JPS648685B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for manufacturing a thick-walled hot-rolled high-strength steel strip coil that exhibits little strain aging deterioration and has excellent toughness and separation resistance. More specifically, the present invention uses Nb-containing steel with a composition characterized by low P, trace Ti, and low N, is rapidly cooled after controlled rolling, and is further coiled at a significantly lower temperature than conventional methods. The present invention relates to a method for manufacturing a thick-walled hot-rolled high-strength steel strip coil having little strain aging deterioration and excellent toughness and separation resistance. Here, "separation" refers to layered openings observed on the fracture surface of an impact test piece, and the occurrence of such separation means a decrease in ductility. In particular, the present invention is useful as a method for manufacturing low-temperature steel plates used for line pipes, structures, etc. with a thickness of 4.5 mm or more, or when separation is inferior in properties such as ductility or toughness in the thickness direction. This is useful as a manufacturing method for low-temperature steel plates that are used in situations where force is applied in the thickness direction due to the structure. (Prior art) As disclosed in Japanese Patent Publication No. 58-19724, the method for manufacturing hot-rolled high-strength steel sheets used for such purposes has been to perform controlled rolling on Nb-containing steel. There is a method of obtaining toughness and at the same time improving separation resistance by reducing P to 0.01%. (Problem to be solved by the invention) Separation is thought to be caused by high-temperature winding, and the above patent publication also states that it is disadvantageous in terms of work efficiency, process, and equipment, but the hot rolling is completed. The possibility of post-cooling and winding at a low temperature is also suggested. However, the inventors conducted various studies on this point, including follow-up tests, and found that quenching after hot rolling leaves a large thermal strain on the steel sheet, which causes strain age hardening, and eventually reduces separation resistance. It turned out that the improvement was not satisfactory. (Means for Solving the Problems) That is, the present invention aims to provide a method for manufacturing a hot-rolled coil with excellent toughness (including measures against strain aging deterioration) and separation resistance. To achieve this, controlled rolling is applied to the Nb-added steel, followed by rapid cooling and low-temperature winding. At the same time, the separation resistance is improved by lowering the P content, and as a countermeasure against strain aging, the steel composition is adjusted. and trace amount of Ti
The addition of nitrogen reduces the amount of nitrogen. In other words, the present invention aims to improve separation resistance by rapidly cooling to an extremely low temperature range that has not been conventionally carried out and then winding the steel. They identified the following. According to the research results of the present inventors, a method for manufacturing a hot-rolled high-strength steel coil with excellent toughness can be obtained as a synergistic effect of the above-mentioned measures. Here, the present invention is expressed in weight%: C: 0.01 to 0.25%, Si: 0.7% or less, Mn 0.5 to 1.8%, P: 0.025% or less, S: 0.010% or less, Nb: 0.005 to 0.15%, Ti : 0.005 to 0.05%, Sol.Al: 0.01 to 0.10%, N: 0.0050% or less, and if necessary, V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50%. Hereinafter, steel having a composition consisting of one or more of Mo: 0.30%, B: 0.0050% or less, and Ca: 0.010% or less, and the remainder substantially Fe, is processed at a processing rate of 50% or more at a temperature of 1000°C or less. It is characterized by hot rolling, finishing the hot rolling at a temperature of 850 to 700°C, then quenching at a cooling rate of 5°C/sec or more, and winding at a temperature of less than 500°C to 200°C. , a method for manufacturing a strong hot-rolled coil having a plate thickness of 4.5 mm or more. In the present invention, the reason why the steel composition is limited as described above is as follows. C: A preferable element that increases strength. From the viewpoint of strength, the lower limit is set at 0.01%. But 0.25%
If the content is higher than this, welding performance and low-temperature toughness will deteriorate, so the upper limit is set at 0.25%. Preferably it is 0.15% or less. Si: Effective in increasing strength through solid solution hardening. The upper limit is set at 0.7% based on weldability. Mn: An effective element for toughening through solid solution hardening, transformation hardening, and fine grain hardening. The lower limit is 0.5%, which is the amount at which such hardening can be expected.
The upper limit is set at 1.8% to ensure weldability. P: P is an element that is unfavorable for separation performance, and it is better to have as little amount as possible.
In the present invention, the upper limit is set at 0.025% in consideration of the fact that the adverse effects of P can be considerably reduced by low-temperature winding, as will be described later, and in consideration of economic efficiency.
Preferably, it is 0.010% or less. S: Combines with Mn to form A-based inclusions, improving toughness,
It is an element that not only reduces ductility but also tends to induce separation, so it is better to have as little as possible. Considering economic efficiency, the upper limit is set to 0.010%, but preferably 0.005% or less. Nb: A preferable element that provides excellent low-temperature toughness due to a large increase in strength with the addition of a small amount and a synergistic effect with controlled rolling. The lower limit is set at 0.005% as the amount at which such effects can be expected greatly, and the upper limit is set at 0.15% from the viewpoint of weldability. Ti: When coiling at a significantly low temperature as in the present invention, strain aging deterioration due to solid solution N tends to occur, but this is a preferable element as a countermeasure against such strain aging deterioration. It is also generally a preferable element as a countermeasure against cracks in CC. Therefore, the lower limit is set at 0.005% as the amount at which the effect is expected.
shall be. The upper limit is the amount at which the above effect is saturated.
