JP3440894B2 - High strength hot rolled steel sheet excellent in stretch flangeability and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet suitable for use in automobile bumper parts, impact beam and other strength parts, and particularly has a tensile strength TS of 780 MPa or more and stretch flangeability. The present invention relates to an excellent high-strength hot-rolled steel sheet and a method for producing the same.

[0002]

2. Description of the Related Art With the recent movement toward weight reduction of automobile bodies, attention has been paid to the expansion of application sites of high-strength steel sheets. Among them, high-strength steel sheets with a strength of more than 1000 MPa are put into practical use for bumper parts, impact beams, etc. used to suppress the deformation of the cabin during a collision. In reality, all of these steel plates are manufactured through a cold rolling process, except for thick products having a plate thickness of more than 3.2 mm. The main reason for this is that in the case of a cold rolled steel sheet, out-of-plane deformation of the steel sheet, that is, disorder of the shape, can be suppressed relatively easily by the in-furnace roll during continuous annealing, and the shape of the product is good. Is. On the other hand, plate thickness 3.
As a high-strength steel sheet with a thickness of 2 mm or less, particularly 3.0 mm or less, the main reason why hot-rolled steel sheets could not be applied until now is that in the cooling process after hot rolling,
This is because effective tension could not be applied to the steel sheet, and the disorder of the steel sheet shape could not be suppressed unlike the cold-rolled steel sheet.

The reason why the hot-rolled steel sheet has not been put into practical use as a thin high-strength steel sheet in the above plate thickness range is that it is disadvantageous in securing the material in addition to the above-mentioned steel sheet shape. That is, it is usually difficult to obtain a uniform and fine structure equivalent to the case of cold rolling / annealing in the state of hot rolling, and this difference is reflected in the difference in workability between the two. However, this difference is most noticeable in the local ductility represented by stretch flange processing, which is also reflected in bending processing, hole expanding processing, and the like. By the way, regarding the hole expanding workability of the high-strength hot-rolled steel sheet, some proposals have been made so far. For example, JP-A-61-19733 and JP-A-62-196336 disclose that a bainite phase is excellent as a metal structure in consideration of stretch flangeability. This is because the hole expandability (stretch-flange formability) is improved when a simple C-Si-Mn-based component system is subjected to hot rolling and then accelerated cooling to form a structure mainly composed of bainite. is there. On the other hand, as a means of achieving high tensile strength without performing accelerated cooling after hot rolling, hardenability-improving elements such as Cu, Ni, Cr, and Mo, which have been conventionally adopted in the field of thick steel plates, are used. A method of adding is known.

[0004]

However, the hole expandability of the steel sheet manufactured by the manufacturing method disclosed in the above-mentioned JP-A-61-19733 and JP-A-62-196336 is surely the ferrite-marten. Although it is superior to steel sheets with a site structure, it has not yet reached the level where it can fully meet today's required levels. Moreover, in this conventional technique, the variation of the structure is relatively sensitive to the variation of the hot rolling condition, so that the variation of the material tends to be large, which hinders the continuous or automated press forming. It was also

In addition, the method of adding hardenability improving elements such as Cu, Ni, Cr and Mo requires a large amount of expensive alloy elements, which is not only disadvantageous in terms of cost but also in terms of recycling. There was also the problem of making scrap management complicated. Further, in this method, it is necessary to add the alloying elements to such an extent that the martensite single phase is completely obtained, and if the addition amount is insufficient, the resulting structure is a mixed structure of ferrite and martensite, Alternatively, a structure containing a small amount of pearlite or bainite in part was obtained, and it was not easy to obtain the desired good stretch flangeability.

As described above, the tensile strength is 780 MPa or more,
Especially, high-strength hot-rolled steel sheet of 780 to 1300MPa with a thickness of 3.0
Manufactured as a normal hot-rolled steel sheet from a thin sheet of mm or less 3.
It has been extremely difficult to produce a hot-rolled steel sheet having high strength and good stretch flangeability in a wide thickness range up to a thickness of more than 0 mm, and having a quality that can be practically used.
Therefore, it is strongly desired to develop a manufacturing technique for hot-rolled steel sheet that solves such a problem, and in particular, in view of cost reduction of the steel sheet, the advent of a technique for producing a low-alloy-based composition in which the amount of alloying elements is minimized. Was required .

