RU2225457C1 - Steel with improved hardenability for cold die forging - Google Patents

Abstract

FIELD: metallurgy, namely constructional steels designed for making complex-profile heat-treatable parts by cold die forging process. SUBSTANCE: high-strength steel includes next relation of components, mass %: carbon, 0.28 - 0.35; manganese, 0.90 - 1,40; silicon, 0.01 - 0.17; sulfur, 0.005 - 0.020; chrome, 0.01 - 0.25; vanadium, 0.01 -0.07; molybdenum, 0.01 - 0,10; nickel, 0.01 - 0,10; niobium, 0.005 - 0.02; titanium, 0.01 - 0.04; boron, 0.0005 - 0.0050; aluminum, 0.02 - 0.06; nitrogen, 0.005 -0.015; iron, the balance. Next conditions are satisfied: (12/C - Mn/0.55) less or equal to 20; 500 x (Ti/24 - N/7) + 2.2 more or equal to 0; 40 less or equal to (C/0.01 + B/0.001) more or equal to 33. Rolled bars of such steel with diameter up to 35 mm may be hardened in their whole volume. EFFECT: possibility for making complex-profile hardened parts by cold die forging. 2 tbl

Description

 The invention relates to the field of metallurgy, in particular, to the development of structural stampable high-strength steel, intended for the manufacture of thermally improved complex parts obtained by cold forming.

 Known structural steel containing (wt.%): Carbon 0.16-0.25%, silicon 0.13-0.32%, manganese 0.95-1.35%, boron 0.001-0.005%, titanium 0.02-0.08%, chromium 0.10-0.27%, copper 0.15- 0.25%, vanadium 0.02-0.035%, molybdenum 0.06-0.17%, nitrogen 0.004-0.006%, nickel 0.08-0.025%, phosphorus 0.025-0.040%, tungsten 0.16-0.25%, the rest is iron [1]. The disadvantage of this steel is its low manufacturability and an unsatisfactory level of structural strength parameters with thermal improvement.

 The closest in technical essence and the achieved effect to the proposed steel is steel containing (wt.%): Carbon 0.18-0.27%, silicon 0.20-0.42%, manganese 0.60-1.0%, chromium 0.8-1.3%, nickel 0.45-0.79% , boron 0.0005-0.003%, titanium 0.02-0.05%, vanadium 0.01-0.06%, molybdenum 0.18-0.28%, zirconium 0.01-0.06%, calcium 0.001-0.008%, aluminum 0.005-0.025%, sulfur 0.010-0.060%, rest iron, with ∑ (Ti + V + Zr) = 0.05-0.12% [2].

 The disadvantages of the known steel are the wide range of variation of carbon, manganese, chromium, which does not allow to obtain a stable level of mechanical properties. The presence of zirconium in steel, although it favorably affects the characteristics of hardenability, however, makes its production in some cases not technologically advanced due to the poor digestibility of this element in steelmaking. It is also shown that in this steel without losing the level of consumer properties it is possible to exclude such expensive elements as zirconium and calcium. The analyzed composition also does not take into account the protection factor of boron from binding to nitrides, which at an industrially obtained level of nitrogen in steel will not allow to obtain increased characteristics of its hardenability.

 The objectives of the invention are to increase the hardenability characteristics and to ensure through hardenability of long products with a diameter of up to 35 mm

Примеси: фосфор до 0.025%, медь до 0.20%.The objectives are achieved in that the proposed steel containing carbon, silicon, manganese, chromium, nickel, vanadium, titanium, aluminum, molybdenum, nitrogen, boron, sulfur, the rest of the iron additionally contains niobium and nitrogen (in the absence of calcium and zirconium) in the following the ratio of components, wt.%:
Carbon - 0.28-0.35
Manganese - 0.90-1.40
Silicon - 0.01-0.17
Sulfur - 0.005-0.020
Chrome - 0.01-0.25
Vanadium - 0.01-0.07
Molybdenum - 0.01-0.10
Nickel - 0.01-0.10
Niobium - 0.005-0.02
Titanium - 0.01-0.04
Boron - 0.0005-0.0050
Aluminum - 0.02-0.06
Nitrogen - 0.005-0.015
Moreover

Impurities: phosphorus up to 0.025%, copper up to 0.20%.

