JPS5818973B2 - Method for manufacturing high-strength cold-rolled steel sheet with excellent press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet with good press formability, and provides a preferred method for manufacturing a high-strength cold-rolled steel sheet with good surface properties and excellent press formability. The object of the present invention is to appropriately obtain high-strength steel sheets that are suitable for use in automobile outer panels and other parts, and have good surface properties such as paintability and solderability, and are free from scale defects.

It has been known from Japanese Patent Publication No. 31089/1989 to produce cold-rolled high-strength steel sheets with good deep drawability using Ti stabilized steel with controlled addition amounts of SizMn and P. In order to obtain high strength while maintaining the high V value of Ti stabilized steel, Si
0.5 to 2.5 parts are added.

However, in products to which a large amount of Si is added, hot-rollable surface defects and temper color occur during annealing, making it difficult to obtain surface properties suitable for use in automobile outer panels as described above. There is a disadvantage that it is.

Also, in JP-A-50-129424, Mn is added to Ti stabilized steel by 1.0 to 3.0 or Mn is added to 0.
Continuously add 8 to 3.0% and 0.8 to 2.0% of Si
Although annealed cold-rolled high-strength steel sheets with good press formability have been provided, these also have the same disadvantage that preferred surface properties cannot be obtained.

The present invention was created after repeated studies in view of the above-mentioned circumstances.

That is, first of all, the above-mentioned temper color is an oxide film formed during annealing of cold rolled steel sheets, and the more severe this is, the more difficult it is to paint.
The non-conductor property deteriorates, and this oxide film is Si2
When the Mn content is low, it is an Fe oxide, but as the Si2Mn content increases, it changes to an oxide containing SitMn, which has a stronger affinity with 0 than F4 such as F'e2 5i042Mn2Sj04, and forms a refractory oxide. The greater the amount of these, the thicker the oxide film becomes and the more severe the temper color becomes.

However, bright annealing of cold-rolled sheets is usually performed in a mixed atmosphere of H2 + N2, but the thickness of the oxide film containing Si and Mn as described above increases as the H2 partial pressure decreases and as the O2 partial pressure increases. Also, the reaction with the atmosphere progresses as the temperature increases and the time increases, so the oxide film becomes thicker.

According to the results of annealing experiments in various cycles on cold-rolled sheets with different amounts of 812Mn in an atmosphere gas of 7% H2 + 93% N2 and a dew point of -40°C, which is commonly used in the field, it was found that the paintability when batch annealing was The limit temper color without deterioration of non-damage properties is S i <0.4%, Mn
If both of these coexist near the upper limit, Si + Mn (need to be 1.0%, but since continuous annealing is heating for a short time, this
j.

The hermitage tolerance becomes looser, Si < 0.7%, Mn
<1.0%, and if they coexist close to the upper limit, 1/'iS i 0Mn (If it is 1.4%, it has been confirmed that temper color to the extent that it impairs paintability and solderability does not occur. It was done.

In addition, the faster the material is cooled, the more difficult it is for the temper color to appear, so it is preferable to cool it as quickly as possible.Quick cooling in jet water is most effective, and furthermore, if pickling is performed after this water cooling, temper color will occur to some extent. However, it can be completely removed.

It should be noted that Si has a negative effect not only on temper color but also on scale defects in hot ductility, so it is necessary to appropriately limit Si. S iQ2 FeO eutectic scale is formed, and this scale is difficult to peel off even by descaling with high-pressure water in the hot-rolling line, resulting in scale defects called red scale on the surface of the hot-rolled sheet. Because it is deeply engraved, it tends to remain even after pickling, causing surface defects on cold-rolled sheets.

To prevent such scale defects, limit the amount of Si, heat the slab sufficiently in a heating furnace to thicken the scale, and raise the temperature at the entrance of the finishing mill to a sufficiently high temperature to make it easier to remove the scale. It is necessary to.

In the end, it was concluded that in order to solve the above-mentioned temper color and hot ductility surface defects, it is necessary to strictly control the amounts of Si and Mn added, and to perform recrystallization annealing using continuous annealing rather than batch annealing. reached.

The present invention was devised based on the results of extensive studies as described above, and has been developed to appropriately improve the surface texture, which is a problem with cold-rolled high-strength plates made of Ti-stabilized steel, and to improve tensile strength. The strength is 40 kg/- or more, the deep drawability evaluation index te is 1.5 or more, the overhang performance evaluation index n value is 0.20 or more, and it is also easy to paint. We succeeded in obtaining a steel plate with favorable performance in terms of solderability and solderability.

