JPH0557330B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing an ultra-low carbon cold rolled steel sheet suitable for use as a cold rolled steel sheet for automobiles. This is intended to advantageously improve the fatigue properties of the parts. (Conventional technology) Cold-rolled steel sheets used as steel sheets for automobiles include:
First of all, excellent deep drawing workability is required.
In addition, various properties such as surface beauty, strength, dent resistance, weldability, paintability, and corrosion resistance are also required. In particular, recent automotive steel sheets need to be compatible with a wide variety of designs, and are placing particular emphasis on improving the r value for deep drawability, as well as lowering yield stress and increasing work hardening rates from the perspective of shape fixability. It's here. Many manufacturing techniques for deep drawing steel sheets developed from this point of view have been disclosed so far. (Problem to be solved by the invention) However, another important characteristic required of automotive steel sheets is workability during spot welding, which cannot be avoided during assembly work after processing, and mechanical properties of the welded part. However, in ultra-low carbon steel sheets, there remains a problem in that spot weldability is generally inferior to that of low carbon steel. However, no technique has been reported so far that has succeeded in improving spot weldability. In general, from the viewpoint of workability, especially deep drawability and shape fixability during press forming, it is better to improve elongation (E1) and rankford value (r value), and to lower YS (low YR). The manufacturing technology for this purpose has been realized through ultra-low carbonization. However, on the other hand, when such steel plates are subjected to spot welding, they are inferior to conventional steel in terms of strength, and moreover,
As shown in the figure, the range of appropriate welding conditions is shifted to a higher welding current side than for conventional steel types, creating a new problem in that the spot welding machine wears out more quickly. Furthermore, although spot welds made of ultra-low carbon steel have good high-cycle fatigue properties, they still have a problem in that they have poor low-cycle fatigue properties. This invention advantageously solves the above problems, and even when ultra-low carbon steel plates are subjected to spot welding, there is no deterioration in strength, and the spot weldability, especially the fatigue properties of the spot welds, are excellent. The purpose of this study is to propose an advantageous manufacturing method for ultra-low carbon cold-rolled steel sheets. (Means for Solving the Problems) First, the solution history of this invention will be explained. It is well known that softening by reducing the amount of C and improving the texture by adding Ti, Nb, etc. are effective ways to improve workability, that is, r value and El. As a result, this type of steel is generally very soft, which has been considered to be extremely important for improving deep drawability. On the other hand, since ultra-low carbon steel has very few solid solution elements and no yield elongation occurs, it has been thought that temper rolling is not necessarily necessary. This is because the purpose of temper rolling for ultra-low carbon steel is different from that for low carbon steel, and is only for shape correction and surface adjustment, so in the case of ultra-low carbon steel, temper rolling may not be necessary at all. Alternatively, it has been thought that a very light pressure is sufficient. Under these circumstances, the inventors proceeded with research and development of a technology to improve the spot weldability of ultra-low carbon steel. As a result, we found that steel plates that have become too soft are easily deformed by the pressure applied from the electrode during spot welding, resulting in abnormally low contact resistance between the electrode and the steel plates or between the steel plates. (See Figure 11). This decrease in electrical resistance is considered to be a major cause of deviations in the range of appropriate welding conditions during spot welding of ultra-low carbon steel sheets. On the other hand, since ultra-low carbon steel sheets have few impurities and exhibit very large grain growth during heating, coarsening of the crystal grains in the spot weld zone, and thus softening, is also thought to be a factor inhibiting weldability. Therefore, as a result of intensive research to solve the above problem, the inventors added Ti, Nb, and B simultaneously to ultra-low carbon steel, and controlled the coexistence state of these three elements within a specific range. It was found that regulating the strength of spot welds is extremely effective in improving the strength of spot welds. We have also discovered that the manufacturing process, especially the temper rolling process, plays an important role in fully demonstrating its effects. To explain this point in more detail, the effect of increasing the hardness of the weld zone can only be obtained when the three elements Ti, Nb, and B coexist, and even if one of these additive elements is missing, the weldability will be improved. No improvement effect was observed. Even in conventional deep drawing steel sheets
Many component systems such as Ti and Nb addition, Ti-B system, Nb-B system, etc. have been proposed, but all of them have the effect of improving the material quality of each element alone (for example,
It is aimed only at improving El, r value, etc.
