JPS60181228A - Manufacture of hot rolled steel material - Google Patents

Abstract

PURPOSE:To enable the accurate estimation of the quality of a steel material after hot rolling under any conditions when a plate, a hot strip or the like is manufactured by hot rolling, by attaining the desired strength according to a strength estimating attaining the desired strength according to a strength estimating equation including terms determined by structures and grain size. CONSTITUTION:In order to attain the desired strength, hot rolling is carried out by setting the grain size of ferrite and the strength and volume fraction of each of ferrite, pearlite, bainite and martensite structures so as to satisfy the strength estimating equation (where TS is tensile strength, sigma deg. is a parameter expressing the strength of each structure, V is volume fraction, F is ferrite, P is pearlite, B is bainite, M is martensite, and each of A1-A6 is a constant). By this method, factors related directly to strength and handled, and the relation among them is prescribed, so the strength of a steel material can be accurately estimated independently of changes in conditions during rolling and cooling or the structure of the steel material.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing steel products such as thick plates and hot strips by hot rolling.

(Problems with the Prior Art) The material quality of a steel material is generally determined by a term determined by the structure and a term determined by the grain size. For example, the tensile strength (TS) can be expressed as in equation (1).

TS=fCσF, σP, σB, σM, vl-+VP
, vl , Vrl l dF ) (1) Here, σ is a parameter representing the strength of each structure, ■ is the volume fraction, d is the grain size, F is ferrite, P is pearlite , M indicates martensite.

The present invention provides a means for accurately estimating or calculating the strength of steel materials. Conventionally, the only methods for estimating the strength of steel materials are simple multiple regression models using variables such as composition, hot rolling end temperature, coiling temperature, or cooling stop temperature, which directly determines the strength. There were no studies that dealt with factors such as the strength of the structure, volume fraction, and ferrite grain size. In other words, the strength was estimated using indirect manufacturing factors, and even though it was an inexact model with poor accuracy, it was still usable because conventional multiple regression models only estimate products from one rolling mill. The manufacturing conditions are as follows: rolling starts from a certain heating condition, rolling conditions including the finishing temperature (which determines the austenite grain size before transformation) are determined from the product thickness, and the cooling temperature range (which governs the transformation behavior) This is because the rolling process and cooling rate were automatically determined from the rolling end temperature and coiling temperature.

Therefore, it can be said that the conventional model was a special model that could only be applied in specific cases such as those mentioned above, and it was attempted to expand the range of materials for rolled materials by broadly changing the rolling conditions and subsequent cooling conditions. It cannot meet the demands of the new era. However, according to the present invention, since the factors that directly determine the strength are dealt with and the relationship between the two is defined, it is possible to estimate the material quality of the hot-rolled steel material under all conditions and with an accuracy that cannot be obtained in the past. be.

(Objective of the Invention) The object of the present invention is, based on the above-mentioned knowledge, to eliminate all the conventional drawbacks and to develop an innovative hot rolling method that performs hot rolling by controlling the essential factors that control the material properties of steel materials. and a cooling control method.

(Structure of the Invention) The structure of the present invention for achieving the above object is as follows.

In order to obtain the target strength, hot rolling is performed by setting the ferrite grain size and the strength and volume fraction of various structures of ferrite, pearlite, bainite, and martensite that satisfy the strength estimation formula below. Method of manufacturing rolled steel products.

TS=AI +A2 Xσy' XVF +A3 Xσ
p XVp+A4 ×σB' xv, +A5 The model will be explained below. There is the following known theory regarding the strength of composite structure steel.

Theory 1: For example, according to Nakane et al.'s [Tetsu to Hagane e8 (1982) P2OJ], if the strength ratio of the soft phase to the hard phase is 2.3 or less, a constant strain model is established, and the strength can be expressed by the following formula. I can do it.

σ=σ1V1 +σ2 V2 where σ1 is the strength of the soft phase and vl is the volume fraction.

Further, σ2 is the strength of the hard phase, and ■2 is the volume fraction.

Theory 2: For example, M, F. Ashby's rPhil May, 1
4 (In2) According to P1157J, the strength when the hard phase is dispersed can be written in the form of the following equation.

σ−σ. +ao x J”i where σ.l (10 is a constant term, ■ is the volume fraction of the hard phase.

The present invention combines these known theories to formulate the relationship between strength and texture, and the main points are as follows. That is, (1) there has been no attempt to estimate the strength by combining the above two theories.

■No one has specifically shown which of the above two theories falls into the category of ferrite grains, pearlite, bainite, and martensite.

■There is no knowledge that the strength of ferrite-pearlite steel, ferrite-pearlite-bainite steel, ferrite-bainite steel, ferrite-martensitic steel, or any other combination of steel can be estimated using a single formula.

From Theory 1, it can be easily inferred that the constant strain model holds true for ferrite and pearlite.

