JPS6026808B2 - Method for manufacturing thick-walled hot-rolled high-strength steel strip with excellent low-temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a thick hot rolled high tensile strength steel strip which is required to have high strength of 50k9f/fence or higher and low temperature resistance as a material for line pipes, structures for industrial machinery, etc.

Generally, in order to obtain a thick-walled hot-rolled high-strength steel strip with a thickness of 6 ribs or more having the above characteristics, controlled rolling is carried out using Nb-containing steel, but Nb-containing steel has an effect of suppressing austenite recrystallization. Since this is large, a coarse upper bainite structure extending in the rolling direction is likely to be mixed in.

As a countermeasure for this, controlled rolling is being carried out with an emphasis on austenite pongee granulation through low-temperature heating, but when it undergoes rolling after rolling, bainite structure is likely to be mixed in, and due to low-temperature heating, Nb It also has the unavoidable drawback of a decrease in strength due to coarsening of (CN). The present invention was completed as a result of intensive study of controlled rolling and controlled cooling methods for V-containing steels, which have a large amount of solid solubility, while comparing them with Nb-containing steels. The feature is that controlled rolling and controlled cooling are performed using V-containing steel that does not contain Nb so as to maximize the effect of V on pongee graining and low-temperature transformation promotion.

0.08%C-1.5%Mn-0.13%V in the attached drawings
The results of investigating the effect of coiling temperature on the mechanical properties of steel (0.07%C-1.6%Mn-0.04%N uniform steel B) using hot rolling simulation experiment method are as follows. show.

According to this, it can be seen that the influence of the coiling temperature is significantly different between V steel and Nb steel. In N-type steel, when the steel is rolled at a low temperature of 550° C. or less, its strength increases and large-scale verticalization occurs.As a result of microstructural investigation, it was found that this formation of arms was due to the inclusion of the bainite structure. On the other hand, V steel has a peak of arm formation at 600 qo, but both excellent low-temperature toughness and high strength are obtained by low-temperature coiling at 500 to 300 qo. This is because the peak of arm formation at 600°C is due to the formation of secondary precipitated arms of V(CN) that occurs during slow cooling after coiling.
In normal low-temperature winding (550 to 600 qo), Nb steel has been used more favorably than V steel because this arming is large.
It has been found that in low-temperature winding of 500 to 300°C, V steel has better strength and low-temperature sardine resistance than Nb steel.

This is because, according to the results of microstructural observation, a bainite structure is mixed in all cases, but the bainite structure in V steel is very different from that in Nb steel, with a small amount and a large number of fine grained polygonal ferrite. It is divided by
In other words, this is interpreted to be because the secondary precipitation strengthening of V (CN) is suppressed in low-temperature winding of 500 pm or less, and the pongee grain strengthening effect and low-temperature transformation strengthening effect of V are exerted. On the other hand 3
It is thought that the formation of arms during winding at a temperature lower than 00oo is due to the disappearance of the self-tempering effect due to slow cooling after winding. The drawing shows the above-mentioned V steel legs and Nb steel legs with approximately 1,200 pieces each.
℃, reduce the pressure by 75% at 1100 qo or less and
Finish rolling at ℃ and set the cooling rate to 5 to 10℃/
This figure shows the influence of the winding temperature on the tensile strength (TS) and the Charpy fracture surface transition temperature (vT) of a hot-rolled sheet with a thickness of 11 (s).

Next, the reason for limiting the range of steel components in the present invention will be explained.

C: C brings about an increase in strength through an increase in the amount of low-temperature transformation, but if the amount of C is too large, the amount of low-temperature transformation is too large, causing a severe deterioration of low-temperature ballability.

Therefore, in order to obtain a high strength of 50 kof/mau or higher, the lower limit is set to 0.02%, and the upper limit is set to 0.02% in consideration of the deterioration of the properties of sardines at low temperatures.
It shall be 15%. Si: Si is effective in increasing strength through solid solution strengthening, but the upper limit is set at 0.00000000000000000000000000000000000000000000000000000000 level in consideration of deterioration of weldability.
It shall be 60%.

Mn: Mn is effective in increasing strength through both solid solution strengthening and low-temperature transformation strengthening, but the lower limit is set at 0.5% in order to obtain the specified high strength, and the upper limit is set at 2% in consideration of deterioration in weldability.
．． It shall be 20%. S: S combines with Mn to form nonmetallic inclusions, resulting in a decrease in Charpy shell phenergy in the direction perpendicular to rolling.

Therefore, it is desirable to reduce it as much as possible, but from the economic point of view, the upper limit is set at 0.010%. Sol nail: Sol mountain is effective as a deoxidizing agent, but the deoxidizing effect is 0.10%
Therefore, the upper limit is set to 0.10%.

V: V is a particularly important element in the present invention.

Generally, the purpose of adding V in low alloy steel is to utilize the precipitation strengthening effect of V(CN), but this causes large precipitation arms in thick hot rolled high tensile strength steel strip.

The purpose of adding V in the present invention is to aim for an increase in strength while ensuring good low-temperature grade properties by utilizing the grain strengthening effect and low-temperature transformation strengthening effect by combining the rolling conditions and cooling conditions described below. . Therefore, the lower limit is set at 0.10% at which the above effect is expected, and the upper limit is set at 0.30% at which the effect is saturated. Cu
, Ni, Cr, Mo: has the same effect as Mn.