That is, the device according to the present invention has C(0,02 ratio, Si:0.
20-0, 70%, Mn: 0.60-1.00%
, and this S i +Mn (1.4%, P
:0.01O~0.040%, S(0.010%, 50
t, AI, : 0.020-0.090, TibaTi/
A slab containing C in the range of 5 to 20, the balance/d-Fe and unavoidable impurities is hot rolled at a heating temperature of 1200°C or higher and a finishing temperature of 870°C or higher, and after pickling, a slab containing 50% or more After being cold rolled at a cold rolling rate, it is recrystallized by heating and soaking in a continuous annealing furnace at a temperature of 730 to 850° C. for 10 to 120 seconds, and then rapidly cooled from the subsequent temperature.

To explain in more detail the lid according to the present invention, the reasons for limiting the component composition are as follows.

It is necessary to reduce C to 0.02% or less by degassing, and in the present invention, 5 to 20 Ti is added as Ti/C in order to completely fix solid solution C as TiC and increase the Te value. In order to obtain a cost-effective product by reducing the amount of Ti added, it is necessary to reduce the amount of C to 0.02% or less.

For Si, even if Mn and P, which are reinforcing elements other than Si, are added up to the upper limit, it is not possible to secure a tensile strength of 40 Kq/rran2 or more. This is necessary to obtain the desired tensile strength.

However, if the Si content is 0.7% or more, a temper color that deteriorates paintability, solderability, etc. is generated due to continuous annealing, and this is the upper limit.

Even if the reinforcing elements other than Mn are contained up to the upper limit, Mi can maintain a tensile strength of 40/wn2.
It is necessary to contain 6% or more, and ■, 0%
If this is the case, temper color will occur in the same way as described in the case of Si, and this will be the upper limit.

However, although the above-mentioned upper limits are sufficient for each of Si and Mn, if both of them are allowed to coexist near their respective upper limits, temper color may also occur, so this fried shell has Si+Mn (1°4 %.

P is Si and Mn within their respective upper limits, and Si + MH (
By adding 1.4%, tensile strength 4
0 Kg/rran2 can be secured at the very limit, but if a margin of Sj and Mn K is provided, it is effective to supplement P as a reinforcing element.

To obtain this strengthening effect, the lower limit is 01010, but if this P is too large, the r value will deteriorate, so 0.
The upper limit is Section 04.

Since S is an element that forms MnS and reduces ductility, it is preferably as low as possible and is limited to 0.010 or less.

At is in the range of 0.020 to 0.090% in sot and AL.

That is, since At is used as a deoxidizing agent, it is 0.0
If it is less than 20%, the Ti yield will be poor and 5in2 inclusions may occur, so this is the lower limit.If it is more than 0.090%, surface defects may occur, so this is the upper limit. shall be.

Ti is in the range of 5 to 20 in T i /C.

In other words, the purpose of adding Ti is to increase the Te value and improve deep drawability.The reason why the r value increases with the addition of Ti is that solid solution C and N are fixed as TiC and TiN during hot rolling. This is because the matrix is purified, so there are more 111 planes that are advantageous for deep drawability during recrystallization annealing after cold rolling, and Ti/C of 5 or higher is required to exhibit such an effect.

Moreover, even if it becomes 20 or more, the above effect is saturated and it becomes uneconomical in terms of cost, so this is set as the upper limit.

The manufacturing conditions for the above-mentioned steel shall be specified respectively for hot rolling, cold rolling and annealing.

That is, steel with the above components is melted in a converter and made into a slab by ingot formation or continuous casting.
In order to reduce the content to 2% or less, the steel may be treated with a vacuum degassing device after being tapped from the converter.

In any case, the obtained slab should be heated to 1200°C or higher, because the above-mentioned slab contains Si, so the primary 5in2-FeO system is heated during heating. Scale is generated and causes a scale defect called red scale during the hot rolling process, but when the heating temperature is 1200°C or higher, the scale becomes thicker and the high-pressure water in the hot rolling rough rolling line is heated. When descaling, it becomes easy to separate the content, so red scale can be prevented.

In addition, the slab temperature at the entry side of the finishing rolling mill during hot rolling is also 100.
It is necessary to maintain the temperature at 0°C or higher, that is, even if primary scale is removed by descaling on the entry side of the rough rolling mill line, secondary scale generated before entering the finishing rolling mill line will be removed from the entry side. This is important for further descaling, and as mentioned above, by heating at a temperature of 1200°C or higher, if normal rough rolling conditions are met, it is necessary to ensure a slab temperature of 1000°C or higher at the entrance of the finishing mill. I can do it.

Furthermore, it is necessary to finish finish rolling at 870°C or higher, and since the C content of the above-mentioned composition is extremely low at 0.02% or less, the Ar3 point is 870°C or higher, and this Ar3 point or higher is If finish rolling is not performed in the temperature range, the beam drawability of the product will deteriorate, so the lower limit of the finishing temperature has been set at 870.
°C.

Note that the winding temperature is not particularly limited, but in consideration of pickling properties, it is preferably 700° C. or lower.

Steel hot-rolled under the above-mentioned hot-rolling conditions has a 50%
% or more.

