JPS58733B2 - Method for manufacturing non-temperature high tensile strength hot rolled steel strip for processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-rolled steel strip, and particularly to a method for producing a high-strength steel plate with excellent toughness, cold workability, weldability, etc.

High-strength hot-rolled steel sheets for cold working require a high level of strength and excellent workability at the same time, but in general, increasing strength tends to reduce workability, so it is important to improve these in a harmonious manner. It is desired.

In other words, as the strength increases, workability factors such as elongation, stretchability, stretch flangeability, etc. decrease, and
Problems with deterioration of shape fixability due to increased springback during molding and equipment due to increased processing force,
Various obstacles arise, such as mechanical disadvantages.

It is known that these problems regarding workability become significant when the yield stress of the material is relatively high.

By the way, so-called high-strength hot-rolled steel sheets used for cold working generally have a tensile strength of 60 kg/mm2 or more and a yield point of 45 kg/mm2 or more, but in this case, increasing the tensile strength also increases the yield point. Not only does this increase, but the ratio of the latter to the former (yield ratio) also becomes higher, and as a result, formability often becomes dramatically difficult.

Therefore, a realistic and preferable high-strength steel is one that has as low a yield stress as possible despite having high tensile strength in its raw state, while having a yield limit that rises to a predetermined level after processing. It can be said that it is something.

In order to achieve the above, various attempts have been made in the past, but they have not yet achieved satisfactory results and are often accompanied by other drawbacks such as poor weldability.

For example, a well-known method is to add reinforcing elements to an iron slab in order to increase the strength of the steel plate.

This method uses Nb, V, TiB, etc. as the main strengthening elements, and utilizes the precipitation strengthening and grain refining effects of these elements as the main strengthening mechanism, while supplementary additions of Si, M
The strength is increased by the solid solution hardening (strengthening) effect of n, Ni, Cl, Cr, Mo, etc., but when this method is used, the rate of increase in yield stress is large compared to the tensile strength. However, there is a drawback that the yield ratio exceeds 70%, which is considered as a practical guideline, and reaches 80 to 95%.

In order to improve this drawback, as is well known, it is effective to increase the C content or the amount of solid solution strengthening elements added, but in the case of the former, it has a very negative effect on workability toughness and weldability. In the latter case, a relatively large amount of addition is required to increase the strength, resulting in an increase in C equivalent and a concomitant deterioration in toughness and weldability. There is a limit to the decline.

Next, as the other side, a method using a bainite structure is also known.

, this method is based on the properties of the bainitic structure, i.e., high strength can be obtained with relatively low C equivalent chemical components and a large number of free dislocations introduced during transformation, so that no soil yield point appears during the tensile test. However, ferrite, pearlite,
This is a method to obtain a steel plate with high strength and low yield point by utilizing the fact that it has a lower yield than structural steel.

However, if only a normal bainite structure is used,
As is well known, the rate of decrease in elongation relative to the increase in strength is extremely large - that is, the yield point is too low - and this deteriorates the processing method.

An object of the present invention is to eliminate the drawbacks of the prior art and provide a method for producing a high-strength hot-rolled steel strip that has excellent toughness, cold workability, weldability, etc. in a low content of reinforcing elements. It is in.

In order to achieve the above object, the present invention provides that when hot rolling an iron slab containing reinforcing elements etc., the content of the reinforcing elements etc. is C: 0.02-0.12% (weight, same as below)
, Mn: 1.0-2.0%, Si: 1.0% or less, Mo
: 0.05 to 1.0%, Al: 0.10% or less, S: 0
．． 020% or less, and during the hot rolling, the slab was heated at 1000 to 1300°C, and the finish rolling was performed at a starting temperature of 9500°C or less, but at a finishing temperature of TFS or higher as described later, and a total downstream rate of 50. % or more, cooling is performed within the temperature range of TFS to TBS described below at a cooling rate of VC or higher, and winding is performed to the MS point of the steel ~ (TBS - 30°C
) is characterized in that it is carried out sequentially within the temperature range.

