JPH0987748A - Production of non-if steel-based nonaging dead soft cold-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a dead soft steel sheet having excellent strain aging resistance and workability by the existing equipment using a B-added non-IF steel base dead soft steel by specifying the compsn. of a dead soft steel sheet, the coiling temp. after hot rolling and the cooling rate in continuous annealing. SOLUTION: A dead soft steel having components contg., by weight, 0.0007 to 0.0017% C, <=1.5% Si, 0.05 to 2% Mn, 0.005 to 0.15% P, 0.001 to 0.02% S, 0.04 to 0.13% Al, 0.001 to 0.003% N, 0.0005 to 0.0024% B, and the balance Fe and satisfying 0.5 to 0.8 B/N, is used. The coiling temp. after hot rolling in this steel is regulated to 400 to 590 deg.C, and the average cooling rate from 650 to 50 deg.C in the cooling stage in continuous annealing is regulated to 0.5 to 7 deg.C/sec.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet suitable for press working used in the fields of automobiles, home electric appliances, buildings and the like, and electrolytic zinc plated with zinc or alloyed zinc for the purpose of rust prevention. The present invention relates to a method for manufacturing a galvanized steel sheet that has been plated or hot dip galvanized.

[0002]

2. Description of the Related Art As a conventional ultra-low carbon cold-rolled steel sheet, at least Ti and Nb which have a strong bonding force with the interstitial solid solution elements (C, N) in the steel and easily form carbonitrides. A so-called IF steel (Intersti
Tial Free Steel) is well known. Since this steel does not contain an interstitial solid solution element that causes deterioration of strain aging resistance and workability, it has non-aging and extremely good workability.

In recent years, the progress of degassing technology has facilitated the melting of ultra-low carbon steel, so that IF steel has come to be used in large quantities as the main grade of cold-rolled steel sheet for press working.

However, IF steel has the following problems. First, since expensive Ti and Nb are added, the material cost increases. Secondly, when Ti or Nb is added, the recrystallization temperature becomes high, so high temperature annealing is essential. Thirdly, in a steel to which Ti having a strong tendency to form an oxide is added, surface defects due to oxide inclusions are likely to occur. Fourthly, the strength of the crystal grain boundary is reduced due to the absence of solid solution C and N, and brittle cracking occurs during secondary processing.

For the purpose of solving such problems of IF steel, for example, Japanese Patent Publication No. 58-49622 and Japanese Patent Publication No.
Japanese Unexamined Patent Publication No. 11-11294 proposes a non-IF steel ultra low carbon steel to which a trace amount of B is added without adding Ti or Nb. Further, in JP-A-6-93376 and JP-A-6-93377, the strain aging resistance is improved by further tightening the restrictions on the C content of the non-IF steel ultra low carbon steel containing B. A method of improving workability by adding P and devising cooling conditions after hot rolling is disclosed. Further, in JP-A-6-212354, the contents of Mn and P of a similar B-added non-IF steel ultra-low carbon steel are controlled so that Mn + 20P is 0.3% or more, and hot rolling finish is performed. Improving the strain aging resistance by increasing the cooling rate immediately after rolling to refine the structure of the hot-rolled sheet to improve the r-value after cold rolling / annealing, and by precipitating AlN by hot coiling after hot rolling. A method has been proposed.

[0006]

However, in the methods described in Japanese Patent Publication No. 58-49622 and Japanese Patent Publication No. 61-11294, technical examination is not sufficient, and a high quality material can be stably obtained. I can't. In particular, the stability of strain aging resistance is not a level that can be practically used.

The methods described in JP-A-6-93376 and JP-A-6-93377 do not improve the strain aging resistance enough to withstand practical use. In addition, we recommend quenching after finish rolling under the cooling conditions after hot rolling, but a large modification of the cooling equipment is required to achieve this. Further, in the B-added ultra-low carbon cold-rolled steel sheets described in these patent publications, it is not possible to stably prevent the appearance of a characteristic very sharp upper yield point by aging at room temperature, as shown below. this is,
It is extremely unfavorable in terms of workability at the actual vehicle press site.

