JPS6137332B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a precipitation-strengthened low yield ratio hot-rolled steel plate having a thickness of about 4.5 mm to 15 mm and a yield strength of 30 Kg/mm ² or more. Steel pipes for the purpose of transporting oil or gas fuel have become subject to increasingly strict standards from the viewpoint of safety. One of these is the yield ratio (the value obtained by dividing the yield strength by the tensile strength), and it has come to be believed that the lower the ratio, the better in order to prevent pipes from breaking. Particularly in recent years, when high-strength steel is used to increase wheel transport efficiency, the upper limit on yield ratio is severely restricted. Furthermore, this situation is because the pipe diameter is not 200
It is noticeable in welded steel pipes of about 800 mm. This is because this class of steel pipes generally has a large ratio of plate thickness to diameter, is subject to large work hardening during bending during pipe manufacturing, and suffers from compressive strain due to crimping during welding, as well as expansion and expansion.
Since the pipe undergoes processing such as sizing, the yield ratio of the steel pipe as a product is quite high. Generally, work hardening significantly increases the yield strength, but the tensile strength does not increase so much, so work hardening increases the yield ratio. Therefore, the yield ratio of the steel plate or steel strip used as the material has come to be required to be extremely low, such as 0.8 or less, or even 0.75 or less in extreme cases, in order to allow for the equivalent amount of work hardening. . On the other hand, the manufacturing method of steel plates as steel pipe materials is by plate mills or hot strip mills, but high-strength steels with yield strength of 30 kg/mm ² or higher are produced by Nb, V, Ti, etc.
Use precipitation strengthening elements such as However, precipitation strengthening is generally known to increase the yield ratio. This is thought to be because fine precipitates impede the movement of dislocations. Furthermore, when manufacturing with a hot strip mill, forced cooling is performed using cooling water after finish rolling to increase productivity. Therefore, the cooling rate when passing the transformation point becomes faster,
The crystals become fine grains. As the grain becomes finer, both the yield strength and tensile strength increase, as known from the Petsch equation.
Since the degree of increase in yield strength is large, the yield ratio becomes high. Due to the above reasons, the yield ratio is low and the yield strength is 30.
It has been difficult to produce precipitation-strengthened high-strength steel sheets of Kg/mm ² or more using hot strip mills. As a result of various experiments and research, the present inventors have found that even in steel containing precipitation-strengthening elements such as Nb or V, by limiting the composition and rolling under specific hot-rolling conditions. Even in a hot strip mill, we succeeded in producing high-strength steel with a yield ratio of 0.8% or less, and if necessary, 0.75 or less, and excellent elongation. That is, the gist of the present invention is that C0.15 to 0.25% by weight, Si 0.7% or less, Mn 1.5% or less,
If it contains Al0.005-0.080%, Nb0.005-0.03%, or V0.01-0.04%, or a combination of Nb and V, the sum should be 0.005-0.05%, and the remainder Steel consisting of Fe and unavoidable impurities is made into a slab in the normal process, heated to over 1200℃, and then processed in a hot strip mill.
Rough rolling is completed at 1000°C or higher, finish rolling is completed at 830°C or higher and 900°C or lower, followed by cooling at an average cooling rate of 5 to 20°C/sec, and winding into a coil at 550°C to 700°C. A method for manufacturing a low yield precipitation strengthened hot rolled steel sheet with a yield ratio of 0.8 or less and a weight of C0.15.
~0.25%, Si0.7% or less, Mn1.5% or less, Al0.005
~0.080%, further Nb0.005~0.03%, or
If it contains 0.01 to 0.04% of V, or a combination of Nb and V, the sum should be 0.005 to 0.05%, and in addition, Ni 0.5% or less, Mo 0.5% or less, Cu 0.5
% or less, REM 0.1% or less, Zr 0.1% or less, Ca, 0.02