Therefore, the object of the present invention is to solve the problems of the prior art and to provide excellent stretch flangeability.
And to provide a high-strength hot-rolled steel sheet and a manufacturing method thereof thin that. Another object of the present invention is to provide an inexpensive manufacturing technique capable of manufacturing such a high-strength hot-rolled steel sheet for a hot-rolled steel sheet having a plate thickness of 3.5 mm or less even with a low alloy composition. is there. Furthermore, the present invention is, as a target value of the specific properties of the steel sheet, and an object thereof is to provide a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more above.

[0008]

Means for Solving the Problems In order to achieve the above-mentioned object, the inventors have conducted intensive studies and studied from the aspects of steel composition, manufacturing conditions and the like. As a result, in terms of material, even if expensive alloying elements are not necessarily used, if the steel is adjusted to have an appropriate composition range, it can be manufactured by combining it with appropriate hot rolling-cooling conditions. It was found that a structure mainly composed of inite was formed and good mechanical properties were stably obtained. It was also found that, among these manufacturing conditions, it is particularly important to control the cooling pattern after hot rolling. That is, until now, cooling on the hot run table has focused on the coiling temperature and the average cooling rate from the start of cooling to the coiling temperature, but the cooling rate in each temperature range (that is, the hot run table). The cooling rate at each position on the table) was not considered. However, according to such a conventional method, the mechanical properties largely fluctuate due to the change of the metal structure, and it is impossible to obtain a material having a uniform material that can be practically used. On the other hand, the inventors of the present invention continuously cool the coiling temperature after hot rolling to a coiling temperature on the hot run table without interruption in the middle while maintaining a predetermined cooling rate (relatively slow cooling). Have been found to be extremely effective in solving the above problems. Then, it was concluded that the desired target can be achieved by combining this with appropriate steel composition and hot rolling conditions (slab heating temperature, finish rolling temperature).

The present invention has been completed based on such findings, and the gist thereof is as follows. (1) C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, Mn: 1.5 to
3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al:
0.150 wt% or less, N: 0.0200 wt% or less, and Nb:
It contains 0.001 to 0.20 wt% and Ti: 0.005 to 0.20 wt%, either one or two, and the balance is composed of Fe and unavoidable impurities, and has a metal structure of fine bainite with an average grain size of 3.0 μm or less. A high-strength hot-rolled steel sheet having excellent stretch-flangeability, which is characterized by having a structure having an area ratio of 90% or more .

(2) A high-strength hot-rolled steel sheet excellent in stretch-flangeability, characterized in that, in the steel sheet described in (1) above, B: 0.0005 to 0.0040 wt% is further contained in addition to the above components.

(3) In the steel sheet according to (1) or (2) above, in addition to the above components, Cu: 0.02 to 1.0 wt%,
Ni: 0.02-1.0 wt%, Cr: 0.02-1.0 wt%, Mo: 0.02
~ 1.0 wt% of 1 type or 2 or more types in total 1.
A high-strength hot-rolled steel sheet having excellent stretch-flange formability, characterized by containing 0 wt% or less.

(4) In the steel sheet according to any one of (1) to (3) above, in addition to the above components, Ca: 0.0005 to
A high-strength hot-rolled steel sheet having excellent stretch flangeability, which is characterized by containing 0.0050 wt%.

(5) The high-strength hot-rolled steel sheet according to the above (1) to (4), which has a composition having an aspect ratio of bainite grains of 1.5 or less and is excellent in stretch flangeability.

(6) C: 0.05 to 0.30 wt%, Si: 1.0 wt%
Below, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S:
0.005 wt% or less, Al: 0.150 wt% or less, N: 0.0200 wt
% Or less, Nb: 0.003 to 0.20 wt%, Ti: 0.005
~ 0.20wt% of a steel slab containing one or two of them is heated at 1200 ° C or less, then hot-rolled at a finish rolling end temperature of 800 ° C or more, and within 2 seconds after the rolling is finished. Of high-strength hot-rolled steel sheet with excellent stretch-flange formability, which is characterized by starting cooling at a cooling rate of 20 to 150 ° C / sec and continuously cooling to the coiling temperature and coiling at 300 to 550 ° C. Production method.