However, TFs is the starting temperature of pro-eutectoid ferrite transformation in a quasi-equilibrium state (℃), TBS is the starting temperature of ultrafine bainite transformation (℃), and VC is the ultrafine bainite composition ratio of 60%.
The lower limit (° C./S) of the cooling rate required to achieve the above is given by each posterity.

In the present invention, the reinforcing elements contained in the iron slab include Mn, Si, and Mo, which are generally known as solid solution hardening elements.
In addition, C1 is a reinforcing element, AI is a deoxidizing element, S is an unavoidable element, and Nb and V are examples of reinforcing elements that may be added as necessary.

It broadly refers to Ti, examples of solid solution hardening elements such as Cr, Ni, and Cu, Zr, Mg, Ca, rare earth elements, and B as examples of shape control agents for sulfides in steel.

The effects of the reinforcing elements and the reason for the content according to the present invention are as follows.

C: 0.02% or more is required for strengthening effect, but 0.1%
If it exceeds 2%, it not only adversely affects workability, toughness, and weldability, but also is unfavorable for forming the ultrafine batonite structure described below.

Mn: Has the effect of making it easier to obtain an extremely fine bainite structure by strengthening, lowering the Ar3 transformation point, delaying the ferrite-pearlite transformation, lowering the bainite transformation temperature, etc., and for this purpose, 1.0% or more is added. preferable.

However, if it exceeds 2.0%, it is not preferable because it deteriorates workability, toughness, and weldability, and also develops rolling texture and increases the anisotropy of mechanical properties.

Si: As a solid solution hardening element, it has the effect of increasing the strength without significantly increasing the yield ratio, but on the other hand, it has the effect of increasing the Ar3 transformation temperature and the bainite transformation temperature, and therefore 1.
If it exceeds 0%, it is not only unfavorable for obtaining an extremely fine bainite structure, but also causes a decrease in commercial value due to deterioration of surface properties.

Mo: The most important element in obtaining an ultrafine bainite structure.

In other words, since it has the effect of significantly retarding ferrite and pearlite transformation, it is possible to improve the ultrafine bainite structure ratio, and it is possible to roll at a lower temperature under the control of the cooling rate during finish rolling, which will be described later. Therefore, by achieving a finer bainite structure, it is advantageous in terms of yield ratio, strength, workability, and toughness.

0.05% or more is required to exhibit this effect, but if it exceeds 1.0%, not only will manufacturing costs increase unnecessarily, but the transformation will be significantly delayed, resulting in martensite (MS).
) phase is generated, which adversely affects workability, toughness, etc.

A1: From the viewpoint of workability and toughness, it is used as the main deoxidizing element, but its use in excess of 0.10% is not only unnecessary, but also causes an increase in alumina-based inclusions. Undesirable.

S: It is desirable to reduce as much as possible in order to improve workability, toughness, and weldability, but it is also related to manufacturing costs, so it is relatively easy to achieve with commercial steel and it is possible to substantially reduce the negative effects. The upper limit is 0.020% as the range that can be reduced.
shall be.

Nb, V, Ti (Group

However, the total amount of these elements added is 0.20%.
If the value exceeds 0.20, the above effect not only becomes saturated, but also the yield ratio increases significantly due to precipitation hardening, which is not preferable for obtaining the low yield ratio that is the objective of the present invention.
It needs to be %.

Cr, Ni, Cu (Y group): As solid solution hardening elements, it is possible to improve the strength without significantly increasing the yield ratio, and because it lowers the bainite transformation temperature, it is easy to obtain an extremely fine bainite structure. However, not only are these elements expensive, but adding them in large amounts has a negative effect on workability, toughness, and weldability.
．． The upper limit is 0%, and it may be added as necessary.

Zr, Mg, Ca, rare earth elements (group 2): Effective for controlling the shape of sulfides in steel, and using an appropriate amount is preferable because it improves the anisotropy of mechanical properties, but adding a large amount may reduce the amount of inclusions. This is not preferable because it actually increases the amount.

