JPH0213013B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is mainly aimed at automobile applications, and
It has a tensile strength of 60Kgf/mm2 or ^more at about 1.6 to 4.0mm,
This invention relates to a method for producing high-strength hot-rolled steel sheets with excellent workability and weldability. Conventionally, high-strength hot-rolled steel sheets with a tensile strength of 60 Kgf/mm ² or higher have been manufactured using a precipitation strengthening system in which Nb, V, and Ti are added to Si-Mn and strength is provided by the precipitation strength of these carbonitrides. It had been. However, this method is economically problematic because it uses expensive Nb, V, Ti, etc., and its use is limited due to the low elongation characteristic of precipitation strengthening. A material that is expected to overcome this situation is so-called dual phase steel, which is composed of ferrite and martensite. It has truly favorable properties from the viewpoint of workability, such as a low yield point, high work hardening, and high elongation. The original manufacturing method for this steel was to heat it to the α/γ2 phase region and then rapidly cool it, and continuous annealing equipment was suitable for this process.
However, in general, there is no continuous annealing equipment for hot-rolled steel sheets, so development research is being conducted on manufacturing as hot-rolled steel sheets, that is, non-temperature treatment, and although several methods have been proposed, they put too much stress on process conditions. Problems arise such as the addition of alloying elements and the need for many alloying elements.
It has not yet been put into practical use from the viewpoint of economy and characteristics. The present invention was developed and put into practical use against the background of the above. The gist of the present invention is as follows. (1) C: more than 0.15 to 0.25%, Si: 1.5% or less, Mn:
After forming a slab of steel containing 0.7 to 1.5%, P: 0.01% or less, S: 0.005% or less, and Al: 0.01 to 0.10%, with the balance being Fe and unavoidable impurity elements,
It is characterized by heating to 1200℃ or less, hot rolling, finish rolling at Ar ₃ transformation point to 930℃, then cooling at an average cooling rate of 15℃/S or more, and winding at 350 to 560℃. A method for manufacturing high-strength hot-rolled steel sheets with a tensile strength of 60Kgf/mm ² or more and excellent workability and weldability. (2) C: more than 0.15 to 0.25%, Si: 1.5% or less, Mn:
0.7-1.5%, P: 0.01% or less, S: 0.005% or less, Al: 0.01-0.10%, and Ca: 0.0005-
0.0050%, REM: 0.005~0.015%, Mg:
A slab of steel containing one or more of 0.0005 to 0.010% and the remainder Fe and unavoidable impurity elements is then heated to 1200°C or less,
60Kg characterized by hot rolling and finish rolling at Ar ₃ transformation point ~ 930℃, then cooling at an average cooling rate of 15℃/S or more and coiling at 350-560℃
A method for producing high-strength hot-rolled steel sheets having a tensile strength of f/mm ² or more and excellent workability and weldability. In other words, a structure consisting of fine ferrite and dense pearlite is obtained through a combination of component regulation centered on C, Si, and Mn and specific hot rolling conditions to ensure strength and elongation. Although the elongation at this time is inferior to that of composite structure steel, it is better than precipitation-strengthened steel, and it can withstand forming sufficiently for applications such as automobile members. The workability of hot-rolled steel sheets for automobiles is not limited to elongation.
Stretch flangeability is also an important factor. Therefore, in order to thoroughly reduce sulfide-based inclusions and, in some cases, make their shape spheroidal, Ca,
Add one or more of REM and Mg. Spot weldability is the next most important property for hot rolled steel sheets for automobiles. There is a simple way to judge spot weldability based on the peeling condition when the welded part is peeled off vertically, but more precisely it is determined by cross tensile strength (JIS.Z3137).
Problems include not peeling off from the bonded surface and fatigue strength. In order to improve such spot weldability, it has been found that limiting the amount of C in the steel and significantly reducing the P and S contents are effective. Based on the above points, we specified the ingredients and hot rolling conditions, and by creating an exquisite combination, we were able to manufacture a high-strength hot rolled steel sheet that balances workability, spot weldability, and economic efficiency. Next, the reason for limiting the numerical values of each component of the present invention will be described. C is 60Kgf/mm ² as ferrite/pearlite steel
It is necessary to ensure the above strength, and for that purpose, a minimum content of more than 0.15% is required. But 0.25%
If it exceeds 60Kgf/mm2, the pearlite part increases too much, and even if a tensile strength of 60Kgf/ ^mm2 or more is taken into account, the ductility deteriorates significantly, and the spot weldability also deteriorates. Therefore, C is over 0.15~
It was set at 0.25%. Si forms a substitutional solid solution in the ferrite phase and is effective in increasing strength. Furthermore, it has the effect of increasing the degree of work hardening of ferrite and increasing its toughness. But 1.5%