(7) C: 0.05 to 0.30 wt%, Si: 1.0 wt%
Below, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S:
0.005 wt% or less, Al: 0.150 wt% or less, N: 0.0200 wt
% Or less, Nb: 0.003 to 0.20 wt%, Ti: 0.005
~ 0.20wt% of the steel slab containing one or two of them is heated at 1200 ℃ or less, then the finish rolling start temperature is 950 to 1050 ℃ and the finish rolling end temperature is 800 ℃ or more. Rolling, starting cooling within 2 seconds after the completion of rolling, continuously cooling to a coiling temperature at a cooling rate of 20 to 150 ° C / sec, and coiling at 300 to 550 ° C. A method for producing a high-strength hot-rolled steel sheet having excellent flangeability.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons for limiting the above-mentioned gist structure will be described. C: 0.05 to 0.30 wt% C is an element effective for strengthening the transformation structure, and its effect is exhibited by adding 0.05 wt% or more. However, if it exceeds 0.30 wt%, the nugget portion of the spot welding becomes significantly hard and the weldability deteriorates, making it difficult to apply it as a steel sheet for automobiles. Therefore, the C content is limited to the range of 0.05 to 0.30 wt%. From the viewpoint of stabilizing the tensile properties of the steel sheet,
A range of 0.20 wt% or less is preferable.

Si: 1.0 wt% or less Si is a useful element used for increasing the temper softening resistance when utilizing transformation structure strengthening. On the other hand, Si
Has the effect of increasing the hot deformation resistance of steel, 1.0 wt%
If it is added over the range, the tendency becomes particularly remarkable, and it becomes difficult to carry out hot rolling to a thin product aimed at by the present invention. Therefore, the amount added is 1.0 wt% or less. For applications where surface scale defects are a problem, it is desirable to control the content to 0.8 wt% or less.

Mn: 1.5 to 3.5 wt% Mn is an element effective in preventing hot cracking due to S, and it is desirable to add it in accordance with the amount of S. Mn is
Since it has the effect of refining the crystal grains, it is also necessary from the standpoint of improving the material quality. In particular, in the present invention, in the low-temperature transformation phase mainly composed of bainite, the strength of the steel is increased by the hardenability improving action of Mn, and TS: 780 M
Secure at least Pa. In order to exert these effects, at least 1.5 wt% must be added. As the amount of Mn added increases, more stable strength can be obtained and the uniformity of the material also improves. However, even if added in excess of 3.5 wt%, not only the effect is saturated, but also hot deformation resistance increases, making it difficult to reduce the thickness by hot rolling.
Further, excessive addition deteriorates the weldability and the formability of the welded portion. For these reasons, the upper limit of the amount added is 3.5 wt%. For applications where better corrosion resistance and moldability are required, a range of 3.2 wt% or less is desirable.

P: 0.02 wt% or less P is generally added as a solid solution strengthening element of the ferrite phase in a high-strength steel sheet having a relatively low strength two-phase structure. It is not possible to expect solid solution strengthening of P in steel sheets with a pressure of MPa or more. When the content of C, Mn, etc. is large, P hardens the steel and reduces the stretch flangeability. Furthermore, P has a strong tendency to segregate at a specific position in the plate thickness direction, and causes weld embrittlement due to segregation. For these reasons, the P content is 0.02 wt% or less,
Preferably, it should be limited to 0.01 wt% or less.

S: 0.005 wt% or less S is a harmful element which exists as an inclusion in the steel, reduces the ductility of the steel sheet, and causes deterioration of corrosion resistance.
In particular, as in the present invention, when the strength is high, the notch sensitivity increases, so it is necessary to reduce the amount of MnS-based inclusions that are stress concentration sources as much as possible. Therefore, it is necessary to reduce S as much as possible, and the upper limit is 0.005 wt%. It should be noted that 0.00 is used in applications where particularly good workability is required.
It is desirable to set it to 2 wt% or less.

Al: 0.150 wt% or less Al is an element added as a deoxidizing element and is useful for improving the cleanliness of steel and refining the structure. In order to exert such effects, it depends on the deoxidation technology of molten steel, but it is generally necessary to add 0.010 wt% or more. However, excessive inclusion causes deterioration of surface properties and reduction of strength of the steel sheet.
Therefore, Al is added in the range of 0.150 wt% or less. In addition,
From the viewpoint of material stability, it is desirable to add in the range of 0.010 to 0.080 wt%.

N: 0.0200 wt% or less When N exceeds 0.0200 wt%, the heat ductility of the steel is lowered, internal defects and surface defects of the steel sheet are likely to occur, and slab cracking during continuous casting is caused. Increase the occurrence of. Therefore, the upper limit of N is 0.0200 wt%. The range of 0.0020 to 0.0150 wt% is preferable from the viewpoints of material stability and yield improvement in consideration of the entire manufacturing process. Since this N has the effect of lowering the transformation point of steel, it is effective to add it within the above range when it is desired not to roll at a transformation point greatly in the production of a thin steel sheet.

