JPH0218363B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The technical ^contents described in this specification are mainly related to the manufacturing method of high-strength steel sheets that are suitable for automotive structural members. The purpose of this invention is to propose development effects for the advantageous production of P-added high-strength mixed-structure steel sheets with good ductility. In recent years, tensile strength has been added to strength components such as bumpers and door guard bars from the perspective of automobile safety and weight reduction.
High-strength steel plates of 50Kg ^f /mm2 or ^higher are often used. Materials used for such applications are required to have high tensile strength and yield point, as well as good ductility. (Prior art) Steel sheets to which precipitation fine grain strengthening elements such as Nb, Ti, and V are added have been used for such applications, as shown in Japanese Patent Application Laid-open No. 48-81721, but steel sheets containing precipitation-refined grain strengthening elements such as Nb, Ti, and V have been used, but the ductility is not sufficient. Subsequently, mixed-structure steel sheets consisting of ferrite, which is characterized by high ductility, and low-temperature transformation products such as martensite have been developed and are being widely used.These mixed-structure steel sheets have high tensile strength and high They have a common property of being ductile but at the same time having a low yield point.
As shown in No. 19441 and No. 58-22332, mixed structure steels generally have a yield ratio defined as the ratio of yield point to tensile strength, which is as low as 65 or 60% or less. As mentioned above, precipitation fine-grain strengthened high-strength steel sheets with additions of Nb, Ti, V, etc. have a high yield point and tensile strength, but have poor ductility, while mixed-structure steel sheets have good tensile strength and ductility. However, it has been impossible to manufacture thin steel sheets that have a low yield point and are suitable for applications that require both a high yield point and high ductility. On the other hand, high-strength steel plates using P have been proposed for a long time, such as in Japanese Patent Application Laid-Open Nos. 50-23316 and 50-60419. These disclose that P-added cold-rolled steel sheets are subjected to skin pass rolling after continuous annealing, but the structure before skin pass rolling consists of ferrite carbide, making it impossible to obtain a mixed structure containing low-temperature transformation products. Furthermore, JP-A-58-22332 and JP-A-57-137452 mentioned above disclose how to obtain a mixed structure, which is the basis of the present invention, for cold-rolled thin sheets and hot-rolled thin sheets, respectively. However, a low yield ratio was inevitable. (Problems to be Solved by the Invention) Normally, skin pass rolling is suitable as a method for increasing the yield point of a steel plate after annealing.
Generally, skin pass rolling is accompanied by deterioration of ductility. However, when this method was actually applied to a mixed texture steel sheet, it was discovered that mixed texture steel sheets manufactured under specific conditions had little deterioration in ductility even when subjected to skin pass rolling. It is an object of the present invention to utilize the above knowledge to establish a method for manufacturing a high-strength steel plate that has both a high yield point and high ductility. (Means for Solving Problems) This invention has a basic composition of C: 0.03 to 0.20 wt%, Mn: 0.8 to 3.0 wt%, P: 0.04 to 0.15 wt%, S: 0.01 wt% or less, Al: 0.10 wt% or less. , a hot-rolled thin sheet having a mixed structure consisting of 5% or less of a low-temperature transformation product phase and the remaining ferrite phase, produced as hot-rolled or by continuous annealing after subsequent cold rolling; or The tensile strength method consists of heat-treating the cold-rolled sheet to form the mixed structure, and then masking pass rolling it within the elongation rate SK (%) that satisfies the following formula according to the initial yield ratio YR ₀ obtained by the heat treatment. 5.25-0.075×YR ₀ SK ( ^% ) 3 (%). First, the experimental results that led to the completion of this invention will be explained. A mixed structure cold-rolled steel sheet (1.2 mm thick) made of ferrite-martensite produced under the conditions shown in Table 1 was subjected to skin pass rolling to an elongation rate of 5%, and its tensile properties were investigated. The results are shown in Figure 1.