It shall be 0.05%. Preferably it is 0.010-0.03%. Sol.Al: Added as a deoxidizing agent. The lower limit is 0.010% where the effect occurs, and the upper limit is 0.10% where the effect is saturated. N: When winding at a significantly low temperature as in the present invention, strain aging deterioration due to solid solution N tends to occur. Therefore, it is better to have as few as possible. Considering economic efficiency, the upper limit is set at 0.0050%. Furthermore, in the present invention, at least one of the following elements may be added, if necessary, in order to improve the mechanical properties of the hot rolled steel sheet, particularly to strengthen it. V: A preferable element that causes a large increase in strength even when added in a small amount. The upper limit is set at 0.15% considering economic efficiency. Cu, Ni, Cr: These are elements effective for toughening. Considering economic efficiency, the upper limit is set at 0.50% for each. Mo: An element effective for toughening. Considering economic efficiency, the upper limit is set at 0.30%. B: In the case of winding at a significantly low temperature after quenching as in the present invention, B is a preferable element that increases strength even when added in a very small amount. The upper limit is set at 0.0050%, where the effect is saturated. Ca: This element is not only preferable for low-temperature toughness and ductility by controlling the morphology of inclusions, but also for preventing separation. However, if it is too large, the amount of inclusions in the steel increases, resulting in deterioration of toughness and ductility. Therefore, the upper limit is set at 0.010%. The reason for limiting the hot rolling conditions in the present invention is as follows. Rolling conditions: First, the reason why hot rolling is performed at a working rate of 50% or more at 1000°C or less is that if the temperature is higher than this or lower than this, the introduction of deformation bands into the austenite is insufficient and the final This is because the resulting structure is not refined and sufficient toughness cannot be obtained. Also, the reason why rolling is finished at a temperature of 850 to 700℃ is because if the temperature is higher than this, the number of ferrite grains per unit area generated from the deformation zone introduced into the austenite will decrease, so the final result will still be obtained. The tissue does not become finer. In addition, if the temperature is lower than this range, a large amount of ferrite will be produced and rolling will continue, which not only increases the anisotropy due to warm-worked ferrite, but also causes problems in rolling operations such as bumping in tandem rolling mills. These troubles occur frequently, making rolling difficult. Cooling/winding conditions: After the above controlled rolling, quench cooling at 5°C/sec or more,
The reason why the film is then wound at a temperature of less than 500°C to 200°C is that separation is likely to occur due to embrittlement due to grain boundary segregation of P at a winding temperature slower than this cooling rate or higher than this temperature. Furthermore, at a coiling temperature lower than 200°C, a so-called two-phase structure containing martensitic structure occurs, resulting in a significantly low yield ratio. It also deteriorates and separation occurs frequently. Further, at a coiling temperature lower than 200° C., solid solution C remains in the steel even with the chemical composition specified in the present invention, and the strain aging resistance is not good. On the other hand, if the coiling temperature is 500° C. or higher, even if the cooling rate after hot rolling is 5° C./sec or higher, good separation resistance cannot be achieved. FIG. 1 shows the steel that is the object of the present invention.
0.10%C-0.25%Si-1.35%Mn-0.008%P-
0.001%S-0.065%Nb-0.015%Ti-0.03%Al-
This is a graph showing the effect of coiling temperature on the toughness and separation of 0.0025% N steel. Heating temperature during hot rolling: 1150°C, cumulative reduction rate below 1000°C: 80%, hot rolling finish. The temperature: 750°C, finished plate thickness: 7.0mm, and the cooling rate until hot rolling and winding were all within the range of 10 to 50°C/sec. The "total length of separation-like cracks on the tensile fracture surface" is
As shown in FIG. 2, it was expressed as the total length (l ₁ +l ₂ ) of the separation-like cracks 2 observed on the fracture surface of the tensile test piece 1. Figure 1 also shows that when steel with a specific composition is hot-rolled and coiled under the conditions of the present invention, it has excellent separation resistance, toughness, and a relatively high yield ratio (70
%) is clearly achievable. Next, the present invention will be further explained by examples. Example Steel types A to Q, whose steel compositions are shown in Table 1, are melted in the usual manner to produce slabs (CC slabs).
Hot rolling and subsequent heat treatment were performed under the hot rolling conditions shown in the table. In each table, conditions outside the range defined in the present invention are marked with an asterisk (*). The mechanical properties of each test rolled material obtained were determined. The results are also summarized in Table 2. As understood from Tables 1 and 2, the hot rolled steel sheets obtained by the manufacturing method of the present invention all have excellent properties in terms of toughness, strain aging resistance, and separation resistance. There is. In particular, when comparing Tests Nos. 1 to 3 and Tests Nos. 9 to 22, which are examples of the present invention, it can be seen that the Ti-added low N steel is superior in terms of separation resistance. Note that in Test Nos. 1 and 2, which were processed under conditions outside the range specified in the present invention, the Ti content was low or the N content was high, so the strain aging resistance was poor, and Test No. 3 The case where Ni is added instead of is shown, but the strain aging performance is similarly not good. Although the reason is unknown, the separation performance in test Nos. 1 to 3 was also poor. Similarly, in Test Nos. 4 to 6, the low temperature toughness was not good, and similarly, in Tests Nos. 7 and 8, the separation resistance performance was not good. Furthermore, even in the case of the present invention, as can be seen from the data shown in Test Nos. 9 and 10, P
Test No. 9, which had a lower amount, had better separation resistance. In addition, when quenching after rolling and winding at a low temperature as in the present invention, a curtain wall cooling method with a slit-type nozzle is preferable to the conventional laminar flow method as a cooling method on a hot run table. Desirable from the standpoint of cooling efficiency and uniform cooling.

