JPH0941040A - Production of high strength cold rolled steel sheet excellent in strength-flanging property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of an abnormal structure in a steel sheet and to impart excellent stretch-flanging properties thereto by specifying the compsn. of a low carbon steel and executing annealing after cold rolling at a two phase region temp. SOLUTION: The compsn. of a steel is composed of, by weight, 0.04 to <0.20% C, 0.50 to <2.00% Mn, <=1.50% Si, <=0.10% P, <=0.005% S, <=2.00% Cr, and the balance Fe with inevitable impurities. Preferably, at least one kind of element selected from the groups composed of <=1.0% Cu, <=0.0020% Ca, <=0.05% Ti, <=0.05% Nb, <=1.0% Ni and <=0.0020% rare earth metals is added thereto. The steel sheet having the above compsn. is subjected to cold rolling, is thereafter annealed in a two phase region, is subjected to cooling in such a manner that it stays in the temp. ragion of 650 deg.C to a temp. T deg.C at which pearlitic transformation stops for >=10sec and is subjected to water cooling so as to regulate the cooling time from the T deg.C to 450 deg.C to <=5sec.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-rolled steel sheet having excellent stretch-flangeability, which can be suitably used as an original plate for a complicated stamped part.

[0002]

2. Description of the Related Art In order to secure stretch flangeability, it is necessary to control the generation of inclusions and hard phases to suppress the generation of cracks and voids in punched cross sections. High-strength cold-rolled steel sheet is usually manufactured using continuous annealing equipment, but if rapidly cooled in a state where untransformed austenite remains, a hard structure such as martensite is generated and stretch flangeability deteriorates. . By tempering this martensite, it is possible to reduce the deterioration of its characteristics, but this causes problems such as an increase in cost due to an increase in the number of steps.

As will be described later, the present inventors have found that
As described above, it was found that holding a steel sheet in the process of transformation often produces an abnormal structure containing fine martensite, which deteriorates the stretch flangeability.
Therefore, in order to obtain a cold-rolled steel sheet having a good stretch flangeability, it is necessary to avoid generation of such an abnormal structure. However, a manufacturing method that takes this point into consideration has not been heretofore known, and therefore an unexpected stretch flange defect may occur, resulting in a decrease in yield.

[0004] For example, Japanese Patent Publication No. Sho 49-1690 and Japanese Patent Publication No. Sho 22-22285 describe a method for producing a hot / cold rolled steel sheet containing Cr and having excellent stretch flange formability. There is no stipulation of annealing conditions, and it is not always possible to obtain sufficient stretch flangeability. JP 5
Even in JP-A 2-46323 and JP-A-56-75521, cooling control paying attention to pearlite transformation is not specified, and therefore sufficient stretch-flangeability cannot be obtained.

Japanese Unexamined Patent Publication (Kokai) No. 58-167750 describes a cold-rolled steel sheet made of polygonal ferrite, bainite and martensite. Since such a steel sheet contains hard martensite, it has a stretch flange. It is disadvantageous to sex. Japanese Patent Laid-Open No. 3-277742 discloses A
Although a method of annealing at c ₃ ± 50 ° C. is described, it is not always possible to obtain sufficient stretch-flangeability because cooling is not performed in consideration of precipitation of pseudo-pearlite.

As described above, although various methods for manufacturing cold-rolled steel sheets having improved stretch flangeability have been proposed in the past, a manufacturing method that takes into consideration the generation of abnormal structure is not known. . Therefore, the present inventors, when producing a high-strength cold-rolled steel sheet in continuous annealing equipment, to suppress the occurrence of abnormal structure, to establish a method of producing a steel sheet having excellent stretch flangeability, various I did a research.

First, as shown in FIG. 1, a steel sheet of steel type H1 in Table 1 was held at 730 ° C., which is a temperature between Ac ₁ and Ac ₃ , for 5 minutes, and then a temperature t lower than Ac ₁ ( The holding temperature was maintained for 10 seconds, and then water quenching was performed. FIG. 2 shows the pearlite fraction in the second phase in the steel sheet subjected to such heat treatment. Perlite is found in the second phase only when the holding temperature t is between 650 ° C and 540 ° C. From the result of FIG. 2, when the steel sheet of the steel type H1 is heat-treated under the conditions shown in FIG. 1, the pearlite transformation stop temperature T is 540 ° C.

