JPS60204828A - Manufacture of tough and hard hot rolled steel strip having separation resistance - Google Patents

Abstract

PURPOSE:To improve the separation resistance and toughness by hot rolling a contg. prescribed percentages of C, Si, Mn, P, S and sol.Al under prescribed conditions, rapidly cooling the resulting strip, and coiling it. CONSTITUTION:A steel contg. by weight, 0.01-0.25% C, <=0.7% Si, 0.5-1.8% Mn, <=0.015% P, <=0.01% S and 0.01-0.1% sol.Al is refined. The steel is hot rolled at >=50% total draft in a temp. range of <=1,000 deg.C and 850-750 deg.C finishing temp. The resulting strip is rapidly cooled at >=5 deg.C/sec high cooling rate and coiled at 200-500 deg.C to obtain a tough and hard hot rolled steel strip of >=4.5mm. thickness having superior separation resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a strong hot-rolled copper strip having extremely excellent separation resistance and a relatively high yield ratio.

<Industrial Application Fields> In recent years, strong hot-rolled steel strips, which exhibit high strength and excellent toughness as hot-rolled, have been used for various structural members with a yield point design, mainly for lines and f-beams. Especially when the plate thickness is 4.5 van
Demand for the above-mentioned high toughness hot-rolled steel strips has begun to show an increasing trend.

By the way, such a strong hot-rolled copper strip is manufactured by applying controlled pressure IA to steel to which Nb, which is a toughening element, is added.
However, when a test piece is cut from this hot-rolled steel strip and subjected to a Charpy impact test, a unique layered opening called a separation is often observed on the impact fracture surface. This was thought to be the cause of the decrease in shock absorption energy.

And, unlike the case of steel plates that are staged at a plate factory,
Seven J? Hot-rolled steel sheets produced by controlled rolling? Ration occurs not only on impact fracture surfaces, but also on tensile p-rupture surfaces, and countermeasures against this problem have become strongly desired.

Therefore, various studies on separation have been conducted, and it is believed that the above-mentioned separation that occurs in hot rolled steel strips is mainly caused by grain boundary segregation of P that occurs during slow cooling after coiling the copper strip. It has been strongly inferred that this is a type of tempering brittleness.

<Prior art> For this reason, in the past, for example, the
Measures to improve separation resistance by reducing the P content in steel as much as possible, as seen in Publication No. 24, have been proposed, and steps have been taken to improve the properties of strong hot-rolled steel strips. Ta.

However, conventional methods for improving secession and maturation resistance, including the method proposed in the above patent publication, are based on the idea that the addition of Nb is essential for producing high-toughness hot-rolled steel strips. However, it was only aimed at Nb-added hot-rolled steel strips, and even with the measures mentioned above, it was not possible to completely suppress the ration phenomenon.

<Purpose of the Invention> In view of the above-mentioned current situation, the present inventors have developed a hot-rolled steel strip that does not cause rations and has superior strength and toughness compared to conventional materials. In order to advance the process at a lower cost, we conducted extensive research on various steel types, not just Nb-added steel, including their hot rolling conditions, cooling conditions after hot rolling, and winding conditions. As a result, we came to the following findings.

<Findings> (a) The presence of Nb, which is a toughening element, also has an extremely negative effect on the separation resistance performance of hot-rolled steel strips, and even though they are Nb-free steels, they contain P and S. If the amount of steel is kept as low as possible, it is rapidly cooled immediately after hot rolling, and coiled at a significantly lower temperature than previously thought possible, a hot-rolled steel strip with almost no separation occurs. be obtained.

The reason why separation resistance performance deteriorates in Nb-added steel is not clear, but ferrite grains in Nb-added steel are easier to extend and arrange in the rolling direction than in Nb-free steel. Since such phenomena play a major role in the occurrence of separation, it is inferred that it is 4.

(b) To impart a specified high toughness to Nb-free steel,
It is necessary to carry out a combination of specific controlled rolling, subsequent quenching, and low-temperature winding.On the contrary, if these are set under specific conditions, even Nb-free steel can be fully satisfied. What resilience can achieve.

