JPH0477049B2 - - Google Patents

Description

[Detailed description of the invention]

Technical Field This invention relates to a technology for improving the bending properties and stretch-flange formability of high-strength cold-rolled steel sheets with a strength level of 50 kgf/mm ² or higher. Background technology When it comes to high-strength cold-rolled steel sheets with a tensile strength of about 50 kgf/mm2 or ^more , the surface hardness generally increases rapidly, so cracks are likely to form on the surface due to mold galling during press forming, resulting in bending cracks and stretch flanges. There was a problem that cracks were likely to occur. In addition, this type of steel plate uses Si, Mn and P to ensure strength.
Since relatively large amounts of reinforcing elements such as these are added, a large amount of nonmetallic inclusions are likely to be produced, and there remains a problem in that cracks caused by such inclusions occur frequently. Purpose of the Invention The present invention advantageously solves the above-mentioned problems, and has the advantage of using a 50kgf/mm class ² or higher high-strength cold-rolled steel sheet that does not cause bending cracks or stretch flange cracks even during press forming. The purpose of this research is to propose a new manufacturing method. Structure of the Invention This invention was developed after thorough research on press cracking, and in order to achieve the intended purpose, a thin decarburized layer is formed on the surface of the steel plate, and
This is based on the knowledge that it is extremely effective to suppress the amount of S in unavoidable impurities. That is, in this invention, C: 0.05 to 0.20% by weight
(hereinafter simply shown in %), Si: 0.05-1.50%, Nn:
1.00-3.50%, P: 0.020-0.100% and S:
Contains 0.010% or less, and sometimes additionally Nb: 0.010
~0.050%, Ti: 0.010~0.150%, and the remainder is iron and unavoidable impurities. After hot rolling, the slab is wound into a coil, and then cold rolled. In the manufacturing method of high-strength cold-rolled steel sheets, which consists of a series of steps in which continuous annealing is performed at a temperature range above the recrystallization temperature.
By performing the process at the following temperature, the steel plate surface has a thickness of 1
This is a method for producing a high-strength cold-rolled steel sheet with a strength level of 50 kgf/mm ² or more, which is characterized by forming a decarburized layer of ~200 μm, and has excellent bending properties and stretch-flange formability. This invention will be specifically explained below. First, the reason why the composition of the steel sheet is limited to the above range in this invention will be explained. C: 0.05-0.20% If the C content is less than 0.05%, the tensile strength (TS) will decrease after continuous annealing, so in order to compensate for this,
The C content has the disadvantage of requiring large amounts of strength elements such as Si, Mn, P, Ti, Nb and V, while if it exceeds 0.20%, the fatigue strength and static strength of the spot welded joint will decrease. It was limited to the range of 0.05-0.20%. S: 0.010% or less Si: 0.05-1.50% Si effectively contributes to improving bendability and stretch flangeability, but its effect is only achieved at 0.05%.
On the other hand, if it is added in excess of 1.50%, the effect not only reaches saturation, but also hardens the steel and makes hot rolling and cold rolling difficult. It was limited to a range of 1.50%. Mn: 1.00-3.50% Mn is a useful element that contributes to improving strength, but if it is less than 1.00%, the effect of addition is poor;
If it exceeds 3.50%, the strength improvement will reach saturation, and the hot-rolled base plate will harden significantly, making cold rolling particularly difficult, so it was limited to a range of 1.00 to 3.50%. P: 0.020 to 0.100% P is a useful element that effectively contributes to strengthening steel when added in small amounts, but if it is less than 0.020%, the effect of addition is poor, while if it exceeds 0.100%, the effect just reaches saturation. 0.020 to 0.100, as there is a large risk that segregation will become apparent and workability will deteriorate.
% range. If a large amount of S is contained, stretch cracks and bending cracks will occur during press forming, so it is preferable to reduce S as much as possible, but up to 0.010% is acceptable. Furthermore, in this invention, Nb is used as a reinforcing element for steel.
One or two selected from among and Ti can be added in the following range. Nb: 0.010-0.050% Nb is an element that can effectively improve the strength of steel with the addition of a small amount.
The effect reaches saturation when it exceeds 0.010
It was supposed to be added in a range of ~0.050%. Ti: 0.010-0.150% Ti is also an element that can effectively improve the strength of steel when added in small amounts, just like Nb.
If the amount is less than 0.010%, the effect of addition is poor, while if it exceeds 0.150%, the effect reaches saturation, so it was added in the range of 0.010 to 0.150%. Figure 1 shows C: 0.08%, Si: 1.10%, Mn: 1.90.
%, P: 0.095%, Al: 0.050% and O: 0.0021
%, S of steel sheets with various compositions with S ranging from 0.001 to 0.018%.
The results of an investigation into the relationship between quantity and cleanliness are shown.
The cleanliness was measured in accordance with JIS G 0555. As is clear from the figure, when the amount of S exceeds 0.010%, the cleanliness deteriorates rapidly, but the deterioration in the cleanliness means that bending cracks and stretch flange cracks are more likely to occur. means. Now, the greatest feature of this invention is that it forms a decarburized layer on the surface of the steel sheet.By forming such a decarburized layer, even high-strength cold-rolled steel sheets of 50 kgf/mm grade ² or higher can be press-formed. This made it possible to effectively prevent bending cracks and stretch flange cracks, which were sometimes feared to occur. However, if the thickness of the decarburized layer is less than 1 μm, the soft layer will be too thin and will not be effective in improving bending properties and mold-gathering properties.On the other hand, if it exceeds 200 μm, it will not only be difficult to obtain the target strength; The decarburized layer was limited to a range of 1 to 200 μm because the processing time for forming the decarburized layer would be long, which would impair economic efficiency. The decarburized layer is formed by winding the hot-rolled coil at a high temperature to form a decarburized layer on the surface of the hot-rolled coil. At this time, if the winding temperature is below 680℃,
Since the diffusion rate of C in the steel decreases and FeO, which is preferable for decarburization, immediately transforms into Fe ₃ O ₄ , it is difficult to form a sufficient decarburization layer, so the coiling temperature should be higher than 680°C. is necessary. In addition, the temperature at the exit side of the finishing stand during hot rolling is 950 to 850.
℃, the upper limit of the subsequent winding temperature is industrially sufficient to be 850°C or less. Examples Examples of the present invention will be described below. Six types of steel slabs having the chemical composition shown in Table 1 were hot-rolled, then wound into a coil, and then cold-rolled and continuously annealed to produce high-strength cold-rolled steel plates. Of the six types of steels A to F, excluding steel E, a decarburized layer with the thickness shown in Table 1 was formed during the manufacturing process. Formation was performed by hot rolling and then winding at a high temperature of 720°C. Note that the comparative example was decarburized by continuous annealing in an annealing atmosphere with a dew point of 10°C. Next, the mechanical properties of each of the obtained steel plates were investigated. The results are also listed in Table 1. For yield stress, tensile strength and elongation, No. 5 test pieces were prepared in accordance with JIS Z 2201.
Tests were conducted based on JIS Z 2241. Here, the test pieces were taken from three locations at 0° (L direction), 90° (C direction), and 45° (D direction) with respect to the rolling direction, and the test results are shown as the average value of L + C + 2D / 4. . Regarding bending properties, bending test pieces were prepared according to JIS Z 2204, and examined using a metal material bending test method according to JIS Z 2248. These bending characteristics are as follows: The inner bending radius due to C bending is 8.0t, 2.5t, 2.0t, 1.5t, 1.0t, respectively, where the plate thickness is t.
A bending test was conducted for each case of 0.5t and 0t, and the presence or absence of bulges on the outside of the curved portion was observed, and evaluation was made at the limit bending radius at which no bulges would occur. Furthermore, stretch flange formability was evaluated by a hole expansion test.

