JPH05125448A - Manufacture of high strength cold rolled steel sheet excellent in strength and ductility - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet excellent in strength and ductility by subjecting the continuously cast slab of steel having a specified compsn. to heat treatment under specified conditions, thereafter subjecting it to hot rolling and cold rolling into a sheet material and thereafter executing continuous annealing under specified conditions. CONSTITUTION:The continuously cast slab of steel having a compsn. contg., by weight, 0.05 to 0.15% C, 0.5 to 2.5% Si, 0.5 to 3.0% Mn and 0.005% S or furthermore contg. 0.0002 to 0.0020% CaO is subjected to hot rolling into a steel sheet, which is cooled in the temp. range of (the starting temp. of ferritic transformation) to (the starting temp. of ferritic transformation - 70 deg.C) in this steel at >=5 deg.C/sec average cooling rate and is thereafter coiled round a coil at >=600 deg.C. Scales on the surface of the above hot rolled steel sheet are pickled and removed away. After that, it is subjected to cold rolling to regulate its sheet thickness into final one, and continuous annealing is executed. At this time, the cold rolled steel sheet is held in the temp. range of the Ac1 transformation point to the Ac3 transformation point at >=20 deg.C/sec cooling rate and is held to the temp. range of 350 to 480 deg.C for >=60sec. The high strength cold rolled steel sheet having >500N/m<2> tensile strength can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-rolled steel sheet having a tensile strength of 500 N / mm ² or more and excellent strength and ductility characteristics.

[0002]

2. Description of the Related Art Higher strength steel sheets for automobiles have hitherto been made in order to meet the needs of automobiles and users who desire low fuel consumption by reducing the weight of the vehicle body. However, recently, fuel consumption itself has been taken into account due to global environmental problems, and due to the social demand to reduce fuel consumption in order to regulate carbon dioxide emissions as a measure against global warming, it has become even more important than ever. The importance of high strength steel sheets has come to be pointed out. The weight reduction of automobile parts is performed by reducing the plate thickness while maintaining various strength characteristics (impact strength, strength as a rigid body, welding strength, fatigue strength, etc.) necessary for the parts. Therefore, even if the strength is increased, the formability deteriorates, which means that it cannot be used for the part concerned, and it is naturally an important factor that a steel material with high strength and excellent workability is used. Becomes

At present, most of the steel sheets used for body parts manufactured by press molding have a tensile strength of 30.
It is from 0 to 450 N / mm ² . Therefore, tensile strength 500-700 with workability of this strength class level
There is a great need for high-strength steel sheets with N / mm ² . On the other hand, a method for producing a high-strength steel sheet containing retained austenite has been proposed as a material having high strength and excellent workability. This is the transformation-induced plasticity (Transfo
rmulation Induced Plasticit
(y: TRIP), which is in the limelight as a material that far exceeds the strength and ductility characteristics of the dual phase steel, which is the most excellent conventional high strength steel sheet. Various techniques for manufacturing this steel sheet have been proposed. Among them, as a specific example of the technique for obtaining a strength of about 600 N / mm ² class, Japanese Patent Laid-Open No. 1-7
9321, Japanese Patent Application Laid-Open No. 1-79322, Japanese Patent Application Laid-Open No. 1-168819 and the like.

[0004]

However, all of these techniques have some problems in industrial production. In Japanese Patent Laid-Open No. 1-168819, there is no device for stably obtaining a high-grade material, so the annealing temperature for obtaining the best material is limited to a narrow range, and from the viewpoint of industrial production. Cannot be called a completed technology. In Japanese Patent Laid-Open No. 1-79322, it seems that the hot rolling method is partly devised, but in hot rolling, it is necessary to wind the coil around a coil at a temperature not higher than the Bs point so that the bainite has an area ratio of 50 to 100%. However, this means that for cooling by water cooling, winding is performed at the boundary between film boiling and nucleate boiling, and it is hard to say that this is a technique that can be stably obtained. As described above, the conventional method for producing a high-strength cold-rolled steel sheet containing retained austenite is the property obtained as the final product, that is, the Dual Phase steel having the highest strength and ductility among the high-strength steel sheets to date. Although it greatly exceeded, it was due to the stable manufacturability on the manufacturing side that it was not supplied to the market. The problem remains to improve this and stably manufacture a high-strength cold-rolled steel sheet containing retained austenite and having excellent strength and ductility characteristics.