(C) Therefore, when Nb-free steel with minimal P and S contents is subjected to controlled rolling under specific conditions, it is rapidly cooled to a significantly lower coiling temperature than conventional methods, and then coiled. To obtain a high-performance strong hot-rolled steel strip having extremely excellent separation resistance and having sufficiently satisfactory strength and toughness.

<Structure of the Invention> This invention was made based on the above findings, and C: 0.01 to 0.25%, preferably more than 0.5% to 0.25% (hereinafter referred to as component ratio). h is weight -), Si: 0.7% or less, Mn: 0.5 to 1.8%.

P: 0.015% or less, s: o, oio% or less.

aoL, At: 0.01 to 0.10%, and if necessary, V: 0.15% or less,
Ti: 0.03% or less.

Cu: 0.5% or less, Ni: 0.5% or less.

Cr: 0.5% or less, Mo: 0.30% or less.

B: 0.0050% or less, Ca: 0.010% or less, and the balance: Fe and other unavoidable impurities. Cumulative rolling reduction rate at 1000°C or less: More than 50%.

Finishing temperature: After hot rolling at 850-750°C, it is immediately quenched at a cooling rate of 5°C/see or higher, and rolled up at a temperature range of less than 500°C to 200°C, resulting in excellent separation resistance. 4, 5
This method is characterized by the ability to stably form a strong hot-rolled copper strip with a relatively high yield ratio and a plate thickness of over 300 mm.

Next, in the method of the present invention, the reason for numerically limiting the amount of the steel composition and the hot rolling/coiling conditions as described above will be explained.

A2 Composition Component (a) CC The C component has the effect of significantly improving the strength of L steel by low-temperature winding as in the method of the present invention. Therefore, in order to ensure the desired strength, the lower limit of its content t-0,0
1%, but preferably more than 0.5% in order to fully utilize the reinforcing effect of C. On the other hand, if the C content exceeds 0.25%, welding performance deteriorates, so the C content was set at 0.01 to 0.25%.

Si component is an element that works effectively to increase the strength of steel through solid solution hardening, even when added in a small amount.
If the content exceeds %, it will cause deterioration of weldability.
The Si content was set at 0.7% or less.

(c) Mn The Mn component has the effect of improving both the strength and toughness of steel through solid solution hardening, transformation hardening, and fine grain hardening, but if its content is less than 0.5%, this effect will not be achieved. A sufficiently satisfactory effect cannot be obtained, and on the other hand, if the content exceeds 1.8%, weldability deteriorates.
．． It was set at 8%.

(d) P Since P is an undesirable impurity element that deteriorates separation resistance performance, it is better to minimize the amount of P as much as possible, but considering economic efficiency, the upper limit of the P content is set at 0.015%.
It was determined that However, it is desirable to suppress the content to 0.010% or less.

(e) S S is not a measure that causes A-based inclusions to occur due to the combination of hermitages and reduces the toughness and ductility of steel, but S
The upper limit of the content 'k was set at 0.010%.

However, it is desirable to suppress the sail to 0.05% or less.

(f) Aot, Al 5ot, At components are necessary for deoxidizing steel, and in order to ensure a sufficient deoxidizing effect, it is necessary to contain 0.01 or more. On the other hand, even if the content exceeds 0.10%, an improvement effect of 9 or higher cannot be obtained.
sot, At The shoulder included was determined to be 01 to 0.10 饅.

(gl V, Ti, Cu, Ni, Cr, Mo, B and C
a These ingredients have the effect of increasing the strength of steel and improving the toughness, ductility, and cracking resistance of continuously cast slabs, so they can be added in amounts of lai or more if necessary. The reason for limiting the amount of elements added will be explained.

○・V ゛V acid component is a preferable element because it is a preferable element that greatly increases the strength of steel when added in a small amount, but 0.15
(2) The content was set at 0.15% or less, since it would be economically disadvantageous to contain more than 0.15%.

