JPS63203721A - Production of hot rolled steel sheet having excellent hydrogen induced cracking resistance and stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To produce a non-heattreated hot rolled steel sheet having excellent hydrogen induced cracking resistance and stress corrosion cracking resistance by subjecting an ingot having a compsn. consisting of C, Si, Mn, Al, Nb, P, S and Fe to hot rolling and coiling at specific temps. CONSTITUTION:The ingot contg. 0.01-0.4wt.% C, 0.01-0.5% Si, 0.01-2.0% Mn, 0.01-0.1% Al, 0.01-0.1% Nb, <=0.1% P, <=0.03% S, contg., if necessary, at least one kind among 0.001-0.01% Ca, 0.01-0.1% V, 0.01-0.1% Ti, and 0.01-1.0% Cr, and consisting of the balance Fe and inevitable impurities is hot rolled. The rolled steel is subjected to finish rolling at >=850 deg.C. The steel sheet is thereafter cooled from a 950-700 deg.C temp. range at 5-40 deg.C/sec cooling rate and is coiled at 650-350 deg.C. The mixed grain structure of 10-50% area rate occupied by ferrite particles having >=5mum grain size is then formed. The non-heattreated hot rolled steel sheet which withstands severe NACE corrosion conditions is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a non-tempered hot rolled steel sheet having excellent resistance to hydrogen-induced cracking and stress corrosion cracking.

In recent years, there has been a demand for high-quality steel plates that have high strength and toughness, as well as excellent resistance to hydrogen-induced cracking and stress corrosion cracking.

As is already well known, the factors that most significantly affect such corrosion cracking of steel sheets are macro and micro segregation in the steel and elongation of nonmetallic inclusions. Therefore, conventionally,
For example, some methods of manufacturing high-quality steel sheets such as those described above are in practical use by adding Ca to clean steel with extremely low S and extremely low P to sufficiently control the morphology of nonmetallic inclusions. has been made into However, when using this method, the manufacturing cost is extremely high, and it is technically almost impossible to completely spheroidize M n S, so it is difficult to spheroidize M n S. In addition, A-based MflS with a length of 2 μm or less stretched in the rolling direction remains in the steel sheet and becomes the starting point of hydrogen-induced cracking.

Another known method is to reduce the amount of C and Mn and to reduce micro-segregation during casting by diffusion soaking, but this method requires long-term heating at high temperatures;
Productivity is low, and the resulting steel sheets have poor mechanical properties such as elongation and toughness. Furthermore, methods of adding Cu or Go have been proposed in order to suppress the infiltration and diffusion of hydrogen into steel, but in addition to the high cost of steel production, hydrogen-induced cracking is unlikely to occur in severe acidic corrosion environments. Not prevented.

On the other hand, from the viewpoint of metallographic structure, a method has been proposed in which the steel sheet is subjected to controlled cooling after finishing rolling to obtain a uniform bainite structure or a uniform fine ferrite-bainite structure, but the cooling operation is not easy and Low-temperature transformation product phases, such as martensite, which are harmful to hydrogen-induced cracking resistance, may be generated, and even though they are effective in suppressing the propagation of hydrogen-induced cracking, it is difficult to suppress the occurrence of hydrogen-induced cracking. That is difficult.

Moreover, none of the above methods can avoid the occurrence of hydrogen-induced cracking under NACB@corrosion conditions, which is a severe corrosive environment.Especially, when the material strength is high, hydrogen-induced cracking cannot be prevented. This is extremely difficult.

In order to solve the above-mentioned problem, the inventors conducted detailed and extensive research on hydrogen-induced cracking and stress corrosion cracking in steel sheets. During hot rolling, the steel slab is finish-rolled in the austenite recrystallization-non-recrystallization transition temperature range of 850°C or higher to form a predetermined mixed grain structure, thereby improving hydrogen-induced cracking resistance and resistance. We discovered that it is possible to obtain non-heat-treated hot-rolled steel sheets with excellent stress corrosion cracking resistance at low cost.
This led to the present invention.

5. 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 for achieving a high carbon content of 0.01-0.4% by weight and 0.01-0.01% Si by weight. 5%, Mn 0.01-2.0%, AJo 01-0.1%, Nb 0.01-0.1%, P 0.11% or less, S 0.03% or less, balance from iron and unavoidable impurities. When hot rolling a steel billet, after finish rolling at a temperature of 850°C or higher, 9
By winding at a cooling rate of 5 to b°C from a temperature range of 50 to 700°C, a mixed grain structure is formed with an area ratio of 10 to 50% occupied by ferrite grains with a grain size of 5 μm or more.

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 imparting the required strength to steel, and in the present invention, it is necessary to add a small amount (0.01%. However, when adding too much, it may affect the toughness of the steel. It impairs weldability, and in the case of continuously cast materials, it also causes abnormal center segregation, and furthermore, it promotes the cathode reaction in a corrosive environment, so the upper limit of its addition amount should be set at 0.゜4%.