When exceeding , these effects tend to be saturated;
In addition, Si scale deteriorates pickling properties and also impairs economic efficiency, so the amount of Si added is set to 1.5% or less. Mn creates a dense ferrite-pearlite structure and improves the strength and ductility of steel.
0.7% is necessary. However, if the amount of Mn added is too large, it will exhibit a layered structure, deteriorating ductility and increasing cost, so the upper limit was set at 1.5%. Next, from the point of view of spot weldability, it was necessary to thoroughly reduce P, and it was set to 0.01% or less. Furthermore, S content must be thoroughly reduced from the viewpoint of spot weldability and stretch flangeability, and is set to 0.005% or less. In order to improve stretch flangeability, it is necessary to reduce sulfide inclusions, and for this purpose it is necessary to reduce the amount of S as mentioned above. In addition to reducing this, it is preferable to make it spherical. For this reason, it is preferable to add one or more of Ca, REM, and Mg to form a sulfide with less plasticity. Each
Below 0.0005%, 0.005%, and 0.0005%, the effect of spheroidization is small, while 0.0050%, 0.015%, and
If it exceeds 0.010%, the effect of spheroidization will not only be saturated, but also increase oxide inclusions and deteriorate ductility, so if one or more of Ca, REM, and Mg are added, 0.0005 to 0.0050% and 0.005 to 0.015% respectively. %,
It needs to be 0.0005-0.010%. Al is necessary as a deoxidizer. If it is less than 0.01%, the effect will be small, and if it exceeds 0.10%, alumina inclusions will increase and the ductility of the steel will deteriorate. Next, regarding hot rolling conditions, hot rolling conditions are a very important component in the combination of ingredients in the present invention. First, the heating temperature needs to be 1200°C or less. In the steel of the present invention, from the viewpoint of ductility, Ti, Nb,
No V added. Therefore, austenite is difficult to become fine grains during hot rolling, and the temperature range in which it is not recrystallized is also small. Therefore, unless the austenite grains in the state before rolling are made small, a fine final structure cannot be obtained. Therefore, it is necessary to heat it at a low temperature. This also results in the advantage of energy saving. In order to obtain a more thorough microstructure, the heating temperature is preferably 1150°C or lower.
The lower limit of the heating temperature is preferably as low as possible for hot rolling, but is usually around 1050°C. Next, the finish rolling finishing temperature needs to be between the Ar ₃ transformation point and 930°C. When the temperature exceeds 930°C, the austenite before transformation becomes coarse and exhibits a coarse bainitic structure, which deteriorates ductility. Further, if rolling is performed below the Ar ₃ transformation point, ferrite transformation occurs and the ferrite is processed and deteriorates ductility. Next, cooling from the end of finish rolling to coiling is important in that the structure is created by transformation during this period, and for this purpose it is necessary to maintain an average speed of 15°C/S or more. If it is less than 15°C/S, it becomes a coarse ferrite/pearlite structure, which not only does not provide strength, but also tends to form a band-like structure, which also deteriorates stretch flangeability. 25 to ensure more stability and strength
It is preferable to set it as C/S or more. The winding temperature is an important component in obtaining the properties unique to the present invention in relation to the components and the heating temperature. 560
If the coiling temperature exceeds ℃, a fine ferrite/pearlite structure cannot be obtained and strength cannot be ensured. On the other hand, if the coiling temperature is less than 350°C, the rolling speed will be limited, which will reduce productivity, and the shape will be distorted, requiring a straightening process, which will impair economic efficiency. Therefore, the winding temperature was set at 350 to 560°C. However, if the winding temperature is below 400℃, the productivity will drop slightly compared to the normal winding temperature.
Preferably, the temperature is over 400°C. On the other hand, in order to stably obtain a fine structure, the winding temperature is preferably 480°C or lower. The fine ferrite-pearlite structure referred to herein is a structure consisting of polygonal ferrite having a diameter of about 5 to 10 μm and fine pearlite existing at the grain boundaries, and pearlite does not exhibit a complete layered structure. In some cases, island-like martensite or bainite may also be present. The reasons for limiting the numerical values of the constituent elements have been described above, and the steel slab used here may be made by either the blooming-ingot-forming method or the continuous casting method, but in terms of economy, it is preferable to use the continuous casting method. It is also preferable to warmly charge the slab into a heating furnace in order to save energy. Next, the present invention will be explained using examples. Steel having the components shown in Table 1 was melted in a converter, continuously cast into a slab, and then hot rolled.

