JPS58136717A - Manufacture of high tensile hot rolled steel strip containing vanadium - Google Patents

Abstract

PURPOSE:To manufacture the titled steel strip with superior strength, ductility and toughness by holding a continuously cast slab of a low carbon steel consisting of Si, Mn, S, Al, V, N and the balance essentially Fe at a specified temp., hot rolling the slab, and coiling the resulting strip after rapid cooling. CONSTITUTION:A steel slab consisting of, by weight, 0.02-0.15% C, <=1.50% Si, 0.5-2.20% Mn, <=0.010% S, 0.005-0.020% sol.Al, 0.02-0.30% V, 0.0050- 0.0200% N and the balance Fe with inevitable impurities is manufactured by a continuous casting method. The slab is held at the A3 point - 1,180 deg.C for a fixed time and hot rolled at <=1,100 deg.C and >=70% draft. After finishing the rolling at 850-680 deg.C, the resulting strip is rapidly cooled to 650-500 deg.C at >=3 deg.C/sec cooling rate and coiled at the temp.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a vanadium (V)-containing hot-rolled high-strength steel strip that is suitable as a material for line pipes, automobile structures, etc. and has high strength and excellent ductility and low-temperature toughness. This relates to a manufacturing method.

In recent years, from the viewpoint of energy saving and controlled rolling, which aims to finely control the rolling stage under specific conditions, the low-temperature heating rolling method, in which hot rolling is carried out by heating the material to be rolled to a relatively low temperature, has been developed. Y♀, 0.02~o, o
For the production of hot-rolled products of high-strength steel containing 8% Ae (hereinafter, when expressing the compositional component m of steel, CH is a weight factor) and further containing Nb or V, or both. However, when applying the low-temperature heating rolling method to these steel types, Nb (C) tends to form inside the steel, especially in Nb-containing steel.
Since the solubility product of the compound N) is small, coarse precipitates of undissolved Nb(ON) are present during low-temperature heating, which poses a troublesome problem of causing a significant decrease in strength. On the other hand, in the case of Nb-containing steel, from the above-mentioned perspective, the disadvantages seen in Nb-containing steel should not occur because the V(CN) compound has a large solubility product in steel. However, in reality, as shown in Figure 1, even with Nb-containing steel, when subjected to low-temperature hot rolling, the strength of the Nb-containing steel decreases significantly. .

Figure 3 shows that Nb-containing steel and ■-containing steel were each hot-rolled at a temperature below 1100°C with a rolling reduction of 85%, and the rolling was completed at soo°C to form a plate with a thickness of 11. It is a diagram showing the influence of heating temperature on the strength of hot-rolled simulation material, and as Nb-containing steel, C:
008%, Si: 0.3% + Mn': 1. ,2
% r b : 0007 % r Nb : 0
03% + 5- AQ: 0.04%, N: 0.0104
%, i'e and remaining impurities, ■ As the impregnated steel, C: 0.11%, Si',
0.3%, Mn: ], :S%.

S: 0.007, v: o, os%, /V: 0.0 poor)
%, N20.0110% + Fe, and impurities: the remainder.

From the above-mentioned viewpoints, the present inventors developed a high-strength hot-rolled steel sheet with good ductility and low-temperature toughness by low-temperature hot-rolling, while maintaining the high strength obtained by ordinary high-temperature hot-rolling as much as possible. In order to obtain a high As a result of conducting various studies on rolled materials and hot rolling conditions while adding basic considerations to the causes of strength reduction due to hot rolling, we have obtained the knowledge shown in (a) to (C) below. be.

a) The improvement in the strength of V-containing steel is based on precipitation strengthening in which VN precipitates during slow cooling after hot rolling and coiling.
Even if VN is dissolved in the base material during low-temperature heating, if a certain amount or more of AC is present in the steel, AQN will precipitate, which will reduce the amount of solid solution N. Even if it is coiled at an appropriate temperature after hot rolling, less VN will precipitate during slow cooling after coiling, resulting in a large decrease in strength.

FIG. 2 (al and (bl) have a basic composition of C':0.
10%, Si: 0.3%, Mn: -L, 3%,
Continuously cast slabs of V20.10%, N:OO], 00%, Fe and impurity steel containing various amounts of Al were rolled at a temperature of 1100°C or less at a reduction rate of 85%. This graph shows the influence of the amount of Ag and the heating temperature on the mechanical properties of a hot-rolled simulation material of 11 mm thickness that was rolled at 600°C after rolling at 800°C. %AN -V steel has a significant strength decrease due to low temperature heating, but 0015%7V
The above-mentioned influence of Al was confirmed from the result that there was little decrease in strength in the -V steel and the O,OO 2% Al -V steel.