To that extent, the effects of adding Ti, Nb, and B elements were mutually independent and merely additive. Moreover, these effects could be well explained by the conventional theory of recrystallization texture formation. However, in order to achieve the desired effect of this invention, it is essential to add appropriate amounts of Ti, Nb, and B3 elements, that is, to coexist in a delicate balance of the three elements. This is significantly different from the case of Nb or B-containing deep drawing steel sheets. In other words, although many steel sheets for deep drawing with addition of Ti, Nb, or B have been proposed, they all focus only on improving deep drawability.
Only steels with excessive addition of Ti, Nb, B, etc., or steels with an inappropriate balance among the three components, are used in terms of spot weldability. Therefore, conventional steels do not have the good spot weldability that this invention aims for. It is thought that this was not possible. In addition, this invention improves the fatigue properties of spot welds, especially the fatigue properties at low cycles, without sacrificing any other properties by setting the reduction rate in temper rolling higher than usual. This was a successful improvement. The gist of the invention is as follows. (1) C: 0.004wt% (hereinafter simply expressed as %) or less,
Si: 0.1% or less, Mn: 0.5% or less, P: 0.025%
Below, S: 0.025% or less, N: 0.004% or less,
Al: 0.01~0.10%, Ti: 0.01~0.04%, Nb:
Contains 0.001 to 0.010% and B: 0.0001 to 0.0010%, and the following formulas (1) to (4) (11/93)Nb−0.0004≦B≦(11/93)Nb+0.0004 ……(1) Ti> (48/12)C+(48/14)N...(2) Nb<1/5・(93/48)Ti...(3) C+(12/14)N+(12/11)B>0.0038
......(4) in a range that satisfies the requirements, and the remainder is essentially Fe at a finishing temperature of 700.
~900℃, coiling temperature: 300~600℃, then cold rolling at a reduction rate of 60~85%, and then continuous annealing at a temperature range of above the recrystallization temperature and below 780℃. A method for producing an ultra-low carbon cold-rolled steel sheet with excellent spot weldability, which comprises applying heat rolling to a rolling reduction of at least [plate thickness (mm) + 0.1]% and no more than 3.0%. This invention will be specifically explained below. First, the experimental results that formed the basis of this invention will be explained. FIG. 1 shows the results of an investigation into the effects of the addition of Ti, Nb, and B, which are particularly important components in this invention, on spot weldability. In the above experiment, the test steels used were general deep drawing steel sheets, C: 0.04%, Si: 0.01%,
Mn: 0.20%, P: 0.01%, N: 0.0040%, Al:
Low carbon steel plate containing 0.036%, C: 0.002%, Si:
0.1%, Mn: 0.1%, P: 0.01%, S: 0.01%,
Based on Al: 0.02%, N: 0.002-0.003%,
Furthermore, conventional Ti-added ultra-low carbon steel sheet with Ti: 0.06% added, Ti: 0.03%, Nb: 0.005%, B:
Ti− according to the invention with the addition of 0.0007% respectively
Nb-B added ultra-low carbon steel plate was used. In addition, spot welding is performed using RWMA (Resistance
Welder Manufacturers', Association) recommended values, the sample size was set to 0.8 x 30 x 30 mm,
Pressure force: 190Kg using 4.5mmφ CF type electrode
I went with f. The lower limit of the appropriate welding current is based on the point where the nugget system formed by welding is 3√mm or more (where t is the thickness of the specimen), while the upper limit is defined at the point where expulsion occurs. . As is clear from the figure, the appropriate welding current of conventional Ti-added ultra-low carbon steel is significantly higher than that of low carbon steel, which places a heavy burden on welding equipment, whereas the present invention For Ti-Nb-B-added ultra-low carbon steel, not only the lower limit of the appropriate welding current is almost the same as that of low-carbon steel sheets, but also the upper limit of the appropriate welding current, which is regulated by spatter, is on the higher current side than for low-carbon steel. , the appropriate welding current range is further expanded than that of low carbon steel. Such an effect is considered to be due to optimization of the softness of the steel plate. Figure 2 shows the YS of steel plate.
