JPS6196030A - Manufacture of high strength and high toughness hot rolled steel plate having superior resistance to hydrogen induced cracking and stress corrosion cracking - Google Patents

Abstract

PURPOSE:To obtain a high strength and high toughness hot rolled steel plate having superior resistance to hydrogen induced cracking and stress corrosion cracking by subjecting a steel billet contg. specified amounts of C, Si, Mn and Al to rough rolling, regulated finish rolling, cooling at a regulated cooling rate and slight rolling-down. CONSTITUTION:The steel consisting of, by weight, 0.03-0.3% C, 0.01-1.0% Si, 0.5-2.0% Mn, 0.005-0.5% Al and the balance Fe with inevitable impurities is rough-rolled and finish-rolled at 900-800 deg.C at >=30% total draft in a temp. range below an austenite recrystallization temp. The resulting steel plate is cooled to 750-650 deg.C at 15-80 deg.C/sec average cooling rate, and it is slightly rolled down at 3-20% reduction of area. By such method, a large number of fine cementite grains are deposited in the ferrite grains while the precipitation of cementite on the ferrite grain boundaries is suppressed. The steel may further contain the proper amount of Nb, Ti, V, Cr, Cu, Mo, Ni, Ca, etc., is necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength, high-toughness hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance.

In recent years, there has been a growing demand for high-quality steel sheets that not only have high strength and toughness, but also have excellent resistance to hydrogen-induced cracking and stress corrosion cracking. As is already well known, the factors that have the greatest effect on corrosion cracking of steel plates are:
Due to macro- and micro-segregation in steel and elongation of non-metallic inclusions, high-quality steel sheets such as those mentioned above have traditionally been made with S and P.
Steel plates made of quenched and tempered steel with reduced quenching and homogeneous tempered steel such as tempered bainitic steel are used. However, such a method has a drawback in that high-quality steel sheets cannot be mass-produced at low cost because of poor productivity and economic efficiency.

For this reason, in recent years, especially against the background of advances in steelmaking technology, the mass production of ultra-low S and low P steels, for example, with an S content of 0.002% or less and a P content of 0°010% or less, has been realized. Finally,
Some methods have been put into practical use for manufacturing these high-quality steel machines without heat refining with relatively high productivity. However, for cracks that involve an extremely large number of factors, such as hydrogen-induced cracking and stress corrosion cracking, for example, the amount of S may be reduced to o, as described above.

Even if it can be suppressed to 0.2% or less, the influence of hydrogen generated on the surface of the steel sheet and penetrating into the steel cannot be avoided.

In order to solve these problems, recently, from the viewpoint of the chemical composition of steel, C and M have been added to suppress segregation.
Cu' to suppress hydrogen infiltration and diffusion into the steel, reducing nji? ) Adding Co, adding Nb and Ti to ensure effective hydrogen trap sites,
Furthermore, methods such as adding CaJPREM have been proposed. On the other hand, from the viewpoint of metal structure, in addition to improving the ingot casting structure, the final metal structure is changed from a ferrite-pearlite structure to a fine ferrite-bainite structure. It has been proposed to control the metamorphosis into a bainite structure.

In particular, with respect to clean steel, it is known that the material properties of prepared materials are improved by controlling the latter transformation structure.

However, in the case of relatively thick steel materials such as steel materials for line pipes, it is difficult to uniformly obtain low-temperature transformation products such as bainite structure, so in practice it is necessary to control the chemical composition and to Research is mainly being carried out in the direction of making the structure finer. That is, in this ferrite-pearlite steel, the band-like structure, which is mainly a pearlite band, has a detrimental effect on the toughness, hydrogen-induced cracking resistance, and stress corrosion cracking resistance of the steel material.
Hot rolling is controlled to create an extremely fine structure and isolate the pearlite structure. However, according to these conventional methods, hydrogen-induced cracking and stress corrosion cracking of steel sheets cannot be suppressed under severe corrosive environments such as NACE.