[Table] The ○ mark in the code is a component according to the present invention.
Hot rolling conditions are shown in Table 2. Among the steels in Table 1, symbols A to D are steels of the present invention. Steels with codes E and F are C
The steel with the symbol G differs from the present invention in the amount of P, and the steel with the symbol H differs in the amount of S. Also, under hot rolling conditions
Nos. 1 to 3, 8, 11, 13 to 17 are the conditions based on the present invention, No. 4 is the finishing temperature, No. 5 is the heating temperature,
Nos. 6 and 10 differ from the present invention in the winding temperature, and Nos. 7, 9, and 12 differ in finishing and average cooling rate between windings.

[Table] The hot rolling conditions of No. marked with an ○ are the methods according to the present invention.
The thus produced steel strip was pickled and cut into plates on a cutting line. At that time, 1% temper rolling was performed. After that, it was subjected to a material test. A JIS2201, No. 5 test piece was used for the tensile test. The cross tension of spot welded joints complied with JIS Z3137. Welding is done at a single point, and the conditions are: electrode 8.0mmφ, current application time.
28 cycles, holding time 28 cycles, pressure 500Kg
The current was set near the optimum value in terms of cross tensile strength. In the case of a plate thickness of 2.3mm, it was approximately 1300A. In addition, the hole expansion test uses a method of expanding a sheared hole with a diameter of 20 mm using a conical punch, and the hole expansion ratio is calculated by dividing the hole diameter at the time the crack penetrates the plate thickness by the original hole diameter (20 mm). And so. The results of the material tests are shown in Table 3. Table 3 also shows the material test results for typical dual phase steels and precipitation strengthened steels (both plate thicknesses are 2.3 mm).

【table】

[Table] From Table 3, steels No. 1 to 3, 8, 11, and 13, which complied with the composition and hot rolling conditions of the present invention, have a tensile strength of 60 Kgf/mm ² or more and an elongation superior to precipitation-strengthened steel. . These No. steels also showed good values in spot welding cross tensile strength. However, it is a low C material.
Although it is slightly lower than No. 14, C
This is a good value considering that the amount is over 0.15%. As for the fracture type, there were some that showed some fracture within the nugget. In addition, the hole expansion ratios are both good values of 1.4 or higher. On the other hand, steels Nos. 4 and 5 of Comparative Examples have slightly higher tensile strength but extremely poor elongation and low hole expansion ratio. Tensile strength of 60Kgf/mm Class ² cannot be obtained under the conditions of No. 6, 7, 9, 10, and 12, and tensile strength of 60Kgf/mm Class ² cannot be obtained even with No. 14 steel, which has a small amount of C. . Further, steels No. 15 to 17, which are different in composition from the present invention, have defects such as extremely low cross tensile strength, low elongation, and low hole expansion ratio. Further, as described above, the hot rolling conditions according to the present invention are economically favorable since there are no factors inhibiting productivity and there is little yield loss in shape, etc. Finally, the steel strip according to the present invention may be used as it is in the form of a black peel, or may be used after being pickled. Alternatively, the plate may be cut on a shear line. At that time, the shape may be adjusted using a leveler or temper rolling, and curling may be corrected.

Claims (1)

[Claims] 1 C: more than 0.15 to 0.25%, Si: 1.5% or less, Mn:
0.7-1.5%, P: 0.01% or less, S: 0.005% or less,
After forming a slab of steel consisting of Al: 0.01-0.10% and the balance Fe and unavoidable impurity elements, it is heated to 1200℃ or less, hot rolled and finish rolling is completed at the Ar ₃ transformation point ~930℃, After that, the average cooling rate is 15℃/S
A method for producing a hot-rolled steel sheet having a tensile strength of 60 Kgf/mm ² or more and excellent workability and weldability, which comprises cooling at a temperature of 350 to 560°C and winding. 2 C: more than 0.15 to 0.25%, Si: 1.5% or less, Mn:
0.7-1.5%, P: 0.01% or less, S: 0.005% or less,
Al: 0.01~0.10%, further Ca: 0.0005~0.0050%,
After forming a slab of steel containing one or more of REM: 0.005 to 0.015% and Mg: 0.005 to 0.010%, with the remainder being Fe and unavoidable impurity elements,
Heating to below 1200℃ and hot rolling to Ar ₃ transformation point ~
Finish rolling at 930℃, then average cooling rate
A method for producing a hot-rolled steel sheet having a tensile strength of 60 Kgf/mm ² or more and excellent workability and weldability, characterized by cooling at 15°C/S or more and winding at 350 to 560°C.