for hot forming a steel blank into a product, such as an automotive part, as claimed in claim 1, comprising the following steps:
d) During hot forming, the heated steel blank starts at a starting temperature T2 above Ar3 and is cooled to an interrupt temperature T3 in the range 400-550 ° C with at least 25 ° C / s. Cooling at speed V2 and molding the product;
e) immediately cooling the product from the interruption temperature T3 to the outside temperature at a cooling rate V3 of 0.2 to 10 ° C./s, and the interruption temperature T3 and the cooling rate V3 are In fractions,
Bainitic ferrite 55-90%
5-15% residual austenite
Martensite 5-30%
Wherein the method is selected to have a multiphase microstructure comprising:

  The steel blank as starting material for carrying out the method according to the invention is obtained indirectly from standard casting methods or from steel strips or steel materials. In the hot rolling of a cast ingot, the preheating temperature may be about 1100-1250 ° C. Using traditional hot rolling and rolling conditions, a steel ingot can be rolled into a sheet product having a thickness of 3-5 mm. The finish rolling temperature is about 850-880 ° C. After hot rolling, the steel sheet is cooled to a coil winding temperature of 550 to 700 ° C. at an average cooling rate of 5 to 50 ° C./s. At coil winding temperatures below 550 ° C., excess martensite or bainite is formed, excessively increasing the strength of the hot rolled steel sheet. Excessive strength increases act as a burden on the production of cold rolled steel sheets during subsequent cold rolling, causing problems such as poor appearance. At the time of cold rolling, the hot-rolled sheet wound with a coil is pickled and cold-rolled. Cold rolling can also be performed at a rolling reduction of about 30-75% under standard conditions. However, considering prevention of heterogeneous recrystallization, the cold rolling reduction is preferably adjusted to 40 to 70%. After cold rolling, the sheet thickness is about 1-2 mm, depending on the product requirements. After cold rolling, the sheet is rewound into a blank of a size suitable for hot forming according to the present invention. Cold rolled sheets are a preferred starting product in the process according to the invention.

  In the present cooling method performed during hot forming, the two-step cooling pattern and the interrupted cooling temperature are adjusted appropriately.

Advantageously, the heated steel blank used in step d) has an austenitic structure. In a preferred embodiment for obtaining such an austenitic structure, the method according to the invention comprises:
a) heating the steel blank to an austenitizing temperature T1, higher than Ac3, preferably Ac3 + 20 ° C. to Ac3 + 60 ° C., more preferably at a heating rate of 10-25 ° C./s;
b) soaking the steel blank within this range, preferably a soaking time t1 of 1 to 5 minutes;
c) optionally moving the heated and soaked blank to a hot forming facility.

  These austenitization steps to obtain the austenitic structure are performed at a temperature T1, higher than Ac3, more preferably Ac3 + 20 ° C. to Ac3 + 60 ° C., in steel sheets, blank strips, for example in the furnace or in the hot forming equipment itself. This is done by heating. Preferably the heating rate is in the range of 10-25 ° C./s. This heating step is followed by soaking at a temperature higher than Ac3, preferably Ac3 + 20 ° C to Ac3 + 60 ° C, for a short time, preferably 1-5 minutes (Ac3 is a transformation of ferrite to austenite in hypoeutectoid steel. Temperature completed by heating). The austenitizing temperature (T1) and soaking time are selected to ensure complete dissolution of the carbide. Then, if applicable, the so-heated steel blank is moved to a hot forming apparatus, for example a hot forming press. If applicable, the travel time is usually adjusted to a short travel time, for example less than 10 seconds, to prevent the blank from being cooled below the start temperature T2 of the next hot forming step. The hot forming starting temperature T2 and co-cooling should be above the Ar3 point (typically 780-830 ° C.), for example to prevent ferrite phase transformation during migration. (Ar3 is the temperature at which austenite begins to convert to ferrite due to cooling of the steel.) During hot forming, the blank is deformed and at the same time an interrupted quenching temperature T3 of 400-550 ° C, from 25 ° C / s Cool at a high cooling rate V2 to avoid the formation of hypoeutectoid ferrite and pearlite. When the cooling rate is less than 25 ° C / s, pearlite may be generated during rapid cooling. Immediately, ie without holding the blank at a temperature of about T3 for a predetermined time, for example, isothermal austemper at T3, the steel blank is cooled to room temperature with a further cooling rate V3 of 0.2-10 ° C / s Let Cooling rates higher than 5 ° C./s within this range are advantageous for achieving high total elongation (At). A range of 0.5 to 5 ° C./s is preferable from the viewpoint of high ultimate tensile strength (UTS). By adjusting the interruption temperature T3 in the final process and the further cooling rate V3 in combination, the volume fraction of the various phases in the final microstructure of the product thus produced is effectively adjusted. The interrupted quenching temperature T3 is within the bainite transformation range. If T3 is too high, ie generally above 550 ° C., some pearlite structure may be formed during the subsequent slow cooling process at cooling rate V3. If T3 is too low, ie below the Ms point, a bainite structure is obtained in an amount insufficient for the final microstructure. If V3 is too fast, there will be too much martensite in the final microstructure, making it impossible to ensure sufficient elongation. If V3 is too slow, too little martensite is obtained to ensure high strength. In addition, the combination of T3 and V3 is a multiphase microscopic product in which the resulting product comprises 55-90% bainitic ferrite, 5-15% retained austenite, and 5-30% martensite in the volume fraction. Choose so that a product comprising the structure is obtained. A low martensite content within this range, typically less than 10%, gives a relatively high total elongation. A relatively high UTS is achieved when the multiphase microstructure comprises 55-85% bainitic ferrite, 5-15% retained austenite, and 10-30% martensite. In general, the higher the interruption temperature T3, the lower V3 must be to obtain this multiphase microstructure.

