JPH0768583B2 - High-tensile cold-rolled steel sheet manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength cold-rolled steel sheet having a two-phase structure, and more specifically, to the occurrence of surface flaws in a steel sheet based on the oxidation of Si and the formability and chemical conversion treatment. 45 to excellent mechanical properties, press formability and spot weldability while preventing deterioration of
The present invention relates to a method for producing a high-strength cold-rolled steel sheet of 65 kg / mm ² grade.

In recent years, as a high-strength cold-rolled steel sheet having good mechanical properties, particularly good press formability during use, one having a two-phase structure has been adopted. Dual-phase steel is a steel that is strengthened by dispersing hard martensite grains in a soft ferrite ground, and is generally obtained by heating in a ferrite-austenite two-phase region and then cooling at an appropriate cooling rate. . Steel sheets of dual phase steel have low yield stress and high tensile strength. Moreover, since the large elongation is maintained even at high strength, the moldability is very good. Conventional dual phase steel is
Based on Mn, Cr and Mo are used to precipitate the martensite phase, while Si is used as a strengthening element and a precipitation element for the ferrite phase.

By the way, in the production of ordinary cold-rolled steel sheet, the slab obtained by ingot casting or continuous casting is sent to a reheating furnace for hot rolling performed prior to cold rolling, where the slab is heated for a long time. To do. In the dual-phase steel, since a relatively large amount of Si is contained for the above reason, in the reheating step, a low melting point oxide composed of three elements of Si-Fe-O is heated near the slab surface. It forms at the austenite grain boundaries.
Therefore, the grain boundaries are finely eroded on the surface of the steel sheet after the hot rolling is finished, and even if such defects are removed by a maintenance process such as descaling or pickling during rolling, fine cracks are generated in the steel itself. If it has occurred or if the cleaning is insufficient, it will remain as a surface flaw. As a result, the appearance becomes poor, and problems such as cracks occur in severe press molding.

Further, Si and Mn solid-soluted in the steel of the dual phase steel easily form Si-O and Mn-O-based oxides during annealing after cold rolling, which causes dust generation and welding during spot welding. It causes the generation of the weld metal and causes a decrease in the strength of the weld zone. Further, even during chemical conversion treatment, the reaction of chemical conversion treatment film formation is hindered, and as a result, sufficient corrosion resistance cannot be obtained.

The difficulty associated with the above reheating step is that so-called direct rolling without omitting such reheating (ingot or continuous cast steel slab or slab immediately or in a short-time heat retention furnace for heat retention and reheat) It can be avoided by a rolling method in which the material is placed in a recuperating furnace and then hot-rolled), but the defects associated with oxide formation during annealing cannot be overcome by such a method.

Therefore, an object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet of a dual-phase structure steel having excellent mechanical properties, spot weldability and chemical conversion treatability, which do not exhibit the above-mentioned drawbacks.

(Structure of the Invention) In the production of a cold-rolled steel sheet of dual-phase steel, the present inventors
By limiting the contents of C, Si, and Mn of steel, limiting the reheating conditions before hot rolling, and further annealing after cold rolling by continuous annealing under specific soaking conditions and atmospheres. The present invention has been completed based on the finding that various difficulties due to the oxidation of Si and Mn as described above can be prevented.

The present invention contains C: 0.01 to 0.10%, Si: 0.70 to 1.0%, Mn: 1.8 to 3.0%, Al: 0.005 to 0.10% by weight, and if necessary, Ca, Ti, Mg and rare earth metals are added. 1
For steel slabs containing one or two or more 50 ppm or less, the surface temperature is 10 if reheating hot pieces (hot charge) or reheating cold pieces when directly rolling.
After performing reheating such that the residence time at 50 ° C or more is within 2 hours, hot rolling and cold rolling are sequentially performed, and then H ₂ 0.3 to 1
A high-strength cold-rolled steel sheet having a two-phase structure characterized by performing continuous annealing for 20 to 120 seconds at a soaking condition of 700 to 880 ° C in an atmosphere containing 2% of dew point of -60 to -10 ° C. In the manufacturing method.

The reasons for limiting the steel composition and manufacturing conditions of the present invention as described above will be described below.

Steel composition: If C is less than 0.01%, smelting becomes difficult, while 0.10
%, The spot weldability deteriorates, so 0.01 to 0.
It was set to 10%.

