JP2005163055A - Method for producing high workability and extra-high strength cold-rolled steel sheet excellent in delayed fracture resistant characteristic after forming - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high workability and extra-high strength cold-rolled steel sheet excellent in delayed fracture resistant characteristic after forming. <P>SOLUTION: After casting a steel slab composed of, by mass%, 0.1-0.2% C, 0.01-1.8% Si, 1-2.5% Mn, 0.001-0.05% P, 0.0001-0.005% S, 0.005-0.05% Al, 0.0001-0.005% N and the balance substantial Fe, this steel slab is immediately or once cooled, and after heating, hot-rolling and pickling, the steel sheet obtained by performing the cold-rolling is heated and annealed at 700-900°C and cooled at ≥10°C/sec cooling speed and heated so as to rise the temperature at ≥10°C/sec heating speed to ≥(0.3×heat treatment temperature)°C with an induction-heating and the heat treatment is applied at 150-500°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a high workability ultra-high strength cold-rolled steel sheet having excellent delayed fracture resistance after forming.

  Conventionally, for ultra-high-strength cold-rolled steel sheets, material strength can be ensured by various strengthening measures, but workability tends to decrease with increasing strength. In other words, in high-strength cold-rolled steel sheets, due to structural inhomogeneities, local mixing of hard and soft phases, workability is greatly reduced with increasing strength, increasing strength, ductility and bendability, etc. Actually, it is difficult to achieve both the processing characteristics.

  Recently, for example, Patent Document 1 discloses, as a steel sheet excellent in bending workability, residual austenite having a soft layer of C: 0.1 wt% or less in the surface layer portion and the balance being less than 10 vol%. A steel sheet composed of a composite structure of a low-temperature transformation phase or ferrite, Patent Document 2 discloses a steel sheet in which the number of inclusions having a diameter of 5 μm or more is defined, and Patent Document 3 includes a combination of specific steel components and production conditions. A high-strength steel sheet having excellent stretch flangeability has been disclosed, and it has finally become possible to find knowledge regarding improvement of workability.

  However, Patent Document 1, Patent Document 2 and Patent Document 3 do not describe any knowledge about delayed fracture characteristics.

  Regarding delayed fracture characteristics, for example, Patent Document 4 discloses a steel sheet in which the composition, plate thickness, and plate shape are defined, and Patent Document 5 discloses a steel sheet in which the volume ratio of steel components and martensite is defined. However, these do not describe any knowledge about processability and moldability.

As for the manufacturing method, Patent Document 6, Patent Document 7 and Patent Document 8 disclose a continuous annealing facility provided with an induction heating device and a manufacturing method in which tempering is performed by induction heating. However, there is no knowledge of material properties that are important for formability and workability, and the actual situation is that only descriptions of equipment columns, sheet passability, shape, and variations are described.
Japanese Patent Laid-Open No. 5-195149 JP 2002-363694 A Japanese Patent Laid-Open No. 11-350038 JP-A-6-1222936 Japanese Unexamined Patent Publication No. 9-111398 JP-A-2-74823 Japanese Patent Laid-Open No. 10-121150 JP 2003-27136 A

  As described above, at present, an ultra-high-strength cold-rolled steel sheet having both good workability and high strength and excellent delayed fracture resistance has not been obtained.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a method for producing a high workability ultra-high strength cold-rolled steel sheet having excellent delayed fracture resistance after forming.

  In order to solve the above-mentioned problems, the present inventors have intensively studied from the aspects of steel composition, production conditions, metal structure, and the like. As a result, the steel component is controlled within an appropriate range, the cooling rate from the cooling start temperature during continuous annealing and the heating rate during heating and heating after cooling are controlled, and the temperature raising means is limited, Is optimized, and it has been found that an ultra-high strength cold-rolled steel sheet having excellent workability and excellent delayed fracture characteristics after forming can be obtained.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  In mass%, C: 0.1 to 0.2%, Si: 0.01 to 1.8%, Mn: 1 to 2.5%, P: 0.001 to 0.05%, S: 0.0001 to 0.005%, Al: 0.005 to 0.05%, N: 0.0001 to After casting a steel slab containing 0.005% and the balance substantially consisting of Fe, immediately or once cooled, heating, hot rolling, pickling, cold rolling, and then steel plate obtained from 700 to Heat annealing at a temperature of 900 ° C, cooling at a cooling rate of 10 ° C / second or more, and heating up to (0.3 x heat treatment temperature) ° C or higher by induction heating at a heating rate of 10 ° C / second or more And a method for producing a high workability ultra-high strength cold-rolled steel sheet having excellent delayed fracture resistance after forming, characterized by heat treatment at a temperature of 150 ° C to 500 ° C.

