JPH07100818B2 - Method for producing cold-rolled steel sheet excellent in normal temperature non-aging property and bake hardenability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet which is excellent in non-aging at room temperature and bake hardenability and has good workability.

(Prior Art) Cold-rolled steel sheets used as materials for outer panels of automobiles are required to have excellent deep drawability, bulging properties, shape properties, and non-aging at room temperature. Paint baking (usually around 170 ° C) during automobile manufacturing process to improve
After that, the property that the yield stress of the steel sheet increases, that is, bake hardenability, is often required.

By the way, both normal temperature aging and bake hardening are phenomena that occur when solid solution C remaining in the steel sheet at the time of shipping the steel sheet diffuses and segregates into dislocations. Therefore, if a large amount of solid solution C is left in order to increase the amount of bake hardening, the room temperature non-aging property deteriorates. Many techniques have been developed to date to obtain the contradictory characteristics of non-aging at normal temperature and high bake hardening, but many of them have a common technical idea.

That is, a certain amount of solute C must diffuse to dislocations in order for the steel to harden or stretcher strain to occur. The diffusion distance of C at 100 ° C x 1 hour (heat treatment condition when evaluating non-aging at room temperature) is 1300Å, but at 170 ° C x 20 minutes (heat treatment condition when evaluating bake hardenability) it becomes 5700Å . That is, if the same amount of solute C remains, more solute C can reach the dislocation in the latter case. Therefore, if an appropriate amount of solid solution C remains after annealing, stretcher strain does not occur at 100 ° C x 1 hour, but 170 ° C x 2
At 0 minutes, there is an amount of solid solution C capable of causing bake hardening. Most of the techniques in the past have examined methods for leaving this appropriate amount of solid solution C.

As a specific example, Japanese Patent Publication No. 59-2726 discloses a technique for obtaining an appropriate amount of residual solid solution C by annealing at a two-phase region and coagulating cementite to reduce the precipitation site of C after cooling. Is. Furthermore, Japanese Patent Publication No. 57-57937 discloses a method in which the cooling rate is also defined. Further, Japanese Patent Publication No. 60-17004 discloses a method of using Nb-added ultra-low carbon steel to control a cooling rate up to 650 ° C. after continuous annealing so that an appropriate amount of solute C remains.

(Problems to be Solved by the Invention) However, although such a conventional technique has a good non-aging property at room temperature, it has a small bake hardening amount and cannot sufficiently satisfy the user's requirements.

(Means for Solving Problems) Therefore, the inventors of the present invention have conducted detailed studies to solve the above problems, and as a result, have found that Mo, Sn, and Sb are added to ordinary aluminum-killed steel.
It has been found that by adding any one of them, a higher bake hardenability than before can be obtained while maintaining non-aging at room temperature.

That is, the gist of the present invention is that C: 0.01 to 0.04%, Mn: 0.05 to 0.4%, P: 0.03% or less by weight%, Total A
l: 0.02 to 0.08%, and (a) Mo: 0.06 to 0.35% (b) Sn: 0.01 to 0.1% (c) Sb: 0.01 to 0.1%, and the balance contains Fe and unavoidable Recrystallization temperature or higher after cold rolling of hot-rolled steel sheet containing mechanical impurities
Heating and cooling in a temperature range of ₃ points or less of Ar, 250 ° C to 400 ° C
A method for producing a cold rolled steel sheet excellent in normal temperature non-aging property and bake hardenability, which comprises performing continuous annealing in which each of the overaging treatments is performed in the following temperature range and then temper rolling.

The present invention will be described in detail below. First, the reasons for limiting each component in the steel targeted by the present invention will be described.

First, if C is not present in an amount of 0.01% or more, the degree of supersaturation of C becomes insufficient after continuous annealing, and the room-temperature non-aging property deteriorates. Also,
If it exceeds 0.04%, the r-value and ductility are deteriorated.
Was 0.01 to 0.04%.

The lower the amount of Mn, the higher the r value. Therefore, it is preferably as low as possible, but it is necessary to set it to 0.05 to 0.4% in order to make S that causes hot brittleness harmless.

The lower the amount of P, the better the r value, so 0.03% or less.