St is added as a strong deoxidizing agent, and when solidified in the matrix, it has the effect of significantly improving the elongation and ductility of steel. In order to effectively exhibit this effect, it is necessary to add at least 0.01%, but if too much is added, undesirable effects such as deterioration of weldability, deterioration of cleanliness, and generation of surface scale may occur. Therefore, the upper limit of its addition amount is set at 0.5%.

Mn has a remarkable effect on improving the strength and toughness of steel, and in order to effectively obtain such an effect, Mn must be added at least 0.
It is necessary to add 0.01%.

However, if too much is added, micro-segregation becomes noticeable and abnormal structures and layered structures are formed, deteriorating hydrogen-induced cracking resistance and toughness, and furthermore, weldability.
The upper limit is set at 2.0%.

A1 is an element necessary as a deoxidizing agent like Si,
At least 0.01% is added, but if added in excess, the toughness deteriorates and casting defects become noticeable, so the upper limit is set to 0.1%.

Nb is useful in retarding austenite recrystallization and is useful in improving the strength of steel.

In order to effectively obtain such effects, it is necessary to add 0.01% or more. However, even if it is added in an excessive amount, the above effect will be saturated and it is not preferable from the economical point of view of steel manufacturing. Therefore, the upper limit of the amount added is set at 0.1%. Moreover, especially Ar
, finish rolling temperature range above the transformation point, i.e. about 800-1
At 000℃, when the amount added is less than 0.01%, all of the processed austenite recrystallizes before cooling, while when it exceeds 0.1%, it is difficult to obtain recrystallized austenite. , Therefore, if the amount of Nb is 0. O
If it is not in the range of 1 to 0.1%, a mixed state of recrystallized and unrecrystallized austenite cannot be obtained after finish rolling and before cooling.

P promotes 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, its content is set to 0.1% or less. do.

Further, S is a harmful element that combines with Mn to form A-based inclusions and becomes a starting point for hydrogen-induced cracking. Ca on steel
Even if MnS is added, it is impossible to completely eliminate MnS, so it is advantageous to keep its content as low as possible.
In the present invention, the S content in the steel is 0.003% or less.

In the present invention, in order to further impart strength and corrosion resistance to the steel sheet, at least one element selected from the group consisting of V, TI, and Cr may be added as necessary.

These elements are added to increase the strength of the steel plate and improve its toughness, but if they are added in excess, the above effects will be saturated, and furthermore, this will impair the economic efficiency of steel manufacturing.
The upper limit of the amount added is 0.1% for V, 0.1% for Ti, and 1.0% for Cr. Moreover, since the above-mentioned effects cannot be obtained when the amount of each of these elements added is less than 0.01%, the lower limit is set to 0.01%.

Furthermore, in the present invention, Ca can also be added to the steel. Ca controls the morphology and composition of sulfide-based inclusions in steel, and is particularly effective when satisfying Ca/S≧2, and can improve hydrogen-induced cracking resistance. However, when added in excess, it forms clusters,
Since it deteriorates the quality of the steel sheet, the upper limit is set at 0.01%.

In order to obtain the effect of addition, it is necessary to add 0.001%, so the lower limit is set to o, oot%.

According to the method of the present invention, when hot rolling a steel billet having the above-mentioned chemical composition, after finishing rolling at a temperature of 850°C or higher, the cooling rate is set at 5 to 70°C from a temperature range of 950 to 700°C. By winding it up at 650 to 350°C, a mixed grain structure is obtained in which the area ratio of ferrite grains having a grain size of 5 μm or more is 10 to 50%.

First, in the method of the present invention, in hot rolling a steel billet, finish rolling is performed at a temperature of 850° C. or higher. When the finish rolling temperature is lower than 850°C, austenite remains unrecrystallized, so it becomes a relatively uniform fine grain structure after austenite transformation, and as described later, the desired mixed grain structure cannot be obtained. First, it is impossible to obtain a structure in which an appropriate amount of coarse ferrite and fine ferrite or bainite coexist, which is effective in improving hydrogen-induced cracking resistance.

Next, in the cooling process following such hot rolling,
When 5 to b deviate from the temperature range of 950 to 700°C, the desired structure cannot be obtained.

Furthermore, when the cooling rate is lower than 5°C/sec, the volume of pearlite or low-temperature transformation product phase increases, and this low-temperature transformation product phase is harmful to hydrogen-induced cracking resistance and coarse ferrite grains. The area ratio of is 50% or more.

On the other hand, if the cooling rate is higher than 40°C/sec, it becomes difficult to stop the cooling of the steel sheet and control the coiling temperature, and low-temperature transformation product phases such as martensite are likely to appear in the micro-segregation areas. When the zero winding temperature is lower than 350°C, the area ratio occupied by ferrite grains with a grain size of 5 μm or more becomes less than 10%, making it impossible to obtain a suitable mixed grain structure, resulting in increased hydrogen-induced cracking. , a low-temperature transformation phase that is harmful to hydrogen-induced cracking resistance is significantly generated. On the other hand, 6
When the temperature exceeds 50°C, the entire structure becomes coarse grained.