  Preferably, the steel in the manufactured product, in particular TBF steel, comprises bainite-based ferrite free of carbides, retained austenite with a high carbon concentration, and more preferably a relatively small amount of martensite. Characterized by a multiphase structure. In the multiphase structure, the matrix structure (matrix) includes a fine plate of bainite ferrite substantially free of carbides. The formation of such a microstructure is due to the fact that cementite precipitation during the bainite transformation is suppressed by alloying the steel with a sufficient amount of Si and / or Al. Solubility is very low and generally slows the growth of cementite from austenite. Carbon rejected from bainite-based ferrites is concentrated in residual austenite, thereby transforming to martensite upon cooling after bainite transformation or stabilizing it to room temperature.

  The advantages of this type of microstructure are diverse. Bainitic ferrites in TBF steel have ultrafine particle sizes, usually up to about 15 micrometers in length and up to 0.3 micrometers in thickness (typically about 10 μm in length and about It exists in the form of a 0.2 μm plate. Furthermore, strength and ductility are improved at the same time. In contrast to existing TRIP (transformation-induced plasticity) steels, high flow stress is due to the small thickness of bainite lath and the absence of hypoeutectoid polygonal ferrite. Residual austenite is presumed to give additional TRIP effects and be useful in improving total elongation. The presence of martensite islands leads to high strength and high instantaneous cure rate. If a synergistic effect between the transformation-induced plasticity obtained by retained austenite and the fine plate-type bainite ferrite in TBF steel is ensured, a dramatic improvement in the elongation of TBF steel can be obtained. Toughness and impact performance are also improved.

  Moreover, in order to reach the high strength and high ductility of the steel, the constituents of the various phases in the final microstructure are preferably 55% to 90% bainite-based ferrite and 5% to 15% retained austenite in the volume fraction. %, And 5-30% martensite.

  The multiphase microstructure according to the invention can be obtained by designing a steel with a special composition and by careful adjustment of the thermomechanical treatment method as described above.

  Thus, in other words, in a first aspect, a preferred embodiment of the present invention is a method of hot forming a steel blank into a product, wherein the steel blank is heated to an austenitizing temperature T1, preferably Ac3 + 20 ° C. to Ac3 + 60 ° C. And soaking the steel blank into the austenitizing temperature range so that the carbide is completely dissolved, heating the soaked blank while preventing the transformation of the ferrite phase, Introducing into the hot press, the steel blank is hot formed, starting at a starting temperature T2 higher than Ar3, forming the shape product, and thus hot forming the product to the interruption temperature T3 within the bainite transformation range Cooling at a cooling rate that avoids the formation of hypoeutectoid ferrite and pearlite, and also the product from the interruption temperature T3 to ambient temperature, thus the volume fraction in the manufactured product , Bainitic ferrite 55 to 90%, residual austenite is 5% to 15%, martensite comprises a cooling to be 5-30%, to a method.