In order to solve the above-mentioned problems, the present inventors conducted detailed and extensive research on hydrogen-induced cracking and stress corrosion cracking in steel plates, and found that the occurrence and propagation of hydrogen-induced cracking and stress corrosion cracking in clean steel were as follows. In this process, carbides mainly composed of cementite precipitated at grain boundaries play an extremely important role, and by appropriately controlling the precipitation sites and shapes of these carbides, hydrogen-induced cracking resistance and stress corrosion cracking resistance can be improved. The inventors have discovered that it is possible to obtain a hot-rolled steel sheet that is not only excellent in other mechanical properties, but also has excellent other mechanical properties, and has thus arrived at the present invention.

The method for producing a hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance according to the present invention includes, in weight percent, C 0.03-0.3%, Si 0.01-160%, Mn 0.5-0. After rough rolling a steel billet consisting of 2.0% Al, 0.005 to 0.05% Al, and the balance iron and unavoidable impurities, the total rolling reduction in the austenite non-recrystallized region was reduced to 30%.
With the above, after finishing rolling at a temperature of 900 to 800°C, cooling to a temperature of 750 to 650°C at an average cooling rate of 15 to b seconds, and further light rolling at a processing rate of 3 to 20%. By this method, it is possible to suppress the precipitation of cementite at the ferrite grain boundaries and to cause a large amount of cementite to precipitate finely within the ferrite grains.

First, the reason for limiting the chemical composition of the steel used in the method of the present invention will be explained.

C is an essential element for ensuring the strength of steel, and in the present invention, at least 0.03% is added. However, when added in excess, it impairs the toughness and weldability of the steel, and in the case of continuously cast materials, it can cause abnormal center segregation, and it also has the effect of promoting cathode reactions in a corrosive environment. Therefore, the upper limit of its addition amount is set at 0.3%.

3i is added as a strong deoxidizing agent, and also has the effect of improving the elongation and ductility of steel by being solidified in the base material. In order to effectively express such an effect, at least 0.0
It is necessary to add 1%, but when adding too much, undesirable problems such as deterioration of weldability, reduction in cleanliness, and generation of surface scale occur, so the upper limit of the amount added is set at 1%. Set to 0%.

Mn is added in an amount of at least 0.5% in order to impart the required strength and toughness to the steel in the present invention.

However, if it is added in an excessively large amount, micro-segregation becomes noticeable and an abnormal structure is formed, deteriorating the hydrogen-induced cracking resistance and toughness, so the upper limit is set at 2.0%.

Al, like Si, is an element necessary as a deoxidizer,
At least 0.005% is added, but if added in excess, the toughness deteriorates and casting defects become noticeable, so the upper limit is set to 0.05%.

P causes micro-segregation in steel, promotes the generation of abnormal structures in the center of the steel ingot, and is harmful to hydrogen-induced cracking resistance. Therefore, in the present invention, the content is preferably 0.0
3% or less.

Further, S is harmful because it combines with Mn to form A-based inclusions, which become a starting point for cracking. When Ca is added to steel and the CaS content increases, it may form clusters and induce cracks. Therefore, in the present invention, the S content in the steel is preferably 0.01% or less.

In the present invention, in order to further impart strength and corrosion resistance to the steel plate, Cr, T t -, Cu, M
At least one element selected from the group consisting of O% N b s V and Ni can be added.

Cr has the effect of increasing corrosion resistance in addition to increasing the strength of steel, but when added in excess, penetrators are generated during welding and significantly impede weldability, so the upper limit should be set at 1.
．． Set to 0%.

Ti precipitates fine carbonitrides and acts as an effective trap site for hydrogen in the steel, further improving the hydrogen-induced cracking resistance of the steel. However, if too much is added, the carbonitrides will become coarse and will even encourage cracking, so the upper limit of the amount added is set at 0.05%. .

Cu prevents hydrogen from penetrating into steel and significantly improves corrosion resistance in a slightly acidic corrosive environment. In order to effectively exhibit such an effect, it is necessary to add at least 0.1%. However, if added in a large amount exceeding 1.0%, hot embrittlement will occur, so the upper limit of the amount added should be set at 1.0%.
%.