FIG. 1 shows the yield point behavior after mild strain aging at room temperature. In the figure, (a) is the yield point behavior of a normal cold-rolled steel sheet, and (b) is the yield point behavior of an ultra-low carbon cold-rolled steel sheet with B added. In a normal cold-rolled steel sheet, a sharp upper yield point does not appear in aging when recovery of yield point elongation (hereinafter referred to as YPEl) is about 0.3%. The yield point behavior of B-free or simple ultra-low carbon cold-rolled steel sheet is also the same as in (a).
On the other hand, in the B-added ultra-low carbon cold-rolled steel sheet, a sharp upper yield point appears even when the recovery of YPE1 is as small as 0.3%. The stress difference between the upper and lower yield points is 15MP
When the stress difference between the upper yield point and the lower yield point is large, nonuniform strain distribution is promoted during press forming. Therefore, YPEl is 0.3%
Even if it is very small, stretcher strain occurs in a real vehicle press with large parts.

[0009] In JP-A-6-212354, as in the case of JP-A-6-93376, no attention is paid to the peculiar yield point behavior of the B-added ultra-low carbon cold-rolled steel sheet, so that the strain aging resistance Improvement is not enough. Further, rapid cooling after hot rolling and finish rolling for improving workability is also not preferable because a large modification of the cooling equipment is required.

The present invention has been made to solve the above problems, and uses a non-IF steel type ultra-low carbon steel containing B, which does not contain expensive Ti or Nb, and is superior in strain resistance aging to the existing equipment. It is an object of the present invention to provide a method for producing an ultra low carbon cold rolled steel sheet having workability and workability.

[0011]

Means for Solving the Problems The above-mentioned problems are expressed in terms of% by weight,
C: 0.0007 to 0.0017%, Si: 1.5% or less, Mn: 0.05 to 2%, P: 0.005 to 0.15
%, S: 0.001 to 0.02%, Al: 0.04 to
0.13%, N: 0.001 to 0.003%, B: 0.
0005 to 0.0024%, the balance being Fe and unavoidable impurities, and having a component of B / N: 0.5 to 0.8, and a winding temperature after hot rolling of 400 to 590.
650 ° C during the cooling process in continuous annealing.
To 50 ° C., the average cooling rate is 0.5 to 7 ° C./sec.

Further, C: 0.0007 to 0.0012
%, Mn: 0.05 to 0.1%, P: 0.005 to 0.
009% and S: 0.001 to 0.009% are preferable because the strain aging resistance is significantly improved.

The reasons for limiting the manufacturing conditions of the present invention will be described below. C: Strain aging has a great influence, and if it exceeds 0.0017%, good strain aging resistance cannot be secured. 0.000
If it is less than 7%, the manufacturing cost in steelmaking becomes very high. It is preferably 0.0007 to 0.0012%.

Si: It is necessary to add Si for imparting strength to high-strength steel sheets, but if it exceeds 1.5%, workability and surface properties deteriorate.

Mn: To prevent red hot embrittlement, addition of 0.05% or more is necessary. In a high-strength steel sheet, it is necessary to add it in order to impart strength, but if it exceeds 2%, workability deteriorates. When it is 0.05 to 0.1%, the strain aging resistance is further improved, and therefore it is particularly preferable.

If P is less than 0.005%, the cost for removing P in steelmaking becomes very high. In a high-strength steel sheet, it is necessary to add it in order to impart strength, but if it exceeds 0.15%, workability and weldability deteriorate. 0.005-0.00
When it is 9%, the strain aging resistance is further improved, so that it is particularly preferable. The cause is not always clear, but it is considered as follows. That is, there is a site between C and P.
If P tends to segregate at the grain boundaries due to the occurrence of competing, the amount of C segregating at the grain boundaries decreases. When the amount of P is decreased, the amount of segregation of C at grain boundaries is increased, and the amount of solute C in the grains is decreased, so that the strain aging resistance is improved. The cause of the improvement in strain aging resistance due to the decrease in the amount of Mn described above is presumed to be due to the same phenomenon.

If S is less than 0.001%, the cost for removing S in steelmaking becomes very high. If it exceeds 0.02%, the workability deteriorates. If it is 0.001 to 0.009%, the amount of Mn can be made 0.1% or less, and thereby the strain aging resistance is further improved, which is particularly preferable.

Al: Although added as a deoxidizing agent, in the present invention, it also acts to fix the solid solution N as AlN. Solid solution N
Is first fixed as BN by B, and the remaining part is Al
N. If it is less than 0.04%, the amount of solid solution N remaining after the precipitation of BN is small, so the precipitation of AlN during annealing becomes unstable, and N aging is caused. If it exceeds 0.13%, the surface quality of the slab is deteriorated and the cost is increased.

If N is less than 0.001%, the cost for lowering N at the steelmaking stage becomes very high. If it exceeds 0.003%, the precipitation amount of BN or AlN increases, and the grain growth during annealing is hindered.

