JP4492105B2 - Manufacturing method of high-strength cold-rolled steel sheet with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a method for producing a high-strength cold-rolled steel sheet having excellent stretch flangeability.

In recent years, the use of high-strength thin steel sheets for automobile parts has been studied in order to improve the safety of automobiles and reduce the weight of vehicle bodies. Since steel sheets for automobile structural members used as automobile parts are press-formed, characteristics such as elongation and stretch flangeability are required. However, the method of improving elongation and the method of improving stretch flangeability are in a contradictory relationship, and it has been difficult to improve both at the same time.
On the other hand, Patent Document 1 and Patent Document 2 are excellent in stretch flangeability with a strength of 780 MPa or more by forming a structure containing fine bainite with an average crystal grain size of 5.0 μm or less at a fraction of 80% or more. A manufacturing method for obtaining a high-strength steel sheet is disclosed.

In Patent Document 3, as a method for producing a high-strength steel sheet excellent in stretch flangeability, a tempering treatment is performed at a temperature of 350 to 600 ° C., and the hardness ratio of the ferrite phase and the low-temperature transformation generation phase is reduced. A method for improving local elongation is disclosed.
In addition to the above, Patent Document 4 and Patent Document 5 disclose manufacturing methods using induction heating.
JP 2001-226741 A JP 2001-220647 A Japanese Unexamined Patent Publication No. 7-59726 JP-A-2-74823 Japanese Patent Laid-Open No. 10-121150

  However, in Patent Document 1 and Patent Document 2, in order to obtain a high-strength steel sheet excellent in stretch flangeability with a strength of 780 MPa or more, the manufacturing conditions are strictly defined mainly in the hot rolling process, so the manufacturing restrictions There is a problem with productivity.

  In Patent Document 3, tempering treatment at 400 ° C. or higher sharply lowers the strength, so that a large amount of alloying element is required and the cost is increased. In addition, the higher the tempering temperature, the higher the energy cost in production and the lower the productivity. Therefore, the tempering process at 400 ° C. or higher has a problem from the viewpoint of productivity.

  In Patent Document 4 and Patent Document 5, there is no knowledge about material properties such as stretch flangeability, and only descriptions of equipment rows, plate-through properties, shapes, variations and the like are described.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a method for producing a high-strength cold-rolled steel sheet having excellent stretch flangeability.

  As a result of diligent research to solve the above-mentioned problems, the inventors of the present invention have made 100 ° C./s from a rapid cooling start temperature to 100 ° C. or less after recrystallization annealing on a high-strength cold-rolled steel sheet (TS ≧ 590 MPa). Rapid cooling at the above cooling rate, followed by rapid heating at 10 ° C / s or higher and tempering treatment improves the deformability of martensite, which is a low-temperature transformation phase, and improves elongation and stretch flangeability. I found it.

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

[1] At mass%, C: 0.03-0.2%, Si: 2% or less, Mn: 0.5-3%, P: 0.1% or less, S: 0.01% or less, SolAl: 0.01-0.1%, N: 0.005% The steel containing the following, melting the steel consisting of the remainder Fe and inevitable impurities , then hot-rolled, pickled, cold-rolled steel plate obtained by recrystallization annealing at a temperature above the Ac1 transformation point , Rapid cooling start temperature : When quenching from 550 ° C to 100 ° C at a cooling rate of 100 ° C / s or more and tempering temperature is T ₁ (° C), up to 0.5 x T ₁ (° C) or more, 10 High-strength cold-rolled steel sheet with excellent stretch flangeability, which is rapidly heated at a heating rate of ℃ / s or higher and tempered at a temperature of 200 to 500 ℃ (excluding 250 ℃ or less). Manufacturing method.

  [2] A method for producing a high-strength cold-rolled steel sheet excellent in stretch flangeability, characterized by further comprising one or two of Cr: 1% or less and Mo: 1% or less in the above [1] .

  [3] In the steel described in [1] or [2] above, one or more of V: 0.05 to 0.2%, B: 0.0002 to 0.002%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% or A method for producing a high-strength cold-rolled steel sheet excellent in stretch flangeability, comprising two or more kinds.

  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 high-strength cold-rolled steel sheet is preferably a tensile strength of 590 MPa or more, preferably used for parts such as a door impact beam and a bumper used for the purpose of improving the safety against collision of an automobile. A cold-rolled steel sheet of 780 MPa or more.