The given combinations of alloying elements make it possible to obtain in the rolled metal of the proposed steel (long products with a diameter of up to 35 mm) after thermal improvement (quenching from a temperature of at least 900 ^° C followed by tempering from a temperature of 480-600 ^° C) a homogeneous finely divided martensite structure with a favorable combination of strength characteristics and plasticity.

Carbon and carbonitride-forming elements are introduced into the composition of this steel in order to provide a finely dispersed grain structure, which will increase both its strength level and provide a given level of ductility. In this case, niobium controls processes in the austenitic region (determines the tendency to growth of austenite grain, stabilizes the structure during thermomechanical processing, increases the recrystallization temperature and, as a result, affects the nature of the γ-α transformation), while the effect of vanadium is manifested at temperatures below And ₁ , since it is in this region that the interval of intense release of vanadium carbonitride is located. Vanadium also contributes to the hardening of steel during thermal improvement. The upper limit of the carbon content (0.35%), vanadium (0.07%) and niobium (0.02%) is due to the need to ensure the required level of ductility of steel, and the lower (0.28%, 0.01% and 0.005%, respectively) is provided by the required level of strength of this steel.

 Manganese, molybdenum and chromium are used, on the one hand, as solid solution hardeners, and on the other hand, as elements that significantly increase the stability of supercooled austenite and increase the hardenability of steel. Moreover, the upper level of the content of these elements (respectively 1.4% Mn, 0.10% Mo, 0.25% Cr) is determined by the need to ensure the required level of ductility of steel, and the lower (respectively 0.90% Mn, 0.01% Mo, 0.1% Cr) is determined by the need to provide the required level strength and hardenability of steel.

 Nickel in the specified range (0.01-0.10%) affects the characteristics of hardenability and toughness of steel.

 Silicon refers to ferrite-forming elements. The lower limit on silicon - 0.01% is due to steel deoxidation technology. A silicon content above 0.17% will adversely affect the ductility characteristics of steel.

 Sulfur determines the level of ductility of steel. The upper limit is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit is due to issues of manufacturability.

 Boron contributes to a sharp increase in the hardenability of steel. Moreover, the upper limit of the boron content is determined by considerations of ductility of steel, and the lower - the need to ensure the required level of hardenability.

 Aluminum and titanium are used as deoxidizers and protect boron from binding to nitrides, which contributes to a sharp increase in the hardenability of steel. So, the lower level of the content of these elements (0.02 and 0.01, respectively for aluminum and titanium) is determined by the requirement to ensure hardenability of steel, and the upper level (0.06 and 0.04) is determined by the requirement to ensure a given level of ductility of steel.

 Nitrogen, an element involved in the formation of carbonitrides, while the lower level of its content (0.005%) is determined by the requirement to ensure a given level of strength, and the upper level (0.015%) is determined by the requirement to ensure a given level of ductility and hardenability.

в противном случае не обеспечивается защита бора от связывания его в нитриды и резко снижаются характеристики прокаливаемости стали.To ensure complete binding of nitrogen to nitrides such as TiN and AlN as a result of reactions
[Ti] + [N] = TiN
the following ratio of elements is required:

otherwise, boron is not protected from binding to nitrides and the hardenability characteristics of steel are sharply reduced.

определяет условия сохранения в стали более 50% "эффективного" бора, что обеспечивает заданные характеристики прокаливаемости стали.Ratio

determines the conditions for preserving in steel more than 50% of the “effective” boron, which ensures the specified characteristics of hardenability of steel.

определяет условия обеспечения заданного уровня характеристик прокаливаемости стали и параметры технологичности.Ratio

determines the conditions for ensuring a given level of hardenability characteristics of steel and processability parameters.