  Advantageously, the steel has the following elements: C0.15-0.45 (by weight), Si0.6-2.5, Mn1.0-3.0, Mo0-0.5, Cr0-1.0, P0.001-0.05, S <0.03, Ca <0.003, Ti0.1 or less and V0.1 or less, and the balance Fe and other inevitable impurities. If desired, the steel may contain less than 1.5% Al, partially replacing the same amount of Si, provided that the sum of Si and Al is in the range of 1.2-2.5%. Preferably, for better control of the multiphase microstructure, the amount of Mn and Cr satisfies Mn + Cr ≦ 3%, while preferably the amount of C and Mo also satisfies C + 1 / 3Mo ≦ 0.45%.

  Below, the function of each element in a steel composition is demonstrated. The content is expressed as% by weight of the total composition.

  C is an element that ensures high strength and ensures retained austenite. C is added in an amount of 0.15% or more to form the desired multiphase microstructure and achieve ultra high strength and ductility. On the other hand, if the C content exceeds 0.45%, it is difficult to obtain a multiphase microstructure in the method according to the present invention comprising a subprocess that is continuously cooled. In addition, the toughness and weldability of the steel sheet are likely to be impaired. C is preferably present in an amount of 0.2-0.4%, more preferably 0.2-0.35%.

  Mn is one of the main elements of the steel composition according to the present invention. The function of Mn is to stabilize austenite and obtain the desired multiphase microstructure. If Mn is less than 1.0%, the effect is not well characterized. On the other hand, when the content exceeds 3%, a complete martensite structure can be easily formed. As a result, steel hardens and becomes brittle during press molding. Furthermore, Mn is an element useful for lowering the Ac3 temperature. A higher Mn content is advantageous for lowering the temperature required for hot press forming. The Mn content is limited within the range of 1.0 to 3.0%, preferably 1.5 to 2.5%, more preferably 1.6 to 2.5%, and still more preferably 1.7 to 2.4%.

  Si is an element effective for reinforcing a solid solution, and is useful for suppressing the formation of carbides due to decomposition of retained austenite. In order to obtain high ductility, toughness and formability, carbide formation (transition carbide or cementite) should be avoided as much as possible. Since Si suppresses the precipitation of brittle cementite during bainite formation, it improves formability and toughness. A minimum of 1.0% Si is required to form carbide free bainite. However, it is also known that Si impairs galvanizing properties due to the formation of oxides attached to the steel substrate. Therefore, the upper limit of Si is controlled to less than 2.5%. The Si content is advantageously 1.2 to 2.5%, more preferably 1.4 to 2.0%.

  Al is also an effective element for suppressing the formation of carbides, particularly due to the decomposition of austenite. Partial replacement of Si with the same amount of Al has been shown to effectively retard cementite formation without adversely affecting hot dip coatability in TBF steel. However, the high concentration of Al increases the possibility of generating polygonal ferrite, which is less effective than fine plate-like ferrite in terms of strength. If Si is completely replaced by the same amount of Al, the strength-ductility balance is significantly impaired. When added, the amount of Al is limited to 1.5% or less.

  P is an element useful for maintaining the desired retained austenite, and the effect is exhibited when the amount of P is 0.001% or more, more preferably 0.005% or more, but P is added in excess. Sometimes the workability of steel is impaired. Therefore, the P content is preferably limited to 0.05% or less.

  S is a harmful element that forms sulfide-based inclusions, such as MnS, which initiates crack formation and impairs workability. Therefore, it is preferable to reduce the amount of S as much as possible. Therefore, S is suppressed to 0.03% or less.

  Mo and Cr help improve the hardenability of the steel and promote the formation of bainite ferrite. At the same time, Mo and Cr are elements that have a similar effect of stabilizing retained austenite. Therefore, Mo and Cr are very effective for process control. It is advantageous to contain 0.05% or more of each of Mo and Cr. However, if each of Mo and Cr is added in excess, the effect is saturated and further addition is not economical. Therefore, the amount of Mo is 0.5% or less, and the amount of Cr is 1% or less.

