JPH04168217A - Manufacture of high toughness and high tensile strength hot rolled steel sheet excellent in uniformity - Google Patents

Abstract

PURPOSE:To manufacture a hot rolled steel sheet excellent in uniformity, in a finish rolling stage for a sheet bar after coarse rolling, by specifying rolling and cooling conditions. CONSTITUTION:In the hot rolling of continuously cast slab stock, the finish rolling of a coil-shaped sheet bar after coarse rolling is started at >=50% draft at <=950 deg.C and in the range of 70 to 800 deg.C rolling finishing temp. Subsequently, the edges of the sheet bar coil finished with coiling are connected, and finish rolling is continuously executed. In succession, it is rapidly cooled at >=5 deg.C/S cooling rate and is coiled in the temp. range of 450 to 600 deg.C. In this way, the high toughness and high tensile strength hot rolled steel sheet excellent in uniformity can be manufactured.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to a method for manufacturing a high-toughness, high-tensile strength hot-rolled steel sheet that has excellent uniformity in terms of toughness and tensile properties, and is particularly applicable to line pipes, structures, etc. This method is useful as a method for producing low-temperature steel plates.

(Prior Art) Conventionally, as a method for manufacturing high toughness/high tensile strength hot rolled steel sheets used for line pipes, structures, etc., for example, as disclosed in Japanese Patent Publication No. 58-19724, Nb
As disclosed in Japanese Patent Application Laid-open No. 61-34116, a method is disclosed in which high toughness is obtained by controlled rolling of steel added with P, and at the same time, separation resistance is improved by lowering P. In addition to the above, there is a method of preventing deterioration due to strain aging and obtaining high toughness by adding Ti and low winding temperature.

Since these are based on controlled rolling of Nb-containing steel, they require control of the rolling reduction in the austenite low temperature range and a low rolling end temperature.

This is a rolling process that is difficult to control, and even if proper rolling is performed with appropriate ingredients, there may be a temperature drop from the rolling tip to the rolling end or a local temperature drop at the rolling tip or rear end within the same coil. Because the rolling temperature differs in the longitudinal direction of the coil, the uniformity of material properties is poor, and there are parts in a single coil where predetermined properties cannot be obtained, which often results in poor product yield.

In addition, so-called zooming up, which increases the rolling speed as a method of reducing the temperature difference, increases the variation in frictional resistance between the rolls and the surface of the steel sheet during rolling, and reduces rolling in the longitudinal direction and thickness direction of the sheet. There are problems such as distortion becoming inconsistent and uniform characteristics not being obtained.

(Problem B to be Solved by the Invention) This invention provides high-toughness and high-tensile heat with excellent uniformity by uniformizing the temperature of the rolled material when performing controlled rolling in finish rolling of hot rolling. The purpose is to manufacture rolled steel plates.

(Means for Solving the Problems) This invention involves hot rolling of a steel billet material having a chemical composition that provides high toughness and high tensile strength, by reversing the rolling tip and trailing end in rough rolling of the material to be rolled in finish rolling. In addition, by sequentially connecting the rolled parts and performing controlled rolling, the rolling temperature is made uniform over the entire length of the rolled material, and hot-rolled sheets with excellent uniformity are manufactured. That's right.

1. C: 0.01 wt% or more, 0.3 wt% or less Si: 0.5 wt% or less Mn: 0.5 wt% or more, 2.0 wt% or less
A1: 0.01wt% or more, 0.10wt% or less P
: 0.025wt% or less S: 0.010wt% or less and N: 0.0
050 wt% or less, and Ti = 0.005 wt% or more and 0.100 wt% or less,
and Nb...0.005%1t% or more, 0.100w
When hot rolling a continuous cast billet material containing one or two selected from t% or less, with the remainder consisting of iron and unavoidable impurities, the sheet bar that has gone through the rough rolling stage is once rolled into a coil. After that, finish rolling is started from the end of the winding at a rolling reduction rate of 50% or more at 950°C or less and a rolling end temperature of 700°C or more and 800°C or less, and at the rear end, The winding end of the succeeding sheet bar coil is connected and finish rolling is performed continuously.Following this finish rolling,
2, cooling at a cooling rate of /S or more, 450°C or more, 600
A method for producing a high-toughness, high-tensile strength hot-rolled steel sheet with excellent uniformity, which is characterized by winding at a temperature range below ℃.