■ Ti The Ti component has the effect of suppressing cracking of continuous casting slabs, but if it is contained in excess of 0.03%, it will promote cracking of continuous casting slabs, so the T1 content should be 0.03%.
- The following has been established.

(iipcu, Ni, and Cr) These components are effective elements for improving the strength and toughness of steel, but in consideration of economic efficiency, the content of each component was determined to be 5% or less.

[Co., Ltd.] MO The Mo component is an element added as a measure against separation, but in the method of this invention, it is rather
It is added to aim for the toughening effect that it has.

However, in consideration of economic efficiency, the upper limit of MO content t
It was set as 0.30%.

■B The B component is an effective element that increases the strength of steel when added in a very small amount, but its effect is saturated with the addition of 0.0050%, so the B content is o, o
It was set as less than oso%.

(2) Ca The Ca component is an effective element for improving toughness and ductility by controlling the morphology of inclusions, but it is also a preferable element as a countermeasure against rations and rations.

However, if the Ca content exceeds 0.010%, the number of inclusions in the steel increases and the toughness and ductility deteriorate, so the Ca content was set at 0.010% or less.

B. Hot rolling/coiling 9 conditions (a) Cumulative rolling reduction rate at 1000°C or below When the 1gM rolling reduction at 1000°C or below falls below 50%,
Since the grain refinement of austenite does not proceed sufficiently and the final structure does not become the desired fine grain structure, sufficient toughness cannot be obtained. For this reason, 10
It was determined that a cumulative reduction rate of 50% or more should be ensured in a temperature range of 00°C or lower.

(b) Hot-rolling finishing temperature If the hot-rolling finishing temperature exceeds 850°C, the grain refinement of forstenite will not proceed and the desired fine grain structure will not be obtained.On the other hand, in the temperature range below 750°C Then, as a result of austenite not recrystallizing or rolling down after ferrite transformation, even steels that do not contain Nb will have an extended grain ferrite structure similar to that of Nb-added steels, which is not inconvenient from the viewpoint of toughness. However, a problem arises in that separation is more likely to occur. Therefore, the hot rolling finishing temperature was set at 850 to 750°C.

(C) Cooling rate If the cooling rate after hot rolling is slower than 5° C./see, separation will easily occur due to embrittlement due to the segregation of P in the steel at grain boundaries.

Therefore, the cooling rate after completion of hot rolling was set at 5° C./see or higher.

(d) Coiling temperature If the coiling temperature is 500°C or higher, even if the cooling rate after hot rolling is 5°C/see or higher, or Vi
On the other hand, if the winding temperature is lower than 200°C, it is not a problem in terms of separation resistance, but the hardness of the ferrite underground Since the increase in the amount of molten C and the inclusion of martensitic structure cause a significant deterioration of toughness and a significant decrease in the yield ratio, the winding temperature should be set to less than 500°C to 2.
The temperature was set at 00°C.

Figure 1 shows 0.15% C, which is the steel subject to the present invention.
-0,25%5t-1,35%Mn -0,008%P
-0,002%S -0,03%At -0,0018%
It is a graph showing the influence of the coiling temperature on the toughness and hardness of N steel, hot rolling heating temperature: 1200°C.

$ stacking efficiency 267% at 1000°C or less, hot rolling finishing temperature: 820°C, finishing plate thickness: 11 m, cooling rate between coiling after hot rolling is all 5 to b. Regarding the items in the box This is what I researched.

In addition, the "total length of seven groove-like cracks on the tensile stress fracture surface" is the total length of each of the separation-like cracks 2 observed on the fracture surface of the tensile stress test piece 1, as shown in Figure 2. It is expressed as the total length Ctt+tz).

Figure 1 also shows that when steel with a specific composition is hot-rolled and coiled under the conditions of the present invention, excellent separation resistance, toughness, and a relatively high yield ratio (7011) can be achieved. it is obvious.

Next, this invention will be specifically explained using an example and a comparative example.

<Example> First, steels A to 0 having the compositions shown in Table 1 were produced by a conventional method.