Total Al not only has a role as a strong deoxidizer, but also fixes N in steel as AlN to prevent normal temperature aging due to solid solution N, so 0.02% or more is necessary. However, if it exceeds 0.08%, the r-value will deteriorate, so Total
Al is 0.02 to 0.08%.

Next, the reason for adding Mo, Sn, and Sb, which are the most important features in the present invention, will be described.

As mentioned above, the room temperature non-aging property and the bake hardening amount are uniquely determined by the amount of solid solution C remaining at the time of shipment, but the inventors of the present invention maintain higher room temperature non-aging property while maintaining the same room temperature non-aging property. An additive element having a curability was found. That is, for steels having the components shown in Table 1 as test materials, steelmaking, ingot making, hot rolling (finishing temperature 880 ° C, winding temperature 710 ° C), cold rolling (75% reduction) Etc. After forming a cold-rolled sheet with a thickness of 0.8 mm, heat at 810 ° C for 70 seconds, cool at about 100 ° C / sec, and then perform overaging treatment at 350 ° C for 4 minutes, then 1% I applied a skin pass.

For each sample, non-aging at room temperature was evaluated by yield elongation after artificial aging treatment (100 ° C x 1 hour). This is because the stretcher strain does not occur until the yield elongation is 0.2% or less. Also, the bake hardenability is [Yield stress after 2% tensile-170 ° C x 20 minutes heat treatment].
-[Stress at 2% tension] was used for evaluation. FIG. 1 shows the amount of bake hardening with respect to the added amount of each element, and FIG. 2 shows the yield elongation after artificial aging. As can be seen from both figures, when these elements are added to Ammikilled steel, the amount of bake hardening increases, but the yield elongation after artificial aging remains at 0.2% or less and hardly changes. It can be seen from Fig. 1 that the bake-hardening amount starts to increase remarkably when Mo is 0.06% or more and when Sn is Sn.
It is 0.01% or more, and in the case of Sb, it is 0.01% or more. As the amount of these elements added increases, the amount of bake hardening increases, but the workability deteriorates. Third is the effect on r-value.
It is a figure. From the same figure, the r value rapidly deteriorates in the case of Mo exceeding 0.35%, Sn exceeding 0.1%, and Sb exceeding 0.1%.
From the above, Mo is 0.06% to 0.35%, Sn is 0.01 to 0.1%, and Sb is
It was set to 0.01 to 0.1%.

In order to investigate the cause of the phenomenon that the bake hardening amount increases by adding Mo, Sn, Sb, the amount of solid solution C remaining in the test material used in this experiment was measured by the internal friction method. It was found that the amount of solid solution C increased as the number of elements increased, and the correspondence relationship between the amount of solid solution C and the bake hardening amount was in agreement with the previously reported relationship. Therefore, it can be said that the increase in the bake-hardening amount due to the increase in the amount of each additional element is due to the increase in the residual solid solution C. The reason why the amount of residual solid solution C increases is not critical, but it is considered that these elements may delay the diffusion of C or suppress the progress of carbide precipitation. Further, although the baking hardening amount is increased (that is, the amount of residual solid solution C is increased) by increasing the amount of these additive elements, the reason why the room temperature non-aging property is not deteriorated is not clear at present. The present inventors presume as follows.

That is, in general, when light-working a steel, if there is an obstacle in the movement of dislocations in the steel, the dislocation density becomes higher for the same workability as compared with the case where it does not. This is because in order to secure the total amount of strain applied to the steel, the amount that cannot be obtained by the moving distance of dislocation must be compensated by increasing the dislocation density. Mo, Sn, and Sb all have larger atomic radii than Fe. When these elements form a solid solution, they hinder the movement of dislocations and increase the dislocation density during skin pass rolling. It is considered that this is the reason why the room-temperature non-aging property does not deteriorate despite the increase in the residual solid solution C.

Next, the reasons for limiting the manufacturing conditions will be described. First, the cold rolling reduction rate in the cold rolling after the completion of the normal hot rolling is preferably 6.0% or more in order to secure the workability, particularly the r value. Setting the heating temperature in the continuous annealing to the recrystallization temperature or higher is a necessary condition for softening the steel. The heating temperature because r value deteriorates when annealed beyond the Ar ₃ point is less than the recrystallization temperature Ar ₃ point. The heating time is not particularly limited, but if heated for a long time, abnormal grain growth may occur and rough skin may be caused. The cooling rate after heating is not particularly limited, but 5 ° C./sec or more is desirable in order to shorten the time of overaging treatment.