They may be added as necessary within a total amount of elements of 0.1% or less, which is a range that does not cause such adverse effects.

B: Addition of a trace amount has the effect of delaying ferrite-pearlite transformation, and if necessary, it may be added in an amount of 0.020% or less.

Next, the iron slab containing the above-mentioned strengthening elements etc.
The reason for heating at 1300° C. is to enhance the effect in subsequent treatment steps.

That is, in order to achieve the object of the present invention, it is necessary to obtain an extremely fine bainite structure in the next finish rolling process;
For this purpose, it is preferable that the initial austenite grains in the iron slab be small.

Therefore, it is desirable that the heating temperature of the slab be lower, but 10
If it is less than 00°C, it is not preferable because a predetermined finish rolling end temperature cannot be ensured.

On the other hand, if the temperature exceeds 1300'C, the initial austenite grains may become coarsened, resulting in misalignment, and it may be necessary to adjust the temperature to ensure a predetermined finish rolling start temperature after rough rolling, resulting in a reduction in rolling efficiency. This is undesirable as it may cause a decrease in

Next, the reason why the finish rolling conditions and the cooling conditions in the process are within the scope of the present invention will be described.

As already stated (for the purpose of the present invention, the yield ratio is practically 7
0% or less and to increase the tensile strength. To achieve this, the relationship between the ultrafine bainite structure ratio and the yield ratio (see Figure 1) and the relationship between the tensile strength and the ultrafine bainite structure, which have been studied by the present inventors, are required. From the knowledge that there is a substantially linear positive relationship with the bainite structure ratio, it is known that the ultrafine bainite structure ratio needs to be 60% or more.

In order to do this, as is clear from the continuous cooling transformation curve diagram (omitted), first of all, the cooling rate from the austinite temperature region does not cut the pro-eutectoid ferrite-pearlite transformation nose, that is, the critical cooling. It is necessary to perform rapid cooling at a rate higher than the cooling rate, and secondly, the temperature range that requires this rapid cooling must be from at least the temperature at which pro-eutectoid ferrite transformation begins to the temperature at which bainite transformation begins at the critical cooling rate. However, the ultrafine bainite structure in the present invention is produced by fine austenite grains to which processing strain has been introduced, and its transformation characteristics are vastly different from the bainite transformation obtained by ordinary heat treatment. , it is impossible to quantitatively infer this using conventional transformation curve diagrams.

Therefore, in order to carry out the quantitative estimation described above, it is essential to introduce new parameters that are not based on conventional transformation curve diagrams.

As a result of intensive study, the present inventors have determined that the above-mentioned (1) to (3)
TFS, TBS, and VC shown in the formula were found as effective parameters.

As is clear from each equation, these parameters are mainly regulated by the chemical composition of the steel, but are also influenced by changes in the slab heating temperature, finish rolling temperature, rolling reduction, etc.

However, these effects do not need to be a particular problem as long as the processing conditions of the present invention are met.

By using the above parameters and performing finish rolling and cooling under the conditions of the present invention, an extremely fine bainite structure ratio of 60% or more can be achieved.

That is, Fig. 2 shows the ultrafine bainite structure percentage when the quenching end temperature of steel with the same chemical composition according to the present invention is kept constant at TBS, and the cooling rate V and quenching start temperature (TI) are varied. This diagram schematically shows the changes.

As is clear from this figure, at any cooling rate, as the quenching start temperature increases, the ultrafine bainite structure rate increases and tends to be saturated near the temperature where the quenching start temperature is almost the same as TFS. .

The value of the saturated ultrafine bainite structure ratio increases as the cooling rate increases, and when the rapid cooling start temperature is near TFS, v-■
It is confirmed that it reaches approximately 60% when c.