The results of an investigation into the relationship between the welding current range and the welding current range are shown below. The test materials were steel slabs with various C contents ranging from 0.002% to 0.4% (with the exception of Si:
0.01%, Mn: 0.1-0.3%, P: 0.01-0.02%,
S: 0.01~0.02%, N: 0.002~0.005%, Al: 0.01
~0.04%, Ti: 0.03%, Nb: 0.005%, B:
0.0007%) to 1100-1250℃, then finishing temperature: 700-1000℃, winding temperature: 450-700℃.
After hot rolling at ℃, cold rolling at a reduction rate of 60 to 85%, and then continuous annealing at a temperature range of 700 to 880 ℃ to obtain various YSs. Spot welding was carried out under the same conditions as in Fig. 1, except that the sample thickness was 0.7 mm, the welding time was 7 cycles, and the pressure was 175 Kgf. As is clear from the figure, the appropriate welding current range is strongly influenced by the YS of the steel plate, and when the YS becomes lower than 19 Kgf/mm ² , the appropriate welding current range shifts significantly to the high current side. In order to harden a steel sheet while maintaining good deep drawability, it is effective to add Ti, Nb, and B in combination to ultra-low carbon steel. Table 1 shows the results of investigating the mechanical properties of low carbon steels and ultra-low carbon steels with various compositions. The composition and manufacturing conditions of the test steel are the same as those shown in FIGS. 3 and 4. However, regarding the amount of Ti, Nb, and B, Ti: 0.02~0.04%, Nb: 0.005~
0.008%, B: 0.0005 to 0.0008%, as appropriate.

[Table] As is clear from the table, although the steel sheet containing the three elements Ti-Nb-B has significantly improved YS compared to other ultra-low carbon steel sheets, El and r The values are almost unchanged and there is no deterioration in deep drawability. In this respect, the low carbon steel sheet has almost the same YS level as the Ti-Nb-B composite addition steel sheet, but its deep drawability is significantly inferior to the ultra-low carbon steel. Next, FIG. 3 shows the results of investigating the hardness of welded parts when spot welding was performed on various steel plates shown in Table 1 above. As is clear from the figure, Ti according to the present invention
Although the -Nb-B composite steel sheet is an ultra-low carbon steel, it has a base metal hardness comparable to that of low carbon steel, but although it is an ultra-low carbon steel sheet, it has Ti,
When either Nb or B was missing, only low base material strength was obtained. The Ti--Nb--B composite addition steel obtained according to the present invention also has the advantage that the nugget portion has a higher hardness than other ultra-low carbon steels. In general, if the hardness of the spot weld or its vicinity is low, the spot weld will break before the base metal and the weld strength cannot be sufficiently increased. The hardness of the welded part of the carbon steel plate was insufficient. In this regard, once the hardness of the weld increases to such an extent that the base metal breaks before the spot weld, even if the hardness increases further, in principle it will not affect the strength of the spot weld. This invention steel plate and low carbon steel plate are
This corresponds to this state. However, simply adding Ti, Nb, and B does not necessarily mean that the above effects can be obtained; there are appropriate addition ranges for each component depending on several metallurgical interactions. First, we will show the results of investigating the coexistence effect of Nb and B. FIG. 4 shows the relationship between the amounts of Nb and B added and the hardness of the spot weld (nugget). The sample steel has a plate thickness of 0.8 mm and a C value of 0.0015 to 0.004.
%, Mn: 0.13-0.33%, S: 0.008-0.025%,
P: 0.011-0.018%, Al: 0.022-0.035%, N:
Based on 0.0011-0.0033%, Ti: 0.015-0.037%, B and Nb 0-0.0010%, 0 respectively.