FIG. 3 shows the influence of the residence time at the temperature in the range from 650 ° C. to the temperature T on the hole expandability (λ). Hole expandability (λ), which is the expansion ratio of the hole diameter when a punched hole is expanded by a conical punch until a crack occurs, and the larger λ, the better the stretch flangeability. As is clear from FIG. 3, the longer the residence time at the temperature in the range from 650 ° C. to the temperature T, the higher λ. However, in practice, the upper limit of this residence time is about 20 seconds.

FIG. 4 shows the effect of the residence time at the temperature range from T to 450 ° C. on the hole expandability (λ) for the residence time of 12 seconds. The shorter the residence time, the better the hole expandability (λ). This is because
Although it is not completely clear, untransformed austenite that could not be transformed into pearlite from 650 ° C to the stop temperature T of pearlite transformation became a structure detrimental to stretch-flangeability between temperature T and 450 ° C. It is presumed that it will be transformed. If the holding time in the temperature range from the temperature T to 450 ° C is short, the harmful tissues are reduced,
Good stretch flangeability. On the contrary, from the temperature T to 45
Even if the holding time at temperatures up to 0 ° C is short, if the amount of untransformed autesnite is large, the amount of harmful structures and the amount of hard low-temperature transformation products are large, so from 650 ° C to T When the holding time is short, the stretch flangeability deteriorates. In principle, if the temperature range from the temperature T to 450 ° C. is cooled very quickly, no abnormal structure should be precipitated, but the cooling rate above the experiment deteriorates the flatness of the plate due to thermal strain, The cost is increased due to the flatness correction.
Usually, it is practically in the range of 1 to 5 seconds.

As will be described later, the stop temperature T of the pearlite transformation depends on the composition of the steel. It also depends on the C concentration in austenite when held between points Ac ₁ and Ac ₃ . Thus, according to the present invention, the temperature T is measured according to the composition of the steel and the heat treatment, and the residence time at the temperature in the range of 650 ° C. to T is set to 10 seconds or longer, and the temperature in the range of T to 450 ° C. is set. A high-strength cold-rolled steel sheet having excellent stretch-flangeability can be obtained by cooling so that the residence time at the temperature is 5 seconds or less.

[0011] Conventionally, as described above, it is not possible to set the optimum manufacturing conditions that fluctuate depending on the composition of the steel and the conditions of heat treatment, and therefore it is not possible to manufacture a cold-rolled steel sheet having a good stretch flangeability at a high yield. . FIG. 5 is a diagram showing the influence of the Cr amount on the temperature T, and FIG. 6 is a diagram showing the influence of the C concentration in the austenite on the temperature T. The C concentration in austenite fluctuates by changing the temperature change of austenitization in the two-phase region even if the steel composition is the same. It is clear that the temperature T varies depending on the composition of the steel and the heat treatment conditions.

[0012]

SUMMARY OF THE INVENTION The present invention has been made based on the above-mentioned findings, and when manufacturing a high-strength cold-rolled steel sheet in a continuous firing facility, it suppresses the occurrence of an abnormal structure and is excellent. It is an object of the present invention to provide a method for producing a steel sheet having stretch flangeability.

[0013]

A method for producing a high-strength cold-rolled steel sheet excellent in stretch-flangeability according to the present invention is C 0.04% or more by weight%, less than 0.20% by weight, and Mn 0.5.
0% or more, less than 2.00% by weight, Si 1.50% by weight or less, P 0.10% by weight or less, S 0.005% by weight or less, Cr 2.00% by weight or less Residual iron and inevitable impurities After cold rolling the steel sheet, it is annealed in the two-phase region, cooled so that it stays at a temperature between 650 ° C. and the temperature T at which pearlite transformation stops for 10 seconds or more, and then from T to 450 ° C. It is characterized in that it is cooled so that the residence time at the temperature is 5 seconds or less.