Next, Figure 3 shows the same steel as in Figure 2, but the quenching start temperature is TF.
The cooling rate V and the quenching end temperature (Tl
) is schematically shown to show the change in ultrafine bainite structure ratio when changing 1. It is something that

As is clear from this figure, at any cooling rate, as the quenching end temperature decreases, the ultrafine bainite structure rate increases significantly, and tends to be saturated near the temperature where the quenching end temperature is almost the same as TBS. .

It is confirmed that this saturation value is 60% or more when the cooling rate is higher than VC in the vicinity of TBS, but 60% cannot be secured when it is lower than VC.

From the above, in the present invention, cooling is performed from TFS to TB.
The reason for carrying out the cooling rate below vc within the temperature range of S is understood.

Next, to explain the finish rolling, which is a pre-cooling step, the reason why the upper limit of the finish rolling start temperature is set at 950°C is that if it exceeds this, the reduction in the non-recrystallization temperature region becomes insufficient, resulting in This is because poor introduction of strain results in a relatively coarse bainite structure mixed in, which deteriorates workability and toughness.

In addition, the reason why the finish rolling end temperature is set to be higher than TFS is that in a lower temperature range, under normal rolling method, rolling will be applied in the two-phase region (r+a), resulting in deterioration of workability and toughness. This is because it is easy.

However, in the treatment under the chemical composition and rolling conditions of the present invention, unless any control is performed, the finish rolling end temperature will be TF
In order to avoid this, the cooling rate is controlled even during finish rolling to keep the finish rolling end temperature at TFS.
It is necessary to maintain the above level.

The total rolling reduction during finish rolling needs to be at least 50% or more in order to obtain a sufficiently uniform ultrafine bainite structure.

Next, the final process, winding, will be described.

The thermal history of the steel after being cooled by the above method is preferably such that an extremely fine bainite structure is generated through isothermal transformation as much as possible, so this thermal history can be easily obtained in a winding process such as in a hot rolling hot strip mill, for example. The case is favorable.

However, as in the present invention, when the cooling conditions are set such that the ferrite/pearlite transformation temperature region is rapidly cooled and its progress is suppressed as much as possible, transformation occurs rapidly after winding, and the coil temperature increases due to the latent heat of transformation. It may rise to exceed the extremely fine bainite transformation temperature range and reach the pearlite transformation temperature range.

In such a case, the percentage of ultrafine bainite structure decreases, resulting in a significant decrease in strength and increase in yield ratio, and it may not be possible to secure the desired mechanical properties.

The effect of temperature rise due to latent heat of transformation after winding is that if the chemical composition is within the range of the present invention, the winding temperature is (TBS - 30℃
) This can be resolved by doing the following.

On the other hand, the reason why the winding temperature is set to be higher than the MS point is that there is a risk that the martensitic phase may be partially mixed if it is not done.

Under the rolling conditions of the present invention, the austenite grains are sufficiently refined, so that the proportion of martensite phase is 10%.
If it is less than this, the material quality will not be significantly damaged, but if it exceeds this, it will have a negative effect on workability and toughness, and if it is mixed in a particularly large amount, it can cause defects in the shape of the steel sheet, so in any case, martensitic phase Contamination should be avoided as much as possible.

Examples Various iron slabs with chemical components shown in Table 1 (Examples 1 to 10)
, Comparative Examples 1 to 4) were processed under the rolling conditions shown in the same table to obtain hot-rolled steel sheets with a thickness of 6 mm, and their mechanical property values were determined, and the results shown in Table 2 were obtained.

As is clear from Table 2, although the tensile strength (TS) of the present examples is high (especially Examples 5 to 10), the yield strength is 0.2% (which is essentially considered as the yield point). In addition to being relatively low (therefore, the yield ratio is less than 70% of the practical range), it has a good balance between strength and elongation (El), and is extremely good in terms of impact resistance, workability, and weldability. It is understood that it is excellent.

On the other hand, among the comparative examples using the conventional method, when the cooling rate in the finish rolling process is slow (Comparative Example 1) and when the coiling temperature is high (Comparative Example 2), the tensile strength is extremely low and the yield ratio is also extremely high. However, this can be said to be due to insufficient formation of ultrafine bainite.