[0005]

To solve these problems, the present invention provides a steel of a specific component with a specific hot rolling and cold rolling from the viewpoint of industrially producing a stable and high standard. This is to be solved by taking the subsequent continuous annealing method. The mass of the skeleton is C: 0.05% to 0.15% Si: 0.5% to 2.5% Mn: 0.5% to 3.0% S ≦ 0.005% is required. According to the above, 0.0002 to 0.0020% of Ca is contained, and the balance substantially consists of Fe.

[0006]

[Operation] Next, the operation of each requirement and the reason for limiting the numerical value will be described. C: C is an important element for the formation of a retained austenite phase, and if it is less than 0.05%, a sufficient amount of retained austenite cannot be obtained and therefore good workability cannot be exhibited. In this sense, the higher the amount of C, the better.
The upper limit of the range for obtaining the target 500 to 700 N / mm ² strength characteristics is 0.15%. Preferably,
It should be in the range of 0.08 to 0.12%. Si: Si is an important element for allowing austenite to remain in the austempering treatment near 400 ° C.
It is considered that this is because Si has an effect of suppressing transformation accompanied by precipitation of coarse cementite during transformation near this temperature and stabilizing austenite. Such Si
In the present invention, the effect is not exerted at 0.5% or less, while if it exceeds 2.5%, the effect is saturated and only the economical efficiency is impaired. Preferably, it should be in the range of 0.8 to 1.8%.

Mn: Mn must be added in order to impart a certain hardenability, because it has the effect of delaying the transformation of bainite in Mn and the effect of lowering the martensite transformation start temperature of austenite. It is considered to be an effect to be exhibited. In the steel of the present invention, the content range of Mn is specified in order to effectively exhibit these. If it is less than 0.5%, sufficient austenite cannot be secured, and if it exceeds 3.0%, the low-temperature transformation products are unnecessarily increased and the strength is increased. More preferably, it should be 0.8 to 1.8%. S: S is an element that deteriorates stretch flange formability in the steel of the present invention, so it must be thoroughly reduced. Therefore, it is made 0.005% or less. Further, if Ca is contained in an amount of 0.0002 to 0.0020%, the effect is more clearly exhibited. The subsequent hot rolling conditions and the continuous annealing conditions after cold rolling are extremely important in the present invention together with the limitation of the components.

In the hot rolling process, it is necessary to perform cooling at an average of 5 ° C./s or more in the temperature range of (ferrite transformation start temperature) to (ferrite transformation start temperature-70 ° C.) and wind it at 600 ° C. or more. In the present invention, this is a necessary condition for stably obtaining high strength ductility characteristics in the final product after continuous annealing. The present inventors have found that a measure for stably obtaining a high strength ductility characteristic is
It was discovered that not only the optimization of the composition and the continuous annealing conditions but also the optimization of the hot rolled structure is important, and the optimum structure under the conditions that can be adopted more stably in the current manufacturing process. Was repeatedly examined. As a result, the cooling conditions and coiling temperature after hot rolling finish rolling were limited. If it is wound at a temperature lower than 600 ° C., the low temperature transformation product is mixed in the hot rolled structure, which deteriorates the cold rolling property and narrows the annealing temperature condition, which is an industrially unsuitable condition.

When winding at 600 ° C. or higher, the inventors of the present invention have considered that the condition for obtaining a highly stable material property must be the cooling condition after finish rolling, and have conducted repeated studies. Needless to say. As a result, (ferrite transformation start temperature) to (ferrite transformation start temperature-70
It has been found that it is optimal to apply an average cooling rate of 5 ° C./s or more in the temperature range of (° C.). The reason for this has not been clarified in many areas, but it is considered that the degree of Mn or C dispersion at this stage probably affected the structure distribution of the final product. If the average of 5 ° C./s or more is not applied at a temperature exceeding the ferrite transformation start temperature −70 ° C., the elongation at break of the final product deteriorates.