【table】

[Table] (Note) *: Outside the scope of the present invention

【table】 [Brief explanation of the drawing]

Figure 1 is a graph showing the effect of coiling temperature on the toughness and separation of steel; and Figure 2 is a graph to explain how to display the "total length of separation-like cracks on a tensile fracture surface." FIG. 1...Tensile test piece, 2...Separation-like cracks.

Claims (1)

[Claims] 1% by weight: C: 0.01 to 0.25%, Si: 0.7% or less, Mn 0.5 to 1.8%, P: 0.025% or less, S: 0.010% or less, Nb: 0.005 to 0.15%, A steel having a composition consisting of Ti: 0.005 to 0.05%, Sol.Al: 0.01 to 0.10%, N: 0.0050% or less, and the remainder substantially Fe is hot rolled at a processing rate of 50% or more at 1000°C or less, Hot rolling is completed at a temperature of 850 to 700°C, then quenched at a cooling rate of 5°C/sec or more, and coiled at a temperature of less than 500°C to 200°C, with a thickness of 4.5 mm or more. A method for manufacturing a strong hot-rolled coil with plate thickness. 2 In weight%, C: 0.01-0.25%, Si: 0.7% or less, Mn 0.5-1.8%, P: 0.025% or less, S: 0.010% or less, Nb: 0.005-0.15%, Ti: 0.005-0.05%. , Sol.Al: 0.01 to 0.10%, N: 0.0050% or less, and further contains V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30%, A steel having a composition consisting of one or more of B: 0.0050% or less, Ca: 0.010% or less, and the remainder substantially Fe is hot rolled at a processing rate of 50% or more at 1000℃ or less to produce 850 A plate with a thickness of 4.5 mm or more, characterized in that hot rolling is completed at a temperature of ~700°C, then quenched at a cooling rate of 5°C/sec or more, and coiled at a temperature of less than 500°C - 200°C. A method for manufacturing a strong hot-rolled coil having a high thickness.