  According to the present invention, a high-strength cold-rolled steel sheet having excellent stretch flangeability can be obtained. Since the steel plate of the present invention has high strength and is excellent in elongation and stretch flangeability, it can be applied as a difficult-to-form member such as an automobile structural member, which has conventionally been difficult to apply high strength steel plates. It becomes. Furthermore, when the high-strength cold-rolled steel sheet of the present invention is used as an automobile structural member, it contributes to reducing the weight of the automobile, improving safety, etc., and is extremely useful in industry.

The present invention defines the following components, further defines annealing conditions, specifically, after recrystallization annealing, rapidly cools from a rapid cooling start temperature to 100 ° C. or less at a cooling rate of 100 ° C./s or more, and then anneals. When the return temperature is T ₁ (° C.), it is characterized by rapid heating up to 0.5 × T ₁ (° C.) or higher at 10 ° C./s or more, and these are the most important requirements in the present invention. It is. By defining the components and annealing conditions in this manner, the deformability of martensite, which is a low-temperature transformation phase, is improved, and a high-strength cold-rolled steel sheet having excellent stretch flangeability 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: 0.03-0.2%
C is an important element for strengthening the martensite phase of the quenched structure. If C is less than 0.03%, the effect of increasing the strength is insufficient. On the other hand, when C exceeds 0.2%, good weldability cannot be obtained. From the above, C is set to 0.03% or more and 0.2% or less.

Si: 2% or less
Si is a solid solution strengthening element and is an effective element for obtaining a steel plate having high strength and high elongation. However, if it exceeds 2%, a large amount of Si oxide is formed on the surface of the steel sheet and the chemical conversion property is deteriorated, so Si is made 2% or less.

Mn: 0.5-3%
Mn is an important element for suppressing the formation of ferrite in the annealing zone in a continuous annealing furnace. If it is less than 0.5%, the effect is not sufficient. On the other hand, if it exceeds 3%, slab cracking occurs in the continuous casting process. From the above, Mn is 0.5% or more and 3% or less.

P: 0.1% or less, S: 0.01% or less
P and S are impurities in the steel of the present invention, and are preferably lower when considering the workability of the steel sheet. Therefore, P is 0.1% or less and S is 0.01% or less.

Sol.Al:0.01-0.1%
Al is used as a deoxidizing material, but if it is less than 0.01%, a sufficient deoxidizing effect cannot be obtained. On the other hand, if it exceeds 0.1%, the effect of Al addition is saturated and uneconomical. Therefore, Sol.Al is set to 0.01% or more and 0.1% or less.

N: 0.005% or less
N is an impurity contained in the crude steel, and deteriorates the formability of the raw steel sheet. Therefore, it is desirable to remove and reduce it as much as possible in the steel making process. However, if N is reduced more than necessary, the refining cost increases, so 0.005% or less, which is substantially harmless, is set.

Cr: 1% or less
Cr is added as necessary to increase the strength of the steel sheet. In some cases, such as improved corrosion resistance. In order to obtain the above effect without impairing the effect of the present invention, Cr is preferably added in a content of 1% or less.

Mo: 1% or less
Mo is a precipitation strengthening element, but if it is too much, the ductility is lowered and the price is also expensive. For these reasons, it is preferable to add Mo at a content of 1% or less.
V: 0.05-0.2%, B: 0.0002-0.002%, Nb: 0.005-0.1%, Ti: 0.005-0.1%
V, B, Nb, Ti nitride-forming elements can be contained within the range that does not deteriorate the characteristics and manufacturability for the purpose of strength adjustment, etc., V: 0.05% to 0.2%, B: 0.0002% It is preferable to add one or two or more in a content of 0.002% or less, Nb: 0.005% or more and 0.1% or less, Ti: 0.005 or more and 0.1% or less.

  The remainder other than the above is substantially made of Fe. Here, the balance being substantially Fe means that what contains other trace elements including unavoidable impurities can be included in the scope of the present invention unless the effects of the present invention are lost.

  Next, the manufacturing method of this invention is demonstrated.

  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 Ar3 or higher in order to improve elongation and stretch flangeability. If the final rolling temperature is lower than the Ar3 point, the ferrite grains become extremely coarse due to the two-phase structure at the final rolling stage, and a steel sheet with good workability may not be obtained even if cold rolling and annealing are performed.

  Next, the obtained hot-rolled sheet is cooled and wound up. The winding temperature is preferably 620 ° C. or lower in order to improve elongation and stretch flangeability.

  Next, pickling and cold rolling are performed to obtain a desired thickness. The cold rolling rate at this time is preferably 50% or more in order to improve elongation and stretch flangeability.