For overaging treatment, if the temperature exceeds 400 ℃, the amount of residual solid solution C will increase too much and the room temperature non-aging property will deteriorate.
Set to 0 ° C or less. Also, at temperatures below 250 ° C, the overaging time must be extended. Therefore, the temperature of the overaging treatment should be 250 ° C or higher and 400 ° C or lower. The time of overaging treatment is not particularly limited, but is preferably 30 minutes or less due to the nature of continuous annealing. Temper rolling is an essential process to eliminate stretcher strain. Although the rolling reduction is not particularly limited, it is preferably about 1%.

The steel used in the method of the present invention can be made into a cold-rolled steel sheet through various steps of ordinary steelmaking, continuous casting or ordinary ingot casting, slabbing, hot rolling and cold rolling.

Next, the effects of the present invention will be described more specifically with reference to Examples.

(Example) After the cold-rolled sheet having a thickness of 0.8 mm was manufactured through the normal process for the steel shown in Table 2, continuous annealing and overaging treatment were performed under the conditions shown in Table 3 to obtain a 1% skin pass. After that, the mechanical properties before aging shown in Table 3 are examined, and the yield elongation and the bake hardening amount after artificial aging treatment (100 ° C x 1 hour) are [2% strain -170 ° C x 20 minutes. Later yield stress] −
It was measured under the condition of [stress at 2% strain]. In addition, all these mechanical characteristic values are values measured by JIS No. 5 test pieces by an Instron type tensile tester.

In Table 3, trial numbers No. 1 to No. 11 are examples of the present invention, and No. 12 to N.
o.18 is a comparative example. All of the examples of the present invention satisfy the condition that the yield elongation after artificial aging is 0.2% or less and the stretcher strain does not occur, and the bake hardening amount is 4.8 kg f / mm ²
It is as high as ~ 6.2kg f / mm ² . Moreover, the r value is 1.4 or more, and the workability is excellent. On the other hand, Comparative Example No. 15,
No.17 and No.18 are the yield elongation after artificial aging is below 0.2%, bake hardenability amount as low as ^{3.7kg f / mm 2 ~4.0kg f /} mm 2. In addition, No. 16 has a high bake hardening amount of 5.2 kg f / mm ² ,
Yield elongation after artificial aging is 0.5%, which does not satisfy room temperature non-aging. Comparative Examples No. 12 to No. 14 are examples in which the additive element is contained in a large amount so that they are not aged at room temperature and have high bake hardenability, but the r value is deteriorated.

(Effects of the Invention) As is clear from the above examples, according to the present invention, it is possible to manufacture a cold-rolled steel sheet having excellent non-aging at room temperature and high bake hardenability, and the industrial effect is extremely remarkable. is there.

[Brief description of drawings]

Figure 1 shows the effect of the addition amount of each element of Mo, Sn and Sb on the bake hardening amount, and Figure 2 shows the effect of the addition amount of each element of Mo, Sn and Sb on the yield elongation after artificial aging. FIG. 3 and FIG. 3 are diagrams showing the influence of the addition amounts of Mo, Sn, and Sb elements on the r value.

Claims (1)

[Claims] 1. By weight%, C: 0.01 to 0.04%, Mn: 0.05 to 0.4%, P: 0.03% or less, Total Al: 0.02 to 0.08%, and (a) Mo: 0.06 to 0.35% (b) Sn : 0.01-0.1% (c) Sb: 0.01-0.1%, and the balance is at least the recrystallization temperature after cold rolling the hot-rolled steel sheet consisting of Fe and unavoidable impurities.
Heating and cooling in a temperature range of ₃ points or less of Ar, 250 ° C to 400 ° C
A method for producing a cold-rolled steel sheet excellent in normal temperature non-aging property and bake hardenability, which comprises performing continuous annealing in which each is overaged in the following temperature range, and then temper rolling.