  In the above-mentioned means, “the balance is substantially Fe” means that an element containing other trace elements including inevitable impurities can be included in the scope of the present invention unless the effects of the present invention are lost. means. Moreover, in this specification, all% which shows the component of steel is mass%.

  In the present invention, the ultra-high-strength cold-rolled steel sheet is a cold-rolled steel sheet having a tensile strength of TS980 MPa or higher, desirably a tensile strength of TS1180 MPa or higher.

  According to the present invention, a high workability ultra-high strength cold-rolled steel sheet having excellent delayed fracture resistance after forming can be obtained. The present invention is particularly effective for ultra-high-strength cold-rolled steel sheets having a tensile strength TS of 980 MPa or more, and is suitable for automobile parts that are roll-formed or press-formed.

  In the present invention, the steel components shown below are controlled, and the annealing rate, in particular, the cooling rate from the cooling start temperature at the time of continuous annealing is 10 ° C./second or more, and the heating rate at the heating temperature rise after cooling is ( 0.3 × heat treatment temperature) It is characterized by controlling the temperature up to 10 ° C./second or more and limiting the temperature raising means to induction heating, which are the most important requirements in the present invention. Thus, by controlling the components and annealing conditions, the structure is optimized (a structure obtained by tempering a structure composed only of a low-temperature transformation phase obtained by quenching from an austenite single phase region, or a ferrite phase and a It becomes a two-phase structure steel composed of the returned low-temperature transformation phase), and an ultra-high strength cold-rolled steel sheet having excellent workability and excellent delayed fracture characteristics after forming can be obtained.

  Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the chemical components of steel in the present invention are as follows.
C: 0.1-0.2%
C is indispensable for strengthening steel using the low temperature transformation phase. In general, the strength of the low temperature transformation phase tends to be proportional to the amount of C, and C is required to be 0.1% or more to obtain a tensile strength of TS980 MPa or more. Furthermore, as the amount of C increases, it is easier to ensure the tensile strength. In addition, workability such as ductility tends to decrease. From the above, C is 0.1% or more and 0.2% or less.
Si: 0.01-1.8%
Si is an element that improves ductility and contributes to strength improvement, and its effect is not exhibited at less than 0.01%. On the other hand, even if the content exceeds 1.8%, the effect is saturated. Moreover, when it contains excessively, the electrical resistance at the time of resistance welding increases, weldability is inhibited, and there is a tendency to deteriorate the chemical resistance and corrosion resistance after coating. From the above, Si is made 0.01% to 1.8%.
Mn: 1 to 2.5%
Mn contributes to the refinement of crystal grains through the action of lowering the Ar3 transformation point, and has the action of increasing the strength-ductility balance and the strength-hole expansion ratio λ balance. It is also an important element to suppress surface cracking caused by hot brittleness due to S. Thus, Mn is an austenite stabilizing element, and in order to stably obtain a low temperature transformation phase in the cooling process from austenite existing during heat annealing from the viewpoint of securing strength (TS), Mn is 1% This is necessary. On the other hand, if the content exceeds 2.5%, the structure becomes non-uniform due to segregation of Mn, etc., the material uniformity decreases, and the workability and delayed fracture resistance after molding tend to deteriorate. From the above, Mn is set to 1% to 2.5%.
P: 0.001 to 0.05%
P is an element that contributes to strengthening of the steel sheet by forming a solid solution in the steel. On the other hand, it is also an element that lowers the bond strength of grain boundaries by segregation to the grain boundaries and degrades workability, and also reduces chemical conversion treatment properties, corrosion resistance, etc. by concentration on the steel sheet surface. When P exceeds 0.05%, the above-mentioned influence appears remarkably. However, excessive reduction of P is accompanied by an increase in manufacturing cost. Accordingly, P is set to be 0.001% or more and 0.05% or less.
S: 0.0001 to 0.005%
S is an element that adversely affects workability. When S increases, it exists as inclusion MnS, and in particular, the local ductility of the material is lowered and the workability is lowered. Moreover, weldability also deteriorates due to the presence of sulfides. Such an adverse effect can be avoided by setting S to 0.005% or less. In addition, when S is 0.005% or less, the press workability can be remarkably improved. However, excessive reduction of S is accompanied by an increase in manufacturing cost. Accordingly, S is set to be 0.0001% or more and 0.005% or less.
Al: 0.005-0.05%
Al is an element effective for improving the yield of deoxidation and carbide-forming elements, and 0.005% or more must be added to exert this effect. Moreover, it is an essential element for improving the cleanliness of the steel sheet, and from this point, Al is required to be 0.005% or more. When Al is less than 0.005%, the removal of Si-based inclusions is incomplete, there are many delayed fracture starting points, and delayed fracture is likely to occur. On the other hand, when Al is added in excess of 0.05%, not only the effect is saturated, but also workability deteriorates and problems such as an increased tendency of surface defects occur. From the above, Al is made 0.005% or more and 0.05% or less.
N: 0.0001 to 0.005%
When the content of N is large, a large number of nitrides are formed, which becomes the starting point of delayed fracture and is liable to be delayed. Therefore, N must be limited to 0.005% or less. However, excessive reduction of N is accompanied by an increase in manufacturing cost. Accordingly, N is set to be 0.0001% or more and 0.005% or less.