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

Here, in the production method of the present invention, a steel sheet having a two-phase structure of a ferrite phase and a martensite phase or a martensite single-phase structure is obtained, in which the deformability of martensite which is a low-temperature transformation phase is improved. Therefore, in the recrystallization annealing to tempering treatment, after recrystallization annealing, quenching is performed at a cooling rate of 100 ° C./s or more to 100 ° C. or lower, and then the tempering temperature is set to T ₁ (° C.), 0.5 × T ₁ (° C) or higher is rapidly heated at 10 ° C / s or higher. These are the most important requirements in the present invention. This will be described in detail below.

  First, after soaking (recrystallization annealing) above the Ac1 point, it is cooled to the quenching start temperature. If the recrystallization annealing temperature is less than Ac1, an austenite phase cannot be obtained while maintaining a high temperature, and therefore a martensite phase cannot be obtained after rapid cooling, and high strength cannot be achieved. The soaking time is not particularly limited, but if it is less than 10 seconds, there is a high possibility that undissolved carbide is present and the abundance of the austenite phase may be reduced. After the recrystallization annealing, the cooling to the rapid cooling start temperature is not particularly limited, and for example, means such as a gas jet can be used. The rapid cooling start temperature is preferably 550 ° C. or higher. If it is less than 550 ° C., the second phase is not sufficiently martensitic transformed, and bainite may be mixed.

  Next, rapid cooling is performed at a cooling rate of 100 ° C./s or higher from the rapid cooling start temperature to 100 ° C. or lower. If the cooling rate from the rapid cooling start temperature to 100 ° C is less than 100 ° C / s, pearlite and bainite may not precipitate and martensite transformation may occur. To obtain high strength, the alloy addition amount must be increased. A new problem will occur. Note that water cooling is preferable as a cooling method at this time. However, the cooling method is not limited to water cooling, and gas jet cooling, mist cooling, roll cooling and the like can be used alone or in combination.

Next, after rapid cooling, the temperature is raised to the tempering temperature. At this time, assuming that the tempering temperature is T1 (° C), the temperature rise to 0.5 × T ₁ (° C) (rapid heating completion temperature) or higher is performed at a heating rate of 10 ° C / s or higher. And When heating rate is less than 10 ° C. / s, or rapid when the heating completion temperature such less than 0.5 × T ₁ is not occur carbides finely dispersed precipitates in a martensitic phase, deformability of the martensite phase is conventional As a result, a sufficient stretch flangeability effect cannot be obtained. From the above points, the temperature rising rate is preferably 20 ° C./s or more. The rapid heating completion temperature is preferably 0.8 × T ₁ or more in order to obtain a high tempering effect in a short time. Furthermore, since rapid heating further improves stretch flange characteristics, it is desirable to use induction heating. However, the method is not limited to induction heating, and methods such as radiant heating and direct fire heating can also be used.

  Next, tempering is performed after the temperature rise. Tempering temperature shall be 200 ℃ or more and 500 ℃ or less. When the tempering temperature is less than 200 ° C., the martensite phase is not sufficiently tempered and the ductility is low. On the other hand, when the tempering temperature exceeds 500 ° C., when the tempering process is performed, the strength rapidly decreases. Further, in order to obtain a higher strength cold-rolled steel sheet with TS ≧ 780 MPa, the tempering temperature is preferably 200 ° C. or higher and 400 ° C. or lower.

  As described above, λ is improved by rapid heating after recrystallization annealing and rapid cooling. Although the details about the improvement of λ are unknown, it is thought that carbides are finely dispersed and precipitated by rapid heating, and the deformability of the martensite phase is increased.

  Next, cooling is performed after tempering. The cooling method and cooling rate at this time are not particularly limited.

  From the above, a high-strength cold-rolled steel sheet having excellent stretch flangeability according to the present invention can be obtained.

  Steel slabs having the chemical composition shown in Table 1 were produced by continuous casting, and the slabs were reheated to 1250 ° C. and then hot rolled to a plate thickness of 2.8 mm. Hot rolling was performed at a finishing temperature of 850 to 900 ° C and a winding temperature of 500 to 600 ° C. Subsequently, pickling and cold rolling were performed to obtain a steel plate having a thickness of 1.2 mm. The obtained steel sheet was recrystallized and annealed at a soaking temperature shown in Table 2 in a continuous annealing furnace, quenched at a cooling rate of 100 ° C./s or higher in water quenching, and martensified at a volume ratio shown in Table 2. After obtaining a composite structure steel having a site phase, tempering treatment was performed at the tempering temperatures shown in Table 2. Induction heating was used for heating to the tempering temperature.