Next, these strips were hot-rolled under the conditions shown in Table 2, and wound up to produce a hot strip having a thickness of 211 strips.

Test pieces were cut out from each of the hot-rolled steel strips obtained in this way, and their mechanical properties and erosion resistance were examined, and the results shown in Table 2 were obtained. Ta. Incidentally, the separation resistance performance was measured by the same method used to obtain the results shown in FIG.

As is clear from the results shown in Table 2, all hot-rolled copper strips obtained by the method satisfying the conditions of the present invention have
Excellent separation resistance performance and high strength and toughness of 70%
In contrast, the hot-rolled steel strip obtained by the test number 1 method using corner-added steel as the material and the steel with high P and S contents have an extremely high yield ratio. All of the hot-rolled steel strips obtained by the method of Test No. 2 had poor separation resistance, and also had low hot-rolling efficiency at temperatures below 100°C, and poor hot-rolling finishing temperature and post-hot rolling temperature. Test numbers 3 to 3 in which the cooling rate of deviates from the conditions of the present invention
It can be seen that the hot-rolled steel strip obtained by method 5 is inferior in toughness, strength, or ration resistance.

Overall effect〉 As mentioned above, according to the present invention, in the neck of hot rolling,
Hot-rolled steel sheets suitable for yield point design that have excellent separation resistance and toughness can be manufactured stably at low cost, making them suitable for lines/4'-eaves and other structures in low-temperature environments. It can be used in structural materials where force is applied to the plate in the direction of the plate (because separation tends to occur when the properties in the thickness direction are poor), and it exhibits excellent properties, making it suitable for industrial use. Moreover, useful effects are brought about.

[Brief explanation of drawings]

Figure 1 is a graph showing the influence of the winding temperature on steel toughness and separation I11, and Figure 2 explains how to display the ``total length of separation-like cracks on the tensile υ fracture surface.'' FIG. In the drawings: 1...Tensile test piece, 2...Separation-like division. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo TL or 1 Name Academic 1 Figure Port 2E/J ('C) Manufacturing method for strong and tough hot-rolled copper strips 3, relationship with the amended case Patent applicant representative Norifumi Kumagai 4, agent Address 8th floor, Soyasu Daini Building, 23-chome, Kanda Nishikicho, Chiyoda-ku, Tokyo Talk about 〒+ot (03) 233-1676/1677
Contents of the amendment 1) "No. 1" and "Table 2" on pages 16 and 17 of the specification are corrected as shown in the attached sheet. that's all

Claims (2)

[Claims] (1) C: 0.01 to 0.25% by weight. St: 0.7- or less. Mn: 0.5-1.8%. P: 0.015% or less. S: 0.010% or less. Sol, Al: 0.01-0.10% Balance: Fe and other unavoidable impurities, Cumulative rolling reduction rate at 1000°C or less: 50% or more. The finish is characterized by hot rolling at a temperature of 850 to 750°C, followed by immediate cooling at a cooling rate of 5°C/sec or more, and winding at a temperature range of less than 500°C to 200°C. , a method for producing a strong hot-rolled copper strip having a thickness of 4.5 or more and having excellent seration/elongation resistance. (2) In terms of polymerization ratio, C: 0.01 to 0.25%. St: 0.7% or less. Mn: 0.5-1.8%. P: 0.015% or less. S: 0.010% or less. Contains 5otJ,t: 0.01 to 0.10%, and further contains V: 0.15% or less. Tf: 0.03% or less. Cu: 0.5% or less. Ni: 0.5- or less. Cr: 0.5% or less. Mo: 0.30% or less. B: 0.0050% or less. Ca: Contains one or more of 0.010% or less, and the remainder: Fe and other unavoidable impurities. Cumulative reduction rate at 1000°C or less: 50 inches or more. After hot rolling at a finishing temperature of 2850 to 750°C, immediately quench at a cooling rate of 5°C/sec or more and winding at a temperature range of less than 500°C to 200°C, 4. A method for producing a strong hot-rolled copper strip having a thickness of .5 m or more and having excellent separation resistance.