Si is necessary to some extent to make it a dual phase steel,
If it is less than 0.70%, a two-phase structure having a good balance between tensile strength and elongation cannot be obtained. However, when Si is more than 1.0%, the above-mentioned deterioration due to the oxidation of Si becomes remarkable, so Si
Was limited to a narrow range of 0.70 to 1.0%.

Mn also needs at least 1.8% to obtain a two-phase structure, while Mn
If n exceeds 3.0%, spot weldability and chemical conversion treatability deteriorate, so it was limited to 1.8 to 3.0%.

If at least 0.005% of Al is not contained, SiO ₂ inclusions will not decrease. However, even if Al over 0.10% is present,
Only high cost, little effect, 0.005 ~
It was set within the range of 0.10%.

Of the unavoidable impurities, if P exceeds 0.04%, the formability is deteriorated due to the formation of a band-like structure, so P is preferably 0.04% or less. Further, if S is more than 0.02%, the formability is deteriorated due to the formation of MnS, so S is preferably made 0.02% or less.

In order to prevent the deterioration of the formability due to the formation of MnS, one or more of Ca, Ti, Mg or a rare earth metal may be added if necessary, but the total amount is 50 ppm.
If it exceeds the above range, the amount of inclusions will become excessive and adverse effects will occur. Therefore, when adding these, the total amount should be 50 ppm or less.

Manufacturing conditions: The slab used in the method of the present invention may be one obtained by either ingot-slumping or continuous casting. When it is not necessary to reheat the slab by the direct rolling, the slab obtained by the ingot-agglomeration or continuous casting is immediately sent to the hot rolling step as it is.

In the case of reheating of hot pieces or cold pieces, that is, in the case of slabs that require reheating before hot rolling, the residence time at a surface temperature of 1050 ° C or higher in this reheating is within 2 hours. And This is because, as described above, when reheating at high temperature becomes long, grain boundary oxidation due to the Si-Fe-O system becomes remarkable. The attached drawings show the relationship between the residence time at a surface temperature of 1050 ° C or higher and the surface deterioration after hot rolling. In the figure, the horizontal axis represents direct rolling, is direct rolling, and uses a short-time recuperative furnace for heat retention and reheating, indicates the case where reheating of hot pieces or reheating of cold pieces is performed, The numbers below indicate the residence time (hr) when the surface temperature is 1050 ° C or higher. The symbols on the vertical axis mean that the surface deterioration after hot rolling is as follows: ◎: None at all ○: Slightly △: Degraded, but practically unimpeded ×: Degraded Is not suitable for practical use.

Moreover, each dot means one test. From the attached drawings, it can be seen that when the reheating time at a surface temperature of 1050 ° C. or more exceeds 2 hours, the grain boundary oxidation by the Si—Fe—O system becomes so remarkable as to impair practical use. In this connection, the same applies to the case of reheating with hot charge.

The slab that has been directly fed or reheated under the above conditions is hot-rolled according to the method of the present invention. The conditions for hot rolling may be ordinary conditions. That is, the finishing temperature is Ar
_The temperature is higher than ₃ points, usually about 800 to 900 ° C, and the winding temperature is 400 to 700 ° C, preferably about 550 to 750 ° C. If the coiling temperature is lower than 400 ° C, the curing becomes excessive and the coiling operation itself and the next step become difficult. On the other hand, the winding temperature is 70
If the temperature exceeds 0 ° C, a band-like structure and generation of secondary Si-Fe-O-based oxides will appear.

After hot rolling, descaling is performed and the resulting strip is cold rolled. The rolling reduction at this time is preferably at least 40% in order to secure desired high tension, plate thickness accuracy and flatness. The upper limit of the rolling reduction is not particularly limited, but may be limited depending on the desired plate thickness and surface accuracy.

The steel sheet obtained by cold rolling is then subjected to continuous annealing to develop a two-phase structure and to secure desired mechanical properties. It has been found that the conditions of this annealing are important for obtaining various desired performances in the method of the present invention. According to the method of the present invention, annealing involves soaking at a temperature of 700 to 880 ° C for 20 to 12 ° C.
It is carried out for 0 seconds and is carried out by continuous annealing in an atmosphere containing 0.3 to 12% of H _{2 and} having a dew point of −60 to −10 ° C. In batch annealing,
The temperature at the coldest point is maintained for 2 hours or more at a temperature of 700 to 800 ° C., and annealing is performed in an atmosphere having a dew point of −60 ° C. to −10 ° C. containing H ₂ 0.3 to 12%.