2. C: 0.01 wt% or more, 0.3 wt% or less St: 0.5 wt% or less Mn: 0.5 wt% or more, 2.0 wt% or less 5ol
A1: 0.01 wt% or more, 0.10 wt% or less P
: 0.025wt% or less S: 0.010wt% or less and N: 0.0
050 wt% or less, Ti...0.005 wt% or more, 0.100 wt% or less, and Nb...0.005 wt% or more, 0.100
wt% or less, and Cr: 0.5wt% or less, Ni: 0.5wt% or less, Mo: 0.3wt% or less, V: 0.15wt% or less, B: 0.0050wt% or less, Cu: 0.50wt% or less, Ca: 0.010wt% or less, and REM: 0
．． When hot rolling a continuous cast billet material containing one or more selected from the following at 0.10 wt% or less, with the remainder being iron and unavoidable impurities, the sheet bar that has undergone the rough rolling stage is It is wound into a coil shape, and then finish rolling is started from the end of the winding at a rolling reduction rate of 50% or more at 950°C or less and a rolling end temperature of 700°C or more and 800°C or less. , the winding ends of the following sheet bar coils are connected, finish rolling is performed continuously, and following this finish rolling, 5°
Rapid cooling at a cooling rate of c/3 or higher, 450℃ or higher, 600℃
A method for producing a high-toughness, high-tensile strength hot-rolled steel sheet with excellent uniformity, which is characterized by winding at a temperature range below ℃.

Here, the seat bar connection method is upset welding,
High frequency heating welding or the like may be used, and there is no particular limitation.

(Function) First, the reason for limiting the component composition in this invention will be described.

C: A component for improving strength, and must be contained in an amount of 0.01 wt% or more in order to improve strength.

However, if it exceeds 0.30 wt%, weldability and low-temperature toughness deteriorate. Therefore, its content is 0.01wt
% or more and 0.30wt% or less.

Si: Effective as a deoxidizing agent, but if it exceeds 0.5 wt%, toughness will be impaired. Therefore, its content is 0,
The content shall be 5 wt% or less.

Mn2 is an effective component for improving strength and toughness, with 0
．． 5 wt% or more is required. However, 2.0%1t%
If it exceeds this, weldability will deteriorate. Therefore, the content of Q is 5 wt% or more and 2.0 wt%.

Ti: Significantly increases strength by adding a small amount, and also has solid solution N
This is a preferable component that prevents deterioration of toughness due to strain aging. The effect can be expected at 0.005 wt% or more, but it becomes saturated when it exceeds 0.100 wt%. Therefore, its content is 0.05 wt% or more, 0.1
00t% or less.

Nb: Like Ti, it is a preferred component that significantly increases strength when added in a small amount and improves toughness through a synergistic effect with controlled rolling. The effect can be expected at 0.005 wt% or more, but it becomes saturated when it exceeds 0.10 mt%.

Therefore, its content is 0.005 ent% or more, 0.005 ent% or more. ．． 010wt% or less.

SolAl: Used for deoxidizing steel. Its effect is 0
．． It occurs at 0.010 wt% or more, and becomes saturated when it exceeds 0.100 wt%. Therefore, its content is 0.010wt
% or more and 0.100wt% or less.

P: Because it can deteriorate low-temperature toughness and cause hot cracking during welding, its content is 0.025 im.
t% or less.

S: Since it combines with Mn to form A-based inclusions and deteriorates toughness and ductility, its content is set to 0.010 wt% or less.

N: Since it deteriorates toughness, it is better to have as little as possible, but considering economic efficiency, the content is set to 0.010 wt% or less.

Furthermore, in this invention, in addition to the above-mentioned component composition examples, for example, Cr, Ni, Mo, V, B,
Cu.

One or more selected from Ca and REM are contained.

The main purpose of containing these components is to improve strength and toughness, and the amount of each added is within a range that does not impede weldability or toughness.