In the present invention, the steel sheet to be used is, in addition to the above-mentioned elements, Cu 1.0% by weight or less and C, if necessary.
a 0.0020% by weight or less, Ti 0.05% by weight or less,
Nb: 0.05 wt% or less REM: 0.0020 wt% or less, Ni: 1.0 wt% or less, may contain at least one element selected from the group consisting of.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION First, the chemical composition of a steel sheet used in the present invention will be described. C is necessary to secure the required strength in the obtained steel sheet, and when the addition amount is less than 0.04% by weight, the strength is insufficient. However, if the addition amount is 0.20% by weight or more, the amount of the hard cementite phase increases, and the hard low-temperature transformation product increases, resulting in insufficient stretch flangeability. Particularly, in the present invention, the amount of C is preferably in the range of 0.08 to 0.16% by weight.

[0016] Mn is also necessary to secure the required strength in the obtained steel sheet, and when the addition amount is less than 0.50% by weight, the strength becomes insufficient. However, the amount added is 2.
When it is more than 00% by weight, hard martensite is apt to occur and stretch flangeability is deteriorated. Particularly, in the present invention, the amount of Mn is preferably in the range of 0.80 to 1.60% by weight.

When the amount of Si exceeds 1.50% by weight, residual austenite which is detrimental to stretch flangeability and unstable is likely to be produced. When the amount of P exceeds 0.10% by weight, it becomes brittle even if the steel becomes brittle. When S exceeds 0.005% by weight, the precipitates of S, which are harmful to the stretch-flange formability, increase and the stretch-flange formability deteriorates.

[0018] Cr is added as needed to increase the strength of the steel sheet. However, when the amount added exceeds 2.00% by weight, precipitation of pearlite becomes difficult in the normal continuous annealing process, and a large amount of untransformed austenite remains in the temperature range where a structure detrimental to stretch-flangeability is generated. , Untransformed austenite transforms into an abnormal structure that is harmful to stretch flangeability. If pearlite is sufficiently deposited,
Since the amount of untransformed austenite decreases, if the temperature range in which the abnormal structure precipitates is cooled as fast as possible, the amount of the abnormal structure can be suppressed to such an extent that the stretch flangeability does not deteriorate. Preferably, the Cr content is 1.20% by weight or less.

Since Cu and Ni do not adversely affect the effects of the present invention by their addition, they may be added if necessary. Cu is useful for precipitation strengthening, improvement of corrosion resistance, etc., but there is a problem that hot working cracks and surface cracks easily occur due to addition of Cu. Therefore, in order to suppress this, it is useful to add Ni in the same amount from half of Cu. However, when the added amount of Cu exceeds 1%, not only the above effects are saturated, but also economically disadvantageous. The amount of Ni added is also preferably 1% or less from the viewpoint of cost.

Ca and REM are useful for improving stretch flangeability. By combining with the above elements according to the present invention, stretch flangeability can be further improved. However, when excessively added, inclusions increase, so the addition amount is preferably in the range of 0.0020% by weight or less.

Since Ti and Nb do not adversely affect the effects of the present invention, they may be added if necessary. Ti
Is useful for improving the stretch-flangeability by improving the properties of S precipitates, and Nb is useful for increasing the strength of the steel sheet. However, both elements are excessive. However, since the product price will be increased even if added to, the amount of each added should be 0.05% or less.

According to the method of the present invention, after cold rolling a steel sheet having the above-mentioned components, annealing is carried out in the two-phase region, and the temperature is in the range from 650 ° C. to the pearlite transformation stop temperature T for 10 times. While cooling to stay for more than a second,
A high-strength cold-rolled steel sheet having excellent stretch-flange formability is obtained by cooling the pearlite transformation at a temperature in the range from the stop temperature T to 450 ° C. for 5 seconds or less.

According to the present invention, it is necessary to carry out continuous annealing in the two-phase region. When the continuous annealing is performed in the austenite single phase region, in the steel sheet having the chemical composition defined in the present invention, it becomes difficult to precipitate pearlite and it is difficult to suppress the formation of an abnormal structure. In order to suppress the generation of the abnormal structure, it is necessary to cool the temperature range in which the abnormal structure is precipitated extremely rapidly, so that the flatness of the plate becomes poor and the cost for correcting the flatness increases. When continuous annealing is performed at a low temperature at which reverse transformation to austenite does not occur, recrystallization takes time, so it is necessary to reduce the strip running speed, resulting in poor productivity.