The spot welding conditions were the same as those shown in FIG. As is clear from the figure, Nb: 0.001 to 0.010
%, B: In the range of 0.0001 to 0.0010%, the hardness of the welded part (nugget part) is high, especially when the above component composition range is satisfied, and the amount of B is (11/93)Nb±
Particularly good results were obtained in the range of 0.0004 (%). The above results show that the hardness of the spot weld is maximum when B and Nb exist in approximately the same number of atoms, indicating that some kind of interaction may exist between Nb and B in the steel. However, it cannot be determined at present whether this is a direct interaction between a substitutional solute atom and an interstitial solute atom in a solid solution state. Note that the change in the mother plate material due to the combined addition of Ti, Nb, and B is also considered to be due to the interaction of Nb and B described above. In other words, due to the above interaction, the grain size of the hot-rolled sheet becomes finer, and the grain size of the annealed sheet also becomes relatively finer, so YS increases, and at the same time, the fine homogenization of the grain size of the hot-rolled sheet increases the r value. improvement and
It is thought that this also brings about an improvement in El. Next, the reason why the component composition of the present invention is limited to the above range will be explained. C: To soften the steel and improve the El,r value,
It is advantageous to reduce the C content as much as possible.
If the C content exceeds 0.0040%, the material begins to deteriorate significantly, so the upper limit of the C content was set at 0.0040%. Si, Mn: Both Si and Mn contribute effectively as deoxidizing agents, but if they are contained in excess, they will cause damage to ductility.
was limited to 0.5%. P, S: Both are impurity elements, and it is desirable to reduce them as much as possible, but both are acceptable if they are about 0.025% or less. N: Like C, N not only reduces workability but also deteriorates aging resistance, so it was limited to a range of 0.004% or less. Al: It is necessary to add 0.01% or more as a deoxidizing agent. However, adding too much leads to an increase in inclusions, which has a significant negative effect on the quality of the material, so the upper limit was set at 0.10%. Nb: Nb is a useful element that, when coexisting with B, refines the structure of the spot weld and increases the hardness of the weld. Moreover, the combined addition of Nb and Ti ensures high El and high r values and also effectively contributes to improving YP. The effect appears above 0.001%, but 0.010%
If it exceeds %, it will cause an excessive increase in YP and a decrease in El, so limit it to 0.001 to 0.010%. However, as the ratio of Nb to Ti increases, NbC
The amount of Nb precipitated increases and the material deteriorates easily, and the hot rolling winding temperature must be set to a high temperature of 600°C or higher, so the addition of Nb needs to be reduced in balance with Ti. In particular, when the atomic ratio of Nb to Ti exceeds 0.2, the deterioration of the material becomes significant, so it is necessary to set the atomic ratio to Nb/Ti<1/5, or the weight ratio to Nb<1/5・(93/48)Ti. be. Figure 5 shows the results of investigating the influence of Nb/Ti (atomic ratio) on El. As is clear from the figure, when Nb/Ti becomes 0.2 or more, El decreases rapidly. B: Adding a small amount of Nb in the presence of Nb is useful for increasing the strength and YS of spot welds and base metals. The results can be seen when adding 0.0001% or more, but since adding too much leads to deterioration of the material, the upper limit was set at 0.0010%. However, in order to fully and satisfactorily exhibit the above effects, it is not enough for the amount of B to simply satisfy the above range; as shown in Figure 4 above, it is necessary to balance the amount of Nb (11 /93)Nb−0.0004≦B≦(11/93)Nb
It is important to limit the range to +0.0004. Ti: The above-mentioned effect of improving spot weldability by Nb and B cannot be realized without the presence of Ti, as shown in FIG. 5 above. This means that in order for Nb and B to have sufficient interaction, N,C
This is because most of the elements in the steel, such as Nb or B, which are fixed as precipitates, need to be fixed with Ti. For this reason, it is necessary to add Ti at least C+N (number of atoms).