In addition, Mo5Nb, V, and Ni are added to increase the strength of steel and improve its toughness, but if they are added in too much, the above effects will be saturated, and furthermore, considering economic efficiency, The upper limit is M.

0.2% for Nb, 0.1% for Nb, 0.1% for ■, and 0.2% for Ni.

Furthermore, in the present invention, Ca can also be added to the steel. Ca is effective in controlling the morphology and composition of sulfide inclusions in steel, and in particular, when Ca/S≧2 is satisfied, the sulfide inclusions become completely spheroidized, resulting in an excellent Hydrogen-induced cracking resistance can be imparted. However, when added in excess, it forms clusters and performance deteriorates, so the upper limit is set to o, o o so o%.

According to the method of the present invention, a steel piece having the above-mentioned chemical components is roughly rolled and then treated under predetermined conditions, thereby achieving high strength with excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance. A high toughness hot rolled steel sheet can be obtained. That is, the total reduction rate of the steel slab in the austenite unrecrystallized region is 30
% or more and finish rolling at a temperature of 900 to 800°C, then cooled to an average cooling rate of 15 to 0°C, and further lightly rolled at a processing rate of 3 to 20%.

First, in the method of the present invention, after rough rolling a steel billet,
The total rolling reduction in the austenite non-recrystallized region is 30% or more. This is because if it is smaller than 30%, a fine structure cannot be obtained and the toughness deteriorates. The finish rolling temperature is in the range of 900 to 800°C. When the temperature is higher than 900°C, it is difficult to sufficiently reduce the austenite non-recrystallized region, so ferrite grains become coarse and toughness decreases. On the other hand, if the temperature is lower than 800°C, the precipitated ferrite will be strengthened, and as a result, the texture will develop, separation will occur frequently, and the toughness in the thickness direction will deteriorate.

After this finish rolling, supersaturated C solidified in the ferrite
In order to precipitate , it is immediately quenched to a temperature with an average cooling rate of 15~b. If the cooling rate is too slow, C is concentrated at the interface during cooling and highly pure ferrite is precipitated, so that cementite is likely to precipitate at the grain boundaries. Although the upper limit of the cooling rate is not particularly limited, it is set to 80° C./sec from the viewpoint of actual operation.

After this rapid cooling, in the method of the present invention, in order to precipitate cementite finely and in large quantities within the grains,
Light reduction is performed at a processing rate of 3 to 20% in a temperature range of 650°C. This is because in this temperature range, the largest amount of Cl precipitates in the ferrite, and by applying a light pressure reduction in this temperature range, cementite is precipitated in the finest form and in the largest number within the ferrite grains. If the light rolling temperature is outside the above range, a large amount of cementite will precipitate at the grain boundaries, making it impossible to obtain the desired physical properties. The rolling reduction rate needs to be at least 3% in order to finely precipitate cementite, while if it exceeds 20%, the precipitated ferrite phase will be processed, and as a result, the development of texture will become significant.

In addition, in the present invention, the cooling stop temperature or the winding temperature is 6
If the temperature is 50° C. or lower and higher than 650° C., coarse growth of cementite occurs and elongation of grain boundary cementite occurs.

As described above, by processing a steel billet having a predetermined chemical composition according to predetermined conditions, a fine ferrite structure is generated without forming a texture, and cementite is finely and in large numbers in the ferrite. As a result, it is possible to obtain a hot-rolled steel sheet that not only has excellent mechanical properties but also excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance.

The present invention will be explained below with reference to Examples.

Example 1 After rough rolling a steel billet having the chemical composition shown in Table 1, it was rolled in an austenite non-recrystallized area under the conditions shown in Table 2, and finished rolled at a predetermined temperature. A steel plate having a thickness of 8.8 mm was obtained by quenching, light reduction, and winding according to the method (invention method).

For comparison, a steel plate with a thickness of 8.8 mm was obtained by finishing rolling at the temperature shown in Table 2 according to the usual method, and winding without performing rapid cooling and subsequent light reduction. (comparative law).