In addition, from the results shown in Figure 2 (a) and (bl), 0015%A+, -V steel and O, OO2%AA
Although the -V steel has higher strength than the 005% AQ -V steel heated at low temperature, it has an elongation equal to or higher than that of the -V steel, and in terms of the Charpy fracture transition temperature, both - It was clear that the temperature was below 50°C, which is an excellent value.

b) When V-containing steel is bloomed and rolled, VN tends to be strain-induced precipitated by the indenter of the bloomer, so even if it is heated to the temperature range used in low-temperature heating rolling of 3 to 1180°C AC prior to tone rolling, it will not become coarse. VN remains undissolved and causes a decrease in strength, but when a continuous casting slab is used, this phenomenon can be suppressed as much as possible.

C) Therefore, if a slab of low AQ steel with appropriate amounts of N and ■ added is obtained by continuous casting, then subjected to low-temperature hot rolling and then slightly quenched, good strength can be obtained. , hot-rolled products with ductility and toughness are capped.

Therefore, this invention was made based on the above knowledge, and by a continuous casting method, C: 0.02-015%, Si: 1.50% or less, Mn: 0.5-2.20%, S. :0.01
0 or less, sot, AE-0,005 to 0.020%, V: 0.0
2 to 0.30%, N: 0.0050 to 0.0200%, and if necessary, Nb: 0.10% or less, Mo: 0.5% or less, Cu: 0.5% Hereinafter, a steel slab containing one or more of the following: Ni: 0.5% or less, Cr: 1.0% or less, Ca: 0.0100% or less, the remainder being Fe and unavoidable impurities. After obtaining this, it was heated to A3 point ~ 1180℃
9 - Maintain the temperature within the temperature range for a certain period of time, then perform hot rolling at a temperature of 1100°C or lower with a processing rate of 70% or more, and complete the rolling in the temperature range of 850 to 680°C. Cooling of ℃/5ecJ or more
Rapid cooling with J IAI', rolling yield is 650~! 500℃
After the steel is cooled down to 1Ω, it is coiled within that temperature range to obtain a V-containing 4 high tensile strength hot rolled steel strip with excellent strength, ductility and toughness.

In addition, in the method for manufacturing a V-containing high-strength hot-rolled steel strip of the present invention, a continuously cast slab is used as the material to be rolled in order to prevent strain-induced precipitation of VN. Even so, Ar3 points J′J.

If the material is cooled down and then heated to 3 to 1180°C, there will be many nuclei of υVN precipitated, and coarse VN is likely to be generated during heating, which may result in loss of strength and effectiveness. Therefore, preferably, the slab obtained by the continuous casting method is directly heated to A3 to 118 points without being cooled.
It is preferable to employ so-called hot charging, in which hot rolling is performed after holding the material in a temperature range of 0° C. for a certain period of time.

Next, in the method for producing a hot-rolled steel strip of the present invention, the compositional component amount, heating holding temperature, and rolling temperature of the steel.

Processing rate, rolling end temperature, cooling rate, and winding temperature 10- -] The reason for limiting it as mentioned above will be explained.

■ C The C component has the effect of increasing the amount of pearlite and the amount of low-temperature transformation products, resulting in an increase in strength, but when its content is 0.
．． If it is less than 0.02%, the required 50 kg/
It is not possible to obtain high strength of m4 or more, and on the other hand, 0.15
If the content exceeds 0.02% to 0.15%, it will cause deterioration of low temperature toughness and ductility.
limited to.

■ The 5I Sl component is effective in increasing the strength of steel through solid solution strengthening, but if it is contained in excess of 1.50%, weldability will deteriorate, so the content should be reduced to 1.5%.
It was limited to 50% or less.

■ Mn The Mn component has the effect of increasing the strength of steel through solid solution strengthening and low-temperature transformation strengthening, but if its content is less than 0.5%, it is not possible to obtain the specified high degree of hardness.
Since weldability deteriorates if the content exceeds 0.05% to 220%.