Nb: 0.003 to 0.20 wt%, Ti: 0.005 to 0.20
wt% These elements are extremely important elements that contribute to the refinement and homogenization of the structure. In the present invention, by combining with a relatively low temperature slab heating temperature, the target fine crystal structure of 3.0 μm or less is obtained. To be able to obtain. This effect is at least 0.003 wt% for Nb and Ti
Although it can be obtained by adding 0.005 wt% or more, if any element is added in excess of 0.20 wt%, the effect is saturated and the risk of slab cracking during continuous casting increases. Therefore, Nb is 0.003 to 0.20 wt and Ti is 0.005 to 0.
Add in the range of 20wt%.

B: 0.0005 to 0.0040 wt% B, which will be described below as an optional additive element, effectively contributes to the refinement of the microstructure of the steel sheet and suppresses the ferrite transformation of the steel. Therefore, in order to obtain a high strength steel sheet, It is extremely effective. These effects are exhibited by adding 0.0005 wt% or more. On the other hand, even if added in excess of 0.0040 wt%, the effect will be saturated. Therefore, B is 0.0005
Add as needed in the range of ~ 0.0040wt%.

Cu: 0.02 to 1.0 wt%, Ni: 0.02 to 1.0 wt%
%, Cr: 0.02 to 1.0 wt%, Mo: 0.02 to 1.0 wt% in a total amount of 1.0 wt% or less. These elements delay the transformation after the completion of hot rolling, and the strengthening by the transformation structure can be effectively utilized. The strength of the steel sheet can be increased. The effect is obtained by adding 0.02 wt% or more, but if added excessively, it increases the deformation resistance during hot rolling, deteriorates chemical conversion treatability and surface treatability in a broader sense, and further hardens the weld. This leads to a decrease in weldability. Therefore, the addition amount of these elements is 1.0 wt for each element.
%, And the total amount is 1.0 wt%. It should be noted that any of the elements behaves in the same manner when added alone or in combination.

Ca: 0.0005 to 0.0050 wt% Ca is an element useful for detoxifying S in steel.
In the case of a structure mainly containing fine bainite with a relatively large Mn content, the improvement of stretch-flange formability by adding Ca is particularly large. Although such effects are exhibited by adding 0.0005 wt% or more, addition of more than 0.0050 wt% not only saturates the effect, but rather tends to deteriorate the surface properties and deteriorates the surface treatment characteristics. There is a risk of Therefore, the amount of Ca added should be in the range of 0.0005 to 0.0050 wt%. Considering the balance of various qualities, 0.0010 to 0.0035wt
The range of% is preferred.

Fine Bainite Structure The metal structure in the present invention must be mainly composed of fine bainite (90% or more in area ratio is bainite). The distinction between bainite and martensite (not tempered) is judged from the strength difference, but it is difficult to distinguish between bainite and tempered martensite. Therefore,
In the present invention, paying attention to the precipitation state of carbide, bainite when the carbide is mainly precipitated in the grain or at the lath boundary, on the other hand, tempered martensite when it is frequently precipitated also in the former austenite grain boundary And When the relationship with the stretch-flange formability was examined by such a structure judging standard, the structure mainly composed of bainite showed much better stretch-flange formability even with the same strength. The reason for this was presumed to be that carbides precipitated at the former austenite grain boundaries, and also coarsely precipitated carbides, adversely affect this property.

Average Grain Size and Aspect Ratio of Bainite Structure The finer the bainite structure, the better the stretch flangeability, and in this sense, the regulation of the crystal grain size is also an important requirement. The average grain size of the bainite structure here is
It is calculated according to the method for determining the average grain size of ferrite (JIS G0552), and is calculated from the average of the values measured over the entire thickness of each plate thickness section in the rolling direction and the direction orthogonal to the rolling direction. When the average particle diameter measured in this way is 3.0 μm or less, stretch flangeability is remarkably improved. Even in the conventional precipitation-strengthened steel plate, there is an example in which a bainite structure of 3.0 μm or less is partially obtained, but it partially contains a coarse structure, and the average grain size value is 3.0 in the entire plate thickness. μ
None of them fell below m. The bainite structure preferably has no mixed grains, that is, no coarse grains having a grain size of more than 10 μm are present. Further, the average grain size of the bainite structure is preferably 2.5 μm or less when a better stretch flangeability is required. The aspect ratio of bainite grains is preferably 1.5 or less from the viewpoint of workability. Here, the aspect ratio refers to the ratio of the major axis and the minor axis of bainite grains, the major axis generally in the rolling direction and the minor axis in the plate thickness direction.