  In addition to the above component ranges, the steel of the present invention can contain the following elements as long as the effects of the present invention are not lost.

  Ti and Nb are not preferred to be contained in a large amount because they form carbonitrides, but delay fracture is suppressed by refining crystal grains and contributing to homogenization of the structure. Therefore, Ti and Nb can be contained in the range of 0.001% to 0.1%. Cu, Ni, Cr, and Mo are elements that contribute to strength, and can be contained within a range of 0.01% to 0.5%. Ca is effective in controlling the shape of MnS, and B is effective in suppressing delayed fracture through grain boundary strengthening due to preferential segregation at grain boundaries. However, the effect tends to be saturated. Therefore, Ca and B are preferably contained in the range of 0.0001 to 0.005%.

  Next, a manufacturing method will be described.

  From the molten steel adjusted to the above chemical composition range, a slab is melted by continuous casting or ingot forming. Subsequently, the obtained slab is cooled and then reheated or hot rolled as it is. The final rolling temperature in hot rolling is preferably 850 ° C. or higher in order to improve the hole expansion rate and the critical bending radius. At a final rolling temperature lower than 850 ° C, a two-phase structure is formed at the final rolling stage, resulting in significant coarsening of ferrite grains, resulting in a non-uniform structure, and a steel sheet with good workability even when cold rolled or annealed is obtained. It may not be possible.

  Next, the obtained hot-rolled sheet is cooled and wound up. The coiling temperature is preferably 450 ° C. or higher in order to reduce the rolling load during cold rolling and improve cold rolling properties.

  Next, pickling and cold rolling are performed to obtain a desired thickness. The cold rolling rate at this time is preferably 30% or more in order to promote recrystallization of ferrite and improve ductility.

  Next, heat annealing and tempering treatment are performed on the steel sheet obtained as described above.

  In the production method of the present invention, after heat annealing, the crystal grains are homogenized by quenching at a cooling rate of 10 ° C./s or higher, and then the temperature is increased to (0.3 × heat treatment temperature) ° C. or higher, at a temperature increase of 10 ° C./second or higher. The metal structure is made extremely uniform by heating at an increased rate by induction heating and heat treatment. These are the most important requirements in the present invention. This will be described in detail below.