  Ti and V have the effect of forming strengthening precipitates and refining the microstructure. The amount of each of Ti and V is 0.1% or less, preferably 0.05% or less.

  Ca is an element effective for controlling the form of sulfide in steel and improving workability. It is recommended to contain 0.0003% or more of Ca. However, if added in excess, the effect is saturated. Therefore, the preferred amount is 0.0003-0.003%.

  Regarding the practical production method, the above heat treatment can be performed by heating and cooling in a continuous annealing facility (CAL), a hot press forming facility, a salt bath, and the like.

  Advantageously, the first combined deformation / cooling step d) is performed in a hot forming facility. In a preferred embodiment, the second cooling step e) takes place in air or in stock outside the hot forming facility.

  Preferably, the steel of the molded product has an ultimate tensile strength (UTS) of at least 1400 MPa, advantageously at least 1500 MPa, more preferably at least 1600 MPa, most preferably at least 1700 MPa.

  Advantageously, the steel of the shaped product has a total elongation of at least 8%, preferably at least 10%, more preferably at least 12%, most preferably at least 14%.

  The heat treatment method can be easily performed by using hot (press) forming with standard hot forming equipment, with only improved simultaneous cooling process including cooling interruption temperature and speed. After austenitization, the heat blank is inserted into a hot press die set where the blank is molded and cooled. The moving time from the furnace to the press forming tool (for example, 5 to 10 s) is controlled so that the starting deformation temperature is surely higher than Ar3. The cooling rate V2 of the steel part in the forming tool depends on the deformation and on the quality of contact between the tool and the steel blank. The molding tool is cooled by circulating a liquid, for example, and is cooled so that the cooling rate is sufficiently high (> 25 ° C./s) when it is rapidly cooled in a press mold. The interrupted cooling temperature is controlled by the time for separating the press tool. The molded product is then removed from the mold and cooled to room temperature in air. Molded products, such as sheets, can be stacked and then cooled to ambient temperature in air.

  After cooling to room temperature, the parts are built into the car body structure. Next, a paint baking process is performed. The paint baking cycle does not affect the properties.

In a second aspect, the present invention is preferably a steel product formed by the method according to the invention, wherein the steel is in a volume fraction,
Bainitic ferrite 55-90%,
5-15% residual austenite,
Martensite 5-30%
A steel product having a microstructure comprising

  The details and particularly advantageous and / or preferred embodiments of the compositions and microstructures described above with respect to the method according to the invention are equally applicable to the second aspect of the invention. Preferably, the steel has an ultimate tensile strength of at least 1400 MPa, advantageously at least 1500 MPa, preferably at least 1600 MPa, more preferably at least 1700 MPa, and / or a total elongation of at least 8%, preferably at least 10%, more Preferably it is at least 12%, most preferably at least 14%.

According to a third aspect, the present invention is a steel strip, sheet or strip for use in a thermomechanical treatment, in particular in the hot forming method of the invention as described above, the composition being in weight%. ,
C 0.15-0.45
Si 0.6-2.5
Mn 1.0-3.0
Al 0-1.5
Mo 0 ~ 0.5
Cr 0-1.0
P 0.001-0.05
S <0.03
Ca <0.003
Ti <0.1
V <0.1
The balance Fe and inevitable impurities,
Provide a strip with Si + Al = 1.2-2.5%.

  Preferably, the relationship of Mn + Cr ≦ 3% and C + 1 / 3Mo ≦ 0.45% is applied.

A preferred composition of a steel strip, sheet or blank according to the third aspect of the invention, in which the alloying elements are present, expressed in weight percent,
C 0.2-0.4, preferably 0.2-0.35 and / or Si 0.8-2.0, preferably 1.2-1.8 and / or Mn 1.5-2.5, preferably 1.7-2.4 and / or Mo 0.05-0.5 and / or Cr 0.05-1.0 And / or P 0.005-0.05 and / or Ca 0.0003-0.003
Comprising.

Products obtained from steel strips, sheets or blanks show a high tensile strength by rapid cooling after heat treatment, and a high yield strength, especially for paints, after heat treatment. Based on these advantages, the excellent impact properties of the steel product according to the invention are obtained. Furthermore, the steel product of the present invention advantageously exhibits good adhesion to the coating layer. Moreover, other advantages of the steel product of the present invention are a good surface appearance after coating and excellent corrosion resistance.