FIG. 1 shows the results of examining the relationship between stretch flangeability (hole expansion test) and average grain size of bainite structure. The test steel plate is 0.08 wt% C-0.21 wt% Si-3.
0 wt% Mn-0.040 wt% Al-0.0030 wt% N-0.025 wt% Nb
A steel slab of -0.015 wt% Ti-0.0025 wt% B-0.0020 wt% Ca was used, with the slab heating temperature varied from 950 to 1300 ° C, the finish rolling temperature from 750 to 980 ° C, and the cooling rate varied from 10 to 200 ° C. By adjusting the winding temperature, bainite can be adjusted to 90%.
% Hot rolled steel sheet with a thickness of 2.0 mm (tensile strength TS is 960 to 1200 MPa). From FIG. 1, it can be seen that the hole expandability is remarkably improved by setting the average grain size of the bainite structure to 3.0 μm or less. In addition, it has been confirmed that this hole expansion ratio does not simply correlate with TS,
Even if the TS is the same, the finer structure can improve the hole expansion ratio. The hole expansion test complies with the standards of the Iron and Steel Federation, and punches out a 10 mmφ hole (clearance 12.5
% Constant), and molding with a conical punch having an apex angle of 60 °.

[0030]

Next, manufacturing conditions will be described. The slab is preferably manufactured by a continuous casting method in order to prevent macroscopic segregation of the components, but it is also possible to manufacture it by an ingot making method or a thin slab casting method. After manufacturing, the slab is cooled to room temperature and then reheated.In addition to the conventional process, the slab is inserted into a heating furnace as a hot piece and then rolled, or it is directly fed or slightly heat-retained. Energy-saving process steps such as direct rolling can be applied without any problems. However, from the viewpoint of uniform and finer initial structure, even when direct rolling is performed,
It is desirable to reheat after finishing the γ → α transformation.

Slab heating temperature (SRT): 1200 ° C. or less The slab heating temperature has a great influence on the γ grain size. Conventionally,
In the production of high-strength steel sheet such as the present invention, Nb, in the steel of the component added carbonitride forming elements such as Ti,
In order to effectively use the precipitation strengthening, it is necessary to completely solution-treat these elements in the initial state, and the SRT is generally set to a high temperature of about 1250 ° C or higher. On the other hand, the present inventors regulated the SRT to 1200 ° C. or less even though it is a high-strength steel containing Nb and Ti, so that a part of the added Nb and Ti remains in an undissolved state. It was found that the homogeneity and miniaturization of the hot rolled structure are remarkably improved by this. In this case, the deformation resistance tends to increase as compared with the conventional high SRT method, but the degree of increase in the deformation resistance is relatively small because the dynamic recrystallization occurs in the rough rolling process. In the present invention, although the precipitation strengthening action may be reduced, the reduced amount can be compensated by making the structure a uniform and fine bainite-based structure, and it is also advantageous in that the structure is significantly refined and uniformized. It brings the point. In order to make the structure more uniform and finer, it is desirable that the SRT is in the range of 1100 ° C. or lower, and further 1050 ° C. or lower.

Finish rolling start temperature (temperature at the entry side of the finish rolling mill): 950 to 1050 ° C. In the present invention, dynamic recrystallization is caused by rough rolling, and
By promoting it even in at least 1 to 4 passes of finish rolling, an increase in deformation resistance in finish rolling can be suppressed.
Further, the dynamic recrystallization is not only effective in reducing the deformation resistance during rolling, but also becomes equiaxed grains by the recrystallization, and it is possible to advantageously achieve the aspect ratio of bainite grains of 1.5 or less. In order to promote dynamic recrystallization in finish rolling, the finish rolling start temperature is important.
By setting the temperature to 950 to 1050 ° C, dynamic recrystallization is promoted, and it becomes possible to suppress an increase in deformation resistance.

Finishing rolling end temperature (outlet side temperature of finishing rolling mill): 800 ° C or higher By setting the hot finishing rolling end temperature to 800 ° C or higher, a uniform and fine hot-rolled sheet structure can be obtained. However, when the finish rolling finish temperature is lower than 800 ° C, the structure of the steel sheet is expanded and becomes non-uniform, and a part of the worked structure remains, which increases the risk of causing various defects during forming. Therefore, the finish rolling finish temperature shall be 800 ° C or higher.
In addition, in order to further improve the mechanical properties, it is desirable that the temperature is 820 ° C or higher. The upper limit of the finish rolling end temperature is not required to be specified, but it is usually 950 ° C or lower due to the relationship with SRT.