% or less, and the remainder is Fe.
Steel consisting of steel and unavoidable impurities is made into a slab in the normal process, heated to 1200℃ or higher, then rough rolled in a hot strip mill at 1000℃ or higher, and then heated at 830℃ or higher and 900℃ or lower. Finish rolling is finished,
followed by cooling at an average cooling rate of 5 to 20°C/sec,
Manufacturing method of precipitation-strengthened hot rolled steel sheet with a low yield ratio of 0.8 or less, which is characterized by winding into a coil at 550℃ to 700℃, and the weight of C0.15 to 0.25%, Si0.7% or less, Mn1 .5% or less, Al0.005~0.080%, and more
If it contains Nb0.005~0.03% or V0.01~0.04%, or contains a combination of Nb and V, the sum is
The steel is made into a slab using a normal process, with the balance being Fe and unavoidable impurities, heated to 1200℃ or higher, and then roughly rolled at 1000℃ or higher in a hot strip mill. exit,
Finish rolling is completed at 830°C or higher and 900°C or lower, followed by cooling at an average cooling rate of 5 to 20°C/sec, and then 550°C to 900°C.
Low yield ratio precipitation with a yield ratio of 0.8 or less, characterized by winding the coil into a coil at 700°C, then unwinding the coil, straightening it while giving a maximum surface bending strain of 0.6 to 2.0%, and cutting it into a cut plate. Manufacturing method and weight of strengthened hot rolled steel sheet: C0.15-0.25%, Si0.7% or less,
Mn1.5% or less, Al0.005~0.080%, and
If it contains Nb0.005~0.03% or V0.01~0.04%, or contains a combination of Nb and V, the sum is
0.005 to 0.05%, and one or two of the following: Ni 0.5% or less, Mo 0.5% or less, Cu 0.5% or less, REM 0.1% or less, Zr 0.1% or less, Ca 0.02% or less.
After making the steel into a slab using the normal process, it is heated to 1200°C.
Heat as above, then use a hot strip mill
Rough rolling is completed at 1000°C or higher, finish rolling is completed at 830°C or higher and 900°C or lower, followed by cooling at an average cooling rate of 5 to 20°C/sec, and winding into a coil at 550°C to 700°C. , then unwind the coil to maximum surface bending strain
This is a method for producing a precipitation-strengthened hot-rolled steel sheet with a low yield ratio of 0.8 or less, which is characterized in that the sheet is cut after straightening while applying bending deformation of 0.6 to 2.0%. In addition, the composition system of the steel according to the present invention does not interfere with measures to reduce inclusions in steel manufacturing, but rather tends to reduce inclusions. Detection defects can also be significantly lowered. Furthermore, the steel plate according to the present invention has various excellent features as a material for steel pipes, such as good plate thickness accuracy due to the use of hot strip mill products, and therefore good dimensional accuracy as a steel pipe. Next, the effects of the constituent elements of the present invention and the reason for limiting the numerical values will be explained. First, regarding the ingredients, C is set higher than in conventional steels of this type. The reason for this is to increase the pearlite volume fraction and increase the strength due to pearlite, and to lower O (oxygen) from the C-O (carbon-oxygen) balance in steel manufacturing to make the steel cleaner.
If it is less than 0.15%, the effect is insufficient, and
If it exceeds 0.25%, the toughness of the steel will deteriorate, and the carbonitrides of Nb and V will not dissolve sufficiently in solid solution during slab heating, and the precipitation strengthening elements will not be effective. Next, Si and Mn form a substitutional solid solution in the steel to strengthen it as a solid solution. Si and Mn
exceeds 0.7% and 1.5%, respectively, the toughness of the steel,
In particular, it deteriorates the toughness of welded parts. Appropriate amounts of Si and Mn improve the toughness, ductility and weldability of steel, so if these properties are strictly required, Si should be in the range of 0.1-0.5% and Mn in the range of 0.7-1.2%. It is preferable. Also, Al is necessary for deoxidation. Particularly when used in electric resistance welded steel pipes as in the case of the present invention, the ends of the steel plates become weld abutments, and large inclusions at those locations become fatal defects. Therefore, it is necessary to sufficiently deoxidize with Al and to float sufficiently so that the deoxidized products do not remain in the molten steel. For this purpose, the amount of Al is important and is 0.005%