B: The purpose of addition is the following three. (1)
Fine-grained structure of hot-rolled sheet, (2) Reduction of adverse effect of solute N on aging resistance and workability, (3) Securing strength of welding heat affected zone. If it is less than 0.0005%, these three effects cannot be obtained. If it exceeds 0.0024%, due to the relationship with the above N amount, B may be contained in excess of the amount precipitated as BN, and the above-mentioned peculiar yield point behavior unfavorable for press molding is exhibited.

B / N: This is an extremely important factor for avoiding the peculiar yield point behavior of B-added ultra-low carbon steel and effectively eliciting the effect of B addition. If this value is less than 0.5, the effect of adding B cannot be obtained. That is, it becomes difficult to reduce the grain size of the hot-rolled sheet structure and Δr increases,
During annealing, a large amount of fine AlN precipitates and ductility decreases.
Further, the hardenability deteriorates and the weldability also deteriorates.

The upper limit of 0.8 is provided to avoid a peculiar yield point behavior. The reason is considered as follows. 0.8 is 1: 1 in atomic ratio. B is BN
However, if it is more than this, B becomes excessive. The excess B segregates at the grain boundaries. As in P, B and C
The site composition of the above works. However, this alone cannot explain the sensitive yield point behavior. I have to think that the following phenomena are occurring. The starting point of the deformation is the grain boundary, which is the stress concentration part. When the deformation starts from the grain boundary, there are many starting points of the deformation, so that the sharp upper yield point does not appear. But,
If B is segregated at the grain boundaries, it becomes difficult for the grain boundaries to become the starting point of deformation for some reason. In such a state, the deformation is started simultaneously in the grain, so that a sharp upper yield point appears.

Winding temperature after hot rolling: If it is less than 400 ° C., stable winding operation cannot be performed. If it exceeds 590 ° C, the crystal grains of the hot-rolled sheet are coarsened. Note that winding at 590 ° C. or lower has the following merits. That is, solid solution N in excess of the amount deposited as BN cannot be deposited as AlN during winding, but is deposited as fine AlN during annealing. The fine AlN serves as an effective precipitation site for solid solution C in the cooling process during annealing.

Cooling rate in continuous annealing: This is one of the gist of the present invention, and the cooling rate from 650 ° C. to 50 ° C. is sufficiently slow in order to reduce the amount of solid solution C in the grain that controls the aging property. Need to However, if it is less than 0.5 ° C./sec, the productivity is significantly reduced. On the other hand, if it exceeds 7 ° C./sec, the time period during which solid solution C diffuses into the grain boundaries and segregates during cooling and remains in the grains, resulting in deterioration of strain aging resistance. This is a point that is greatly different from IF steel.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The slab may be directly hot-rolled without being reheated after continuous casting, or may be hot-rolled after being reheated in a heating furnace. Alternatively, it may be cast in the form of a thin slab and directly finish-rolled without undergoing rough rolling. The heating temperature at the time of reheating in the heating furnace may be a usual temperature range of 1000 to 1250 ° C., but a lower temperature is preferable because sulfides formed during heating become large, and grain growth during annealing is good.

The finish temperature for hot rolling is a normal condition of Ar ₃
It should be more than a point. Cold rolling rate is in the normal range 65-95%
Good.

Annealing is a dedicated equipment for annealing (continuous annealing line)
However, the annealing may be performed in the annealing equipment included in the hot dip galvanizing line.

The annealing temperature is in the normal temperature range of 650 to 650.
It may be 880 ° C. The elongation of temper rolling may be in the usual range of 0.3 to 2%.

[0029]

【Example】

(Example 1) Steels A to N having the chemical components shown in Table 1 were melted by a degasser. Then, a cold rolled steel sheet was produced under the following production conditions. Slab heating temperature: 1200 ° C., hot rolling: finishing plate thickness 4 mm, finishing temperature 870 ° C., winding temperature 560 ° C., cold rolling: finishing plate thickness 0.8 mm (cold rolling rate 80%), continuous annealing:
Heating rate: approx. 13 ° C / sec, soaking 800 ° C x 30 sec, 650
Average cooling rate of 2.5 ° C / sec at ~ 50 ° C, temper rolling: 0.5% elongation.

Then, the tensile properties and spot welding strength after aging were investigated. The tensile test was performed according to JIS2241.
The spot welding strength was evaluated by tensile shear strength. The prescription condition is 40
Acceleration aging of 14 days at ℃. Spot welding conditions are: Tip of tip: DR type 6 mmφ, pressure: 200 kgf, energization: 1
2 cycles × 8 KA.