■S 8 minutes combines with Mn to form non-metallic inclusions, causing a decrease in Charpy shelf energy in the direction perpendicular to rolling and a decrease in local ductility, so it is desirable to reduce it as much as possible, but in particular If the O, Oi content exceeds 0%, the toughness tends to deteriorate significantly, so the content should be reduced to O, O],
It was limited to below 00%.

■ sot, Affi sot, M, component is usually 0.0% in steel as a deoxidizing element.
It is said that it is good to contain 0.005% or more, but in AM guild steel, generally 0.02 to 0.07
% is added. However, when performing low-temperature heating and low-temperature rolling, sufficient grain refining of steel can be obtained even without the grain refining effect of 5at-AQ, and the Si content can be reduced to 1
When the value is as high as 5%, a sufficient deoxidizing effect can be obtained with only Sl, so there is no need to add AQ to achieve complete deoxidation. For this reason, it is not necessary to add a large amount of soL and AQ, but if the content of soL and AQ is less than 0005, the number of A-based Mn5j inclusions increases, the Charpy shelf energy decreases, and low-temperature toughness deteriorates. As a result, the uses of steel become extremely limited. On the other hand, so
b. If the AQ content exceeds 0.020%, A
gN precipitates preferentially, leading to a decrease in the amount of VN precipitated,
This results in a decrease in strength. Because of all this, 5ot-A+! The content was limited to 0005-0020%.

(2) The component (V) combines with solid solution N and has the effect of ensuring the strength of the steel through its precipitation strengthening. That is, V(CN)
The solubility product of Nb (CN) is larger than that of Nb (CN), etc., and a large amount of Nb can form a solid solution in the substrate during low-temperature heating. The combination produces large precipitation strengthening due to VN, but if the content of Since no further improvement in the effect could be expected, the content was limited to 0.02 to 0.30 parts.

13- ■ N The N component precipitates as V N by combining with ■ and has the effect of strengthening steel, but its content is
If O is less than 50%, especially when the content of S('1) and AQ is close to 0.02%, it will be consumed as AeN and the desired effect cannot be expected from the above action. If the content exceeds 0.0200%, steel flakes will easily occur, so the content should be reduced to O, O.
It was limited to O50-0.0200%.

However, in order to maximize the precipitation strengthening of VN, more is better, and it is preferably contained in an amount of 0.0100% or more.

■ Nb The Nb component has the effect of refining ferrite grains during controlled rolling with low-temperature heating, resulting in good ductility and excellent low-temperature toughness. Since no improvement in the effect was observed, the content was limited to 0.010% or less.

■ Mo, Cu, Ni, and Cr 14- Mo, Cu, Ni, and Cr components have the effect of improving the strength of steel like Mn, but Mo, Cu, and Ni
is contained in excess of 05, and CR is 10%
If the content exceeds 0.05%, the weldability of any of them will deteriorate. Limited.

σi) Ca Ca component contains AI! It combines with 203 and MnS to reduce A-based inclusions and B-based inclusions, improves the Charpy shelf energy in the direction perpendicular to rolling, and also improves local ductility. If C-based inclusions are included, the number of C-based inclusions in the steel will increase, which will conversely reduce shelf energy and local ductility, so the content was limited to 0.0100 modulus or less.

(i ■ Point A3 ~ 1) Reason A for holding the continuous cast piece at 80℃
If the temperature is maintained at a temperature lower than 8 points, the solubility product of V (CN) is extremely low, and not only precipitation strengthening due to addition cannot be expected, but also rolling occurs in the ferrite region, resulting in a significant decrease in ductility. On the other hand, holding the temperature higher than 180°C is not only undesirable for the purpose of energy saving, but also makes it impossible to obtain the desired good strength, ductility, and low-temperature toughness because the initial austenite grains during heating are large. For these reasons, it was decided to maintain the temperature in the range of 80°C from point A3 to point A.

@ Reason for reducing at a processing rate of 'i'o% or more at temperatures below 1100℃ Under other conditions, it is not expected that the austenite grains will become much finer, so at a temperature below 1100℃
It was decided to reduce at the above processing rate.