Ti/(48/12・C+48/14・N)>1 must be satisfied. Furthermore, unless the absolute amount of Nb and B is added in an amount of 0.01% or more, the effect of adding Nb and B will not be fully exhibited because the solid solution elements will be insufficiently fixed. Also, regarding deep drawability, Ti≧0.01%
Although a high r value and a high El value can be obtained, excessive addition of Ti has an adverse effect on adjusting YS by temper rolling. Therefore, the upper limit of Ti addition was set at 0.04%. The presence of an appropriate amount of Ti has the effect of suppressing the appearance of fine precipitates containing Nb, so it is necessary to raise the coil winding temperature after hot rolling (>600℃) as with normal Nb addition. It is economically advantageous because there is no such phenomenon, and excessive softening due to crystal grain growth can also be prevented. Based on the above findings, Ti is 0.01 to 0.04% and Ti/(48/
12.C+48/14.N)>1. In order to maximize the above effects,
It is more advantageous to add the Ti content to the minimum necessary limit. Figure 6 summarizes the results of investigating the effect of Ti content on weld hardness over a wide range of ingredients. Here, the component range and welding conditions were almost the same as in the case of FIG. 4. As a result, the data could be roughly divided into three categories based on the range of Ti content. That is, Ti≦(48/12・C+48/
In the range of 14 N), there were some with high hardness and some with very low hardness, and there was a large variation. The reason for this is probably that the yield of B decreased due to the decrease in Ti content, and the interaction effect of Nb-B was insufficient. on the other hand
When Ti>(48/12・C+48/14・N), the hardness was at least Hv≧180, which was sufficient hardness. And among them, Ti<(48/12・C+
48/14・N+48/32・S), it was confirmed that the hardness of the welded part was stable at a very high level. This is because sufficient hardness can be obtained if Ti is added to the minimum necessary amount, that is, at least equivalent to C and N.
Furthermore, it has been shown that when S is added in an amount greater than the equivalent amount, the hardness of the welded part tends to decrease. The reason for this is thought to be that when Ti is present in a sufficient (excess) amount relative to C, N, and S, the effect of Nb, which would form precipitates with some C, is almost eliminated. Therefore, in this invention, Ti>(48/
12・C+48/14・N), a certain effect can be obtained, but in order to obtain an even better effect, the amount of Ti should be adjusted to Ti<( 48/12・C+48/14・N+48/32・
S) is preferably limited to a narrow range. Note that even when Ti, Nb, and B are added, C,
If the contents of N and B were too low, the hardening of the welded part was insufficient. Figure 7 shows the effects of interstitial solid solution elements C, N, and B on the weld hardness of various steels.
The results of the study on the impact of In the figure, C+ means that all elements are converted to C content.
12/14・N+12/11・B was taken on the horizontal axis. As is clear from the figure, C+12/14・N+
When 12/11.B is less than 38 ppm, the effect of refining the structure becomes insufficient and sufficient weld hardness cannot be obtained. Therefore, in this invention, C, N and B are contained in an amount of C+12/14.N+12/11.B of 38 ppm or more. Next, the reasons for limiting the manufacturing conditions of this invention will be explained. The relationship between YS and the appropriate welding current range for ultra-low carbon steel is as shown in Figure 2 above, but even though the compositions are the same, the appropriate current range shifts to a higher value due to a decrease in YS. . Accordingly
Changes in the welding current value can be suppressed by increasing YS, but at this time it is important not to deteriorate other characteristics such as the r value and El. Therefore
In the Ti-Nb-B system, the following restrictions on manufacturing conditions are required to ensure good material quality. That is, when hot rolling a steel slab adjusted to the proper composition according to the present invention, it is necessary to finish roll it at 700 to 900°C and then wind it at a temperature in the range of 300 to 600°C. Here, the lower limit of finishing temperature is r due to residual strain.