  A practical embodiment of the method according to the invention is shown schematically in FIG. 1, which plots the relationship between temperature and time. The steel blank is heated at a heating rate of 15 ° C. to an austenitizing temperature T1 of Ac 3 + 20 ° C. to Ac 3 + 60 ° C. and soaked to that temperature for a soaking time t1. The blank thus heated and soaked is moved from the furnace to the hot forming facility, during which some air cooling occurs. Care should be taken that the temperature T2 does not fall below Ar3 before hot blanking the blank. After hot press forming, the blank thus formed is cooled to an interruption temperature T3 at a rate higher than 25 ° C. Next, air cooling is performed.

  The invention is further illustrated by the following examples.

  Steels A to F having the composition specified in Table 1 were cast into ingots of about 25 kg (100 × 110 × 330 mm). The ingot was reheated and then roughly hot rolled into a slab having a thickness of 40 mm. Finish hot rolling is carried out at the following process parameters: preheating 1200 ° C for 30 minutes, multiple passes to a thickness of 4mm (48-27-18-12-8-6-4mm), and finish rolling temperature 860 ± Performed at 20 ° C. Regulated cooling was performed to 600 ° C at a speed of 30 ° C / s described in the runout table, and a commercially used coil winding method was simulated. After the coil winding process, the steel plate contains a microstructure composed of ferrite and pearlite and has a tensile strength of less than 700 MPa. The plate was then cold rolled into a 4 to 1 mm sheet.

  Important temperatures, such as Ac3 and Ms, were measured by standard expansion analysis to facilitate measurement of the temperature in the process according to the invention.

  The resulting cold rolled sheet was subjected to heat treatment using a CASIM simulator. Specifically, the steel sheet is heated to 870-920 ° C. at a rate of 15 ° C./s, the sheet is held at this temperature for 2 minutes, then cooled to a break temperature of 400-550 ° C. at a rate of 50 ° C./s, then Cooling at a speed of 0.2-10 ° C / s, different air cooling conditions were simulated.

  The tensile test was performed using a JIS 5 tensile test sample, and the tensile strength and elongation of the sample having the necessary microstructure were measured. The volume fraction of bainite and / or martensite in the microstructure was estimated by combining metallographic characterization with expansion analysis. The volume fraction of retained austenite was measured using TEM. Other measurements are standard. Table 2 shows the alloys of the present invention having the tensile test results and the required microstructure constituents.

  Experimental results show that the microstructure and properties are substantially independent of T1 as long as the austenitizing temperature T1 is higher than Ac3, preferably between Ac3 + 20 ° C. and Ac3 + 60 ° C. It has also been demonstrated that the movement temperature T2 and cooling rate V2 do not significantly affect the microstructure and properties as long as T2 is higher than Ar3 and V2 is 25 ° C / s or higher, preferably less than 100 ° C / s. It was. However, the microstructure and properties are strongly dependent on the interruption temperature T3 and the cooling rate V3. The C content and the alloying element content have a great influence on the selection of T3 and V3. From these results, the necessary multiphase microstructure conditions for the alloy according to the invention can be obtained by carefully adjusting T3 and V3. For alloys with higher C content or higher Mo content, the cooling rate V3 can be adjusted to the lower range 0.25-2 ° C./s, for alloys with lower C or lower Mo The cooling rate V3 can be adjusted to a higher range of 2-10 ° C./s. For a given alloy composition at a fixed T3 temperature, a higher cooling rate results in less bainite ferrite, but relatively higher martensite. Therefore, higher strength can be obtained. A lower cooling rate results in more bainite ferrite but relatively less martensite and higher elongation is achieved. The higher the T3 temperature, at a given cooling rate, there is relatively more bainite-based ferrite in the final microstructure, resulting in higher strength but lower elongation.