The hot-rolled steel sheet produced by the method of the present invention has a grain size of 5μ microstructure.
It is necessary to have a mixed grain structure in which the area ratio occupied by ferrite grains of m or more is 10 to 50%. If the area ratio occupied by these ferrite grains is less than 10%, the starting point of hydrogen-induced cracking is likely to occur.On the other hand, if it exceeds 50%, an ordered structure of large ferrite grains is formed, and hydrogen-induced cracking propagates. becomes easier. In addition, the strength also decreases.

According to the method of the present invention, finish rolling in the recrystallized-unrecrystallized transition region of austenite and subsequent cooling under predetermined conditions are important technical means.

In other words, as a result of this method, recrystallized austenite preferentially recrystallized starting from nonmetallic inclusions is converted into relatively large ferrite grains, and as a result, fine inclusions that serve as starting points for cracking are reduced to hydrogen-induced cracking and stress corrosion. Hydrogen-induced cracking and stress corrosion cracking are likely to be suppressed because it has a high probability of being present in ferrite, which has a strong cracking suppressing effect, and a fine bainite structure with high HIC propagation resistance is formed around it.

As mentioned above, according to the method of the present invention, a steel billet to which a predetermined amount of Nb and other alloying components have been added is hot-rolled to form austenite recrystallization at a temperature of 850°C or higher. By finishing rolling in the crystal transition temperature region, it is possible to obtain a non-11 quality hot rolled steel sheet that is excellent in both hydrogen-induced cracking resistance and stress corrosion cracking resistance.
Even under the ACEIli food environment, no cracking occurs.

EXAMPLES The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

When hot rolling the steel slabs having the chemical components shown in Table 1, they were finish rolled under the conditions shown in Table 2 and cooled to produce hot rolled steel sheets. From this hot rolled steel plate, the length is 100fi,
A test piece with a width of 20 m and a thickness of 1 (In) was taken, and the entire surface was finished.

These test pieces were subjected to NACE (5% common salt and 0.5% acetic acid)
After being immersed for 96 hours in an aqueous solution containing hydrogen sulfide and saturated with hydrogen sulfide, each specimen was examined for hydrogen-induced cracking (HIG) by microscopic observation of six cross sections and an ultrasonic flaw detector. In addition, stress corrosion cracking (SC) was performed using the same method as above under conditions where a stress of 70% of the yield strength was applied to each test piece.
C) was investigated.

The results of HIC resistance and SCC resistance are shown in Table 2.
In the table, the evaluation of HIC resistance is: ○: No cracking, 68
Crack length ratio is less than 3%, ×: Crack length ratio is 3% or more, where W is the plate width and a is the crack length Evaluation of SCC resistance is 08 No cracks, 62 Cracks are observed in some parts but do not penetrate the test piece, ×: Cracks pass through the test piece and the test piece is broken.

Furthermore, Table 2 also shows the state of mixed grains in the obtained steel sheets.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amount of Nb added, finish rolling end temperature, and recrystallized austenite-unrecrystallized austenite transition region. Figure 2 is a graph showing the relationship between the cooling rate after finish rolling and the area of ferrite with a grain size of 5 μm or more. It is a graph showing the relationship with the rate. Patent Applicant: Kobe Steel, Ltd., Representative Patent Attorney: Ittsu Makino Department Figure 1 Nb-1<*1 Figure 2 Clearing Speed C'111! #)

Claims (2)

[Claims] (1) C0.01-0.4% by weight, Si0.01-0.5%, Mn0.01-2.0%, Al0.01-0.1%, Nb0.01-0.1% , P 0.1% or less, S 0.03% or less, When hot rolling a steel billet consisting of the balance iron and unavoidable impurities, after finish rolling at a temperature of 850°C or higher, 9
By cooling at a cooling rate of 5 to 40°C/sec from a temperature range of 50 to 700°C and winding at 650 to 350°C, the particle size is 5.
A method for producing a hot-rolled steel sheet having excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance, characterized by forming a mixed grain structure with an area ratio of 10 to 50% occupied by ferrite grains of μm or more. (2) In weight% (a) C0.01-0.4%, Si0.01-0.5%, Mn0.01-2.0%, Al0.01-0.1%, Nb0.01-0 .1%, P0.1% or less, S0.03% or less, and (b) Ca0.001-0.01%, V0.01-0.1%, Ti0.01-0.1% , Cr0.01 to 1.0%, and finishing at a temperature of 850°C or higher when hot rolling a steel billet containing at least one element selected from the group consisting of: After rolling, 9
By cooling at a cooling rate of 5 to 40°C/sec from a temperature range of 50 to 700°C and winding at 650 to 350°C, the particle size is 5.
A method for producing a hot-rolled steel sheet having excellent hydrogen-induced cracking resistance and stress corrosion cracking resistance, characterized by forming a mixed grain structure with an area ratio of 10 to 50% occupied by ferrite grains of μm or more.