In continuous annealing, it is first necessary to maintain the temperature in the temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point for 10 seconds or more. This is because it is necessary to make the ferrite / austenite two-phase state in this annealing process. If the temperature is higher than the Ac ₃ transformation point, all austenite is formed during annealing, and the austenite coarsens, so that it is difficult to finally retain austenite and desired properties cannot be obtained. Further, austenite cannot be obtained by annealing below the Ac ₁ transformation point. afterwards,
It is necessary to cool the temperature range of 600 to 480 ° C at a cooling rate of 20 ° C / s or more. This is an important requirement as a pre-stage treatment for preventing transformation of pearlite by quenching in this temperature range and performing subsequent austempering. If it is less than this cooling rate, austenite unnecessarily undergoes transformation accompanied by precipitation of carbides, and desired strength and ductility characteristics cannot be obtained. Although the upper limit cooling rate is not particularly specified, it is considered to be up to about 200 ° C./s in consideration of temperature controllability. Preferably 50-120 ° C /
There should be a cooling rate of s.

After that, it is necessary to maintain the temperature range of 350 ° C. to 480 ° C. for 60 seconds or more. This is a necessary condition for leaving austenite in the final product stage by performing the optimum austempering treatment. 35
If cooled to a temperature lower than 0 ° C., austenite is consumed due to the formation of martensite, the austenite of the final product is not secured, and it is hardened, so that desired characteristics cannot be obtained. Also, 480 ° C
If it exceeds, C is consumed due to the formation of extra carbides, and the desired characteristics cannot be obtained. Preferably,
It should be in the range of 380-450 ° C. Further, it is necessary to maintain the temperature range for 60 seconds or more. This is a condition necessary for performing the austempering treatment itself by utilizing the bainite transformation, and if it is less than this time, optimum austenite for ductility cannot be obtained. It should preferably be in the range of 250 to 600 seconds. If a time longer than this is adopted, the bainite transformation cannot be unnecessarily advanced and the amount of austenite cannot be secured.

The product according to the method of the present invention has a residual austenite content of 5 to 10% and a ferrite content of 70% or more, and is composed of the remaining low-temperature transformation product. To date, most of the high-strength steel sheets containing retained austenite and having excellent workability are mostly in Japanese Patent Laid-Open No. 61-157625 and Japanese Patent Laid-Open No. 01-1-1.
As disclosed in Japanese Patent No. 59317, the tensile strength is 800 N /
It has been proposed as a product of mm ² or more or its manufacturing technology, in which case the retained austenite is, for example, 15%.
The above is included. In the case of the present invention, it is the result of studying the measures for obtaining the desired characteristics in a stable manner in industrially manufacturing those having a tensile strength lower than that, and after defining the component range. Even if the amount of retained austenite is less than 10%, the TRIP effect is sufficiently exerted, and conversely, even if the amount of retained austenite exceeding 10% is secured, in the case of this austenite, the austenite which is not reflected in the elongation property is obtained. It is unsuitable. In the present invention, the cold piece may be inserted into the heating furnace in hot rolling, or the hot piece may be rolled as it is. The finish rolling temperature should preferably be above the Ar ₃ transformation point. For cooling after finish rolling, the effect of the present invention is not lost even if water cooling such as laminar or spraying is used or a cooling medium instead thereof is used. The pickling after hot rolling may be with sulfuric acid or hydrochloric acid. The subsequent cold rolling is tandem,
Either reverse is acceptable. The cold rolling rate may be produced within a range that can be industrially produced, but it is preferably 50% or more. In the temper rolling after continuous annealing, if the shape is bad, it is enough to correct it, and a leveler or the like may be used. In addition, this product may be further electroplated. In that case, either Zn plating or Zn-Ni plating may be used.

[0013]

Example After the steels shown in Table 1A were made into slabs by continuous casting, hot rolling was performed under the conditions shown in Table 1B. Two seconds after finishing rolling, water cooling was performed by laminar cooling. The cooling rate was 40 ° C./s on average. Hot rolled sheet thickness is 3.
It was set to 0 mm. Then, it was pickled and cold rolled to 1.0 mm.
When continuously annealing this cold rolled coil, continuous annealing was performed under the conditions shown in Table 1B. All of the heat retention (annealing) in continuous annealing is within the scope of the invention, and 600 to 600 after annealing.
The cooling rate in the temperature range of 480 ° C was 60 ° C / s. After that, 0.3% temper rolling was performed and used for the test.