The results are shown in Table 2. In addition, steel plates A to Table 2
N is manufactured from each of the steels A to N shown in Table 1.

Steel plates A, B, C and D show the effects of B / N. Steel plates A and B whose B / N is out of the upper limit include
Although the recovery amount of YPE1 due to aging is as small as 0.3%, a sharp yield point appears. Steel plate C, which is the steel of the present invention, has no sharp yield point. Steel plate D with B / N out of the lower limit
Has no problem in aging, but has a low spot welding strength and a large Δr, and thus has a problem in weldability and workability.

Steel plates E, F, G, and H show the influence of the amount of C. In both cases, since B / N is within the range of the present invention, no sharp yield point appears even when aged. But YP
The influence of C amount appears on the recovery amount of El. The lower the amount of C, the smaller YPE1. In the steel plate H whose C amount is out of the upper limit,
YPEl becomes about 1%, and problems such as stretcher strain occur during press working. The influence of the C content also appears in the workability, and the lower the C content, the higher the El and r values.

The steel plates I and J have the effect of reducing the amounts of Mn and S. It can be seen that compared with the steel plate F having almost the same C content, the recovery amount of YPE1 is further reduced and the El and r values are improved due to the lower Mn and lower S.

Steel plate K is obtained by further reducing the amount of C with respect to steel plates I and J. The steel plate L is obtained by further reducing the P amount with respect to the steel plate K. By taking such measures, more excellent strain aging resistance and workability can be obtained.

The steel plates M and N are made to have high strength by adding Si, Mn and P. The addition of the strengthening element does not impair the effect of suppressing the peculiar yield point behavior according to the present invention. Although the workability deteriorates due to the increased strength, the same characteristics as the IF steel-based high strength steel sheet can be obtained.

[0037]

[Table 1]

[0038]

[Table 2]

(Example 2) Using the steel L in Table 1, the influence of the cooling rate in annealing was investigated. Manufacturing conditions are the same as in Example 1 except for the average cooling rate from 650 ° C to 50 ° C.

The results are shown in Table 3. The influence of the cooling rate mainly appears on the recovery amount of YPE1 after aging. In the steel plates L-1 and L-2 whose cooling rate is outside the upper limit of the present invention, YP
The recovery amount of El is large, and problems such as stretcher strain occur. Steel plates L-3 and L, which are cooling rates of the present invention
It can be said that -4 is practically non-aging. In particular, steel plate L
-4, like IF steel, is completely non-aging.

[0041]

[Table 3]

Example 3 Using the steel K in Table 1, the influence of the coiling temperature after hot rolling was examined. The conditions other than the winding temperature are the same as in Example 1.

The results are shown in Table 4. The influence of the winding temperature mainly appears in Δr. Steel plates K-1 and K-2 whose winding temperatures are out of the upper limits of the present invention have a large Δr as a practical steel, which causes a problem such as a large ear during press working. Δr of steel plates K-3 and K-4, which is the winding temperature of the present invention
Is a value comparable to that of a normal cold-rolled steel sheet and does not cause such a problem.

[0044]

[Table 4]

[0045]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, it is possible to add non-I containing B without adding expensive Ti or Nb.
It is possible to provide a method for producing an ultra-low carbon cold-rolled steel sheet having excellent strain aging resistance and workability by using the existing equipment, using the F steel ultra-low carbon steel.

[Brief description of drawings]

FIG. 1 is a diagram showing a yield point behavior after strain aging at room temperature for a relatively short time.

Claims (2)

[Claims] 1. C: 0.0007 to 0.00 in% by weight
17%, Si: 1.5% or less, Mn: 0.05 to 2%,
P: 0.005-0.15%, S: 0.001-0.0
2%, Al: 0.04-0.13%, N: 0.001-
0.003%, B: 0.0005 to 0.0024%, the balance being Fe and inevitable impurities, and B / N:
It has a component of 0.5 to 0.8, the coiling temperature after hot rolling is 400 to 590 ° C., and the average cooling rate from 650 ° C. to 50 ° C. in the cooling process in continuous annealing. Is 0.5 to 7 ° C./second. A method for producing an ultra low carbon cold rolled steel sheet. 2. C: 0.0007 to 0.0012%, M
n: 0.05 to 0.1%, P: 0.005 to 0.009
%, S: 0.001 to 0.009%, The method for producing an ultra low carbon cold rolled steel sheet according to claim 1, wherein S: 0.001 to 0.009%.