If it is less than 0.080%, it cannot be deoxidized sufficiently, and if it exceeds 0.080%,
Since the amount of deoxidation products mainly consisting of Al ₂ O ₃ remaining in the steel increases, the amount of Al was set to 0.005 to 0.080%. If tighter inclusion control is required, Al is 0.01
~0.04% is preferred. Next, Nb and V strengthen steel as precipitation-strengthening elements, so they are essential for high-strength steels with a yield strength of 30 Kg/mm ² or more. However, as mentioned above, precipitation strengthening usually increases the yield ratio, so it is kept as low as possible while taking into consideration the balance with other conditions. In other words, the upper limit of Nb is 0.03
%, V exceeds the upper limit of 0.04%, and when Nb and V are used in combination, the upper limit of 0.05% is exceeded, the effect of precipitation strengthening becomes too large, and a yield ratio of 0.8 or less is not achieved even considering other conditions. Not done. To achieve a yield ratio of 0.75 or less, the above values should be 0.025%,
(for Nb), 0.03% (for V), 0.04% (for Nb+
V) is preferred. Furthermore, precipitation strengthening is not achieved when the lower limits of these elements are less than 0.005% (in the case of Nb), 0.01% (in the case of V), and 0.005% (in the case of Nb+V). Furthermore, if necessary, Ni0.5% or less, Mo0.5% or less, Cu0.5%
Below, REM 0.1% or less, Zr 0.1% or less, Oa 0.02%
One or more of the following may be added.
Since Ni and Mo have the effect of lowering the yield ratio as solid solution strengthening elements, one or more of them may be added. If each content exceeds 0.5%, weldability deteriorates and the steel becomes extremely expensive, which is undesirable from an economical point of view. Further, since Cu improves corrosion resistance, it may be added as necessary. If it exceeds 0.5%, the effect will be saturated, so the upper limit is 0.5%. Furthermore, REM, Zr, and Ca make the sulfide inclusions non-plastic during hot heating, resulting in spheroidization, which improves the ductility and toughness in the direction perpendicular to the rolling direction, so they may be added as appropriate. The upper limit is determined at the point where the effect is saturated. In addition to the above, for applications that require special inclusion measures, S as an impurity element should be added to 0.015%.
Hereinafter, it is preferable that O is 0.0030% or less, and that Mn is 3 to 6 times that of C and 3 to 9 times that of Si. Next, the hot rolling conditions will be explained. First, regarding the heating temperature, it is necessary to set it to a high temperature of 1200° C. or higher in order to sufficiently dissolve the carbonitride of Nb or V as a solid solution.
Particularly in the case of the present invention, since the carbon content is set high in order to lower the yield ratio, it is difficult for carbonitrides to form a solid solution, so the heating temperature is important. Next, the rolling temperature is very important because in combination with the ingredients it gives a low yield ratio. Rough rolling must be performed at a temperature of 1000°C or higher. 1000℃
If rough rolling is performed at less than 100%, the austenite grains become fine, and the subsequent finish rolling and cooling result in very fine ferrite grains, making it impossible to lower the yield ratio. Preferably, rough rolling should be completed at 1040°C or higher.
Next is finish rolling, but if this temperature is too low, the frequency of nucleation of ferrite grains in austenite grains will increase, and as a result, the ferrite grains will become too fine and the yield ratio will increase. In this sense, it is necessary to perform finish rolling at a temperature of 830°C or higher. However, since fine ferrite grains are favorable for toughness, the finish rolling temperature cannot be increased unnecessarily.