Таким образом, заявляемое техническое решение соответствует критерию "новизна".Comparative analysis with the prototype allows us to conclude that the claimed composition differs from the known lack of calcium and zirconium and the introduction of new components, niobium and nitrogen, as well as the ratios

Thus, the claimed technical solution meets the criterion of "novelty."

 The analysis of patent and scientific and technical information did not reveal solutions having a similar set of features that would achieve a similar effect - an increase in the hardenability characteristics of steel.

 Therefore, the claimed combination of features meets the criterion of "significant differences".

 Below are examples of the implementation of the invention, not excluding others, in the scope of the claims.

где M₁ и М₂-средние значения сравниваемых величин; S₁ ² и S₂ ² дисперсии среднего; \
t $\genfrac{}{}{0ex}{}{\text{0,005}}{\text{kr}}$ (α) - критическое значение критерия Стъюдента при уровне значимости 0.95 и числе степеней свободы -α.
Определение характеристик прокаливаемости (критический диаметр D₅₀) проводили методом торцевой закалки цилиндрических образцов диаметром 25.0 мм и длиной 100 мм с заплечиками согласно ГОСТ 5657. Перед изготовлением образца заготовки прошли термическую обработку в камерных печах по следующему режиму: нормализация, 900^oС, 1 час, воздух. Испытывали по два образца на плавку. Закалка образцов проводилась струей воды в специальной установке. В связи с необходимостью предотвращения окисления и обезуглероживания торца образца, непосредственно соприкасающегося со струей воды при закалке, нагрев образцов в камерных печах (без защитной атмосферы) проводили в специальных стаканах. Торец образца ставился на специальную графитовую пластину. Образец нагревался в камерной печи до температуры 900^oС. Продолжительность прогрева образца до температуры закалки составляла 30-50 минут. Отклонения от заданной температуры закалки не превышало ±5^oС. Выдержка образца при температуре закалки после нагрева составляла 30 мин. Время с момента извлечения образца из печи до начала охлаждения не превышало 5 с. Образец находился под струей воды до полного охлаждения (порядка 15÷20 мин). Температура охлаждающей воды составляла 20±5^oС. Для замера твердости по всей длине закаленного образца сошлифовывались две диаметрально противоположные площадки на глубину 0.5±0.1 мм. Площадки сошлифовывались при обильном охлаждении водой. Шероховатость поверхности площадок была не грубее 7-го класса чистоты по ГОСТ 2789. Не допускались прижоги, вызывающие структурные изменения металла. Для построения кривой прокаливаемости стали замер твердости начинали на расстоянии 1.5 мм от закаленного торца в осевом направлении. Первые 16 замеров от торца образца производили с интервалом 1.5 мм, а затем через 3 мм. Если на определенном расстоянии от торца образца твердость не меняется, то измерения производили через один интервал, а затем прекращали испытания. С целью обеспечения точной фиксации мест измерения твердости было специально сконструировано и изготовлено приспособление. В случае необходимости повторного измерения твердости на площадке, на которой были сделаны замеры, площадку перешлифовывали. Глубина съема металла при повторной шлифовке составляла 0.1÷0.2 мм. Твердость определяли по Роквеллу (HRC) в соответствии с требованиями ГОСТ 9013. Для каждой пары точек, находящихся на одинаковом расстоянии от торца образца на двух противоположных площадках, подсчитывали среднее арифметическое значение твердости.Under experimental conditions, 10 melts of experimental steels were smelted, the chemical composition of which is shown in Table 1. Billets of samples of the studied steels with a size of 14 • 14 • 300 mm were heat-treated in laboratory furnaces of the SNZ type according to the following modes: quenching from 900 with a holding time of 50 minutes and cooling in water. Vacation at a temperature of 580 ^o With a shutter speed of 120 minutes. The thickness of the workpieces and cooling conditions during quenching provided through hardenability of the workpieces. Mechanical characteristics were determined on tangential samples. Tensile tests at room temperature were carried out on type I specimens, GOST 1497-84 on an INSTRON-1185 testing machine with strain-strain registration. The loading speed of the sample is 5 mm / min. The strength characteristics σ _b and σ _0.2 and the viscosity δ and φ were determined.
The average values of the characteristics were calculated according to the test results of at least three samples per point. The significance of the differences in the average values of the analyzed values was evaluated using the Student criterion, calculated as follows:

where M ₁ and M ₂ are the average values of the compared values; S ₁ ² and S ₂ ^{2 the} variance of the average; \
t $\genfrac{}{}{0ex}{}{\text{0.005}}{\text{kr}}$ (α) is the critical value of the Student criterion with a significance level of 0.95 and the number of degrees of freedom -α.
The hardenability characteristics (critical diameter D ₅₀ ) were determined by end hardening of cylindrical samples with a diameter of 25.0 mm and a length of 100 mm with shoulders according to GOST 5657. Before manufacturing the sample, the billets were heat treated in chamber furnaces according to the following regime: normalization, 900 ^o С, 1 hour air. Two samples were tested for melting. Quenching of the samples was carried out by a stream of water in a special installation. In connection with the need to prevent oxidation and decarburization of the end face of the sample directly in contact with the water jet during quenching, the samples were heated in chamber furnaces (without a protective atmosphere) in special glasses. The end face of the sample was placed on a special graphite plate. The sample was heated in a chamber furnace to a temperature of 900 ^o C. The duration of heating of the sample to the quenching temperature was 30-50 minutes. Deviations from the set quenching temperature did not exceed ± 5 ^° C. The sample exposure at quenching temperature after heating was 30 min. The time from the moment the sample was removed from the furnace to the start of cooling did not exceed 5 s. The sample was under a stream of water until completely cooled (about 15–20 min). The temperature of the cooling water was 20 ± 5 ^o С. To measure hardness along the entire length of the hardened sample, two diametrically opposite sites were ground to a depth of 0.5 ± 0.1 mm. The sites were ground with plenty of water cooling. The surface roughness of the sites was not coarser than the 7th grade of cleanliness according to GOST 2789. Burns were not allowed, causing structural changes in the metal. To construct the steel hardenability curve, hardness measurements were started at a distance of 1.5 mm from the hardened end in the axial direction. The first 16 measurements from the end of the sample were made with an interval of 1.5 mm, and then through 3 mm. If the hardness does not change at a certain distance from the end of the sample, then the measurements were made at one interval, and then the tests were stopped. In order to ensure accurate fixation of hardness measuring points, a device was specially designed and manufactured. If it is necessary to measure hardness again at the site on which measurements were taken, the site was resurfaced. The depth of metal removal during repeated grinding was 0.1–0.2 mm. The hardness was determined according to Rockwell (HRC) in accordance with the requirements of GOST 9013. For each pair of points located at the same distance from the end of the sample at two opposite sites, the arithmetic mean value of hardness was calculated.

 The mechanical properties are presented in table 2.

 As can be seen from table 2, the proposed steel in comparison with the known has higher hardenability characteristics.

SOURCES OF INFORMATION
1. USSR copyright certificate 1406208, C 22 C 38/54, 10.30.1986

 2. USSR author's certificate 768849, C 22 C 38/54, 03/06/1978 (prototype).

Claims (1)

High-strength steel containing carbon, silicon, manganese, chromium, nickel, vanadium, titanium, aluminum, molybdenum, boron, sulfur and iron, characterized in that it additionally contains niobium and nitrogen in the following ratio, wt.%: Carbon 0.28-0.35 Manganese 0.90-1.40 Silicon 0.01-0.17 Sulfur 0.005-0.020 Chrome 0.01-0.25 Vanadium 0.01-0.07 Molybdenum 0.01-0.10 Nickel 0.01-0.10 Niobium 0.005-0.02 Titanium 0.01-0.04 Boron 0.0005-0.0050 Aluminum 0.02-0.06 Nitrogen 0.005-0.015 Iron Else moreover

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