That is, these are as follows.

Cr, Ni: These components are effective for improving strength and toughness, but if both are contained in excess of 0.5 wt%, weldability deteriorates. Therefore, both upper limits are 0.5
Let it be wt%.

Mo2 is an effective component for improving strength and toughness, but 0.3
If the content exceeds wt%, weldability will deteriorate.

Therefore, the upper limit is set to 0.3 wt%.

(2): This is a component that can be expected to significantly improve strength when added in a small amount, but if it is contained in an amount exceeding 0.15 wt%, the effect is saturated and weldability deteriorates. Therefore, we set the upper limit to 0.15
Let it be wt%.

B: A component that improves hardenability and improves strength and toughness, but if it is contained in an amount exceeding 0.0050 wt%, the effect is saturated and weldability deteriorates. Therefore, the upper limit is set to 0.005 wt%.

Cu: An effective component for improving strength and toughness, but 0.5
If the content exceeds 0 wt%, weldability will deteriorate.

Therefore, the upper limit is set to 0.5 wt%.

Ca, REM: This component is not only preferable for low-temperature toughness and ductility by controlling the morphology of inclusions, but is also preferable as a countermeasure against separation. However, both 0.010 w
If the content exceeds t%, weldability and toughness will deteriorate. Therefore, both upper limits are set at 0.010-t%.

Next, the hot rolling conditions of this invention and subsequent cooling,
The winding conditions etc. will be described below.

Hot rolling: The key point of this invention is to wind up the sheet bar after rough rolling into a coil shape.

In other words, by winding the rough-rolled sheet bar into a coil shape and then performing finish rolling while unwinding, the leading and trailing ends of the material to be rolled during rough rolling are reversed during finish rolling. .

For this reason, the local temperature drop at the trailing edge of the rough rolling tip is inevitable, and under the temperature gradient that inevitably occurs from the rolling leading edge to the trailing edge, the material to be rolled has a low temperature during finish rolling. The material is rolled from the trailing end side during rough rolling, and the temperature of the entire length of the material to be rolled is made uniform during finish rolling.

Moreover, by winding it into a coil after rough rolling, there is an effect of uniformizing the temperature. In particular, the localized temperature drop at the rolling tip during rough rolling is reheated by being rolled into the coil after rough rolling, and the temperature is made uniform. At this temperature, finish rolling is performed.

Furthermore, by winding the sheet bar after rough rolling into a coil, it is easy to connect it to the preceding sheet bar, and this connection allows rolling without rolling edges in finish rolling, except for the first and last rolled material. , Therefore, there is no local temperature drop at the rolling end during finish rolling.

By doing this, it is possible to eliminate the deviation of the rolling temperature at the leading and trailing ends of the rolled material and the unevenness of rolling strain caused by increasing the rolling speed to adjust the rolling temperature, which has been observed in the past. A uniform material can be obtained.

Next, under the rolling conditions, set the rolling reduction rate to 950°C or less!
The reason why it is set to 50% or more is that if the temperature is higher than this and if the processing rate is lower than this, the introduction of deformation bands into the austenite will be insufficient, the final structure will not be refined, and it will not be sufficiently refined. This is because strength and toughness cannot be obtained.Furthermore, the reason why the rolling end temperature range is set to 700°C or more and 800°C or less is that if the temperature is higher than this range, ferrite per unit area will be generated from the deformed band introduced into the austenite. Since the number of grains decreases, the final structure will not become finer, and if the temperature is lower than this range, a large amount of ferrite will be generated and rolling will continue, so it is difficult to warmly process ferrite. The anisotropy based on
This is due to deterioration of toughness.

Cooling rate: If the cooling rate is slow, due to grain boundary segregation of P (brittleness is likely to occur), grain growth of ferrite grains occurs, making it impossible to ensure strength and toughness.For this reason, the cooling rate is 5℃/S. or more is required.