  First, heat annealing is performed at a temperature of 700 to 900 ° C. If the annealing temperature is lower than 700 ° C, it is difficult to completely remove the influence of the structure after cold rolling, resulting in a layered structure, a so-called band-like non-uniform structure, and the elongation and bendability deteriorate, and delayed fracture after forming. Is likely to occur. Furthermore, the fraction of the ferrite phase which is a soft phase increases, and it becomes difficult to ensure high strength. On the other hand, when the annealing temperature is higher than 900 ° C., the ferrite transformation is delayed due to the rapid coarsening of the austenite grain size. Therefore, the fraction of the hard low-temperature transformation phase in the finally obtained structure increases, El decreases, and workability deteriorates. Therefore, the annealing temperature is 700 ° C. or higher and 900 ° C. or lower.

  Next, cooling is performed at a cooling rate of 10 ° C./second or more. If the cooling rate is slower than 10 ° C./sec, ferrite is excessively generated during continuous cooling, TS is lowered, and it is difficult to ensure strength. In addition, the steel plate residence time at a high temperature is prolonged, and elements such as Si and Mn are concentrated on the steel plate surface, the surface properties are deteriorated, workability is lowered, and delayed fracture is likely to occur. On the other hand, in order to produce a hard low-temperature transformation phase from austenite, it is desired that the cooling rate is fast, but at 1000 ° C./second or more, there is no significant difference in the obtained structure, and there is no change in characteristics. Therefore, the cooling rate is 10 ° C./second or more. Preferably, in order to further refine the crystal grains obtained and improve the hole expansion rate in addition to the uniform crystal grains, the temperature is set to 50 ° C./second or more and less than 1000 ° C./second. The cooling stop temperature is 70 ° C or lower. Above 70 ° C, a sufficient amount of low-temperature transformation phase cannot be obtained, and TS980MPa or more cannot be achieved.

  In the present invention, the cooling rate is defined as (cooling start temperature−70 ° C.) / (Time required for cooling from the cooling start temperature to 70 ° C.). The cooling start temperature is preferably 600 ° C. or higher. In addition, cooling and gas cooling can be used between the annealing temperature and the cooling start temperature.

  The cooling method is not particularly limited. Although water cooling is preferable, gas jet cooling, mist cooling, roll cooling, or the like may be used, or a plurality of cooling methods may be combined. In performing water cooling, a method of rapidly cooling the steel strip by immersing the heated steel strip in water or spraying water is preferable. In order to increase the cooling rate when immersed in water, it is desirable to jet water in the water tank. The water may be warm water or may contain a water-soluble substance such as a salt. Further, it may be an emulsion containing finely dispersed oil.

  Next, the temperature is raised by induction heating up to (0.3 × heat treatment temperature) ° C. or higher at a temperature increase rate of 10 ° C./second or higher. When the rate of temperature increase is slower than 10 ° C./second, the carbides become coarse and the distribution of carbides is localized in the process of tempering the low-temperature transformation phase generated after annealing and cooling. For this reason, uniform deformation is hindered at the time of molding, locally high strains or voids are generated, and there are many origins of delayed fracture, which deteriorates delayed fracture resistance. A more preferable heating rate is 20 ° C./second or more in order to obtain a more uniform structure and improve delayed fracture characteristics.

  If the temperature raised by induction heating is less than 0.3 x heat treatment temperature, after induction heating is completed, heat treatment with a temperature gradient in the steel sheet is performed as in the conventional temperature rise heating, so the effect of induction heating is sufficient. It cannot be expressed. In order to further exhibit the effect of induction heating, the temperature raised by induction heating is preferably 0.5 × heat treatment temperature or higher.

  In the induction heating, there is no difference in thermal history between the thickness direction and the width direction of the steel sheet, and the entire steel sheet can be heated uniformly without a temperature gradient. In heating other than induction heating, a temperature gradient is generated between the steel sheet surface layer and the center, resulting in a non-uniform structure. The reason why a uniform structure cannot be obtained by heating other than induction heating is that the surface layer is heated first, and the center side is like the residual heat of the surface layer and is heated slowly and gradually in the thickness direction and width direction. This is because the heat treatment is performed with a gradient. Therefore, in order to obtain a steel sheet having excellent delayed fracture resistance after forming, heat treatment by induction heating is performed to obtain a very uniform structure. The output required at this time depends on the efficiency, but the induction heating equipment is preferably 1500 kw or more as a whole equipment. One or more facilities may be installed in parallel.