However, if the amount is too large, the effect will be saturated and it will be economically disadvantageous, as well as having disadvantages such as deterioration of weldability, which is not preferable. Therefore, the upper limit is set at 0.5% for each. C
a: Ca is due to the reduction effect of A-based inclusions and B-based inclusions,
This results in an improvement in shell phenergy in the direction perpendicular to rolling.

However, if the amount added is too large, the number of C-based inclusions in the steel increases, which conversely causes a decrease in shell phenergy. Therefore, the upper limit is set to 10 melon blood. The method of the present invention is characterized by hot rolling as described below using copper having the above-mentioned composition. This is because, if the amount of rolling is smaller, the pongee graining of austenite does not proceed much, and coarse bainite structures that form arms due to rapid cooling after rolling are likely to be mixed in.

Further, the upper limit of the rolling finishing temperature is set to 850° C. because finishing at a higher temperature reduces the amount of fine-grained polygonal ferrite mixed in, but also causes a large amount of coarse bainite structure to be mixed in. The lower limit is set to 680 qo because finishing at a lower temperature than this will process a large amount of ferrite, which will not only increase the anisotropy based on the texture but also deteriorate the rutting property. In addition, after rolling, it is rapidly cooled at a rate of 5 to 10 p0/s and coiled at a rate of 500 to 300 q0, but the upper limit of the cooling rate is 100°C/s. This is because, if the winding is carried out, not only will the fine grain strengthening effect and low-temperature transformation strengthening effect, which are the purpose of V addition, become unavailable, but there is also a high possibility that secondary precipitation arms of V(CN) will occur. is 100℃/
This is because, if it exceeds s, the transformation of ferrite during cooling is significantly suppressed, and the bainite structure becomes extremely large, which also deteriorates the toughness. In addition, if the material is wound at a temperature lower than 300q0, the self-tempering effect due to slow cooling after winding is lost, resulting in large arms. Next, an example of a hot rolling simulation experiment of the present invention will be shown. Hot rolling was carried out using copper having the chemical composition shown in Table 1 under the conditions shown in Table 2, and the resulting mechanical properties are shown in Table 3.

[11] is a Nb steel, and under the rolling and cooling conditions according to the present invention, a coarse bainite structure is mixed and the low-temperature grainability is not good.

In addition, ■, '31, '4', and {51 are the same V steels, but ■ and [51, which belong to the present invention, have high strength and excellent low-temperature resistance, and All V-containing steels have the expected good properties. In addition, in comparison (11) where the cooling rate is significantly lower, there is an extremely large amount of bainite structure, and although the strength is high, the grain quality is greatly deteriorated. Table 1 Chemical composition (wt) Table 2 Hot rolling conditions (finished plate thickness 11 side) Table 3 Mechanical properties

[Brief explanation of the drawing]

The attached drawing shows V steel (0.08%C-1.5%Mn-0.1
3%V) and N uniform wire (0.07%C-1.6%Mn-0.
4% Nb) is a chart showing the results of a hot rolling simulation experiment investigating the influence of coiling temperature on mechanical properties.

Claims (1)

[Claims] 1 C0.02 to 0.15%, Si 0.60% or less, M
n0.50-2.20%, S0.010% or less, sol
Al 0.10% or less, V 0.10-0.30%, balance F
The steel consisting of e and impurities is rolled down at a working rate of 70% or more at 1100°C or less, and the rolling is finished at 850 to 680°C,
After that, rapid cooling was performed at 5 to 100°C/s, and 500 to 30°C
A method for producing a thick-walled hot-rolled high-strength steel strip with excellent low-temperature toughness, characterized by winding at 0°C. 2 C0.02-0.15%, Si0.60% or less, M
n0.50-2.20%, S0.010% or less, sol
Al 0.10% or less, V 0.10-0.30%, Cu 0.50% or less, Ni 0.50% or less, Cr 0.5
Steel consisting of one or more of 0% or less, Mo 0.50% or less, the balance Fe and impurities is rolled at a working rate of 70% or more at 1100°C or less, and the rolling is finished at 850 to 680°C. , then rapidly cooled at 5 to 100°C/s to 50°C.
A method for producing a thick-walled hot-rolled high-strength steel strip with excellent low-temperature toughness, characterized by winding at 0 to 300°C. 3 C0.02-0.15%, Si0.60% or less, M
n0.50-2.20%, S0.010% or less, sol
Al 0.10% or less, V 0.10-0.30%, Ca1
11.00ppm or less, steel with the balance Fe and impurities
Reducing at a processing rate of 70% or more at 00℃ or less,
A method for producing a thick hot-rolled high-strength steel strip with excellent low-temperature toughness, characterized in that rolling is completed at 80°C, followed by rapid cooling at 5 to 100°C/s and coiling at 500 to 300°C. 4 C0.02-0.15%, Si0.60% or less, M
n0.50-2.20%, S0.010% or less, sol
Al 0.10% or less, V 0.10 to 0.30%, and one or more of Cu 0.5% or less, Ni 0.5% or less, Cr 0.5% or less, Mo 0.5% or less, and Steel consisting of 100 ppm of Ca or less, the balance Fe and impurities is rolled down at a working rate of 70% or more at 1100°C or less, and 8
Finish rolling at 50~680℃, then 5~100℃/
A method for producing a thick hot-rolled high-strength steel strip with excellent low-temperature toughness, the method comprising rapidly cooling the steel strip at a temperature of 500 to 300°C.