[0014]

[Table 1A]

[0015]

[Table 1B]

To: Ferrite transformation start temperature (° C.) under the above conditions FT: Hot rolling finish rolling end temperature (° C.) T ₁ : End temperature of cooling by water cooling started 2 seconds after finish rolling (° C.) CT : Winding temperature (° C) ST: Annealing temperature in continuous annealing (° C) T ₂ : End point temperature of cooling at the same cooling speed that was performed after rapid cooling at 600 to 480 ° C T ₃ : T Hold for 320 seconds after T _2. Temperature of steel material after aging YP: Yield point strength (N / mm ² ) TS: Tensile strength (N / mm ² ) El: Elongation at break (%)

The tensile test was carried out using a No. 5 test piece described in JIS Z2201 according to the method described in Z2241. The stretch flange formability was evaluated by a hole expansion test. This is a numerical value obtained by expanding a hole with a diameter of 20 mm punched with a punching clearance of 10% with a 30 ° conical punch, measuring the hole diameter when a crack penetrates the plate thickness, and dividing this by the die diameter at the time of punching. It was evaluated by. After finishing rolling, cooling by water cooling was started at 1.5 seconds to 50 ° C / s, cooling by water cooling was completed at 670 ° C, and the film was wound at 620 ° C. The test results of the product are shown in Table 1B.

The steels of the present invention A-1, B-1, C-1,
All of D-1 were in the range of TS = 500 to 700 N / mm ² , and had excellent elongation at break and stretch flangeability, which was d / do ≧ 1.5. Next, the case where the manufacturing conditions are variously changed using A steel will be described. The manufacturing conditions are shown in Table 2A. Hot rolling heating temperature = 115
0 ° C., FT = 830 ° C., hot rolled plate thickness = 3.0 mm, cold rolled plate thickness = 1.0 mm. In addition, the Ac ₁ transformation point of A steel is 7
At 50 ° C, the Ac ₃ transformation point is 850 ° C. After continuous annealing,
It was subjected to temper rolling of 0.3% and subjected to a tensile test.

[0019]

[Table 2A]

CR ₁ : Cooling rate after finishing rolling (° C /
s) To: Ferrite transformation start temperature (° C) under the conditions T ₁ : Ending temperature of cooling started after finishing rolling (° C) CT: Winding temperature (° C) ST: Annealing temperature in continuous annealing (° C) CR _2: 600 to 480 (or T ₂₎ the cooling rate in the temperature range _{℃ (℃ / s) T 2} : end temperature of the cooling at _{CR 2 ℃ / s (℃)} t: T 2 after the retention time ( s) T _3: T ₂ after t seconds temperature after holding (℃) underline indicates that it is outside the invention range. Table 2B is Table 2A
It is a test result of the product in. The present invention steel A-1,
All of A-5 and A-6 showed characteristics of break elongation ≧ 35% at TS = 600 N / mm ² .

[0021]

[Table 2B]

[0022]

INDUSTRIAL APPLICABILITY As described above, the present invention can be fully applied as a material for automobiles, and by doing so, weight reduction of automobiles can be achieved, thereby contributing to global environmental protection. Further, it can be applied to high-design car design, and can meet the needs of car buyers. As described above, the product according to the present invention can contribute to a wide range of society from global scale to individuals, and its effect is great.

Claims (1)

[Claims] 1. C: 0.05% to 0.15% Si: 0.5% to 2.5% Mn: 0.5% to 3.0% S≤0.005% by mass ratio Then, when 0.0002 to 0.0020% of Ca is contained and the balance is substantially steel and steel is made into a slab by continuous casting and then hot rolled, (ferrite transformation start temperature) to (ferrite transformation start temperature -70) In the temperature range of (° C.), cooling is performed at an average of 5 ° C./s or more, and winding is performed at 600 ° C. or more. Then, after pickling, cold rolling, and continuous annealing, the temperature is kept for 10 seconds or more in the temperature range of Ac ₁ transformation point or more and Ac ₃ transformation point or less, and the temperature range from 600 ° C to 480 ° C is 2
Cool at a cooling rate of 0 ° C./s or more, and then 350 ° C. to 4
A method for producing a high-strength cold-rolled steel sheet, which has a tensile strength of 500 N / mm ² or more and excellent strength and ductility characteristics, which is obtained by maintaining the temperature range of 80 ° C. for 60 seconds or more.