In addition, when the finish rolling start temperature is set to be as high as that in the normal rolling method (Comparative Example 3), even if other conditions meet the conditions of the present invention, elongation occurs, and the absorbed energy (E-
20° C.), transition temperature (vVrs), and notch elongation, which is an example of processing characteristics, are both significantly deteriorated.

This is considered to be due to insufficient refinement of one grain and the formation of a coarse bainite structure.

As is clear from the above, when using conventional methods, it is impossible to simultaneously achieve improvements in tensile properties, especially in the comprehensive sense of tensile strength, yield ratio, and impact properties, toughness, workability, and weldability. , this is possible according to this embodiment.

The hot-rolled steel strip obtained in this example is suitable for various processing applications including steel materials whose upper limit of yield ratio is regulated, such as line pipes and casing pipes.

As explained above, according to the present invention, (1) a copper strip with excellent properties can be produced simply by hot rolling, and the resulting copper strip (2) is a high-strength steel with a low yield ratio. (3) It is especially advantageous when processing low strain areas; (4) Although it uses structural reinforcement as a strengthening mechanism, it follows a processing heat treatment method, so it has low workability in terms of strength.
Various effects can be obtained, such as a good balance of toughness and (5) high strength despite low alloying element content, resulting in excellent weldability.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the relationship between the yield ratio and the ultra-fine bainite structure ratio, Figure 2 is a diagram explaining the influence of the rapid cooling start temperature and cooling rate on the ultra-fine bainite structure ratio, and Figure 3 *
FIG. 3 is a diagram illustrating the influence of the quenching end temperature and cooling rate on the field fine bainite fraction.

Claims (1)

[Claims] 1. When hot rolling an iron slab containing reinforcing elements, etc., the content of the reinforcing elements, etc. is changed to C: 0.02 to 0.12% (
weight, the same below), Mn: 1.0 to 2.0%, Si: 1
．． 0% or less, Mo: 0-05 to 1-0%, Al: 0.1
0% or less and S: 0.020% or less, and during the hot rolling, the slab was heated to 1000 to 1
Finish rolling at 300°C, with a starting temperature of 950°C or lower, but with a finishing temperature of TFS or higher, and a total rolling reduction of 50°C.
% or more, cooling is performed sequentially within the temperature range of TFS to TBS described below at a cooling rate of VC or higher, and winding is performed within the temperature range of the MS point of the steel to (TBS - 30 ° C.). However, TFS (temperature at which pro-eutectoid ferrite transformation starts in semi-normal state °C) - -204 [%C] -14 (%Mn) +5
8C%5i)-23C%Ni)+24[%Cr)+22
C%Mo)-14(%Cu)+(0,8-(%C))・
(-75[%Mnl+47[%5i)-30(%Cr)
+20C%Mo)-8,8C%Cu:l)+910,T
BS (starting temperature of ultrafine baitonite transformation °C) = -15
8[%C]-11[%Mn]+35(%Si,)l-1
7C%Ni]-25[%Cr]-19[%Mo), vc
(Lower limit of cooling rate required to make ultrafine baitotonite composition ratio 60% or more, °C/5) -12-3 (%C] -
S, SC%Mn)+13.2(%5i)-4,7[%N
1) -19,5(%Cr)+25exp(-2,5[%
Mo)) -6,6 (% Cu) + 15) A method for producing a non-thermal high tensile strength hot rolled copper strip for processing. 2 In claim 1, it is further provided that 0.2% or less of one or more elements selected from group X consisting of Nb, V, and Ti as reinforcing elements and/or Cr, Ni, and C
1.0% of one or more elements selected from the Y group consisting of u
It is characterized by containing 0.1% or less of one or more elements selected from the following and/or two groups consisting of Zr, Ca, Mg, and rare earth elements and/or 0.020% or less of B. A method for producing a non-tempered high tensile strength hot rolled copper strip for processing.