If the temperature exceeds 900°C, coarse and mixed grains of ferrite are produced, which is unacceptable from the viewpoint of the toughness of the steel. These rolling conditions are determined in consideration of the following cooling conditions in order to lower the yield ratio. Like a plank mill,
In a process in which air cooling is performed after finish rolling, the cooling rate when passing the transformation point is low and relatively large ferrite grains are obtained by transformation, so the yield ratio is relatively low. For hot strip mills, it is necessary to limit the cooling rate for this reason; if the average cooling rate exceeds 20°C/sec, a yield ratio of 0.80 or less cannot be achieved. Preferably it is 15°C/second or less. The lower limit of the average cooling rate is 5°C/sec. 5℃/
If the cooling rate is less than a second, the productivity will decrease and the purpose of manufacturing with a hot strip mill will be lost. Finally, the coiling temperature is 550°C, which is the temperature range in which carbonitrides of Nb, V, or both are precipitated most minutely.
Need to be ~700℃. If the temperature is lower than 550°C, precipitation will not be sufficient, and if the temperature exceeds 700°C, the precipitates will become coarse and precipitation strengthening will not be sufficiently achieved. Note that the upper limits of the slab heating temperature and rough rolling completion temperature are not particularly specified. This is because the effect is exhibited up to the upper limit that can be achieved with a normal hot strip mill.
The respective upper limits are approximately 1330°C and 1150°C. Although the reasons for limiting the constituent elements of the present invention have been described above,
To add a few more points, the steel according to the present invention is generally tapped in a converter. Thereafter, it is preferable to clean the steel by vacuum degassing using the DH method or the RH method. Next, it enters the hot rolling process, either by ingot making and blooming into a slab, or by continuous casting into a slab. A hot rolled coil is then obtained according to the present invention.
The hot-rolled coil is directly subjected to the tube-making process, or the coil is unwound and cut into plates, which are then subjected to the tube-making process. When unwinding a coil to make a cut plate, it is necessary to straighten the cut plate by repeated bending (leveler processing) so that the maximum surface bending strain is 0.6 to 2.0%. This is preferable because it provides good yield ratio and low yield ratio. Next, the effects of the present invention will be explained using examples. Example 1 First, the influence of the components in the present invention will be explained in Example 1. Steel containing the chemical components shown in Table 1 was melted in a converter, ingot-formed and bloomed, and hot-rolled according to the present invention. . The hot rolling conditions were to heat the slab at 1230℃,
Subsequently, unidirectional rough rolling was performed using a 5-stand rough rolling mill. The rough rolling finish temperature is 1050°C. This was followed by finish rolling using a seven-stand tandem rolling mill. The final rolling temperature is 850℃ and the product thickness is 8.0mm. Subsequently, it was cooled at an average cooling rate of 10°C/sec and wound into a coil at 630°C. The mechanical properties of the material at that time are shown in Table 2. A JISZ22015 test piece was used as the tensile test piece. In the table, YS, TS, El, and YR represent yield strength, tensile strength, elongation, and yield ratio (YS/TS), respectively. The direction of the tensile test piece was perpendicular to the rolling direction. In Table 2, symbols A, H, and I marked with a circle indicate components based on the present invention that were produced by a method based on the present invention. Symbols B to F have different components from those of the present invention. As is clear from this table, according to the present invention, a high tensile strength steel plate having a YR of 0.8 or less and a yield strength of 30 Kg/mm ^{2 or} more can be obtained. Furthermore, as shown in FIG. 1, the steel sheet according to the present invention has an excellent El for the same TS, and can withstand various types of straightening, sizing, etc. of steel pipes.