Cooling after hot finish rolling Cooling after hot finish rolling in the present invention is performed at a cooling rate of 20.
It shall be continuously cooled to the coiling temperature at ~ 150 ° C / sec. The purpose of controlling the cooling after rolling in this way is to finally obtain a stable and uniform bainite structure. In the present invention, in this step, without interrupting the cooling as in the past, from the exit side of the finish rolling mill on the so-called hot run table, forced cooling with cooling water is continued until the coiling temperature is reached. Achieved by cooling. The cooling rate when performing the above cooling is 20 to 150 ° C in the entire temperature range until the coiling temperature is reached.
The range is / sec. At a speed lower than this range, sufficient strength cannot be obtained, while at a speed higher than this range, the strength variation in the width direction and the longitudinal direction of the steel sheet increases.

For cooling after hot rolling, water cooling is started immediately after rolling is completed, and so-called slow cooling having a heat transfer coefficient lower than usual is applied to achieve both material uniformity and shape uniformity. It is effective in the above. If such cooling is not started within 2 seconds after the end of rolling, the processing strain added by rolling is eliminated, and effective micronization of the structure cannot be achieved.
It becomes a non-uniform structure in which a coarse structure is mixed. For this reason,
It is necessary to start cooling within 2 seconds after the completion of rolling. When the hot-rolled steel sheet having a plate thickness of 3.5 mm or less targeted by the present invention is cooled on a hot run table, the heat transfer coefficient at the time of cooling is a large value, and the total length in the longitudinal and width directions of the steel sheet is 20 to It becomes difficult to maintain the cooling rate of 150 ° C / sec, and the uniformity of the material deteriorates. In addition, if the cooling rate is not uniform, the shape of the steel sheet is disturbed, the cooling rate becomes more uneven, and the uniformity of the material deteriorates. Considering these things, the heat transfer coefficient during cooling is
It is preferably 1000 W / m ² · K or less. Also, when cooling on the hot run table, in order to prevent overcooling of the edge part, if cooling water is masked at both ends in the width direction so that the cooling water does not directly contact the edge part of the steel plate, uniform cooling can be achieved. The above effect is exhibited more effectively.

Winding temperature: 300 to 550 ° C. By lowering the winding temperature after hot rolling, the strength can be increased. Then, if it is wound at a temperature of 550 ° C or lower, the target tensile strength of 780 MPa can be satisfied. However, if the steel sheet is wound at a temperature lower than 300 ° C, the shape of the steel sheet deteriorates, and it becomes difficult to correct the shape thereafter, which causes problems in actual use. In addition, the uniformity of the material tends to deteriorate. Therefore, the coiling temperature after hot rolling shall be 300-550 ℃. If higher material uniformity is required, a temperature range of 350 ° C or higher is desirable.
In addition to this, the shape of the steel plate is preferably 25 mm or less in terms of wave height in consideration of prevention of catching troubles, flaws, etc. in a processing line after press forming or the like. The wave height is based on the Japan Iron and Steel Federation Standard,
The wave height will be measured on the surface plate.

The steel sheet of the present invention can be manufactured by a process which takes in the above conditions. However, it is possible to use the steps described below individually or in combination to supplementarily adopt the sectional shape, dimensional accuracy and material of the steel sheet. It is desirable because the uniformity can be further improved. First, the leading material and the trailing material are joined at the entrance side of the finish rolling mill and continuously rolled. By continuously rolling in this manner, there is no so-called unsteady rolling portion at the leading and trailing ends of the material to be rolled, and stable hot rolling conditions can be achieved over the entire length and width of the steel sheet. And such rolling contributes greatly to the improvement of the cross-sectional shape of the steel sheet. Then, it becomes possible to stably improve the steel plate shape over the entire length on the hot run table, it becomes easy to obtain uniform cooling conditions in the longitudinal direction and the width direction of the steel plate, which is advantageous in obtaining a uniform fine structure. Become. The joining method on the entry side of the finish rolling mill is not particularly specified, but examples thereof include an induction heating welding method, a pressure welding method, a laser welding method, and an electron beam welding method. By continuously rolling the preceding material and the following material in this manner, even when the rolled steel sheet is cooled on the hot run table, it is possible to constantly apply tension to the steel sheet, so that the shape of the steel sheet is kept good. It is also possible to prevent uneven cooling due to the bad shape of the steel sheet. Further, according to this rolling method, since the tip of the material to be rolled can be passed through stably, it is difficult to apply it from the viewpoint of the threading property and the biting property in the ordinary single-shot batch rolling. It becomes possible to carry out hot rolling with a coefficient, that is, hot rolling using a large amount of lubricant, and it is possible to reduce the rolling load. At the same time, since the surface pressure of the roll can be reduced, the life of the roll can be extended. Further, the reduction of the friction coefficient during rolling is extremely effective for uniforming the structure in the plate thickness direction. As described above, in a thin hot-rolled steel sheet, it is extremely effective to join the preceding material and the following material and continuously roll them.