Tensile test pieces (JIS No. 14 A test piece, diameter 6, cut in C direction), Charpy test pieces (5 mm thickness, 2 mV notch, C direction) of each steel plate obtained in this way. (cut out), hydrogen-induced cracking test piece (length 100, width 2
A stress corrosion cracking test piece (length: 75 mm, width: 15 mm, thickness: 51 mm, surface finish) was prepared and subjected to each test.

Hydrogen induced cracking (HIC resistance) test was conducted using 5% common salt and 0 acetic acid.
．． It was immersed in an aqueous solution containing 5% hydrogen sulfide and saturated with hydrogen sulfide for 96 hours, and each steel type was evaluated by inspecting 6 cross sections and using an ultrasonic flaw detector. The evaluation criteria were Q: no cracks, Δ: crack length ratio less than 3%, and ×: crack length ratio of 3% or more. Here, the crack length ratio is when W is the plate width and a is the crack length, and the stress corrosion cracking resistance (5cc resistance) is measured under the condition of applying stress of 70% of the yield strength. Ta.

After the test, the surface was observed with a 10x microscope to check for surface cracks, and the evaluation criteria were O: no cracks, Δ: cracks observed, and ×: severe cracks.

Table 2 shows the mechanical properties, impact properties, hydrogen-induced cracking resistance, and stress corrosion cracking resistance of the steel sheets. It is clear that the steel plate produced by the method of the present invention not only has superior mechanical properties and impact properties, but also superior hydrogen-induced cracking resistance and stress corrosion cracking resistance compared to steel plates produced by conventional methods. be.

Procedural correction divination (voluntary) December 10, 1982

Claims (3)

[Claims] (1) Consists of 0.03 to 0.3% by weight of C, 0.01 to 1.0% of Si, 0.5 to 2.0% of Mn, 0.005 to 0.05% of Al, and the balance consisting of iron and inevitable impurities. After rough rolling the steel billet, the total rolling reduction in the austenite non-recrystallized area is 30%.
After finishing rolling at a temperature of 900 to 800°C,
Temperature 750-650 at average cooling rate 15-80°C/sec
A method for producing a high-strength, high-toughness hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance, which comprises cooling the steel sheet to 0.degree. C. and lightly rolling it at a processing rate of 3 to 20%. (2) In weight% (a) C0.03-0.3%, Si0.01-1.0%, Mn0.5-2.0%, AE0.005-0.05%, and (b) At least selected from the group consisting of Nb 0.1% or less, Ti 0.05% or less, V 0.1% or less, Cr 1.0% or less, Cu 1.0% or less, Mo 0.2% or less, and Ni 0.2% or less. 1
After rough rolling a steel billet consisting of seed elements, balance iron and unavoidable impurities, the total rolling reduction in the austenite non-recrystallized area is reduced to 30%.
After finishing rolling at a temperature of 900 to 800°C,
Temperature 750-650 at average cooling rate 15-80°C/sec
A method for producing a high-strength, high-toughness hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance, which comprises cooling the steel sheet to 0.degree. C. and lightly rolling it at a processing rate of 3 to 20%. (3) In weight% (a) C0.03-0.3%, Si0.01-1.0%, Mn0.5-2.0%, Al0.005-0.05%, and (b) At least selected from the group consisting of Nb 0.1% or less, Ti 0.05% or less, V 0.1% or less, Cr 1.0% or less, Cu 1.0% or less, Mo 0.2% or less, and Ni 0.2% or less. 1
After rough rolling a steel billet consisting of a seed element, (c) Ca 0.0050% or less, the balance iron and unavoidable impurities, the total rolling reduction in the austenite non-recrystallized region is 30%.
After finishing rolling at a temperature of 900 to 800°C,
Temperature 750-650 at average cooling rate 15-80°C/sec
A method for producing a high-strength, high-toughness hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance, which comprises cooling the steel sheet to 0.degree. C. and lightly rolling it at a processing rate of 3 to 20%.