0 It is difficult to obtain fine-grained ferrite when finishing at a temperature exceeding 850°C, while finishing at a temperature lower than 680°C results in significant deterioration of ductility due to processing of ferrite. Rolling end temperature 850-680℃
limited to.

q ■ No rapid cooling of 3℃/Sec or more, fi 50
Reasons for coiling at ~500°C If the cooling rate after rolling is slower than 3°C/sec, or if coiling is performed at a temperature higher than 650°C, ■ fine VN precipitates that are effective for strength, which is the purpose of addition. is not obtained,
Furthermore, if the wire is wound at a temperature lower than 500.degree. C., the precipitation of VN will be greatly suppressed and high strength will still not be obtained. For this reason, it was decided to perform rapid cooling at 3° C./see or higher and then wind it up in a temperature range of 650 to 500° C.

Next, the present invention will be explained by comparing examples and comparative examples.

First, steel slabs having the chemical composition shown in Table 1 were obtained by continuous casting. Next, this slab was subjected to a hot rolling simulation experiment under the conditions shown in Table 1 to obtain a hot rolled coil with a finished plate thickness of 11 m. Measuring the mechanical properties of each hot rolled coil obtained in this way,
The results are also shown in Table 1.

In Table 1, test number 1 is a hot-rolled low-N steel with a normal M content, and the hot-rolled steel strip obtained in this way has a weight of 50 kg/rru? It has a low tensile strength of less than r. Test number 2-5
These are the same low Al1-V-N steels, but the hot rolling conditions for those with test numbers 2 to 4 are outside the scope of this invention, so they have high strength, good ductility, and excellent low temperature properties. While toughness was not obtained, good results were obtained in test number 5, where the hot rolling conditions were within the scope of the present invention.

As described above, according to the present invention, it is possible to obtain a high-strength hot-rolled steel strip that has high strength, good ductility, and excellent low-temperature toughness by low-temperature hot rolling with low energy consumption. This brings about excellent industrial effects.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the influence of heating temperature on the strength of hot rolled simulation materials of Nb steel and V steel, and Figure 2 (a
) and (b) are diagrams showing the influence of M content and heating temperature on the mechanical properties of a hot rolled simulation material of V-containing steel. 19- Discard 1 figure 1050 1150 1250 force U
Sacrificial Warm Sword (0c) No. (a) +050 1250 aa = Warm Skin (0C) 2 (b) 1050 1250 Force a Heat Bu X A'' (℃n

Claims (4)

[Claims] (1) By continuous casting method, C: 0.02~:'0.15%, Sl: 1.50% or less, Mn: 0.5~2.20%, S: 0.010% or less, soL, M: 0.005-0.020%, V:
After obtaining a steel slab consisting of 0.02 to 0.30%, N: 0.0050 to 0.0200%, Fe and unavoidable impurities, and residue (more than weight %), this is A3.
The material is maintained within a temperature range of 1180°C for a certain period of time, and then hot rolled at a temperature of 1100°C or less at a processing rate of 70% or more, and the rolling is completed in a temperature range of 850°C to 680°C. After that, the rolled material is rapidly cooled at a cooling rate of 0°C/sec or more, and once the rolled material has cooled to a temperature of 650 to 500°C, it is coiled in that temperature range. A method for producing a vanadium-containing high-tensile hot-rolled copper strip. (2) The method according to claim 1, wherein the slab obtained by the continuous casting method is maintained at a temperature range of A3 point to 1180°C for a certain period of time without being cooled, and then hot rolled. A method for producing a vanadium-containing high-tensile hot-rolled copper strip. (3) Continuous casting method: C: 0.02-0.15%, Si: 1.50% or less, Mn: 0.5-2.20%, S: 0.010% or more. sot, Alt: 0.005-0.020%, V: 0
．． 02-030%, N: 0.0050-0.0200%, and further includes: Nl): 0.10% or less, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0 Contains one or more of the following: .5% or more, Cr: ], O% or less, Ca: 0.0100% or less, P'e and unavoidable impurities, the balance 9. (by weight) After obtaining a steel slab of A3
The material is maintained within a temperature range of 11.80°C for a certain period of time, then hot rolled at a temperature of 1100°C or less at a processing rate of 70% or more, and the rolling is completed in a temperature range of 850°C to 680°C. Then, the rolled material is rapidly cooled at a cooling rate of 3°C/sec, and when the rolled material is cooled to a temperature of 650 to 500°C, it is coiled in that temperature range. An excellent method for producing vanadium-containing high-tensile hot-rolled steel strip. (4) The slab obtained by continuous casting is kept in the temperature range of A3 point to 1180°C for a certain period of time without being cooled, and then hot rolled! 1! iπ [The method for producing a vanadium-free high-tensile strength hot-rolled steel strip according to claim 3.