The upper limit was determined from the viewpoint of suppressing the deterioration of the r value, and from the viewpoint of preventing the deterioration of the r value due to coarsening of crystal grains. Furthermore, if the winding temperature becomes too high, the effect of improving weldability due to the coexistence of Nb-B will be significantly weakened, so the upper limit of the winding temperature was set at 600°C. However, if the winding temperature is too low, it will interfere with subsequent steps, so the lower limit was set at 300°C. Next, the purpose of cold rolling is to impart an appropriate cold strain necessary for forming a recrystallized texture. Therefore, the lower limit of the rolling reduction ratio was set at 60% to obtain sufficient rolling strain. However, if the rolling reduction ratio is too high, the load on the rolling mill becomes too large and productivity decreases, so the upper limit of the rolling reduction ratio was set at 85%. Regarding the CAL annealing temperature, it needs to be higher than the recrystallization temperature. However, if the annealing temperature is too high, the steel becomes too soft, making it difficult to obtain the desired effect of the present invention, so the upper limit was set at 780°C. Next, in this invention, skin-pass rolling is performed, and this skin-pass rolling process is particularly important in this invention. FIG. 8 shows the results of an investigation into the influence of the temper rolling reduction rate on the weldable limit current value. The lower limit of weldable current was investigated using deep drawing mild steel plates of various compositions with a thickness of 0.7 mm as test materials. As is clear from the figure, the effect of temper rolling reduction is particularly large for Ti-Nb-B steel, and at a reduction rate of [plate thickness (mm) + 0.1]% or more, welding is more effective than for low carbon steel. A phenomenon in which the lowest possible current value became lower was observed. Next, the effect of temper rolling reduction on low cycle fatigue properties was investigated. All test materials were cold-rolled annealed steel plates with a thickness of 0.8 mm, and steel A was a common steel plate.
0.04%C low carbon Al killed steel, B steel is 0.003%C
Ultra-low carbon steel without the addition of Ti, Nb, etc., and C
The steel contains 0.002% C-, which satisfies the composition range of this invention.
It is 0.031%Ti-0.007%Nb-0.0006%B steel. The temper rolling reduction rate was set to the standard 0.3%.
However, although D steel is a steel manufactured under the same composition and conditions as C steel, the skin pass rolling reduction rate was set to be as high as 1.3% only for this D steel. Welding was carried out for 8 cycles, welding current: 7.5kA, and pressure: 200Kgf. The additional mode in the fatigue test was 0-Tension, that is, complete oscillation tensile shear fatigue. And the test stop is JIS Z
3136, when a fatigue crack with a length equivalent to the nugget diameter was observed from the steel surface. The results obtained are shown in FIG. As is clear from the figure, the fatigue strength of Steel B, which is an extremely low carbon steel, is lower than that of Steel A, which is a normal low carbon steel. In this regard, although steel C contains Ti-Nb-B and has a low temper rolling reduction of 0.3%, although its fatigue strength in the high cycle range is somewhat improved, the fatigue strength in the low cycle range is still low. low. On the other hand, in steel D, in which the temper rolling reduction was set to a high value, the fatigue strength not only in the high cycle region but also in the low cycle region was significantly improved. And this effect is similar to the case in Figure 8.
It was confirmed that excellent properties can be obtained when the Ti--Nb--B type material is temper-rolled under high pressure. Therefore, in this invention, the rolling reduction ratio: [plate thickness (mm) +
It became necessary to perform temper rolling with a steel content of 0.1% or more. However, if the rolling reduction rate becomes too high, the deterioration of the material will become significant, so the upper limit is 3.0.
%. The reason why skin pass rolling at the above rolling reduction effectively improves the fatigue properties of spot welds has not been clearly elucidated, but the residual stress during skin pass rolling is It is presumed that changes in the distribution in the thickness direction have some influence. (Function) The functions and effects of the combined addition of Ti, Nb, and B, which are particularly important in this invention, are summarized as follows. First, Ti is necessary to ensure a certain material quality and to fix N. Nb supplements the material quality improvement effect of Ti, and B
The coexistence of this material greatly contributes to the remarkable refinement of the structure. Although B alone has almost no effect on refining the structure, its effect is remarkable when it coexists with Nb.