  Here, FIG. 1 is a graph showing the relationship between the cooling rate after annealing and the temperature rising rate after cooling with respect to delayed fracture resistance. In FIG. 1, a slab of 0.142% C-1.15% Si-2.11% Mn-0.015% P-0.0007% S-0.040% Al-0.0035% N, slab heating temperature: 1250 ° C, finish rolling temperature: 910 ° C, Winding temperature: 580 ° C, cold rolling reduction: 50%, heating, hot rolling, pickling, cold rolling, then annealing temperature: 860 ° C, cooling start temperature: 700 ° C, cooling rate : 2 to 900 ° C./second, rate of temperature increase by induction heating or radiant tube heating: 1 to 100 ° C./second, heat treatment temperature: 360 ° C., heat retention time: 900 seconds, annealing was performed to obtain a steel plate. From FIG. 1, it can be seen that cracking does not occur at the cooling rate after annealing of 10 ° C./second or more and the heating rate is 10 ° C./second or more using induction heating, and the delayed fracture resistance is excellent. From FIG. 1, it is preferable that the cooling rate after annealing is 50 ° C./second or more, and the temperature rising rate after cooling is 20 ° C./second or more.

  Next, heat treatment is performed at a temperature of 150 ° C. to 500 ° C. When the heat treatment temperature is lower than 150 ° C., the softening of the low-temperature transformation phase generated by rapid cooling becomes insufficient and the workability is poor. Furthermore, in the case of low-temperature heat treatment, internal strain resulting from distortion of the crystal lattice remains during rapid cooling after annealing, and there are many origins of delayed fracture, and delayed fracture is likely to occur. When the temperature is higher than 500 ° C., the low-temperature transformation phase is decomposed into ferrite and carbide, TS is rapidly lowered, and it is difficult to ensure high strength. Therefore, the heat treatment temperature is set to 150 ° C. or more and 500 ° C. or less. In order to achieve both workability and excellent delayed fracture resistance, the heat treatment temperature is preferably more than 300 ° C and less than 500 ° C. In addition, the heat retention time is not particularly specified when the heat treatment is performed, but the heat can be retained for about 60 to 1500 seconds. In addition, after the heat insulation heat treatment, cooling such as furnace cooling, gas cooling, and water cooling can be performed.

  As described above, a high workability ultra-high strength cold-rolled steel sheet having excellent delayed fracture resistance after forming according to the present invention can be obtained. The microstructure of the ultra-high-strength cold-rolled steel sheet obtained by the present invention is tempered with a structure composed only of a low-temperature transformation phase obtained by quenching from an austenite single-phase region, or with a ferrite phase. It is a two-phase steel composed of low-temperature transformation phase. The low temperature transformation phase is martensite, bainite, residual austenite, etc. obtained by quenching from austenite.

  Further, in the present invention, since heat treatment, that is, tempering treatment is performed by induction heating, the obtained ultra-high-strength cold-rolled steel sheet is adjacent to or separated from each of the ferrite phase and the tempered low-temperature transformation phase. It has a metallographic feature that there is little nano hardness distribution for each crystal grain of the same phase. That is, the difference between the maximum nanohardness and the minimum nanohardness of the ferrite phase adjacent to or away from each other is within 20 GPa, and the difference between the maximum nanohardness and the minimum nanohardness of the tempered low-temperature transformation phase existing adjacent or away from each other is 20 GPa. Is within. This is presumably because, in the heating process of induction heating, rapid heating is performed and the temperature gradient is small, so that there is little concentration unevenness in the vicinity of the crystal grain boundaries and in the crystal grains, and the entire steel sheet is uniformly heat-treated.

  On the other hand, the conventional heat treatment method cannot perform the heat treatment uniformly. In general, the working environment of thin steel sheets is different from steel materials such as bolts, and a machining process for automobile parts is essential. Therefore, in conventional steel having a non-uniform structure, strain is introduced during molding into automobile parts and the like, and micro-cracks and the like are generated at the crystal grain interface and the like, and there are many sites where hydrogen is absorbed and accumulated. . Therefore, the delayed fracture resistance after molding is poor.