Further, it is advantageous to use an edge heater for heating the widthwise end portion of the material to be rolled (sheet bar) on the entry side of the finish rolling mill in order to make the temperature of the material to be rolled uniform in the width direction. is there. In the present invention, since the uniformity of the steel sheet temperature during rolling and cooling on the hot run table is important, the width direction end portion where the temperature tends to decrease is heated at the entrance side of the finish rolling mill to produce a steel sheet. It is preferable to make the temperature distribution in the width direction uniform. In addition, since the temperature tends to drop even at the longitudinal end of the material to be rolled, the heating device that can heat the material to be rolled (sheet bar) over the entire width at the entrance side of the finish rolling mill (hereinafter referred to as a sheet bar heater). ), It is preferable to heat the temperature-reduced portion at the end in the longitudinal direction to make the longitudinal temperature distribution of the material to be rolled uniform. In addition, when rolling after the joining, the sheet bar may be wound into a coil on the entrance side of the joining device. At this time, especially regarding the outermost winding and the innermost winding of the coil. It is particularly preferable to use the seat bar heater because the temperature easily drops. When heating the material to be rolled using an edge heater or a sheet bar heater, it is recommended that the temperature difference in the final finish rolling be 20 ° C or less. It will vary somewhat depending on the composition.

According to the method described above, TS is 780 MPa.
Has characteristics on more than, even better stretch flangeability, it is possible to provide uniform in the longitudinal direction and the width direction of the steel sheet. Also, in order to perform gentle cooling in the hot run,
It is possible to manufacture a hot rolled steel sheet having an excellent shape. Further, it is possible to further improve the uniformity of the material by using the rolling method of joining the preceding material and the following material to finish rolling and the heating of the sheet bar by the edge heater and the sheet bar heater together. .

[0041] Also, the steel sheet after hot rolling, is used to remove the oxide layer of the surface by pickling, also further skin pass rolling for surface roughness control, or leveler processing for straightening such To be used. Further, it is also possible to use the black leather as it is without performing pickling. Furthermore, it is also possible to perform various electroplating and hot dipping on the surface layer.

[0042]

Example 1 A steel having the composition shown in Table 1 and the composition having the balance substantially Fe was melted. This steel slab was hot-rolled under the conditions shown in Table 2 to a final finished plate thickness of 1.6 mm and pickled to obtain a test steel plate. The obtained hot rolled steel sheets were subjected to microscopic observation, tensile test, bending test, and hole expansion test. JIS No. 5 test pieces were used for tensile properties . Also, hole expandability test conforms to Iron and Steel Federation standard, machined holes 10mmφ by punching (12.5% clearance constant), was carried out by molding a conical basin apex angle 60 °. The results of these tests are shown in Table 3. The tensile properties of the same hot-rolled steel sheet without pickling were also examined, but no difference in properties due to the presence or absence of pickling was observed. Also, there are 3 points in the longitudinal direction of the steel plate (15m position from the tip, center position in the longitudinal direction, 15m position from the tail end) and 5 points in the width direction (center position in the width direction, 25mm position from each edge, 100mm from both edges). A total of 15 samples were taken at the position (position), and the uniformity of the material was evaluated by measuring the magnitude of the variation in tensile strength.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

As shown in Tables 1 to 3, in all the steel sheets of the invention examples, bainite occupied 90% or more in area ratio, and had an average grain size of 3.0 μm or less. Also,
TS is equipped with a characteristic on the 780 MPa or more, it can be seen that meets the desired characteristic. Further, the bending workability and the hole expansion rate were also sufficient. The bainite referred to here is one in which carbides are mainly precipitated in the grains or at the lath boundaries, and are less precipitated at the former austenite grain boundaries.