Winding temperature: If the winding temperature is high, the strengthening by precipitates such as Nb and Ti cannot be effectively utilized, and embrittlement due to grain boundary segregation of P tends to occur, so it must be kept at 600°C or less. If the winding temperature is low, it tends to lead to defects in the coil shape, resulting in poor yield, and solid solution C and N tend to remain in the steel, which is not good for strain aging properties, so a temperature of 450°C or higher is required. .

Next, among the rolling conditions, experimental results regarding the rolling end temperature, which greatly affects toughness, will be described below.

Warm C: 0.07wt%, Si: 0.14w
t%, Mn2 1.42wt%, Nb:
0.04wt%, SolAl: 0.02wt%, P
: 0.008wt%, S: 0.002wt%,
A steel billet material having a component composition of N: 0.003 wt% was rolled to a thickness of 5 mm under the following hot rolling conditions in which the rolling end temperature was varied, and the toughness of the obtained hot rolled sheet was investigated.

Rolling start temperature: 1150℃ Winding temperature of sheet bar after rough rolling: 1000℃950
Rolling reduction ratio F0% at temperatures below 0.degree. C. Rolling end temperature -670.degree. C. to 830.degree. C. Cooling rate: 30.degree. C./S Product winding temperature 2500.degree. C. The results of these investigations are summarized in FIG. 2.

As is clear from FIG. 2, when the rolling end temperature is within the limited range of 700°C to 800"C of the present invention, excellent toughness is exhibited with vTs of 1100"C or less; At temperatures outside of this range, vTs is 1100°C or higher, resulting in poor toughness. Therefore, in order to obtain high toughness,
The rolling end temperature must be in the range of 700°C to 800°C.

(Example) Smelted in a 15-cm upper converter furnace, C: 0.08wt%, Si: 0.14wt%, M
n: 1.4wt%, Ti: 0.03wt%, N
b: 0.04wt%, SolAl: 0.02w
t%, Pro. 008wt%, S: 0.002wt
%, N: 0.004 wt%, and hot rolling was performed using the compatible steel of the present invention to obtain a hot-rolled sheet coil.

Hot rolling conditions are as follows: Rolling start temperature: 1150°C, Coiling temperature of sheet bar after rough rolling: 1000°C 195
Rolling ratio near 0% below 0°C, rolling end temperature near 50”C1, cooling rate: 30°C/S, product winding temperature: 500°C1, finishing plate thickness = 5 mm, and in the finish rolling, rough A hot-rolled sheet coil was manufactured by a method in which the leading and trailing ends of the coil of a sheet bar wound up after rolling were connected to the preceding material and the succeeding material, respectively, and rolling was performed continuously, and this was used as an example adapted to the present invention.

On the other hand, as a normally rolled material, under the above hot rolling conditions,
A comparative example was a hot-rolled sheet coil that was rolled independently without winding the sheet bar after rough rolling or connecting it to the ground-rolled material.

The toughness and tensile properties of these hot-rolled sheet coils in the longitudinal direction were investigated. The results of this investigation, ie, changes in toughness and tensile properties in the longitudinal direction of the coil, are shown in Figure 1.

As is clear from FIG. 1, in the comparative example, the toughness and tensile properties deteriorated significantly at both ends of the coil, and YS showed a tendency to gradually increase from the rolling tip side to the rolling end side.
Poor uniformity over the entire length of the coil.

On the other hand, in the adapted example, although there were some changes at the ends of the coil, uniform characteristics were obtained over the entire length, indicating excellent uniformity.

z旌1 21 compatible steel types of the present invention and 3 comparative steel types, a total of 24 steel types, having the chemical composition shown in Table 1 and melted by a conventional method, were rolled under the hot rolling conditions shown in Table 2, A hot-rolled plate with a thickness of 5 m was used.

In addition, in Table 1, P, S, and N are steel symbols that are outside the limited range of this invention, and E and F are shown as comparative steels, and Table 2
Hot-rolled sheets manufactured using these steels (sample Nα1
0 to 12) were shown as comparative examples together with hot rolled sheets (sample Nos. 4 to 9) whose hot rolling conditions were outside the limited range of the present invention.

The tensile properties and toughness of these hot rolled sheets were investigated.