It is also known that Theory 2 can be applied to ferritic and martensitic dual phase steels. However, for steels with three or more phases containing martensite, theory 1. Neither theory 2 is applicable. In this way, both Theory 1 and Theory 2 have a narrow scope of application and can only be applied under limited conditions.

However, recently, multiphase steels containing martensite have been developed one after another by making full use of rolling technology and cooling technology. Against this background, expanding the application of strength estimation formulas has been raised as an urgent issue. The present invention meets this need, and the inventor's analysis revealed that by combining Theory 1 and Theory 2, a prediction formula that can be applied to all cases can be obtained.

First, the structures that appear in ordinary steel can be roughly divided into ferrite,
It can be classified into four types: pearlite, bainite, and martensite. Therefore, when formulating the relationship between these structures and strength, it is necessary to determine which of the above two theories is applicable. Therefore, first, in order to determine the application target of the constant strain model of theory 1, the Ickers hardness of various tissues was measured and the ratios were compared. Since ferrite is the softest of the four types of structures, the ratio values are shown in FIG. 1 based on this hardness.

As can be seen from FIG. 1, the hardness ratio of pearlite and bainite to ferrite is 2.0 or less. i.e. ferrite, pearlite.

The constant strain model of Theory 1 can be applied to Boehnerite. What remains is martensite, which falls within the category of Necessity Theory 2.

From the above, it can be seen that the model formula for estimating the strength of steel can be written as follows.

TS = σfVf + (Tp Vp + σ3 VB + kL particles - (2) It is clear that the strength of ferrite can generally be written as follows.

10 σf = σ0 child 10 k, d i 1.910. (3
) (2) and (3), TS = σofV(-+σPvp + (TB VB
57−÷+kfd4 (4) Parameters σI, σP0. If σJ is used, σof=BIXσi +B2−...−(5)σp=13
3 ×σδ +B4 ・・・・・・(6)σ,=B5X
σ; +B6 ......(7) Therefore, (4) is T S = AI + A2 X % XVF + Aa
X (fp X Vp+A4×σ, o xvB +A
5X (C-10+A6X dF...(8)).

Furthermore, an example in which the coefficients of A0 to A6 are determined by the least squares method is shown below.

TS=-10,425+ 0.289XHFXVF+
0.342XHp XVp + 0.287XHB X
VB + 58.05-÷ x, r; + 1.259X dF (
9) However, H is Ickers hardness, and here it is used as a parameter expressing hardness in terms of strength. The unit of TS is "kg/mm", d is mm, and the contribution rate of the formula is R = 0.984 for the target material in the wide range shown in Table 1.
Met. As is clear from the contribution rate of the equation, equation (8) can estimate the TS with extremely high accuracy. or,
It is also possible to know the combination of the strength 1 volume fraction of the structure and the ferrite grain size necessary to obtain the required material from equation (8), and then determine the rolling conditions and cooling conditions to create them.

Here, the volume fraction of the tissue is determined using the isothermal transformation diagram (TTT).
There is an estimation method based on , and there is a method for estimating parameters representing tissue strength, such as hardness, from components and cooling conditions. Regarding the ferrite grain size, the γ grain size was measured using online ultrasonic waves or X-rays as a direct measurement method. There is a method of estimating the ferrite grain size.

(Example) Next, an example according to the present invention will be shown. The component systems of the sample materials in the examples are all C--Mn-based, and the details are shown in Table 1.

As can be seen from Table 1, no matter how the structure and ferrite grain size change, according to the present invention, TS can be estimated with very good accuracy. As mentioned above, this causes
For example, predict the strength using equation (8), or use equation (9) to determine the required strength, volume fraction, and ferrite grain size from the required strength, and perform hot rolling and subsequent cooling regulation to achieve these values. becomes possible.

As described above, the present invention is the first to formulate the relationship between the structure and strength of steel materials, and this method has great power in estimating strength with high accuracy.

(Effects of the invention) As is clear from the above explanation, no matter how the rolling conditions and cooling conditions change and the structure of the steel material changes, according to the present invention, the strength can be estimated accurately. Determine the volume fraction and ferrite grain size, and roll with this in mind.
Cooling conditions can be specified. As a result, the strength estimation accuracy in hot rolling is greatly improved, making it possible to perform extremely efficient hot rolling, resulting in significant effects such as improved yield and reduced costs.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the ratio of hardness between the soft phase and the hard phase for each type of structure. Patent Applicant Representative Patent Attorney Tomoyuki Yafuki (and 1 other person) 3 1st γtJ[J (“ 01 (’1 1 o’clock turn) Naon)”

Claims (1)

[Claims] To obtain the target strength, hot rolling is performed by setting the ferrite grain size and the strength and volume fraction of various structures of ferrite, pearlite, bainite, and martensite that satisfy the following strength estimation formula. A method for producing hot rolled steel material characterized by: TS=Al +A2 XσF XVy +A3 XσP
'XVP + A4 x σ: XVB +A5 86: Constant