In the above table, the following abbreviations are used.
BF = Bainitic ferrite
M = Martensite
Ar = retained austenite
Vickers hardness measured at a load of HV5 = 5 kgf
YS = yield strength
UTS = ultimate tensile strength
At = total growth

Claims (16)

A method of hot forming a steel blank into a product,
d) During hot forming, the heated steel blank is started at a starting temperature T2 above Ar3 and cooled to an interruption temperature T3 in the range 400-550 ° C with a cooling rate V2 of at least 25 ° C / s. The process of molding the product,
e) immediately cooling the product immediately from the interruption temperature T3 to the ambient temperature at a cooling rate V3 of 0.2 to 10 ° C./s,
The product at which the interruption temperature T3 and the cooling rate V3 are obtained in this way is a volume fraction,
Bainitic ferrite 55-90%
5-15% residual austenite
Martensite 5-30%
Selected to have a multiphase microstructure comprising. Optionally manufactured from steel strip, sheet, the blank is in weight percent and has the following composition:
C 0.15-0.45
Si 0.6-2.5
Mn 1.0-3.0
Al 0-1.5
Mo 0 ~ 0.5
Cr 0-1.0
P 0.001-0.05
S <0.03
Ca <0.003
Ti <0.1
V <0.1
The method according to claim 1, further comprising Fe and inevitable impurities, wherein Si + Al = 1.2 to 2.5%.   The method according to claim 2, wherein Mn + Cr ≦ 3% and C + 1 / 3Mo ≦ 0.45%. The composition is, by weight percent, C 0.2-0.4 and / or Si 0.8-2.0 and / or Mn 1.5-2.5 and / or Mo 0.05-0.5 and / or Cr 0.05-1.0 and / or P 0.005-0.05 and / or Ca 0.0003-0.003
The method according to claim 2 or 3, comprising: The composition is, by weight percent, C 0.2-0.35 and / or Si 1.2-1.8 and / or Mn 1.7-2.4
The method according to any one of claims 2 to 4, comprising:   The method according to any one of claims 1 to 5, wherein the bainite-based ferrite is substantially free of carbides and the retained austenite has a high carbon concentration.   The method according to claim 1, wherein the bainite-based ferrite particles have a maximum length of 15 μm and a maximum thickness of 0.3 μm. Before the hot forming step d)
a) heating the steel blank to an austenitizing temperature T1 higher than Ac3, preferably Ac3 + 20 ° C. to Ac3 + 60 ° C., preferably at a heating rate of 10-25 ° C./s;
b) soaking the steel blank in the above range, preferably a soaking time of 1 to 5 minutes;
c) The method according to any one of claims 1 to 7, further comprising a step of moving the heated and soaked blank to a hot forming facility, if desired.   The method according to claim 5, wherein step a) is carried out in a continuous annealing facility, a hot forming facility, a salt bath or the like.   The method according to claim 1, wherein step d) is performed in a hot forming facility.   The method according to any one of claims 1 to 10, wherein step e) is performed in air outside the hot forming facility. Preferably, a steel product formed by the method according to any one of claims 1 to 11, wherein the steel is in a volume fraction,
Bainitic ferrite 55-90%
5-15% residual austenite
Martensite 5-30%
A steel product having a microstructure comprising   The steel has an ultimate tensile strength of at least 1400 MPa, advantageously at least 1500 MPa, preferably at least 1600 MPa, more preferably at least 1700 MPa, and / or at least 8%, preferably at least 10%, more preferably at least 12%, most 14. Steel product according to claim 13, preferably having a total elongation of at least 14%. A steel strip, sheet or blank for use in a thermomechanical treatment method, preferably a method according to any one of claims 1 to 12, wherein the composition of
C 0.15-0.45
Si 0.6-2.5
Mn 1.0-3.0
Al 0-0.6
Mo 0 ~ 0.5
Cr 0-1.0
P 0.001-0.05
S <0.03
Ca <0.003
Ti <0.1
V <0.1
A steel strip, sheet or blank with the balance Fe and inevitable impurities and Si + Al = 1.2-2.5%.   15. A steel strip, sheet or blank according to claim 14, wherein Mn + Cr ≦ 3% and C + 1 / 3Mo ≦ 0.45%. By weight% C 0.2-0.4, preferably 0.2-0.35 and / or Si 0.8-2.0, preferably 1.2-1.8 and / or Mn 1.5-2.5, preferably 1.7-2.4 and / or Mo 0.05-0.5 and / or Cr 0.05-1.0 and / or P 0.005-0.05 and / or Ca 0.0003-0.003
The steel strip, sheet or blank according to claim 14 or 15, wherein

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