Example 2 0.15 wt% C-0.20 wt% Si-1.8 wt% Mn-0.009 wt% P-
0.001 wt% S-0.039 wt% Al-0.0025 wt% N-0.025 wt
Using a steel slab of% Nb-0.0015 wt% B-0.0020 wt% Ca as a raw material, hot-rolled steel sheets (pickling materials) having a thickness of 2.6 to 1.2 mm were manufactured under the respective manufacturing conditions shown in Table 4. When continuous hot rolling is applied, a sheet bar having a thickness of 25 mm obtained by rough rolling is joined by heating and melting and welding the leading end of the preceding material and the leading end of the following material on the entry side of the finishing rolling mill. , And finish rolling was continuously performed. Similar tests were carried out on these test steel sheets, and the results shown in Table 5 were obtained. Tables 4 and 5
In each of the steel sheets of the invention examples, bainite occupies 90% or more of the area ratio (other structures are pearlite, martensite, or ferrite), and the average grain size is 3.0 μm or less without any mixed grains. It was a different organization.
Also, TS is provided with a characteristic of the 780 MPa or more, good bending workability showed hole expansion ratio. In the invention examples, the plate crown of the steel plate (difference in plate thickness between the center in the width direction and the position of the edge 25 mm) was 40 μm or less, which was good. Further, regarding the steel sheet of the invention example and the cold rolled sheet of 1.4 mm (continuously annealed material),
A small diameter electric resistance welded tube was produced. As a result, although the steel sheet of the invention example had to be adjusted to the optimum welding conditions, it could be produced without any problem in terms of forming and product properties, like the cooling plate.

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

As described above, according to the present invention,
A thin high-strength hot-rolled steel sheet having excellent stretch flange formability can be provided. Further, according to the present invention, by optimizing the chemical composition and hot rolling conditions, the shape is uniform,
It is possible to provide a high-strength hot-rolled steel sheet having high uniformity of material. Therefore, the high-strength hot-rolled steel sheet according to the present invention can be used in place of the conventional high-strength cold-rolled steel sheet in terms of quality,
It greatly contributes to energy saving in the manufacturing process and reduction of product costs such as strength members of automobiles and impact beams (beam pipes).

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the grain size of bainite and the hole expansion rate.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Nobuo Yamada No. 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Steel Co., Ltd. Chiba Works (72) Takao Uchiyama No. 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Chiba Works Co., Ltd. (56) Reference JP-A-8-199298 (JP, A) JP-A-10-195591 (JP, A) JP-A-6-264184 (JP, A) JP-A-6-145894 ( (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46

Claims (7)

(57) [Claims] 1. C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, M
n: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al: 0.150 wt% or less, N: 0.0200 wt% or less,
In addition, it contains any one or two of Nb: 0.003 to 0.20 wt% and Ti: 0.005 to 0.20 wt%, and the balance is composed of Fe and unavoidable impurities, and the metal structure has an average grain size of 3.0 μm or less. A high-strength hot-rolled steel sheet having excellent stretch-flangeability, which is characterized by having a structure having a fine bainite area ratio of 90% or more . 2. The high-strength hot-rolled steel sheet having excellent stretch-flange formability, which further comprises B: 0.0005 to 0.0040 wt% in addition to the above components. 3. The steel sheet according to claim 1, further comprising Cu: 0.02 to 1.0 wt%, Ni: 0.02 to 1.0 wt%, Cr: 0.02 to 1.0 wt%, Mo: 0.02 in addition to the above components. A high-strength hot-rolled steel sheet having excellent stretch-flange formability, characterized by containing one or more of 1.0 wt% to 1.0 wt% in a total amount of 1.0 wt% or less. 4. The steel sheet according to any one of claims 1 to 3, which further contains Ca: 0.0005 to 0.0050 wt% in addition to the above-mentioned components, and is excellent in stretch flangeability. Strength hot rolled steel sheet. 5. The high-strength hot-rolled steel sheet according to claim 1, wherein the bainite grains have an aspect ratio of 1.5 or less. 6. C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al: 0.150 wt% or less, N : 0.0200 wt% or less, and Nb: 0.003 to 0.20 wt%, Ti: 0.005 to 0.20 wt% Any one or two of the slabs are heated at 1200 ° C or lower, and then the finish rolling finish temperature. Hot rolling at 800 ℃ or more, start cooling within 2 seconds after rolling, continuously cool to the coiling temperature at a cooling rate of 20 to 150 ℃ / sec, and coil at 300 to 550 ℃. And a method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability. 7. C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al: 0.150 wt% or less, N : 0.0200 wt% or less and Nb: 0.003 to 0.20 wt%, Ti: 0.005 to 0.20 wt%, and a slab containing one or two of them is heated at 1200 ° C. or lower, and then finish rolling start temperature. 950 ~ 1050 ℃, finish rolling finish temperature 800 ℃
The hot rolling is performed as described above, cooling is started within 2 seconds after the completion of rolling, and the material is continuously cooled to the coiling temperature at a cooling rate of 20 to 150 ° C / sec and wound at 300 to 550 ° C. And
A method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability.