JPH0112814B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high-strength steel plate whose yield ratio is lower than that of steel of the same strength level. Generally, steel with a high yield ratio has a small uniform elongation, so when deformation occurs during an earthquake or the like, the elongation is locally concentrated in one place and it is easy to break. For this reason, steel materials with a high yield ratio tend to be difficult to use in the fields of civil engineering and architecture. In addition, it is extremely rare to add B (boron) to steel that is rolled without quenching and tempering, or is subjected to normalizing treatment, and only British fortiweld steel is known. There are only This invention produces high-strength steel plates with a low yield ratio using simple manufacturing processes and equipment, using low-alloy steel plates containing a trace amount of B, by simply rolling them as they are or by tempering them after rolling if necessary. As a result of studying the method, we learned that the slab heating temperature greatly controls the hardenability improvement effect of B, and by controlling the slab heating temperature and rolling conditions, we can produce steel sheets with the above characteristics with stable quality. It was discovered that it can be manufactured at low cost. Here, the tensile strength is 80 as a low yield ratio steel plate.
85% or less at Kg/mm ² , 80% or less at tensile strength 70Kg/mm ² ,
It also refers to a material with a tensile strength of 60Kg/mm ² and 75% or less. The low yield ratio high tensile strength steel plate of this invention has C 0.13~
0.35%, Si 0.05~0.90%, Mn 0.75~2.50%, Mo
0.05~0.80%, Nb 0.01~0.08%, solAl 0.03~
0.15%, B 0.0002~0.0015%, and Cu 0.10~
2.00%, Ni 0.10-2.50%, Cr 0.10-2.50%, V
A slab containing one or more of 0.01 to 0.10% is heated to 1150℃ or less, and then heated to 900℃.
The rolling process is completed at 850 to 650℃ by adding at least 30% of the thickness reduction strain [(plate thickness at 900℃ - final plate thickness) / plate thickness at 900℃] to achieve the specified plate thickness. It is characterized in that it is manufactured by being left to cool as a steel plate, or by finishing rolling at 850 to 650°C and then tempering it at 550°C or less. That is, B improves hardenability when it is dissolved in solid solution and segregated at grain boundaries, but it combines with N to form BN and loses its hardenability improvement effect. However, the slab temperature is 1150
When the temperature exceeds .degree. C., AlN in the steel dissolves into solid solution, the amount of solid solute N increases, and BN is generated during the cooling process after rolling, and the hardenability improving effect of B disappears. At temperatures and rolling conditions that take advantage of the above-mentioned effects of B to prevent solid solution of AlN and formation of BN, the microstructure of the steel sheet becomes acicular and the yield ratio decreases. It is also well known that by setting the rolling conditions to low-temperature, high-pressure finishing, a fine structure is formed and low-temperature toughness is improved. In this invention, it is required that a reduction strain of at least 30% or more be applied at a temperature of 900° C. or less. Furthermore, the rolling finishing temperature is set at 850 to 650°C because recrystallization occurs at finishing temperatures higher than 850°C, making it impossible to improve toughness by rolling.
Ferrite-Austenite 2 below 650℃
This is because the rolling process occurs in the phase region, and the ferrite is processed, increasing the yield point and preventing a low yield ratio. The yield ratio of the high-strength steel plate manufactured by this invention method is
The target is 85% or less. Tempering is performed after rolling when a low yield ratio and good toughness are required. However, the higher the tempering temperature, the higher the yield ratio, so in order to keep the upper limit of the yield ratio below 85%, the tempering temperature should be 550°C or below. The reason why a steel plate having a low yield ratio can be obtained in this invention will be explained below. The yield phenomenon occurs at low stress because the higher the mobile dislocation density, the easier it is for the dislocation to start moving and undergo plastic deformation due to a smaller stress. Since the yield ratio is expressed as yield point/tensile strength, it is necessary to lower the yield point and increase the tensile strength in order to obtain a low yield ratio. For this purpose, the condition can be achieved by using a quenched structure of bainite or martensite. When bainite and martensite transform from austenite, mobile and immobile transitions occur at a high density on their habit planes. As high stress is applied after yielding, these become entangled with each other and become immobile transitions, and eventually cause netting and break, but ultimately, the higher the immobility transition density, the higher the tensile strength without causing netting even under high stress. Show that. This invention provides a composition in which a microstructure with a high density of mobile dislocations such as bainite and martensite can be obtained by allowing the steel to cool after rolling. Further, as mentioned above, rolling and cooling as is is preferable in order to lower the yield ratio, but there are also applications where toughness needs to be further improved, in which case tempering treatment is performed at a lower temperature. When tempering is performed, carbides precipitate on the mobile transition, turning the mobile transition into an immobile transition and increasing the yield point (thus increasing the yield ratio), but when the tempering temperature is lowered, the yield ratio is 85%. It is suppressed from exceeding. Next, the reason why the chemical composition of the steel sheet is limited in this invention will be explained. If C is less than 0.13%, the required strength cannot be obtained. However, if it exceeds 0.35%, weldability deteriorates, which is not preferable. Si content of 0.05% or more is required to ensure strength, but if the content exceeds 0.90%, the toughness will be significantly degraded. Mn content of 0.75% or more is required to increase strength and toughness. However, if it exceeds 2.50%, it should be avoided as it can cause an abnormal structure, resulting in two-piece cracking or lamellar tear, which can cause trouble during welding. When Mo coexists with B, hardenability can be ensured even at a slow cooling rate comparable to normalizing, so it is necessary to contain Mo at 0.05% or more, but if it exceeds 0.80%, the hardenability will not improve significantly. Nb is effective in refining crystal grains and improving low-temperature toughness and yield point when contained in trace amounts of 0.01% or more.
If it exceeds 0.08%, low-temperature toughness and weldability will deteriorate, so it was set at 0.01 to 0.08%. SolAl must be contained in an amount of 0.03% or more in order to fix N and ensure the hardenability improvement effect of B. However, if it exceeds 0.15%, it is not preferable because it may cause cracking of the steel ingot. B is an element that can be obtained at a low price and is a main constituent element of this invention steel in order to obtain the necessary strength. That is, when B is contained in a trace amount of 0.0002% or more, the hardenability is greatly improved at a rolling heating temperature of 1150° C. or less. However, if it exceeds 0.0015%, boron nitride will be produced at a cooling rate comparable to normalizing, and the hardenability will deteriorate, so the appropriate amount is 0.0002 to 0.0015%. In addition to the above-mentioned constituent elements, this invention steel also contains Cu,
Contains one or more of Ni, Cr, and V. Cu content of 0.10% or more increases the strength, but if it exceeds 2.00%, it is undesirable because it induces weld cracking. Ni increases low-temperature toughness and bond toughness when it is contained in an amount of 0.10% or more, but the effect does not improve much when it is contained in an amount exceeding 2.50%, which is not economically preferable. Cr content of 0.10% or more improves hardenability and thus improves strength after rolling, but if it exceeds 2.50%, weldability deteriorates. V content of 0.01% or more causes precipitation hardening and improves strength, but if it exceeds 0.10%, toughness deteriorates. The reason why the heating temperature of this invention steel is limited to 1150° C. or less is to improve the hardenability effect of B contained as described above, thereby preventing solid solution of AlN and suppressing the formation of BN. In addition, in order to improve low-temperature toughness by refining austenite grains through rolling, 900
℃ or less, the thickness reduction rate (plate thickness at 900℃ - final plate thickness / plate thickness at 900℃) must be 30% or more to achieve low-temperature, strong pressure strain. At that time, the range of finishing rolling temperature should be 850 to 650°C. At finishing temperatures higher than 850°C, recrystallization occurs and the effect of rolling is reduced. Furthermore, if the finishing temperature is lower than 650°C, the partially transformed ferrite will be processed and the yield point will increase, making it impossible to achieve the low yield ratio that is a feature of this invention. Next, an example of this invention method will be described. Component steels belonging to this invention having the chemical compositions shown in Table 1 (sample steels A to E) and comparative steels that do not contain B (sample steel F)
The slabs of ~H) were rolled to a thickness of 15 mm by changing the heating and rolling conditions as shown in Table 2, and the as-rolled and tempered steel plates were subjected to tensile and impact tests and compared. . The results are shown in Table 2.

【table】

【table】

[Table] As can be seen from the above table, Steel A and Steel H have almost the same composition except for the difference in B content, but when they are treated in the same manner using the method of this invention, that is, No. 1 steel plate and No. 1 steel plate. Comparing No. 16 steel sheets, there is a large difference in strength and toughness, demonstrating the effect of B content. In addition, the comparison between No. 4 steel plate and No. 6 steel plate of B steel, No. 7 steel plate and No. 8 steel plate of C steel, and No. 9 steel plate and No. 10 steel plate of D steel is shown below. The effects of strain on strength and toughness were further investigated for No. 1 steel plate of A steel and No. 1 steel plate of A steel.
The comparison between No. 3 steel plate and No. 9 steel plate and No. 11 steel plate of D steel shows the influence of slab heating conditions on strength and toughness.
The excellent effects of the steel plate manufacturing method of this invention are shown. As described above, according to the method of the present invention, it is possible to easily produce a high-strength steel plate that has a low carbon equivalent, has excellent strength and toughness, has a lower yield ratio than steel at the same strength level, and can be used satisfactorily as rolled. can be manufactured.

Claims (1)

[Claims] 1. C 0.13-0.35%, Si 0.05-0.90 in weight ratio
%, Mn 0.75~2.50%, Mo 0.05~0.80%, Nb
0.01~0.08%, solAl 0.03~0.15%, B 0.0002~
0.0015%, and Cu 0.10~2.00%, Ni 0.10~
A slab containing one or more of 2.50%, Cr 0.10 to 2.50%, and V 0.01 to 0.10%, with the balance consisting of Fe and unavoidable impurities, is heated to 1150°C or lower and heated to 900°C or lower to at least 30% A method for producing a low yield ratio high tensile strength steel plate, which is characterized by rolling at a thickness reduction rate of 850 to 650°C, and then allowing it to cool. 2 Weight ratio: C 0.13-0.35%, Si 0.05-0.90
%, Mn 0.75~2.50%, Mo 0.05~0.80%, Nb
0.01~0.08%, solAl 0.03~0.15%, B 0.0002~
0.0015%, and Cu 0.10~2.00%, Ni 0.10~
A slab containing one or more of 2.50%, Cr 0.10 to 2.50%, and V 0.01 to 0.10%, with the balance consisting of Fe and unavoidable impurities, is heated to 1150°C or lower and heated to 900°C or lower to at least 30% Rolled at a thickness reduction rate of , finished rolling at 850 to 650℃, and then
A method for manufacturing high-strength steel sheets with a low yield ratio, characterized by tempering at a temperature of 550℃ or less.