The steel plate subjected to the above heat treatment was subjected to the following tests to evaluate YP (MPa), TS (MPa), El (%), and hole expansion rate λ (%). The results are shown in Table 3.
Tensile test: A JIS No. 5 test piece was sampled by cutting at a right angle to the rolling direction of the steel sheet, and the test was conducted in accordance with JIS Z2241.
Hole expansion test: The test was conducted in accordance with Japan Iron and Steel Federation Standard JFST1001-1996.

  From Table 3, it can be seen that the test material numbers G to L, which are examples of the present invention, have a higher hole expansion ratio λ than the comparative examples, and have good elongation and stretch flangeability. On the other hand, Comparative Examples A to F do not perform rapid heating, and the rate of temperature increase to the tempering treatment temperature is less than 10 ° C./s and is outside the scope of the present invention, so the deformability of the martensite phase is low compared to the present invention examples. The stretch flangeability is inferior.

  Using steel having the composition of steel number 4 in Table 1, cold rolling was performed under the same conditions as in Example 1. Next, the obtained steel plate was heat-treated under the conditions shown in Table 4. In the heat treatment, water quenching was rapidly cooled at a cooling rate of 100 ° C./s or higher, and various methods were used for heating to the tempering temperature.

  The heat-treated steel sheet was subjected to a tensile test and a hole expansion test in the same manner as in Example 1 to evaluate the performance. The results obtained are shown in Table 5.

From Table 5, the test material numbers 1, 2, 4, 9, 11, 12, 13, 14, 16, 17, 18 which are examples of the present invention all have a tensile strength of 780 to 90 MPa, and TS × ( El × λ) ^1/2 is 30000 or more and has good elongation and stretch flangeability.

On the other hand, for example, sample No. 7, which is a comparative example, has a recrystallization temperature of 650 ° C. and less than the Ac1 point, so that the strength and the hole expansion rate are low and the stretch flangeability is inferior. Since test material number 10 has a low tempering temperature of 50 ° C., sufficient tempering of the martensite phase cannot be obtained, the hole expansion rate is low, and stretch flangeability is poor. For test material numbers 3, 5, 6, 8, and 15, the rate of temperature increase to the tempering temperature is 10 ° C / s or less, so the deformability of the martensite phase is lower than the example of the present invention, and the stretch flangeability is poor. ing. As described above, the comparative example is inferior in the characteristics of TS, El, or λ, and TS × (El × λ) ^1/2 is less than 30000.

Based on the above results, FIG. 1 shows the relationship between the rate of temperature increase during tempering and TS × (El × λ) ^1/2 . According to FIG. 1, it can be seen that there is a temperature increase rate range during the tempering treatment in order to obtain a 980 MPa grade steel sheet with an excellent balance of properties. That is, in order to set the value of TS × (El × λ) ^1/2 as a reference for a 980 MPa class steel plate having an excellent property balance to be 30000 or more, the rate of temperature increase is 10 ° C./s or more. Even if the heating method is radiant heating or direct flame heating, the value of TS × (El × λ) ^1/2 is 30000 or more if the rate of temperature rise is 10 ° C./s or more. However, the temperature rise by induction heating is preferable because it shows a more excellent hole expansibility.

FIG. 6 is a graph showing the relationship between the temperature rise rate during tempering and TS × (El × λ) ^1/2 (Example 2).

Claims (3)

In mass%, C: 0.03-0.2%, Si: 2% or less, Mn: 0.5-3%, P: 0.1% or less, S: 0.01% or less, SolAl: 0.01-0.1%, N: 0.005% or less Then, the steel composed of the remaining Fe and inevitable impurities is melted, then hot rolled, pickled, and then cold rolled, and the steel plate obtained is subjected to recrystallization annealing at a temperature equal to or higher than the Ac1 transformation point, and rapid cooling is started. Temperature : Rapid cooling from 550 ° C to 100 ° C at a cooling rate of 100 ° C / s or more, and when tempering temperature is T ₁ (° C), up to 0.5 x T ₁ (° C) or more, 10 ° C / s A method for producing a high-strength cold-rolled steel sheet with excellent stretch flangeability, characterized by performing rapid heating at the above heating rate and tempering at a temperature of 200 to 500 ° C (excluding 250 ° C or less) . The high-strength cold-rolled steel sheet with excellent stretch flangeability according to claim 1, wherein the steel further contains one or two of Cr: 1% or less and Mo: 1% or less. Method. The steel according to claim 1 or 2 further contains one or more of V: 0.05 to 0.2%, B: 0.0002 to 0.002%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%. The method for producing a high-strength cold-rolled steel sheet having excellent stretch flangeability according to claim 1 or 2.

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