  Using the slab shown in Table 1, slab heating temperature: 1250 ° C, finish rolling temperature: 880 ° C, winding temperature: 580 ° C, cold rolling reduction: 50%, after heating, hot rolling, pickling, Cold rolling was performed, followed by annealing under the conditions shown in Table 2 to produce a steel plate. The cooling stop temperature at this time was set to 70 ° C. or lower. About the obtained steel plate, the material test of the following item was conducted and the material characteristic was investigated. The results are also shown in Table 2.

  (1) Using a JISZ2201 No. 5 test piece with the rolling direction and 90 ° as the longitudinal direction (tensile direction), a tensile test based on JISZ2241 was performed and evaluated.

  (2) Bending characteristics: A 40 mm wide × 200 mm long test piece with the rolling direction as the longitudinal direction was used for evaluation by performing a bending test according to JISZ2248. The test was performed at N = 3, and the bending radius at which no crack occurred at the bending tip portion was defined as the critical bending radius for all N = 3.

(3) Hole expansion rate: Implemented based on the Japan Iron and Steel Federation Standard JFST1001. When a hole with an initial diameter d ₀ = 10 mm was punched and the 60 ° conical punch was raised to widen the hole, the punch was stopped when the crack penetrated the plate thickness, and the punched hole diameter d after crack penetration was measured, The hole expansion rate (%) = ((d−d ₀ ) / d ₀ ) × 100. Tested at N = 3 and determined by simple average.

  (4) Delayed fracture time: A method conforming to JISZ2248 is used to prepare a 180 ° U-bend sample with a tip of R = 10 mm, and the spring back is tightened with bolts, and fixed so that the interval between U-bend samples is 20 mm. Then, it was immersed in 0.1 N hydrochloric acid, and the time until cracking was measured. Crack determination is visual. No cracking occurred when the specimen was immersed for a maximum of 240 hours and no cracking occurred. The test was conducted at N = 3, and the shortest crack generation time among N = 3 was defined as the delayed fracture time.

  (5) Nano hardness measurement: Using a nano hardness tester manufactured by Tribo Scope, with a load of 0.05 to 0.20 gf on the electropolished surface, the displacement of the diamond triangular pyramid with an indentation angle of 115 ° is 40 to 80 nm. The hardness was measured and calculated as nano hardness Hn = maximum load Fmax / indentation cross-sectional area A. Each component phase was measured at N = 10, and the difference in nano hardness = maximum nano hardness−minimum nano hardness was obtained.

  From Table 2, all the material properties are good in the examples of the present invention. In particular, there is no crack in the material test for delayed fracture time, and in the nano hardness measurement, the difference in nano hardness is as small as 20 GPa, and high workability ultra-high strength cold-rolled steel sheet with excellent delayed fracture resistance after forming. It can be seen that is obtained.

  On the other hand, in the comparative example in which either the component or the annealing condition is out of the scope of the present invention, the difference in nano hardness exceeds 20 Gpa or cracks occur, the delayed fracture time is long, and the high resistance to delayed fracture after molding is excellent. A workable ultra-high strength cold-rolled steel sheet has not been obtained.

  Other than automotive parts that require delayed fracture resistance after molding and high workability, they are also suitable as strength members for impact beams, door guard bars, and bumper reinforcements, for example.

It is a figure which shows the relationship between the cooling rate after annealing with respect to delayed fracture resistance, and the temperature increase rate after cooling.

Claims (1)

In mass%, C: 0.1-0.2%, Si: 0.01-1.8%, Mn: 1-2.5%, P: 0.001-0.05%, S: 0.0001-0.005%, Al: 0.005-0.05%, N: 0.0001- After casting a steel slab containing 0.005% and the balance substantially consisting of Fe, immediately or once cooled, heated, hot rolled, pickled, cold rolled steel sheet obtained,
Heat annealing at a temperature of 700-900 ° C,
Cool at a cooling rate of 10 ℃ / second or more,
(0.3 x heat treatment temperature) Up to ℃ or higher, heated by induction heating at a temperature increase rate of 10 ℃ / second,
A method for producing a high workability ultra-high strength cold-rolled steel sheet having excellent delayed fracture resistance after forming, characterized by heat treatment at a temperature of 150 ° C to 500 ° C.

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