However, since the effect of microstructural refinement due to the coexistence of Nb-B is very strong, it is important to suppress the content of both to the necessary minimum and balance of each component. The reason why such an effect is obtained has not yet been clearly elucidated, but it is thought to be as follows. During spot welding, a portion of the steel plate melts, and the temperature in the vicinity also becomes quite high. At that time, in general, the crystal grains of ultra-low carbon steel become significantly coarsened. This is the reason why the structure of the welded part of ultra-low carbon steel was not healthy in the past, and was the biggest reason why the strength of the welded part was low. However, in the steel of this invention, it has been confirmed that the structure near the welded part does not become coarser, but rather becomes finer. This is δ→γ or γ
→It is presumed that this is because the Nb-B atom pair strongly suppresses the nucleation and growth of transformation during α transformation. Here, the structure of the weld zone is not an equiaxed grain, but an acicular structure,
It shows an extremely rare structure for an ultra-low carbon steel. The greatest feature of this invention is that this microstructure effect can be obtained within a range that does not cause deterioration of the material. For information on steel sheets additively added with Ti, Nb, and B for the purpose of improving deep drawability, secondary work brittleness, etc., and the manufacturing direction thereof, please refer to Japanese Patent Publication No. 60-47328,
It has been proposed in JP-A-59-74232, JP-A-59-190332, JP-A-59-193221, and JP-A-61-133323, but all of them are based on Ti, Nb, and B, respectively. This is an attempt to obtain good deep drawability by utilizing the action and effect, and it cannot be expected to improve spot weldability or the fatigue properties of spot welds, which is an important aspect of this invention. . For example, in the above publications, B is added only to improve bake hardenability and secondary workability, while Nb is added to suppress room temperature aging.
Furthermore, Ti is added primarily for the purpose of improving material quality. Therefore, the effect of adding each component is, in principle, simply additive;
The limiting conditions of Ti+Nb<0.04% in JP-A-133323 and Ti+Nb<0.06% in JP-A-59-190332 and JP-A-59-193221 are only for adding good chemical conversion treatability to the steel sheet. do not have. Therefore, the idea of a composite addition that takes into account the interaction of Ti, Nb, and B to achieve the excellent spot weldability that is the objective of this invention is not seen at all in these proposals, and it is no surprise that However, B in this invention: 0.0001 to 0.0010%, Nb: 0.001
~0.010%, Ti: 0.01~0.04%, and B: (11/
93) Nb−0.0004~(11/93)Nb+0.0004%, Ti
(48/12・C+48/14・N>1, Nb<1/5・
(93/48) Only a steel plate with a composition different from the composition specified for Ti is disclosed. This becomes even clearer when looking at the examples of the above-mentioned publications. Furthermore, the scope of claims related to Ti in each of the above publications, Ti<48/14・
It is clear from the fact that N and Ti<48/12.C+48/14.N do not satisfy the present invention at all. Needless to say, since the target steel sheet properties and composition systems are different, the manufacturing process of the steel sheets is also different. For example, taking the winding temperature as an example, JP-A-59-74232 requires a temperature of 650°C or higher;
-47328, JP-A-59-190332, JP-A-59-193221
No. 61-133323 also recommend winding at a temperature exceeding 600°C. It is well known that increasing the winding temperature improves the material quality to some extent, but it also brings with it various disadvantages such as reduced descaling and surface properties. This also aims to improve the drawbacks that occur when such high-temperature winding is applied. (Example) Example 1 A continuous cast slab having the composition shown in Table 2 was
After heating to 3.2°C, hot finish rolling was performed at 880°C.
After forming a hot-rolled sheet with a thickness of mm, it was rolled up at 550℃. Then, it was cold-rolled at a reduction rate of 75% to form a cold-rolled sheet of 0.8 mm, and then continuously annealed at a temperature of 750°C.
Thereafter, temper rolling was performed at a reduction rate of 1.2%. Table 3 shows the results of examining the mechanical properties, minimum appropriate welding current, and welding strength of each steel plate thus obtained. In addition, each mechanical property is calculated from the rolling direction and from the rolling direction.