If the soaking temperature is lower than 700 ° C or the soaking time is less than 20 seconds, a two-phase structure cannot be obtained. On the other hand, the soaking temperature is 88
If the temperature exceeds 0 ° C or the time is longer than 120 seconds, oxide formation due to Si or Mn dissolved in steel occurs, and the appearance and various performances deteriorate. If the H ₂ content in the annealing atmosphere is less than 0.3%, Fe oxidation will occur. The lower the dew point of the annealing atmosphere, the better. However, in practice, it is extremely difficult to set the dew point at a temperature lower than -60 ° C, so the lower limit of the dew point was set to -60 ° C. If the H ₂ content of the atmosphere exceeds 12% or the dew point is higher than −10 ° C, Si or
The oxide formation by Mn comes to be recognized. Other conditions for annealing may be ordinary conditions. Specifically, the heating temperature to the soaking temperature is 2 to 20 ° C / sec, and the cooling rate is 2 to 40 ° C / se.
c. The overaging treatment after annealing may be performed at 200 to 450 ° C. for 2 to 8 minutes, and the cooling rate after overaging may be about 1 to 20 ° C./sec. After the continuous annealing, the steel sheet may be continuously galvanized if necessary.

Example A steel sheet having a chemical composition shown in Table 1 was produced by continuous casting with a width of 95
A slab having a thickness of 0 mm and a thickness of 250 mm was obtained and treated under various conditions according to the method of the present invention to produce a cold rolled steel sheet.
Specifically, the slab was fed directly to a hot rolling mill by direct rolling, by using a recuperating furnace in direct rolling, or through reheating treatment of a cold slab to obtain a strip having a thickness of 3.0 mm. This strip was then subjected to cold rolling and continuous annealing to obtain a cold rolled steel plate. Classification of slabs used, hot rolling conditions, plate thickness after cold rolling, annealing conditions (including overaging conditions) and mechanical properties and other properties of the obtained steel plates (surface grain boundary oxidation, spot weldability, The chemical conversion treatability) is summarized in Table 2.

As can be seen from the results in Table 2, when the reheating treatment is performed before hot rolling, if the heating time at the surface temperature of 1050 ° C or higher exceeds 2 hours (Comparative Examples 2, 3, 4), the Si on the steel plate surface is -Fe-
Grain boundary oxidation due to the O-based oxide is remarkable, and the appearance and formability are deteriorated, and the chemical conversion treatment is also deteriorated. When the annealing time exceeds 120 seconds (Comparative Example 1), Si in steel is
Or, the spot weldability and chemical conversion processability are significantly deteriorated due to the oxidation of Mn. In Comparative Example 5 using steel type No. 7 having a high Si content of steel, even if the conditions of reheating before rolling, rolling and annealing are all within the scope of the present invention, oxidation of Si in steel is caused. Similarly, spot weldability and chemical conversion treatability are deteriorated. On the other hand, in the example of the present invention, a high-strength cold-rolled steel sheet having a good balance of mechanical properties, surface grain boundary defects, spot weldability, and chemical conversion treatability is obtained.

As described above, the method of the present invention can avoid the occurrence of surface defects and the deterioration of formability and spot weldability due to the oxidation of Si and Mn, and also has high mechanical strength and chemical conversion treatability. It can be said that it is an advantageous method as a method for producing a general-purpose high-strength cold-rolled steel sheet because the steel sheet can be economically produced.

[Brief description of drawings]

The accompanying drawings are diagrams showing the relationship between the slab classification and the reheating temperature, that is, the residence time of the slab surface temperature of 1050 ° C. or more in the reheating of the cold slab and the grain boundary oxidation of the steel sheet surface after hot rolling.

Claims (1)

[Claims] 1. Containing C: 0.01 to 0.10%, Si: 0.70 to 1.0%, Mn: 1.8 to 3.0%, Al: 0.005 to 0.10%, and if necessary, 1 of Ca, Ti, Mg and rare earth metals.
For steel slabs containing 50 or less kinds of steel or two or more of them added, if they are directly sent by rolling, they remain the same, and if reheating of hot slabs or reheating of cold slabs, surface temperature 1050 ℃ or more residence time is 2 hours After subjected to reheating to be within performed sequentially hot rolling and cold rolling, and then soaking conditions in an atmosphere of a dew point of -60 to-10 ° C. containing H ₂ 0.3 to 12% is seven hundred to eight hundred and eighty ° C. 20 ~
A method for producing a high-strength cold-rolled steel sheet having a two-phase structure, which comprises performing continuous annealing for 120 seconds.