The reason why this cold rolling ratio is set to 50 coefficient or more is because the high Lankford value r that is the object of the present invention cannot be obtained unless cold rolling is performed by at least 50% or more.

Furthermore, the IJ tube cold rolled as described above is recrystallized by continuous annealing.

This continuous annealing furnace is preferably one that can perform a cycle consisting of heating, water quenching, and tempering.

Also, the atmosphere in the heating and soaking furnace is 4 to 4 times N2 in N2.
It is necessary to create a reducing atmosphere with a 10% mixture, and the recrystallization heating soaking temperature for such continuous annealing is 73%.
The temperature should be in the range of 0 to 850°C.

In other words, this temperature range is chosen because if it is below 730°C, complete recrystallization will not occur, and if it is above 850°C, the surface quality will deteriorate due to the occurrence of temper color. .

Furthermore, the holding time for heating and soaking as described above is 10 to 1
The holding time should be 20 seconds, and the lower limit of the holding time of 10 seconds is the minimum time necessary to complete recrystallization, and the upper limit of 120 seconds is a longer time than this. If you do so, the line speed will decrease.

That is, this not only leads to a decrease in productivity, but also causes a decrease in surface properties due to prolonged heating.

The above-mentioned material is then rapidly cooled from that temperature, and the reason for this rapid cooling from that temperature is to make the surface quality as good as possible.

Although quenching in jet water is most preferable for rapid cooling so that the lid can be folded, it has been practically confirmed that good results can be obtained even with gas cooling.

However, when quenching in jet water as mentioned above, quenching distortion remains and the shape of the plate may deteriorate, but in such cases, the shape can be improved by tempering at 300 to 400°C. .

In addition, strips that have undergone the above annealing conditions are actually made into products after being subjected to pressure regulation of about 0.5%.
The product obtained through this series of processes is a cold-rolled high tensile strength steel that has good surface properties suitable for use in automobile exterior panels and good deep drawability as a Ti stabilized steel. It becomes a steel plate.

To explain specific examples according to the present invention, steels having the composition ranges according to the present invention and comparative steels as shown in Table 1 below were prepared.

After tapping, these steels were continuously cast into slabs and hot-rolled at a heating temperature of 1250°C, a finishing temperature of 900°C, and a winding temperature of 600°C to obtain hot-rolled plates with a thickness of 2.8 mm.

This material was then pickled and rolled at a cold rolling rate of 72% to 0.8 rr.
The product thus obtained was passed through a continuous annealing line, heated to 820°C, soaked for 60 seconds, then quenched in jet water, and then tempered at 300°C for 60 seconds. The pressure was adjusted by .5% to make a product.

That is, the mechanical properties and surface properties of the product thus obtained were as shown in Table 2 below.

Note that the mechanical properties in Table 2 are in the direction perpendicular to the rolling direction (C
This is the value in the JIS No. 5 tensile test 1 test for direction), and as for the method of judging the superiority or inferiority of the surface properties, we first examined the presence or absence of temper color by visual inspection, and then conducted a test for paintability. The paintability test was performed by applying ED coating to the steel plate after phosphate treatment, and applying JIS-Z
・Based on the test method specified in 2371, salt water spray was applied to
Superiority or inferiority was determined based on the state of rust occurrence after 40 hours of use.

Regarding solderability, a mixture of solder and flux is applied to the surface of the steel plate and heated from the back side to melt the solder.
Superiority or inferiority was determined based on whether the particles spread on the surface or were repelled and turned into particles.

That is, from the results shown in Table 2, the cold-rolled high-strength steel manufactured by rolling the steel with the composition range according to the present invention under specific hot-rolling and cold-rolling conditions and passing it through a continuous annealing line has a deep It was confirmed that the steel sheet has excellent drawability and stretchability, as well as excellent surface properties, and is suitable for use in outer panels for car bodies and other applications.

According to the present invention as described above, it is possible to appropriately prevent hot-rollable surface defects and the occurrence of temper color during annealing in cold-rolled high-strength steel with good deep drawability using Ti stabilized steel, and improve paintability. . . . It is an invention that can appropriately produce a high-strength cold-rolled steel sheet that has excellent surface properties such as roughness and does not cause scale defects, and is an invention that has great industrial effects.

Claims (1)

[Claims] IC: 0.02% or less, Si: 0.20 to 0.70
%, Mn: 0.60-1.00%, this SizMn is 1.4% or less, P: 0.010-0
．． 040 person in charge. S: 0.010% or less, sol, 8l: 0.020~
0.090 and Ti in the range of 5 to 20 in terms of Ti/C is heated to 1200°C or higher, hot-rolled at a finishing temperature of 870°C or higher, and then further cooled at a reduction rate of 50% or higher. Manufacture of a high-strength cold-rolled steel sheet with excellent press formability, which is characterized by being rolled, then heated and soaked at 730 to 850°C for 10 to 120 seconds, and then subjected to an annealing treatment to give a good surface quality. Method.