[Table] It is clear from Figure 1 that the yield ratio of any material increases rapidly as the elongation rate in the skin pass increases. The critical skin pass elongation rate at which the yield ratio is 70 _% or more is determined by the following formula: SK (%) The following relationship is given: =5.25−0.075×YR ₀ (%). On the other hand, as the skin pass elongation rate increases, the elongation tends to decrease as shown in Figure 1.
In additive steels A and B, ductility deterioration is small when the elongation rate of the skin pass is 3% or less, and it deteriorates rapidly only when it exceeds 3%. It can be seen that Comparative Steel C, which contains virtually no P, suffers from significant ductility deterioration due to skin passes. This experiment investigated the effect of skin pass on the case where a mixed structure was obtained by heat treatment after cold rolling as shown in the above-mentioned Japanese Patent Application Laid-Open No. 58-22332. It has been confirmed that when a similar skin pass is applied to the hot-rolled thin sheet with a mixed structure shown in the above-mentioned Japanese Patent Application Laid-Open No. 57-137452, substantially the same increase in yield ratio is secured. (Function) Next, the reason for limiting the range of components of this invention will be explained. C: 0.03-0.20wt% C is important because it controls tensile strength through the amount of low-temperature transformation products such as martensite after continuous annealing, and high tensile strength can be obtained when the C amount is lower than 0.03wt%. Not only that, but also the Ac1 transformation point rises rapidly, making it difficult to control the temperature during annealing, so the lower limit is set to 0.03%.
And so. On the other hand, increasing the amount of C is preferable because it increases the strength, but if it exceeds 0.20 wt%, spot weldability deteriorates rapidly, so the upper limit was set at 0.20 wt%. Mn: 0.8-3.0wt% Mn is a solid solution strengthening element and is necessary to ensure strength, but in this invention it is also important for the formation of low-temperature transformation products together with P, and as a premise of this invention minimum 0.8wt to obtain a mixed tissue of
% is required. Increasing the amount of Mn is preferable in order to increase strength, but if too much is used, it will increase costs and deteriorate weldability, so the upper limit was set at 3.0 wt%. P: 0.04-0.15wt% P has solid solution strengthening and low-temperature transformation formation promotion effects,
Further, it is a desirable element because it is inexpensive. In particular, in this invention, although the reason is unknown, it is an important element because it has a large effect of reducing ductility deterioration caused by skin passes.In order to bring about this effect, 0.04wt% or more is required;
An excessive amount of P exceeding 0.15 wt% causes embrittlement and deterioration of weldability, so the upper limit was set at 0.15 wt%. S: 0.01wt% or less S exists in steel as MnS and causes ductility deterioration, so it is preferable to reduce it as much as possible. However, if it is difficult to achieve an extremely low S content, for example, rare earth elements such as Ce and La or Ca may be added. Since ductility deterioration can be prevented by adding sulfide and controlling the shape of the sulfide, the upper limit was set at 0.01 wt%. Al: 0.10wt% or less Al is necessary as a deoxidizing element, but excessive Al forms alumina clusters, deteriorates the surface quality, and increases the risk of hot cracking, so the upper limit was set at 0.10wt%. . The above-mentioned limited amounts of C, Mn, P, S, and Al are the basic components of the high-strength steel sheet of the present invention, but in addition, 2% or less of Si, 1% or less of Cr and The object of the present invention is not impaired even if Mo, 0.1% or less of Nb, Ti, and V are added individually or in combination. In particular, the addition of Nb, Ti, V, etc. has the effect of refining the precipitate grains even in mixed-structure steel sheets, resulting in increased strength and yield ratio in as-annealed state, and as a result, it is necessary to increase the yield ratio to 70% or more. This is preferable in order to reduce the skin pass elongation rate. Steel with a limited composition as described above is hot-rolled or cold-rolled and then continuously annealed to produce a hot-rolled steel sheet or a cold-rolled steel sheet, respectively. The hot rolling or annealing conditions in this case are as follows. First, for hot-rolled thin sheets, the heating temperature of the steel slab, which is melted so that the above-mentioned component ranges are satisfied, is 1100 to 1250°C, which is almost the same as in the case of normal rolling. Further, after the final finish rolling is completed at 750 to 900°C, cooling is performed at a normal cooling rate (10 to 200°C/sec), and no special regulation of the cooling pattern is required. Coil winding temperature (CT) after hot rolling is 450℃
The following is desirable. In other words, the initial relief ratio YR ₀ after cooling is determined almost solely by CT within the range of the hot rolling conditions described above, and when coiling is performed at a temperature higher than 450°C, pearlite transformation occurs, resulting in a decrease in TS.
YR ₀ is 70% or more. When the CT is below 450°C, in the case of the steel with the composition of this invention, more than 70% of ferrite is generated until the time of winding, so C is concentrated in the austenite phase, and combined with the effect of Mn, it occurs after the winding or after the winding. Martensitic transformation occurs before desorption. Next, the cold-rolled sheet is subjected to normal hot rolling, pickling, cold rolling, and then continuous annealing. Hot rolling is carried out under normal conditions, but low temperature winding of 600°C or less is preferred in order to obtain high strength.
Next, the annealing conditions are important; first, the heating temperature must be set to Ac1 point or higher in order to obtain the austenite phase, which is the parent phase of the low-temperature transformation product phase. In addition, at Ac1 point or higher, the higher the temperature, the more the amount of γ phase increases, and as a result, the amount of low-temperature transformation product phase increases after cooling, and higher strength can be obtained. Temper color and pick-up occur at the same time as saturation occurs, so the temperature should be kept at 950°C. Here, pick-up refers to a phenomenon in which oxide scale that has fallen from a preceding steel strip in a continuous annealing line or the like adheres to the succeeding steel strip. When containing elements such as Nb, Ti and V, α-
Since higher strength can be obtained on the low temperature side of the γ region, α-γ
Low temperature annealing in the range is preferred. It is necessary to hold the heating time for 10 seconds or more in order to reveal a predetermined amount of γ phase, and to hold it for more than 10 minutes, it is necessary to lengthen the soaking zone of the annealing furnace or reduce the threading speed. Accordingly, the upper limit was set at 10 minutes in order to increase costs in both cases. Cooling from this heating temperature is important because it has a large effect on the resistance to secondary processing embrittlement. That is, if the cooling rate is less than 15° C./sec, even if a mixed structure is obtained, the secondary processing brittleness is not improved. Furthermore, the tensile strength of a mixed structure steel sheet increases as the cooling rate increases, and in order to obtain higher strength with the same composition,
A cooling rate of s or more is preferred. Note that the above cooling rates are all average rates between 600 and 300°C, but in order to improve ductility, it is necessary to reduce the amount of solid solute C in ferrite, and for that purpose, cooling at a high temperature of 600°C or higher is necessary. It is preferable to perform slow cooling at a cooling rate of 20°C/sec or less per area. A particularly important point of the present invention is to subject the P-added high-strength hot-rolled steel sheet or cold-rolled steel sheet obtained as described above to skin pass rolling within a specific range. Generally, skin pass rolling is performed for the purpose of eliminating yield elongation remaining after hot rolling or annealing, and adjusting the shape and surface properties of the steel sheet. However, the skin pass rolling of this invention, in addition to adjusting the steel sheet shape and surface texture, particularly applies strain to the steel sheet to increase the yield point, and improves the yield ratio which was less than 65% or 70% when hot rolled or continuously annealed. There is an important objective to increase this to 70% or more. According to many experimental results, the minimum skin pass elongation rate SK required to achieve a yield ratio of 70% or more
(%) is found to have a correlation with the initial yield ratio YR ₀ before the skin pass, and the skin pass elongation rate SK (%) determined by the following formula is set as the lower limit. SK (%) = 5.25 - 0.075 x initial yield ratio % As the skin pass elongation rate increases, the yield ratio increases as shown in Figure 1. In this case, the change in tensile strength of the steel plate is small, and the change in yield ratio corresponds to the change in yield point. Therefore, in order to obtain a high yield point, it is preferable to increase the skin pass elongation rate, but as shown in FIG. 1, if the skin pass elongation rate is 3% or more, the elongation deteriorates rapidly, so the upper limit was set at 3%. (Example) Table 2 shows the composition, manufacturing conditions, and tensile properties of a mixed structure thin steel sheet obtained according to the present invention in comparison with a comparative example.