【table】

【table】

【table】

[Table] Example 2 Next, in Example 2, the effects of hot rolling conditions will be explained. Steel having the components shown in Table 3 was tapped in a converter and subjected to ingot making and blooming processes to form slabs. Subsequently, after heating at 1250°C, hot rolling was performed under the hot rolling conditions shown in Table 4. The thickness of each plate was 8.0 mm. Thereafter, a bending deformation with a maximum surface bending strain of 10% was applied on the straightening line to produce a straightened cut plate. Numbers 1 and 2 in the table are according to the present invention. Number 3 differs from the present invention in rolling temperature, and number 4 differs in cooling rate after finishing. As shown in the mechanical properties in Table 4, according to the present invention, a high tensile strength steel plate with a stable low yield ratio can be obtained. This steel plate was subsequently formed and welded into an electric resistance welded steel pipe, and then expanded and sized to become a product steel pipe. However, the overall quality of the steel pipe, including mechanical properties, roundness, and straightness, was good. Ta.

【table】

[Table] As described above, the present invention uses precipitation-strengthening elements to sufficiently reduce the yield ratio, which has become important in recent years as a quality characteristic for steel pipes, and has a yield strength of 30 Kg/mm ² or higher. This is a method that allows stable production.

[Brief explanation of the drawing]

Figure 1 shows the relationship between strength and ductility, with elongation on the vertical axis and tensile strength on the horizontal axis. In the figure, the ○ mark represents the steel plate according to the present invention, and the ● mark represents the comparative steel plate.

Claims (1)

[Claims] 1 Contains by weight C 0.15 to 0.25%, Si 0.7% or less, Mn 1.5% or less, Al 0.005 to 0.080%, further Nb 0.005 to 0.03%, or V 0.01 to 0.04%, or Nb, If V is included in combination,
The sum should be 0.005 to 0.05%, and the remainder should be
Steel consisting of Fe and unavoidable impurities is made into a slab in the normal process, heated to 1200℃ or higher, then rough rolled in a hot strip mill at 1000℃ or higher, and finished at 830℃ or higher and 900℃ or lower. Finish rolling,
followed by cooling at an average cooling rate of 5 to 20°C/sec,
A method for producing a precipitation-strengthened hot-rolled steel sheet with a low yield ratio of 0.8 or less, which is characterized by winding it into a coil at 550°C to 700°C. 2 Contains C 0.15 to 0.25%, Si 0.7% or less, Mn 1.5% or less, Al 0.005 to 0.080%, and further Nb 0.005 to 0.03%, or V 0.01 to 0.04%, or a combination of Nb and V In case,
The sum should be 0.005 to 0.05%, and one or more of the following: Ni 0.5% or less, Mo 0.5% or less, Cu 0.5% or less, REM 0.1% or less, Zr 0.1% or less, Ca 0.02% or less steel, the balance of which is Fe and unavoidable impurities, is made into a slab in a normal process, heated to a temperature of 1200°C or higher, and then rough rolled in a hot strip mill at a temperature of 1000°C or higher,
Finish rolling is completed at 830°C or higher and 900°C or lower, followed by cooling at an average cooling rate of 5 to 20°C/sec, and then 550°C to 900°C.
A method for manufacturing a precipitation-strengthened hot rolled steel sheet with a yield ratio of 0.8 or less, which is characterized by winding it into a coil at 700℃. 3 Contains C 0.15 to 0.25%, Si 0.7% or less, Mn 1.5% or less, Al 0.005 to 0.080%, and further Nb 0.005 to 0.03%, or V 0.01 to 0.04%, or a combination of Nb and V In case,
The sum should be 0.005 to 0.05%, and the remainder should be
After making steel consisting of Fe and unavoidable impurities into a slab in the normal process, it is heated to 1200℃ or higher, then rough rolling is completed at 1000℃ or higher in a hot strip mill, and then the steel is heated at 830℃ or higher and 900℃ or lower. Finish rolling is completed, followed by cooling at an average cooling rate of 5-20°C/sec, winding into a coil at 550°C-700°C, and then unwinding the coil and bending with a maximum surface bending strain of 0.6-2.0%. A method for producing a precipitation-strengthened hot-rolled steel sheet with a low yield ratio of 0.8 or less, which comprises straightening the sheet while applying deformation and then cutting the sheet. 4 Contains C 0.15 to 0.25%, Si 0.7% or less, Mn 1.5% or less, Al 0.005 to 0.080%, and further Nb 0.005 to 0.03%, or V 0.01 to 0.04%, or a combination of Nb and V If so, the sum should be 0.005 to 0.05%, and one or two of the following types: Ni 0.5% or less, Mo 0.5% or less, Cu 0.5% or less, REM 0.1% or less, Zr 0.1% or less, Ca 0.02% or less After forming the steel containing the above and the balance consisting of Fe and unavoidable impurities into a slab in a normal process, it is heated to 1200°C or higher, and then rough rolling is completed at 1000°C or higher in a hot strip mill,
Finish rolling is completed at 830°C or higher and 900°C or lower, followed by cooling at an average cooling rate of 5 to 20°C/sec, and then 550°C to 900°C.
A low yield ratio with a yield ratio of 0.8 or less, which is characterized by winding the coil into a coil at 700°C, then winding the coil, straightening it while giving a maximum surface bending strain of 0.6 to 2.0%, and cutting it into a cut plate. A method for producing precipitation-strengthened hot-rolled steel sheets.