Thus, according to the present invention, continuous annealing is performed in two steps.
After performing in the phase region, as shown in FIG. 3, the stretch flangeability is improved by cooling so as to stay for 10 seconds or more from 650 ° C. to the temperature T at which the pearlite transformation stops. be able to. This improvement in stretch-flangeability is considered to be because austenite transforms into pearlite, and the amount of autesnite that transforms into a structure detrimental to stretch-flangeability decreases.

Next, according to the present invention, as shown in FIG. 4, the temperature from the temperature T at which the pearlite transformation is stopped to 450
The stretch-flangeability can be improved by cooling so that the residence time up to ° C is 5 seconds or less. The cause of the improvement in stretch-flangeability is not completely clear, but it is considered that the amount of abnormal tissue harmful to stretch-flangeability is reduced.

[0026]

EXAMPLES Steel sheets having the chemical compositions shown in Tables 1 and 2 were heat treated under the conditions shown in Tables 3 to 5, and as a result, the mechanical properties of the obtained steel sheets are shown in Tables 3 to 5. The steel sheet according to the present invention has excellent stretch flangeability. When the amount of Cr is large like the steel of the steel type C6, it becomes difficult to precipitate pearlite, and the stop temperature of pearlite precipitation cannot be measured. Further, in the steel of the C7 steel type, the amount of alloying elements is small and the pearlite transformation does not stop. The method of the present invention can be applied only to steel whose pearlite transformation stop temperature can be measured.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

[Table 5]

[0032]

According to the method of the present invention, when a high-strength cold-rolled steel sheet is manufactured by continuous annealing equipment, it is possible to suppress the occurrence of abnormal structure and manufacture a steel sheet having excellent stretch-flangeability. Thus, by using the steel sheet according to the present invention, it becomes possible to reduce the thickness of the original plate of the pressed part, which can contribute to the weight reduction of the vehicle body of an automobile, for example. Further, the properties of the steel sheet according to the present invention are not impaired even if hot dip galvanizing or the like is applied thereto.

[Brief description of drawings]

FIG. 1 is a graph showing heat treatment conditions under which an experiment of a test material was performed.

FIG. 2 is a graph showing the relationship between the holding temperature after two-phase region annealing and the pearlite fraction in the second phase.

FIG. 3 is a graph showing the influence of the residence time at a temperature in the range from 650 ° C. to the pearlite transformation stop temperature T on the stretch flangeability (λ).

FIG. 4 is a graph showing the effect of residence time at a temperature in the range from T to 500 ° C. on stretch flangeability (λ).

FIG. 5 shows the effect of Cr on the stop temperature T of pearlite transformation.
It is a graph which shows the influence of quantity.

FIG. 6 is a graph showing the effect of C concentration in austenite on the stop temperature T of pearlite transformation.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/50 C22C 38/50

Claims (2)

[Claims] 1. C wt.% 0.04% or more, less than 0.20 wt%, Mn 0.50% or more, less than 2.00 wt%, Si 1.50 wt% or less, P 0.10 wt% % Or less, S 0.005% by weight or less, Cr 2.00% by weight or less After cold rolling a steel sheet composed of balance iron and unavoidable impurities, annealing is performed in the two-phase region, and pearly transformation stops at 650 ° C. High-strength cold with excellent stretch-flangeability, characterized by cooling so that it stays at a temperature between T and 10 seconds or more, and so that the staying time from T to 450 ° C. becomes 5 seconds or less. Manufacturing method of rolled steel sheet. (A) C 0.04% or more, less than 0.20% by weight, Mn 0.50% or more, less than 2.00% by weight, Si 1.50% by weight or less, P 0.10% by weight. Hereinafter, S 0.005 wt% or less, Cr 2.00 wt% or less, and (b) Cu 1.0
At least 1 selected from the group consisting of wt% or less, Ca 0.0020 wt% or less, Ti 0.05 wt% or less, Nb 0.05 wt% or less, REM 0.0000 wt% or less, Ni 1.0 wt% or less After cold rolling a steel sheet containing the other elements and the balance iron and unavoidable impurities, annealing is performed in the two-phase region,
Elongation characterized by cooling so that it stays at a temperature between 0 ° C. and a temperature T at which pearlite transformation stops for 10 seconds or more, and so that the staying time from T to 450 ° C. becomes 5 seconds or less. A method for producing a high-strength cold-rolled steel sheet having excellent flangeability.