【table】

[Table] As is clear from the test results shown in Table 1,
All of the steel plates (A to C) obtained according to the present invention have significantly better bending properties and stretch flange formability than the comparative examples (D to F). Effects of the Invention As described above, according to the present invention, the above-mentioned cracks can be avoided in high-strength cold-rolled steel sheets with a strength level of 50 kgf/mm ² or higher, where conventionally there was a concern that bending cracks and stretch flange cracks would occur during press forming. This is advantageous because the occurrence can be completely prevented.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the amount of S in steel and the cleanliness.

Claims (1)

[Claims] 1 Contains C: 0.05 to 0.20% by weight, Si: 0.05 to 1.50% by weight, Mn: 1.00 to 3.50% by weight, P: 0.020 to 0.100% by weight, and S: 0.010% by weight or less, with the remainder being iron and A high-strength cold-rolled steel sheet made of a series of processes in which a slab containing unavoidable impurities is hot-rolled, then wound into a coil, then cold-rolled, and then continuously annealed at a temperature above the recrystallization temperature. In the manufacturing method of
By performing the process at the following temperature, the steel plate surface has a thickness of 1
A method for producing a high-strength cold-rolled steel sheet having a strength level of 50 kgf/mm ² or more and having excellent bending properties and stretch-flange formability, which is characterized by forming a decarburized layer of ~200 μm. 2 Contains C: 0.05 to 0.20% by weight, Si: 0.05 to 1.50% by weight, Mn: 1.00 to 3.50% by weight, P: 0.020 to 0.100% by weight, and S: 0.010% by weight or less, and further Nb: 0.010 to 0.050%, Ti: A slab containing one or two selected from 0.010 to 0.150%, with the remainder consisting of iron and unavoidable impurities, is hot rolled, wound into a coil, then cold rolled and then recrystallized. In the manufacturing method of high-strength cold-rolled steel sheets, which consists of a series of processes in which continuous annealing is performed at a temperature range above 680°C, the coiling following hot rolling is performed at a temperature exceeding 680°C, then 850°C.
By performing the process at the following temperature, the steel plate surface has a thickness of 1
A method for producing a high-strength cold-rolled steel sheet having a strength level of 50 kgf/mm ² or more and having excellent bending properties and stretch-flange formability, which is characterized by forming a decarburized layer of ~200 μm.