【table】

[Table] (Effects of the invention) According to the present invention, the yield point and tensile strength are equivalent to those of conventional precipitation fine-grain strengthened high-strength steel sheets, as well as superior ductility, and the tensile strength is equivalent to that of conventional mixed structure steel. It is clear that ductility and higher yield points are obtained.

[Brief explanation of drawings]

FIG. 1 is a comparative graph showing the effect of improving the yield point of the test steels shown in Table 1 by skin pass rolling.

Claims (1)

[Claims] 1 C: 0.03 to 0.20 wt% Mn: 0.8 to 3.0 wt% P: 0.04 to 0.15 wt% S: 0.01 wt% or less Al: 0.10 wt% or less, as hot rolled or a hot-rolled thin sheet or a cold-rolled thin sheet having a mixed structure consisting of 5% or more of a low-temperature transformation product phase and the remaining ferrite phase, which is produced by continuous annealing after cold rolling. A tensile strength of 60 Kgf/mm ² or more, consisting of heat treatment followed by skin pass rolling within the range of elongation SK (%) that satisfies the following formula according to the initial yield ratio YR ₀ obtained by the heat treatment,
A method for producing high-strength steel plates with a yield ratio of 70% or more and good ductility. Note 5.25−0.075×YR ₀ SK (%) 3%