1 in the direction of 45℃ and in the direction of 90° from the rolling direction:
It is shown as an average value with a ratio of 2:1. In addition, spot welding uses a CF type electrode with a diameter of 4.8 mm, and welding time: 8
The welding was performed at a cycle and pressure of 200Kgf, and the welding strength was evaluated using the value at a welding current of 7.5kA.

【table】

【table】

【table】

[Table] As is clear from Table 3, Ti according to the present invention
-Nb-B composite additive ultra-low carbon steel sheet (steel types A to E)
In all cases, not only the values were good and the deep drawability was excellent, but also the lower limit of the appropriate welding current value was wide in spot welding, and the spot welding strength was also sufficient. On the other hand, all of the comparative examples which deviated from the appropriate range of the present invention had poor spot weldability. Example 2 Steel slabs having the composition of steel type A in Example 1 were processed under various conditions shown in Table 4 to obtain cold rolled plates (all plate thicknesses were 0.8 mm). Table 5 shows the results of examining the mechanical properties and spot weldability of each of the cold-rolled sheets thus obtained.

【table】

[Table] As is clear from Table 5, all of the steel plates (Nos. 1 to 3) obtained according to the method of the present invention not only have excellent deep drawability but also have high low cycle welding fatigue strength. While excellent spot weldability was exhibited, in cases where the manufacturing conditions were outside the appropriate range (Nos. 4 and 5), both mechanical properties and spot weldability were poor. (Effects of the Invention) Thus, according to the present invention, it is possible to obtain an ultra-low carbon cold-rolled steel sheet that has excellent spot weldability and particularly high weld fatigue strength without impairing formability.

[Brief explanation of the drawing]

Figure 1 is a graph showing the effects of the addition of Ti, Nb, and B on spot weldability, Figure 2 is a graph showing the relationship between YS of steel plate and appropriate welding current range, and Figure 3 is Figure 4 is a graph showing the effect of the addition of Ti, Nb, and B on the hardness of the weld zone.
Figure 5, a graph showing the relationship between the addition amount of B and the hardness of the spot weld, shows the effect on El of the steel plate.
A graph showing the influence of Nb/Ti. Figure 6 is a graph showing the influence of Ti content on weld hardness.
Fig. 7 is a graph showing the influence of C, N, and B on the weld hardness, Fig. 8 is a graph showing the relationship between skin pass rolling reduction rate and lower limit of weldable current, and Fig. 9 is a graph showing the influence of C, N, and B on the weld hardness. Graph showing the influence of steel components and temper rolling reduction on the fatigue strength of spot welds, No. 10
The figure shows a comparison of the range of appropriate welding conditions for conventional low carbon steel and ultra-low carbon steel.
It is a graph showing the relationship between YS and electrical resistance value of a steel plate.

Claims (1)

[Claims] 1 C: 0.004wt% or less, Si: 0.1wt% or less, Mn: 0.5wt% or less, P: 0.025wt% or less, S: 0.025wt% or less, N: 0.004wt% or less, Al. : 0.01 to 0.10wt%, Ti: 0.01 to 0.04wt%, Nb: 0.001 to 0.010wt% and B: 0.0001 to 0.0010wt%, and the following formulas (1) to (4) (11/93)Nb− 0.0004≦B≦(11/93)Nb＋0.0004……(1) Ti＞(48/12)C+(48/14)N……(2) Nb＜1/5・(93/48)Ti…… (3) C+(12/14)N+(12/11)B>0.0038...Contains within the range that satisfies (4), and the remainder is substantially
Finishing temperature of steel slab with Fe composition: 700~
Hot rolled at 900℃, coiling temperature: 300~600℃, then cold rolled at a reduction rate of 60~85%, and then continuously annealed at a temperature range of above the recrystallization temperature and below 780℃. After treatment [Plate thickness (mm) + 0.1]
A method for producing ultra-low carbon cold-rolled steel sheets with excellent fatigue properties at spot welds, characterized by performing temper rolling at a rolling reduction of % or more and 3.0% or less.