The above investigation results are shown in Table 2 together with the hot rolling conditions.
(low rolling reduction of 950°C or less) Sample No. 5 (low rolling end temperature) Sample No. 6 (high rolling end temperature) Sample No. 7 (high winding temperature) Sample No. 8 (low winding temperature) Sample High toughness could not be obtained in the samples with Fk1.9 (slow cooling rate after rolling), and in the chemical composition of the steel, P, S, and N were each outside the upper limit of the preferred range of this invention. Sample 10.1
High toughness cannot be obtained in each sample of No. 1.12.

In contrast to the above comparative examples, all the applicable examples of the present invention have high toughness,
High strength is obtained.

(Effects of the Invention) This invention provides the following advantages: When hot rolling a steel billet material having a chemical composition for high toughness/high tensile strength hot rolled steel sheet, the material to be rolled after rough rolling is wound into a coil shape, By making the leading and trailing ends of the coils loose and the leading and trailing ends of the finish rolling, and by connecting multiple coils and performing continuous finish rolling, we can produce high-toughness, high-strength hot-rolled steel sheets with excellent uniformity over the entire length of the coils. According to the present invention, not only a high-toughness, high-tensile-strength hot-rolled copper plate can be stably obtained, but also the walking performance can be improved.

[Brief explanation of the drawing]

FIG. 1 is a graph showing changes in toughness and tensile properties in the longitudinal direction of the coil for examples adapted to the present invention and comparative examples, and FIG. 2 is a graph showing the relationship between rolling end temperature and toughness.

Claims (1)

[Claims] 1. C: 0.01 wt% or more, 0.3 wt% or less Si:
0.5 wt% or less Mn: 0.5 wt% or more, 2.0 wt% or less SolAl
: 0.01wt% or more, 0.10wt% or less P: 0.0
25 wt% or less S: 0.010 wt% or less, and N: 0.0050 wt% or less, and Ti...0.005 wt% or more and 0.100 wt% or less, and Nb...0.005 wt% Above, 0.100
Contains one or two selected from wt% or less,
When hot rolling continuously cast billet material, the remainder of which consists of iron and unavoidable impurities, the sheet bar that has gone through the rough rolling stage is wound into a coil, and then finish rolling is performed from the end of the winding.
Start when the rolling reduction rate is 50% or more at 950°C or less and the rolling end temperature is in the range of 700°C or more and 800°C or less, and finish by connecting the winding end of the subsequent sheet bar coil to the rear end. High toughness with excellent uniformity characterized by continuous rolling, followed by finish rolling, rapid cooling at a cooling rate of 5°C/S or more, and coiling at a temperature range of 450°C or higher and 600°C or lower.・Manufacturing method for high-tensile hot-rolled steel sheets. 2.C: 0.01wt% or more, 0.3wt% or lessSi:
0.5 wt% or less Mn: 0.5 wt% or more, 2.0 wt% or less SolAl
: 0.01wt% or more, 0.10wt% or less P: 0.0
25 wt% or less S: 0.010 wt% or less, and N: 0.0050 wt% or less, Ti...0.005 wt% or more, 0.100 wt% or less, and Nb...0.005 wt% or more, 0.100
wt% or less, and Cr: 0.5wt% or less, Ni: 0.5wt% or less, Mo: 0.3wt% or less, V: 0.15wt% or less, Contains one or more selected from B: 0.0050 wt% or less, Cu: 0.50 wt% or less, Ca: 0.010 wt% or less, and REM: 0.010 wt% or less, and the remainder is When hot rolling continuously cast billet material with a composition of iron and unavoidable impurities, the sheet bar that has gone through the rough rolling stage is wound into a coil shape, and then finish rolling is performed from the end of the winding.
Start when the rolling reduction rate is 50% or more at 950°C or less and the rolling end temperature is in the range of 700°C or more and 800°C or less, and finish by connecting the winding end of the subsequent sheet bar coil to the rear end. High toughness with excellent uniformity characterized by continuous rolling, followed by finish rolling, rapid cooling at a cooling rate of 5°C/S or more, and coiling at a temperature range of 450°C or higher and 600°C or lower.・Manufacturing method for high-tensile hot-rolled steel sheets.